The more I think about the challenges facing us (humanity) and the opportunities to use biomimicry for innovation in the built environment, the more I believe that we can come up with super cool solutions using biomimicry for any challenge, but unless the fundamental assumption of everyone at the design table is that our built environment is dependent upon, participates in and can support thriving local ecosystems, we will produce solutions that will ultimately fall short of embodying the shift we want and need to see in the way we live life on this planet.
I also believe that once designers come to the table with a basic scientific understanding of our entwinement with the life around us, a whole new world of creative opportunity opens up to not just design and build a structure that solves for human needs, but rather design and build a multifunctional, responsive structure that is a participant in a complex web of life. The next question then becomes, what else can the structure do?! Biologists at the design table can help work with designers to answer that question.
There is incredible thought leadership and work being done around the world to try to reconcile how we can put into words and practice these ideas of shifting a built environment designed to sit upon a landscaped into one that lives within it. The related articles at the end of this post were shared with me by biomimicry colleagues (thanks Josh Stack, Jane Toner and Norbert Hoeller!) and are on my reading list to help me wrap my head around how these ideas fundamentally change our approach and how we move forward.
My thought is, imagine if a region could get together to establish that fundamental assumption for itself – bringing together designers and decision-makers from all functions and scales of the built environment to agree that all design should strive to support fundamental ecosystem functions using local native ecosystem metrics. Each participant in this collective leadership could influence their own piece of the puzzle (playing out in various industries and scales) while at the same time considering and building in mechanisms for how their piece fits into, can respond to and support the whole. Can it be done?
At Biomimicry Chicago we are boldly imagining such a future for the Chicago region through our Deep Roots Initiative which we are kicking off with our Deep Roots Workshop April 21. We want to explore these ideas and see if/how we can put these ideas to practice. There is incredible work being done in Chicago in trying to address multiple challenges having to do with various ecosystem functions at multiple scales. We have an opportunity to come together to understand how they are all interrelated from an ecological perspective, define what is ecologically “sustainable” for the region and set an overarching framework of goals to strive for. Our subsequent measures of progress as we intentionally restore ecosystem functions in our built environment will then have a scientific basis for assessing whether or not we as a region are truly on the path toward “sustainability.”
The more minds thinking about this, the better. I encourage you to feel free to share more resources in the comment thread below. Only together can we change our story!
Following up on something I have been thinking about since my post on inspiration for the Global Biomimicry Design Challenge on climate change, I thought I’d share an example of my thought process on using natural models for initial brainstorming. This is my first pass and I haven’t dug deep into the science, but am testing the waters on a high-level idea. So bear with me as I try to wrap my head around this one – energy and associated system cycles. I have more questions than answers as my thoughts are only (maybe not even) half-baked – maybe you can help me out. Or feel free to use my ideas to add to yours!
Last week I was talking with my colleague about various major categories of ecosystem functions. Her diagram had five categories, including “energy” and “carbon”. In looking at the diagram however, I realized that this perspective separates out two components that are fundamentally part of, but not even all of, one system. Does combining the conversations of carbon sequestration and energy efficiency into a comprehensive discussion about the entire system around energy beyond just the carbon cycle, with a comparison to the natural model, provide an avenue to identify missed opportunities to balance things out?
When we talk about energy it is almost always purely in the mindset of procurement/consumption. Energy flow is one way – we dig it up/suck it up/soak it up/stick a turbine in it and gobble it up. What’s the result? We put that energy to work for us in various ways that fuel our activities – cooking, transporting, building, farming, etc. The end result is that that energy once used is gone, but the benefits we reap from consuming it might live on in the form of something made (cooked food, a product, a house, a road…). Doing more with one unit of energy is how we improve efficiency. In the sustainability realm the conversation about “energy efficiency” is sometimes shifted to “carbon management” in recognition of energy consumption (specifically when it’s carbon-based) as a component in the larger carbon cycle.
When we talk about carbon sequestration it’s often a kind of nebulous, unseen phenomenon that most people don’t understand. We know it’s part of the carbon cycle and is a component we have increasingly realized we need to address because there’s this vast amount of carbon dioxide accumulating in our atmosphere and changing our climate. So we also relate carbon sequestration to energy in the realm of the need to pull back out the carbon dioxide emitted during the burning of fossil fuels and organic matter to help balance the carbon cycle. But this discussion is not often expanded to be related to a comprehensive picture of energy beyond a discussion of carbon dioxide. And while carbon dioxide is our main concern, maybe an analysis of the whole system could identify opportunities we might otherwise miss.
In nature, energy procurement and consumption is fed by the sun, but the story of energy is not just about carbon dioxide. It involves an intricate dance of several inputs and outputs in the system enable it to stay balanced in perpetuity – everything is used and recycled with the exception of heat. Not true of our current human system. Even when we look to understand photosynthesis for the conversion of radiant heat to energy to try to replicate that natural model (solar panels), we choose to basically ignore the whole sequestering of carbon dioxide, use of water and releasing of vast amounts of oxygen, water and carbon dioxide thing that occurs in photosynthesis too – we’re just interested in the conversion of energy from one form to another. Are we missing vast components of a balanced system and thus opportunities to greatly improve our design? What if we tried to mimic the functions of the entire natural system of inputs and outputs to restore balance?
I’ve already talked about how our energy systems have knocked the carbon cycle out of balance. So, using biomimicry, if we want to use the plant energy cycles – complete with the inputs and outputs – as a model for our energy systems, we need to understand nature’s energy system first and then draw metaphors. Easiest thing to do is to draw a picture!
The following diagram shows an overall simplified cycle of inputs and outputs involved in photosynthesis, plant growth and energy flows supporting the food web (that’s us in the “animal” block). (Photosynthesis is the process in which radiant energy is turned into chemical energy in the form of sugars, which are the building blocks for plant structure (starches) as well as immediate energy for plant growth. That stored energy in plants is the energy that gets passed up the food chain from herbivores to carnivores and everything in between.)
The above diagram shows how the byproducts of each step contribute to critical resources for other steps in the process, creating a closed loop with the exception of the renewable energy input of the sun and outputs of heat. It’s brilliant.
Contrast that with examples of our energy systems. The following diagrams also show simplified energy flows in human-designed systems.
Okay, so now we have an overall idea of how these both work. Notably for me, neither of the human-designed energy systems result in closed loop cycles of inputs and outputs. The solar obviously resembles more closely the procurement and conversion of radiant energy at a site, similar to a plant. I don’t know enough about energy systems to know how to wrap my head around the conversion of radiant energy to electricity vs. chemical energy – that’s above my pay grade for this blog post! So let’s keep it simple for now (but if you know, let me know a good resource to find out more!).
In looking at the coal-fired power plant example, you’ll notice that the inputs include coal, oxygen (O2) (oxygen needed for combustion of coal) and water. In looking at the energy cycle of the food web, you’ll notice that the inputs and outputs are similar to that of an animal – animals consume glucose (stored energy) from plants or other animals that have eaten plants. Animals also consume oxygen which is needed for chemical reactions that result in growth (for the metabolic process). So we might draw the following metaphors included in the above diagram:
Oxygen = oxygen (needed for a chemical reaction
Coal = stored chemical energy (sugars) (this is the fuel)
Power plant = animal
Use of electricity to build structures = metabolism (growth)
Battery storage = maybe ATP? (adenosine triphosphate, or “the ‘molecular unit of currency’ of intracellular energy transfer”)
(Since I have not spent more than today on this, my metaphors might be off. What do you think?)
If you agree for now that we might draw a metaphor between animals and our human-made power plants, what does that mean for our overall cycle? To me it means our current design is missing a plan for the majority of the system needed to maintain the required balance for stable system functioning (as evidenced through climate change). The question is, how can we think about our energy systems more holistically and model them after the original power plants and energy webs?
If we go with the above, and our current energy system design only includes the “animal” component of the larger system, what if we expand our discussion of “energy” to include the entire cycle of inputs and outputs to understand how we can design an energy system that fits within the natural balance to maintain climate stability? Who uses energy in the system and how? What do they produce as a byproduct of using that energy? What questions does that raise about our systems?
Need for balance of inputs & outputs: the consumption and sequestration of carbon dioxide (CO2) and water (H2O) with the release of oxygen (O2), carbon dioxide and water. If fossil fuels and other biomaterials aren’t burned for energy at all, obviously this changes the equation and reduces the burden on the system to sequester carbon dioxide (once restored to a balance from the current state, and this of course ignores whatever inputs and outputs for making the product (e.g., a solar panel)). But since we aren’t flipping the switch on fossil fuels any day soon, we need to find ways to bring the carbon cycle back into balance. And what about the release of oxygen in the process – what part of our system might generate oxygen as a byproduct?
Forms of energy: What would a system look like that relies on local real-time renewable conversion of radiant energy to, and storage of, chemical energy that is in a form readily accessible for use (as opposed to use of non-renewable storage of radiant energy captured in fossil fuels and turned to electricity)? Lots of solar cells (produced with solar energy sources!) and batteries? Any other options?
If plants (real plants, not human power plants!) are the consumers of carbon dioxide in our natural model, what would be the equivalent of a plant in human energy systems? Manufacturers creating raw materials? Does that reveal the missing link in our system – manufacturers who convert radiant energy on site to fuel their own manufacturing processes (core needs) as well as build raw materials (which form the basis of structures) from carbon dioxide? If so, we clearly need to rethink the potential of a hugely (over) abundant (free!) resource – carbon dioxide – as a building block for materials. Some materials manufacturers are already thinking this way, but if this is the key to balancing the cycle, we need some serious widespread innovation using carbon as a fundamental building block of many more our materials.
Can we use the energy flow of a food web to think more about how the supply chain beyond materials manufacturers plays a role – what’s the equivalent of a herbivore (e.g., a manufacturer turning materials into some form of product?), omnivore or carnivore in human systems? Do they exist in the same type of balance we see in land-based food systems (i.e., does it turn out we have an overabundance of “carnivores” requiring high energy inputs?)? If so, by increasing energy efficiency are we creating more “herbivores” ??
We’ve cut down a lot of plants – trees to be more exact. Whenever you see green, you are looking at the sequestration of carbon dioxide in materials. It would be foolish to think we shouldn’t also be restoring natural systems to leverage their ability to pull carbon out of the atmosphere. But to what extent? This thinking is reflected in E.O. Wilson’s Half Earth initiative.
Oh, so many more questions than answers! 🙂
My brain is spinning so we’ll have to leave additional pondering for another day. Next steps for me if I were to pursue this further would be to do a deep dive into the science to find out more specifically how this works each step of the way. Next I would then recheck my metaphors and make sure that everything actually makes sense – for every single part of the system. This is where the fun happens – you never know what you’ll find out.
What flaws do you see in my thinking? If any, how would you rewrite those metaphors in line with your thinking? What can you add? How can we build on this? How might you go deeper? What are the right metaphors? What is the natural model equivalent to “electricity”? Or am I totally off base? I’m excited to see what comes out in the Project Drawdown initiative to see if/how their recommendations line up (or not) with this thinking.
Never have I been able to sit down with a design, go through the Life’s Principles (LPs) evaluation and check off every single box. Until last night.
I had the honor last weekend of seeing a screening of the film Sustainable as a part of the Chicago region’s One Earth Film Festival. In the filmmakers’ words, Sustainable is “a film about the land, the people who work it and what must be done to sustain it for future generations.” But to be more specific, the film follows a family in central Illinois who made the conscious decision to farm sustainably. They happened to meet the famous chef Rick Bayless as he was trying to find the type of high quality local food he found in Mexico, and you might say that the rest is history. This family now coordinates a group of family farmers to provide many of the top chefs in Chicago with sustainable local foods on a weekly basis, and there is significant collaboration between farmers and chefs all while significantly improving the farmland/environment – it is a win-win-win situation.
I am not a film critic, but in my amateur opinion I do highly recommend it to everyone and anyone – and fortunately it’s available to a wide audience as it’s available on Netflix and Amazon (yay!).
Of course, this is a biomimicry blog, so I was going to sit down to write about the film topic in the context of Life’s Principles (or Nature’s Unifying Patterns) – how we use biomimicry not just during the brainstorming creative phase, but also through the entire design process (and for existing designs) by using LPs as an evaluation tool.
At every step in the biomimcry process we can use LPs to ask critical questions that help us to think more holistically about the entire context of a design to identify opportunities for improvements that we might not otherwise see. And while many LPs are pretty self-explanatory, having someone on your team with a deep understanding of all twenty-six LPs can be incredibly useful during this type of evaluation. Being able to practice using LPs for evaluation forces you to realize what you do or actually don’t understand about each LP and only makes you a stronger biomimic.
When I sat down and took a look at my sheet of all twenty-six LPs, I literally could check off each one when thinking about how these pioneers of the sustainable local food movement in Chicago have grown (and continue to grow) and operate their farm, business and network – their sustainable food movement.
I realize blogs are often a one-way street, but I’d love to hear your thoughts. Will you share and contribute your thoughts and ideas to the evolving sustainable food movement? (caveat – your ideas have to have a rationale using at least one LP or NUP!) Please feel free to either leave them in a comment below or send them to me in an email at email@example.com. Let’s collaborate, have fun, use our collective expertise to help valuable movements like this one. Let’s change our story together!
Think “flame retardant” and your next thought would not likely be “cellular respiration” and “food”. Yet it turns out a process occurring all the time in our cells (in you right now!) has invaluable lessons that have the potential to radically shift the flame retardant industry through not only new technology, but a completely radical supply chain based on raw materials from food. Yes, that’s right, food. I’ll discuss the hows and whys in Part 3 of my #SystemReset series, and demonstrate the potential paradigm shift this solution represents.
As I mentioned in Parts 1 and 2 of my #SystemReset series, using a biomimicry systems approach (including the innovation methodology and Life’s Principles) to rethink the approach to solving for the core function of a product allows innovators to quickly find existing working whole-system solutions in nature as starting points to create viable alternative human product category system solutions. These alternatives introduce radical disruption at a systems level – both affecting the product category ecosystem and beyond but also redefining (for the better) the environmental, social and economic impact of that product category – while still delivering a viable solution that addresses the core functional needs of the product category.
Nobody’s dream of the future includes their house burning down. But the fact is there are gadgets and appliances and lights throughout our homes that have the potential to catch fire, and enough accidents occur that fire is a top five leading cause of accidental death worldwide. What burns? Homes, cars, airplanes, communications equipment…you name it. The response of course is to incorporate into or douse materials with flame retardants to slow the spread of fire to enable people to escape in time. Flame retardants can be found in mattresses, couches, carpet padding, curtains, flooring, composite materials, packaging, insulating materials, glues, fabric finishes and surface coatings, plastics (oh, so many plastics) and electronic components (think computers and TVs, including cords). Great news, right? Yes and no. While they can present a clear benefit in reducing the severity of a fire and increasing the time for escape from fire in our homes and places of work, some of the major classes of conventional flame retardants can pose significant human and environmental health risks.
Your mind might now be full of questions: So how is nature’s approach different from conventional flame retardants? And why do we even care? What alternatives are already out there and how is a biomimetic solution different? If a new group flame retardants shifts the flame retardant industry, what are the potential range of impacts? Let me tackle them one by one, and as always, leave a comment below to share thoughts, ask questions and continue the discussion!
The Dominant Design in Flame Retardants
The flame retardant industry is complex. The market is dominated by three major manufacturers of flame retardants – Israel Chemicals, Chemtura and Albemarle. The largest portion of the market demand comes from the construction industry and related building materials and interior finishes. Additional demand and anticipated growing markets include construction products, electrical and electronic products, wire and cable, motor vehicles, textiles, aircraft and aerospace (check out this report I drew this information from for more info on the industry). Flame retardants are used in an incredible variety of materials from foams to plastics, wood, fabrics, paper, insulation, paints, etc. Certain families of flame retardants are typically used on some types of materials and not others, depending on the material and flame retardant properties. This is no small industry, and the variety of applications is overwhelming.
Depending on the type of flame retardant and material being protected, flame retardants can make up anywhere from less than 1% to 30% of the content of a material. For example, cellulose insulation is about 20% flame retardant by weight, plastic television and computer cases are often 10–20%, and polyurethane foam cushioning can be up to 30%. One of the challenges in creating a flame retardant is developing one that can be added to a host material without altering the properties of the host material. Because of this challenge, flame retardants are often added without chemical bonding to the host material. In addition, as some flame retardants contain volatile organic compounds, the chemicals off-gas during decomposition and evaporation (e.g., while they are sitting in your home, car and office). These challenges result in the potential for the flame retardant particles to migrate out of the host material and/or release gases into the environment.
If you’re like me, this all makes you wonder, how often and to what degree do you come into contact with flame retardants everyday? And, more importantly, what is your risk?
What are the risks?
People are primarily exposed to flame retardants in two ways – breathing it in (inhalation) or inadvertently eating it (ingestion). The flame retardants migrate out of the materials in our homes and collect in dust, which we breathe in and put in our mouths without knowing. Young children who are often on and/or close to the floor are particularly at risk of ingesting or breathing in dust containing flame retardants and are consistently found to have the highest levels of flame retardants in their systems, particularly in the Unites States. In a 2014 study of 40 child care centers in California, researchers discovered 100 percent of the centers contained both PBDE and non-PBDE flame retardant compounds, including tris(2,3-dibromopropyl)-phosphate, which was named as a presumed carcinogen in 1970 and banned in children’s sleepwear in 1977 (but not banned for other uses – facilities with foam furniture “significantly higher levels of PBDEs compared to those without foam products”).
These flame retardants not only contribute to development of cancer but are also linked to endocrine disruption, diabetes and hyperthyroidism; developmental problems for children such as decreased birth weight; neurological defects such as antisocial behavior, memory loss and lower IQ; undescended testicles; infertility in adults and DNA mutation (for information, check out these articles from the New York Times and Washington Post). (Anecdotally, it also makes me think about all the dogs I’ve known – you know, the ones with their noses sniffing and licking the floors of our houses – very few have died of anything other than cancer.)
When combusted, flame retardants can also degrade into even more highly toxic compounds such as dioxins, furans and formaldehyde. Primarily, it is the toxic smoke released by fires that kills or causes harm by attacking the respiratory systems of humans, posing the most significant risks to firefighters who breathe in the toxic smoke. A 2010 study in the UK revealed that at the same time that the increased use of synthetic polymers since 1955 was accompanied by an increase in fire deaths and injuries (peaking around 1980 at double the number of deaths and declining back to 1955 levels in the 2000s), the ratio of deaths from smoke and toxic gas inhalation increased by a factor of 20. As summarized in Chapter 4 of the book Polymer Green Flame Retardants (2014), the conclusions drawn from the study were that the increase of deaths from inhalation of smoke and toxic gases occurred “at least in part by the higher fire toxicity of certain polymers, and particularly those containing gas phase flame retardants.”
The broad impact of flame retardants on human health due to the pervasiveness of fire retardants in homes is unknown, particularly in the United States where measurement of the amount of flame retardants in a typical United States home is measured in ounces and pounds. Despite the known toxicity of these chemicals, due to the structure of our political and regulatory system these flame retardants are placed onto the market without a thorough study of associated health risks. Several of the halogenated flame retardants (e.g., PBDEs, polybrominated biphenyls (PBBs), and Tris-dibromopropyl phosphate) have been banned in the US and Europe, although they’ve been replaced by similar but unstudied organohalogen compounds. In the case of PBDEs, it “took nearly 40 years of exposure to PBDEs to accumulate sufficient evidence for action to be taken” (Chapter 4, Polymer Green Flame Retardants (2014)). The question then remains, why should we wait for other flame retardants to be banned – why not replace them with safer alternatives?
Much information can be found on the internet regarding more information on health risks associated with flame retardants and preventative measures that can be taken in the home, car and workplace. A good resource is the Green Science Policy Institute, which conducts scientific research and focuses on reducing the use of flame retardants. They provide easy-to-understand information on their website. In addition, two books I referenced in this section and found useful (although highly technical) were Polymer Green Flame Retardants edited by Constantine D. Papaspyrides & Pantelis Kiliaris (2014), and Fire Toxicity edited by A. A. Stec and T. R. Hull (2010). Wikipedia also has a summary and list of major studies regarding the health risks of flame retardants.
Another good thing to know – California passed Technical Bulletin (TB) 117-2013 which went into effect January 1, 2014. TB 117-2013 states that manufacturers may transition from the open flame test process adopted and mandated in 1975 to the new methods for smolder resistance of cover fabrics, barrier materials, resilient filling materials, and decking materials for use in upholstered furniture. This means when you buy new furniture, you can find a tag on it that states whether or not the manufacturer used flame retardants to achieve compliance with flammability regulations.
The search for alternatives
Because of the known health risks of the aforementioned flame retardants, some in the flame retardant industry have been researching and moving towards more benign alternatives in recent years. In fact, according to an industry report, “Halogenated retardants, represented by bromine- and chlorine-based products, are being phased out across the globe due to their perceived environmental and human health risks, creating an opportunity for suppliers of alternative flame retardants to replace them.” Now is the time for market disruption.
As mentioned above, alumina trihydrate now makes up almost 40 percent of the US market demand, and this report also indicates that “non-halogenated phosphorus compounds and other types with more environmentally friendly profiles will benefit from industry efforts to forestall increased regulatory scrutiny.” (note, that quote was written pre- the 2016 U.S. presidential election). While more eco-friendly flame retardants can be less toxic in their use and during combustion than those mentioned above, many still have human and environmental health and safety impacts.
For example, alumina trihydrate is produced during aluminum production. Aluminum is found in bauxite ore which is found throughout the world in tropical and semitropical zones. Bauxite ore is largely mined in open cast mines (strip mines). This type of mining consists of clearing vegetation then stripping and storing the top layer of native soil, mining the top few meters of bauxite ore, replacing the native soil in the (now depressed) land and replanting vegetation (the last two steps are best practice but not always completed). Approximately 40-50 square kilometers of new land are cleared each year for bauxite mining.
Alumina is then refined from bauxite ore in a process known as the Bayer process. The Bayer process includes heating the ore in a solution of caustic soda, water and other chemicals depending on the ore, and precipitating out the aluminum from the rest of the ore. The crystallized alumina are chemically bonded to water, so the crystals are then heated to 2000 degrees F to separate the water from the alumina. The byproduct of the process is bauxite tailings. For every ton of alumina produced, approximately 1 to 1.5 tonnes of bauxite tailings are generated, or approximately 150 million tonnes in 2015.
While alumina trihydrate is found to be relatively benign as a flame retardant with respect to human health during use and combustion, the production of alumina trihydrate is less than benign. While the aluminium industry for many years has been trying to address environmental challenges throughout the industry, the reality is that production of new aluminum requires open cast mining which destroys habitat, creates dust and erosion, disrupts hydrology, and can have social justice issues; and processing which requires large amounts of water and energy, produces bauxite tailings which can pollute water and land, and generates significant greenhouse gas emissions. In addition, if looking up the supply chain, the use of caustic soda presents its own problems – the process to produce caustic soda is energy intensive and can use significant amounts of mercury (depending on the process used), and necessarily generates equal amounts of chlorine, the majority of which is used to manufacture organic and inorganic compounds.
Therefore, while the industry and potentially health experts may be satisfied with the use of alumina trihydrate as a flame retardant, the negative environmental impacts associated with the product’s life cycle render the flame retardant as far from benign. The same might be true for other “eco-friendly” flame retardants such as one based on resorcinol bis(diphenyl phosphate). Many eco-friendly flame retardants are not yet on the market and need ongoing research to find scalable production and application methods, such as those made from dopamine, milk proteins or other bio-based materials.
Significant research is going into more eco-friendly flame retardant alternatives. But considering the significant health risks associated with conventional flame retardants, we need alternatives in the market sooner rather than later. And those alternatives that are not only eco-friendly in use but also in raw material production and manufacture could significantly shift the industry for the better.
Note – some efforts to find non-toxic flame retardants include research on using natural flame retardants sources from plants or animals. For example, one research effort discovered that milk proteins (caseins, found in whey, a byproduct of cheese manufacturing), when applied to certain fabrics, are effective flame retardants and do not produce toxic off-gassing during combustion. (A commercial application of this type of product, however, is not yet available). While this research uses natural chemicals and could possibly have potentially similar effect on the entire supply chain for the flame retardants it could possibly replace, this example is not biomimicry. However, all approaches that provide this type of paradigm-shifting research are valuable in the overall effort to shift away from flame retardants that are harmful to humans and/or the environment.
How would nature fight fire?
Nature deals with fire all the time, so many plants and animals have developed defenses to protect themselves against fire. What the function of fighting fire comes down to is avoiding combustion – by either sacrificing some part of their structures, deflecting, or slowing down and/or cooling a fire. One place you probably wouldn’t even think to look for knowledge about thwarting combustion is within each cell of our bodies. Cells need to convert nutrients into biochemical energy in a process known as metabolism – it’s how we grow. During metabolism, energy in the form of heat is generated basically rendering it a combustion reaction, and yet we don’t spontaneously combust! (Or at least melt into a puddle of goo.) Why is that?
It turns out that metabolic chemical reactions occur in separate steps in our cells, and while some steps generate heat (exothermic reactions), other steps absorb energy (endothermic reactions). During a key stage in the process – the citric acid or Krebs cycle – energy is generated through the oxidation of (or binding of oxygen to) acetates which is then captured as chemical energy in a molecule (adenosine triphosphate or ATP) and stored for later use as an energy source for other metabolic processes.
There are three main processes of the Krebs cycle that are applicable to the properties necessary for stopping fire. During the initial generation of energy in the Krebs cycle, carbon dioxide is released when acetate molecules are oxidized – thus taking away oxygen (fuel) and producing carbon dioxide (which does not burn). In addition, the process of capturing chemical energy in the formation of ATP is endothermic, meaning it absorbs energy, which results in cooling. Another relevant aspect of the Krebs cycle is that during the formation of ATP, water is formed which also has a cooling effect. Thus both functions of cooling and extinguishing are present in the Krebs cycle.
The “Flame Retardants from Food” solution
The Kreb’s cycle was an exciting revelation for innovation scientists and engineers working on an eco-friendly flame retardant at Trulstech, Inc. They had before them a proven, working example of a process in which oxygen and energy (e.g., heat) were consumed – just the steps that are necessary for putting out fires. So they asked, if this chemical process is happening all the time in our bodies, can we not only emulate the citric acid cycle chemical processes, but also develop the solution emulating the life-friendly chemistry so that the flame retardant is non-toxic and biodegradable? The answer is a big fat exciting YES!
Trulstech developed Molecular Heat Eater (MHE)®, a non-toxic, environmentally friendly and biodegradable flame retardant containing only food grade chemicals (in approved concentrations). Based on innovation technology emulating the Krebs cycle transfer of energy processes, MHE® both cools and extinguishes flames to stop the spread of fire in polymer, plastic and fiber materials. The MHE® technology solves key challenges posed by traditional flame retardants.
MHE® functions in multiple ways to slow or extinguish a fire. MHE® is specifically formulated according to the material physics of each host material (polymers, plastics, fibers) and its transfer of energy; specifically, MHE® is formulated according to what chemicals are released from the host material and at what temperatures this chemical breakdown occurs. The MHE® technology uses weak acids in combination with strong alkalis to create acidic complex salts. Additional carbon compounds are also added. When exposed to high heat, the salts are oxidized (meaning oxygen binds to the salts), effectively taking away the fire’s fuel (oxygen); carbon dioxide and water are released from the reaction. Carbon dioxide is a non-flammable gas and water has a cooling effect. In addition, MHE® is designed to bind to the host material at the specific temperature at which the host material starts to break down; this chemical binding is an endothermic or cooling reaction which absorbs energy, thus also lowering the temperature. For all materials, MHE® causes an intumescent or charring reaction in which the material swells as a result of the release of carbon dioxide and heat exposure, increasing in volume and decreasing in density. The char (carbon) is a poor conductor of heat thus retarding heat transfer to the unexposed material. The result is that the retardant can break the chain reaction, pyrolysis, which is a precondition for starting fire. The strategy of binding MHE® to the host material significantly reduces migration of the flame retardant into the environment.
MHE® is produced in biodegradable micro-sized particles that are applied in the form of a liquid, gel or powder. Trusltech does not produce powder below 2 microns in size because they do not want to make nanoparticles with potential health issues. The size range of 2 to 10 microns reduces the weight load because it increases the surface area of the flame retardant, thus speeding up the chemical reactions during exposure to heat (so less retardant is required). MHE® currently can be applied to most types of materials such as polyester, polyamide, PVC, polyurethane, bitumen, rubber, polyolefines, latex, cellulose-based materials, PVAc, PVA and lacquer. MHE®-treated materials have met flammability standards in markets around the globe.
True to the solution found in nature, life-friendly chemistry is used to produce MHE®. MHE® is manufactured using food grade chemicals in quantities acceptable to applicable regulatory entities and manufactured from raw materials such as grape pomace and citrus fruits. In the quantities used, these chemicals are harmless to humans, completely biodegradable in the environment, and do not break down into harmful substances when exposed to fire. In addition, because the raw materials are plant-based, the raw material life-cycle is carbon neutral. Food grade chemicals are also widely available in all markets, challenging the chemical industry’s grip on pricing and barriers to market entry. In this way, MHE® solves the risks to human health and the environment posed by other flame retardants on the market, and provides a completely alternative supply chain solution that is better for our environment. MHE® is currently on the market under the name Apyrum sold by Deflamo in Europe and the former Republics of the Soviet Union.
Impact on the flame retardant product category ecosystem
As noted above, raw materials for MHE® are sourced from food products, the products are harmless when used in approved quantities during raw material sourcing, production, manufacture, use and disposal – the MHE® alternative literally provides a different approach or indirect impact on pretty much the entire product category ecosystem and associated goods and services.
Raw materials are sourced from food, even waste products of food that can be sourced local to the production facility – a far cry from the supply chain needed for producing the raw materials for flame retardants with mineral, organic and inorganic compounds. Industries affected by this shift would include the aluminum mining industry, chlorine producers (who also happen to produce caustic soda), petrochemical manufacturers and their supply chains…you get the picture. The supply chain for this type of flame retardant has the potential to shift value from the conventional supply chain players to local farmers, adding value to local communities.
Worker safety is improved and exposure to toxic chemicals is reduced at all stages of production. As MHE® is mixed into the host material at the chemical manufacturer plant, material manufacturers have lower costs because they no longer require the equipment to mix the flame retardant and material on site, store toxic flame retardant and catalyst chemicals, maintain associated complex environmental regulatory compliance or pay the high cost for disposal of toxic chemicals.
Of course consumers benefit greatly by this type of flame retardant due to decreased health risks – reduced long-term exposure to toxic chemicals and even more toxic gases in the case of exposure to smoke from a fire. In complementary goods and services, the health industry would be impacted in the long run as costs associated with care for the significant health impacts associated with conventional flame retardants would be non-existent. Flame retardants in this family of chemicals that are combusted in a fire remain non-toxic when they turn to into a gas phase, and health impacts associated with smoke inhalation go down. In addition, if a product never catches fire, the flame retardant will eventually biodegrade, leaving no negative impact on the environment.
Not surprisingly, Trulstech has had limited success in gaining the interest of well-established, vertically integrated flame retardant manufacturers. With a racially disruptive technology, Trulstech is searching for different leverage points to disrupt the market. It can’t happen soon enough!
With an estimated 2.8 million metric tons of flame retardant produced annually by 2018, and with halogenated products being phased out due to their human and environmental risks, there is an opportunity for disruption as alternative flame retardants fill the void. Adoption of flame retardants like MHE® thus has the potential to radically shift the flame industry landscape with completely new science that enables effective resistance to combustion at a competitive cost without causing harm to humans or the environment. Talk about a complete #SystemReset!
I would love to see the life-cycle comparison of MHE® with conventional flame retardants, including alumina trihydrate, to understand the full economic and environmental impact at an industry scale of this paradigm shift if taken to scale. Check out this example of LCAs on flame retardants – the last few slides referencing the UK study I understand to include an LCA of MHE – would love to see it! But while this is fantastic, I want to try to grasp the full impact of shifting the entire industry. What would this world look like? Can we paint a clear picture? Any takers?? Let’s put real numbers to this #SystemReset.
When I launched into my #SystemReset series last fall, I felt pretty good about the information I had about the context of three product categories (plastics, flame retardants and anti-bacterials). But some systems are more complex than others, and certainly the fire retardant industry is complex and technical – I’ve been slowed by research that leads me into ever-deepening circles of inquiry! And, of course, me being me I want to make sure I’ve got it right. So, instead of sharing my own work this week, I wanted to share with you great work by other people – a super helpful video shared this week that helps to expand our understanding of the biomimicry process, as well as a couple of fascinating books coming out this spring. Check them out!
Biologically Inspired Design for Industry: An Evolving Practice
I found this webinar to provide a thorough and helpful case study example from Kimberly-Clark of how this team has been shifting biomimicry from idea to implementation, and the lessons learned. Of course, I’ve included a permanent link on my Resources page.
The Center for Biologically Inspired Design (CBID) at Georgia Tech, in combination with Kimberly-Clark Corporation, recently completed two biologically inspired design projects. These projects successfully generated two new active research lines for improving product performance. Join Michael Helms, PhD, from Georgia Tech, and Marsha Forthofer, Kimberly-Clark, to learn more about the projects and discuss the conditions that enabled (and inhibited) the success of the projects including:
expectation setting across different design domains
design team knowledge and skill requirements
translating biological concepts into actionable, funded research
New Books Spring 2017
Teeming: How Superorganisms Work to Build Infinite Wealth in a Finite World
My friend, Dr. Tamsin Woolley-Barker, PhD, has authored a book about her area of expertise – superorganisms – and what we can learn from them. Drawing on fundamental lessons learned from multiple superorganisms, she provides insight into how organizations can restructure to be adaptable, resilient and integral to the functioning of a system.
The most successful species are those that adapt to change, and the same is true in business. But there are limits to vertical growth, and our hierarchical structures can only grow so tall before complexity and instability overwhelm them. Today’s global organizations need a new way to sense and respond to change. Earth’s most ancient and successful societies – the ants and termites, and vast fungal networks underground – have already solved the problem. For hundreds of millions of years, they have worked in huge cities – tens of millions strong – compounding their wealth from one generation to the next with no management whatsoever. With just four simple principles – Collective Intelligence, Distributed Leadership, Swarm Creativity, and Regenerative Value – Teeming shows how these simple individuals pool their diverse and independent experiences to create rich hotspots of abundance and exquisite resilience to change. We can do it too.
Drawdown, The Most Comprehensive Plan to Reverse Global Warming
Not biomimicry per se although the premise of the idea is that we need to balance our carbon cycle like the rest of life does (and Dayna Baumeister of Biomimicry 3.8 is a Scientific Advisor!), but I am excited to see what comes out of the research Project Drawdown has done over the last few years in the book that will summarize it all, Drawdown, The Most Comprehensive Plan to Reverse Global Warming. Considering the Biomimicry Institute’s Global Design Challenge focus is on climate change, it will be fascinating to learn from the winners of the competition what biomimicry might add to the list of approaches we can use to balance our carbon cycle. Let’s do this!
To be clear, our organization did not create or devise a plan. We do not have that capability or self-appointed mandate. In conducting our research, we found a plan, a blueprint that already exists in the world in the form of humanity’s collective wisdom, made manifest in applied, hands-on practices and technologies. Individual farmers, communities, cities, companies, and governments have shown that they care about this planet, its people and its places. Engaged citizens the world over are doing something extraordinary. This is their story.
A couple years ago I did a project where I looked for biological models that might help answer the question, How does nature adapt to increasing scarcity of water, nutrients and energy? In light of the Biomimicry Institute’s 2017 Global Design Challenge focus on climate change, I thought I’d share what I found in the hopes it inspires the innovation we so urgently need in this area.
Aside from two weeks of frigid temperatures and snow in December, I’m not sure winter is coming to Chicago this year. People I talk to around the world have anecdotes that are similar – weather patterns are just not the same. And as we all know, despite our current government’s attempt to silence its own scientists, the notion of climate change does not rest on anecdotes but on solid science. And the science tells us that climate change is accelerating. Whether we choose to “believe” it or not is irrelevant. The question is, what are we going to do about it? (For a description of how humans are impacting global climate cycles, see my previous post.)
To be a part of the solution, the Biomimicry Institute’s 2017 Global Design Challenge (currently ongoing – deadline April 30!) is focused on climate change. They have two general categories for which they are looking for innovations:
Helping communities adapt to or mitigate climate change impacts (i.e. those forecasted or already in motion), and/or
Reversing or slowing climate change itself (e.g. by removing excess greenhouse gasses from the atmosphere).
These are not small challenges. Where does one start?
Because these problems are so complex and will require systemic changes that are comprised of behavioral changes from the individual to system levels, breaking down the challenge into more specific areas of inquiry can help challenge teams focus on biologized questions that might give a team natural models that will inspire concrete results. When brainstorming natural models it’s important to look at all scales, from single organisms to ecosystems. Diversity in scale and context can begin to provide a better picture of how individual behaviors add up to system dynamics, and potentially give innovative insights into critical leverage points in the system.
A sampling of biological models for adaptation
A couple years ago I did a team project where we looked for biological models that might help answer the question, How does nature adapt to increasing scarcity of water, nutrients and energy? I came up with the following natural models and design principles. I hope these might help you and your team in your challenge innovation process!
Click on the title of each box to link to summaries of the strategies and key references I used to develop these design principles provided farther below. Please feel free to reach out to me for more information on any of these biological models.
Photosynthetic plasticity enables CAM plants to acclimatize in response to dynamic environments through a variety of rapid, flexible and reversible photosynthetic processes.
Employ a variety of rapid, flexible and reversible resource management methods that activate or deactivate in response to short-term variations in resource availability enabling survival during periods of scarcity and exploitation of resources during periods of abundance.
Phenotypic plasticity of the dandelion maintains fitness across a wide variety of light intensity environments by modifying structural, chemical and reproductive traits in response to local light levels.
Modifying structural, chemical and growth mechanisms in response to energy intensity levels enables optimized access to and use of available energy resources across a wide variety of systems.
The Creosote bush maximizes water utilization by adjusting growth patterns, chemical processes and allocation of resources to optimally maintain function in response to unpredictable water availability, enabling survival under severe drought.
Maximize resource utilization by adjusting growth patterns, vital functional processes and allocation of resources to optimally maintain function in response to unpredictable resource availability, enabling survival under severe resource scarcity.
Plants and fungi co-adapted in nutrient-poor native soils increase their fitness by developing more expansive symbiotic structures to maximize uptake and exchange of scarce resource(s) most limiting to growth.
Local entities adapted to resource-limited environments increase success by developing more expansive symbiotic local resource uptake networks and exchange nodules specifically designed to maximize access to and availability of the local resource(s) most limiting to growth.
The process of ecological succession secures increasingly scarce nutrients by establishing progressively complex interdependent structures and processes that increasingly minimize nutrient outflow and lock up nutrients in organic matter that stays within the system, resulting in a closed-loop nutrient cycling system.
Secure increasingly scarce resources by establishing progressively complex interdependent system components that increasingly minimize resource outflow and incorporate resources into system component materials that do not leave the system, resulting in a closed-loop resource cycling system.
While most plants open their pores during the day to absorb carbon dioxide (CO2) from the air to conduct photosynthesis (converting sunlight energy to sugars needed for growth) in a process called C3, CAM plants open up their pores and take in CO2 at night when conditions are cooler and more humid to reduce by approximately one-tenth the amount of water loss that occurs when pores are open. CAM plants then store the CO2 until daylight when photosynthesis occurs in sunlight behind closed pores. There are four phases identified in the CAM process, and depending on environmental conditions, CAM plants may employ all four phases including opening their pores in early and late daytime light (in times of sufficient water) or only one phase when the pores never open day or night (in severely dry conditions). CAM plants from desert habitats, such as cactuses and succulents, where intense permanent stressors are intense heat, sunlight and lack of water, have evolved very little variation in their use of CAM – even when moved to more humid habitats they still open their pores only at night.
However, CAM plants adapted to dynamic environments where conditions are variable, such as orchids living on tree branches in the tree canopy in tropical forests, have the ability to modify the CAM phase they are expressing in response to changing environmental stresses, sometimes within a matter of hours. Some CAM plants (CAM/C3 intermediates) are also able to switch completely from using the CAM process to using the C3 photosynthetic process in response to environmental conditions. Stressors in the environment which might trigger changes in process include interactions between temperature, sunlight, water, carbon dioxide, nutrients and sometimes salinity. The photosynthetic plasticity of tropical CAM plants allows fast, flexible and readily reversible responses to allow for acclimation in response to dynamic stresses enabling CAM plants to occupy many niches with high species diversity in a wide range of tropical habitats where resource availability is variable.
Lu¨ ttge U. 2010. Ability of crassulacean acid metabolism plants to overcome interacting stresses in tropical environments. AoB PLANTS 2010: plq005, doi:10.1093/aobpla/plq005. Epub 2010 May 13. doi: 10.1093/aobpla/plq005
Borland, et.a al. 2011. The photosynthetic plasticity of crassulacean acid metabolism: an evolutionary innovation for sustainable productivity in a changing world. New Phytol. 2011 Aug;191(3):619-33. doi:10.1111/j.1469-8137.2011.03781.x. Epub 2011 Jun 16.
Kluge, M., Razanoelisoa, B. and Brulfert, J. (2001), Implications of Genotypic Diversity and Phenotypic Plasticity in the Ecophysiological Success of CAM Plants, Examined by Studies on the Vegetation of Madagascar1. Plant Biology, 3: 214–222. doi:10.1055/s-2001-15197
Osmond, CB. 1978. Crassulacean Acid Metabolism: A Curiosity in Context. Annual Review of Plant Physiology. Vol.29:379-414 (Volume publication date June 1978) doi:10.1146/annurev.pp.29.060178.002115
Dandelions are able to survive under a wide variety of environments. Dandelions most often produce seeds without needing a pollinator, resulting in offspring that are clones (genetically identical) of the parent plant. Therefore, their ability to be significantly adaptive to local environments rests not in changes occurring at the genetic level but as a result of their phenotypic plasticity – i.e., their ability to change their structural and functional traits in response to the limiting (scarce) resources in a local environment. Trait changes may include altering traits such as photosynthetic processes, leaf angles, and flowering time. In response to decreasing availability of light in shaded understories, dandelions produce significantly longer, more rounded “shade” leaves with a greater surface area to collect scarce light resources; taller flower stems to increase access to potential pollinators; and they speed up the time it takes to flower, create and disperse mature seeds.
Molina-Montenegro, M.A., Peñuelas, J., Munné-Bosch, S. et al. “Higher plasticity in ecophysiological traits enhances the performance and invasion success of Taraxacum officinale (dandelion) in alpine environments.” Biol Invasions (2012) 14: 21. doi:10.1007/s10530-011-0055-2
Creosote is the most drought-tolerant plant in North America, using a wide range of adaptations to live in harsh habitats where sometimes it is the only plant living. Creosote can live with no rain at all for more than two years and individuals can live to be thousands of years old. Creosote essentially maintains two mechanisms for growth and reproduction to maximize utilization of a scarce resource – through an annual growing season when rains are typically present (in the spring), and opportunistically when brief rains occur at other times of the year.
Under extremely dry conditions, Creosote is able to continue to grow and reproduce with minimal water loss. In decreasing water availability, the bush is able to adjust its osmotic potential significantly lower, meaning it increases chemical concentrations in its leaves which increases its ability to move water from soil to roots to leaves (but is also taxing on the plant). The extremely low osmotic potential enables the bush to draw water up to its leaves from underground sources through its deep “tap” root (up to about 10 feet in depth). With a low osmotic potential the leaf cells are able to maintain enough water to continue with reduced photosynthesis such that the plant continues to grow and even flower in drought, albeit in a limited capacity. In periods of water stress Creosote allocates resources to reproduction at a cost to vegetative growth.
However, the Creosote bush is also able to quickly capitalize on brief rains when they occur. Creosote has a second thin fibrous root pattern that exists just under the soil surface that can spread out up to 50 square yards from the base of the plant. New cell growth on these surface roots are incredibly efficient at soaking up surface water, appearing to grow inches in day. With the sudden increase in water availability, Creosote is able to adjust its osmotic potential appropriately and increase photosynthesis and thus growth, including a quick burst of flowering. In addition, the bush will continue at an accelerated growth rate while water is available, allocating additional resources towards vegetative growth. Because Creosote blooms both seasonally and opportunistically, plants during late summer monsoons can be putting out new shoots, blooming, and setting seed at the same time. Once the water is gone, the plant will again adjust growth and chemical processes to withstand drought conditions.
MEINZER, F. C., RUNDEL, P. W., SHARIFI, M. R. and NILSEN, E. T. (1986), Turgor and osmotic relations of the desert shrub Larrea tridentata. Plant, Cell & Environment, 9: 467–475. doi:10.1111/j.1365-3040.1986.tb01762.x
Sharifi, M. R., et al. “Effect of Manipulation of Water and Nitrogen Supplies on the Quantitative Phenology of Larrea Tridentata (Creosote Bush) in the Sonoran Desert of California.” American Journal of Botany, vol. 75, no. 8, 1988, pp. 1163–1174., www.jstor.org/stable/2444099.
Plants and fungi that co-adapt in nutrient-poor native soils improve their survival and reproduction by developing larger mutually beneficial structures to maximize uptake and exchange of the locally scarce resource(s) that are most limiting to growth. Fungi grown in the soils in which they have evolved grow larger fibrous branching growth structures for nutrient uptake and many more resource exchange nodules on the roots of plants also adapted to the same local nutrient-poor soils. These fungi structures are specifically adapted to maximize the exchange of the limiting resources in their local soil conditions. For example, if phosphorus is scarce in home soils, the fungi develop large structures to maximize the uptake and exchange of phosphorus with their host plants. However, if this same fungi were moved to a foreign soil location where nitrogen was scarce and most limiting to growth, the fungi would not only develop fewer growth and exchange structures, but would still maximize uptake of phosphorus, thereby not maximizing the survival and reproduction of plants deficient in nitrogen in the foreign soil. Therefore, survival and reproduction of plants and fungi is greatly increased when plants and fungi are grown together in their home soils because of their strategy to mutually maximize the uptake and exchange the locally limiting resource(s).
Collins Johnson, Nancy, et al. “Resource limitation is a driver of local adaptation in mycorrhizal symbioses.” PNAS, vol 107 no. 5, February 2, 2010, pp.2093-2098. doi: 10.1073/pnas.0906710107
Klironomos, J. N. (2003), Variation in Plant Response to Native and Exotic Arbuscular Mycorrhizal Fungi. Ecology, 84: 2292–2301. doi:10.1890/02-0413
Ecosystem development following a disturbance where no life is present (such as after lava flows) is called primary succession. At the start of primary succession, nutrients necessary for plant life are locked up in the bedrock (such as phosphorus) or in the air (such as nitrogen). Pioneering plants that are first on the scene are able to tap into these nutrients by fracturing and wedging into rocks with their root systems. As soils develop around the plant roots from a buildup of decaying plant and weathered rock materials, these pioneering plants also fix nitrogen from the air to the soils. At this stage, however, a high rate of decomposition due to higher temperatures from sun exposure breaks down nutrients in the system at a high rate. Many of the nutrients gathered on site are lost in surface water runoff and leaching as the soils are shallow and not well compacted. As additional abundant quick-growing, quick to seed plants are established, the increasing soil layer serves to reduce water runoff and promote the slow percolation of water, increasing the availability of water and nutrients. Decomposition (breaking down) of organic matter, plant roots and soil organisms add carbon dioxide to soils, helping to increase the acidity of soils which serves to break down minerals into nutrients available for uptake by plants.
Once enough nitrogen is present in soils, an additional diversity of plants start to grow on site. Increasing diversity of plant (and animal) life results in an increasing variety and density of physical structures including leaf sizes, height of plants, and roots structures. The more complex system structure is increasingly able to absorb additional gases from the air while also creating deeper, more compact soils that trap nutrients on site. In addition, with the increasing canopy of leaves and larger root volumes, water runoff is again decreased not only because the soils are able to hold more water, but because the plants also return water as vapor to the air, decreasing the amount of water making it down to the soil and flowing off site. Decreased runoff decreases the loss of nutrients. However, due to the high rate of nutrient loss (particularly phosphorus) during primary succession, and as more species colonize the site, fewer nutrients are available relative to the increasing organic matter on site. The increasing resource scarcity results in increasing competition as well as mutualisms between organisms to gain access to increasingly scarce nutrients. Plants that grow more slowly and are able to capture nutrients in their roots and mass, such as shrubs and trees, gain competitive advantage and push out the smaller, faster-growing, nitrogen-fixing plant species.
As ecological succession continues toward a mature state where species diversity is greatest but nutrients limiting to growth are scarce, nutrients become locked up in the soils and plant and animal matter, with the highest amount of nutrients locked up in decomposing matter like leaf litter. At this stage the ecosystem structure has mature trees that block much of the sunlight making it down to the forest floor, which slows the rate of decomposition as well as reduces water loss. The rarest of nutrients necessary for plant growth, such as phosphorus, are cycled tightly within the ecosystem’s physical and chemical structures. This type of nutrient cycling is considered “closed loop”, meaning the loss of nutrients is minimized and the diverse array of species in all niches of the ecosystem take part in the upcycle of nutrients throughout the system again and again. By locking up scarce resources necessary for life within the ecosystem structure, the system avoids collapse.
Allenby, B. R. and Cooper, W. E. (1994), Understanding industrial ecology from a biological systems perspective. Environ. Qual. Manage., 3: 343–354. doi:10.1002/tqem.3310030310
Gorham, Eville, et al. “The Regulation of Chemical Budgets Over the Course of Terrestrial Ecosystem Succession.” Annual Review of Ecology and Systematics, vol. 10, 1979, pp. 53–84., www.jstor.org/stable/2096785.