& Environmental Sustainability amid Human Domination of Global Ecosystems

David Tilman

Abstract: Concern about the loss of Earth’s biological diversity sparked two decades of research of Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 unprecedented intensity, intellectual excitement, and societal relevance. This research shows that bio- diversity is among the most important factors determining how ecosystems function. In particular, the loss of biodiversity decreases the productivity, stability, and ef½ciency of terrestrial, freshwater, and ma- rine ecosystems. These research ½ndings come at a time of rapidly increasing threats to global biodiversity resulting from agricultural land clearing, climate change, and pollution caused by globally accelerating demand for food and energy. The world faces the grand, multifaceted challenge of meeting global de- mand for food and energy while preserving Earth’s biodiversity and the long-term sustainability of both global societies and the ecosystems upon which all life depends. The solutions to this challenge will require major advances in, and syntheses among, the environmental and social sciences.

The existence of life is the de½ning feature of Earth, and diversity is the most striking aspect of DAVID TILMAN, a Fellow of the American Academy since 1995, is life on Earth. Since the origins of life three billion Regents Professor in the Depart- years ago, the biological diversity of life, or its bio- ment of , and diversity, has been on an upward but jagged trajectory Behavior at the University of Min- along which the formation of new species has ex- nesota; he is also a Professor in the ceeded, with but few exceptions, the loss of exist- Bren School of Environmental Sci- ing species. The exceptions were major ence and Management at the Uni- events, attributable to catastrophic occurrences versity of California, Santa Barbara. His publications include The Func- such as meteor impacts and globally massive vol- tional Consequences of Biodiversity: canic activity. Earth now has on the order of ½ve Empirical Progress and Theoretical million species, all descended from the same ances- Extensions (edited with Ann P. tor. This biodiversity has been an enduring source Kinzig and Stephen W. Pacala, of wonderment and scienti½c mystery from the era 2002), Spatial Ecology: The Role of of great naturalist-explorers, such as Darwin and Space in Population Dynamics and Wallace, to the present. Interspeci½c Interactions (edited with Peter Kareiva, 1997), Resource Com- Earth also has seven billion people who, in meet- petition and Community Structure ing their needs for food and energy, have become a (1982), and more than two hundred globally dominant force affecting Earth’s ecosys- articles in scienti½c journals. tems and threatening their biological diversity. The

© 2012 by the American Academy of Arts & Sciences

108 rapid acceleration in human global im- achieve greater environmental sustain- David pacts through land-clearing and the de- ability as well as ensure full and equitable Tilman struction of natural habitats, fossil fuel lives for all peoples. Some of the major combustion and climate change, nutrient mysteries that remain are discussed later pollution, and other activities has led to in the essay; as is always the case in sci- projections that humanity may be in the ence, many more mysteries await their process of causing species at a discovery. rate rivaling some of the largest extinc- I also address social and cultural issues tion events found in the fossil record.1 that arise from the application of new sci- These projections have raised a series of enti½c knowledge to society. These are questions and concerns, the most funda- perhaps the greatest challenges because

mental of which are: What caused the advances in scienti½c knowledge can con- Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 evolution of Earth’s amazingly great bio- tribute to achieving societal goals only if diversity? Does the loss of this biodiversi- the knowledge is accepted and adopted by ty matter? Are there practices that could society. How, though, is this done? The consistently and stably sustain the habit- ethical precepts, laws, and customs of a ability of Earth while meeting the food, society are the result of hundreds to thou- energy, and other needs and demands of sands of years of often-slow cultural evo- the nine to ten billion people who will lution. What would or could motivate the populate the planet by mid-century? If rapid changes in customs, laws, and ethics such practices are discovered, what poli- that may be needed now that human activ- cies, ethics, or approaches could lead to ities have become the dominant global their global adoption? force affecting how ecosystems function? The ½rst question, on the origins of bio- diversity, has been a mainstay of evolu- In 1958, Charles Elton, the great Oxford tionary research at least since Darwin. ecologist, hypothesized that stability is The question of whether biodiversity loss greater in ecosystems containing a diverse really matters was ½rst raised in the 1980s. set of species. He worked in the style of By the mid-1990s, it had ignited a wave of ecological research that was popular in ecological research of unprecedented his day, undertaking qualitative compar- intensity, intellectual excitement, contro- isons of habitats that differed in their versy, and societal relevance. In so doing, diversity: species-rich meadows versus it helped transform the discipline of ecol- nearby monocultures of crop plants, for ogy into a more mechanistic and predic- example, or isolated and depauperate tive science in which hypotheses are tested islands versus highly diverse mainland against the outcomes of multiple ½eld ex- communities. He further suggested that periments; observations in multiple eco- habitats with high biodiversity are less systems, both natural and those experi- susceptible to invasion by exotic species. encing human impacts; and the predic- A century earlier, Darwin had indicated tions of alternative mathematical theories. that greater plant diversity is associated In this essay, I consider the biodiversity with greater primary productivity, but revolution and its aftermath by summa- this insight lay dormant until rediscov- rizing its major discoveries, controversies, ered in 1993 by Sam McNaughton.2 and resolutions. The scienti½c revolution At about the same time that Elton was that has occurred, as remarkable as it has carrying out his research, G. E. Hutchin- been, is only the initial step toward the son, the noted aquatic ecologist at Yale, discoveries that are needed if society is to observed how paradoxically high the

141 (3) Summer 2012 109 Biodiversity & diversity of many ecosystems seems to By 1973, when the second edition of his Environmental be.3 Then-current mathematical theory book Diversity and Stability in Model Ecosys- Sustainability predicted that the number of coexisting tems was published, May suggested an species should be no greater than the alternative resolution to the debate–that number of distinct resources for which ecosystem properties might grow more the species compete. In contrast, even in stable with diversity even as population seemingly simple habitats such as the stability declined–but his insight was well-mixed open waters of lakes and the overlooked. Indeed, for the next two de- oceans, the number of coexisting species cades, most ecologists, myself included, of algae was often an order of magnitude considered diversity of little relevance greater than the number of limiting nutri- to stability or other ecosystem processes.

ents for which they competed. Hutchin- Then-current research, much of it per- Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 son’s paradox sparked my fascination formed with well-replicated ½eld experi- with biodiversity. I dedicated the ½rst two ments, focused on the mechanisms of decades of my career to understanding interaction among a few species and on how species compete with each other, and how the traits of each species influence the how and why such competitive interac- dynamics and outcome of interactions tions so often lead to the coexistence of among those species. Higher-level ques- many species rather than to domination tions about how the number of interact- by one or a few species. ing species might have an impact on the Elton’s ideas flourished for a decade or functioning of ecosystems were set aside two, only to be put aside as the discipline while ecologists worked to transform the began to develop a tradition of experi- ½eld into a more mechanistic and predic- mental, observational, and theoretical re- tive science. search. Scientists now sought the mech- anistic construction of a species-based During this period, a few scholars con- understanding of the dynamics of multi- tinued to study biodiversity. In 1981, Paul species communities and ecosystems. and Anne Ehrlich, evolutionary ecologists New theory played a pivotal role in this at Stanford, published their book Extinc- transition. Robert May, the brilliant physi- tion: The Causes and Consequences of the Dis- cist-turned-ecologist from Princeton and, appearance of Species. They raised concern later, Oxford, presented elegant mathe- about how human activities are threaten- matics showing that the stability of com- ing global biodiversity and how loss of this munities of competing species declines biodiversity could harm the functioning as communities become more diverse.4 of ecosystems and the services they pro- May’s mathematical demonstration that vide to society. Edward O. Wilson, the the dynamics of individual species become Harvard evolutionary biologist, also wrote less stable at higher biodiversity led to extensively on this issue and was award- debate on how diversity affects the stabil- ed a Pulitzer Prize for his 1992 book, The ity of natural ecosystems. After reviewing Diversity of Life. The work of the Ehrlichs, more than two hundred papers on the Wilson, Peter Raven, Sam McNaughton, issue, Daniel Goodman criticized the Stuart Pimm, and others had so elevated superorganismal perspectives then in global concerns about the loss of biodiver- vogue in ecosystem ecology and conclud- sity that the United Nations convened an ed that Elton’s diversity-stability hypoth- “Earth Summit” in Rio de Janeiro in 1992. esis was not supported by a preponder- This gathering led to the international ance of evidence.5 Convention on Biological Diversity.

110 Dædalus, the Journal ofthe American Academy of Arts & Sciences Shortly afterward, Hal Mooney and productive than those that are more David Detlef Schulze organized a small meeting diverse.8 By 1997, several papers had Tilman of ecologists to synthesize and evaluate reported similar effects of plant diversity how the functioning of ecosystems might on primary productivity based on well- depend on biodiversity. The supporting replicated biodiversity experiments per- evidence, though scattered and scant by formed in ½eld conditions.9 the standards of the discipline, was suf½- As should occur in science whenever cient to reignite my interest in this ques- evidence seems to challenge current ideas, tion as well as the interest of almost every- this growing body of evidence was met one else who attended the meeting. The with skepticism. In the ½rst paper to ques- resulting edited book presented intriguing tion the apparent effects of plant biodi-

concepts suggesting that ecosystem func- versity on primary productivity, Michael Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 tioning could be linked to biodiversity.6 Huston raised doubts about the ability of Once rejected, an idea rarely regains the experiments to reject an alternative traction in science because its seeming cause called sampling effects.10 He present- flaws are well known. Yet two papers pub- ed the intriguing hypothesis that the effects lished in in 1994 reopened debate come not from diversity per se, but from over the diversity-stability hypothesis. the greater probability that a highly pro- The ½rst, “Biodiversity and Stability in ductive plant species would be present at Grasslands,” explored how the stability of higher diversity. If the productivity of a grassland plant communities in response plot is determined mainly by the growth to a major drought depends on plant of its most productive species, Huston diversity.7 John Downing and I had ap- reasoned, then the greater productivity proached these data with more than our observed in higher diversity plots might usual level of scienti½c skepticism. We merely mean that they have a greater tried hundreds of different analyses, each chance of containing a highly productive aimed at rejecting the hypothesis that species. That same year, David Wardle and greater plant diversity leads to greater collaborators published a study of a set of ecosystem stability. We instead found that small islands showing that island produc- every analysis supported that hypothesis. tivity is more dependent on ½re frequency Results from more than two hundred plots and other factors than on plant biodiver- showed that stability, measured as resis- sity.11 Next, in their paper “The Statistical tance to the effects of a major disturbance, Inevitability of Stability-Diversity Rela- is a sharply and signi½cantly increasing tionships in Community Ecology,” Dan function of plant diversity. In particular, Doak and collaborators offered an alter- during the drought, the productivity of native hypothesis to explain the apparent grassland plots containing one to three effect of diversity on ecosystem stability.12 species fell to about one-tenth of pre- The biodiversity revolution was under drought levels, whereas plots containing way, and what ensued was more than a ½fteen to twenty-½ve species had their decade of discovery characterized by nu- productivity fall to only about half of their merous rounds of debate and resolution predrought levels. Three months later, driven by the interplay of experimental Shahid Naeem and collaborators pub- results, novel analyses, theoretical predic- lished the paper “Declining Biodiversity tions, and observations in natural ecosys- Can Alter the Performance of Ecosys- tems. Ecology, as a science, had come of tems,” which reported that simpler and age. Of all the grand debates that have oc- less diverse laboratory food webs are less curred in ecology, the biodiversity debate

141 (3) Summer 2012 111 Biodiversity & was the ½rst to be so thoroughly tested Most of these biodiversity experiments, Environmental via the interplay of numerous focused though, lasted only one or two years. The Sustainability experiments, new theory, and quantita- few long-term experiments that have been tive ½eld observations. One after another, done reveal that the initial effects of bio- novel hypotheses were proposed, tested, diversity on productivity increase through modi½ed, and synthesized as more than time.19 For instance, results from the one hundred different biodiversity experi- longest-running biodiversity experiment, ments were performed around the world. which my collaborators and I established This large number of experiments opened in Minnesota in 1994, show that the an- up ecology to meta-analysis, a new tool nual biomass production of the highest- that greatly contributed to the biodiversity diversity plots (those planted with sixteen 13

synthesis. As this occurred, it became species) increased through time much Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 increasingly clear that the loss of biodiver- more than the average biomass of the sity has many more and larger impacts, same sixteen species growing in mono- some via newly discovered pathways, on cultures. In particular, in the third and ecosystem functioning than had ever fourth years of the experiment, the high- been envisioned. An idea cast aside in the diversity plots had, on average, 92 percent 1970s had turned into one of the most greater production than the monocul- highly studied and well-understood con- tures. This ½gure increased to 157 percent cepts of ecology. by years 8 and 9, and to 190 percent by years 17 and 18, which are the source of The evidence that led to a new biodiver- our most recent data. sity paradigm came from a confluence of Stability. The stability of an ecosystem results of experiments and theory.14 To process is a measure of the constancy of provide a flavor of this work, and espe- the process in response to disturbance. cially its ½ndings, I briefly summarize Greater ecosystem stability thus means ½ve types of ecological processes that are that the process is better buffered and now known to be affected by the loss of less variable. Natural and managed eco- biodiversity. systems experience a wide variety of Productivity. The growth of plants pro- intensities and types of perturbations: vides the “primary productivity” that is climatic variation (cool or warm and/or the basis of all ecosystem functions. wet or dry periods); disease or pest out- Experiments have shown that, on average, breaks, ½re, erosion, landslides and other plots planted with highly diverse mixtures physical disturbances; and shifts in the of plant species annually produce about structure of food chains, such as from loss 70 to 100 percent more aboveground bio- of top predators. Long-term biodiversity mass–that is, they have greater primary experiments have provided direct tests of productivity–than plots planted with the dependence of ecosystem stability on monocultures of these same species.15 plant diversity. For instance, year-to-year The positive effect of plant diversity on variation in annual biomass production ecosystem productivity has been observed was lower in higher-diversity plots in in ecosystems ranging from temperate both a European grassland experiment grasslands16 to tropical, Mediterranean, and our Minnesota grassland biodiversity and boreal ecosystems.17 In experiments experiment,20 showing that higher diver- in which ½sh species diversity was manip- sity leads to greater stability of primary ulated, treatments with greater numbers of productivity in systems experiencing ½sh species produced more ½sh biomass.18 year-to-year climate variation. Similarly,

112 Dædalus, the Journal ofthe American Academy of Arts & Sciences observational studies show that the stabil- ally similar to established abundant spe- David ity of the productivity of marine ½sheries cies. Additional work shows that the Tilman is higher in those ½sheries that have major factor inhibiting invasion is low greater ½sh species diversity, and that availability of the limiting soil nutrient, greater numbers of genetic varieties of and that more diverse plant communities wheat lead to less variation in yields as reduce soil nutrient concentrations to well as higher yields.21 lower levels.24 Disease. Most pathogens and disease are Biodiversity and Agriculture. Four crops– speci½c to one or a few species. As a result, maize, wheat, rice, and soybeans–provide the rate of disease transmission from an about 80 percent of the food calories con- infected individual to a susceptible indi- sumed globally. Because of the rapidity

vidual of the host species is proportional with which crop pathogens and pests Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 to the population density of the host. This evolve and overcome plant defenses, the fundamental principle of epidemiology sustainability of these crop yields is high- suggests that the incidence of disease for ly dependent on continued breeding for a given host species should decline when resistance to the latest varieties of patho- the host species is living in a more diverse gens and pests. For instance, “IR8,” the rice community. Because each plant species variety that began the Green Revolution would be less abundant than in monocul- in Asia in 1967, had its yield fall 24 percent ture, the incidence of species-speci½c plant over the subsequent thirty years because diseases should, on average, decline as of pathogens and pests. Nine subsequent plant diversity increases. This expectation rice varieties had their yields decline by is supported by results of numerous bio- similar rates after their introductions.25 diversity experiments. For instance, fungal For crop breeding to stay ahead of pests pathogens that grow on the surfaces of and pathogens, breeders must have an leaves are much less abundant at higher immense storehouse of genetic variants, plant diversity.22 Transmission rates for at least some of which are resistant to diseases of many other types of species, emerging pathogens and pests. Even more including amphibians, corals, ½sh, and genetic diversity is needed to ½nd new birds, are similarly lower when the biodi- genetic combinations that increase crop versity of the host community is greater.23 yields. Thus, although most such crops are Resistance to Invasion. Charles Elton’s grown as monocultures (and perhaps observations (discussed above) led him particularly because they are grown in to suggest that more diverse ecosystems such a way), genetic diversity is of great are less easily invaded by exotic species. economic and societal value. Biodiversity experiments have provided Biodiversity can be used as a tool to broad support for this hypothesis. An increase crop yields in some situations.26 experiment in which seeds of numerous For instance, Youyong Zhu and collabo- nonresident plant species were added to rators found that growing two varieties plots in a biodiversity experiment that of rice in alternating sets of rows (a prac- differed in both their species numbers tice called intercropping) greatly decreases and their functional group compositions incidence of a signi½cant fungal pathogen showed two marked effects. First, the that attacks a highly valued variety but to added species were less likely to invade which the second variety is resistant. not only when the diversity of the estab- Long Li and collaborators observed that lished plant community was high but also intercropping of faba beans and maize when the invading species were function- increases maize yields by 40 percent and

141 (3) Summer 2012 113 Biodiversity & faba bean yields by 25 percent; they also invade, as an invading species would have Environmental found that this over-yielding is caused by to survive and grow on the resources left Sustainability differences in the rooting depths and sea- unconsumed by established species. sonality of growth of two crops, as well as Two hypotheses have received increas- by faba beans’ ability to mobilize other- ingly robust support from biodiversity wise unavailable phosphorus. While many experiments. First, species coexist with crop combinations do not over-yield, Li other competing species precisely because reports that combinations that do over- the species have trade-offs; any trait that yield are planted on 28 million hectares in increases the ability of individuals in a China. Intercropping is rarely practiced in species to deal with one limiting factor Western nations today, but its ability to must necessarily make them less able to 28

increase yields in particular cases might deal with some other limiting factor. Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 offer bene½ts (although intercropping Second, changes in biodiversity have con- also introduces a number of challenges sistent and predictable impacts on many for mechanized agriculture that need to aspects of ecosystem functioning; the be solved). trade-offs among the species that share a habitat mean that larger numbers of these After the initial experimental demon- species will, on average, be better at deal- strations that biodiversity affects numer- ing with limiting factors in that habitat. ous aspects of ecosystem functioning, Thus, the very processes that have allowed attention shifted to why biodiversity Earth to accumulate such a large number matters. The ½rst discussions centered of species also mean that greater diver- on the role that sampling effects might sity would affect ecosystem functioning play versus the importance of niche dif- in exactly the ways that have now been ferences among species. Application of a observed experimentally. variety of increasingly sophisticated ana- An important corollary of “biodiversi- lytical techniques has shown that sam- ty matters” is that “species matter.” This pling effects (later called selection effects) point has arisen repeatedly, both from are generally unimportant and that niche results of biodiversity experiments and differentiation effects (also called comple- from ecological theory. An important mentarity) are the predominant cause. demonstration of the “if diversity matters, This ½nding was especially evident in species must matter” hypothesis was instances where species had several years offered by Anthony Ives, Kay Gross, and to interact and thus the effects of their Jennifer Klug, who showed theoretically interactions were well established. that ecosystem stability depends not on The understanding of why biodiversity the number of species per se, but on the matters was also illuminated by mathe- differences among the species.29 Because matical theory. In particular, a sequence multiple competing species can stably co- of papers showed that when competing exist only if they have trade-offs in their species have trade-offs in their traits that traits, the biodiversity of an ecosystem allow them to coexist stably, the net result (when enumerated by the simple metric is that ecosystem stability and productiv- of the number of species present) affects ity increase with diversity.27 In addition, ecosystem processes precisely because the more diverse ecosystems reduce limiting species differ from each other. resources to lower levels, both contribut- ing to their greater productivity and Human well-being is highly dependent reducing the abilities of other species to on nature. The total land surface of Earth

114 Dædalus, the Journal ofthe American Academy of Arts & Sciences is 13 billion hectares, of which 4 billion It seems likely that biodiversity may play David hectares are in the Arctic or Antarctic, or a central role in achieving greater sustain- Tilman are desert or tundra. The vast majority of ability, but this is a hypothesis in its in- the remaining 9 billion hectares is heavily fancy. The fate of this hypothesis–and of used by people, with about 5 billion global biodiversity–is at present uncertain. hectares serving as agricultural lands, roughly one-fourth of which is farmed The next ½fty years are likely the ½nal and the rest used for livestock production. period of rapid expansion in human pop- Much of the remaining land is forested, ulation and consumption. Global popula- with about 1.5 billion hectares being tion, which had increased 270 percent in actively managed for tree production the twentieth century, is likely to increase

globally. Thus, two of humanity’s most from its current seven billion people to Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 essential needs–food and shelter–are about nine or ten billion, a 35 percent directly dependent on the productivity change, by the middle of this century, at and stability of about 75 percent of Earth’s which point global population growth usable lands. Moreover, because green- may halt. This astounding population in- house gases are released from fossil fuel crease, though, is small compared to the combustion, there is increased interest in increases in per-capita global consumption also using land to produce biomass for (measured as per-capita Gross Domestic conversion into biofuels with low green- Product) across this same time period. house gas emissions. During the twentieth century, the real Society depends on nature not only for (inflation adjusted) buying power of a goods such as food, timber, and energy, but typical person increased by 360 percent, also for a variety of ecosystem services.30 and it is projected to increase by about We need potable water, a resource that is 150 percent during the next ½fty years as produced by intact grassland and forest the peoples of many developing nations ecosystems, and that is harmed by some gain “middle class” incomes. agricultural and industrial activities. In- The double-whammy of greatly in- tact ecosystems minimize flooding; they creased population and even more great- are a major storehouse of organic carbon ly increased consumption per individual that would otherwise be released into the has already turned humans from being atmosphere as carbon dioxide (CO2) if the one of many species on Earth to being the land were cleared; they create the fertile dominant force affecting all ecosystems. soils on which agricultural productivity Moreover, human environmental impacts depends. are likely to double or triple by mid- One of the great challenges facing century because of the anticipated global humanity is to ½nd ways to meet its needs increases in both per-capita consump- for food, timber, energy, and other goods tion and population size.31 To meet an while maintaining the ability of managed estimated doubling in global demand for and natural ecosystems to provide vital food may require that about 1 billion ecosystem services. Discovery and adop- hectares of tropical forest and grasslands tion of better management practices will be cleared for crop production and that be essential to optimizing the production agricultural fertilization, which can cause of goods and ecosystem services from serious water pollution, increase about managed lands, and thus increasing the 170 percent. Land-clearing leads to the long-term sustainability of the full range loss of biodiversity and is a major source of goods and services that people need. of greenhouse gas release. Moreover,

141 (3) Summer 2012 115 Biodiversity & agriculture itself accounts for about 37 per- to cause a 100 to 110 percent increase in Environmental cent of total human-caused greenhouse global demand for crops is that per-capita Sustainability gas releases, and such releases would more meat consumption increases with income, than double as food demand doubled. In and each kilogram of meat protein requires comparison, all forms of transportation that livestock be fed from 3 to 20 kilograms combined account for only 20 percent of of crop protein. This range in values oc- global greenhouse gas emissions. curs because animals differ greatly in the Global energy demand is increasing at ef½ciency with which they convert grain least as rapidly as is food demand, and protein into edible animal protein, with most of this increased demand is being farm-raised ½sh being about eight times met by the combustion of fossil fuels. The more ef½cient than cattle, and poultry

net result of these food and energy trajec- being about four times more ef½cient Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 tories will be major climate changes, the than cattle. Direct human consumption irreversible loss of a signi½cant portion of of grain protein is even more ef½cient. Earth’s biodiversity, and greatly decreased Dietary shifts toward non-livestock pro- provisioning of numerous vital ecosystem teins would provide environmental bene- services. Although there are insights to ½ts, as would advances in the ef½ciencies be gained from articulating the environ- with which livestock convert feed into mental problems that human activities meat. Thus, there are contributions to be are causing, it is even more important to made toward achieving greater environ- ½nd solutions. mental sustainability across disciplines The science, social science, and business as divergent as the culinary arts (via the of sustainability are all in their infancy. creation of delicious but environmental- What we see now is the embryo of an un- ly ef½cient entrées) and animal nutrition. known organism. Its development will be Lessening the Impacts. Modern societies guided by the creativity and careers of the are highly dependent on energy. The three next generation. In that spirit, I offer a few greatest impacts of fossil fuel combustion thoughts about the challenges and possi- come from the release of greenhouse gases bilities ahead as we seek viable solutions. (which cause climate change), of ½ne par- Ef½ciency. The expanding human domi- ticulate matter (which causes respiratory nation of the globe will affect biodiversity, problems and increases mortality), and of climate, and numerous ecosystem services mercury (which causes health problems). largely in terms of increased demand for Wind and solar power are alternatives food and energy. There are two equally that reduce these impacts, but adoption of important types of solutions to this prob- these technologies has been slow because lem. The ½rst type focuses on decreasing of challenges related to cost and reliability. demand for food and energy, and the sec- Almost all current vehicles require liquid ond on meeting these demands in ways fuels. Electric vehicles may be the solu- that lessen environmental impacts. De- tion, but in order to achieve meaningful mand can be reduced by increases in deployment, advances in battery technol- ef½ciency. Energy ef½ciency is a familiar ogy must be made that would increase topic, but food ef½ciency is not. About a mileage range to somewhere between quarter to a third of global food produc- three hundred and ½ve hundred miles. tion is wasted, with the causes of this Air transport may always depend on wastage differing among societies. The liquid fuels. The challenge is to create liq- major reason why a projected 35 percent uid fuels that are greenhouse gas neutral increase in global population is expected and that do not compete with food crops

116 Dædalus, the Journal ofthe American Academy of Arts & Sciences for fertile land. If biofuels did compete for or facilitating species. Similarly, the 1.5 bil- David fertile land, their greenhouse gas bene½ts lion hectares of managed forests may yield Tilman would likely be eliminated because of the more timber and other forest products if greenhouse gas emissions associated with they are planted to the right mixtures of additional land-clearing to meet global tree species. Again, these possibilities have food demand. Or, even worse, escalating never yet been pursued commercially, food prices might harm the diets of the much less globally. world’s poorest people, in effect having Mysteries and Paradoxes. The path toward airplanes and vehicles outcompete the achieving environmental sustainability already malnourished poor for food. is ½lled with mysteries and paradoxes. Using, Not Losing, Biodiversity. Biodiver- Mysteries motivate science, leading to

sity might provide a solution for this prob- advances in fundamental science and Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 lem. Consider an as-yet untested possi- technological breakthroughs. Although I bility: the production of carbon-negative have focused on such scienti½c advances biofuels. As already mentioned, biomass in this essay, we also need fundamental production can be increased by 70 to 200 advances in our understanding of our- percent when highly diverse mixtures of selves. Humans are unique among all species are planted. The greatest reported species in how dependent our welfare is yield increases are from the diverse mix- on culturally transmitted knowledge. Our tures of native plants that my collabora- lives depend on knowledge accumulated tors and I have grown in Minnesota on during the ten thousand–year history of highly degraded soils that were no longer agriculture and on advances in public suitable for agriculture.32 Although we health, civil engineering, and medicine. observed no detectable increase in soil We are now so highly dependent on knowl- carbon and nitrogen stores for the mono- edge that many people dedicate the ½rst culture plots, the highest diversity plots twenty-½ve to thirty years of their lives to removed from the atmosphere and stored obtain the knowledge needed for a profes- as soil organic carbon about 4.4 tons of sional career. The pursuit of such training CO2 per hectare per year. As we reported by women and men is perhaps the most in the paper “Carbon-Negative Biofuels important force that is causing global from Low-Input High-Diversity Grassland population growth to slow. Biomass,” this biomass could be used to Human behavior, though, is often con- produce liquid transportation fuels that fused or even paradoxical. Why do so are carbon-negative. Because of carbon many members of the most knowledge- sequestration in the degraded soils, we dependent species on Earth act in ways calculated that the net effect of growing that ignore, or even deny, knowledge? the biomass, making the fuels, and com- Why do individuals refuse to accept mod- busting the fuels would be a reduction in ern scienti½c knowledge as relevant or atmospheric CO2. This possibility, though, even as valid? At the same time that med- is yet to be pursued. ical science has shown that healthy diets Biodiversity might also help us better and active lifestyle can extend lives by a meet growing demand for food and forest decade, people around the world are be- products. The research showing that more coming more overweight and more in- diverse ½sheries are more productive sug- active than ever before. People complain gests that we might be able to harvest more about the high cost of gasoline and yet seafood from aquaculture operations that preferentially buy expensive vehicles that have the right combination of competing have low fuel ef½ciency. People whose lives

141 (3) Summer 2012 117 Biodiversity & have been saved by novel antibiotics that erated by humanity. So, too, must it be Environmental overcame drug resistance that had evolved humanity that discovers and embraces Sustainability in a pathogen often deny the existence of the solutions. This effort will require that evolution. Others deny climate change. we learn more not just about the environ- I do not make these points to disparage ment but also about ourselves. I can imag- anyone, and I deeply value intellectually ine no issue more worthy of pursuit than honest skepticism on any topic. Rather, I the grand, multifaceted challenge of help- mention them because the environmental ing society live sustainably on Earth. issues that Earth faces are problems gen-

endnotes Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 1 Paul Ehrlich and Anne Ehrlich, Extinction: The Causes and Consequences of Disappearance (New York: Random House, 1981), 294; Stuart L. Pimm and Peter Raven, “Extinction by Num- bers,” Nature 403 (2000): 843–845; Peter M. Vitousek et al., “Human Alteration of the Glob- al Nitrogen Cycle: Sources and Consequences,” Ecological Applications 7 (1997): 737–750. 2 Samuel J. McNaughton, “Biodiversity and Function of Grazing Systems,” in Biodiversity and Ecosystem Functioning, ed. Ernst-Detlef Schulze and Harold A. Mooney (Heidelberg, Ger- many: Springer-Verlag, 1994), 525. 3 G. Evelyn Hutchinson, “The Paradox of the Plankton,” The American Naturalist 95 (1961): 137–145. 4 Robert May, Diversity and Stability in Model Ecosystems, 2nd ed. (Princeton, N.J.: Princeton University Press, 1973), 265. 5 Daniel Goodman, “The Theory of Diversity-Stability Relationships in Ecology,” The Quarterly Review of Biology 50 (1975): 237–266. 6 Biodiversity and Ecosystem Functioning, ed. Schulze and Mooney. 7 David Tilman and John A. Downing, “Biodiversity and Stability in Grasslands,” Nature 367 (1994): 363–365. 8 Shahid Naeem et al., “Declining Biodiversity Can Alter the Performance of Ecosystems,” Nature 368 (1994): 734–737. 9 David Tilman, David Wedin, and Johannes Knops, “Productivity and Sustainability Influenced by Biodiversity in Grassland Ecosystems,” Nature 379 (1996): 718–720; David U. Hooper and Peter M. Vitousek, “The Effects of Plant Composition and Diversity on Ecosystem Processes,” Science 277 (1997): 1302–1305; David Tilman et al., “The Influence of Functional Diversity and Composition on Ecosystem Processes,” Science 277 (1997): 1300–1302. 10 Michael A. Huston, “Hidden Treatments in Ecological Experiments: Re-evaluating the Ecosys- tem Function of Biodiversity,” Oecologia 110 (1997): 449–460. 11 David A. Wardle, Olle Zackrisson, Greger Hornberg, and Christiane Gallet, “The Influence of Island Area on Ecosystem Properties,” Science 277 (1997): 1296–1299. 12 Daniel F. Doak et al., “The Statistical Inevitability of Stability-Diversity Relationships in Community Ecology,” The American Naturalist 151 (1998): 264–276. 13 Bradley J. Cardinale et al., “Effects of Biodiversity on the Functioning of Trophic Groups and Ecosystems,” Nature 443 (2006): 989–992; Patricia Balvanera et al., “Quantifying the Evi- dence for Biodiversity Effects on Ecosystem Functioning and Services,” Ecology Letters 9 (2006): 1146–1156; Bradley J. Cardinale et al., “The Functional Role of Producer Diversity in Ecosystems,” American Journal of Botany 98 (2011): 572–592.

118 Dædalus, the Journal ofthe American Academy of Arts & Sciences 14 See ibid. as well as David Tilman, Clarence L. Lehman, and Kendall T. Thomson, “Plant David Diversity and Ecosystem Productivity: Theoretical Considerations,” Proceedings of the Nation- Tilman al Academy of Sciences 94 (1997): 1857–1861; David Tilman, “The Ecological Consequences of Changes in Biodiversity: A Search for General Principles,” Ecology 80 (1999): 1455–1474; Michel Loreau, “Biodiversity and Ecosystem Functioning: Recent Theoretical Advances,” Oikos 91 (2000): 3–17; Clarence L. Lehman and David Tilman, “Biodiversity, Stability and Productivity in Competitive Communities,” The American Naturalist 156 (2000): 534–552; Michel Loreau, “Linking Biodiversity and Ecosystems: Towards a Unifying Ecological The- ory,” Philosophical Transactions of the Royal Society-Biological Sciences 365 (2010): 49–60. 15 Cardinale et al., “Effects of Biodiversity on the Functioning of Trophic Groups and Ecosys- tems”; Balvanera et al., “Quantifying the Evidence for Biodiversity Effects on Ecosystem Functioning and Services”; Cardinale et al., “The Functional Role of Producer Diversity in Ecosystems.” Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 16 Ibid. as well as Tilman, Wedin, and Knops, “Productivity and Sustainability Influenced by Biodiversity in Grassland Ecosystems”; Hooper and Vitousek, “The Effects of Plant Com- position and Diversity on Ecosystem Processes”; Tilman et al., “The Influence of Function- al Diversity and Composition on Ecosystem Processes.” 17 Monserrat Villa et al., “Species Richness and Wood Production: A Positive Association in Mediterranean Forests,” Ecology Letters 10 (2007): 241–250; Daniel Piotto, “A Meta-Analysis d Comparing Tree Growth in Monocultures and Mixed Plantations,” Forest Ecology and Man- agement 255 (2008): 781–786; Alain Paquette and Christian Messier, “The Effect of Biodi- versity on Tree Productivity: From Temperate to Boreal Forests,” Global Ecology and Bio- geography 20 (2011): 170–180. 18 Michael P. Carey and David H. Wahl, “Determining the Mechanism by which Fish Diversi- ty Influences Production,” Oecologia 167 (2011): 189–198. 19 David Tilman et al., “Diversity and Productivity in a Decade-Long Grassland Experiment,” Science 292 (2001): 843–884; Bradley J. Cardinale et al., “Impacts of Plant Diversity on Bio- mass Production Increase through Time because of Species Complementarity,” Proceedings of the National Academy of Sciences 104 (2007): 18123–18128; Peter B. Reich et al., “Impacts of Biodiversity Loss Escalate through Time as Redundancy Fades,” Science 336 (May 4, 2012): 589–592. 20 David Tilman, Peter B. Reich, and Johannes Knops, “Biodiversity and Ecosystem Stability in a Decade-Long Grassland Experiment,” Nature 441 (2006): 629–632; Andrew Hector et al., “General Stabilizing Effects of Plant Diversity on Grassland Productivity through Popula- tion Asynchrony and Overyielding,” Ecology 9 (2010): 2213–2220. 21 Nathan R. Franssen, Michael Tobler, and Keith B. Gido, “Annual Variation of Community Biomass is Lower in More Diverse Stream Fish Communities,” Oikos 120 (2011): 582–590. Salvatore Di Falco, Jean-Paul Chavas, and Melinda Smale, “Farmer Management of Produc- tion Risk on Degraded Lands: The Role of Wheat Variety Diversity in the Tigray Region, Ethiopia,” Agricultural Economics 36 (2007): 147–156. 22 Charles E. Mitchell, David Tilman, and James V. Groth, “Effects of Grassland Plant Species Diversity, Abundance, and Composition on Foliar Fungal Disease,” Ecology 83 (2002): 1713– 1726. 23 Felicia Keesing et al., “Impacts of Biodiversity on the Emergence and Transmission of Infec- tious Diseases,” Nature 468 (2010): 647–652. 24 Joseph E. Fargione and David Tilman, “Diversity Decreases Invasion via Both Sampling and Complementarity Effects,” Ecology Letters 8 (2005): 604–611. 25 Kenneth G. Cassman et al., “Meeting Cereal Demand while Protecting Natural Resources and Improving Environmental Quality,” Annual Reviews of Environmental Resources 28 (2003): 315–358.

141 (3) Summer 2012 119 Biodiversity & 26 Youyong Zhu et al., “The Use of Rice Variety Diversity for Rice Blast Control,” Scientia Agri- Environmental cultura Sinica 36 (2003): 521–527; Long Li et al., “Diversity Enhances Agricultural Produc- Sustainability tivity via Rhizosphere Phosphorus Facilitation on Phosphorus-De½cient Soils,” Proceedings of the National Academy of Sciences 104 (2007): 11192–11196. 27 Tilman, Lehman, and Thomson, “Plant Diversity and Ecosystem Productivity”; Tilman, “The Ecological Consequences of Changes in Biodiversity”; Loreau, “Biodiversity and Ecosystem Functioning”; Lehman and Tilman, “Biodiversity, Stability and Productivity in Competitive Communities”; Loreau, “Linking Biodiversity and Ecosystems.” 28 Ibid. 29 Anthony R. Ives, Kay Gross, and Jennifer L. Klug, “Stability and Variability in Competitive Communities,” Science 286 (1999): 542–544. 30 Gretchen C. Daily, ed., Nature’s Services (Washington, D.C.: Island Press, 1997), 375. Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/108/1830517/daed_a_00166.pdf by guest on 29 September 2021 31 Jonathan A. Foley et al., “Solutions for a Cultivated Planet,” Nature 478 (2011): 337–342; David Tilman, Christian Balzer, Jason Hill, and Belinda L. Befort, “Global Food Demand and the Sustainable Intensi½cation of Agriculture,” Proceedings of the National Academy of Sciences 108 (2011): 20260–20264. 32 David Tilman, Jason Hill, and Clarence Lehman, “Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass,” Science 314 (2006): 1598–1600.

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