Biodiversity is Nature’s backup system and thus the bedrock foundation of social-environmental sustainability. Backup systems, in the biological sense, comprise the various functions of different species that act in concert as an environmental insurance policy. To maintain this insurance policy, an ecosystem needs three kinds of diversity: biological, genetic, and functional. is the richness of species in any given area. constitutes different degrees of flexibility in the way a species adapts to change. The most important aspect of genetic diversity, from our human point of view, is that it buffers the extremes in the variability of the way ecosystems behave, particularly in the medium and long term. And equates to the variety of biophysical processes the species richness of an area allows to take place. The upshot is that healthy ecosystems act as shock absorbers in the face of potential, catastrophic disturbance.
BIODIVERSITY AS ECOLOGICAL BACKUPS AND SHOCK ABSORBERS
To better understand this concept, think of each of these kinds of diversity as the individual leg of an old-fashioned, three-legged milking stool. When so considered, it soon becomes apparent that if one leg (one kind of diversity) is lost, the stool will fall over. Fortunately, however, biological diversity passed forward through genetic diversity effectively maintains functional diversity.
This backup results in a stabilizing effect similar to having a six-legged milking stool, but with two legs of different kinds of wood in each of three locations So, if one leg is removed, it initially makes little difference which one because the stool will remain standing. If a second leg is removed, its location is crucial because, should it be removed from the same place as the first, the stool will fall. If a third leg is removed, the location is even more critical, because the loss of a third leg has now pushed the system to the limits of its stability, and it is courting collapse—in terms of the value we, as a society, placed on the ecosystem in the first place. Remove one more piece and the ecosystem to shift dramatically, perhaps to our long-term social detriment.
By analogy, there are two different species with similar functions but varying degrees of adaptability in three separate locations within a given landscape. If the population of one species becomes locally extinct, it initially makes little difference which one or in which location because all local ecosystems will continue to function and serve the local people. If, however, a second species becomes locally extinct, its location is crucial because, should it be disappear from the same place as the first, the ecosystem would be unable to function as before. Now, if one more population of the two species becomes extinct in one of the three local areas, for whatever reason, its location is even more critical, because the loss of a population in the third area would pushed all three local ecosystems to the limits of their stability—in terms of the value the local people placed on them in the first place. Finally, if a fourth population were to become extinct, its loss would cause the local ecosystem to shift, perhaps to the long-term detriment of the people who counted on that particular ecosystem service for their livelihood.
In a broader sense, by continuing to overfish the world’s oceans, clear-cut the world’s forests, over-graze the world’s grasslands, fragment once-contiguous habitats, and generally despoil the world through the continued pollution of its air, soil, and water—we humans are causing large numbers of species to become extinct (and their feedback loops and ecological services with them), all to our progressive social-environmental impoverishment and that of all generations to come.
We must, therefore, understand and account for the continuance of life’s biological backup systems, those multi-species relationships that act simultaneously as environmental shock absorbers and insurance policies, whereby ecosystems are largely protected against major functional shifts and thus our loss of their ecological services. Another major challenge with the loss of the individual services performed by different species is the gradual dismantling of Nature’s self-reinforcing feedback loops, which collectively constitute the functional engine of every ecosystem. How, you might wonder, do species act in concert to create a network of feedback loops within an entire forest.
THE FEEDBACK LOOPS OF LIFE
Biologist Louise Emmons made a critical observation with respect to this question: “For want of a squirrel, a seed was lost; for want of a seed, a tree was lost; for want of a tree, a forest was lost; for want of a forest . . . ” In other words, the tremendous biodiversity of the tropical rain forests could be lost without cutting a single tree! How? Let’s consider the rain forest in Gabon, Africa, whose fascination to Emmons lies in “its stunning complexity.” In this forest, says Emmons, “You can stand anywhere and be surrounded by hundreds of organisms that are all ‘doing something,’ going about their living in countless interactions—ants carrying leaves, birds dancing, bats singing, giant blue wasps wrestling with giant tarantulas, caterpillars pretending they are bird droppings, and so on.”1
In Gabon (an equatorial state bordering the Atlantic Ocean in west-central Africa), Emmons found that nine species of squirrels all live together in one forest. Each is a different size; three have specialized diets or habits, which leaves six that feed on nuts, fruits, and insects and could therefore be potential competitors for food. But a closer look reveals that three of the six species—one large, one medium, and one small—live exclusively in the canopy of the forest, where the largest one, a “giant” squirrel, feeds primarily on very large, hard nuts, while the smaller ones eat proportionally smaller fruits and nuts. The other three species—again one large, one medium, and one small—live exclusively on the ground, where they eat the same species of fruits and nuts as do their neighbors in the canopy, except that they eat the fruits and nuts once they have fallen.
Fruit can be found on the trees throughout the year in the tropical forest of Gabon, but any one species of tree produces fruit for only a short period each year. To support three species of squirrels, eight species of monkeys, and eight species of fruit-eating bats (and so on) in the canopy, this forest must have a wide variety of species of trees and lianas (high-climbing, usually woody vines), each of which produces fruits and nuts in its own rhythm. The varying sizes of the fruits and nuts can support different sizes of squirrels with different tastes, whereas these same fruits and nuts when they fall to the ground can feed a whole analogous array of species.
Clearly tropical rain forests are amazingly rich in species of trees—not just any trees, however, but those whose fruits are eaten and whose seeds are dispersed by birds and mammals. Not surprisingly, therefore, tropical rainforests are also rich in mammals and birds—not just any mammals and birds, however, but those that eat fruits and disperse their seeds. There are, for example, 126 species of mammals within a single area of forest in Gabon. Furthermore, the life cycle of each species is interdependent with the life cycles of the other species. The enormous number of vertebrate animals appears to be supported by the large number of species of plants that act as sources of food all year.
If all this biodiversity of vegetation is to be maintained, each individual tree must succeed in leaving offspring. Seeds and tender young seedlings are among the richest foods available to forest animals, and their succulence greatly increases their chance of being eaten by the large numbers of hungry animals searching for food around the bases of fruit- and nut-bearing trees. Similarly, such organisms as fungi, worms, and insects soon accumulate where the seeds and seedlings are concentrated and spread from one seed or seedling to another.
Under such circumstances, seeds carried away from such concentrations of hungry organisms are more likely to succeed in germinating than seeds that remain in place. Another major benefit of seeds being carried away from the parent tree is the likelihood they will fall in places with different conditions. A new condition might be a pocket of better soil on a mound created by termites or in a spot where a dead tree has created a hole in the canopy that emits sunlight.
It is certainly no accident that 80 to 95 percent of the species of trees in tropical rainforests produce seeds that are dispersed by birds and mammals. By dispersing those seeds, the birds and mammals also maintain the rich diversity of species of trees, which not only formed their habitat in the first place but also perpetuate it. This is an ideal example of a self-reinforcing feedback loop.2
Before leaving this topic, let’s consider biodiversity from a more biophysical point of view. For example, the spatial distributions of 36 to 51 percent of the tree species in three diverse Neotropical, primary forests in Colombia (La Planada), Ecuador (Yasuni), and Panama (Barro Colorado Island) show strong associations with the distribution of soil nutrients. These results indicate that the availability of belowground resources plays an important role in the assembly of communities of tropical trees at local scales. With respect to climate change, the response of tropical forests will depend on the nutritional strategies individual species.
A group of tropical trees (which form the forest canopy) and ferns (which live in the understory) rely on a common pool of inorganic nitrogen, rather than specializing in the extraction of different nitrogen pools. Moreover, these tropical species abruptly changed their dominant source of nitrogen in unison in response to a climate-driven change in precipitation. This threshold response indicates a coherent strategy among topical species to exploit the most available form of nitrogen in their soils. Such apparent community-wide flexibility indicates that diverse species within tropical forests can physiologically track changes in the cycling of nitrogen brought about by shifts in the climate.
This kind of flexibility is critical when it comes to changing land use in the tropics, where vast areas of habitat have been damaged and biodiversity has been reduced. Yet the effects of dwindling biodiversity are rarely considered in terms of their innate relationships to ecosystem processes. Data from studies in Puerto Rico, Southern China, Dominica, and Nicaragua indicate that functional diversity—not simply species richness—is important in maintaining integrity, despite the fluxes of nutrients and energy. High species richness may, nevertheless, increase the resilience of tropical ecosystems following disturbance by increasing the number of alternative pathways along which resources can flow.3
Although I could extol the wonders of Nature seemingly forever, at this juncture suffice it to say that we must understand, account for, and protect the continuance of life’s backup systems—those multi-species relationships that act simultaneously as environmental shock absorbers and insurance policies. As such, biodiversity, in all its myriad feedback loops, protects ecosystems from changes that would degrade or discontinue the biophysical services that form the natural wealth we humans rely on for our well-being. If you are wondering what I mean by “natural wealth,” professor David Pearson, from the University of Arizona, Tempe, expresses it nicely:
• Ecosystem processes: Biodiversity [which includes both genetic and functional diversity] underpins the processes that make life possible. Healthy ecosystems are necessary for maintaining and regulating atmospheric quality, climate, fresh water, marine productivity, soil formation, the cycling of nutrients, and waste disposal.
• Ethics: No species and no single generation have the right to sequester Earth’s resources solely for its own benefit.
• Aesthetics and culture: Biodiversity is essential to nature’s beauty and tranquility. Many countries place a high value on native plants and animals. These contribute to a sense of cultural identity, spiritual enrichment, and recreation. Biodiversity is essential to the development of cultures.
• Economics: Plants and animals attract tourists and provide food, medicines, energy, and building materials. Biodiversity is a reservoir of resources that remains relatively untapped.4
When, therefore, we humans tinker willy-nilly with an ecosystem’s composition and structure to suit our short-term economic desires, we risk losing species, either locally or totally, and so reduce the ecosystem’s biodiversity, then its genetic diversity, and finally its functional diversity in ways we might not even imagine. By decreasing biodiversity, for whatever reason, we simultaneously decrease myriad biophysical feedback loops and thereby progressively lose existing choices for manipulating our environment in such a way that we increase—rather than decrease—our quality of life. Such loss of choice can affect our economic viability both directly and indirectly because lost biodiversity can so alter an ecosystem that it is rendered incapable of producing what we once valued it for or what we, or the next generation, could potentially value it for again. Maintaining biodiversity and its backup systems is thus a critical link in the reciprocal relationship between ecosystems and communities—social-environmental sustainability.
Finally, this series of essays is not only meant to tell a wonderful story of our incredible home planet but also to help you, the reader, understand that the array of species—collectively biodiversity—evolved exceedingly slowly through the long reaches of time, but can be lost within a millisecond (geologically speaking) through extinction, which is forever. Moreover, as each species becomes extinct, its irreplaceable biophysical functions, which we take for granted as ecological services, are also lost unto everlasting. It is absolutely essential, therefore, that we understand and accept the fact that our social-environmental well-being, and that of every generation, rests with the vital protection of biodiversity worldwide. That responsibility is ours, as the adults and leaders of today. To all generations of children we bequeath the consequences of our thoughts, decisions, and actions—for better or worse. How shall we choose?
Series on Biodiversity:
1. Louise H. Emmons, “Tropical Rain Forests: Why They Have So Many Species, and How We May Lose This Biodiversity without Cutting a Single Tree,” Orion 8 (1989): 8–14.
2. Louise H. Emmons, “Tropical Rain Forests: Why They Have So Many Species, and How We May Lose This Biodiversity without Cutting a Single Tree.” op. cit.
3. The preceding three paragraphs are based on: (1) Whendee L. Silver, Sandra Brown, and Ariel E. Lugo, “Effects of Changes in Biodiversity on Ecosystem Function in Tropical Forests,” Conservation Biology 10 (1996): 17–24; (2) Robert John, James W. Dalling, Kyle E. Harms, and others, “Soil Nutrients Influence Spatial Distributions of Tropical Tree Species,” Proceedings of the National Academy of Sciences 104 (2007): 864–869; and (3) Benjamin Z. Houlton, Daniel M. Sigman, Edward A. G. Schuur, and Lars O. Hedin, “A Climate-Driven Switch in Plant Nitrogen Acquisition within Tropical Forest Communities,” Proceedings of the National Academy of Sciences 104 (2007): 8902–8906.
4. From an advertisement in the Arizona State University Magazine (received in 2007) for David L. Pearson’s 2001 book on tiger beetles published by Cornell University Press and written with Alfried P. Vogler.
Text © by Chris Maser 2012. All rights reserved.