At their core, the problems we face today are no different from those our ancestors faced: how to find a balance between what we take from the biosphere and what we leave behind for our descendants. But while our distant ancestors were incapable of affecting the Earth System as a whole, we are not only able to do that, we are doing it. Humanity now faces a choice: we can continue down a path where our demands on Nature far exceed Nature’s capacity to supply them on a sustainable basis; or we can take a different path, one where our engagements with Nature are not only sustainable but also enhance our collective well-being and the well-being of our descendants.

To pursue a sustainable future will require a transformative change in our mode of thinking and acting.

The biosphere, which is the part of Earth that is occupied by living organisms, is a self-organising, regenerative entity. Its rhythms, for example the seasons, shape the regeneration patterns of the living world. But living systems in turn make use of the non-living, or abiotic, constituents of the biosphere and transform them. Water, carbon and nitrogen cycles are expressions of that. Because the ability to regenerate is a characteristic of living systems, the biosphere’s regeneration is key to the sustainability of the human enterprise. Regenerations of the living world at various scales and periodicity are synchronised via natural processes that are still not understood well.55

Biological diversity, or biodiversity for short, means the diversity of life. Its decline disrupts biospheric processes, for example, the processes governing the climate system. The sustainability of our engagement with Nature is thus ultimately about the functioning of the biosphere, not just the living part of it.

Chapter 10 looked at the distribution of well-being across time and the generations. This chapter examines the distribution in space rather than time, of components of the Impact Equation introduced in Chapter 4. That is, it breaks down the Impact Equation – a global condition of sustainability – to examine how both our demands on the biosphere and the biosphere’s supply are distributed geographically and among income groups.

On the demand side, there is significant variation in human activity (y), population (N) and efficiency in our engagement with the biosphere (α), both within and between countries. On the supply side, the stock of the biosphere (S) is spatially varied – there exists an inherent heterogeneity in ecosystems and natural resources throughout the world – and the intensity of depletion varies between and within countries and regions.

Ecosystem structure and functioning is the focus of much ecological research because many ecosystem properties such as production, energy flow, nutrient cycles, and stability lie at the core of understanding ecological processes. Net primary production (NPP) is primarily influenced by climate and nutrients. On a global scale, NPP in terrestrial biomes tends to be greatest near the tropics, where the combination of constant and moderately high temperature and adequate rainfall promote plant growth. NPP in marine biomes peaks at about 40° S latitude, which is associated with large areas of upwelling and high nutrient availability. On a regional and local scale, the availability of nutrients such as nitrogen and phosphorus influence terrestrial, marine, and freshwater production. Ecosystem structure is based on the interactions between producers, consumers, detritivores, and decomposers. A substantial but variable amount of energy is lost with each transfer from one trophic level to the level above, which has the effect of limiting food chain length (FCL). In some aquatic systems, longer food chains are associated with CO2 export from the water into the atmosphere, and with the biomagnification of toxic substances.

Anthropologist Richard Leakey sent Jane Goodall to Gombe (now Gombe Stream National Park) to study chimpanzees in the wild. As an anthropologist, he was keenly interested in human behavior, and believed that chimpanzees would provide a window to understanding it. It took Goodall six months of crawling around in the woods before any chimpanzees would allow her to get close enough to observe them. But her persistence paid off, as she was able to document chimpanzees showing some very human behavior including tool making, cooperative hunting and war making. Partway through her career, she elected to devote the rest of her career to environmental activism and education, and Gombe research was continued by a growing community of researchers including her student, Anne Pusey. Pusey was fascinated by mother–infant relationships, by developmental changes in juveniles as they matured, and by how chimpanzees manage to avoid breeding with close relatives. Other researchers at Gombe studied the relationship between rank and reproductive success, and how disease was influencing survival rates in three different populations in the region. Unfortunately, life table studies indicate that disease and a lack of immigrants into the region are threatening the viability of this iconic group of chimpanzees.

A species’ behavioral, developmental, and reproductive life history will influence how quickly it can recover after a population crash. Some species can recover very quickly, while others, such as the North Atlantic right whale, cannot recover quickly, because even under ideal conditions they develop slowly and have very low reproductive rates. Ecologists have described various life history classification schemes that identify important tradeoffs in resource allocation, and focus attention on interesting life history questions. The quantitative relationship between metabolic rate and body size can help ecologists understand some life history tradeoffs, such as the relationship between number and size of offspring. There is a fundamental tradeoff between parental investment in any one reproductive event and the number of lifetime reproductive events, which in some cases can lead to a semelparous reproductive life history. Variable environments can select for phenotypic plasticity, which can lead to organisms with similar genotypes expressing alternative behavioral, developmental or reproductive life history traits. In some cases, phenotypic plasticity may help species adjust to rapidly changing environmental conditions, including climate change.

Organisms may compete for a great variety of limiting resources, such as food and habitat and, in the case of plants, light and pollinators. Direct mechanisms of competition, as highlighted by interactions between yellow crazy ants and hermit crabs on Tokelau, include resource and interference competition, while indirect mechanisms of competition that are mediated by other species are also widespread in ecological communities. Introductions of species into novel environments allow ecologists to study competitive interactions in real time. Interspecific competition can lead to competitive exclusion when two or more species occupy similar niches. A variable environment, niche shift, and niche partitioning can promote species coexistence. Theoretical models, such as the Lotka–Volterra competition model, help identify conditions in which two or more competing species can coexist. When conservation ecologists introduce two or more species as biological control agents, they must consider potential competitive interactions among the introduced species, keeping in mind the factors that promote the coexistence of the introduced species.

Humans have profoundly changed nutrient cycles on a global, regional, and local level. Agricultural runoff carrying heavy loads of nitrogen and phosphorus compounds caused eutrophication of the Black Sea. This led to a series of events that culminated in the annual formation of a dead zone within the Black Sea, and the consequent loss of biological diversity of several trophic levels. The nitrogen cycle depends heavily on the activities of microorganisms to fix nitrogen, and to transform nitrogen in the processes of nitrification, ammonification, denitrification, and anammox. Technological advances such as the Haber–Bosch process have vastly increased the amount of reactive nitrogen entering ecosystems, leading to increases in agricultural production, but also polluting many aquatic systems. The phosphorus cycle is similar to the nitrogen cycle, in that globally there are vast stores of phosphorus compounds, but most of it is inaccessible to organisms. In contrast to the nitrogen cycle, there is only a small atmospheric component to the phosphorus cycle; most phosphorus becomes available through weathering of rocks. Both nutrient cycles are similar in one very important way; nitrogen and phosphorus are recycled many times between organisms and the environment before exiting an ecosystem.

Dan Janzen and Winnie Hallwachs, his wife and colleague, have spent two lifetimes studying ecological interactions between organisms, mostly at Area de Conservacion Guanacaste (ACG) in northwestern Costa Rica. Early in his career, Janzen investigated many basic questions in evolutionary community ecology. One study of plant reproductive success and life history strategies showed that legume species use one of two alternative strategies to reproduce successfully – producing huge numbers of tiny defenseless seeds or small numbers of large, well-defended seeds. A second study explained high biological diversity in rainforests as arising because baby plants survive poorly near their parents (because seed predators consume them there), and only become established a considerable distance away from them. He also emphasizes that current selection pressures may differ from historical pressures, so it is critical to understand ecosystems in the context of their evolutionary history. Both Janzen and Hallwachs have now shifted their focus to inventorying the diversity of Lepidoptera, their parasitoids and host plants at ACG, so that their complex interactions can be understood by researchers and by students who use ACQ as a natural classroom.

Island biogeography theory views island species richness as an equilibrium of extinction rates and the immigration rates of novel species to an island. At equilibrium, MacArthur and Wilson’s model predicts that species composition will change over time, but species richness will remain relatively stable. In addition, large islands with low extinction rates and high immigration rates will tend to support more species than will small islands. Geographic ecologists also want to understand why particular species or groups of species have a particular geographic distribution. The theories of continental drift and plate tectonics have helped to resolve these questions. More recently, developments in molecular technology have allowed biogeographers to answer numerous questions about species distributions. Landscape ecology explores how variation in landscape structure, such as configuration or scale, influences the distribution and abundance of species. Conservation ecologists are particularly concerned that industrial, agricultural, and urban development have led to increased fragmentation of habitat that is suitable for sustainable wildlife populations. Applying the lessons of island biogeography, ecologists recommend erecting immigration corridors to increase immigration rates of novel species into nature preserves, thereby increasing species richness.

Assuming directorship of the National Oceanic and Atmospheric Administration (NOAA) was one step in Jane Lubchenco’s career that demonstrated her commitment to both basic and applied ecology. In her role as NOAA director, she helped coordinate the efforts of thousands of responders to the Deepwater Horizon spill, and helped evaluate the short- and long-term effects of the spill on marine ecosystems. Lubchenco’s research career began with an investigation into how two species of seastars coexist in intertidal communities. This experience led to a series of comparative studies of intertidal communities off the eastern and western US coastline, and a collaborative study off the Panama coastline. Her research highlighted that ecosystems are structured from the interactions of biotic factors such as herbivory and predation, and abiotic factors such as wave intensity and the presence of refuges to escape predation. A common thread running through her research is that indirect biotic interactions are important and easy to overlook. Field experiences and interactions with many colleagues motivated Lubchenco to get involved in a variety of initiatives that defined the future of ecological research and developed a core of researchers who were effective communicators of ecological applications.

Exploitative interactions can be understood in terms of their lethality and intimacy. Predators and parasitoids cause highest lethality, parasites and parasitoids have highest intimacy with their hosts, while grazers are low on both scales. Exploiters can regulate the populations of their hosts directly by killing or injuring them, or through nonconsumptive processes such as increasing their prey’s stress level and thereby reducing reproductive rates, as has been implicated for the snowshoe hare. Exploiters can also regulate community processes indirectly; for example bats and birds eat arthropods in the forest, which reduces leaf damage by herbivorous arthropods. Prey and hosts use constitutive defenses, such as thorns in plants, and large body size in Serengeti grazers, against exploiters. Some species have evolved induced defenses; for example some plants release toxic chemicals following herbivore attack. The outcomes of exploitative interactions can be predicted by the Lotka–Volterra predation model, which, in its most basic form, predicts that the relative abundance of predators and prey will cycle. A simple model of disease transmission can explain how disease spreads in host populations based on the ease of transmission, the amount of time the host is infectious, and the population size of the host. Both models make numerous simplifying assumptions. Ecologists can incorporate biological complexity into these models, which makes them more realistic, but also more difficult to understand and apply.