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.

Following an extreme disturbance, the ecosystem may go through the process of primary succession, which is characterized by a predictable series of developmental stages that culminate in a climax community – a stable biotic community that represents the final stage of succession. In many cases a disturbance will only kill some of the organisms within the ecosystem. In these cases, the ecosystem may go through a process of secondary succession, in which many factors, including the intensity of the disturbance, the life history traits of colonizing species, and the presence of biological legacies influence the recovery process. Ecologists have described three conceptual models of succession – facilitation, tolerance, and inhibition – that apply under different conditions in different ecosystems. Animals play an important role in the recovery process. Many animal species are excellent dispersers and can quickly return to a disturbed ecosystem. Even if they are unable to establish a breeding population, animals can import seeds or nutrients into a disturbed habitat. Alternatively, animals can inhibit the recovery process by eating seeds or young plants before they get established. In some cases, disturbance can cause ecosystems to experience a regime shift – a very rapid change from one stable state to another.

More than most researchers, Bernd Heinrich’s research is rooted in his background as a naturalist, and his powers of observation. He knew his study species very well, so he was quick to identify anomalous or surprising phenomena. He was particularly attracted to evolutionary puzzles – traits that on the surface appear to be maladaptive. One example of an evolutionary puzzle discussed in this chapter was an observation of caterpillars tossing parts of leaves down from trees (when they could be eating them). A second example was ravens making a ruckus when they find a large carcass, thereby being forced to share the food bonanza with many other birds they attract to the scene. Both studies show how science is an iterative process, which involves testing and rejecting multiple alternative hypotheses. Heinrich brought his research into the laboratory as well, designing ingenious experiments to explore the mechanisms underlying insect thermoregulation. One theme shaping Heinrich’s research is the connection between the natural environment and how natural selection influences behavioral and physiological patterns.

Population ecologists work in three time frames: the past, present, and future. Research in each time frame has its own set of challenges, tools and assumptions. Historical studies of populations often use fossil evidence to make inferences of past distributions and abundance of populations of different species. In Rapa Nui, and other studies of human populations, ecologists also use cultural remains to help with their inferences. Only rarely can ecologists accurately count the numbers of individuals within a present-day population. Instead they rely on a variety of tools and techniques to estimate population size and population growth rates. Ecologists have identified density-independent factors, such as temperature, rainfall, and disturbance, and density-dependent factors, such as competition and disease, that influence population growth. Once growth rates are estimated, ecologists can apply mathematical models to make projections of future population size, which are particularly important for making management decisions about endangered species. Population models are also applied to human populations, allowing planners to anticipate resource needs in regions of the world that will experience substantial changes in population size in future decades.