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.

Threespine sticklebacks, numerous species of disease-causing bacteria, and Darwin’s finches have all shown rapid evolutionary change in response to changing environments. Evolutionary ecologists use a variety of genetic and molecular approaches to study evolutionary change in these and other species. Gene flow, genetic drift, mutation, and natural selection can cause evolutionary change within a population, but natural selection is the only evolutionary process that can lead to adaptation. The benefits and costs of adaptations are environment-dependent and reflect evolutionary tradeoffs, so a trait may be beneficial in one environmental context and costly in a second. Natural selection may lead to speciation when genetic divergence is maintained either by physical barriers to gene flow, or by assortative mating of similar genotypes within a population. Evolutionary ecologists compare morphological, behavioral, and, most commonly, molecular characters in related groups of organisms, and use similarities in these characters to create phylogenetic trees that reflect evolutionary relationships.

Over the years, the Serengeti has been a model ecosystem for answering basic ecological questions about the distribution and abundance of organisms, populations, and species, and about how different species interact with each other and with their environment. Tony Sinclair and many other researchers have addressed some of these questions, and continue to work on understanding important biotic and abiotic linkages that influence ecosystem functioning. In common with all types of scientific inquiry, ecologists use predictions to test hypotheses about ecological processes; this approach is highlighted by Sinclair’s research that explored why buffalo and wildebeest populations were rapidly expanding. Like other scientists, ecologists use observation, modeling, and experimentation to generate and test hypotheses. However, in contrast with much biological inquiry, ecologists ask questions that link numerous levels of the biological hierarchy, from molecular to global ecology.

Animal behavior is subject to the action of natural selection, favoring individuals that behave in ways that maximize their fitness by promoting individual survival and reproductive success. Cultural evolution plays an important role, with behavioral traits of surprising complexity spreading rapidly through a population. Behavioral ecologists measure the costs and benefits of alternative types of behavior to gain an understanding of basic behavioral processes such as territory defense, foraging, and mating. This cost–benefit approach allows quantitative predictions of behavior tightly tied to fitness, such as how long to guard a mate, or how long to forage at a particular location before moving on. Both physiological factors, such as the need to keep eggs warm, and ecological factors, such as the spatial distribution of resources, can influence the evolution of mating systems. In many species, individuals cooperate with each other in procuring food or defending against predators. Hamilton’s model of indirect selection is one possible explanation for the evolution of behavior favoring relatives, including the astounding degree of cooperation in eusocial animals. But in some species, cooperation is common even among unrelated individuals; in these cases game theory models may help explain the evolution of cooperation.

A biome is a large geographical region with characteristic groups of organisms adapted to its environment. Terrestrial biomes are influenced primarily by precipitation and temperature, but also by fire, soils, and nutrients. Aquatic biomes are also influenced by temperature and nutrients, but additionally by salinity, water depth, and current. Consequently, to understand why a certain biome exists in a particular region, it is essential to understand factors that influence that region’s climate, and the processes responsible for the distribution of nutrients. Though tropical rainforests are terrestrial and coral reefs are aquatic, both biomes share many important features. They are hubs of species richness, they have numerous similarities in structure and functioning, and both are threatened by human activities, including acidification. All biomes are dynamic, with shifting borders and changing species composition. Several new biomes have been discovered recently, bringing home the message of how much we still don’t know about how our planet functions. Humans have changed Earth’s properties in so many ways that some researchers argue that ecologists should shift their focus to exploring how human activities influence the distribution of species.

Paleoecological studies can provide some insight into factors influencing a species’ present-day distribution, and its present-day distribution can, in turn, provide some insight into its future distribution. Being able to predict future distributions is very important because climate, an important influence on species distribution, is now changing at a rapid rate. Within a population, individuals may have a random, uniform, or clumped dispersion, though a clumped dispersion is most common because essential resources such as food, light, and undisturbed habitat are often spatially clumped. Distribution patterns change over the short term, as a result of dispersal, and over the long term from factors that influence range expansion and contraction. Abiotic factors, such as climate, soils, light availability and disturbance, and biotic factors, such as behavior, life histories and interactions with other species, can influence the distribution of species. Changes in these factors can lead to changes in distribution, including range expansion, range contraction and extinction. By quantitatively describing a species’ ecological niche, ecologists can understand a species’ present distribution, and may be able to make predictions about its future distribution.

The strong interactions among sea otters, sea urchins, and kelp are the basis of a trophic cascade, a community-level process that is influenced primarily by the actions of predators, and which has indirect effects at least two trophic levels removed from the actions of the predator. Ecologists explore community processes across scales of space and time to investigate whether trophic cascades such as these are common, and whether they are conditional on a particular set of environmental conditions. Ecological communities are structured by species composition and diversity, by the distribution and abundance of each species within the community, by the interactions among species, and by the presence of guilds and functional groups. In addition, community processes are strongly influenced by the actions of dominant and keystone species. Ecologists use food webs to construct a useful but incomplete picture of community structure. By quantifying certain attributes of food webs, such as linkage density and connectance, ecologists can compare food webs to identify unique features of community structure in a particular web. Food web structure and functioning are influenced by both consumers and producers. Ecologists are attempting to identify conditions under which bottom-up versus top-down processes are more likely to influence community structure

Climate change has already profoundly changed the ecological world on all levels of the biological hierarchy. Comparing the past with the present allows researchers to document that changes have happened, and to understand why some groups (e. g. birds vs. mammals in the Mojave Desert) respond differently than others. Climate change has already changed population phenology, and researchers can estimate the speed of phenological change. Population range shifts may occur on two fronts: leading-edge expansion and trailing-edge contraction. Both of these processes are influenced by biotic and abiotic factors. Climate change can also influence the genetic structure and sex ratio of populations – in extreme cases leading to extinction. Changes to the timing of migration or to the emergence of plants and insects can cause phenological mismatch of exploitative or competitive interactions. Prey species are likely to benefit, while predators or herbivores may suffer from lack of food. On a larger scale, both terrestrial and aquatic ecosystems are showing the effects of climate change, even in tropical biomes where warming and drying are not as prominent as they are in more temperate or polar biomes. Though immune to drought, marine biomes are suffering from acidification and from low oxygen levels.

Acclimatization and adaptation are two responses of organisms to a changing environment. Each species functions best in a particular thermal environment, and has adaptations that allow it to survive and reproduce successfully in that environment. The range of thermal conditions tolerated will vary with each species. Animals and plants face somewhat different challenges, as plants cannot move to a new environment, but there is also considerable overlap in how animals and plants deal with a variable thermal environment. Organisms must balance their internal water and solute concentrations within tolerable levels. Terrestrial species in dry environments have adaptations for procuring and conserving water, while aquatic species are more likely to be challenged to maintain the proper solute concentration. For many species, the problem of temperature regulation and water regulation are functionally linked, and there may be tradeoffs in how these two critical processes interact. Environmental temperature, water availability, and solute concentration influence species distribution and abundance. Climate change caused by human actions is already significantly affecting species distribution and abundance and ecosystem functioning.