Autecology refers to how a single species interacts with the environment; its counterpart is synecology, which refers to how multiple species interact with each other. This latter term is mostly congruent with the field of community ecology, the subject of part III of this volume.
Integral to any discussion of autecology is the concept of the niche. This concept has a long and checkered history in the field of ecology, and the term itself has taken on different meanings through time (chapter I.1). In the most general sense, however, we may think of the niche of a population as the way members of that population interact with their environment, both biotic and abiotic. In other words, the term “niche” refers to where organisms live and what they do there.
The first step in considering how organisms interact with their environment is investigating how the specific phenotypic characteristics of members of a population allow them to exist in a particular environment. The environment poses a wide variety of challenges to organisms: for example, they must be able to obtain and retain enough water, withstand high or low temperatures, and obtain enough nutrients to survive. More than a century of research has revealed that species, and even populations of species, are often finely tuned to the specific conditions in the environment in which they live. In recent years, increasingly sophisticated approaches and instrumentation have allowed an exquisitely detailed understanding of the physiological basis of organismal function (chapters I.2–I.4).
Animals—and, in some sense, fast-growing plants—also can influence the way they interact with their environment through behavioral means. For example, animals can choose the habitat in which they occur and thus can determine, to some extent, the environment they experience throughout their lives (chapter I.5). Many organisms move from their birth site at a particular stage in life; although for plants and some animals, dispersal is passive, other species actively choose where to settle (chapter I.6).
Behavior, of course, is a key component of how most animals interact with their environment. Almost all aspects of the natural history of animals have a behavior component. In part I, we consider foraging (chapter I.7) and social behavior (chapter I.8). Other topics are included in parts II and VI of this volume.
Most plants have relatively little ability to determine the environmental conditions they experience. But plants often have another option available—they frequently exhibit substantial phenotypic plasticity, which allows a plant to alter its phenotype in an advantageous way to be better suited to its environment. Scientists have long appreciated this ability in plants, and zoologists have come to realize relatively recently that many animal species exhibit adaptive phenotypic plasticity as well (chapter I.9).
Organisms adapt in yet another way, by molding their life cycle—what is termed “life history”—to the particular environment in which they live (chapter I.10). Thus, species in environments in which resources are abundant and threats are common may have short generation times and early reproduction. Conversely, in environments in which resources are more scarce but threats are not as severe, a more successful strategy may be to defer reproduction and to invest in becoming better competitors for resources, delaying reproduction and ultimately producing fewer, but better provisioned, offspring.
No species occurs everywhere in the world. The behavior and physiological capabilities of a species determine where a species can and cannot occur. In the last few years, advances in remote sensing technology have provided the capability to visualize the distribution of environmental conditions with great precision over large spatial scales (chapter I.11). Combined with records of species occurrences and, ideally, an understanding of species’ physiological capabilities, these geographic information systems approaches have opened new vistas for understanding how and why species occur where they do; these approaches are also of great importance in predicting how species will respond to rapidly changing environmental conditions (see parts IV and V). Of course, the distribution of a species is not only a function of its physiological capabilities and other aspects of its ecology. Rather, Earth geography and history also are important—a species cannot occupy an area that it has never had the opportunity to colonize. Consequently, biological and historical factors combine to determine the geographic range of any species (chapter I.12).
Integral to an understanding of how organisms interact with their environment is the concept of adaptation, the idea that natural selection has molded the characteristics of populations so that they are well suited to the particular circumstances in their environment (chapter I.13). Of course, this is not to say that organisms are optimally adapted to their current conditions, nor that every feature exhibited by a population represents an adaptation for some aspect of the environment. Quite the contrary, natural selection is only one of many processes that affect how populations evolve (chapters I.14 and I.15); in some circumstances, processes other than natural selection will predominate, leading populations to be less well adapted to their current circumstances.
Ecologists are increasingly interested in the evolutionary time scale. On one hand, it has become clear that, in many cases, we can understand the current state of species and of entire communities only by considering their history. Species are not blank slates, to be molded by selection to the optimum configuration for their environment; rather, they have a historical starting point, and selection can work to modify species only from this point (chapter I.13). Similarly, communities, too, have histories—the current state of a community is a result of which species have managed to get to a given locality and how those species interact once there. Methods to incorporate evolutionary information, in the form of phylogenies (or evolutionary trees), are now widely utilized and becoming increasingly sophisticated (chapter I.16). Conversely, evolutionary biologists have clearly demonstrated over the last several decades that evolutionary change can occur very rapidly (chapter I.17). Consequently, ecologists ignore evolution at their own peril—populations can adapt quickly enough that evolution can have effects even on ecological time scales.
Evolution is important in another respect. The components of ecological interactions are species. The study of speciation—how new species arise—has long been the province of evolutionary biologists, but in recent years it has become clear that ecology may play an important role in affecting rates of speciation. In particular, the concept of ecological speciation—the idea that speciation is intimately tied to ecological divergence—has gathered great support (chapter I.18). Hence, in this respect as well, ecological and evolutionary perspectives are strongly intertwined. Finally, over larger time scales, certain groups of organisms diversify greatly, producing not only a large number of species but also occupying a great variety of ecological niches. Some scientists consider this phenomenon, known as adaptive radiation, to be responsible for the majority of life’s diversity (chapter I.19).