HABITAT AND NICHE, CONCEPT OF Kenneth Petren University of Cincinnati I. Contrasting the Habitat with the Niche niche overlap The proportion of available resources II. History of the Habitat and the Niche that are shared by two species. Usually used in the III. Comparing Habitats and Niches context of a single resource that limits population IV. Evolution of the Niche growth. V. The Habitat and Biodiversity VI. Conclusions and New Frontiers WHY ARE SPECIES COMMON in some places but rare GLOSSARY or absent in other places? Early attempts to answer this fundamental question led to the concept of the habitat. community An ecological term referring to a set of A habitat is simply where an organism can be found in species that occur in the same location that have the nature. Habitats are described in terms of geography, potential to affect each other either directly or indi- geology, climate, as well as by other species commonly rectly. found within the same habitat. The habitat concept is community structure The web of potential biological very closely related to another concept used to charac- interactions among members of a community that terize species, the niche. The niche of an organism may be characterized in terms of diversity, complex- can best be described as its role in the community of ity, hierarchy, and stability. organisms around it. Among the many traits that may ecotone A zone of transition between two different comprise a niche are the physical and climatic charac- habitats that may contain a community of organisms teristics that an organism can tolerate, and the resources distinct from either habitat. required for the species to persist, such as food and habitat structure Analogous to community structure, shelter. The habitat of a species is often treated as a but limited to the physical structural aspects of a subset of its niche, and in practice it has grown difficult habitat. The structure of habitats may be character- to draw a distinct line between the two concepts, yet ized by such measures as complexity, heterogeneity, the study of habitats has a history distinct from that of regularity, stratification, and fractal dimensionality. the niche. Today, habitat descriptions are commonly microhabitat Locations within a habitat where organ- used as a practical guide for locating and maintaining isms may carry out important aspects of their lives, species, whereas the niche is a more abstract concept such as places for harvesting food, nesting, or tak- that forms the conceptual foundation for much ecologi- ing shelter. cal and evolutionary theory. Encyclopedia of Biodiversity, Volume 3 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 303 304 HABITAT AND NICHE, CONCEPT OF I. CONTRASTING THE HABITAT WITH the species is referred to as the habitat breadth of the species. Figure 1 shows how two species of plankton THE NICHE can be compared according to a single habitat dimen- sion, depth in the water column. There are potentially As is the case with many ecological concepts, defining limitless gradients or factors that may be involved in habitats and niches concisely and unambiguously is characterizing the habitat of a species. Graphical repre- difficult. Habitat definitions are particularly prone to sentation of more than three variables in a single graph problems of scale. At the larger end of the scale is is difficult, but the concept easily extends into multi- the biogeographical term biome. Some biomes may be ple dimensions. considered the habitat of larger, well-traveled species The niche is a more abstract term than the habitat. such as large birds of prey, but usually the habitat is It encompasses all possible interactions that a species defined more narrowly. On the smaller end of the scale, has with the environment and other species in the com- the term microhabitat is used to describe the places munity. Conceptually, the niche is richer than the habi- where an organism spends part of its time. For instance, tat and forms the foundation for much ecological the- a fish may forage in the microhabitat that occurs near ory. Because of the abstract nature of the concept, the the banks of a river. Thus the concept of the habitat lies definition is perhaps even more difficult to formulate somewhere in between the biome and the microhabitat. for all species. There are striking similarities in the way An ecological community is a suite of species that niches and habitats are displayed. Like a habitat, a niche occur in the same location that at least have the poten- is often defined by axes, and there are multiple axes tial to interact and affect each another either directly that can be compared. or indirectly. These effects are usually measured in The most common type of axes used to characterize terms of population growth or changes in the density a niche include those that define environmental toler- of individuals. Different communities of organisms will ances and resource requirements. Humidity, sunlight, generally reside in different habitats. If most species temperature, wind exposure, and pH are examples of in one community have little chance of affecting the environmental axes that help to define a niche. Resource populations of other species in a different community, axes may include food resources (insects, seeds, bacte- then these communities probably occur in different ria, nutrients), space requirements (breeding sites, ref- habitats. In this way, microhabitats refer to places within a community, and biomes encompass a number of different habitat types. Definition of habitat and com- munity boundaries will depend on the exact species under consideration. For instance, it may be reasonable to distinguish the small mammal communities of grass- land from nearby riparian forest, though an individual jaguar may prey on species in both habitats. The habitat concept was originally applied to single species, but because many species share similar habitats, some general habitat descriptions are applied to many species. Habitat descriptions used in this fashion often incorporate the dominant species commonly found in that habitat. In nature, borders between different kinds of habitats are often not very distinct. Generally, one habitat type will gradually give way to another creating a zone of transition. Communities in the transition zone may be qualitatively different from communities that occur within one habitat type or another. The primary habitat preferences of a species can be displayed graphically. Ordination is the process of characterizing a species with regard to habitat gradients. FIGURE 1 Two hypothetical species of plankton compared along the habitat axis of depth in the water column (top panel). This single Gradients can be displayed as axes on a graph. Usually, habitat axis is correlated with two different niche axes, temperature some measure of density or population growth is plot- and illumination. Species B is found in shallower, brighter, and ted, and the portion of the gradient that is occupied by warmer areas. HABITAT AND NICHE, CONCEPT OF 305 uges, foraging zones). Figure 1 also compares the two of host. These differences make broad comparisons of plankton species in two niche dimensions, illumination niches and habitats difficult, but at any single scale, and temperature, which are correlated with the habitat these concepts can be applied to facilitate meaningful axis, depth. This highlights the similarity of habitat and comparisons among similar species. niche axes, which are often difficult to distinguish clearly. In practice, ecologists focus on a subset of niche II. HISTORY OF THE HABITAT AND dimensions that are the most important in determining the role of a species in the community. Among these THE NICHE are food resources that are essential for survival. These factors limit the population size of the species and there- A. Niche Origins fore determine the distribution and abundance of the It is tempting to think of a niche as a physical place. population. This is the common usage of the word, and there are In spite of the more limited role of the habitat in examples in the early ecological literature that use the theory, the physical nature of habitat axes make them term niche in the purely physical sense. However the useful tools for practical purposes. Habitats can often origins of the ecological niche reside in the more general be adequately described with relatively few variables, observation that no two species are exactly alike. whereas to fully document the niche of a species re- Many naturalists in the latter part of the 19th century quires careful study and a great deal of time. Yet if the turned their attention toward documenting the traits habitat can be accurately captured, it is likely that many that distinguish one species from another. When two the complex niche requirements and interconnections species appeared very similar, it was thought that even- will be contained within the same habitat. As stated tually differences could be found that would distinguish earlier, communities tend to map one to one onto habi- the unique role of each in the community. This idea tats. The utility of this relationship is exploited for was evident even in the writings of Darwin, and over management when there is insufficient time to study time it has evolved into a very general principle: the all aspects of an organism’s ecology before an action principle of competitive exclusion. Exploring the impli- needs to be taken. To preserve a single species, one must cations of this relatively simple notion has dominated also preserve its niche requirements and the complex the study of ecology for much of the 20th century. interconnections with the rest of the community. Pre- The principle of competitive exclusion and the con- serving adequate habitat will often achieve this goal. cept of the niche developed in parallel through the early This principle is important not only for managing target 1900s.