Habitat Selection

by

Douglas Morris

• LAST REVIEWED: 22 September 2021
• LAST MODIFIED: 22 September 2021
• DOI: 10.1093/obo/9780199830060-0082

Introduction

All organisms use habitat that varies across numerous scales of space and time. Consequently, the use of some habitats over others, and the rules that individuals use to make those choices, is a dominant theme in fields ranging from behavioral ecology to evolutionary biology. No standard definition exists for either habitat or habitat selection. To the extent that clear thinking emerges from clear definitions, ecologists should rally around the principle that habitats are defined by the organisms using them. For example, habitats could be distinguished whenever a species’ vital rates at low density differ from one area to another or when the relationship between a species’ density and its fitness in those areas differs. Similarly, effects of space and time might be addressed through a hierarchy of movement behaviors such as foraging (patch), dispersal (habitat), and migration (landscape). We also need to define selection, which is typically used in two fundamentally different ways. To some, selection represents higher use of one or more habitats than expected based on their availability (also called course-grained habitat use). But for many others, selection equates with active and adaptive choice of habitat based on cues that reliably inform individuals of the fitness costs and benefits associated with movement from one area to another. When reading the massive literature on habitat selection, one must know which definitions are being used, what scales are being examined, and, most importantly, what the study aims to test, document, or apply. Much of the theory assumes that organisms should choose habitats in a way that maximizes fitness and that decisions emerge from a struggle for existence (fitness declines with density). Viewed in this light, habitat selection is an evolutionary game that serves as a mechanism for regulating populations in space, including source-sink dynamics, and a process that underlies the distributions and relative abundances of species. Nevertheless, the study of habitat selection is complicated by issues such as distinguishing habitats, determining the best ways to measure a habitat’s quality, evaluating the relative role of active choice as a mechanism causing spatial variation in abundance versus that caused by intrinsic differences in dispersal and population growth, determining cues that organisms use to choose habitats, and assessing how spatial scale and patterns in the landscape limit habitat use.

Foundational Papers

Although naturalists have long known that animals tend to expand their use of habitat with increasing density, no formal theory existed until the middle of the 20th century. Svärdson 1949 is usually credited with the first serious mention that intraspecific competition causes species to expand their habitat use whereas interspecific competition reduces the range of occupied habitats (even though Masaaki Morisita used clever experiments on ant lion habitat preference to develop his first models of density dependence at about the same time; described in Morisita 1969). MacArthur’s landmark contribution (MacArthur 1958) is the first to document density-dependent population regulation associated with the availability of habitat. It spawned a cottage industry on microhabitat separation and species coexistence. Most contemporary theory traces its roots to the ideas in Fretwell and Lucas 1969 (often cited incorrectly as 1970) on ideal habitat selection, which remains a focus of current research. Southwood 1977 is the first to link habitat to life-history. Michael L. Rosenzweig invented and refined his ideas on isolegs (functions identifying densities such that the choice of habitat is equal in the presence of a competing species) during the same period. Rosenzweig 1981 is, perhaps, the clearest exposition on how Rosenzweig viewed habitat selection and why it leads to isolation of competing species in separate habitats (the ghost of competition past). Morris 1988 expounds on the related invention of habitat isodars (functions identifying densities such that the expectation of fitness is equal among occupied habitats), first alluded to in a 1987 publication, and provides explicit definitions for several different ways that habitat regulates populations. Rodenhouse, et al. 1997 concludes that population regulation can occur independently of density if individuals occupy sites of declining quality. The assumption of site preemption in Rodenhouse, et al. 1997 emerged from work by two crucial predecessors (see Shmida and Ellner 1984 and Pulliam 1988, both cited under Source-Sink Dynamics). Readers wishing to trace the history of habitat selection might also wish to consult references in Chalfoun and Schmidt 2012 (cited under Reviews).

• Fretwell, Stephen D., and Henry L. Lucas. 1969. On territorial behavior and other factors influencing habitat distribution in birds. Acta Biotheoretica 19:16–36.

  DOI: 10.1007/BF01601953

  The most famous and influential of three papers on habitat distribution in birds. Developed the ideal-free and ideal-despotic distributions and implicitly defined habitat by the species using it. Perhaps paradoxically the ideal-free distribution, which dominates subsequent literature, was designed as a “reference” for discussing territorial behavior.

• MacArthur, Robert H. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39.4: 599–619.

  DOI: 10.2307/1931600

  Generally regarded as the first quantitative paper demonstrating how differences in microhabitat use promote species coexistence. The paper achieved much of its success by clearly defining a stable equilibrium, documenting density-dependence, contrasting patterns from different areas and at different times, and considering a variety of alternative coexistence mechanisms.

• Morisita, Masaaki. 1969. Measuring habitat value by the “environmental density” method. In Statistical ecology. Edited by Ganapati Patil, Evelyn C. Pielou, and William E. Waters, 379–401. University Park: Pennsylvania State Univ. Press.

  This paper, founded on Morisita’s studies of ant lions two decades earlier, solves the expected density of individuals living at different population sizes in habitats of different qualities. Supplanted by ideal habitat distributions in Fretwell and Lucas 1969, and isodars in Morris 1988, it is, nevertheless, an informative treatment of density-dependent habitat selection. Published in Japanese in 1950 and 1952.

• Morris, Douglas W. 1988. Habitat-dependent population regulation and community structure. Evolutionary Ecology 2.2: 253–269.

  DOI: 10.1007/BF02214286

  The usual reference for habitat isodars, lines in the state space of species densities that represent the solution for ideal habitat selection. The paper explicitly links the characteristics of isodars to quantitative and qualitative differences in habitat and, subsequently, to different forms of population regulation and community structure.

• Rodenhouse, Nicholas L., Thomas W. Sherry, and Richard T. Holmes. 1997. Site-dependent regulation of population size: A new synthesis. Ecology 78.7: 2025–2042.

  DOI: 10.1890/0012-9658(1997)078[2025:SDROPS]2.0.CO;2

  A controversial synthesis proposing that populations can be regulated independently of density by sequential occupation of breeding sites of ever lowering quality. See Ecology 81.4 (2000): 1162–1171 for a provocative exchange among the protagonists.

• Rosenzweig, Michael L. 1981. A theory of habitat selection. Ecology 62.2: 327–335.

  DOI: 10.2307/1936707

  A clear statement of habitat specialists versus generalists, and the consequences of habitat selection, emerging from earlier publications by the same author. A benchmark on the relationships between habitat selection revealed by isolegs and competitive coexistence inferred by isoclines. Uniquely demonstrates how the ghost of competition emerges from habitat selection.

• Southwood, Thomas R. E. 1977. Habitat, the templet for ecological strategies? Journal of Animal Ecology 46.2: 337–355.

  DOI: 10.2307/3817

  Presidential address to the British Ecological Society. Assesses how variation in space and time can organize understanding of species’ strategies and community properties. Attempts to produce a “periodic table” of species’ strategies by combining spatial variation among habitats in time with the variation occurring within single habitats through time.

• Svärdson, Gunnar. 1949. Competition and habitat selection in birds. Oikos 1:157–174.

  DOI: 10.2307/3564710

  The first paper on habitat selection to propose an explicit graphical model for density-dependence and interspecific competition. The paper documents older research on habitat selection, makes explicit reference to adaptive peaks and the evolution of habitat selection, and alludes to habitat expansion through occupation of former sink habitats.