Habitat Selection

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.