Spatial and temporal heterogeneity in habitat or patch suitability and quality can be due to many different factors linked to abiotic, biotic, and social characteristics.
The physical characteristics of the environment that can affect fitness vary from climatic conditions (rain, wind, and temperature regimes), soil nature (e.g., for species that dig burrows), stability of the substratum (e.g., when breeding on a slope or in a tree), level of salinity (for marine species), etc., depending on the species considered. Biotic sources of environmental variation include the availability of biotic resources (e.g., food and nest-building materials), which may be required in sufficient quantity as well as quality (e.g., required nutrients may only be available in specific food items); predators and parasites are often spatially heterogeneous biotic factors.
Finally, social components can play a major role. The density of conspecifics or heterospecifics exploiting the same resources can vary drastically in time and space, and competitors' presence may reduce the fitness of a given individual directly (e.g., when resources are limited) or indirectly (e.g., via the attraction of common predators). Because conspecifics also have to select and secure resources, their distribution among habitats and patches will affect the relative quality of potentially available resources. Conspecific decisions affect fitness gains expected by individuals choosing a particular habitat or patch (i.e., a frequency-dependent process). Conversely, conspecifics' or heterospecifics' presence can also have positive fitness effects, for example, when individuals interact with each other to capture preys, deter predators, build nests, etc., so that when conspecific density decreases below a certain value, individuals' fitness decreases (Allee effect). Conspecifics' presence can also be beneficial by providing information about habitat or patch quality. Both conspecifics' quantity and quality may vary and thus affect breeding habitat quality. In particular, the relatedness between individuals can affect local habitat quality, through kin competition or cooperation.
In many cases, individuals will require several different critical resources simultaneously. All the fundamental ecological requirements for a given activity thus have to be accounted for. For instance, a breeding patch may provide large amounts of food, but lack breeding sites, and thus will not be used. The different factors affecting fitness are also likely to interact with each other. Furthermore, spatiotemporal variations of important factors likely show different patterns at different scales, generating tradeoffs between factors, since the values of the different factors that maximize individual fitness may not occur in the same locations at the same time. These tradeoffs may themselves differ in time and space.
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