Discontinuities in soil composition, plant cover, resources, and individuals are a common pattern in every ecosystem. These discontinuities change in extension and appearance through time.

So we can imagine that every individual perceives every unity of the perceived environment as hospitable or hostile. In Fig. 3.16 a model of hospitality versus hostility for an individual is represented. For instance, in many plants the perception of the neighboring area changes according to season, age, and community composition, consequently changing the model.

Fig. 3.16 A hypothetical A

increase of environmental hostility (from (A) fully B

hospitable to (D) fully hostile) of a matrix as C

perceived by a species. Discontinuity increases according to the hostility gradient

Discontinuities can be observed also at a scale that is larger than the individualbased scale. In this case discontinuities are transformed in patchiness. Moving from discontinuities perceived by individuals at a larger scale, such discontinuities are perceived as a mosaic of favorable or less favorable conditions. In this case the populations are interested in such a process.

In 1988, Pulliam presented a model on distribution of small birds across an environmental continuum (Pulliam 1988, 1996). He recognized that some populations that he called "source" had a positive balance between births and deaths and emigration exceeded immigration (Fig. 3.17). Under other conditions some populations had an opposite trend in which the number of births was not sufficient to balance the number of individual deaths and immigration exceeded emigration. He called this type of population a "sink."

In a population of source type, the surplus of individuals moves to neighboring spaces, including also less favorable habitats. The populations of sink type could go extinct in a short time if an active immigration of individuals from a source population does not balance the individuals that have perished. This continuous flux of individuals from a more favorable to a less favorable habitat is fundamental for the persistence of populations. The Pulliam model is really important to understanding the behavior of populations under different environmental constraints and opens up

Fig. 3.17 Source-sink model of a population in a patchy habitat

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