to chance events. Habitat heterogeneity increases the number of species that will be successful after immigration. Over time, the countervailing forces of extinction and immigration result in an equilibrium level of species richness. Modifications In addition to having an effect on immigration rates, isolation can also affect extinction rates. Populations on islands that are less isolated are less likely to go extinct because individuals from the source population and other islands can immigrate and "rescue" the population from extinction; this is known as the rescue effect. In addition to having an effect on extinction, island size can also affect immigration rates. Species may actively target larger islands for their greater number of resources and available niches; or, larger islands may accumulate more species by chance just because they are larger. This is the target effect. Influencing factors Degree of isolation (distance to nearest neighbour, and mainland) Length of isolation (time) Size of island (larger area usually facilitates greater diversity) The habitat suitability which includes: Climate (tropical versus arctic, humid versus arid, variability, etc.) Initial plant and animal composition if previously attached to a larger land mass (e.g. marsupials, primates) The current species composition Location relative to ocean currents (influences nutrient, fish, bird, and seed flow patterns) Location relative to dust blow (influences nutrients) Serendipity (the impacts of chance arrivals) Human activity Species-area relationships Species–area relationships show the relationship between a given area and the species richness within that area. This concept comes from the theory of island biogeography, and is well illustrated on islands because they are relatively isolated. Thus, the immigrating species and the species going extinct from an island are more limited and therefore easier to keep track of. It is expected that as the area and species richness relationship are directly proportional to one another. For example, as the area of a series of islands increase, there is a direct relationship to the increasing species richness of primary producers. It is important to consider that island species area relationships will behave somewhat differently than mainland species area relationships, however the connections between the two can still prove to be useful. The species-area relationship equation is: . In this equation, represents the measure of diversity of a species (for example, the number of species) and is a constant representing the y-intercept. represents the area of the island or space that is being examined and represents the slope of