|
METAPOPULATION MODELS AND MOSAIC EXTINCTION
The Preferred Habitat of some species occurs in patches. A population of a species is referred to as a Metapopulation if these patches (sub-population, local population, population) are separated by distances which restrict the movement of individuals between them; 1) to the extent of limiting repopulation in the event of extinction of a sub-population. Animals with poor dispersal abilities - relative to the distance between the patches of habitat - are particularly prone to form metapopulations. Terrestrial habitats that typically support metapopulations include isolated wetlands (frogs, fish, water snails); island clusters (tortoises, snakes, fresh water fish); cool forested highlands (snails, arboreal mammals, cool water fish, frogs). Mosaic Extinction can occur where sub-populations are wiped out by chance events, such as frogs in some ponds during a drought, and the ponds are not re-colonised quickly enough, for instance because of their distance or a wall being built between them. The next drought will wipe out even more ponds, and the chance of recolonisation is even less as there are fewer ponds. Under these conditions in a short time the metapopulation will become extinct. In marine environments metapopulations can still occur, even though currents and the continuous aquatic environment facilitate dispersal. Examples are the semi-isolated populations of sub-tropical pipefish in southern Australia (Tiger Pipefish), and inshore pipefish isolated in estuaries by deep water or current direction (Spotted and Wide-bodied pipefish complexes). Habitat reduction and increased fragmentation A decrease in a species total area of habitat will also increase the relative length of edges to area, increase edge proximity, and may result in a greater degree of fragmentation. The habitat could then be in separate (disjunct) patches rather than joined (contiguous). With habitat reduction and the creation of separate isolated patches a metapopulation could be formed. An increase in habitat edge to area can reduce the apparent area of habitat for a species. The population may only inhabit the interior of patches for a certain distance from the edge due to factors including; changed light, temperature, humidity, and increased predation or competition. Spatial Structure and Geography Metapopulations live in patches that are spatially and geographically distributed. The spatial distribution of patches will influence the rates and direction of dispersal (immigration and emigration). The geographical distribution of patches will influence the Essential Habitat components, between patches. For some species, such as migratory birds or fish, Essential Habitats can be very widely dispersed. Both the spatial and geographical distribution of habitat patches must be considered in Population Viability Models. Characteristics of Habitat Patches Metapopulation exist in dispersed patches of Essential Habitat and Preferred Habitat. These habitats can be clearly observed such as isolated ponds rarely joined by flooding for fish. In other cases simple observation may not reveal the patchiness of a landscape for a species. This is particularly the case where many difficult to measure factors interplay.
|
|
|