Table of Contents Executive Summary 2

Habitat Destruction, Alteration, and Fragmentation

Habitat destruction and habitat alteration are two distinct processes, but both have detrimental effects on amphibian populations. Additionally, they both contribute to habitat fragmentation, out of which arises a host of additional problems. Habitat destruction is the complete, and often permanent, loss of an ecosystem and its former biological function. This occurs when wetlands are drained or filled; forests are converted to parking lots, housing developments, agricultural fields, etc.; or when an ecosystem becomes so chemically contaminated that it is uninhabitable. On the other hand, habitat alteration occurs when an ecosystem function is changed, although not completely or permanently. An example of habitat alteration is when trees are cut in a forest as part of a forestry operation. Removal of the canopy will have an effect on the microhabitat and microclimate of the forest, perhaps reducing or eliminating amphibians, but the trees will grow back and suitable habitat will once again exist. The effects of habitat destruction and alteration become most pronounced when they occur on a sufficient scale that habitat fragmentation occurs (Dodd and Smith 2003).

As many amphibian populations exhibit metapopulation structure, fragmentation of both aquatic and terrestrial habitats may lead to numerous issues. As the number of suitable breeding sites decreases, the total number of individuals available to recolonize extirpated populations or form new populations is diminished. This may lead to a reduction in the number of source populations as well. The loss of aquatic habitat may also lead to isolation of populations and the subsequent loss of genetic diversity, as they may be too far apart to allow successful dispersal. Even more important than aquatic breeding sites may be intact terrestrial habitat, because this provides movement corridors that connect local populations. Because amphibians are prone to desiccation, they require moist microclimates provided by intact canopy cover, coarse woody debris, and leaf litter to successfully migrate between habitats. Barriers introduced by fragmentation, such as housing developments, roads, and agricultural fields, may restrict movement (Semlitsch 2003b).

The immediate effects of fragmentation on amphibian populations include the loss of the most vulnerable individuals, populations, and species. As the landscape becomes increasingly fragmented, the natural linkages between habitat patches are lost and barriers to movement are introduced. At this point, they may become isolated from habitats required to complete their life cycles, such as if aquatic breeding sites are cut off from terrestrial sites. There may be an increased risk of predation as edge effects are introduced and cover is reduced. Fragmentation may remain a filter to movement or may become an impenetrable barrier (Dodd and Smith 2003).

Long-term effects of fragmentation include the loss of genetic variability, decreased survivorship, isolation, increased edge effects, and greater susceptibility to stochastic events. Small, isolated populations confined to the tiny pieces of remaining habitat become susceptible to the accumulation of mutations and the loss of adaptive potential. Edge effects become more pronounced and may alter the physical structure and microclimate of the habitat. There is often the loss of migratory routes and dispersal corridors, compounding the problems of isolation. These long-term pressures on the populations, combined with loss of individuals to direct mortality, can become too much for the populations to overcome. They become more vulnerable to extirpation, and possibly extinction, due to random stochastic events, such as disease, drought, etc. (Dodd and Smith 2003).