Effects of fragmentation on large herbivores under variable climates
Fragmentation of landscapes limits the movements of large herbivores, can reduce the forage they acquire, and can cause population declines. Fragmentation includes two components, habitat loss and habitat isolation. Effects of habitat loss on the capacity of areas to support herbivores are clear; reduced area reduces the number of animals supported. Habitat isolation describes changes in access to landscape patches without loss of area. In intact areas animals are able to move between forage patches, but in fragmented landscapes, movement may not be possible. Ephemeral forage production link questions of fragmentation to variable climates. Mobility allows animals to move to find sufficient forage in drought, but fragmentation reduces mobility. Climate change is forecasted in many semi-arid areas to increase the frequency and severity of drought. The ways in which fragmentation and climate variability influence stocking are linked with attributes of the species, spatial heterogeneity and productivity of landscapes, the severity of fragmentation, and the extremes in climate variability. We have used computer simulations to quantify effects of fragmentation under variable climate for large herbivores in Kenya, Tanzania, and South Africa, for wildebeest in Kenya, and in a stylized setting. Here we synthesis results from these analyses into generally applicable findings.
As expected, increased fragmentation reduced the mobility of large herbivores in all scenarios.. More frequent drought exacerbated stresses, but the severity of declines depended upon the intrinsic rate of increase of the herbivore and local variability. Essentially, non-equilibrium rangeland dynamics may best be thought of as a species-specific attribute that varies by patches, rather than an attribute of a rangeland. In Kitengela, Kenya, the movements made by migratory wildebeest no longer appear possible. Fragmentation at levels projected to occur in 2020 causes a 20% decline in the number of wildebeest supported. More frequent drought caused wildebeest numbers to decline, but they had sufficient time in the 10-year simulation to recover. Areas with low primary productivity may have animals at such low densities and moving at their maximum movement rate such that subdivision has little effect. Areas with high productivity allow animals to be supported on what may be small parcels. In general, areas of intermediate productivity appear most sensitive to fragmentation.