Small-scale experimental habitat fragmentation reduces colonization rates in species-rich grasslands

Abstract

Habitat fragmentation is one of the most important threats to biodiversity. Decreasing patch size may lead to a reduction in the size of populations and to an increased extinction risk of remnant populations. Furthermore, colonization rates may be reduced in isolated patches. To investigate the effects of isolation and patch size on extinction and colonization rates of plant species, calcareous grasslands at three sites in the Swiss Jura Mountains were experimentally fragmented into patches of 0.25, 2.25, and 20.25 m2 by frequent mowing of the surrounding area from 1993 to 1999. Species richness in the fragment plots and adjacent control plots of the same sizes was recorded during these 7 years. In agreement with the theory of island biogeography, colonization rate was reduced by 30% in fragments versus non-isolated controls, and extinction increased in small versus large plots. Habitat specialists, in contrast to generalists, were less likely to invade fragments. In the last 4 years of the experiment, extinction rates tended to be higher in fragment than in control plots at two of the three sites. Despite reduced colonization rates and a tendency of increased extinction rates in fragments, fragmented plots had only marginally fewer species than control plots after 7 years. Hence, rates were a more sensitive measure for community change than changes in species richness per se. From a conservation point of view, the detected reduced colonization rates are particularly problematic in small fragments, which are more likely to suffer from high extinction rates in the long run.

Factors and processes affecting plant biodiversity in permanent grasslands. A review

“…Due to changes in land use, including ploughing, abandonment, and drainage of heathland, the total area has strongly decreased and nowadays of many small patches that are more or less connected (Piessens et al 2004;Webb 1998). Habitat fragmentation reduces the area available for certain species and thus leads to shrinkage and isolation of remnant plant populations (Joshi et al 2006;Piessens et al 2004;Saunders et al 1991). Habitat fragmentation is not only habitat loss but also modification of habitat quality, reducing continuous available habitat and increasing edge effects (Burel and Baudry 1999).…”

Evaluating forest fragmentation and its tree community composition in the tropical rain forest of Southern Western Ghats (India) from 1973 to 2004

“…Decreased patch size might have led to a reduction in the size of populations and to an increased extinction risk to the remaining populations. Furthermore, colonization rates may be reduced in isolated patches (Soons and Heil 2002;Joshi et al 2006). Intuitively, forests fragmented by anthropogenic sources are at a higher risk of further fragmentation or removal than forests fragmented by natural causes.…”

Using tree population size structures to assess the impacts of cattle grazing and eucalypts plantations in subtropical South America

“…Several factors can contribute to this situation. Demographics of native tree species in small forest fragments can be altered in many ways (Joshi et al 2006). Large trees are vulnerable in forest fragments because of their tall structure and relatively thick, inflexible trunks and may be prone to uprooting and breakage near forest edges, where wind turbulence is increased (Laurance et al 2000).…”