Effects of habitat loss, habitat fragmentation, and isolation on the density, species richness, and distribution of ladybeetles in manipulated alfalfa landscapes

Abstract

1. Habitat loss and fragmentation are the main causes of changes in the distribution and abundance of organisms, and are usually considered to negatively affect the abundance and species richness of organisms in a landscape. Nevertheless, habitat loss and fragmentation have often been confused, and the reported negative effects may only be the result of habitat loss alone, with habitat fragmentation having nil or even positive effects on abundance and species richness.

2. Manipulated alfalfa micro-landscapes and coccinellids (Coleoptera: Coccinellidae) are used to test the effects habitat loss (0% or 84%), fragmentation (4 or 16 fragments), and isolation (2 or 6 m between fragments) on the density, species richness, and distribution of native and exotic species of coccinellids.

3. Generally, when considering only the individuals in the remaining fragments, habitat loss had variable effects while habitat fragmentation had a positive effect on the density of two species of coccinellids and on species richness, but did not affect two other species. Isolation usually had no effect. When individuals in the whole landscape were considered, negative effects of habitat loss became apparent for most species, but the positive effects of fragmentation remained only for one species.

4. Native and exotic species of coccinellids did not segregate in the different landscapes, and strong positive associations were found most often in landscapes with higher fragmentation and isolation.

5. The opposing effects of habitat loss and fragmentation may result in a nil global effect; therefore it is important to separate their effects when studying populations in fragmented landscapes.