Partitioning the effects of habitat fragmentation on rodent species richness in Southern California

Habitat fragmentation plays a major role in species extinction and the loss of biodiversity around the globe. When fragmentation occurs, the initial loss of overall habitat alone causes species extirpation. However, species that survive this initial loss and persist in the remaining small fragments of habitat continue to experience changes that may lead to their eventual extirpation. Previous research has determined that species richness in habitat fragments is affected by a number of characteristics. These include fragment age, size, and isolation, edge effects, vegetation coverage, habitat heterogeneity, and matrix content. Although most studies focused on one or a few of these characteristics, multiple characteristics work together to affect species richness, showing that the effects of habitat fragmentation are complex. The goal of my study was to partition the complex effects of habitat fragmentation by determining the direct, indirect, and cumulative effects of multiple habitat fragment characteristics on rodent species richness. In 2013, I determined rodent species richness in 25 habitat fragments within a suburban landscape of Thousand Oaks, California. In addition, I measured the following characteristics for each fragment: fragment age, area, isolation, shrub coverage, habitat heterogeneity, perimeter/area ratio, and percent non-urban buffer. Path Analysis was used to test the hypothesized model which described the direct, indirect, and cumulative effect of each habitat fragment characteristic on rodent species richness. Habitat heterogeneity had the greatest direct and total effect on rodent species richness. In addition, fragment area had the greatest indirect effect on rodent species richness through its influence on habitat heterogeneity, suggesting that large fragments containing the greatest diversity of habitats will support the most species. Overall, the path model explained 67% of the variation in rodent species richness among habitat fragments. From a conservation and management standpoint my findings suggest that the most important fragments to protect are the largest and most habitat diverse.