Effects of Natural Fragmentation on Native Forest Structure, Invasive Rat Distributions, and Stable Isotope Signatures after 150 years.

摘要

Empirical studies of forest fragmentation have predominantly focused on short-term effects, providing little of practical value to long-term conservation planning. The kipuka field system of Mauna Loa Volcano, Hawaii provides a unique model system to study long term effects, having more than 140 forest fragments, or kipuka, isolated by lava over 150 years ago. The variables tree biomass, tree species richness, rodent distributions, and stable isotope signatures of rats, were compared with the predictors area and distance from a major highway in 30 kipuka ranging from 0.01 to 53.94 hectares. Edge effects on the tree community were also examined. Tracking tunnels were used to map rodent presence. Rats were captured in several small and large kipuka, in order to compare diets using nitrogen and carbon stable isotope signatures. Area was a significant predictor of tree species richness and structure, but not presence of rats, in the kipuka. Kipuka had distinct edge habitats approximately 30 meters wide, defined by stem density, rather than species composition. Rats were found more often inside kipuka than in the matrix, and this effect was more pronounced around larger kipuka. Rats were more often found near the highway, and on the older of two lava flows. Rats from large kipuka were more enriched in 15N and depleted in 13C than rats from small kipuka, however, baseline isotope values were not established. The difference in isotope signatures may reflect a dietary difference, but appears more likely to reflect differences in baseline nutrient cycling in this system. Large kipuka demonstrated excellent conservation potential, having greater tree biomass and species diversity, but no greater likelihood of hosting rats than small kipuka. Invasive rodent control measures in the landscape could feasibly be limited to the kipuka , reducing the overall cost and effort to protect native resources in this landscape.