Community Composition o Carabid Beetles in the Spruce-fir Forest "Sky-Islands" of the Southeastern Appalachian Mountains

Ortiz, Maria Del Carmen

Understanding the ecological and environmental factors that influence community structure is a fundamental goal of ecology. This thesis presents a study investigating the communities of carabid beetles inhabiting spruce-fir “sky-island” forests in the southern Appalachians, USA. I compare communities between islands and test different island biogeography and assembly rule models. Southeastern spruce/fir forests (dominated by Red spruce, Picea rubens; and Fraser fir, Abies fraseri) are a rare relict habitat type left over from the last glacial maximum of the Pleistocene that now only occur on high elevation sites. Consequently, these evergreen forests are confined to mountaintops and constitute sky-islands surrounded by a matrix of hardwood forests. These islands support several unique species of organisms (including carabid beetles), many of which are endemic to spruce-fir forests. Ground beetles of the family Carabidae are abundant and diverse within the spruce-fir islands. In addition, they are relatively easy to capture and well known taxonomically, allowing for quick and accurate population censuses. All of the carabid beetles included in this study are flightless and are generally incapable of dispersing between mountaintops, possibly contributing to high rates of endemism and species turnover between islands. In total, we collected over 2000 beetles over two years of collections in 2003-2004 from nine different spruce-fir forests located throughout the southern Appalachian Mountains, we also include tree non spruce-fir lower elevation forest sites. I measured and identified all of the collected beetles to the species level: 44 species were identified representing 18 different genera and 11 tribes. Using diversity and community composition data generated from these censuses, I tested basic island biogeography principles such as the relationship between diversity (estimated using several different indices) and area, isolation and elevation. No significant relationships were found between any of these variables and diversity when all beetles were included. However, when I looked just within the tribe Cychrini (Boat beetles), I found a significant increase in diversity and relative abundance at high elevations. I hypothesize that this is due to the boat beetles’ specialized habitat and feeding preferences (boat beetles are specialized predators of snails and slugs) which limit them to cool moist forests. In addition to analyzing patterns of diversity between communities, I also investigated patterns of composition within communities using several prominent assembly rule models including Nestedness, Diamond’s Checkerboard theory, and Hutchinson’s Size Rule hypothesis. The carabid communities did not conform to either the Checkerboard or Size Rule hypotheses but they did exhibit significantly nested patterns especially when the analyses were restricted to just Cychrini or just Pterostichini tribes. Nestedness is generally caused by selective extinction so these results indicate that the communities may still be undergoing relaxation or species loss following their isolation. This study increases our understanding of how ecological communities are structured and has important conservation implications given that spruce-fir forests are an endangered habitat type highly threatened by acid rain, introduced pests, human disturbance, and many other factors. These disturbances will have important consequences for the spruce-fir forests and for the carabid beetle community.