INTRODUCTIONNative ants play important roles in forested ecosystems by translocating and aeriating soil and contributing to litter decomposition and fragmentation. Since its invasion in the 1930's, the red imported fire ant (Solenopsis invicta Buren) has displaced many native species and, consequently, has reduced native biodiversity, thereby altering natural ecosystems. While many studies have been conducted on fire ant populations within different habitat patch types, none have focused on the interface between habitats. As fragmentation of natural systems increases, these interfaces, or ecotones, will become increasingly important. This study focuses on the distribution and interaction of fire ants and native ants along seven ecotones in a post oak savanna.
The goal of this study is to investigate the interaction of native ants and S. invicta along ecotones of a post oak savanna landscape. The richness and abundance of native ants, and their correlation with S. invicta abundance will be compared between different ecotone types.
The objectives of this project are:
Spatial Database DevelopmentMr. Joe Pase (Texas Forest Service) flew the aerial photography mission of Sawdust Ranch in February, 1998. Twelve 9 X 9 in. color infrared (CIR) aerial photographs were acquired at 3,600 ft AGL. The CIR contact prints were created from positive transparencies and scanned at 600 dpi. The scanned images were georeferenced to the Snook DOQQ and resampled to 1m resolution. Images were histogram matched and mosaicked. Intergraph Image Analyst was used for all image processing. The blue X's on the image below represent the locations where aerial photographs were taken.
Landscape ClassificationA patch-corridor-matrix model was used to classify the landscape. In this classification system, seven patch types, a post oak woodland matrix, and a road system corridor were identified. The mosaic of habitat types represented an ideal site for studying habitat interfaces. The ecological classification coverage of Sawdust Ranch is shown below.
Location of each pitfall transect was recorded using a Trimble Pathfinder II GPS unit. The different colored dots on the map below represent transects across different ecotone types.
RESULTSPreliminary results indicate a statistically significant difference in S. invicta abundance between ecotone types. The wood-cultivated field ecotone had a significantly greater abundance of S. invicta than all other ecotones for the Fall 1998 sampling season. The grass-pond ecotone was significantly greater in S. invicta abundance than all but the cleared wood-grass ecotone for the Early Summer 1999 sampling season. When the data were averaged across sampling seasons, the grass-pond ecotone was significantly greater in S. invicta abundance than all but the wood-cultivated field ecotone. A difference in fire ant abundance between sampling seasons exists, but cannot yet be correlated to temporal or seasonal factors.
Eleven genera of native ants were found within the 7 ecotones studied: Camponotus, Crematogaster, Cyphomyrmex, S. (Diplorhoptrum), Dorymyrmex, Forelius, Hypoponera, Monomorium, Paratrechina, Pheidole, and Myrmecina. The most prevalent genera were Forelius, Hypoponera, and Pheidole. Solenopsis invicta abundance was significantly greater than native ant abundance for 6 of the 7 ecotones. Both the Fall 1998 and Early Summer 1999 sampling seasons showed similarities between the abundances of S. invicta and at least one of the native ant genera across the cleared wood-pond ecotone. The chart below displays the number of native ants and fire ants within each ecotone type, averaged over the two sampling seasons.
CONCLUSIONSPreliminary analysis indicates a significant difference in S. invicta abundance across the seven ecotones. Grass-pond and wood-cultivated field ecotones had the greatest abundance of S. invicta. The abundance between ecotones also varied between the different sampling periods. Whether this difference is due to seasonal, temporal, or other changes will be analyzed in the future. Solenopsis invicta abundance was significantly greater than native ant abundance across 6 of the 7 ecotones. Wood-cultivated field is the only ecotone where S. invicta abundance is not significantly greater than all the other native ants found. Differences in ant generic diversity and abundance within each ecotone occurred between the two sampling seasons but has not yet been analyzed.
Future analyses will evaluate:
-native ant diversity and abundance within each ecotone
-distribution of native ants across the ecotones
-effects of S. invicta abundance on native ant diversity
-differences in ecotone structure that may attribute to greater native ant species richness
-differences in ecotone structure that may attribute to greater S. invicta abundance
-variability of data between sampling periods
ACKNOWLEDGEMENTSKent Moore - Owner of Sawdust Ranch
Dr. Douglas Wunneburger - GIS Technical Assistance
Texas Forest Service - Aerial Photography
U. S. Forest Service - Aerial Photography
Dr. Ben Wu - Geostatistician