PARENT SESSION
Poster Session 7: Grasslands
Tuesday, August 9, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal
Promising facilitation of grassland recovery by harvester ants determined by modified Ripley's K spatial analysis.
Nicolai, Nancy*,1, Smeins, Fred1, Feagin, Rusty1, 1 Texas A&M University, College Station, Texas
ABSTRACT- Long-term studies on semiarid grasslands of the Edwards Plateau, Texas show that large-scale vegetation dynamics is driven by large mammalian herbivory, fire and drought. Extended drought causes high mortality in grass species populations and their replacement is mainly by seed dispersal from survivors. Smaller-scale patch dynamics are significantly influenced by Pogonomyrmex barbatus (red harvester ant) because their nests act as drought refugia for grasses owing to a high survivorship adjacent to nests compared to surrounding areas. It was hypothesized that vegetation at these refugia would be the seed source for surrounding grasslands after drought. A spatial pattern should result with more grass seedlings associated with ant nests than if seedlings were distributed randomly, that is from other seed sources. Spatial patterns of grass seedlings were studied after a severe four-year drought ended in July 2002, by collecting seedling and P. barbatus nest numbers along two, 1,130 m line transects. The distribution patterns between harvester ant nests and grass seedlings were evaluated by spatial statistical analysis based on Ripleys K-function. Grass seedlings from 50% of the species were significantly clumped near ant nests. Seedling establishment from grass adjacent to nests occurred up to 25 m away. This spatial pattern was repeated along both transects which implies that after drought a prevailing dynamic exists of an aggregation between nests and grass seedlings; individuals surviving drought at refugia by nests may become the seed source for seedlings in immediately adjacent grasslands.
Key words: refugia, disturbance, spatial analysis, Pogonomyrmex barbatus