Are small, isolated prairie remnants effectively smaller than they look and getting smaller?

摘要

To evaluate the floristic sustainability of three small, isolated tallgrass prairie remnants, spatial and temporal patterns of species and functional group (FG) composition, richness, diversity, and dominance were examined to determine whether there was divergence between edge (outer 10 m) and interior zones. These remnants, all protected and managed nature preserves, are rectangular in shape and range in area from 1.29 ha to 1.77 ha; each is bordered by cropland and unmanaged or mowed ruderal grassland. A stratified sampling design was established using permanently marked transects oriented perpendicular to the long axis of each site. Vegetation data were collected in 50 cm X 50 cm quadrats along each transect; prairies were resampled 5-6 yr following baseline data collection. The null expectations were no differences between edge and interior zones, no differences between edge type (crop and noncrop), and no change over time. Results from linear mixed-effects models identified significant differences between edge and interior zones for native species density, native diversity, dominance, percent native cover, and measures of floristic quality; however, there were no time differences and no significant zone 3 time interactions. Distance from edge to peak levels of species density and diversity was about 15 m yielding core habitats 43% to 50% of remnant size. Beta diversity based on species presence was greater in edge zones compared to interior zones, and declined slightly in repeated samples. Functional group diversity was lower in edge zones compared to interior zones and declined significantly in the repeated samples. Functional groups with significant affiliation to the edge zone were nonnative grasses, woody vines, ferns/allies, and nonnative legumes; FGs affiliated with the interior zone were C-4 grasses, hemiparasites, nitrogen-fixing shrubs, and perennial forbs. One site was sampled three times following 5-6 yr intervals and there were no additional within-site differences in the third sample. These prairies support a rich diversity of native species; however, patterns suggest they are effectively smaller than they appear with a trend of declining functional diversity.