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item Polley, Wayne

Submitted to: Conservation Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2005
Publication Date: 1/27/2005
Citation: Wilsey, B.J., Martin, L.M., Polley, H.W. 2005. Predicting plant extinction based on species-area curves in prairie fragments with high beta richness. Conservation Biology. 1835-1841.

Interpretive Summary: The number of plant species present in grasslands increases as the area of grassland that is examined increases. These simple species-area relationships often are used to predict species loss when grasslands are converted to agriculture or grassland area is otherwise reduced (grassland fragmentation). An important assumption of this approach is that plant populations were randomly distributed throughout the entire grassland before fragmentation. We tested this assumption using species counts from surveys of tallgrass prairie remnants in Iowa. More native plant species were present in prairie fragments than predicted from a species-area relationship developed from historical surveys of intact prairie in Iowa. More species are present in remnant prairies than predicted because the species composition of remnants differ markedly. Remnants differ in species apparently because of differences in soil type. These results indicate that plant species that are native to prairies may best be conserved by protecting several, relatively-small prairie areas that differ in soils or climate rather than by protecting a few large tracts of this grassland type.

Technical Abstract: Species-area relationships and Island Biogeography Theory are commonly used to predict how species richness will decline with fragmentation. There are a variety of largely untested assumptions in these approaches, including the assumptions that populations are distributed uniformly before fragmentation and that local extinctions are due to effects of small population sizes. If populations are not distributed uniformly, then populations can be very abundant locally even while being rare globally. This would cause extinction rates to be smaller than predicted. We tested theory by developing estimates of the number of plant species that should be present in small tallgrass prairie fragments and then tested the uniformity assumption by partitioning species richness into alpha (within site) and beta (among site) components in Iowa prairies and Texas Blackland Prairies. Many more native plant species were present in surveys of prairie fragments (491) than was predicted from theory (27 - 207, depending on the value of the slope of the species-area relationship that was used). A large proportion of the total species richness occurred as among site or beta richness in both Iowa and Texas Blackland Prairie remnants. We suggest that the high proportion of beta richness was responsible for the shallow species-area slope that was observed and lower than expected number of species losses. A better understanding of what determines beta diversity thus may be required to accurately predict effects of fragmentation on the richness of plants. We also suggest that plants in prairie remnants may best be conserved by protecting different prairie types rather than by protecting a few large areas of a single prairie type.