Fifty Years of Change in Forest Understory Composition. II. Is Biotic Homogenization Underway?

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Both the local abundance and regional distribution of species are temporally dynamic. Because of the direct and indirect effects of human activities, local biotas are undergoing homogenization as species tolerant of human disturbance replace those that are not. We tested the hypothesis that more species are becoming less common than are becoming more common at a local level and regionwide. In 1999, we surveyed all vascular understory plants in 1-m2 quadrats at each of 59 sites in northern Wisconsin. All of these sites were originally surveyed by John Curtis and his students 50 years ago. Our analysis is confined to species that were taxonomically unambiguous and present during both sampling periods (n = 93). We used percent frequency as our measure of local abundance and proportion of sites occupied as our measure of regional abundance. At the regional level, the majority of species increased, but the result was not significant (sign test, n = 93, P = 0.14). However, 71.0% of all species declined in their average local abundance, while only 29.0% increased (sign test, n = 93, P = 0.0001). Species fell into one of four categories: locally and regionally declining (31.6%), locally and regionally increasing (11.8%), locally declining, regionally increasing(46.0%), and locally increasing, regionally declining (10.6%). Because a greater proportion of species is declining than increasing locally and regionally, biotic homogenization underway in this forest understory plant community.


Presented at the 86th Ecological Society of America Annual Meeting, Monona Terrace, Madison, WI.

Oral Session #59: Conservation Ecology: Terrestrial Biodiversity.

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