Effects of mesoscale environmental heterogeneity and dispersal limitation on floristic variation in rain forest ferns

Authors

  • MIRKKA M. JONES,

    1. Department of Biology, University of Turku, 20014 Turku, Finland, Department of Biology, University of Missouri-St Louis, 8001 Natural Bridge Road, St Louis, Missouri 63121, USA, and
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  • HANNA TUOMISTO,

    1. Department of Biology, University of Turku, 20014 Turku, Finland, Department of Biology, University of Missouri-St Louis, 8001 Natural Bridge Road, St Louis, Missouri 63121, USA, and
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  • DAVID B. CLARK,

    1. Department of Biology, University of Turku, 20014 Turku, Finland, Department of Biology, University of Missouri-St Louis, 8001 Natural Bridge Road, St Louis, Missouri 63121, USA, and
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  • PAULO OLIVAS

    1. Department of Biological Sciences, Florida International University, Miami, Florida 33199 USA
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Mirkka Jones (fax +358 2333 5730; e-mail mirkka.jones@utu.fi).

Summary

  • 1Field studies to evaluate the roles of environmental variation and random dispersal in explaining variation in the floristic composition of rain forest plants at landscape to regional scales have yet to reach a consensus. Moreover, only one study has focused on scales below 10 km2, where the effects of dispersal limitation are expected to be easiest to observe.
  • 2In the present study, we estimate the importance of differences in some key environmental variables (describing canopy openness, soils and topography) relative to the geographical distances between sample plots as determinants of differences in pteridophyte (ferns and fern allies) species composition between plots within a c. 5.7 km2 lowland rain forest site in Costa Rica.
  • 3To assess the relative importance of environmental vs. geographical distances in relation to the length of environmental gradient covered, we compared the results obtained over the full range of soil types, including swamps, with those for upland soils alone.
  • 4Environmental variability was found to be a far stronger predictor of changes in floristic differences than the geographical distance between sample plots. In particular, differences in soil nutrient content, drainage and canopy openness correlated with floristic differences.
  • 5The decline in mean floristic similarity with increasing geographical distance was stronger than proposed by the random dispersal model over short distances (up to c. 100 m), which is probably attributable to both dispersal limitation and environmental changes. The scatter around the mean was large at all distances.
  • 6Our initial expectation was that the effects of dispersal limitation (represented by geographical distance) on observed patterns of floristic similarity would be stronger, and those of environmental differences weaker, than at broader spatial scales. Instead, these results suggest that the niche assembly view is a more accurate representation of pteridophyte communities at local to mesoscales than the dispersal assembly view.

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