Geographic variation in the structure of salt marsh arthropod communities

Background/Question/Methods

   There is a general consensus that both top-down and bottom-up forces work to structure biological communities. Top-down and bottom-up effects can alter the abundance, biomass or diversity of different trophic levels in a community, and they often interact to impact herbivorous insect populations.  Understanding the relative strengths of these forces is an important key to predicting the structure of biological communities.  

   Salt marshes along the Atlantic and Gulf coasts are similar in many ways, and are inhabited by the same plant and insect species.  However, the Atlantic and Gulf coast areas have different tidal regimes, which may result in the two areas not functioning in exactly the same ways.  In addition, abiotic factors that vary with latitude may lead to differences between south and north Atlantic sites, and abiotic factors that vary with longitude may lead to differences between west and east Gulf Coast sites.

   We hypothesized that the importance of top-down and bottom-up factors in coastal salt marshes might vary geographically.  To test this hypothesis, we characterized abiotic conditions and the plant and arthropod communities at 11 sites along each coast in the late summers of 2009 and 2010.  We also manipulated wrack (dead plant stems) and nutrient availability in 2 x 6 m plots at each site to evaluate geographic differences in community response.  The experiment was established in 2009 and allowed to run to 2010.

Results/Conclusions

   Our sampling documented that some abiotic factors (especially soil organic content) varied geographically.   Plant height, nitrogen content, thatch cover also varied geographically.  Although the total number of arthropods collected did not differ geographically, the trophic composition of samples showed marked variation among geographic regions.  Large-scale differences in latitude and in mean tidal range are likely driving much of this variation.  Arthropod community structure was little affected by wrack addition, but strongly affected by fertilization; and, the effect of fertilization varied geographically for herbivores and prey items. Although salt marshes are superficially similar from Maine to Texas, they may be structured differently throughout this geographic range.  Therefore, extrapolating results from one geographic region to another should be done with caution.