Effects of insect damage and plant density on the spatial patterns of plant populations

Background/Question/Methods: The spatial patterns of plants (i.e., whether they are arranged in a clumped, random, or uniform pattern) can influence both herbivore damage and competition, and thus should influence plant population growth and spread.  However, very little is known about how plant spatial patterns are generated in the first place.  In this study, we test the hypothesis that herbivore damage and plant density influence the spatial patterns of plant populations.  To test this hypothesis, we analyzed the spatial patterns of experimental populations of horsenettle (Solanum carolinense) with a history of exposure to different density and herbivory treatments.  In 2007, twenty-five S. carolinense stems were planted in a uniform pattern into each of 40 plots at one of five initial densities.  Half of the plots were routinely sprayed with an insecticide and the remaining plots were left exposed to natural levels of herbivory.  From 2008-2010, the spatial coordinates of each new ramet was recorded.  We used point-pattern spatial analyses (e.g. Ripley’s K) to describe the degree of clumping within each population at various distance classes.  We conducted separate multivariate regressions to test for relationships between damage and/or density and the degree of clumping of populations.

Results/Conclusions: Spatial analyses of populations show strong effects of damage and strong interactive effects of density and damage on the degree of clumping of S. carolinense stems.  In general, most populations exhibited a random spatial pattern, but there were two scenarios where S. carolinense populations became highly clumped: (1) When plant density is low and insect damage is intermediate and (2) When plant density is intermediate and damage is low.  These findings suggest that damage can interact with density in complex ways to influence the spatial patterns of plant populations.  Potential mechanisms for differences in spatial patterns include distance-dependent survival of clones and/or plastic changes in clonal growth strategies of S. carolinense under various damage and density levels.  Understanding the reciprocal effects of herbivory, density, and plant spatial patterns should also have important implications for the management of noxious weeds through biological control.  Despite its potential importance, consideration of the spatial patterns of populations is currently missing from biocontrol strategies.