Grassland invasion resistance arises due to plant diversity effects on species pattern

Background/Question/Methods

Altering plant species richness changes the extent to which individuals are adjoined by conspecific neighbors (self-association), an effect that is often overlooked in studies assessing plant species richness – invasion relationships.  Such species self-association may be an important determinant of community-scale responses if the effects of heterospecifics and conspecifics on individual performance are unequal and if responses are driven by interactions among close neighbors. In 2012 the plant Species Pattern and Community Ecology (SPaCE) field experiment was established with transplants (64 individuals m^-2; 1 x 1 m plots; n = 5) of 16 tallgrass prairie species.  The SPaCE experiment simultaneously tests effects of plant species richness (2, 4 or 8; functional diversity controlled) and pattern (species randomly assigned to planting positions or grouped in “clumps” of four individuals) on grassland communities.  All community structure treatments were replicated in weeded and non-weeded plots.  Plant aboveground biomass was collected, dried, and weighed and structural equation modeling was used to assess direct and indirect effects of species richness and pattern on invasion.

Results/Conclusions

The effect of species pattern on invasion was not consistent across increasingly species rich communities.  Invaders (non-planted species) were most abundant and most promoted by aggregating resident species in less species rich plots.  When accounting for effects of richness on species self-association through a structural equation model, we found evidence for direct and indirect effects of species richness on invader abundances.  These results indicate that increasing richness likely inhibits invasion due to the addition of more resistant species and changes in local species interspersion and interactions.  These results indicate that efforts to create greater neighborhood species interspersion in reconstructed grasslands, which are often locally species poor, could be rewarded with greater invasion resistance.