Interactions count: plant origin, herbivory and disturbance jointly explain seedling recruitment and community structure

Abstract

Herbivory and disturbance are major drivers of biological invasions, but it is unclear how they interact to determine exotic vs. native seedling recruitment and what consequences arise for biodiversity and ecosystem functioning. Previous studies neglected the roles of different, potentially interacting, guilds of generalist herbivores such as rodents and gastropods. We therefore set up a full-factorial rodent exclusion x gastropod exclusion x disturbance x seed-addition experiment in a grassland community in Central Germany and measured early seedling recruitment, as well as species richness, species composition and aboveground biomass. Gastropod herbivory reduced the positive effect of disturbance on seedling recruitment, particularly for exotic species. Rodent herbivory had weak positive effects on seedling recruitment at undisturbed sites, irrespective of species origin. This effect was likely driven by their strong negative effect on productivity. Interactive effects between both herbivore guilds became only evident for species richness and composition. How many species established themselves depended on disturbance, but was independent of species origin. The fewer exotic species that established themselves increased productivity to a stronger extent compared to native species. Our study highlights that joint effects of disturbance, herbivory and species origin shape early recruitment, while they only weakly affect biodiversity and ecosystem functioning.

Figure 1

Effect of gastropod herbivory (+Gas = gastropod control, −Gas = gastropod exclusion), disturbance (disturbed, undisturbed) and species origin (exotic, native) on (a) the proportion of seedling recruitment (number of recruited seedlings divided by the number of added seeds) and (b) the number of species with seedling recruitment. Note that data shown are the untransformed least square means ± SE of a linear mixed model.

Figure 2

(a) Effect of gastropod herbivory (+Gas = gastropod control, −Gas = gastropod exclusion), disturbance (disturbed, undisturbed) and species origin (exotic, native) on the on the proportion of seedling recruitment (number of recruited seedlings divided by the number of added seeds), (b) effect of rodent herbivory (+Rod = rodent control, −Rod = rodent exclusion) disturbance (disturbed, undisturbed) and species origin (exotic, native) on the proportion of seedling recruitment (number of recruited seedlings divided by the number of added seeds). Note that data shown are the untransformed least square means ± SE of a linear mixed model.

Figure 3

Effect of seed addition (control = no seed addition, exotic = exotic seed addition, native = native seed addition) and disturbance (undisturbed, disturbed) on total species richness (including resident and sown species). Data shown are least square means of repeated-measure linear mixed models ± SE averaged across two years. Note that species richness is given on a square root scale.

Figure 4

Effect of gastropod herbivory (+Gas = gastropod control, −Gas = gastropod exclusion) and rodent herbivory (+Rod = rodent control, −Rod = rodent exclosure) on total species richness (including resident and sown species). Data shown are least square means of repeated-measure linear mixed models ± SE averaged across two years. Note that species richness is given on a square root scale.

Figure 5

Effect of species origin (control = no seed addition, exotic = exotic seed addition, native = native seed addition) and disturbance (undisturbed, disturbed) on aboveground biomass. Data shown are least square means of repeated-measure linear mixed models ± SE averaged across two years. Note that aboveground biomass is given on a logarithmic scale.

Figure 6

The graph exemplifies the experimental design showing one out of five blocks. Each block includes one rodent exclusion plot (indicated by a bold solid line) and rodent control plot (indicated by a bold dashed line) plot. Within each plot, one gastropod exclusion subplot (indicated by a solid line) and gastropod control subplot (indicated by a dashed line) were established. Within each subplot, sub-subplots (depicted with smaller squares) were established that were either subjected to disturbance (shaded squares) or were left undisturbed (open squares). These sub-subplots were furthermore randomly assigned to receive no seeds (C), a seed mixture of 20 exotic species (E) or 20 native species (N).