Guild diversity impacts demographic outcomes of novel species interactions following range shifts

Abstract

Novel competitive interactions between native and range shifting species can precipitate local extinction of native species. However, increased biological complexity within recipient communities may prevent native species loss by decreasing the strength of novel competition experienced by any one species. This phenomenon, termed 'biotic resistance', is commonly applied in invasion ecology, but has received little attention in the context of climate induced range shifts. Here we investigate the effects of biotic resistance in competition between resident native and range-shifting damselflies in a region of Scotland newly colonised by the range-shifter, using competitive mesocosm treatments across multiple life stages and experimental temperatures. Our focal native species (Lestes sponsa) was unaffected by increasing competitive complexity as larvae, showing no fitness benefits in multispecies treatments compared to intraspecific or even interspecific scenarios in the presence of the range shifter. However, multispecies competition with both native and range-shifting species improved adult survival of our focal native species at higher temperatures, compared to interspecific competition with just the range shifter. For our focal range-shifting species (Ischnura elegans), larval growth rate was significantly reduced in multispecies treatments compared to intraspecific or two-species interspecific scenarios, yet adult range shifter survival showed no decrease in multispecies treatments. Furthermore, range shifter larvae displayed improved survival and growth in colder temperatures, compared to a lack of any temperature effects on adult survival. These results suggest that biotic resistance may alleviate the competitive impacts of range shifters on native communities by providing a life stage-dependent benefit to native species while simultaneously decreasing the fitness of range shifters. However, shifting temperatures can cause this interaction to swap between competition and facilitation, creating an environmentally dependent scenario that may benefit both range shifters and resident species, promoting the maintenance of diversity in high latitude communities.

Keywords: biotic resistance; community diversity; competition; complex life cycles; novel interactions; range shifting.

FIGURE 1

Experimental design for the larval competition experiment. Each treatment began with 18 individuals and was replicated three times per temperature. The numbers beside each treatment (L1–L6) refer to the species composition of each as described in text, but treatments are also colour‐coded here by species for ease of visualisation. Green = Lestes sponsa (the focal native species), blue = Ischnura elegans (the focal range shifter), and yellow = Enallagma cyathigerum, a non‐focal native species.

FIGURE 2

Experimental design for the adult competition experiment. Each treatment again began with 18 individuals and was replicated five times. The numbers beside each treatment (A1–A4) refer to the species composition of each as described in text, but treatments are also colour‐coded here by species. Green = Lestes sponsa (the focal native species), blue = Ischnura elegans (range shifter), and yellow = Enallagma cyathigerum.

FIGURE 3

(a) example of a mesocosm used for the larval competition experiment, complete with plastic pond weed. (b) paint dots on the wing of a male Lestes sponsa, used for individual identification. (c) One of the mesh cages used for the adult competition experiments. (d) A post‐experiment female L. sponsa under a UV light showing powder on her thorax, transferred from an interaction with a male L. sponsa.

FIGURE 4

Plotted best fit models for the demographic parameters of survival (left), probability of growth (top right), and growth rate (bottom right) for larval Ischnura elegans. Raw data are plotted as smaller points in each graph, jittered to improve visibility, with the predicted probability value for the response variable (along with its corresponding 95% confidence intervals) plotted as the larger dot and its associated error bars.

FIGURE 5

Survival probability of Ischnura elegans adults, demonstrating differences in survival between males and females across treatments. Raw data are plotted as the smaller points in each graph, jittered for visibility, with the predicted probability values for adult survival and their corresponding 95% confidence intervals plotted as the larger dots and their associated error bars.

FIGURE 6

Plotted best fit models for the demographic parameters of survival (left), probability of growth (top right), and growth rate (bottom right) for larval Lestes sponsa. As above, raw data are plotted as smaller points in each graph, with jitter added among the points for clarity. The predicted probability value for the response variable, along with its corresponding 95% confidence intervals, is plotted as the larger dots and their associated error bars.

FIGURE 7

Survival probability of Lestes sponsa adult males (above) and females (below) showing the interaction of temperature and treatment. Trendlines show the curve of predicted survival for each treatment, with 95% model confidence limits shown in the corresponding colour.

FIGURE 8

Plotted best fit model for mating harassment experienced by female adult Lestes sponsa.