Take me for a ride: Herbivores can facilitate plant reinvasions

Abstract

Herbivores shape plant invasions through impacts on demography and dispersal, yet only demographic mechanisms are well understood. Although herbivores negatively impact demography by definition, they can affect dispersal either negatively (e.g., seed consumption), or positively (e.g., caching). Exploring the nuances of how herbivores influence spatial spread will improve the forecasting of plant movement on the landscape. Here, we aim to understand how herbivores impact how fast plant populations spread through varying impacts on plant demography and dispersal. We strive to determine whether, and under what conditions, we see net positive effects of herbivores, in order to find scenarios where herbivores can help to promote spread. We draw on classic invasion theory to develop a stage-structured integrodifference equation model that incorporates herbivore impacts on plant demography and dispersal. We simulate seven herbivore "syndromes" (combinations of demographic and/or dispersal effects) drawn from the literature to understand how increasing herbivore pressure alters plant spreading speed. We find that herbivores with solely negative effects on plant demography or dispersal always slow plant spreading speed, and that the speed slows monotonically as herbivore pressure increases. However, we also find that plant spreading speed can be hump shaped with respect to herbivore pressure: plants spread faster in the presence of herbivores (for low herbivore pressure) and then slower (for high herbivore pressure). This result is robust, occurring across all syndromes in which herbivores have a positive effect on plant dispersal, and is a sign that the positive effects of herbivores on dispersal can outweigh their negative effects on demography. For all syndromes we find that sufficiently high herbivore pressure results in population collapse. Thus, our findings show that herbivores can speed up or slow down plant spread. These insights allow for a greater understanding of how to slow invasions, facilitate native species recolonization, and shape range shifts with global change.

The Causes and Consequences of Seed Dispersal

“…In addition, intraspecific variation in dispersal can increase spatial spread (Snell et al 2019). Species interactions, such as herbivory, can also alter spatial spreading of populations (Sullivan & Shaw 2023). Dispersal can rapidly evolve during population spread, and this evolutionary change can influence range expansion; on average, theoretical and empirical evidence suggests that eco-evolutionary feedbacks accelerate range expansion (Miller et al 2020).…”

“…In terms of dispersal limitation, several empirical studies suggest that plants are generally dispersal limited and that the degree to which dispersal is distance restricted or spatially contagious depends on the dispersal mode (Tamme et al 2014, Wright et al 2016. In an Iowa grassland, the presence of herbivores had variable effects on distance-restricted dispersal at the species level (Allbee et al 2023), potentially based on the life-history stage at which the herbivores consume plants (Sullivan & Shaw 2023). In a California grassland, dispersal limitation constrained local diversity in a scale-dependent manner, with the largest gains in species richness following species additions occurring at or above 100 m (Germain et al 2017).…”