Closing the gaps for animal seed dispersal: Separating the effects of habitat loss on dispersal distances and seed aggregation

Abstract

Habitat loss can alter animal movements and disrupt animal seed dispersal mutualisms; however, its effects on spatial patterns of seed dispersal are not well understood. To explore the effects of habitat loss on seed dispersal distances and seed dispersion (aggregation), we created a spatially explicit, individual-based model of an animal dispersing seeds (SEADS-Spatially Explicit Animal Dispersal of Seeds) in a theoretical landscape of 0%-90% habitat loss based on three animal traits: movement distance, gut retention time, and time between movements. Our model design had three objectives: to determine the effects of (1) animal traits and (2) habitat loss on seed dispersal distances and dispersion and (3) determine how animal traits could mitigate the negative effects of habitat loss on these variables. SEADS results revealed a complex interaction involving all animal traits and habitat loss on dispersal distances and dispersion, driven by a novel underlying mechanism of fragment entrapment. Unexpectedly, intermediate habitat loss could increase dispersal distances and dispersion relative to low and high habitat loss for some combinations of animal traits. At intermediate habitat loss, movement between patches was common, and increased dispersal distances and dispersion compared to continuous habitats because animals did not stop in spaces between fragments. However, movement between patches was reduced at higher habitat loss as animals became trapped in fragments, often near the parent plant, and dispersed seeds in aggregated patterns. As movement distance increased, low time between movements and high gut retention time combinations permitted more movement to adjacent patches than other combinations of animal traits. Because habitat loss affects movement in a nonlinear fashion under some conditions, future empirical tests would benefit from comparisons across landscapes with more than two levels of fragmentation. K E Y W O R D Sfragment entrapment, individual-based model, long-distance dispersal, mechanistic model, spatial pattern, spatially contagious dispersal

Intrinsic and extrinsic drivers of intraspecific variation in seed dispersal are diverse and pervasive

“…By contrast, a meta-analysis of a worldwide data set (Fontúrbel et al 2015) suggested that fragmentation reduced interaction rates (visitation or fruit removal), but not disperser diversity (abundance or species richness); at a major biome level, fragmentation reduced disperser diversity only in temperate zones but reduced interaction rates in both temperate and tropical zones. Additionally, inter- or intraspecific variation in disperser traits such as movement distance, movement frequency and gut retention time of seeds represent one mechanism explaining how fragmentation can positively or negatively affect dispersal distances (fragment entrapment, Jones et al 2017).…”

Rapid changes in seed dispersal traits may modify plant responses to global change

“…This is most apparent if the absence of the animal disperser leads to dispersal network disruptions (Rogers et al 2017). In a theoretical study, Jones et al (2017) showed that animal traits such as movement distance, activity level and gut retention time interacted with habitat loss such that seed dispersal distances were largest under intermediate habitat loss. Garcia et al (2016) used an isotope-based technique to track bird-mediated seed dispersal and found that small differences in seed and animal traits, for example phenology patterns, could lead to large differences of seed dispersal in response to landscape structure.…”

Effects of zoochory on the spatial genetic structure of plant populations

“…These patterns can be highly consistent over time (Heymann et al., ), increasing the probability of an effect of seed dispersal on SGS. Anthropogenic disturbances that might modify vector behaviour are thus likely to influence seed dispersal patterns and in turn SGS (Jones, Duke‐Sylvester, Leberg, & Johnson, ; McConkey & O'Farrill, ). Our results emphasize the need for future studies on population genetics of animal‐dispersed plants to include ecological and behavioural observations of dispersal vectors as a key for understanding gene flow and spatial distribution of genetic diversity.…”