Interaction between ungulates and bruchid beetles and its effect on Acacia trees: modeling the costs and benefits of seed dispersal to plant demography

Abstract

Integrative studies of plant-animal interactions that incorporate the multiple effects of interactions are important for discerning the importance of each factor within the population dynamics of a plant species. The low regeneration capacity of many Acacia species in arid savannas is a consequence of a combination of reduction in seed dispersal and high seed predation. Here we studied how ungulates (acting as both seed dispersers and herbivores) and bruchid beetles (post-dispersal seed predators) modulate the population dynamics of A. raddiana, a keystone species in the Middle East. We developed two simulation models of plant demography: the first included seed ingestion by ungulates and seed predation by bruchids, whereas the second model additionally incorporated herbivory by ungulates. We also included the interacting effects of seed removal and body mass, because larger ungulates destroy proportionally fewer seeds and enhance seed germination. Simulations showed that the negative effect of seed predation on acacia population size was compensated for by the positive effect of seed ingestion at 50 and 30% seed removal under scenarios with and without herbivory, respectively. Smaller ungulates (e.g., <35 kg) must necessarily remove tenfold more seeds than larger ungulates (e.g., >250 kg) to compensate for the negative effect of seed predation. Seedling proportion increased with seed removal in the model with herbivory. Managing and restoring acacia seed dispersers is key to conserving acacia populations, because low-to-medium seed removal could quickly restore their regeneration capacity.

Effects of seed beetles on the performance of desert legumes depend on host species, plant stage, and beetle density

“…Our study also does not consider other ecological interactions that may have profound effects on germination and seedling growth in nature (Rodriguez-Pérez et al, 2011). For example, seeds cached in rodent dens provide the major source of recruitment for many desert shrubs (McAuliffe, 1990), but damaged seeds may more likely be eaten than cached, as has been demonstrated for squirrel predation on acorns (Steele et al, 1996).…”

Seed production, infestation, and viability in Acacia tortilis (synonym: Vachellia tortilis) and Acacia robusta (synonym: Vachellia robusta) across the Serengeti rainfall gradient

“…To assess whether infestation proportions were associated with MCP, we developed two additional sets of six candidate models each (Table 3), applying MCP on infestation data corresponding with year of seed collection, as above. We compared model fits with the lme function in the nlme package in R (Pinheiro et al 2011) using SITE as a random effect.…”

Low functional redundancy among mammalian browsers in regulating an encroaching shrub ( Solanum campylacanthum ) in African savannah

“…Although it is well known that large herbivores can be important seed dispersers and exert cryptic indirect effects on plants, there is a persistent tendency to treat large-mammal herbivory as a simple pairwise interaction: surprisingly few studies have integrated the positive effects of seed dispersal and indirect interactions with the negative direct effects of folivory [41,42]. We suggest that the diversity of direct and indirect mechanisms that determine the impact of large-herbivore populations on their food plants makes functional redundancy, even in the narrowest sense, extremely unlikely [20,24].…”