Abstract
In savannas across the planet, encroaching woody plants are altering ecosystem functions and reshaping communities. Seed predation by rodents may serve to slow the encroachment of woody plants in grasslands and savannas. Our goals for this study were to determine if rodents in an African savanna selectively removed seeds of an encroaching plant and if foraging activity was influenced by the local vegetation structure or by the landscape context. From trials with two species of seeds (encroacher = Dichrostachys cinerea, non-encroaching overstory tree = Senegalia nigrescens) at 64 seed stations, we recorded 1,065 foraging events by seven species of granivorous rodents. We found a strong positive relationship between rodent activity and the number of seeds removed during trials. Foraging events were dominated by rodent seed predators, with <10.6% of events involving a rodent with the potential for secondary dispersal. Rodents selectively removed the seeds of the encroaching species, removing 32.6% more D. cinerea seeds compared to S. nigrescens. Additionally, rodent activity and the number of seeds removed increased at sites with more grass biomass. Our results suggest a potential mechanistic role for rodents in mitigating the spread of woody plants in grass dominated savannas.
Highlights
Savannas are experiencing an increase in woody vegetation, known as woody encroachment (Stevens et al, 2016a,b)
The diminutive M. minutoides accounted for 10.1% (n = 108) of the activity and we only recorded five foraging events by S. pratensis (0.3%, n = 3), and S. campestris (0.2%, n = 2)
Comparing the proportion of seeds removed by species, we found that rodents removed a greater proportion of D. cinerea than S. nigrescens (β = −1.30, 95% confidence intervals (CI) −2.52 to −0.26, reference = D. cinerea)
Summary
Savannas are experiencing an increase in woody vegetation, known as woody encroachment (Stevens et al, 2016a,b). In the last century, tropical savannas in Africa, South America, and Australia (Stevens et al, 2016b) have seen an increased in abundance of woody plants due to poorly understood and interacting factors, such as atmospheric carbon dioxide, fire regimes, and overgrazing (Sala and Maestre, 2014; Archer et al, 2017). This structural and compositional change can alter the ecosystem functions and services that savannas provide, such as altering carbon storage, decreasing water availability, reducing biological diversity, and disrupting socioeconomic conditions through reduced grazing (Huxman et al, 2005; Soto-Shoender et al, 2018). To maintain savannas and their beneficial services, it is important to understand factors that can limit the establishment of woody encroachers, at the seed germination and establishment stage
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