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

Abstract

Large herbivorous mammals play an important role in structuring African savannahs and are undergoing widespread population declines and local extinctions, with the largest species being the most vulnerable. The impact of these declines on key ecological processes hinges on the degree of functional redundancy within large-herbivore assemblages, a subject that has received little study. We experimentally quantified the effects of three browser species (elephant, impala and dik-dik) on individual-and population-level attributes of Solanum campylacanthum (Solanum incanum sensu lato), an encroaching woody shrub, using semi-permeable exclosures that selectively removed different-sized herbivores. After nearly 5 years, shrub abundance was lowest where all browser species were present and increased with each successive species deletion. Different browsers ate the same plant species in different ways, thereby exerting distinct suites of direct and indirect effects on plant performance and density. Not all of these effects were negative: elephants and impala also dispersed viable seeds and indirectly reduced seed predation by rodents and insects. We integrated these diffuse positive effects with the direct negative effects of folivory using a simple population model, which reinforced the conclusion that different browsers have complementary net effects on plant populations, and further suggested that under some conditions, these net effects may even differ in direction.

Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation

“…African elephants (Loxodonta africana) have strong effects on woody plants due to their physical strength and height (21), causing disproportionate mortality of adult shrubs, by pulling them out (16), and trees, by pushing them over (22). Experiments with size-selective exclosures on savannas showed that elephants accounted for more than 80% of all woody plant loss across all plant height classes (22) whereas exclusion of elephants resulted in 42% more trees (23).…”

“…Temporary reductions in herbivore numbers allow trees to regenerate and grow into taller height classes, escaping herbivory by the time herbivore populations have recovered (17). Fruit and seed consumption might offset the demographic effects of browsing injury by seed dispersal, but the net effect of large herbivore assemblages on seed predation versus dispersal remains unclear (16).…”

“…Exclosure studies in African savannas reveal that species-rich herbivore assemblages may reduce woody species cover by 15-95% (Fig. 1A) (12)(13)(14)(15)(16)(17). This broad range reflects factors such as body size and feeding mode (18) and soil fertility, topography, and hydrology (12,19,20).…”

“…In species-rich herbivore communities, containing large as well as small browsers and a variety of feeding strategies, browsing impacts extend to a wider range of plant growth stages. Demographic bottlenecks imposed by browsing are therefore more difficult to escape (13,14,16). Temporary reductions in herbivore numbers allow trees to regenerate and grow into taller height classes, escaping herbivory by the time herbivore populations have recovered (17).…”

“…1B) (17,24). The removal of large browsers is thought to generally lead to a net increase in abundance of woody plants (20), but this effect depends in part on the compensatory response of smaller herbivores, which can have strong impacts, particularly on the recruitment of woody species (14,16,(25)(26)(27)(28).…”