Low functional redundancy among mammalian browsers in regulating an encroaching shrub (Solanum campylacanthum) in African savannah.
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.
Department of Ecology and Evolutionary Biology, Princeton University, , Princeton, NJ 08544, USA, Department of Zoology and Physiology, University of Wyoming, , Laramie, WY 82070, USA, Department of Biology, University of Florida, , Gainesville, FL 32611, USA, Department of Plant Sciences, University of California, , Davis, CA 95616, USA, Mpala Research Centre, , Nanyuki, Kenya, Department of Zoology, University of British Columbia, , Vancouver, BC V6T 1Z4, USA.
Robert M
RM
Pringle
Jacob R
JR
Goheen
Todd M
TM
Palmer
Grace K
GK
Charles
Elyse
E
DeFranco
Rhianna
R
Hohbein
Adam T
AT
Ford
Corina E
CE
Tarnita
D000818:Animals; D020387:Food Chain; D065948:Grassland; D060434:Herbivory; D058865:Introduced Species; D007630:Kenya; D008322:Mammals / Q000502:physiology*; D008954:Models, Biological; D032322:Solanum / Q000254:growth & development*
biodiversity; bush encroachment; indirect effects; invasive species; megaherbivores; seed dispersal