Browsing herbivores improve the state and functioning of savannas: a
model assessment of alternative land use strategies
Abstract
Changing climatic conditions and unsustainable land use are perceived as
major threats to savannas worldwide. In the past, land use in African
savannas was dominated by livestock-farming as one of the major economic
products, which led to degraded, shrub encroached pastures in many
regions. One response to this widespread degradation is a shift from
land use dominated by cattle to strategies characterized by animal
compositions with more mixed feeding regimes and higher browser
proportions. However, the consequences for ecosystem properties and
processes remain so far largely unclear. We used the ecohydrological,
spatially explicit savanna model EcoHyD to assess the impacts of two
contrasting, herbivore-related land use strategies on a Namibian
savannah: grazing versus browsing herbivores. We varied the densities of
grazers and browsers and determined the resulting composition and
diversity of the plant community, total vegetation cover, soil moisture
and water use by plants. Our results show that properties making plants
less attractive to herbivores were best adapted to different densities
of grazing (cattle) or browsing (pure browsing wildlife) animals. Also,
properties leading to a competitive advantage under limited water
availability were among the dominant ones. Overall, the results are in
line with our expectations: we found heavy shrub encroachment with a
loss of the perennial grass matrix under high stocking rates of cattle.
A novel and unexpected result was that regardless of the density of
browsers, grass cover and plant functional diversity were significantly
higher in wildlife scenarios. This increased grass cover, but also the
higher total cover improved water uptake by plants. We conclude that in
contrast to grazers, browsers even in high densities do not lead to
ecosystem degradation, but rather sustain a diverse vegetation with high
cover of perennial grasses over a long time, implying also a lower
erosion risk and higher provision of ecosystem services.