Abstract:
The scale of resource heterogeneity may
influence how resources are locally partitioned between
co-existing large and small organisms such as trees and
grasses in savannas. Scale-related plant responses may, in
turn, influence herbivore use of the vegetation. To examine
these scale-dependent bi-trophic interactions, we varied
fertilizer [(nitrogen (N)/phosphorus (P)/potassium (K)]
applications to patches to create different scales of nutrient
patchiness (patch size 2 × 2 m, 10 × 10 m, or wholeplot
50 × 50 m) in a large field experiment in intact African
savanna. Within-patch fertilizer concentration and
the total fertilizer load per plot were independently varied.
We found that fertilization increased the leaf N and P
concentrations of trees and grasses, resulting in elevated utilization by browsers and grazers. Herbivory off-take was
particularly considerable at higher nutrient concentrations.
Scale-dependent effects were weak. The net effect of fertilization
and herbivory was that plants in fertilized areas
tended to grow less and develop smaller rather than larger
standing biomass compared to plants growing in areas that
remained unfertilized. When all of these effects were considered
together at the community (plot) level, herbivory
completely eliminated the positive effects of fertilization
on the plant community. While this was true for all scales
of fertilization, grasses tended to profit more from coarsegrained
fertilization and trees from fine-grained fertilization.
We conclude that in herbivore-dominated communities,
such as the African savanna, nutrient patchiness results
in the herbivore community profiting rather more than the
plant community, irrespective of the scale of patchiness. At
the community level, the allometric scaling theory’s prediction
of plant—and probably also animal—production does
not hold or may even be reversed as a result of complex bitrophic
interactions.
