Scale-dependent bi-trophic interactions in a semi-arid savanna: how herbivores eliminate benefits of nutrient patchiness to plants.
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 whole-plot 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 coarse-grained 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 bi-trophic interactions.
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.|Agri-Ecological Services, PO Box 28, Omaruru, Namibia.
Cornelis
C
van der Waal
Department of Experimental Plant Ecology, Institute for Water and Wetland Research, Radboud University, PO Box 9010, 6500 GL, Nijmegen, The Netherlands. H.deKroon@science.ru.nl.
Hans
H
de Kroon
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.
Frank
F
van Langevelde
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.
Willem F
WF
de Boer
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.
Ignas M A
IM
Heitkönig
School of Life Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, 4000, South Africa.
Rob
R
Slotow
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.|Centre for Wildlife Management, University of Pretoria, Private bag X20, Hatfield, Pretoria, South Africa.
Yolanda
Y
Pretorius
Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3A, 6708 PD, Wageningen, The Netherlands.
Herbert H T
HH
Prins
D000818:Animals; D017753:Ecosystem; D065948:Grassland; D060434:Herbivory; D010944:Plants; D014197:Trees
Allometric scaling theory; Grazing; Nutrients; Tree–grass; Trophic cascade