Abstract booklet - gtö – Society for Tropical Ecology

40 PARALLEL SESSION HALL H IV: AFRICAN SAVANNAS BIODIVERSITYPARALLEL SESSION HALL H IV: AFRICAN SAVANNAS BIODIVERSITY 41MONDAY 17:00 Hall H IVGREEN, GREEN GRASS OF HOME: HOW UNGULATES SELECTFEEDING SITES IN A NUTRIENT POOR TALL GRASS SAVANNAH INCOASTAL TANZANIAAnnette Stähli 11WSL/ ETH, Birmensdorf, CH, annette.staehli@wsl.chUnlike the well-studied short grass savannas on fertile soils, tall grass savannasin more humid climates and poorer soils may be difficult habitats to grazefor ungulate herbivores. Grazing patterns in such environments tend to bepatchy but repeated grazing of the same sites over several (dry) seasonsmight help to improve forage quality. These sites (termed “grazing lawns” insome circumstances) may eventually become a crucial resource for severalherbivore species especially bulk grazers. Studying herbivore-vegetationinteractions in northern coastal Tanzania, we compared grass forage qualityand quantity, species composition and grazing intensity on grazed patches andin surrounding apparently ungrazed tall grass vegetation over two dry seasons(short dry season Jan.-Feb. 2010, long dry season July-Sept. 2009). Resultsof a multivariate analysis of 204 vegetation plots, recorded during the shortdry season revealed significant differences in species composition of grazedpatches and nearby tall grass vegetation. Grazed patches were characterizedby significantly higher abundance of two main fodder species (Panicuminfestum and Digitaria milanjiana). Sporobolus pyramidalis and Bothriochloabladhii, two grass species highly avoided by herbivores, were significantly moreabundant in the tall grass vegetation. In both study seasons, grazing intensityon grazed patches was more than two times higher than in surrounding tallgrass vegetation, and mean vegetation height was thus significantly lower. Inboth dry seasons, green vegetation (grasses mixed with some forbs) sampledfrom grazed patches had significantly higher nitrogen and thus crude proteincontents compared to samples from the nearby tall grass savannah. The samewas found in a separate analysis of four grass species known to be preferredby selective grazers. However, N contents of these grass species tended tobe slightly lower than N contents of the total vegetation on grazed patches,presumably because of the presence of N rich leguminous forbs. Overall grazingintensity was positively correlated with nitrogen contents in green vegetation(pooled data of grazed patches and surrounding tall grass).Our findings indicate that in a tall grass savannah, herbivores focus on areaswith particular species compositions offering elevated nutritional value, and bymaintaining grazing over extended periods, are able to boost nitrogen contentseven more.GRASS-GRAZER INTERACTIONS IN AFRICAN SAVANNAECOSYSTEMS: WHERE DOES ’THE PARADIGM’ STAND AND WHEREDO WE GO FROM THERE?Britta K Kunz 1 , Gaelle Bocksberger, Thomas Hovestadt, Ottmar Kullmer,Christine Römermann, Friedemann Schrenk, Georg Zizka, OliverTackenberg1BIK-F, Frankfurt am Main, DE, b.kunz@bio.uni-frankfurt.deAfrican savannas and grass-eating ungulates are considered inextricably linkedto each other at least since the Plio-Pleistocene, when a massive radiation ofungulates occurred, corresponding to a rapid speciation in grasses. Today,African savannas still harbour the highest diversity and biomass of ungulatesworldwide which daily consume large amounts of the aboveground foliage.From the grass’ point of view, grazing represents loss of photosynthetically activematerial. Consequently, the interaction between grasses and grazers generallyis considered an antagonistic one in which both plants and ungulates haveevolved co-adapted traits and strategies in an „arms-race“. However, empiricalfield studies regularly prove that ungulates can play a major role in dispersal ofinconspicuous seeds of grassland species, including grass seeds. Though thisoften is interpreted as ’dispersal by chance’ it might indicate a more mutualisticrelationship then commonly assumed. While a good theoretical frameworkexists on the evolutionary and ecological interactions related to seed dispersalof attractive, nutrient rich fruits or seeds, many basic aspects of the interactionbetween plants with inconspicuous fruits and their seed dispersers are not wellunderstood. Identification of the relevant traits in grass-grazer interactionsand a better understanding of the underlying mechanisms thus would furtherimprove the general theory of seed dispersal by animals. Moreover, it wouldhelp to understand past and current species distribution patterns and to betterpredict future distribution patterns under different scenarios of climatic change.Last but not least a thorough understanding of factors that structure grasslandcommunities will help guiding management efforts.We will review the state of the art of grass-grazer interactions, consideropen questions and suggest future research directions to fill the gaps in ourknowledge concerning this matter.MONDAY 17:15 Hall H IVgtöSociety for Tropical Ecology | Gesellschaft für Tropenökologie e.V. Status and future of tropical biodiversity | Frankfurt, 21 - 24 February 2011gtö