Livestock and wildlife effects on the successional development of a savanna landscape mosaic in East Africa.

摘要

In African savannas, large herbivores, along with rainfall and fire, have historically been considered key drivers of ecosystem dynamics. Recently, attention has focused on an additional driver: landscape heterogeneity, ranging in scale from small nutrient patches to large grassland-woodland mosaics. Herbivores respond to this heterogeneity, and feedbacks can help maintain landscape mosaics. I used descriptive and experimental studies to explore the development of a dominant type of anthropogenic landscape heterogeneity in Kenya, East Africa, focusing on the influence of large herbivores. First, I used chronosequence methodology to describe plant community succession associated with development of abandoned cattle corrals into long-term treeless "glades" in a wooded Acacia drepanolobium savanna. Descriptive data showed that glades were hotspots of increased nutrient levels, improved soil texture, palatable grasses, termite activity, and herbivore use. Herbivore exclusion cages showed that herbivore use reinforced or sped the progression of succession. Second, I used a plant neighbor removal experiment crossed with herbivore exclusion in dry and wet seasons to investigate the interactive effects of herbivory and abiotic stress on plant competition and facilitation in glade plant communities. Facilitation of the early successional species, Cynodon plectostachyus, by the later successional species, Pennisetum stramineum, was highly context dependent, occurring only in the presence of herbivores in the dry season. Pennisetum was competitively dominant during wet times, opposing patterns of dry season facilitation. Thus, the relative strengths of dry and wet season interactions may help determine the overall rate of succession. Third, to test how cattle vs. wildlife affect glade development, I used a large-scale 13-year experiment that excluded different combinations of cattle and wildlife. Long-term vegetation data showed that elephants homogenized the glade landscape mosaic by suppressing growth of large trees that define glade edges. In the herbaceous community, through forage preferences, wildlife suppressed and cattle reinforced succession. Plant neighbor removals indicated that cattle addition intensified facilitation of Cynodon, but dampened facilitation of Pennisetum. Collectively, these studies indicate that large herbivores exert control over the long-term persistence of the glade landscape mosaic through their short-term and long-term effects on herbaceous and woody plant communities.