Defences and responses

Plant-herbivore interactions influence both plant characteristics and feeding patterns of herbivores. In this thesis, I focus on patterns of defence in a wide range of woody species in semi-arid savannas in Botswana, and on responses in woody species to different types of browsing. Methods included both surveys of plant traits relevant to herbivory, and controlled experiments using simulated browsing. On a regional scale gradient, the proportion of spinescent and evergreen species decreased, while fast-growing species increased, with increasing resource availability. On smaller landscape scale gradients, the proportion of spinescent and evergreen species increased, fast-growing species either increased or decreased, while C-based defences generally decreased and nutrients increased, with increasing resource availability. In some species, levels of defence (phenolics) were higher in young trees, compared to mature trees. However, contrary to our predictions, shoots within, compared to above, browsing height on mature trees were less defended (phenolics and tannins). Simulated twig-biting during the dry season (on five species) induced compensatory growth responses in annual shoots during the growth season, and cutting at ground level induced sprouting. Leaf removal late in the growth season (on one species) induced foliage re-growth, although total leaf production during the season did not increase. The application of saliva had a small, but significant, effect on shoot growth following clipping. No evidence was found for a connection between patterns in allocation to defensive traits in shoots of different exposure to browsers, or strength of responses to clipping, and other species characteristics, e.g. leaf fall patterns, growth rate, presence of spines or natural browsing pressure. I suggest further studies focusing on both chemical and structural defences, on different spatial scales and resource gradients. I also conclude that additional studies are needed on potential effects of different traits on different types of herbivores, and stress the advantage of studying a wider range of plant species to obtain more general results

Foraging competition, vigilance and group size in two species of gregarious antelope

In gregarious species, rates of foraging behaviour are often positively related to group size while there is simultaneously a negative relation between group size and vigilance. Although the mechanisms underlying these behavioural patterns are still incompletely understood, decreased predation risk or increased foraging competition in larger groups have been put forward as two possible explanations. Since most empirical tests of these two hypotheses have used manipulative experiments, they have mainly been limited to small-bodied species or to animals in captivity. Here we suggest a time-budget model to test for the causal effects of predation risk and foraging competition that does not necessitate manipulative approaches. We used this method on two species of gregarious antelope, blesbok (Damaliscus pygargus phillipsi) and impala (Aepyceros melampus). We suggest that increased foraging competition in large groups drives the negative relationship between vigilance and group size in these species. This study shows that observational data can be used to test explicit hypotheses on species that are logistically unsuitable for manipulative experiments, and also render support for hypotheses suggesting that factors unrelated to predation risk are involved in shaping the differences in vigilance rates over different group sizes in gregarious species.ab201

Nutrient and secondary metabolite concentrations in a savanna are independently affected by large herbivores and shoot growth rate

Carbon-based secondary metabolites (CBSMs) such as tannins are assumed to function as plant defences against herbivores. CBSMs are thought to be inversely related to growth rate and nutrient concentrations because a physiological trade-off exists between cellular growth and differentiation, but CBSM concentrations can be altered by herbivory-induced changes in the trade-off. We predicted that a significant interaction exists between herbivory and growth phase, such that the effects of large herbivores (or their exclusion) on nutrient or CBSM concentrations are greatest during phases of rapid shoot or leaf growth. Leaf samples were collected during phases of different growth rate from six woody species four years after establishment of a large-scale long-term herbivore exclusion experiment in Kruger National Park, South Africa. Samples were analysed for N, P, condensed tannins and total phenolics. Interactions between growth phase and herbivores were rare. However, the assumption that elevated nutrients and reduced CBSMs occur during fast phases of growth was supported by four species (consistent with the growth-differentiation balance hypothesis), but not the other two. Large herbivores generally did not affect nutrients, but CBSMs in four species were reduced by large herbivores other than elephants, while CBSMs in two species were reduced by elephants. Carbon limitation ultimately prevailed among woody plants taller than 2 m under long-term browsing. Large herbivores and plant growth phase are independent and important determinants of nutrients or CBSMs in African savannas, but the effects depend on the interacting assemblages of species, which poses challenges to the application of current general hypotheses of plant defence

Browser selectivity alters post-fire competition between Erica arborea and E. trimera in the sub-alpine heathlands of Ethiopia

Mammalian herbivores have the potential to alter the competitive relations of woody species, if consumption is unevenly distributed between species. At elevations above 3500 m in the southern Ethiopian highlands, vegetation is dominated by Erica arborea and Erica trimera. Both species can potentially grow into short trees, but are burnt on a rotation of 6 to 10 years, and regenerate by re-sprouting from belowground lignotubers. The regenerating scrub is heavily browsed by cattle. We set up browsing exclosures at three burnt sites to quantify the impact of browsing over a three-year period. When protected from browsing, E. trimera had similar or better height growth than Erica arborea, but in browsed vegetation, Erica arborea instead grew taller. Browsing was more intense on E. trimera in the first years after fire, indicating a difference in palatability between the species. We checked if browse quality differed, by analysing shoot contents of acid detergent fibre, protein, phenolics and tannins. Contrary to expectations the preferred E. trimera contained more acid detergent fibre, less protein and had a higher tannin activity than E. arborea. Although the vegetative growth of E. arborea is favoured relative to E. trimera under high browsing pressure, rapid change in abundance would not be expected, since short-interval fire will repeatedly eradicate any gains in vegetative growth. However, within the typical fire return interval of less than 10 years, E. trimera barely reach a reproductive state, whereas E. arborea flower profusely. Under the current regime of fire and browsing this may in the long run be more important than differences in height growth, leading to a gradual increase in the proportion of E. arborea