1998 Distinguished Ecologist Lecture Series

Sponsored by The Graduate Degree Program in Ecology,
Colorado State University

Norman Owen-Smith

Centre for African Ecology
Department of Zoology
University of the Witwatersrand
South Africa

ABSTRACTS

Megaherbivore populations, vegetation resources and African environments

Norman Owen-Smith
Wednesday, April 22, 1998   4:10 pm   A202 Clark

Overabundant populations within protected wildlife areas have been a feature especially of Africa's "megaherbivores", i.e. elephants, hippos and rhinos. Elephants destroy trees, and in the process change habitats and even landscapes for other species. White rhinos and hippos transform tall grasslands into short grass lawns, likewise changing habitat structure for other grazers; and by suppressing fires may promote bush encroachment. These changes are perceived as adverse for conservation objectives, and have been the justification for culling programmes directed at these large species within national parks. However, the intermediate disturbance hypothesis suggests that to some degree such impacts could be benefical for biodiversity. I have gone further by suggesting that the elimination of megaherbivores by human hunting towards the end of the Pleistocene was the key factor precipitating extinctions among other large mammal species, through consequent habitat changes. In evaluating megaherbivore impacts on vegetation, we need to look beyond the damage incurred by plants to consider the spatial pattern of such impacts within ecosystems. Managerial intervention through water provision and fire control may "homogenize" conditions and worsen the extent of severe vegetation change. How to achieve a stable coexistence between megaherbivore populations and vegetation resources for other species remains a formidable challenge for park managers. Nevertheless, some suggestions as to how this could best be achieved can be made from landscape ecology theory, e.g. by restricting artifical water supplies, or by creating dispersal sinks.

Laws, R.M. 1970. Elephants as agents of habitat and landscape change. Oikos 21:1-15.

Owen-Smith, N. 1988. Megaherbivores. The Influence of Very Large Body Size on Ecology. Cambridge UP. Chapters 12 and 16.

Owen-Smith, N. 1989. Megafaunal extinctions: the conservation message from 11,000 years BP. Conservation Biology 3:405-412.

Owen-Smith, N. 1996. Ecological guidelines for waterpoints in extensive protected areas. South African Journal of Wildlife Research 26:107-112.

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Norman Owen-Smith
Thursday, April 23, 1998   4:10 pm   A202 Clark

Applying optimal foraging theory to herbivores is far more challenging that it is for carnivores. Vegetation resources are heterogeneous in quality as well as in abundance and eating rates obtained. The nutritional value of plant parts depends on allelochemical contents as well as on nutrient concentrations and overall digestibility. Digestive processing time commonly restricts daily food intake, as well as available foraging time. Both food quantity and quality can change drastically over the seasonal cycle. Three approaches towards modeling diet selection have been attempted, with variable success. My "clever ungulate" model was heuristically useful, but not predictively successful. Belovsky's linear programming model appeared amazingly successful, but circular in the way constraint settings were identified. Verlinden and Wiley tried to accommodate the effects of digestive constraints on food choice, but provided no tests. Nevertheless, large herbivores are clearly selective in the vegetation components they consume, and expand their dietary acceptance range over the dry or winter season roughly as expected from theory. The food preference ranking of kudus appeared to accord with a compound index of food value, taking into account eating rate relative to digestion rate, and the balance between nutrients and allelochemicals. Over the seasonal cycle kudus also increased gut fill, extended daily foraging time, and restricted the proportion of foraging time diverted to other activities. In this way they tightly controlled daily energy intakes despite widely varying conditions. Thus assumed environmental and physiological constraints appeared somewhat flexible in their operation. In variable environments, limits may become effective intermittently, with different constraints governing diet selection at different times. Moreover, fixed limits are crude representations of foraging costs that rise non-linearly with increasing foraging effort. This non-linearity makes any attempt to apply average-rate models fundamentally flawed. A dynamic, state-dependent approach to optimization is clearly needed, but challenging to develop.

Owen-Smith, N & P Novellie. 1982. What should a clever ungulate eat? The American Naturalist 119:151-178.

Owen-Smith, N. 1993. Evaluating optimal diet models for an African browsing ruminant, the kudu: how constraining are the assumed constraints? Evolutionary Ecology 7:499-524.

Owen-Smith, N. 1994. Foraging responses of kudus to seasonal changes in food resources: elasticity in constraints. Ecology 75:1050-1062.

Owen-Smith, N. 1996. Circularity in linear programming models of optimal diet. Oecologia 108:259-261.

Owen-Smith, N. 1997. Control of energy balance by a wild ungulate, the kudu, through adaptive foraging behaviour. Proceedings of the Nutrition Society 56:15-24.

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Norman Owen-Smith
Friday, April 24, 1998   4:10 pm   A202 Clark

Classically models of population dynamics have subdivided populations into age or stage classes, with density expressed numerically, and the environment symbolized by a mysterious carrying capacity K. Populations are projected to grow logistically, cyclically or even chaotically, with scant attention to the temporal variability and spatial heterogeneity that are features of the real world. Recently Getz proposed a "metaphysiological" modeling approach, which focused attention on the rates and efficiencies of resource extraction and conversion into consumer populations. This advanced Caughley's modification of the Lotka-Volterra equations for interacting populations, and allowed both r and K to be expressed in measurable parameters. The GPQ modeling approach takes this a step further, by (a) specifying the functional forms of resource gains (G), physiological costs (P) and mortality losses (Q), and (c) explicitly adopting a currency of biomass density. No term is needed for reproduction, and population structure is accommodated as a secondary refinement. This allows population, community and ecosystem processes to be related specifically to adaptive behavioral responses of herbivores to patchy, time-varying environments. For large herbivores the intake ("functional") response is generally sharply threshold ("plateau and precipice") in form, as a result of various constraints plus compensatory adjustments. The mortality function is typically hyperbolic, accelerating with increasing food deficiencies. The outcome is that, for realistic parameter values, the herbivore-vegetation interaction tends to be highly unstable in constant, uniform environments. The GPQ model shows how seasonal variability and heterogeneity in resource quality can stabilize herbivore-vegetation systems. Models ignoring spatial and temporal variability are merely "parables": belief in their predictions is a matter of faith rather than scientific conviction. The GPQ approach offers a simple but realistic conceptual structure that can be elaborated in as much detail as available knowledge allows and the problem demands.

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