Animal exchange ratios: an alternative point of view P. I. D. Kinyua* and J. T. Njoka Department of Range Management, University of Nairobi, PO Box 29053, Nairobi, Kenya Abstract An alternative interpretation is provided of the con- cepts of carrying capacity and exchange ratios, particu- larly suitable for game animal species, based on management models for a given area of rangeland or pasture. It involves modelling animal population dynamics as discrete-time logistic equations. Carrying capacity is then generated endogenously using rainfall as a proxy. The model interaction parameters, also gen- erated endogenously, represent the animal exchange ratios. Because these two parameters are generated endogenously, this approach takes into account all the animals' habitat requirements (food, cover, water and space) simultaneously, unlike other approaches that tend to consider food requirements only.This makes the approach amenable to multi-species situations. It also captures the ecological de¢nition of population growth models where the realized rather than the theoretical carrying capacity is determined endogenously. Key words: carrying capacity, exchange ratios, population dynamics Re¨ sume¨ On apporte une interpre¨ tation alternative des concepts de capacite¨ de charge et de taux d'e¨changes spe¨ ci¢que- ment adapte¨ e aux espe© ces-gibier, base¨ s sur des mode© les de gestion pour une zone donne¨ e d'habitat ou de paª tu- rage. Elle implique la mode¨ lisation de la dynamique des populations animales en e¨ quations logistiques de¨ limi- te¨ es dans le temps. La capacite¨ de charge est alors ge¨ n- e¨re¨ e de manie© re endoge© ne, en utilisant les chutes de pluie comme proxy. Les parame© tres d'interaction mode¨ - lise¨s, ge¨ne¨re¨s aussi de manie© re endoge© ne, repre¨sentent les taux de remplacement des animaux. Comme ces deux parame© tres sont ge¨ ne¨ re¨ s de cette fac , on, cette approche tient compte en meª me temps de toutes les exi- gences de l'habitat des animaux (nourriture, couvert ve¨ge¨ tal, eau et espace), contrairement aux autres approches qui ont tendance a© ne tenir compte que de la nourriture. Ceci rend cette approche adaptable a© des situations touchant de nombreuses espe© ces. Elle inte© gre aussi la de¨ ¢nition e¨ cologique des mode© les de croissance de population ou© la capacite¨ de charge e¡ective plutoªt que the¨ orique est de¨ termine¨ e de manie© re endoge© ne. Introduction The development of the concept of exchange ratio and the related concept of carrying capacity has been con- centrated on livestock and less so on multi-species, including game animals (Scarnecchia, 1985; Scarnec- chia,1990). Exchange ratio refers to the ratio of the number of individual animals or animal units of one species or class of livestock to the number of animals or animal units of another species or class to achieve a speci¢c objective, given speci¢c management objective(s), e.g. multi-species forage harvesting objective (Scarnecchia, 1990). Heady & Child (1994) proposed approximate exchange ratios based on metabolic body weights that could be applied as a guide to herbivores with similar diets, but for animals with di¡erent habitat require- ments, these exchange ratios should be modi¢ed according to dietary overlap among species. However, such modi¢ed exchange ratios should be used with cau- tion (Scarnecchia, 1985) and have limited practical application due to complexities of diet selection by her- bivores. *Correspondence. # 2001East AfricanWild Life Society, Afr. J. Ecol., 39,59^64 59