The effect of the introduction of an exotic species (cattle) into a native African herbivore assemblage was investigated by studying resource partitioning between zebu cattle, wildebeest and zebra. Resource partitioning was investigated by analysing grass sward characteristics (such as sward height and percentage nitrogen in leaves) of feeding sites selected by the different herbivore species. Linear discriminant analysis was used to determine whether a distinction could be made between feeding sites selected by the different animal species or whether the animal species showed overlap in resource use by selecting similar feeding sites. Wildebeest and zebra did not show overlap in resource use except in the wet season when resources were ample. Cattle showed overlap in resource use with zebra in the early wet season and with wildebeest in the early dry season, seasons when food limitation is likely. In the wet season, cattle showed overlap in resource use with both zebra and wildebeest.Implications of these results for competitive relationships between livestock and wildlife are discussed. We suggest that interpretation of overlap in resource use may be different for an assemblage of long-term coexisting native species as compared to an assemblage of native and exotic species. Among native herbivores, overlap in resource use is not expected based on evolutionary segregation. In a native assemblage to which an exotic species has been introduced, however, overlap in resource use can occur under food-limited conditions and consequently implies competition.

In many areas in Africa, seasonal movements of migratory ungulates are restricted and their population numbers decline, for example in the Tarangire region, Tanzania. Here, agriculture restricts migration of ungulates to their wet season ranges. We investigated whether low forage quality or supply are possible causes of population decline of wildebeest and zebra when access to these wet season ranges is restricted and migratory herds have to reside in the dry season range year-round. We simulated grazing through a clipping experiment in the dry season range during the wet season. Clipping negatively affected forage supply and had a positive effect on forage quality by increasing proportions of live and leaf biomass as well as nutrient concentrations in the leaves. However, increase in forage quality in the dry season range due to grazing was not as such that requirements of wildebeest during the wet season, when females are lactating, could be met. We conclude that low forage quality in the dry season range during the wet season could cause the decrease in migratory ungulate populations in the Tarangire region. With this study, the necessity of protecting wet season ranges from expanding human activities to safeguard migratory systems is supported. RésuméDans de nombreuses régions d'Afrique, les déplacements saisonniers des ongulés migrateurs sont entravés et leurs populations déclinent, comme par exemple dans la région de Tarangire, en Tanzanie. Ici, c'est l'agriculture qui limite la migration des ongulés vers les domaines qu'ils fréquentent en saison des pluies. Nous avons étudié si la mauvaise qualité ou la faible quantité du fourrage étaient des causes possibles du déclin des populations de gnous et de zèbres lorsque l'accès à leur domaine de saison des pluies est limité et que les troupeaux migrateurs doivent rester dans les domaines de saison sèche toute l'année. Nous avons simulé le pâturage en menant, en saison des pluies, une expérience de tonte dans l'aire fréquentée en saison sèche. Couper l'herbe avait un effet négatif sur l'apport de fourrage et avait un effet positif sur la qualité du fourrage parce que cela augmentait la proportion de biomasse vivante et de feuilles ainsi que la concentration de nutriments dans les feuilles. Cependant, augmenter par le pâturage la qualité du fourrage dans le domaine de saison sèche n'était pas suffisant pour satisfaire les besoins des gnous en saison des pluies, lorsque les femelles sont allaitantes. Nous concluons qu'en saison des pluies, la qualité médiocre du fourrage dans le domaine fréquenté en saison sèche pourrait causer la diminution des populations d'ongulés migrateurs dans la région de Tarangire. Cette étude conforte la nécessité de protéger les domaines fréquentés en saison des pluies contre l'expansion des activités humaines, afin de sauvegarder les systèmes migratoires.

Autotoxicity and allelopathy affect the respiration and yield of GL66 and GL72, two populations of perennial ryegrass (Lolium perenne L. cv. S23) that were originally selected for contrasting rates of mature-leaf dark respiration under conditions where allelopathic effects could not occur and autotoxic effects were minimal. The aim of this study was to further investigate growth and biomass allocation of these two populations in relation to their autotoxic and allelopathic properties. To this end, plants were subjected to two conditions (monoculture and mixed culture) and two treatments (growth in 'renewed' and 'replenished' nutrient solution, allowing for short- and long-term accumulation of allelochemicals, respectively). The fast-respiring population, GL66, showed a reduced total yield due to allelopathic effects only when long-term accumulation of allelochemicals was allowed (mixed culture, replenished). However, short-term accumulation (mixed culture, renewed) of allelochemicals was sufficient to affect allocation of biomass to leaf sheaths. The slow-respiring population, GL72, suffered from autotoxicity only when long-term accumulation was allowed (monoculture, replenished), and from allelopathy under both short- and long-term accumulation (mixed culture, either renewed or replenished). The predominant allelopathic and autotoxic effect was on dry matter percentage and dry weight of leaf sheaths. We conclude that the roots of both populations release one or more chemical compounds that primarily affect biomass allocation to leaf sheaths, both of the same and of the other population. Sensitivity to the putative inhibitor(s) released by the other population was greater than sensitivity to the inhibitor(s) released by a population's own roots.