Impact of woody encroachment on soil-plant-herbivore interactions in the Kafue Flats floodplain ecosystem

Background/Question/Methods

Encroachment of woody species is considered a serious threat to grazing ecosystems. By changing the availability of light, water and nutrients, woody encroachment can have major effects on the understory layer, and hence alter the availability of forage for herbivores. The overall aim of our study was to determine the consequences of shrub encroachment for: (i) soil carbon, nitrogen, and phosphorus pools and availabilities; (ii) the diversity, quality and productivity of understory herbaceous plants; and (iii) the food supply and population size of the endemic semi-aquatic antelope Kafue lechwe. We performed a meta-analysis on the effects of shrub encroachment on soil nutrients and productivity of understory vegetation, and carried out extensive field studies along cover and age gradients of the encroaching shrubs Dichrostachys cinerea and Mimosa pigra. Additionally, we monitored annual migration of the lechwe population, analyzed annual fluctuation of dung quality and measured lechwe food availability throughout the year.

Results/Conclusions

Encroachment of the Kafue Flats ecosystem by two morphologically different leguminous shrubs has had large effects on soil nutrients and understory vegetation. These effects depend strongly on the canopy structure of shrubs and their ability to fix nitrogen, as well as upon the degree of encroachment. During the period of food scarcity the lechwe– although considered a strict grazer – were found to consume D. cinerea seedpods, with apparently beneficial effects for their nutritional status. The fact that lechwe utilized D. cinerea as food, and also that shrubs increased the nutritional quality of understory grasses, suggest that effect of encroachment has been less negative than previously supposed. Although woody encroachment has reduced herbage production, the Kafue lechwe appear resilient to this decline because they are able to adapt their diet and benefit from shrubs as a dry season food supplement. We show that simply quantifying the extent of woody encroachment does not necessarily yield reliable predictions about its consequences for grazing ecosystems.