Isaac Mapaure

Fatal attraction: vegetation responses to nutrient inputs attract herbivores to infectious anthrax carcass sites

Parasites can shape the foraging behaviour of their hosts through cues indicating risk of infection. When cues for risk co-occur with desired traits such as forage quality, individuals face a trade-off between nutrient acquisition and parasite exposure. We evaluated how this trade-off may influence disease transmission in a 3-year experimental study of anthrax in a guild of mammalian herbivores in Etosha National Park, Namibia. At plains zebra (Equus quagga) carcass sites we assessed (i) carcass nutrient effects on soils and grasses, (ii) concentrations of Bacillus anthracis (BA) on grasses and in soils, and (iii) herbivore grazing behaviour, compared with control sites, using motion-sensing camera traps. We found that carcass-mediated nutrient pulses improved soil and vegetation, and that BA is found on grasses up to 2 years after death. Host foraging responses to carcass sites shifted from avoidance to attraction, and ultimately to no preference, with the strength and duration of these behavioural responses varying among herbivore species. Our results demonstrate that animal carcasses alter the environment and attract grazing hosts to parasite aggregations. This attraction may enhance transmission rates, suggesting that hosts are limited in their ability to trade off nutrient intake with parasite avoidance when relying on indirect cues.

Impacts of mine dump pollution on plant species diversity, composition and structure of a semiarid savanna in Namibia

Effects of mine dump pollution on semiarid savanna vegetation were investigated in Kombat, Namibia. Vegetation structure, species richness, composition and diversity were compared between polluted and control sites. Concentrations of arsenic, chromium, copper, lead and zinc in soils were significantly higher closer to a 99-year-old mine dump than in a control site and exceeded internationally recommended guidelines by as high as 11 times. Mine dump pollution has significantly reduced woody plant densities, species richness and diversity in the affected area due to disappearance of pollution-sensitive species and preponderance of tolerant ones. Species composition and structure of vegetation close to the mine dump significantly changed, possibly due to negative impacts of heavy metals on recruitment as pollution-sensitive species died off, whereas tolerant species invaded the vacated ecological niches. Ordination analyses confirmed a strong pollution gradient, with measured heavy metals accounting for 57.3% of the observed variation in species data, suggesting that other factors are also important determinants in the area. It is recommended that the mine dump and its surrounds should be rehabilitated and regular monitoring should be carried out to remedy the situation and arrest further pollution of the surrounding environment.

Aboveground biomass production of a semi-arid southern African savanna: towards a new model

Following a field campaign to determine the species composition, canopy cover, aboveground annual production and leaf area index (LAI) of the semi-arid savanna of north-western Namibia, we present a production model that can be used by graziers to determine the livestock carrying capacity. The model predicts the annual aboveground net primary production (ANPP) from regression equations of canopy cover by annual production fraction for plant functional classes. We tested the output of the model against another fully independent net primary production (NPP) model, namely the MODIS NPP product. The mean MODIS NPP for the 29 sites was 343 ± 22 kg dry matter (DM) ha−1 y−1 as opposed to 285 ± 142 kg DM ha−1 y−1 for the fAP model that used the regression method (p < 0.01). As a proof of concept, these landscape-scale ANPP values are used to calculate a recommended livestock carrying capacity for the Ehirovipuka Communal Conservancy, a 1 980 km2 communal area with both wildlife and livestock populations. In addition, we also provide details of a field method for predicting landscape-scale LAI from line transect data. This approach can be used to ground reference the LAI values generated from the MODIS LAI product.

A preliminary simulation model of individual and synergistic impacts of elephants and fire on the structure of semi-arid miombo woodlands in northwestern Zimbabwe

Sustainable management of plant-herbivore systems requires an understanding of their long-term dynamics through modeling approaches. A preliminary simulation model was developed using STELLA to predict the impacts of elephants and fire on the structure of semi-arid miombo woodlands of northwestern Zimbabwe. Elephants alone at a density of 0.27 km -2 will convert the woodland into coppice in 120 years due to resulting massive declines of large trees. The same result is achieved in only 10 years if elephant density is at 2 km -2 . The pattern remains similar with simultaneous application of fire once every 4.7 years with elephants at 0.27 km -2 . When elephants are culled at 30% whenever their densities reach 1 km -2 , the woodland does not degenerate into coppice despite fire occurrence once every 4.7 years. Therefore, elephants alone can degrade and maintain semi-arid miombo woodland into coppice, largely due to their damaging impacts on mature canopy trees and fire acts to speed up the process by suppression of an already low recruitment. Fire alone has a lesser influence on woodland structure than elephants because of low fuel loads due to heavy grazing and low grass production as a result of low rainfall and inherently poor soils in the area. A maximum elephant density of 0.1 km -2 is recom-mended to achieve equilibrium in the area.