Determining the rates of vertebrate visitation to and herbivory at megacarcass sites
We have noticed that in many megacarcass sites, a halo of abundant herbaceous vegetation develops from the interactions between consumption and production. The pattern could arise from intense consumption and high-quality production (a phenomena known as a “grazing lawn”) at the periphery of the gravesoil, then high production and less consumption further out, then lower production and lower consumption even further out.
When an elephant first dies, their initial decomposition acts as a massive “fast release” of nitrogen and carbon. At first, this suppresses plant growth due to nitrogen toxicity (too much nitrogen is bad for plants). However, a subsequent “slow phosphorus” release, documented in medium sized carcasses, may lead to increased herbivory in part because large, plant-eating mammals (such as antelope) select for plant nutritional quality, much like we may select the best-looking leafy greens in the supermarket.
Thus, our question is: Are megacarcass sites attractive as feeding sites for large herbivores?
We hypothesize that: There will be a humped distribution in the net effect of herbivory on herbaceous primary production in relation to distance from the center of the gravesoil due to interactions between nutrient toxicity in the gravesoil, fertilization on the periphery, and herbivory on nutrient rich vegetation in the ‘halo’ zone around the gravesoil.
In order to explore this question, we carried out the dung and browse surveys at elephant carcass sites. To map these sites, we marked the center of the carcass or “gravesoil” and ran a 15-meter tape north, followed by a second tape south, then a third and fourth tape east and west. Next, using a 0.25 m quadrat, we walked the length of each line. Any large mammal dung that intercepted our path we weighed and identified (elephant, zebra, ruminant pats, or ruminant pellets), and recorded distance from the center of the grave soil, and direction (N, S, E, W). We also returned the dung to its original location so that these natural processes could continue undisturbed.
In addition, for any woody plants we intercepted, we randomly selected 10 shoots within 2 m above ground and scored for fresh browsing damage (presence/absence) along with direction and distance from the center. Afterwards we ran secondary tapes northeast, northwest, southeast, and southwest repeating the same methodology.
Dung and Browse Surveys:
Though this should provide us a good idea of vertebrate visitation, if these herbivores really are visiting more then they’re likely also eating more of the potentially high-quality graze. In order to get a better idea of at what distance from the carcass they visit more often, we also set up a series of exclosure studies at 10 sites.
Since preliminary data indicate low herbaceous abundance on the gravesoil (up to 5m radius), followed by an increase in height around 10 m, and a return to matrix height at 15 m, we installed three exclosure cages at each megacarcass site at distances of 5 m, 10 m, and 15 m from the center of the gravesoil. Each cage excludes large herbivores from a 1 m2 plot, being constructed of welded steel droppers and chicken mesh. We also marked off adjacent control plots of the same size so we have something to compare each exclosure to.
At the start of the experiment (August 2023), we clipped all vegetation in each plot to ground level, then collected, dried, and weighed them. In the future, we will clip these sites again (likely twice seasonally) and compare growth in grasses and forbs between exclosures and their paired controls. In the near future, we will return to “re-map” each of our 37 sites to see how the dung concentrations and browse abundance have changed in our absence. We will also map new sites in other areas of the park and continue our data analysis.