Grazing herbivores reduce herbaceous biomass and fire activity across African savannas.

Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one-zebra increase in herbivore biomass density (~100 kg/km2 of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.

Hominin turnover at Laetoli is associated with vegetation change: Multiproxy evidence from the large herbivore community.

Vegetation change in eastern Africa during the Pliocene would have had an important impact on hominin adaptation and ecology, and it may have been a key driver of hominin macroevolution, including the extinction of Australopithecus and the emergence of Paranthropus and Homo. The Pliocene paleoanthropological site of Laetoli in Tanzania provides an opportunity to investigate the relationship between vegetation change and hominin turnover because it encompasses the time period when grass cover was spreading across eastern Africa and because hominin species turnover occurred locally at Laetoli, with Paranthropus aethiopicus in the Upper Ndolanya Beds (UNB) replacing Australopithecus afarensis in the Upper Laetolil Beds (ULB). However, it remains unresolved how the vegetation of the UNB and the ULB differed from each other. To examine differences between the two stratigraphic units, multiple proxies-hypsodonty, mesowear, and stable carbon isotopes of tooth enamel (δ13Cenamel)-are used to infer the diets of large herbivores and compare the dietary guild structure of the large herbivore communities. All three proxies indicate an increase in the abrasiveness and C4-content in the diets of the large herbivores in the UNB relative to those in the ULB. After inferring the diets of species based on all three proxies, the large herbivore community of the UNB had a greater proportion of grazers and a smaller proportion of mixed feeders than in the ULB but maintained a similar proportion of browsers and frugivores. The ULB community has few modern-day analogs, whereas the UNB community is most closely analogous to those in modern African grasslands. Thus, hominin turnover at Laetoli is associated with an increase in grass cover within a woodland-grassland mosaic and is part of a broader transformation of the herbivore community structure.

A nonanalog Pliocene ungulate community at Laetoli with implications for the paleoecology of Australopithecus afarensis.

The dietary guild structure of ungulate communities is a useful paleoecological tool for understanding the context of hominin paleobiology and evolution. Ungulates are well represented in the fossil record, and their dietary preferences reflect those of major habitat types. However, paleoecology relies on modern ecological patterns as analogs for recreating ecologies of the past. It has previously been suggested that for much of the Pliocene, no such modern analogs exist for the herbivore communities associated with hominins in eastern Africa. This study aims to determine whether the ungulate community associated with A. afarensis at the Pliocene site of Laetoli, Tanzania, shares similarities with extant communities or whether it lacks a modern analog. Our multiproxy approach using mesowear, hypsodonty, and stable carbon isotopes of tooth enamel to infer the diets of ungulates in the Upper Laetolil Beds shows that this community is dominated by browsers and mixed feeders and has a very low prevalence of grazers and frugivores. This dietary guild composition distinguishes the Upper Laetolil Beds from modern African communities and suggests either that the Upper Laetolil Beds had a unique vegetation structure which was able to support a higher diversity of browsing ungulates than that exists in African ecosystems today or that it retained an ungulate community that was resilient to environmental change. The Upper Laetolil Beds ungulate community is also unique relative to other mid-Pliocene communities in eastern Africa, some of which are similar to extant communities, while others, such as Laetoli, lack modern counterparts. This suggests that A. afarensis was a eurytopic species that inhabited a variety of ecosystems, including those with and without modern analogs. The co-occurrence of both analog and nonanalog communities in the Pliocene suggests that the transformation toward ungulate communities of modern aspect occurred asynchronously in eastern Africa.