Why do the buffalo roam? Determining what drives seasonal strategies in large mammalian herbivores

This blog post is provided by Joel O. Abraham and A. Carla Staver and tells the #StoryBehindThePaper for the paper “Seasonal strategies differ between tropical and extratropical herbivores”, which was recently published in the Journal of Animal Ecology.

The idea for this project came from a project several of us worked on a few years ago, examining the responses of savanna herbivores to drought (published in Ecology and Evolution). From 2014-2016, South Africa was struck by a severe drought, one of the worst in decades, and the charismatic herbivores that inhabit South African savannas responded. Some animals moved to avoid the drought, heading to less droughted portions of the landscape (so-called ‘drought refugia’). Others shifted their diets, consuming comparatively more drought-resistant trees as opposed to drought-sensitive grasses. Interestingly, however, not all herbivores employed these strategies. Only large-bodied grazers – animals like buffalo and zebra – altered their landscape use, and only mixed feeders – elephants and impala – changed their diets during the drought. In other systems, large-bodied grazers migrate seasonally, and mixed feeders are known to shift their diets seasonally. As such, the strategies these animals employed during the drought largely paralleled strategies they already exhibit to navigate seasonality. This finding suggested that herbivore drought responses were constrained to some degree, restricted to herbivores that already had seasonal strategies, and that these strategies were related to herbivore traits. This got us wondering – did these constraints apply to mammalian herbivores more broadly?

Zebras in Kruger National Park during the 2014-2016 drought. Grazing herbivores like these zebras moved to drought refugia to avoid the drought. Photo by Joel O. Abraham.

To answer this question, we first had to compile a dataset of herbivore strategies. To do this, we combed the literature for any records of these herbivore strategies: we pulled together studies reporting seasonal diet composition for as many mammalian herbivores as we could find and collated all records of migration. We also gathered data from the literature on two herbivore traits that had proved key in determining herbivore drought responses – body mass and grass dependence.

However, not all ecosystems are like savannas. Savannas are seasonal, with a defined wet and dry season. This leads to severe, but not absolute, resource bottlenecks during the dry season – the grass stops growing and dries out when the rains stop, and grazing herbivores consume the grass until it is basically gone. In contrast, some of the trees are able to keep their foliage through the dry season, especially those that pull their water from deeper soils.  As such, though grass is seasonally scarce, trees are available across seasons in many savannas. But this is different in other ecosystems – some systems, like tropical rainforests, are less seasonal than this, while others, like monsoonal systems in south-east Asia, experience stronger rainfall seasonality. Meanwhile, temperate and boreal systems have plant phenology strongly structured by temperature rather than by rainfall. These differences in seasonality might contribute to differences in herbivore strategies. To evaluate this possibility, we downloaded species range maps for all the herbivore species from IUCN and, coupled with climate data from WorldClim, quantified various metrics of seasonality across herbivore species ranges.

A kudu eating a savanna shrub. We pulled together data on mammalian herbivore diets and body sizes from existing literature. Photo by Joel O. Abraham.

But seasonality isn’t the only environmental characteristic that might drive seasonal strategies. Differences in plant quality across space have also been shown to play a role in driving seasonal strategies, particularly migration. Plant quality is hard to map out, and no appropriate global dataset exists that we could tap into. However, plant quality is loosely related to soil nutrients. Indeed, the Serengeti wildebeest, an iconic example of large herbivore migration, have been observed to track soil nutrient gradients as they migrate. Conveniently for us, soil nutrients have been mapped globally. So, to figure out if soil nutrients might also be contributing to differences in strategies, we used data from the World Soil Information Services and quantified soil nutrients within herbivore species ranges.

Finally, with these diverse datasets in hand, we could evaluate which variables were most predictive of herbivore strategies. We built a variety of models including these different variables and used model selection to identify which predictors were most important.

A nyala munching in the understory. Mixed feeding herbivores like this nyala eat a mixture of trees and grass all year long and also change their diets seasonally. Photo by Joel O. Abraham.

We found that migration was best predicted by body mass, grass consumption, and latitude. Seasonal diet shifts were best predicted by grass consumption alone, peaking at intermediate levels of grass consumption.

Because latitude proved to be so important, especially for migration, we also compared the distribution of strategies between tropical and extratropical herbivores. Indeed, both migration and diet shifts were more prevalent among extratropical herbivores. Interestingly, extratropical herbivores also exhibited more intermediate diets and body sizes as compared to tropical herbivores.

Hartebeest on the move. In the tropics, grazers like these hartebeest often migrate seasonally. Photo by Joel O. Abraham.

Together these findings painted an interesting picture – the constraints we had found for savanna herbivore strategies did seem to scale up to herbivores in other ecosystems; migration was more prevalent among grazing herbivores and diet shifts were most prevalent among mixed feeders. However, this was only really true of tropical systems – in extratropical systems, herbivore strategies were much more ubiquitous across body size classes and feeding guilds. While this finding was surprising to us at first – we had expected the patterns we observed in savannas to hold for all herbivores – it makes a lot of sense upon reflection: resource bottlenecks are more severe in extratropical systems; pretty much all vegetation is seasonal in temperate, boreal, and polar ecosystems. During winter in the extratropics, most trees lose their leaves, the understory dies back, and the remaining vegetation is covered by snow.

These results suggest really different outcomes for tropical and extratropical herbivores in the face of global change. Because strategies for navigating seasonality are widespread among extratropical herbivores, they may be able to use existing strategies to respond to a changing climate and altered resource distributions. Contrastingly, in the tropics, where the distribution of these behaviors is more restricted, herbivores may be less able to respond to global change; as we saw during the drought, only the herbivores that already exhibit these seasonal strategies may be able to use them in response to novel ecological circumstances. Our findings therefore add to a growing body of literature emphasizing the precarious future of large-bodied tropical herbivores. We will have to take different conservation approaches for tropical and extratropical systems, finding a balance between protecting ecosystems against intensifying herbivore impacts in extratropical systems while protecting large-bodied herbivores in the tropics.

Elephants eating grass. Elephants are one of the few species of tropical herbivores that exhibit both seasonal strategies – they can migrate and shift their diets seasonally. Photo by Joel O. Abraham.

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