Seasons on the Savanna: How Seasonal Diet Changes Impact Population Sizes of Savanna Herbivores

March 17, 2021

Seasons on the Savanna: How Seasonal Diet Changes Impact Population Sizes of Savanna Herbivores

Image courtesy of Vincent Mugaba.

If you have ever seen Disney’s time-honored family feature The Lion King, you likely have a good picture of what an African savanna looks like. The African savanna ecosystem is home to a large and diverse community of megafauna, or large terrestrial mammals, whose populations have undergone serious declines as a result of many complex factors, such as predator-prey relationships, disease, and drought. While the impact of these variables on the population dynamics of savanna herbivores have been well-studied by ecologists, the dietary strategies of these communities have only just recently been investigated as a determining factor of population abundances. 

Seeking to better understand this determinant, Carla Staver, Associate Professor of Ecology and Evolutionary Biology at Yale University, and Gareth P. Hempson, a postdoctoral fellow at the University of Witwatersrand Johannesburg, examined the effects that seasonal dietary changes have on the populations of savanna herbivore species. After composing two different theoretical mathematical models and evaluating data from several African savanna parks, Staver and Hempson found that species that switch their diets seasonally, in addition to species that migrate to find better forage, have increased population sizes and dominate the savannas.

Grazing and Browsing

The savannas of Africa, characterized by trees and grasslands, boast biodiverse habitats that support herbivores such as browsers, who feed on tree leaves and shrubs, grazers, who feed on grass and other low-lying vegetation, and mixed feeders, who alternate between browsing and grazing depending on the season. Browsers, such as giraffes, and grazers, such as zebras and wildebeests, are considered specialists because of their efficiency at eating a particular, albeit limited, diet. On the other hand, mixed feeders, such as impala and elephants, are defined as generalists because of their ability to adapt and survive off a varied diet. 

Savannas are highly seasonal systems, meaning they alternate between a wet season with heavy rainfalls and a dry season with little to no rainfall. During the wet season, grazers enjoy bountiful plant growth. But in the dry season, when grass is sparse, herbivores of the savanna tend to undertake three different practices: mixed feeders alter their food source and switch to browsing, migratory grazers locate new foliage to consume, and nonmigratory grazers suffer from the lack of plant abundance. Not only is grass less abundant during the dry season, but it is also less nutritious, leading species that switch between grass and tree diets to be more likely to obtain their food from trees during the dry season. Because of their reluctance to change diets, nonmigratory grazers experience a substantial decline in population sizes during droughts. “The two types of herbivores that actually do become abundant —mixed feeders and migratory grazers—do so by… actively [having] strategies to ensure that they have a better dry season than they would otherwise,” Staver said. 

Theoretical Models

Reinforcing their theory that migratory grazers and mixed feeders maintain higher population abundances compared to nonmigratory specialists, Staver and Hempson presented two mathematical models in a paper published in Science Advances that illustrate the role that dietary strategies play in population dynamics. The first model, a discrete-time population model, takes into account the variance of population sizes between wet and dry season and finds the overall rate that the populations of herbivores increase by. Specifically, the model uses the geometric mean—a form of averaging found by multiplying rather than adding numbers—of wet and dry season growth rates to establish the comprehensive growth rate of a population.

This model incorporates a few assumptions about vegetation and diet switching; first, it is assumed that grass is more ample and nutritious in the wet season while browse vegetation, such as twigs and leaves, are more bountiful and nourishing in the dry season; second, it is assumed that there is an efficiency cost associated with generalism. Whereas specialists have a particular anatomy that allows them to be very skilled at eating certain types of food, mixed feeders may not be as proficient as grazers at consuming grass or browsers at consuming trees. This raises the question: how does the cost of being less efficient at eating a particular diet compare to the benefits of being able to switch between diets? Staver’s results show that as long as the comparative benefits of grazing in the wet season and browsing in the dry season make up for the costs of switching, then mixed feeders are more successful than specialists at achieving larger population sizes. 

The second model, an application of the well-studied Lotka-Volterra consumer-resource model, represents the interactions between predators and prey, or in this case, the dynamics between herbivores and food resources. While the previous model took for granted the availability of grass and trees and only considered the quality of vegetation, this model couples herbivore population abundance with vegetation availability. What sets this model apart from traditional consumer-resource models is the inclusion of alternating seasons. In accordance with the changing seasons, it was observed that plant and herbivore abundance endure cyclic patterns. While there is some dependence on seasonal vegetation productivity and herbivore digestive and intake efficiency, the model showed that “any type of seasonal diet change, whether from grazing to browsing or a preferred grass forage to a forage reserve will lead to both larger populations and more herbivore biomass,” Staver said. 

Results and Implications

Given these results that mixed feeders and migratory specialists experience population increases as a result of seasonal dietary strategies, what can we expect for the field of ecology and evolutionary biology? Whereas ecologists in the past have predominantly focused on the effects of predators on herbivores, this study represents a “renewed focus on mixed feeders and their implication for vegetation dynamics and the conservation of African savanna ecosystems,” said Staver.

One issue Staver raised is that herbivores of the savannas are increasingly confined to small protected areas that limit the ability for specialists to migrate. “As reserves become smaller and more fenced and more divorced from the matrix around them, the potential for migration potentially decreases, as it has already,” Staver said. As a result, mixed feeding species, such as impala, are able to feed more effectively and become the dominant herbivore in a lot of these diverse African savanna systems.

Especially in the time of the COVID-19 pandemic, Staver’s study demonstrates a profound lesson we as humans can learn from the mixed feeders of the savanna. From developing new business models to altering the way we learn and communicate, businesses and individuals alike are faced with the task of adapting to our new shared reality. While specialization has been historically valued and rewarded in our society, the impala demonstrate that ultimately, in our ever evolving world, generalists may prevail.

Sources:

Staver, A. C., & Hempson, G. P. (2020, October 2). Seasonal dietary changes increase the abundances of savanna herbivore species. Science Advances, 6 (40). 10.1126/sciadv.abd2848
Staver, A.C. (n.d.). Seasonal dietary changes increase the abundances of savanna herbivore species. Staver Lab. https://staverlab.yale.edu/sites/default/files/files/StaverHempson_SciAdv.mp4

Write a comment

Comments

No Comments Yet!

View comments

Write a comment

<