YSM Issue 93.4 Full Magazine

Ecology
FOCUS
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
A breathtaking landscape of the Kruger National Park.
If you have ever seen Disney’s timehonored
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
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addition to species that migrate to find
better forage, have increased population
sizes and dominate the savannas.
Grazing and Browsing
IMAGE COURTESY OF WIKIMEDIA COMMONS
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
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 discretetime
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.
PHOTOGRAPH COURTESY OF PROFESSOR STAVER
This photograph captures a group of impala, a
classic example of a mixed feeder herbivore.
December 2020 Yale Scientific Magazine 23