Dietary migration of Impala rivals the geographical migration of
Serengeti wildebeest
Seasonal dietary changes increase the abundances of savanna herbivore
species

Date:
October 8, 2020
Source:
University of the Witwatersrand
Summary:
A new study shows the Impala's migration is a 'dietary migration',
where they switch from eating mostly grass in the wet season,
to eating more tree leaves or 'browse' during the dry season.



FULL STORY ========================================================================== African savannas are renowned for their huge diversity of wildlife,
yet some animal species are much more abundant than others. What causes
these differences?

==========================================================================
For herbivore species -- plant-eating animals like antelope, zebra and elephants -- the challenge lies in both obtaining enough food to eat
throughout the year, while also avoiding predation by carnivores.

One way to obtain enough food -- and which works extremely well in
places like the Serengeti -- is to migrate over long distances and
track the areas where the best food is available through the seasonal
cycle. This works best for grass eating or 'grazer' species such as
wildebeest and zebra.

On the other hand, being extremely large like an elephant greatly reduces predation risk, with their big bodies also meaning that they can eat
whatever they like, because there is lots of time to digest foods as
they move through their long gut system.

So why then are impala by far the most abundant herbivore in a place
like the Kruger Park? These are animals that do not migrate, and they
are also only medium-sized at best.

Recent research published in Science Advances by Professor Carla Staver
from Yale University and Dr Gareth Hempson from University of the
Witwatersrand has shed new light on this question.

"The key insight emerging from our research is that species like impala
do actually migrate, although not in the sense you would expect," says
Dr Hempson.

"For impala, the migration they undertake is a 'dietary migration',
where they switch from eating mostly grass in the wet season, to eating
more tree leaves or 'browse' during the dry season. This 'mixed feeding' strategy makes a huge amount of sense, because grasses tend to be higher quality and more abundant food in the wet season, but trees tend to
stay greener much longer into the dry season and become the better food
source then." Theoretical models show that for this 'mixed feeding'
strategy to work, the costs of switching between food sources must not
be too high. For example, the mouth shape of a hippo is excellent for
consuming lots of grass quickly, but terrible for picking green leaves out
of a thorn tree. Similarly, giraffe are better off sticking to browsing
from the treetops. The models also suggest that for the mixed feeding
strategy to be advantageous, the 'best' food type must change from season
to season. This is typically the case for grass versus browse in savannas.

Animal census data from 18 protected areas in East and southern Africa
show that being a mixed feeder does indeed have a strong positive effect
on species abundances in savannas, regardless of body size. In fact,
the abundance of mixed feeders is rivalled only by migratory grazers, suggesting that switching your diet from season to season -- either in
terms of what you eat, or where you eat -- is fundamental to achieving
high population sizes.

But what of the benefits of being large-bodied like an elephant? This
research suggests that part of the reason why elephants can
become abundant is in fact due to their broad, mixed diets, but
the anti-predation benefits of being large almost certainly still
stand. However, outsized human poaching effort on very large species
such as rhino and elephant mean that we have little idea of just how
abundant they really can be.

The findings of this research have important implications for the
management and conservation of increasingly threatened herbivore
populations, and for our understanding of the ecology of savannas more
broadly. For example, land use change and the shrinking space available
for animal movement means that mixed feeders -- who can switch their
diet in situ -- are likely to fare better in future than species that historically would have moved long distances to obtain different food
sources. Mixed feeders may also come to play an increasingly important
role in mitigating woody encroachment -- the rapid increase in tree
density in many savannas linked to rising carbon dioxide levels -- thus maintaining more open, grassy spaces to the benefit of grazing species.

"So, next time you're in the Kruger Park and becoming bored of seeing only impala, take a moment to reflect that you are witnessing the outcome of a migration that rivals that of the Serengeti wildebeest -- an extraordinary seasonal dietary migration from grass to browse," says Dr Hempson.


========================================================================== Story Source: Materials provided by University_of_the_Witwatersrand. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. A. Carla Staver, Gareth P. Hempson. Seasonal dietary changes
increase the
abundances of savanna herbivore species. Science Advances, 2020;
6 (40): eabd2848 DOI: 10.1126/sciadv.abd2848 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201008121302.htm

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