How herbivore activity around water affects plant communities

Date:
October 18, 2021
Source:
University of California - Santa Barbara
Summary:
Plants need water to grow. So if there's water, shouldn't there
be more plants? New research shows it's a lot more complicated
than that.



FULL STORY ========================================================================== Plants need water to grow. So if there's water, shouldn't there be
more plants? New research out of UC Santa Barbara and Mpala Research
Centre in Laikipia, Kenya shows it's a lot more complicated than that.


==========================================================================
"You might think that water sources in arid locations have more plants,"
said lead author Georgia Titcomb, a postdoctoral researcher at UC Santa Barbara. "We found that, in really arid locations, herbivores come
for water, and trample and eat the plants, so there is a lot more bare
ground or very well adapted grazing lawn grasses." But that's not the
case everywhere on the savanna.

The researchers gathered data on watering hole communities over the course
of two years to investigate how herbivore activity affects vegetation on
the savannas of central Kenya. They determined that the impacts depend
a lot on environmental conditions. Their study appears as a cover story
in the journal Ecological Applications.

Titcomb is actually a disease ecologist interested in the transmission
of parasites and pathogens among wildlife and domestic animals. She was
curious how water sources could aggregate animals in ways that promote
disease transmission. She concluded that a key step in understanding
this was to characterize the plant communities around water sources, as
plants are an important vector for many parasites as well as a crucial
resource for the animals themselves. After some preliminary analysis,
Titcomb realized that this research would augment previous herbivore
and plant ecology studies conducted in the area.

The team surveyed 17 pairs of sites a total of four times each over
the course of two years. They paired each water source with a control
site 1 kilometer away that had analogous environmental conditions. The researchers tallied vegetation diversity and height around the watering
holes and at the control sites and identified herbivore activity based
on dung. They also characterized the nutrients and physical composition
of soil from each site.

Fieldwork around the watering holes presented a number of challenges. For example, the researchers also had to accommodate the wildlife. "We had to
stop work when elephants came along, for example, and let them drink from
the watering hole," Titcomb recalled. "It was like, 'alright everyone,
leave the transect line, go get in the car and wait.'" The authors found
the effect of herbivores on the plant communities around these water
sources differed depending on the context. Under favorable conditions
for plants, the diversity of plants increased in proximity to water.

This is likely because herbivores kept species in check that would
otherwise dominate the area. This enabled other plant species to gain a foothold, even if the animals did eat more vegetation in the area overall.



========================================================================== Meanwhile, under more arid conditions, increased herbivore activity around water led to fewer plants and plant species. Titcomb suspects this is
because there simply aren't enough resources for plants to be as resilient
in these areas. Only the hardiest species were capable of withstanding
these conditions and pressures. Generally, these were important grasses
that formed grazing lawns frequented by wildlife and livestock alike.

The findings built upon previous research on these water sources, which
has often been conducted in even more arid regions of the savanna,
Titcomb said.

Prior studies generally found that the areas around water sources were
denuded of plants with depleted plant diversity. This paper extends those findings, showing that these plant communities exist along a spectrum
based on environmental conditions in the area.

Due to the nature of the fieldwork, there were a number of related
factors the authors couldn't untangle in this study. Water collects in particular places, influencing soil composition, nutrients and drainage,
which all also affect plant growth. Water also attracts herbivores, which
in turn affect plant survival when they feed or trample on them. All of
these dynamics are tied together.

Sussing out the independent contributions of each of these factors
would require controlled experiments. Nevertheless, the authors feel
this study is an important first step in understanding these systems.

"This work offers critical insight into how changing patterns of
animal movement and aggregation -- such as the gathering of animals
at watering holes -- can change entire communities and ecosystems,"
explained co-author Hillary Young, an ecology professor at UC Santa
Barbara. "However, it also shows that these changes are hard to predict,
and the size and even the direction of the effect can change based on the
local environmental conditions, especially the climate." "Humans are
changing every part of our environment," Titcomb added. "We're re-
sculpting the landscape, and so having some knowledge of those effects
on plant communities will be important when thinking about management
of wildlife and domestic animals." Humans are not only changing the
physical environment, we're also changing the composition of herbivores
that live in these spaces, she pointed out. For many areas, mixed use
-- such as cattle grazing and conservation -- is the only viable way to
support and protect wildlife and the landscapes they live in. It can be
very challenging to maintain a wildlife conservancy without additional
income, Titcomb explained. These findings can inform efforts to manage
wildlife alongside livestock.

"Work like this -- which considers both changes in environmental
conditions and animal movement -- will be critical to understanding and managing ecosystems in this era of global change," Young said.

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Harrison
Tasoff. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Georgia C. Titcomb, Godfrey Amooni, John Naisikie Mantas, Hillary S.

Young. The effects of herbivore aggregations at water sources on
savanna plants differ across soil and climate gradients. Ecological
Applications, 2021; 31 (7) DOI: 10.1002/eap.2422 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211018130349.htm

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