Washington: According to new research, loss of megaherbivores such as
elephants and hippos could allow woody plants, non-grassy herbs and
flowering plants to encroach the grasslands in African national parks.
The study used isotopes in hippopotamus teeth to find a shift in the
diet of hippos over the course of a decade in Uganda’s Queen Elizabeth
National Park following widespread elephant poaching in the 1970s.
Study first author Kendra Chritz said that her method of using hippo
enamel isotopes could help scientists reconstruct past changes in
vegetation in Africa’s national parks, areas with relatively little
ongoing scientific observation.
The results could give ecologists an idea of what could happen to
Africa’s grasslands if elephants, whose populations are steeply
declining, went extinct or reached near-extinction.
“We have a window into what these environments could look like without
megaherbivores, and it’s kind of grim,” Chritz says.
Lowland tropical grasses, such as those in elephant ecosystems, are
part of the C4 class of plants, a reference to the enzyme used to
process carbon dioxide into sugars during photosynthesis.
Corn and sugarcane are also C4 plants. C3 plants, which use a
different enzyme, include trees, shrubs, flowering plants and herbs.
C3 plants compete for resources with C4 grasses in African savannas,
including sunlight.
Elephants and other megaherbivores help keep woody plant encroachment
in check by browsing seasonally on shrubs and trees. But without that
herbivore control, C3 plants can advance on grasslands unimpeded.
The presence of shrubs and trees, which can be seen in aerial
photographs, give only a partial picture of the balance of power
between C3 and C4 plants.
Observing herbs and flowering plants requires ground-level
observation, and records of such observations in many national parks
in Africa, is sparse.
The two plant groups’ metabolic processes treat isotopes of carbon
differently, so that C4 plants have a higher proportion of heavy
carbon isotopes than C3 plants.
As animals, such as hippos, eat plants, and the isotopic signatures of
the plants in their diet are incorporated into the animals’ bodies and
preserved in durable tissue, such as teeth.
The hippos of Queen Elizabeth National Park, Chritz found, had been
indirectly “observing” the plant makeup of the grasslands all along.
Hippo teeth are not easy to come by, Chritz said.
The teeth haven’t been used before for isotope analysis due to the
difficulty of obtaining samples from a wide range of time periods.
One of Chritz’s co-authors, Hans Klingel conducted important research
on the behavior of hippos in Queen Elizabeth National Park in the
1980s and 90s.
He contributed teeth from the 1960s, pre-poaching, and from 2000.
Chritz and her colleagues sampled enamel every centimeter along the
length of each tooth, using known growth rates to correlate sections
of the tooth to different years.
All of these samples taken together recorded diet history from
approximately the last decade of the animal’s life.
But analysis showed that the two teeth displayed very different
isotopic signatures. Chritz needed a third sample, in between the two
time periods.
While in Uganda in 2013, Chritz approached the current game warden
about obtaining another hippo tooth.
The warden showed her a skeleton on display at a park museum from a
hippo that died in 1991 – perfectly within the time range Chritz had
aimed for.
Results showed that the 1960s hippo ate approximately 80 percent C4
plants, and that the percentage of C4 in the later hippos’ diets had
dropped to around 65 percent.
This showed that, within a time scale of only a few decades, C3 plant
encroachment had progressed enough to significantly impact the diets
of the animals in the park.
The results also showed the validity of Chritz’s method and
reconstructed the progression of vegetation changes since the 1960s.
Another surprise in the results was the proportion of C3 plants in the
later hippos’ diets. Initial studies of hippo diets surmised that
hippos only ate grass.
“And few researchers have suggested otherwise. It appears that they’re
actually quite flexible in their diets and adaptable to environmental
change,” Chritz said.
The clear implication from Chritz’s work is that the loss of elephants
and other megaherbivores can lead to rapid environmental and
ecological change.
“We’ve built a record that shows just how drastic the loss of
megaherbivores in a park can be on a very short timescale,” she said
Adding, “Within ten years, we see a big change in what’s happening in
this once diverse grassy area of the park. This is a window into the
future of what could to happen in East African savannas as elephants
continue to be poached at the currently unprecedented rate.”
The study was published in Scientific Reports.