By Bianca T. Esposito, NWNL Research Intern
(Edited by Alison M. Jones, NWNL Director)
NWNL research intern Bianca T. Esposito is a senior at Syracuse University studying Biology and minoring in Economics. Her research this summer is on the intertwined relationships of biodiversity and our water resources. This is Bianca’s second blog on Biodiversity for NWNL. Read her first blog on wild Salmon here.
This blog compares how water impacts the health of sub-Sahara’s Cape buffalo populations to how North America’s bison impact the health of our water resources. This investigation covers three of our NWNL case study watersheds: Africa’s Mara and Nile River Basins, and North America’s Mississippi River Basin.
The Cape buffalo (Syncerus caffer caffer) is found in Kenya’s Mara River Basin savanna and Uganda’s Nile River Basin plains. The bison (Bison bison) used to dominate the Mississippi River Basin’s Great Plains and are still there in scattered small populations. Both species are large, herbivorous mammals that primarily graze on tall-grass ecosystems. However, their habitats and connections to water differ significantly.
Africa’s Cape buffalo migrate seasonally in large herds on cyclical routes dependent on fluctuations in water availability. They move out of areas with limited resources and into areas where moisture and nutrients are available. Cape buffalo also migrate away from their habitat when water levels increase, since flooding restricts their foraging abilities. In these cases, Cape buffalo move to a drier habitat where, in turn, they may experience drought. Either way, when resources become low, their vulnerability becomes high.
A lone Cape Buffalo bull in Kenya’s Mara Conservancy (© Alison M. Jones)
Africa’s famed Serengeti-Mara Ecosystem is located throughout northern Tanzania and extends into Kenya. Much of this region is situated within the Mara River Basin. In the Serengeti National Park, the migration pattern of the Cape buffalo, similar to that of the wildebeest-zebra migration, is dependent on the fluctuation of rainfall each year. Generally, this journey begins in April when Cape buffalo depart their southern plains habitat to head north. This movement is triggered by the onset of heavy rain that floods the plains, reducing the Cape buffalo’s ability to graze. By May the herd is in the northwest Serengeti, where the dry season lasts through July and proximity to the equator allows rainfall to be more evenly distributed, allowing greater opportunities for foraging. Then, in August, the late dry season hits, causing the herd to move further north. On their venture north, they cross the Mara River into Kenya’s Maasai Mara National Reserve. The Cape buffalo remain here enjoying green pastures until November, albeit subject to drought if there’s no rainfall. In December, usually the first rainfall comes which they sense as the onset of the rainy season. They then trek back into Tanzania’s southern plains for the wet season. From January to April, they graze there on plentiful, nutritious grasses.
Herd of Cape buffalo in Kenya’s Mara Conservancy (Creative Commons)
When Cape buffalo inhabit dry lands their reproductive success (also referred to as “recruitment ability”) decreases; but their body condition improves due to what seems to be a fat-storing mechanism that anticipates limited future resources. One benefit of Cape buffalo having to cope with drought is that when food supplies are reduced, they forage through peat layers in dried-up underground channels, releasing nutrients otherwise trapped below ground.
A current major concern for this species is that anthropogenic factors (human activity) causing climate change are expected to increase both water levels and drought, which could push the Cape buffalo outside of their protected areas. In 2017, the Serengeti experienced a drought that lasted over a year causing declines in populations of many species, including Cape buffalo. Drought also causes herds of cattle, goats and sheep outside to enter protected lands to graze, creating a competition for resources between wildlife, livestock and humans in both the Maasai Mara National Reserve and Serengeti National Park. If the Mara River – the only major river in the area – dries up, there would be few resources for ungulates. As well, when droughts end, there is always potential for flash-floods which deter herds from crossing rivers to find greener pastures.
A lone Cape Buffalo bull in Kenya (© Alison M. Jones)
When water is scarce in the Serengeti, a decline of Cape buffalo leads to increased lion mortality. When Cape buffalo lack sufficient food due to drought, they become weak and must travel increased distances to quench their thirst. This leaves the herd fatigued, causing some members to fall behind and thus become more vulnerable to predation. Also, after a drought and the rains begin, Babesia-carrying ticks infect Cape buffalo. Infected buffalo become weak or die, allowing easy predation by lions. Unfortunately, their carcasses transfer babesiosis disease to lions. Alone, this disease is not fatal to the lion. However, babesiosis coupled with canine distemper virus (CDV) is lethal.
Babesiosis from Cape buffalo has caused two major declines in Serengeti lion populations. In 1994, a third of the lion population was lost due to this combination, killing over 1,000 lions.
Lions taking down a Cape buffalo (Creative Commons)
On a smaller scale, in 2001 the Ngorongoro Crater lion population also lost about 100 lions due to this synchronization of disease. Craig Packer, a University of Minnesota biologist, stated, “Should drought occur in the future at the same time as lions are exposed to masses of Babesia-carrying ticks—and there is a synchronous CDV epidemic–lions will once again suffer very high mortality.” He also warns that extreme weather due to climate change puts species at greater risk to diseases not considered a major threat before. Fortunately, mud-wallowing that Cape buffalo use to cool down their bodies is also an effective shield against infiltrating bugs and ticks once the mud dries.
Overall, Cape buffalo rely heavily on rainfall patterns; but climate change is disrupting traditional migratory patterns by raising water levels or causing drought. Both extremes present negative impacts to the Mara River Basin and the biodiversity that inhabits it.
North America’s bison – a bovine counterpart to African Cape buffalo – historically occupied The Great Plains west of the Mississippi River. Early settlers recorded 10 to 60 million bison openly roaming the fields. Like Cape buffalo, bison also migrate in search of food. Their migration paths used to cover vast territory, thus paving the way for many current roads and railroads. A major threat to bison – as with most species – has been habitat loss due to human infringement, as well as well-documented, extensive hunting by new settlers heading west. By 1889, only approximately 1,000 bison remained in North America.
Farmed bison in Texas (© Alison M. Jones)
Due to recent conservation efforts, bison populations are rising; however, not to past numbers. Currently, they are found only in National Parks, refuges and farms. As of 2017, approximately 31,000 pure wild bison remain in 68 conservation herds. “Pure wild bison” are those not bred with cattle for domestication. However, only approximately 18,000 of the remaining population “function” as wild bison. This count excludes very small bison herds used for research, education and public viewing – or bison held in captivity waiting to be culled by protected areas such as Yellowstone National Park due to required limits.
Bison inhabiting the Mississippi River Basin, which drains throughout the Great Plains, have many positive impacts on its waterways and tributaries. Yellowstone Park, where the Yellowstone River drains into the Missouri-Mississippi River system, is the only place in North America where bison continue to freely roam as they used to. In Yellowstone, bison occupy the central and northern area of the park where they migrate by elevation, seasonally choosing food according to abundance, rather than quality. In the winter, they select lower elevations near thermal hot springs or rivers where there is less snow accumulation.
Bison positively affect water supplies when they wallow and paw at the ground. This results in intense soil compaction that creates soil depressions in grasslands. After many years, this soil depression tends to erode since bison don’t like to wallow on previously-created depressions. However, during the rainy season, wetland plants and vegetation grow in these wallows created by bison dust-bathing and trampling. For a short time many species enjoy these ephemeral pool habitats before they disappear in droughts or floods. Meanwhile bison wallows increase species diversity that would otherwise not be present in grasslands.
Bison rolling around in a dry wallow (Creative Commons)
Bison have other positive impacts on water. As they trample through streams, they widen available habitat and alter water quality. Even after a bison dies, it can still contribute to the health of its ecosystem. Their carcasses are a nutritious food source for wolves, coyotes and crows. Studies suggest that bison carcasses take roughly seven years to fully decompose, during which time their remains release nutrients such as phosphorus and carbon into rivers. These nutrients sustain microbes, insects, fish and large scavengers of the area. A bison carcass can also provide sustenance for local fish since maggots, green algae and bacteria grow over their bones during decomposition. Bison carcasses also deposit nutrients into the soil which fertilizes plant regrowth.
Bison can negatively affect water resources, by decreasing native plant diversity due to overgrazing. However, they graze on only grass, which allows forbs (non-woody flowering plants) to flourish, adding biodiversity in grasslands. As well, when bison urinate, they deposit nitrogen into the soil, a key nutrient for grass growth and survival. Their urine also becomes a selectable marker allowing them to return to formerly-grazed pastures during the season. This constant reselection of grassland, allows combustion in ignored, non-grazed pastures, since fire tends to occur in tall grass with nitrogen loss. After fires, the bison are attracted to newly-burned watersheds because of C4-dominated grass which grows in dry environments. Bison select C4-dominated grassy areas because they have low plant diversity, unlike less-frequently burned sites where forbs are abundant. Thus, bison’s pasture preferences allow for more biodiversity, creating healthier watersheds.
Mural near of Native Americans on bison near Masterson, Texas (© Alison M. Jones)
Each of these two similar bovine species have significant, but different, relationships to water availability and quality within their river basins. The African Cape buffalo migration is guided by water fluctuations. This could impact their future since anthropogenically-caused climate change could incur longer and more frequent droughts and increased flood-water levels to an extent that would drive Cape buffalo out of their protected habitats. In contrast, North American bison herds improve the health of waterways in the Mississippi River Basin in several ways. Nutrients from their decomposing carcasses add to the health of tributary streams and rivers; and their mud wallows support greater diversity of wetland and grassland flora.
Whether we look at watersheds in Africa or North America, it is clear that it is as important to study how biodiversity is affected by water availability, as how watershed water quality and quantity affects its biodiversity. Any changes to these ecosystems due to climate change could drastically affect the biodiversity and health of these watersheds.