Life on the Black Cotton

Mpala Ranch has two main soil types–black cotton and red soil–which host vastly different plant communities. Our research takes place on the black cotton soil, where the plant community is dominated by one tree species, the whistling thorn tree (Acacia drepanolobum). Acacias in general are known for having thorny stems and hosting mutualistic ant colonies. The whistling thorn trees have many long, straight spines on the branches, some of which have a swollen, hollow base, which ants can live inside. They also secrete nectar from the base of their leaves that ants can eat. In exchange, the ants may defend the tree against herbivores ranging from other insects to elephants.

The whistling thorn gets its name from the sound the wind makes when it blows through the holes in the hollow thorns. Other trees grown on the black cotton soil, including other acacia species, but the whistling thorn trees dominate. Underneath them grows a variety of grasses.

Many different animals use the black cotton, including wildlife (elephants, giraffe, antelope, buffalo, cheetah, lions), domestic herbivores (cattle, goats, sheep, camel), reptiles, birds, rodents, and insects. Each species affects the plant community in different ways. Grazers eat the grasses, often singling out their preferred species. Browsers pull leaves from the trees. Wood-boring beetles carve tunnels through the tree stems. Elephants will even rip up or knock over full-sized trees, possibly just because just because they find it fun.

These photos were all taken during the dry season, when the grass is brown and the trees drop most of their leaves. In a month or two it will all be vibrantly green again.

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