Keystone species

Pamushana | April 2020

Now, during these lockdown days of few game drives and sightings, it gives us an opportunity to consider some of the broader concepts of ecology and conservation. Here is an overview of keystone species and their importance to our area and its conservation.

What is a keystone species?

keystone species is any organism, from animals and plants to bacteria and fungi that, simply put, is the glue that holds a habitat together. If a keystone species is removed, it sets off a chain of events that turns the structure and biodiversity of its habitat into something different. Although all of an ecosystem’s many components are intricately linked, keystones are the living things that play a pivotal role in how their ecosystem functions.

Why are keystone species so important?

Intact ecosystems perform many vital functions; air and water purification, turning decaying matter into nutrients, preventing erosion and flooding, and moderating climate. Some ecosystems might not be able to adapt to environmental changes if their keystone species disappeared. The population of another species could explode, pushing out other organisms and reducing species diversity. This domino effect is known as a trophic cascade. The removal of a keystone species could allow an invasive species to take over and dramatically change the ecosystem or bring about its complete end.

What kinds of keystone species are there here at Singita Pamushana, Zimbabwe?

Scientists may debate which plants or animals in a particular ecosystem deserve the keystone title due to various complexities, but they are a way to help us understand just how important one species can be to the survival of many others.

Keystone species fall into broad categories:

Predator

By keeping the populations and range of their prey in check, keystone predators, impact other predators as well as other animal and plant species further down the food chain. Examples include lions.

Lions keep prey populations under control and remove sick, weak or genetically compromised animals from the system. Buffalo and zebra are grazers, and without lions, or other large predators to keep numbers in check, overgrazing may occur. Without vegetation to keep the soil in place, the area could become severely eroded.

Prey

Keystone prey serve as a critical food source for predator populations. They are resilient creatures, unlike some other types of prey species that are more susceptible to becoming rare or extinct within an ecosystem. Examples include impala.

Impala are the main prey for wild dogs. If all the impala from our area were removed the wild dog packs would be forced to hunt other prey. They might not be as effective at this and this could lead to starvation, disease and death for the wild dogs. Other predators that prey on impala would suffer too. With apex predators removed from a system there would soon be an imbalance between predator and all prey species.

Ecosystem engineer

An ecosystem engineer is an organism that creates, modifies, maintains, changes or destroys a habitat. They influence the prevalence and activities of other organisms and help define the overall biodiversity of their habitat. Examples include elephants and white rhinos:

Elephants push down, break and uproot vast quantities of small trees and shrubs that would otherwise convert to forest or scrubland. They are ecosystem engineers that preserve open spaces where grasses can thrive. This vegetation supports other herbivores like impala, wildebeest and zebras. In turn, these prey species feed carnivores such as lions, hyenas and cheetahs. It also provides warm, dry soil for smaller animals like mice and shrews to burrow into. Via their dung, elephants also spread plant seeds to new areas – some plants have evolved to the point where they germinate more easily after passing through an elephant’s digestive tract.

White rhinos maintain areas of short grass but if removed from an area the grass is able to grow higher. Other species of grazers are not as effective at keeping the grass short. If there were no white rhinos in our area it would affect fires by increasing their fuel loads and fuel continuity. There would be larger, less patchy fires which could be detrimental.

Mutualists

Mutualists are two or more species in an ecosystem that interact for each other’s benefit. Examples include bees:

Bees take the nectar from flowers, they collect pollen and spread it from one flower to the next, enhancing the odds of fertilization. Nectar and pollen are also the primary food sources for the bees themselves. Without bees the flora would not thrive, nor would the herbivores that survive on it. Without pollen and nectar the bees would die off from the lack of a food source.

Plants

Keystone plants are those that provide a critical source of food and/or shelter for other species. Examples include baobab trees.

Baobabs support the presence of many species and where they live, including bee hives and bird nests, especially vulture nests as vultures prefer to nest in the high branches of the tallest trees. Without vultures, another keystone species, carcasses from kills would not be consumed quickly and efficiently, and this could lead to other scavengers exploiting the food source with catastrophic effects, or the undisposed carcasses potentially becoming a breeding ground for diseases.

All the above examples describe what happens if a keystone species is significantly reduced or removed, but the consequences are just as catastrophic if a keystone species becomes overabundant. An example of this would be too many elephants in a fenced reserve or fragmented national park where they are not able to migrate to different parts of southern Africa in their continual search for preferred nourishment. Over time their impact on their contained environment will become too intense for it to recover, and the ecosystem will be destroyed.

Conservation and ecological management is such an intricate science and it’s a delicate balancing act to maintain such a fascinating and highly complex natural system.