Risk it for the Biscuit
Environmental controls on African herbivore responses to landscapes of fear (2021) Davies et al., Oikos. https://doi: 10.1111/oik.07559
Despite the incredible variation seen in nature when it comes to flora and fauna, it always seems like the two types that most people know are predators and prey. Prey animals being those that eat plants (or other animals), and the predators being those that eat those prey animals. Because prey animals must not only eat food, but try to avoid becoming food for something else, they must always be on the lookout. This watchfulness and awareness is what creates a “landscape of fear” (See Did You Know?), but variation is inherent to the natural world, and there are likely many things that prey animals consider when they pick where they decide to forage. Today’s authors wanted to investigate what factors influence the prey animals choice of foraging areas, and if that selection varies with the environment during the dry season when there isn’t much food available.
Did You Know: The Landscape of Fear
In ecology we love the term “landscape”. This can be in reference to a literal landscape, such as the African plains in this study, or it can be a more abstract concept. These abstract concepts can be used to define how a certain trait value or phenomenon varies over time and space, which tends to give a topography of sorts, which is why it is called a landscape. The landscape of fear defines a range of perceived threats that prey animals face in their day to day lives. For example, a sheltered, empty cave that is secluded and well away from predator signs would appear to be a relatively safe place, while an open plain with a freshly killed gazelle that smells like lions would appear like a much more dangerous area.
What They Did
The authors utilized data from the Hluhluwe-iMfolozi Park in South Africa. Specifically, they used two prey species types (browsers and grazers) and two predator types (ambush predators and coursers). Browsers consume leaves, bark, and green stems from plants above ground-level, while grazers consume plants matter at the ground-level. Ambush predators sit and wait for their prey, while coursers run down prey over long periods of time and long distances. They predicted that in areas with high quality food the animals would have less of a response to predators because they would prioritize eating the comparatively better food over being killed by a predator.
Prey animal observations were conducted by a park ranger and volunteer working together, the ranger would observe the prey animals and the volunteer would record the relevant information (species of the prey, location of the sighting, and group size). These observation events were conducted over the late dry season, with each event covering between 2 and 11 km, for a total of 2715km worth of observations over the entire study.
The distribution and quality of prey animal food was sampled using scans from an aircraft flying over the park. This allowed the authors to quantify the distribution of various types of plant matter for the prey to consumer. Predator data was collected using information from GPS tracking of the two predator types, and the authors then calculated the relative risk of predator encounter for each of their herbivore observations. To put it simply, they quantified how risky it was for a given prey animal to be in a given area at a given time.
What They Found
The authors found a large amount of variation in the responses of the various species to the risk of predators, with some prey animals having minimal responses to predation risk (and some that did not respond to predation risk at all). However, a general pattern that emerged was that medium-sized prey animals (two browsers and two grazers) continued to forage in areas with an increased risk of predator encounter when high-quality food was available.
The two predator types used in this study were the ambush predator (lions) and the coursing predator (African wild dogs). Kills by these two predator species make up ~80% of all of the prey animal deaths in the park, but other predators such as cheetahs and jaguars also prey on animals. The authors make a point that these predators are not likely to have as large of an impact on the prey animals choice of habitat when they forage, but I would be interested in knowing how these other predators factor in to the landscape of fear.
Today’s paper is a fascinating example of the complexity of the natural world. I don’t think that I am alone when I think that it is a given that a prey animal would choose to avoid a risky area, not matter how good the food may be. But, this study has shown that there is more nuance to this, and that in some situations the prey animals will prioritize eating high quality food, no matter the risk associated with doing so. These results imply that food web dynamics will vary across space and time, leaving plenty of opportunity for further study.
Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is currently working like a madman to finish his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.