The assumption that the numbers of smaller predators will decline when large predator populations rise, is not necessarily valid. For mammals in Europe, this relationship appears to exist only in specific cases. Utrecht biologists Thomas van Schaik, Marijke van Kuijk , and Liesbeth Sterck conclude this based on a literature review they conducted, which was recently published in the scientific journal Mammal Review. As a result, the researchers advise conservation practitioners not to rely solely on this commonly proposed theory when assessing the consequences of growing predator populations.
If you are living in the Netherlands, it will not have gone unnoticed: wolves have made a comeback in the country, and their numbers are growing. The return of the wolf is a good example of a remarkable trend seen in both Europe and North America: while in many parts of the world large predators are typically facing adverse conditions, European and American populations are growing. In addition to wolves, the golden jackal, a species not previously found in the Netherlands, has been occasionally spotted in the country since 2016. Furthermore, the lynx, once native to the Netherlands, has been found to live increasingly closer to the country’s borders.
Consequences
Van Schaik, who conducted the literature review as a bachelor’s thesis, indicates that the impact of the rise of such large predators on other predator species in Europe remains largely unclear. Van Schaik: "Studies that look into this question do exist, but they generally focus on one specific area and on a limited number of predator species. We set out to gather as many scientific papers as possible that explored the relationship between the population sizes and distribution of top predators in Europe and the population sizes and distribution of smaller predators. Smaller predators may also adapt their behaviour to deal with the presence of larger predators, so we included studies that looked at behaviour as well. By synthesizing all these papers, we tried to identify broader patterns."
All kinds of relationships
The researchers identified 47 studies that contained information on 38 specific pairs of larger and smaller predators, such as the wolf and the fox, and the badger and the stone marten. Van Schaik: "It is often assumed that a decrease in top predators leads to an increase in smaller predators. Conversely, it is expected that an increase in top predators would result in a decrease in smaller predators. However, we found clear evidence for this relationship in only 10 out of the 38 pairs. The feeding behaviour of the smaller predator turned out to influence this outcome: in these pairs, the smaller predator was often a true hunter and not a scavenger. In the remaining pairs, evidence for a negative relationship between the numbers of larger and smaller predators was limited, conflicting, or absent altogether. Moreover, we also observed cases where the correlation was actually positive, meaning that higher numbers of top predators were associated with higher numbers of smaller predators."
When studying these dynamics, it is important to closely examine the entire predator community.
Thomas van Schaik
Clashes
So how can an increase in top predators negatively impact predators lower on the food chain? Van Schaik: "The top predators might hunt the same prey as the smaller predators, or they steal prey from them. In some cases, larger predators may even kill smaller predators directly. Additionally, the presence of larger predators might cause smaller predators to avoid certain areas to avert confrontation, which could reduce their access to their favourite prey, for example."
However, top predators can also have a positive impact on smaller predators. Van Schaik: "Larger predators leave behind carcasses, which smaller scavengers can feed on. And sometimes, smaller predators may be negatively impacted by a third, medium-sized predator. If this medium-sized predator is subsequently suppressed by a top predator, this can actually benefit the smaller predators."
Complex interactions
The inspiration for the literature review conducted by the Utrecht researchers stemmed from an earlier study, which revealed that diminishing populations of top predators in North America do not always correlate with an increase in populations of smaller predators. Van Schaik states that both studies show that researchers should not just assume that larger predators suppress smaller ones. Van Schaik: "Predator interactions are often complex, with multiple species influencing each other. When studying these dynamics, it is important to closely examine the entire predator community. Additionally, the timeframe of observation is relevant. For instance, smaller predators may initially alter their behaviour in response to the presence of larger numbers of top predators in their area. However, if this is not sufficiently effective, their numbers may still decline. Therefore, the time elapsed since the change in the large predator population can impact the conclusions you draw."
Publication
Understanding mesopredator responses to changes in apex predator populations in Europe: implications for the mesopredator release hypothesis
Thomas van Schaik, Marijke van Kuijk, Elisabeth H. M. Sterck Mammal Review, 22 May 2024. DOI: https://doi.org/10.1111/mam.12357