For millions of years, a variety of large herbivores, or megafauna, influenced terrestrial ecosystems. Among many others, these included elephants in Europe, giant wombats in Australia, and ground sloths in South America. However, these animals experienced a wave of extinctions coinciding with the worldwide expansion of humans, leading to dramatic but still not fully understood changes in ecosystems. Even the survivors of these extinctions strongly declined, and many are currently threatened with extinction.
While there are many case studies as well as theories about the effects of large animals, formal attempts to quantitatively synthesize their effects and establish generality have been lacking.
A new study, conducted by an international team led by researchers from Aarhus University and the University of Göttingen, published in Nature Ecology & Evolution, has gathered numerous individual case studies and analyzed the findings. They show that large animals have a variety of generalizable impacts – impacts that are likely missing from most of today's ecosystems.
The impact of large animals on ecosystems
Among the identified general impacts of large wild herbivores are
- shifts in soil and plant nutrients
- the promotion of open and semi-open vegetation
- the regulation of the population of smaller animals
Moreover, one of the key findings of the studies is that megafauna promote ecosystem diversity by increasing the structural variability in the vegetation.
"The positive impact on variability in vegetation structure is particularly noteworthy, given that environmental heterogeneity is known as a universal driver of biodiversity. While our study mostly looked at the impact of megafauna on small scales, our findings suggest that they promote biodiversity even on the landscape level," says PhD student at Aarhus University Jonas Trepel, who led the study.
Large herbivores change vegetation structure by consuming biomass, breaking woody plants, and trampling smaller plants – impacts that are hypothesized to depend on the animal's body size. Given that the analyzed dataset spanned two magnitudes of body size (45-4500 kg), the researchers were able to test specifically how this important trait shapes the impact of large animals. They found, for example, that megafauna communities which include larger herbivores tend to have positive effects on local plant diversity, while communities composed of smaller species (e.g.
"Large herbivores can eat lower-quality food such as branches and stems, which may result in proportionally greater impacts on dominant plant species and thus give less competitive plants better odds in their struggle for sunlight and space," explains Erick Lundgren, one of the senior authors of the study.
Assistant professor Elizabeth le Roux, who is also one of the senior authors, adds:
"These findings support the expectation that many small herbivores cannot fully compensate for the loss of a few large ones."
The benefits of a meta-analysis
This study is a so-called meta-analysis. This means that the researchers have analyzed data from all available studies on the subject in order to find general patterns. Meta-analyses are especially powerful in their conclusions because they draw on big data pools and make it possible to draw conclusions that go beyond a local context.
While many recent ecological studies have shown or hypothesized the importance of large animals in ecosystems, according to senior author Jens-Christian Svenning, the meta-analytical study is an important step forward by synthesizing direct experimental and semi-experimental evidence from across the globe to assess the generality of these effects quantitatively.
"This global meta-analysis shows that large herbivores have important general effects on ecosystems and their biodiversity," explains professor Jens-Christian Svenning, continuing: "Importantly, our analysis shows that these effects cut across a broad range of ecologically important phenomena, from soil conditions to vegetation structure to plant and animal species composition, affecting not only their general state but also their variation across landscapes."
Jens-Christian Svenning is the director of Center for Ecological Dynamics in a Novel Biosphere (ECONOVO), a Danish National Research Foundation center of excellence at Aarhus University.
Fact box: How did the researchers get these results?
A key aspect of the 297 studies, including 5,990 individual data points, is that the researchers compare adjacent areas with clear differences in the megafauna community (i.e. megafauna present or absent) due to known reasons. The vast majority of studies in the data set are so-called exclosure studies, in which some parts of a field site are fenced up to prevent large animals from entering. By comparing different plots inside and outside of the fences, researchers are then able to assess in which ways megafauna impact the ecosystem.
Importance of ecosystem biodiversity in responding to global change
The identified general importance of large herbivores for ecosystem functioning implies that important functions are missing due to the loss of wild megafauna. This may affect the approach to nature conservation and ecosystem restoration.
"The majority of today's protected areas are missing large animals – and thus also an important range of functions. So even areas we consider to be pristine ecosystems are probably not as natural as we may think. Reintroducing large animals could be a key avenue to make these areas a bit more dynamic and used to disturbances," says Jonas Trepel and continues:
"By increasing the structural variability in an ecosystem, large animals may provide refuges, for example during extreme weather events, but also open up more available niches for other species. This could prevent one or a few species from dominating and allows species with similar ecological attributes to coexist – which in turn would make the ecosystem more resilient. Ultimately, that may help them to deal with the consequences of global change."
Given the important functions that large animals have on ecosystems and their biodiversity, the researchers conclude that it is crucial to not just protect the few remaining megafauna species, but also to reestablish megafauna populations as part of restoration efforts to achieve positive outcomes for Earth's biosphere, not least under the increasingly unprecedented global environmental conditions.
