Image Credit: hbieser, Pixabay Licence, Image Cropped

Introduced herbivores restore Late Pleistocene ecological functions (2020) Lundgren et al., PNAS, https://doi.org/10.1073/pnas.1915769117

The Crux

The fauna of the Pleistocene (also known as the Ice Age) was not that dissimilar to the communities of animals which inhabit our planet now. However, many more large land mammals inhabited all kinds of ecosystems. By the end of the Pleistocene, many of them were extinct, mainly due to climate change impacts (glaciers got larger and restricted their ragne) and prehistoric human impacts like over-hunting, habitat alteration, and introduction of new diseases. The decline of large-bodied herbivores in the Late Pleistocene (LP from here on) led to many ecological changes including reduced nutrient cycling and dispersal, reduced primary productivity, increased wildfire frequency and intensity, and altered vegetation structure. These changes have become our norm.

Scientists usually study species introduction under the premise that they are ecologically novel. However, the introduction of large herbivores has been found to drive changes in the environment, potentially restoring or introducing novel ecological functions similar to pre-extinction Late Pleistocene conditions. This week’s researchers wanted to investigate what sort of role introduced mammals played in restoring ecological interactions by investigating their functional similarity with LP species.

What They Did

The authors created a database using species distribution maps of herbivorous mammals weighing more than 10 kilograms under three categories; LP species, species only present in their native ranges (native–only), and species that were present in their native ranges but also classified as alien elsewhere (inclusives). They considered species distributions since the last interglacial period (~ 130.000 years) and at the continental scale to represent migration that was potentially possible.

Each species was characterized by a number of species traits, which when combined represent their ecological role in the ecosystem: body mass, diet, the way they digest their food (fermentation efficiency), habitat type, and limb morphology. Using Principal Coordinates Analysis (PCoA), they studied changes in trait combinations between species, describing how herbivores, extinct or not, differ from each other. Using this information, they calculated the number and variety of traits a species contributes to an ecosystem, as well as the functional dissimilarity between species. This allowed them to analyze whether or not assemblages of inclusive species were more or less similar to LP assemblages than native-only ones. They also investigated whether introduced species are more closely related (in terms of their traits) to native or extinct species by identifying their nearest neighbor on the PCoA chart. Unique combinations of a species body mass and their diet were used to functionally classify introduced herbivores and to assess whether they restore key metabolic functions in their ecosystem.

Did You Know: Cultural Evolution

While humans are currently causing a large mass extinction event, we have had a considerable effect on other species for hundreds of thousands of year now, ever since Homo erectus left Africa. So why have we had such an effect of wildlife, and why haven’t they evolved to cope with us, as they have with other predators and competitors? One reason is that while biological evolution can be a relatively slow process, cultural evolution is generally a much faster one. Humans underwent cultural evolution, the results of which (tools, organised hunting, etc.) allowed us to have a much quicker effect on species than other predators that came before us.

What They Found

160 of 427 (35%) of all herbivore species over 10 kilograms went extinct in the LP, the majority in North America, followed by South America, Australia, and Europe. In contrast, 33 herbivore species were introduced into new continents, replacing lost species richness by 27% in South America, and about 50% in all other continents. Body mass and diet accounted for the biggest differences between species assemblages in terms of traits, followed by fermentation efficiency. LP extinctions thus led to a shrinkage of trait space, leading to a shift to smaller-bodied herbivores with more efficient fermentation strategies.

Functional richness also reduced significantly due to LP extinctions (62% globally), especially in Australia. Introduced herbivores, on the other hand, replaced lost richness by an average of 39% globally, whereas in Australia it even surpassed the historic values (FIGURE; if not more details). This means that introductions have restored functional richness to a certain level, the question now is if the restored traits are similar to the ones lost. Results show that assemblages including introduced species are 13-39% more similar then native-only assemblages, however especially in Australia traits of introduced species are novel compared to LP assemblages. This turnover of traits is related to the unique marsupial dominated fauna in Australia, while introduced herbivores are mainly ruminants and large grazers.

Here you can see a collection of species that went extinct in various continents during the late Pleistocene, and their closest replacements in terms of their ecological function (Image taken from paper)

Problems

The authors show that certain traits have been restored by introduced species. However, many important traits for extinct taxa, such as sociality, movement patterns, foraging behavior, and vulnerability to predators remain unknown. These traits could potentially affect similarity with LP species and shape how introduced herbivores interact with native species. Furthermore, other ecological contexts like predation and landscape connectivity play an important role in how herbivores affect their ecosystem. While introductions make species assemblages more functionally similar to LP, they do not necessarily restore ecosystems. For example, restricted movement due to habitat fragmentation could lead to increased browsing pressure.

So What?

Human-caused introductions of species are primarily perceived as environmental harm. The authors, however, showed that introduction can counteract global patterns of human-caused extinction by replacing lost functional richness. They demonstrated that the majority of introduced herbivores are similar to extinct species and restore lost functions, especially in continents most impacted by LP extinctions. This restoration has the potential to affect ecosystems. For example, large-bodied browsers in Australia were severely impacted by species extinctions, whereas their restoration may reduce shrub cover, promote grasslands, and reduce wildfire intensity. It is important to note though, that while introduced taxa can replace lost functions, they can also introduce new, and potentially unwelcome ones into ecosystems.

The influence of humans on our ecosystems started thousands of years ago. Thus, the current low diversity in large mammals in many continental areas could be considered as an anthropogenic phenomenon, not a natural one, with important implications for nature management. By considering introduced species inherently as harmful, we potentially overlook important ecological functions that they might have reintroduced in the ecosystem. This indicates that shifting focus from the eradication of introduced species to landscape and predator protection may have broader biodiversity benefits.

Julia Ramsauer is a landscape ecologist currently working on the integration of ecosystem services in the Mediterranean region. You can see here recent work on Ecology for the Masses on her profile at this link, or to listen to the latest episode of her podcast, Environmental Science Careers, you can follow her on Twitter here.