Our story begins in Patagonia, a region of South America spanning the Andes mountains, the Strait of Magellan, the Tierra del Fuego, and five million hectares of forests, wetlands, glaciers, and grasslands. Unsurprisingly, given the variety of habitats encompassed by Patagonia, the area is also home to astounding biodiversity, including elephant seals, the Andean cat, and endangered herbivores such as the guanacos and choiques. In the early 1900s, European colonizers introduced non-native herbivores like the fallow deer, the red deer and cows, which have grown in unchecked populations and now threaten to decimate native plant and animal populations.
The Maqui berry shrub (Aristotelia chilensis) has been used for centuries by the native Mapuche people of Patagonia as a medicinal plant. The health food industry promotes the Maqui berry as an essential antioxidant, and you could probably find it in your local organic co-op as an anti-inflammatory.
Two species depend on the Maqui berry for survival: the White-crested Elaenia and mistletoe. The Elaenia (Elaenia albiceps) eat the Maqui berries and disperse the seeds – shrub produces berries, bird eats berries, bird deposits seeds from berries elsewhere and propagates the Maqui shrub species.
Mistletoe grows on the branches of the Maqui berry shrubs. Also, look at that picture below — Dr. Seuss couldn’t have imagined a better plant. Mistletoe are parasitic plants that steal nutrients and water from their host plant, but they’re not all bad. Recent research suggests many other animal and plant species depend on mistletoe for survival. Our mistletoe, Tristerix corymbosus, plays this keystone role in Patagonian forests.
Now, we know that the berry shrub supports the Elaenia and the mistletoe. The mistletoe, in turn, supports two species. The first is a hummingbird, the Green-backed Firecrown. The hummingbird (Sephanoides sephanoides) survives on the nectar of the mistletoe, while at the same time pollinating the mistletoe and ensuring its survival.
The second species supported by the mistletoe is a mouse-like animal, locally known as the monito del monte, or “little bush monkey”.
The monito del monte is more closely related to Australian marsupials than to any American species. This American marsupial has been so successful because it has cultivated a beneficial relationship with our mistletoe over the past 70 million years. The mistletoe provide fruit for the marsupial, and in return the marsupial deposits the seeds of the mistletoe onto other Maqui berry shrub branches. The monito del monte also make their nests in the crevices of our Maqui berry shrubs, thus making the monito del monte, the green-backed Firecrown, the mistletoe, and the white-crested Elaenia all dependent on the Maqui berry shrubs that are being grazed to the ground by non-native European deer and cows.
Researchers from the University of Tennessee in Knoxville and the Universidad del Comahue in Argentina, led by Mariano Rodriguez-Cabal and M. Noelia Barrios-Garcia, have worked in the Patagonian forests for years, studying the effect of non-native species like the European deer on native plants and animals. However, field experiments proving that non-native species are directly responsible for the loss of native plants and animals are scarce, because food webs are incredibly complex and difficult to quantify. Here, Rodriguez-Cabal et al. saw that the European deer were grazing on the Maqui berry shrubs in such large numbers that it might be possible to see in real-time the disassembly of a complex food web. The grazing of the Maqui berry shrub by the fallow deer could decrease the population numbers of the mistletoe, the hummingbird, the white-crested Elaenia, and the monito del monte.
Rodriguez-Cabal and Barrios-Garcia, with Argentinian collaborators Guillermo Amico and Marcelo Aizen, designed a simple experiment that required a lot of work. At 26 sites in Argentinian national parks, they built fences to surround some Maqui berry shrubs, and at other sites, they let the deer and cow graze on the shrubs. Then, over two summers, they counted the number of mistletoe, hummingbirds and monito del monte around the sites. Their research was not without setbacks. Ecological work is often derailed by the whims of Mother Nature. Halfway through the experiment, the eruption of the volcano Puyehue-Cordon Caulle ruined eight sites with heavy ash, and the researchers had to find eight new sites.
At sites where the European deer were allowed to graze, the researchers saw fewer hummingbirds and monito del monte. However, the Elaenia was more impacted by another non-native animal, the German wasp. The German wasp (Vespula germanica) was introduced to South American in the 1980s, much more recently than the introduction of the European plant-grazers. These wasps are notorious invaders that eat the fruit of native shrubs without dispersing the seeds to new locations, which disrupts the mutualistic association between the shrubs and native birds and mammals.
Within the grazed sites, the researchers temporarily increased the amount of wasps by baiting them with sugar and meat, to see how the wasps affected the presence of the Elaenia and the monito del monte. As expected, the wasps ate the Maqui berries and left none for the Elaenia and the monito del monte. Subsequently, the Elaenia and monito del monte were not as easy to find in the sites where the wasps were drawn to. This also has negative implications for the Maqui berry shrub, which lose leaves to the deer and cow and lose seeds to the wasps.
Researchers often have difficulty proving the non-native invaders are responsible for weakening native populations, such as the negative impact of European deer on the green-backed Firecrown. However, the deleterious effect of the plant-grazers and the wasp invaders could be proven because of the decrease in Elaenia, mistletoe, and Maqui berry shrubs.
Animals and plants are intimately bound in an ecosystem by relationships of food, habitat, and competition. In such tightly linked relationships, non-native animals cannot enter an ecosystem without throwing off this tightrope walk by depleting natural resources for the native animals and plants.
Rodriguez-Cabal, M.A., Barrios-Garcia, M.N., Amico, G.C., Aizen, M.A., Sanders, N.J. “Node-by-node disassembly of a mutualistic interaction web driven by species interactions.” Proceedings of the National Academy of Sciences 110:41, doi: 10.1073/pnas.1300131110