Biting the real grass
When European botanists wander through Southern California, Chile or New Zealand, they should feel very much at home: well-known immigrants from the Old Continent are growing everywhere, and the native flora and fauna has little to counter them. Or is it?
Whether they come from unintended seed admixtures for fields, intentional sowings to supposedly improve pastureland, or even unforeseen escapes from ornamental gardens: Exotic newcomers to foreign ecosystems all too often turn out to be successful invaders, much to the annoyance of conservationists, the native ones Displace flora and fauna. And sometimes drive them to the brink of extinction. In the southern Californian oak savannas, for example, the European representatives already make up almost one hundred percent of the entire grass biomass - there is virtually no room left for the natives.
The invasive species are then not content with merely taking over the ecosystem, but also change its functions and cycles. In this way, they can themselves influence the natural development of these habitats in their favor and establish themselves permanently against displacement. A classic example can be found in Costa Rica, where introduced grasses prevent the regeneration of tropical dry forests because they increase the risk of fire. In contrast to native trees, however, they are fire-resistant, so that they quickly sprout again after destruction. The original woody plants die off, a forest ecosystem becomes a savannah, the species composition of which in this case comes primarily from Africa.
The so-called neophytes and neozoa also have an economic component in addition to the nature conservation component, because they cause economic damage of over 120 billion dollars a year in the USA alone. A lot of money is therefore invested in combating and researching these unwelcome contemporaries, and a little of it probably went to John Parker and his colleagues at the Georgia Institute of Technology in Atlanta.
With an extensive literature review that included 63 studies on 100 invasive and 400 native plant species, they wanted to explore the influence of both exotic and always resident herbivores on those newly mixed communities. Common knowledge was that many of the green newcomers were able to spread so aggressively because, among other things, they had left their predators or pathogenic diseases behind - so there would be no threat to their expansion course from this side.
But regardless of whether it is a matter of deserts or lake ecosystems: In many cases, in addition to herbs, grasses or bushes, people also brought the corresponding general herbivores such as rabbits, pigs, goats, cattle, grass carp or red like deer with. Shouldn't they then primarily attack their known allies from Europe and still keep them in check? Parker's team provides other answers.
Accordingly, the exotic herbivores prefer to feast on the delicate offspring and delicious adult specimens of the original species that are actually foreign to them. On the other hand, they despise the stale home cooking that they were already familiar with from their homeland - and which they might have classified as difficult to digest or even poisonous because of certain ingredients. As a result, they further promote the spread of neophytes, because through selective grazing or defoliation they displace the domestic competition and thus create space for the invaders.
The old settlers, such as North American bison and pronghorn, Australian kangaroos or certain land crabs are completely different: they, in turn, prefer these newly introduced plants to their long-term plant companions when making a possible choice - probably for exactly the same reasons as in the opposite case. And because of these preferences, native herbivores decimate the abundance of invasive species by nearly a third. On the other hand, you were increased by a whopping 65 percent by equally exotic grazing animals.
In addition, the newcomers also pave the way for suitable allies through this food selection, so that their – undesirable – species diversity continues to increase. In doing so, they set in motion, or at least amplified, a trend that will result in a complete transformation of the ecosystem, in which alien species dominate and those originally native are marginalized.
But why do large animals in particular have such an influence on the birth or death of neophytes, which was previously not thought possible? According to Parker and his colleagues, the large herbivorous vertebrates are impressive because of their mobility, the wide range of food they eat and, above all, their ability not only to keep grass, herbs or woody plants small, but to destroy them completely - insects or snails, on the other hand, can only do all this in exceptional cases.
Unintentionally, the European colonial rulers did a double disservice to the nature of the countries they conquered: their large-scale hunting parties not only eradicated many herbivores there or severely decimated them, they also made it easier by replacing bison, moa, Kangaroos and their friends used foreign plants to conquer new territories – many of which are now considered nasty pests.