Grassland fragmentation and plant invasion support generalist arthropods and might harm the specialist of forest steppes

A Hungarian research team, under the leadership of researchers from the Centre for Ecological Research, found contrasting results by investigating the effects of grassland fragmentation and plant invasion on arthropods. The direct effects were positive both on plant diversity and through them on different arthropods groups. Still, these favoured the generalist species and not the specialist ones, adapted to the harsh environment of the forest steppes. Their findings were published in the international journal Science of the Total Environment.

Forest steppes are a mosaic of grasslands and forests in the transition of temperate forest and steppe biomes and occur from the Far East of Russia to the Carpathian Basin in the West, where Hungary lies. They present high nature conservation value. Therefore, their different habitat types are listed in the EU Habitat Directive. However, most of them are threatened by land-use change. Most forest steppes habitats on sensitive sandy soil were converted to plantation forests by exotic pine, a legacy of World War I when Hungary lost two-thirds of its territory along with the mountain areas with spruce important for timber production.

Besides the detrimental effect of fragmentation, plant invasion threatens these less productive but valuable natural forest steppes. Milkweed, the well-known food plant of the charismatic monarch butterfly in North America, is getting increasingly widespread in these habitats, which were introduced for honey-making in the previous two centuries. Although milkweed most often does not fully cover the invaded areas, the well-visible flowering stems emerge markedly from the sparsely vegetated grassland part of the steppes. The researchers supposed and could also prove that milkweed changes the microclimate, which was drier in the invaded areas than in the control ones in the same study fragments, which described a size gradient from 0.2 to 8.7 ha. This size gradient rarely affected the different arthropod groups (bees, butterflies, flower-visiting wasps, flies, true bugs, and spiders); only spider species richness was negatively affected.

Nevertheless, they also showed that milkweed can provide shade with cooler microclimates, thereby supporting plant species richness. Of course, pollinating insect species richness profited from increased native and milkweed abundance. Also, herbivores, such as true bugs, and predators, such as spiders, showed increased species richness for elevated plant richness and associated more complex vegetation structure. Although the latter group, namely the spiders, were indirectly negatively affected by decreasing soil moisture.

In general, all these would suggest that milkweed is not so harmful. However, when the researchers classified the study species into habitat generalists and specialists, they found that the above misrepresentation of positive results is due to the generalist, which profited from the invasion, whereas specialists were not directly affected. Thus, in the long term, the continued invasion would irreversibly change the habitats dominated by generalist arthropods, displacing the valuable specialist ones – explains Péter Batáry, an author of the study. Therefore – recommends Róbert Gallé, the first-author of the study –  besides stopping the fragmentation and carefully maintaining the sensitive habitats, eliminating invasive plants from small habitat fragments is a key to preserve the native biota.