Plant invasion and fragmentation indirectly and contrastingly affect native plants and grassland arthropods.

Plant invasion and habitat fragmentation have a detrimental effect on biodiversity in nearly all types of ecosystems. We compared the direct and indirect effects of the invasion of the common milkweed (Asclepias syriaca) on biodiversity patterns in different-sized Hungarian forest-steppe fragments. We assessed vegetation structure, measured temperature and soil moisture, and studied organisms with different ecological roles in invaded and non-invaded sites of fragments: plants, bees, butterflies, flower-visiting wasps, flies, true bugs, and spiders. Temperature and soil moisture were lower in invaded than in non-invaded area. Milkweed had a positive effect on plant species richness and flower abundance. In contrast, we mainly found indirect effects of invasion on arthropods through alteration of physical habitat characteristics and food resources. Pollinators were positively affected by native flowers, thus, milkweed indirectly supported pollinators. Similarly, we found higher species richness of herbivores in invaded sites than control sites, as species richness of true bugs also increased with increasing plant species richness. Predators were positively affected by complex vegetation structure, higher soil moisture and lower temperature. Furthermore, increasing fragment size had a strong negative effect on spider species richness of non-invaded sites, but no effect in invaded sites. Especially, grassland specialist spiders were more sensitive to fragment size than generalists, whereas generalist spider species rather profited from invasion. Although milkweed invades natural areas, we did not identify strong negative effects of its presence on the diversity of the grassland biota. However, the supportive effect of milkweed on a few generalist species homogenises the communities. The rate of invasion might increase with increasing fragmentation, therefore we recommend eliminating invasive plants from small habitat fragments to preserve the native biota. Focusing also on generalist species and revealing the indirect effects of invasions are essential for understanding the invasion mechanisms and would support restoration efforts.

Winter-Active Spider Fauna is Affected by Plantation Forest Type.

Plantations of non-native trees for commercial use are common practice in Europe. They are known to have severe ecological impacts on arthropod fauna by altering microclimatic conditions and reducing microhabitat diversity. However, the effect of plantation tree species on winter-active fauna is relatively unknown. Spiders are a diverse predatory arthropod taxon with strong effect on their prey populations. The composition of spider communities sensitively indicates changes in habitat structure. We established 40 sampling sites in five non-native pine and five native poplar plantations and collected spiders with pitfall traps for two winters in the Southern part of Hungary. We assessed the average height of vegetation and percentage cover of leaf litter, mosses, herbaceous vegetation, and shrubs to characterize habitat structure. We found species richness and activity density of spiders in the non-native compared to the native plantations, presumably due to the more temperate microclimate in pine than in poplar plantations. However, there was no significant effect of habitat structure and its interaction with forest type on species richness and activity density of spiders. Species composition of non-native and native plantation forests differed significantly. Furthermore, we identified six characteristic spider species of non-native plantations with preference for relatively moist habitat conditions. The single characteristic species, (Agroeca cuprea Menge, 1873) for the native plantations preferred dry and partly shaded habitats. We conclude that the effect of microclimatic differences and prey availability presumably overrides the effect of habitat structure on winter-active spiders.