Insect assemblages of urban grasslands

Abstract

Grassland habitats of the temperate region are of critical importance for the conservation of biodiversity. In Finland, 28% of threatened species, 30% of threatened butterfly species, 25% of threatened vascular plant species, 39% of regionally extinct (RE) species and 70% of threatened bee species are associated with cultural grassland habitats (Ref. 1). In this study, I sampled the vascular plant, carabid beetle and bee assemblages of grassland habitats in the metropolitan region of Helsinki under different levels of urbanization and management, during 2008-2012. I found recorded 252 plant species, and found that there was a correlation between number of plant species and site area, implying that site size and isolation restrict the size of plant assemblages. Also sites with high levels of nutrients had reduced plant species richness, however management by mowing reduced the level of nutrients, even in highly urbanized areas affected by deposition of nitrogen from traffic fumes (Ref. 2). A total of 72 bee species were recorded. The species that managed to persist in urbanized areas were characterized by having greater flight ranges. Species with long colony cycles, small- to medium-sized colonies and late emerging queens (e.g. B. soroeensis) were negatively affected by urbanization, whereas species with the opposite traits (e.g. B. lapidarius) were better able to persist in urban areas. Solitary bee species were sensitive to the amount of urban infrastructure adjacent to the grassland. The carabid material comprised 78 species, with complex patterns of response to level of urbanization. Of those captured in sufficient numbers to test this in a GLMM, 10 were sensitive to urbanization, and two were more abundant under high urbanization. The highest level of species diversity was recorded from dry and managed meadows. NMDS ordination showed that habitat moisture level was a strong determinant of assemblage composition (Ref. 3). I conclude that the primary strategy for promoting the biodiversity of semi-natural grasslands should be to enhance their coverage and connectivity. It is also necessary to ensure the implementation of appropriate management regimes, and to similarly manage adjacent areas of potential grassland habitat. Dry meadows and those with calcareous soils should be prioritized, as they have higher value for many taxa. Further study is necessary on the functions of these taxa and their trophic interactions.

References 1. Rassi, Hyvärinen, Juslén & Mannerkoski (eds.) 2010 The 2010 Red List of Finnish Species 2. Manninen, Forss & Venn 2010 Management mitigates the impact of urbanization on meadow vegetation, Urban Ecosystems 13: 461-481 3. Venn, Kotze, Lassila & Niemelä 2013 Urban dry meadows provide valuable habitat for granivorous and xerophylic carabid beetles. J. Insect. Conserv. 17: 747-764