Human pressure can reduce genetic diversity and elevate trophic position: an arthropod case study

Urban and agricultural areas are covering ever more land, deteriorating animal habitats. So far, there exist a limited number of studies on urbanisation and even fewer studies on the impact of agriculture. To find out how this overall human pressure, that is, hemeroby, affects arthropods, we chose an interdisciplinary approach by studying genetic diversity and trophic position of the spiders Araneus diadematus and Nuctenea umbratica and the ant Formica fuscocinerea. We collected five specimens per species from fifty 500 × 500 m square plots in a medium-sized Central-European city with green surroundings. Firstly, genetic analyses using newly developed microsatellites revealed a significant effect of hemeroby on Araneus diadematus, that is, reduced genetic diversity. Secondly, likewise in Araneus diadematus, the stable isotope δ¹⁵N increased with hemeroby, possibly due to a major food-spectrum change. Thus, we found that hemeroby can negatively impact arthropods. We believe our findings are relevant to urban planning, particularly given that even stronger effects can be expected for larger and more urbanised cities and in less vagile organisms.