Over the last 30 years there has been a great deal of interest in investigating patterns of species co-occurrence across space and time, which may be shaped by interspecific competition for shared resources. A good model of co-occurrence mechanisms is developed among predatory animals along a pollution gradient, where shared resources become more limited in more contaminated areas and the energy budget for detoxification is much higher. Community disassembly by heavy metal pollution may occur when the presence of toxic elements shifts patterns of species co-occurrence from structured to random. On the other hand, limited resources on a pollution gradient should lead to higher competition between dominant species. Disassembly may entail the loss of existing co-evolved interactions among species, which has ramifications for community dynamics and the quality of the functioning of polluted ecosystems. We expect an assemblage dominated by competitive species interactions to exhibit a significant segregation of taxa, whereas one dominated by mutualistic or syntrophic interactions would exhibit an aggregation of taxa. Responses of Carabidae co-occurrence patterns and changes in body size measures to heavy metal concentrations were investigated in a zinc contamination gradient in a Scots pine forest in the vicinity of Olkusz (southern Poland), at 12 study sites. The zinc concentration in the humus layer varied between 108 mg kg-1 dw to 6150 mg kg-1 dw. We used the C-score index, between all possible species pairs in a matrix. The ground beetle assemblages from the reference sites showed a significant segregation pattern. Community disassembly occurred only among assemblages in heavily polluted sites. The average value of skewness and kurtosis were significantly higher in the highly contaminated sites, indicating the greater proportion of small-bodied species in contaminated areas. The Gini coefficient was highest in the low contaminated sites, indicating the body-size inequality of carabid assemblages was greatest in the uncontaminated areas. Our data suggest that increased pollution contributes to the extinction of sensitive forest specialists with large body size and higher competitive abilities, leading to replacement by less sensitive generalists, with smaller body size and that the co-occurrence of species on heavily polluted sites is a result of unstable interactions between species in communities.
Authors:A. Kosewska, E. Topa, M. Nietupski, and R. Kędzior
Carabid beetles and spiders are at the top of the hierarchy of general invertebrate predators, which can help to reduce the abundance of harmful forest pests. They are also frequently used as environmental indicators. In this paper we analyzed the abundance, species richness and changes in carabid beetle and spider assemblages in three treatments of pine forest regeneration – natural, natural with soil prepared by ploughing and artificial with seedlings planted in ploughed soil. The most beneficial forest regeneration treatment variant of forest regeneration for carabid beetles and spiders was the natural regeneration of pine stands without any preceding soil preparation. Both taxa responded strongly to soil ploughing. We also noted the replacement of forest species by less sensitive open area species. In carabid assemblages, large changes in the trophic structure were observed, as predatory species were replaced by hemizoophages in the ploughed treatments.