Detecting the threshold between ornamental landscapes and functional ecological communities: soil microarthropods as indicator species

It is difficult to determine if smaller urban landscapes such as foundation plantings around buildings are just ornamental or if they have crossed a threshold and become functional communities. We hypothesize that one measure of functionality is the emergence of symbiotic relationships and ecosystem engineers, in particular the appearance of mycorrhizae. The mycorrhizae themselves are difficult to detect but a good indicator of their presence would be the grazers of mycorrhizae and their predators, in particular the microarthropods (Oribatid, Mesostigmatid and Prostigmatid mites and Collembolans). We hypothesize that microarthropod diversity and abundance can indicate when the functional threshold has been crossed. In this study microarthropods in the top two cm of soil were compared among urban, mid-urban (transitional) and natural sites on the University of Victoria campus, British Columbia. We found significantly fewer total microarthropods in the urban sites compared with mid-urban and natural sites. When microarthropods were separated by trophic level, we found that low trophic level microarthropods (dominated by Oribatid mites and Collembolans), were significantly more abundant in natural sites, while predaceous microarthropods (Mesostigmatid mites and Pseudoscorpions) were significantly more abundant in the mid-urban sites. We conclude that soil microarthropod diversity and bundance can indicate when a community has become ecologically functional and is not just ornamental.