Biodiversity in a box: three non-native invertebrates preferentially find refugia in green space management infrastructure across urban Los Angeles

Abstract

In Southern California, irrigation infrastructure is a prerequisite for urban green space management, and valve boxes are installed widely to manage water flow. These below-ground, plastic boxes protect valves and manifolds, create space for connecting pipes, and present a scarce ecological resource—elevated humidity and shelter from potential predators. We provide the first systematic survey of the biodiversity of valve boxes and evaluate their role in the establishment of non-native species. We conducted comprehensive surveys of slug and spider refugial habitat elements, including leaf litter, crevices, decaying logs and other cover objects, and valve boxes, across urban Greater Los Angeles and adjacent wildland areas. We found that valve boxes comprised nearly all of the surveyed habitat for three common non-native species, including two slugs in the genus Ambigolimax and a spider in the genus Steatoda. At 83 of 85 sites, we detected these species only in valve boxes and not in any other habitat elements. While valve boxes were significantly more frequent in urban than wildland areas, detections in wildland areas were also largely restricted to valve boxes. All of these species share a preference for elevated levels of humidity, and we speculate that introduced slug and spider taxa within irrigation infrastructure may be a general feature of many urban areas, especially in relatively xeric climates with locally high densities of valve boxes. Under these conditions, irrigation infrastructure likely facilitates the establishment and persistence of non-native species requiring high humidity throughout the urbanized world, and could contribute to their cosmopolitan distribution.