Effects of vegetation differ among three species of fiddler crabs ( Uca spp.)

Abstract

In the mid-upper intertidal zone of Australian mangrove communities, Uca flammula (Crane), Uca signata (Hess) and Uca elegans (George and Jones) live in forest, edge and clearing habitats, respectively, and sesarmids are abundant in forest habitats. To examine why some species associate with, while others avoid vegetated habitats, three physical features that create spatial variation in vegetation cover were experimentally decoupled in the field: i.e. (1) shade cover, (2) above-ground structure and (3) below ground structure. Shade, but not above or below ground structure, significantly affected crab counts. Uca signata, U. flammula and sesarmids invaded shaded treatments; U. elegans avoided them. This suggests that vegetation does not provide structural support for crab burrows or a refuge from predators. Compared to the other species, U. elegans most often employs visual signals and may avoid vegetated habitats because they hinder visibility. The other crabs may benefit from the less stressful conditions found under plant cover as shaded plots were wetter and cooler than exposed plots. Sesarmids may have positive and negative effects on Uca distribution because they harass and sometimes kill Uca, yet their burrowing and feeding activities may have numerous benefits such as enriching the soil. Uca elegans, and possibly U. signata, may avoid living within forests because their waving behaviour during mating makes them conspicuous to predators. The loss of visual signals to attract females may enable U. flammula to successfully co-habitat with sesarmids.