Modeling intertidal crab distribution patterns using photographic mapping among tropical Australian estuaries

Intertidal crabs are abundant in tropical estuaries and have bio-indicator potential. However, the use of intertidal crabs in guiding management actions is limited because high-replication, cost-effective tools to analyze their distribution patterns at large scales are lacking. This study used assemblage modeling and photography to rapidly build formal understanding of the spatial organization of crab communities in the low intertidal zone, between mean sea level at low spring tide and the edge of the mangrove forest, within and among estuaries. A classification and regression tree model revealed seven distinct habitats based on relative occurrence of five species (Uca coarctata, Uca seismella, Macrophthalmus japonicus, Metopograpsus frontalis, and Metopograpsus latifrons) among eight estuaries along 160 km of coast in North Queensland, Australia (18°28'-19° degrees 25' S, 146° 12'-147° 14' E), across four sampling trips between April 2009 and October 2010. Photography provided high-replication sampling across a large area relative to the aim of the study but did not represent the whole intertidal crab community. Complementary hand collections within one estuary allowed the occurrences of three other species (Perisesarma longicristatum, Australoplax tridentata, and Metopograpsus thukuhar) to be fitted into the model. Species occurred across habitats, yet a high occurrence of different species characterized each habitat. The presence of some species not usually found on low intertidal banks suggested connectivity across the intertidal landscape. The model provided a formal basis to add previous small "site or transect specific" scale spatial distribution knowledge as well as other ecological information. Areas with outlying values can be identified as research and management priorities in the absence of other information, although this prioritization should be done with care. The approach may be transferable to other organisms and systems to provide rapid, cost-effective information on the distribution of key fauna where background understanding and resources are limited