The effect of environmental and disturbance factors on the presence of different crab species in Pranburi Forest Park, Thailand

Abstract: The mangrove forest is one of the world’s most threatened ecosystems today and they have decreased significantly in Southeast Asia over the past 40 years. Brachyura (crabs) are important for this ecosystem and they function as ecosystem engineers with their burrowing activities that affects the soil and carbon uptake. There are several factors that can have an impact on the abundance of crabs, two of these are environmental variables or human disturbance factors. Previous studies show that the occurrence and abundance of different crab species can be affected by the type a vegetation in the mangrove forest. The shade from the canopy has also been found to affect the occurrence of some crabs from the steady physical environment that canopy generates. Studies on the effect of human disturbance on the occurrence and abundance of the crab species in this study have not previously been done. The four species that are examined in this study are Scylla serrata (mud crab), Uca vocans (fiddler crab), Sesarma mederi (mangrove crab) and Perisesarma bidens (red claw crab), that live in the study area of Pranburi Forest Park, on the east coast of Thailand. For these species currently little is known about their habitat requirements and if, and how, they are affected by environmental and human disturbance factors. The environmental variables that were used in the analyze was the amount of three different tree species (Rhizophora apiculata, Ceriops tagal, Avicennia marina), water depth and canopy shade. The factors I used to estimate human disturbance was closeness to information signs, resting places and presence of road or river. The results of this study indicate that some species have a higher abundance than others, the sesarmids crabs, S. mederi and P. bidens, have a higher abundance than S. serrata and U. vocans. U. vocans was not included in the analyze because lack of individuals discovered in the fieldwork. I found that the three species analyzed are affected by both environmental variables and human disturbance. More individuals of S. serrata was found where there was a high water depth, a high percentage of canopy shade and less individuals were found in the presence of A. marina and close to the road and river. More individuals of S. mederi was find where there was a high water depth, a high percentage of canopy shade, where R. apiculata ware present and close to the river. More individuals of P. bidens was found where there was a high water depth, a high percentage of canopy shade and far away from the road, information signs and resting places. What could be done by park management to preserve the mangrove forest and the crab species is to plant R. apiculata when reforestation is done and not to cut down trees if the purpose is to maintain a high biodiversity. To reduce number of visitors in the park, opening hours could be set up, though it now is open around the clock.