ความผันแปรของการเข้าสู่พื้นที่ของหอยสองฝาต่างถิ่น Mytilopsis adamsi Morrison, 1946 บริเวณหาดแก้วลากูน จังหวัดสงขลา

Thesis (M.Sc., Ecology)--Prince of Songkla University, 200

The influence of mussel-modified habitat on Fucus serratus L. a rocky intertidal canopy-forming macroalga

The influence of habitat modification by Mytilus edulis L. on the settlement and development of Fucus serratus populations was investigated on rocky shores of the Isle of Anglesey, North Wales. Settlement of fucoids was higher inside mussel habitat than outside on one of two shores studied. The effect of microhabitat on survival of fucoid germlings was examined by transplanting the germlings into and outside mussel habitats, each with and without the exclusion of grazers. Observation showed that periwinkles and top shells were abundant in mussel habitat, while limpets dominated bare rock. Exclusion of grazers greatly enhanced the survival of fucoid germlings in both habitats, indicating that while mussel habitat supports a different grazer assemblage to bare rock, both assemblages are important in limiting fucoid recruitment. The risk of dislodgement was assessed and compared between fucoids growing on mussel shells and bare rock. In situ pull-tests showed that less force was required to detach large fertile thalli growing on mussel shells than those growing on the rock. Adhesion was generally broken between the mussel and the rock rather than between the holdfast and the mussel. These observations indicate that mussels provide an unstable substrate for mature fucoids. Overall results suggest that a negative effect of mussel-modified habitat on fucoids is profound in adults; but the effect is context-dependent in juveniles and can be positive at settlement. Results from a survey on population structure of fucoids across two shores showed that there were greater numbers of large fertile fucoids growing directly attached to rock than on mussel shells, while there was no difference for juvenile fucoids confirming the experimental results. Moreover thalli larger than 60 cm were found only on the rock but not on shells. This finding suggests that a mussel dominated habitat may have a significant impact on reproductive output in fucoid populations

Community level effects of variable recruitment of a key species Mytilus edulis L. in the rocky intertidal

Variation in recruitment is known to affect species demography and population dynamics. There is scant information, however, on how variation in recruitment influences community composition and processes. In the rocky intertidal, mytilid mussels have long been recognised as an important foundation species which can influence the dynamics of the entire community. Although numerous studies have demonstrated the effects of recruitment and post-recruitment processes on structure and dynamics of mussel populations, as well as the effect of mussels as a habitat engineering on community of rocky shores, the understanding of how variation in mussel recruitment might affect community level structure and processes remains unclear. In this Ph.D. research project I used the mussel Mytilus edulis L. as a model species to investigate the effects of variation in recruitment and post-recruitment processes on adult abundance and distribution on rocky shores around the Isle of Anglesey, North Wales, an area known for intense spatial variation in recruitment. I also examined how community composition of key taxa varied in relation to the presence of mussel-modified habitat. Particular emphasis was placed on how mussel-modified habitat influences the abundance and distribution of the dominant canopy-forming fucoid algae Fucus serratus L. which occupies a similar position on the low shore as mussel beds.Recruitment appeared to be a determinant of adult abundance and distribution at the mesoscale (1000s of metres) but not at the smaller, within-shore scale (10s of metres). The relative abundance of adult mussels among three geographically defined coastlines of Anglesey (west, north, and east) reflected the abundance of the recruits. The absence of mussel beds on the west coast of the island can be explained by a combination of low levels of recruitment, poor food supply, probably poor environmental condition, and intense predation. At the community level, community composition varied according to the existence of mussel-modified habitat at different scales. The difference was significant at 1000s of metre scale, significant in one of the two years studied at 100s of metre scale, and not significant at 10s of centimetre scale. Abundance of sessile taxa on primary substrates was lower where mussel habitat dominates, while abundance of mobile species except limpets was higher in the presence of mussel habitat. For mussel-fucoid interactions, mussel habitat influenced fucoids of different life-history phases in different ways. A negative effect of mussel-modified habitat on fucoids was profound in adults but the effect was context-dependent in juveniles and can be positive at settlement. There was a greater number of large fertile fucoids growing directly attached to rock than on mussel shells. This suggests that mussel dominated habitat may have a significant impact on reproductive output in fucoid populations as the abundance of reproducing thalli was reduced. Overall, a series of surveys and experiments in my thesis demonstrates that recruitment of the foundation species M. edulis is an important determinant for community composition on rocky shore