An experimental test of the effects of productivity and environmental stress on a rocky intertidal food web

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Food web dynamics can be controlled by both external factors of primary productivity and environmental stress of ecosystem. However, little is know about how these two factors affect the food web dynamics in natural communities. Here I report a field factorial experiment manipulating the nutrient levels (as an external factor controlling primary productivity), and desiccation stress (as a factor of envilonmental stress), and the presence /absence of macrograzer (limpets ; their grazing is a key process in the food web) in a rocky intertidal algae-grazer food web. The results revealed that desiccation stress and nutrient availability independently influenced the food web dynamics, and their effects occurred without modifying the limpet top-down effect. Desiccation stress directly reduced only foliose algae, and consequently increased competitive inferior filamentous algae and reduced mesograzers (amphipods) through a reduction in food and refuges from desiccation stress. High nutrient availability caused the increase of both filamentous and foliose algae, and consequently caused increase of mesograzers (amphipods). These results suggest that the both nutrient availability and desiccation stress affected the food web dynamics not via top-down process but via bottom-up process, and supported not environmental stress hypothesis but exploitative ecosystem hypothesis.