Changing intra‐ and interspecific interactions across sedimentary and environmental stress gradients

Intensifying land use and accelerated erosion has resulted in increased delivery and deposition of fine terrestrially derived sediment to coastal ecosystems, increasing sediment mud content in estuarine ecosystems. This change in the sedimentary environment is known to have negative consequences for ecosystem functioning. Yet indirect and cyclical relationships between species and the environment are rarely investigated, despite being crucial for understanding ecosystem resilience. We investigate changes in intra- and inter-specific interactions between two key intertidal species (Austrovenus stutchburyi and Macomona liliana) with different but overlapping environmental preferences. We use a large dataset spanning 29 estuaries and 0%-87% mud content to predict the effects of increases in mud. We observed important changes in feedback loops between Austrovenus recent recruits, older individuals, and sediment shell content, and between Macomona and chlorophyll with increasing stress (mud). These loops show different characteristics in the high and low mud content ecosystems and are not apparent in moderate mud. Increases in the number of paths and weaker/more variable relationships between low and moderate mud also occurred. Thus, our moderate (12%-25%) mud content model may be encompassing a transition point in ecosystem dynamics where interaction networks break down and shift to an alternate state.