Co-occurrence of habitat-modifying invertebrates: effects on structural and functional properties of a created salt marsh.

The roles of co-occurring herbivores that modify habitat structure and ecosystem processes have seldom been examined in manipulative experiments or explored in early successional communities. In a created marsh in southern California (USA), we tested the individual and combined effects of two epibenthic invertebrates on nutrient and biomass pools, community structure, and physical habitat features. We manipulated snail (Cerithidea californica) and crab (Pachygrapsus crassipes) presence in field enclosures planted with pickleweed (Salicornia virginica) at elevations matching the plant's lower extent in an adjacent natural marsh. In the 4-month experiment, C. californica altered habitat structure by reducing sediment surface heterogeneity and shear strength (a measure of sediment stability) markedly throughout the enclosures. Both invertebrates had strong negative effects on a group of correlated sediment physicochemical characteristics, including nitrogen and organic matter concentrations and soil moisture. In addition, both invertebrates greatly reduced benthic chlorophyll a, a proxy for biomass of microphytobenthos. Compared to controls, macroalgal cover was up to sixfold lower with crabs present, while snails increased cover at low elevations of enclosures. Unexpectedly, macroalgal cover was eliminated with both species present, perhaps through P. crassipes consumption of larger thalli and C. californica reduction in cover of recruits. Neither species influenced the S. virginica canopy (quantified with an index of branch length and number); however, at the lower elevation of enclosures, the two species together negatively impacted the plant canopy. The two invertebrates' modifications to our experimental marshes led to distinct suites of biotic and physicochemical features depending on their presence or co-occurrence, with the latter producing several unexpected results. We propose that the roles and interactions of habitat-modifying fauna deserve further attention, particularly in the context of efforts to conserve and restore the processes found in natural systems.