Effects of the spatial configuration of habitats on species distributions and population dynamics in unaltered and ditched salt marshes

Parallel grid ditching, a widespread mosquito control practice, alters the structure of the salt marsh tidal network, substituting meandering, dendritic tidal creek systems with highly connected, parallel, linear ditches. It also provides an opportunity to study the effect of landscape pattern on ecological process. Landscape pattern analysis on habitat maps of expansive marshes in southern New Jersey showed that ditching increases the density of intertidal channel edge, decreases the incidence of ponding and the average distance of the marsh surface to channels, and homogenizes overall landscape pattern. I found that both ribbed mussel (Geukensia demissa) density and cordgrass (Spartina alterniflora) height tended to decrease with distance from open water habitats. However, levels of these two responses varied among habitats. Mussel density was greatest next to the bay and along the edge of small natural creeks, while plant height was greatest along the edges of large anthropogenic ditches, suggesting a lack of functional equivalency as habitat between natural and anthropogenic channels. Spatial models applying the empirical field data to a habitat map predicted more homogeneous mussel density and plant height in the ditched than unaltered marsh. In a manipulative experiment to determine mechanisms that may be driving the patterns of mussel distribution, I measured four components of ribbed mussel population dynamics: growth, juvenile recruitment, predation and non-predatory mortality; varying position in the tidal network. All responses except non-predatory mortality were higher closer to channel edges and correlated with flushing rates. Growth was even higher at locations near the edge in ditched than unaltered marshes. Net change in biomass differed among locations showing greater losses at positions with better access to subtidal habitats. Results suggest that tidal network configuration partly determines mussel population dynamics, probably due to differential food and propagule input and predator access. Parallel grid ditching appears to homogenize the landscape pattern of habitats, and through this, it also homogenizes the distribution of salt marsh species. The spatial configuration of the tidal network and interior habitats affects processes and organisms in the marsh and should be considered in salt marsh restoration.