Trade-offs between site fidelity and local dispersal create heterogeneity in consumer-mediated habitat linkages in a disturbed seascape

Background/Question/Methods

The relative advantages and disadvantages of staying in one place (e.g., site fidelity) versus moving elsewhere (e.g., dispersal to utilize temporal resources) have been explored in ecology and evolution for decades. A number of recent ecological frameworks (e.g., metapopulations and metacommunities) have sought to integrate how local and regional effects are linked through dispersal. However, these frameworks have not focused specifically on patterns of habitat use and habitat-specific movement by mobile consumers. With the advent of new telemetry tools, researchers have been able to identify specific (and often surprisingly diverse) patterns of seasonal residency, local movements, and regional dispersal for mobile consumers, especially predators. Plum Island Estuary, in densely populated northeastern Massachusetts, has been impacted by multiple natural and anthropogenic disturbances and presents an ideal opportunity to test the relative importance of residency vs dispersal in consumer-mediated habitat linkages.

Results/Conclusions

By examining acoustically tagged migratory striped bass within Plum Island Estuary, we related residence time at a stationary receiver site (a surrogate for site fidelity) to movements within the estuary  (a measure of dispersal) to quantify how predators can create heterogeneity in consumer habitat linkages. Across four years of acoustic tagging, > 60% of small-adult striped bass, tagged in PIE in summer, suspended their northern coastal migration to forage in PIE > 30 days. Despite moving throughout PIE during the summer, seasonally-present tagged striped bass were consistently clustered in the middle region of PIE at sites with high sandbar area, close to channel networks, adjacent to complex confluences, with intermediate levels of bottom unevenness, and near medium sized drop-offs.  Thus, discontinuities (ecologically meaningful physical breaks) such as bathymetric irregularities, drop-offs, and river confluences, were key habitat features for these fish.  Although many receiver sites within the estuary were visited by tagged striped bass, high predator density and high residence time occurred at only a subset of locations. Yet, even in the locations where tagged predators spent a lot of time (site fidelity), predators dispersed frequently to other locations before returning to high density sites. Thus, at these sites, residence time (fidelity) was greater than dispersal (movements). At irregularly-used locations, dispersal outweighed residence.  Hence, within a feeding estuary in summer, both site fidelity and within-estuary dispersal remain a part of this predator’s behavioral repertoire. This balance of habitat-specific site fidelity and within-estuary dispersal creates heterogeneity in spatially-explicit consumer mediated linkages within a disturbed seascape.