Seascape Ecology by Pittman Simon

Author:Pittman, Simon
Language: eng
Format: epub
ISBN: 9781119084440
Publisher: John Wiley & Sons, Inc.
Published: 2017-09-25T00:00:00+00:00

Figure 8.4 Left: aerial photographs of sections of an eelgrass (Zostera marina) seascape in the lower Chesapeake Bay, Virginia, United States, illustrating different patchiness regimes. From Hovel & Lipcius (2001). Right: simulated eelgrass seascapes created in NetLogo. Colored areas represent seagrass (gray: patch interior; blue: patch edge) and black represents unvegetated sediment.

Source: From Hovel & Regan (2008).

After structuring the seascape patterns, we populated each seascape type with three trophic levels: juvenile blue crabs, larger blue crabs that consume juvenile blue crabs and top-level predators (large fishes). Juvenile blue crabs ‘settled’ into seagrass habitat using one of three routines: along patch edges, within patch interiors, or randomly in seagrass. We compared survival rates for juvenile blue crabs tethered in place versus juvenile blue crabs allowed to move throughout the seascape within each of the four seascape types. Each organism type moved throughout the seascape based on hierarchical sets of rules; juvenile blue crabs prioritized minimizing encounter rates with larger blue crabs, larger blue crabs prioritized minimizing encounter rates with fishes, and fishes either hunted via random walk or were given the ability to detect and move toward their prey. We compared output between models in which larger blue crabs hunted randomly versus when they could detect juvenile blue crabs and move toward them. Juvenile blue crabs were forced to remain in seagrass habitat, whereas larger blue crabs were given a low probability of being outside of seagrass habitat, which allowed them to more rapidly move through the seascape, but came with the cost of increased vulnerability to fish predators. Fishes were assumed invulnerable to predation and were allowed to rapidly move anywhere in the seascape.

The primary finding from our study was that the effects of seascape structure on juvenile blue crab survival depended on their movement ability. Tethered juvenile blue crabs exhibited higher survival in the most fragmented seascapes (very small patches < 1 m2) and lowest survival in continuous seagrass. This counterintuitive pattern matched results from field tethering trials with juvenile blue crabs and likely arose from the reluctance of larger blue crabs (predators of juveniles) to forage in highly fragmented seascapes, due to high predation risk from top predators. In contrast, mobile juvenile blue crabs exhibited low survival in very small patches and high survival in continuous seagrass, because they were able to effectively escape approaching large blue crabs by swimming away into seagrass habitat, an option not available to mobile crabs in small patches. Other aspects of prey and predator behavior also played into patterns of juvenile blue crab survival: larger blue crabs exhibiting directed hunting resulted in reduced juvenile blue crab survival across all seascape types. In contrast, crab settlement behavior had little effect on model outcomes. To add realism to our model, we have now added variation in structural complexity (e.g., seagrass shoot density) among seascape types, as well as growth functions for juvenile organisms and are using this expanded version of the IBM to test how these additional factors help drive relationships between habitat structure and nursery habitat function.

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