Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Density-Dependent Constraints on Spawning and Incubation
Correigh M. Greene and Kimberly R. Guilbault
Population biologists charged with restoring populations of salmonids are faced with the challenge of identifying which life stages will promote the best recovery. Where habitat alterations have strongly impacted populations, the process of determining the best habitat restoration actions involves at least two stages: determining (1) which habitats are most in need of restoration (i.e., are most altered from reference states), and (2) which life stages are most sensitive to changes in habitat improvements (Beechie et al. 2003). Such determinations can in turn lead to specific restoration strategies, if population biologists can identify limiting factors on population recovery. These limiting factors include both density-dependent constraints (populations are limited by interactions with conspecifics at either high or low densities) and density-independent constraints (populations are limited by factors not related to conspecific interactions). Both processes are conceivable for threatened and endangered species, the populations most in need of habitat restoration. For example, high rates of conversion of floodplains to suburban landscapes throughout a river system might reduce the availability of spawning areas and thereby cause competition for spawning sites, even when populations are at greatly reduced levels. Likewise, increased runoff rates resulting from urbanization might also increase the severity of scouring events, thereby limiting the watershed’s productivity in a density-independent manner.