18 Jan 2017
Distribution, ecology, and trophic relationships of a colonial waterbird: the Double-Crested Cormorant

BRANTA — Kate Lyn Sheehan

Distribution, ecology, and trophic relationships of a colonial waterbird: the Double-Crested Cormorant

Institution: Clemson University, South Carolina, USA

Supervisors: Ron J. Johnson & Greg K. Yarrow

Details: PhD, 2013

5416 Limerick Ct.
San Diego, CA 92117

Subject Keywords:Double-crested Cormorant, Parasite Community, Landscape Ecology, Food Web
Species Keywords: Double-crested Cormorant


The realized distribution of an organism is dependent on the environmental characteristics of the landscape, biological interactions within the communities in which it lives, and geographic barriers to dispersal. Changes in habitat can influence landscape characteristics, and consequently, the distribution of organisms within the landscape. The Double-crested Cormorant (Phalacrocorax auritus) is a piscivorous waterbird implicated in human-wildlife conflicts at aquaculture facilities and natural aquatic systems where they compete for resources (fish) with anglers and commercial fishing guides. Impacts of P. auritus on aquatic systems result from their consumption of fish stocks and their contamination of water and soil with guano near nesting and roosting locations. The consequences of top-down and bottom-up forcing associated with P. auritus colonies have been evaluated singularly within particular components of a food web, but they have not been evaluated individually from a community-wide perspective. We observed food chains and trophic networks of communities from lakes where P. auritus breed and compared their composition, biomass, and topologies to those of a mesocosm system where the effects of P. auritus were simulated with the addition of fertilizer and the removal of fishes. When organisms in the lake systems were pooled into trophic levels within food chains, the patterns of relative biomass showed evidence of top-down and bottom-up forcing. In the mesocosms, we also were able to capture differences in topdown, bottom-up, and combined forcing in the topological assessments of trophic networks. The addition of nutrients (bottom-up forcing) was associated with smaller, yet more plentiful fishes. The removal of fishes (top-down forcing during all experimental phases) was associated with a high biomass of fish. The combination of top-down and bottom-up forcing had no impact on the aquatic community when applied at low levels but, at high-intensities, these factors led to a sharp reduction of fish biomass. Thus, the impact of P. auritus in freshwater communities is unlikely to be negative unless their numbers and duration of use is extreme, a rarely realized condition in natural systems.

P. auritus has experienced population declines and rebounds within the last century. An increase in the number of water bodies such as reservoirs, ponds, and aquaculture facilities has changed the cormorant carrying capacity of the North American landscape. Consequently, the distribution of P. auritus has expanded to new geographic areas where foraging and breeding success is high. Human-wildlife conflicts with P. auritus have led to culling programs in many states, with the exception that a resident subspecies of P. auritus that breeds in the southeastern United States is protected from culling and harassment. Because the distribution of multiple subspecies of P. auritus can overlap within southeastern states, it is important to measure differences in habitat use among the subspecies to minimize conflicts in management programs. We developed a species distribution model for two subspecies of P. auritus from their known breeding areas (P. a. auritus from Minnesota and P. a. floridanus from Florida) and transferred those models to South Carolina, where there is question about which subspecies is breeding on reservoir lakes. The models indicate that the breeding habitats of the two subspecies differ. The Florida model correctly predicted nesting locations in South Carolina. The Minnesota model was also able to predict some nesting sites in South Carolina, but with low prediction values that suggested the habitat in South Carolina was not suitable for nesting P. a. auritus. Thus, our models support the presence of the protected P. a. floridanus subspecies breeding in South Carolina.

Landscape characteristics influence the distribution and movements of P. auritus, which in turn reflect the composition and geographic distribution of organisms that they encounter, specifically their helminthic parasites. Trophically transmitted parasites require multiple host species to complete a single revolution of a life cycle. P. auritus obtain helminthic parasites directly from the organisms on which they feed. Thus, the suite of parasites that a host P. auritus contains can indicate the complexity of the aquatic communities from which it fed. We assessed the intestinal parasites of 218 P. auritus that had been collected by state and federal agencies during culling activities. We document 15 types of parasites in P. auritus, many of which had not been previously reported in this species, and others from geographic regions not previously reported. We assessed similarities and differences in the parasite assemblages of P. auritus at local and regional scales, and between migratory (P. a. auritus) and resident (P. a. floridanus) subspecies. The parasite assemblages found within P. auritus were distinct among many sampling locations, among geographic regions, and between resident and migratory subspecies. This appears to be a useful indicator of host grouping and movement and could be investigated further by including additional geographic regions and host species. Moreover, the parasite differences between resident and migratory subspecies add credence to the habitat model finding that the resident subspecies breeding in South Carolina is the protected P. a. floridanus.

Our assessments of local habitat characteristics in relation to the distribution of P. auritus are a new way of demonstrating the differences between sympatric subspecies. Our methodology is able to confirm differences between subspecies that current molecular techniques have been unable to capture. Furthermore, we document how P. auritus predation and defecation can influence the aquatic and parasitic organisms that they encounter in aquatic communities. Because these interactions are restricted to areas where P. auritus occur, our contributions to understanding the distribution of cormorant subspecies and potential impacts on aquatic communities are critical for evaluating management needs and options.

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