Patterns and processes of marine habitat selection: foraging ecology, competition and coexistence among coastal seabirds

Abstract

Changes in the marine ecosystem can affect the distribution, survival, and reproductive success of seabirds. Therefore, a better understanding of factors influencing the marine distribution and abundance of seabirds can provide insight into ecological hypotheses and have important conservation implications. Yet at-sea habitat selection by seabirds has received far less attention than have investigations of their breeding biology. I studied the patterns and processes of marine habitat selection by seabirds in nearshore waters of Pacific Rim National Park Reserve, Vancouver Island, British Columbia. The study focused on comparative analyses among five sympatric species: marbled murrelet (Brachyramphus marmoratus), common murre (Uria aalge), rhinoceros auklet (Cerorhinca monocerata), pigeon guillemot (Cepphus columba) and pelagic cormorant (Phalacrocorax pelagicus). I used a multi-scaled and multi-disciplined approach combining shore-based telescope observations, vessel-based surveys, and developed new techniques for mapping nearshore seabird distributions. Patterns of habitat selection were examined through vessel-based surveys and species-habitat modeling. Vessel-based transects are fundamental to studies of seabird ecology, yet standardized protocols often fail to account for detectability biases. Distance-sampling methods were used to quantify seabird detectability along transects and showed extensive variability (20-80% of birds detected) depending on species, year, and observer. Corrected estimates of bird densities were used in habitat selection modeling, which demonstrated inter-specific and inter-annual differences in species-habitat associations. Most species showed distinct partitioning in habitats, particularly with respect to substrate and along gradients of depth and sea-surface temperature/salinity. Thus, environmental variability is a key factor structuring habitat use and coexistence in this community of piscivorous seabirds. Processes of habitat selection were studied through observations of foraging behaviour, estimates of prey availability, and spatial-statistical analysis of seabird distributions. Marbled murrelets increased foraging effort in years and seasons with scarce prey and poor oceanographic conditions and decreased foraging effort at sites with high prey availability. Despite their flexible activity budgets, increased foraging effort was inadequate to buffer reproductive success in a poor prey year, suggesting that prey availability is a limiting factor in habitat use and population growth for murrelets. Theodolite-based mapping studies examined the fine-scale distribution patterns of murrelets and murres. Nearest neighbour spatial statistics tested for competition over foraging space and showed avoidance of murres by murrelets. The results of these studies have implications for the management and conservation of the imperiled marbled murrelet in British Columbia and elsewhere in their range. I demonstrate a clear link between prey availability and consequences for reproductive success. Habitat selection models provide a step towards identifying critical marine habitats which must be protected under the Species at Risk Act. Murrelets show high forage site fidelity and associations with spatially fixed habitat components (beaches), suggesting that marine protected areas may have an important role to play in the conservation, management and recovery of murrelet populations.