EV-1 Occupancy of yellow-billed and Pacific loons: evidence for interspecific competition and habitat mediated co-occurrence Trevor B. Haynes, Joel A. Schmutz, Mark S. Lindberg, Kenneth G. Wright, Brian D. Uher-Koch and Amanda E. Rosenberger T. B. Haynes (t.haynes@alaska.edu), School of Fisheries and Ocean Sciences, Univ. of Alaska Fairbanks, Fairbanks, AK 99775, USA. – J. A. Schmutz, K. G. Wright and B. D. Uher-Koch, US Geological Survey, Alaska Science Center, Anchorage, AK 99508, USA. – M. S. Lindberg, Inst. of Arctic Biology, Univ. of Alaska Fairbanks, Fairbanks, AK 99775, USA. – A. E. Rosenberger, US Geological Survey, Missouri Cooperative Fish and Wildlife Research Unit, Univ. of Missouri, Columbia, MO 65211, USA. Interspeciic competition is an important process structuring ecological communities, however, it is diicult to observe in nature. We used an occupancy modelling approach to evaluate evidence of competition between yellow-billed (Gavia adamsii) and Paciic (G. pacifica) loons for nesting lakes on the Arctic Coastal Plain of Alaska. With multiple years of data and survey platforms, we estimated dynamic occupancy states (e.g. rates of colonization or extinction from individual lakes) and controlled for detection diferences among aircraft platforms and ground survey crews. Results indicated that yellow-billed loons were strong competitors and negatively inluenced the occupancy of Paciic loons by excluding them from potential breeding lakes. Paciic loon occupancy was conditional on the presence of yellow- billed loons, with Paciic loons having almost a tenfold decrease in occupancy probability when yellow-billed loons were present and a threefold decrease in colonization probability when yellow-billed loons were present in the current or previous year. Yellow-billed and Paciic loons co-occurred less than expected by chance except on very large lakes or lakes with convoluted shorelines; variables which may decrease the cost of maintaining a territory in the presence of the other species. hese results imply the existence of interspeciic competition between yellow-billed and Paciic loons for nesting lakes; however, habitat characteristics which facilitate visual and spatial separation of territories can reduce competitive interactions and promote species co-occurrence. A central theme in community ecology is that competition, particularly among sympatric but taxonomically similar species, plays a key role in species’ ecology (Korpimäki 1987, Samraoui et al. 2012) and distributional patterns (Hutchinson 1957, Hardin 1960). Competition is, in fact, considered not only a primary driver of species’ ecology, but also an evolutionary force, leading sympatric species to use new resources and promoting phenotypic divergence and less competitive sympatry (Schluter 2000). For inter- speciic competition to occur, one species must limit the resource use of another. Investigating competition, there- fore, often takes the form of niche overlap studies (e.g. diet; Korpimäki 1987). However, under many conditions, even with similar resource preferences, competing species can co-exist with minimal in situ overlap – largely due to the potential for species to shift spatial and/or dietary preferences in the presence of competitors (Pontin 1982, Baxter et al. 2004). Interspeciic competition can be reduced by factors such as temporal and spatial separation among species, habitat heterogeneity (allowing more potential for niche- shifts), or when the shared resource is not limited due to other limiting factors for the species (Ritchie 2002). Further, competition may afect distributions at diferent scales with the stronger competitor excluding the other at the patch scale but whose distributions overlap at broader spatial scales (Yackulic et al. in press). Manifestations of competition, therefore, can be subtle and more inter- mittent than other ecological interactions (Wiens 1977), and thus competition has been diicult to demonstrate outside of experimental settings (Connor and Simberlof 1979, Connell 1980, 1983). When experiments are not possible or practical, as is the case with many taxa, data on species distributions can be assessed for evidence of competition by examining whether species co-occur less than would be expected by chance. However, interpreta- tion of the results of this approach can be ambiguous because distributional patterns can be inluenced not only by competitive interactions, but also by difering ecological requirements among species or imperfect detec- tion during sampling (i.e. false absences). Recent advances in occupancy modeling allow for the investigation of species co-occurrence patterns while accounting for difering ecological requirements Journal of Avian Biology 45: 001–009, 2014 doi: 10.1111/jav.00394 Published 2014. his article is a US government work and is in the public domain in the USA Journal of Avian Biology © 2014 Nordic Society Oikos Subject Editor: Wesley M. Hochachka. Accepted 8 January 2014