Combining Demographic Approaches To Identify Sink Dynamics: An Example Using An Endangered Seabird.
Peery, M. 1, Becker, Benjamin2, Beissinger, Steven*,1, 1 Department of Environmental Science, Policy, and Management, Berkeley, CA, USA2 Pacific Coast Science and Learning Center, Point Reyes Station, CA, USA
ABSTRACT- The identification of source sink-dynamics is of fundamental importance for conservation, but is often limited by the inability to determine the role of dispersal on population processes. We demonstrate two ways to identify sink populations and assess the role of immigration on population processes without directly observing individuals dispersing from one population to another using data for the Marbled Murrelet (Brachyramphus marmoratus), an endangered seabird. In the first approach, mark-recapture models developed by Pradel (1996) to estimate the rate of population change (p)and total recruitment (f) can be combined with estimates of local recruitment to measure the rate of immigration. The second approach develops inference from the combined use of different estimators of population growth. Growth rates from p and from counts of individuals estimated over multiple sampling periods c incorporate all demographic processes including immigration, and are compared with estimates from a Leslie matrix model which do not include immigration. p = 1.064 (SE = 0.033) was significantly greater than 1.0, indicating that the population increased during the study period. Recruitment was high (f = 0.182, SE = 0.058) despite the fact the mean birth rate was low (b = 0.044, SE = 0.008) and immigration was estimated to be 0.155 (SE = 0.059). c was 1.058 (SE = 0.047) and also indicated a slowly growing population. A Leslie matrix model parameterized with stage-specific birth and survival populations projected population declines (m = 0.908, SE = 0.054) in the absence of immigration. Our results suggest that Marbled Murrelets in central California represent a classic sink population that appears stable but would decline by 9.2% per year in the absence of immigration from larger populations to the north.
Key words: population model, source-sink, bird, population growth
All materials copyright The Ecological Society of America (ESA), and may not be used without written permission.