Population fluctuations and regulation in great snipe: a time-series analysis.
1. During the last centuries, the breeding range of the great snipe Gallinago media has declined dramatically in the western part of its distribution. To examine present population dynamics in the Scandinavian mountains, we collected and analysed a 19-year time series of counts of great snipe males at leks in central Norway, 1987-2005. 2. The population showed large annual fluctuations in the number of males displaying at lek sites (range 45-90 males at the peak of the mating season), but no overall trend. 3. We detected presence of direct density-dependent mechanisms regulating this population. Inclusion of the density-dependent term in a Ricker-type model significantly improved the fit with observed data (evaluated with Parametric Bootstrap Likelihood Ratio tests and Akaike's Information Criterion for small sample size). 4. An analysis of (a number of a priori likely) environmental covariates suggests that the population dynamics were affected by conditions influencing reproduction and survival of offspring during the summer, but not by conditions influencing survival at the wintering grounds in Africa. This is in contrast to many altricial birds breeding in the northern hemisphere, and supports the idea that population dynamics of migratory nidifugous birds are more influenced by conditions during reproduction. 5. Inclusion of these external factors into our model improved the detectability of density dependence. This illustrates that allowing for external effects may increase statistical power of density dependence tests and thus be of particular importance in relatively short time series. 6. In our best model of the population dynamics, two likely density-independent offspring survival covariates explained 47.3% of the variance in great snipe numbers (predation pressure estimated by willow grouse reproductive success and food availability estimated by the amount of precipitation in June), whereas density dependence explained 35.5%. Demographic stochasticity and unidentified environmental stochasticity may account for the remaining 17.2%.
Kölzsch
Andrea
A
Department of Physics, AGNLD, University of Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany. andrea@agnld.uni-potsdam.de
Saether
Stein Are
SA
Gustafsson
Henrik
H
Fiske
Peder
P
Höglund
Jacob
J
Kålås
John Atle
JA
D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D001947:Breeding; D046091:Charadriiformes / Q000254:growth & development / Q000502:physiology*; D004777:Environment; D005260:Female; D008297:Male; D009664:Norway; D011156:Population Density; D011157:Population Dynamics; D011235:Predatory Behavior; D012621:Seasons; D012744:Sex Ratio; D013269:Stochastic Processes