NEST DEPREDATION, BROOD PARASITISM, AND REPRODUCTIVE VARIATION IN ISLAND POPULATIONS OF SONG SPARROWS (MELOSPIZA MELODIA)

Auk, The, Jul 2006 by Wilson, Scott, Arcese, Peter

ABSTRACT.-

Theory suggests that the persistence of metapopulations will be influenced by the degree of synchrony in the dynamics of their component populations. Various studies have shown that climate can promote synchrony in the size of adjacent populations linked by dispersal, but fewer studies have examined the effects of climate on underlying demographic rates. We studied annual variation in the timing of breeding and reproductive rates in Song Sparrow (Melospiza melodia) on islands linked by dispersal to determine whether biotic factors acting at local scales or climatic factors acting at a regional scale were more influential of variation in demography. The onset of egg laying varied markedly among years but was roughly synchronous across islands within years. Despite this synchrony, island populations varied markedly in reproductive rate, mainly from spatial variation in nest depredation and brood parasitism. In general, populations farther from Vancouver Island and with fewer resident predators experienced less nest depredation and brood parasitism, and higher reproductive rates, than populations closer to Vancouver Island. Our results show that even when climate acts regionally to synchronize reproductive timing in adjacent populations, its effects on reproductive rate may be overridden by biotic factors that vary among populations.

Received 9 October 2004, accepted 30 September 2005.

Key words: brood parasitism, climate, El Niño-Southern Oscillation (ENSO), Melospiza melodia, metapopulation, predation, population synchrony, Song Sparrow, timing of breeding.

Depredación de Nidos, Parasitisme de Cría y Variación Reproductiva en Poblaciones Isleñas de Melospiza melodia

RESUMEN. - En teoría, se espera que la persistencia de las metapoblaciones esté influenciada por el grado de sincronía en la dinámica de las poblaciones que las componen. Varies estudios han mostrado que el clima puede promover la sincronía en el tamaño de poblaciones adyacentes conectadas mediante dispersión, pero relativamente pocos estudios han examinado los efectos del clima sobre las tasas demográficas subyacentes. Para determinar si los factures bióticos que actúan a escalas locales o si los factores climáticos que actúan a escalas regionales tienen una mayor influencia sobre la variación en la demografía, estudiamos la variación anual en el momento de inicio de la reprodución y en las tasas reproductivas en Melospiza melodia en islas conectadas mediante dispersión. El inicio de la postura de huevos varió marcadamente entre años, pero fue aproximadamente sincrónico entre islas en cada uno de los años. A pesar de esta sincronía, las tasas reproductivas de las poblaciones variaron sustancialmente, principalmente como consecuencia de la variación espacial en la depredacion y el parasitismo de los nidos. En general, las poblaciones ubicadas más lejos de Vancouver Island y las que presentaban menos depredadores residentes, presentaron menores niveles de depredación de nidos y de parasitismo de cría, y mayores tasas reproductivas, que las poblaciones ubicadas más cerca de Vancouver Island. Nuestros resultados muestran que aún cuando el clima actúa regionalmente sincronizando el momento en que tiene lugar la reproducción en poblaciones adyacentes, sus efectos sobre las tasas reproductivas pueden ser invalidados por los factores bióticos que varían entre las poblaciones.

SYNCHRONY IN THE dynamics of populations connected by dispersal ("linked populations") should reduce the persistence of metapopulations if synchrony also reduces the tendency for dispersal to recolonize empty patches or offset declines (Harrison and Quinn 1989). Empirical studies report a wide range of correlations in the size of linked populations in nature and identify several factors that can affect synchrony in their dynamics. For example, limiting factors operating at local scales, including predators and stochastic variation in food supply and weather, caused some bird (Smith et al. 1996), butterfly (Thomas and Hanski 1997), and small mammal populations (Moilanen et al. 1998, Franken and Hik 2004) to fluctuate asynchronously over temporal and spatial scales reminiscent of classical metapopulations (Levins 1969). By contrast, other studies of birds (Lindström et al. 1996, Jones et al. 2003), large mammals (Crøtan et al. 2005), sea turtles (Chaloupka 2001), and insects (Hanski and Woiwod 1993) report striking synchrony in the demography or size of linked populations and identify regional variation in climate as a direct or indirect cause. Theory suggests that the degree of spatial autocorrelation in the demography of linked populations with similar density-dependent structures will depend on the interplay of temporal variation in regional climate and biotic limiting factors that act locally (e.g., Moran 1953, Ranta et al. 1997, Lundberg et al. 2000, Ringsby et al. 2002, Gratan et al. 2005). Biotic factors with sufficient influence to override a regional effect of climate might include variation in habitat quality, food supply, competitor or predator abundance, population density, and population age or genetic structure (Newton 1998, Ringsby et al. 2002, Tremblay et al. 2003, Wilson and Arcese 2003).