Cycles, stochasticity and density dependence in pink salmon population dynamics

Complex dynamics of animal populations often involve deterministic and stochastic components. A fascinating example is the variation in magnitude of 2-year cycles in abundances of pink salmon (Oncorhynchus gorbuscha) stocks along the North Pacific rim. Pink salmon have a 2-year anadromous and semelparous life cycle, resulting in odd- and even-year lineages that occupy the same habitats but are reproductively isolated in time. One lineage is often much more abundant than the other in a given river, and there are phase switches in dominance between odd- and even-year lines. In some regions, the weak line is absent and in others both lines are abundant. Our analysis of 33 stocks indicates that these patterns probably result from stochastic perturbations of damped oscillations owing to density-dependent mortality caused by interactions between lineages. Possible mechanisms are cannibalism, disease transmission, food depletion and habitat degradation by which one lineage affects the other, although no mechanism has been well-studied. Our results provide comprehensive empirical estimates of lagged density-dependent mortality in salmon populations and suggest that a combination of stochasticity and density dependence drives cyclical dynamics of pink salmon stocks.     

Density dependent recruitment in the reef fish Chaetodon miliaris

Synopsis Recruitment of reef fish has generally been found to be unrelated to or positively related to adult densities. This paper reports an inverse relationship between the number of recruits of Chaetodon miliaris and the density of conspecific adults. C. miliaris are non-territorial, planktivorous butterflyfish. The study populations occurred on the flanks of patch reefs of about 30 m in diameter. At their peak densities populations consisted of 300–400 C. miliaris. Recruitment (the appearance in the stock of fish less than 3.5cm in total length) occurred primarily from April to June and corresponded to the new moon period in these months. Populations generally showed a steady decline in numbers during months of the year when recruitment was not occurring: the finite rate of decline was approximately 30% per month. Populations varied in the density of adults at the start of the spring-summer period of recruitment, and the magnitude of recruitment to a reef was inversely related to the density of these conspecific adults. Reduction of the densities of adult C. miliaris through trap fishing, resulted in increased recruitment to the fished stock. These results provide evidence that in some. species of reef fish, benthic processes may play an important role in determining the magnitude of recruitment to an adult stock, in contrast to the widely held view that recruitment is in large part a result of chance events in the plankton or that recruitment is directly related to adult densities.     

Ruffed Grouse Winter Habitat Use in Mixed Softwood-Hardwood Forests,Québec,Canada

Abstract: Ruffed grouse (Bonasa umbellus) is a popular small game species in northeastern North America. We assessed female ruffed grouse habitat characteristics during winter of 2001-2002 and 2002-2003 in a region dominated by mixed softwood-hardwood forests by comparing used and random locations. We followed 23 radiotagged adult females in 2 forest sites of the Réserve faunique de Portneuf, Québec, Canada, from late November to mid-April. We described grouse habitat using ground surveys and identified selected habitat characteristics using analysis of variance and logistic regression. Females preferred mixed softwood-hardwood stands > 17 m tall and 61-120 years old. Compared with random locations, grouse locations had more well-developed total canopy cover (>4m;75%), canopy (>4m;35%), midstory (1-4 m tall; 35%), and lower-story (<1 m tall; 23%) coniferous cover, and higher coniferous stem density and tree basal area (dbh > 9 cm; 343 stems/ha and 9.0 m²/ha, respectively). Forest management should maintain mature mixed softwood-hardwood stands (50% coniferous), which are adequate winter habitat for ruffed grouse.     

Geographical gradients in the population dynamics of North American prairie ducks

1. Geographic gradients in population dynamics may occur because of spatial variation in resources that affect the deterministic components of the dynamics (i.e. carrying capacity, the specific growth rate at small densities or the strength of density regulation) or because of spatial variation in the effects of environmental stochasticity. To evaluate these, we used a hierarchical Bayesian approach to estimate parameters characterizing deterministic components and stochastic influences on population dynamics of eight species of ducks (mallard, northern pintail, blue-winged teal, gadwall, northern shoveler, American wigeon, canvasback and redhead (Anas platyrhynchos, A. acuta, A. discors, A. strepera, A. clypeata, A. americana, Aythya valisineria and Ay. americana, respectively) breeding in the North American prairies, and then tested whether these parameters varied latitudinally. 2. We also examined the influence of temporal variation in the availability of wetlands, spring temperature and winter precipitation on population dynamics to determine whether geographical gradients in population dynamics were related to large-scale variation in environmental effects. Population variability, as measured by the variance of the population fluctuations around the carrying capacity K, decreased with latitude for all species except canvasback. This decrease in population variability was caused by a combination of latitudinal gradients in the strength of density dependence, carrying capacity and process variance, for which details varied by species. 3. The effects of environmental covariates on population dynamics also varied latitudinally, particularly for mallard, northern pintail and northern shoveler. However, the proportion of the process variance explained by environmental covariates, with the exception of mallard, tended to be small. 4. Thus, geographical gradients in population dynamics of prairie ducks resulted from latitudinal gradients in both deterministic and stochastic components, and were likely influenced by spatial differences in the distribution of wetland types and shapes, agricultural practices and dispersal processes. 5. These results suggest that future management of these species could be improved by implementing harvest models that account explicitly for spatial variation in density effects and environmental stochasticity on population abundance.     

Predator–vole interactions in northern Europe: the role of small mustelids revised

The cyclic population dynamics of vole and predator communities is a key phenomenon in northern ecosystems, and it appears to be influenced by climate change. Reports of collapsing rodent cycles have attributed the changes to warmer winters, which weaken the interaction between voles and their specialist subnivean predators. Using population data collected throughout Finland during 1986–2011, we analyse the spatio-temporal variation in the interactions between populations of voles and specialist, generalist and avian predators, and investigate by simulations the roles of the different predators in the vole cycle. We test the hypothesis that vole population cyclicity is dependent on predator–prey interactions during winter. Our results support the importance of the small mustelids for the vole cycle. However, weakening specialist predation during winters, or an increase in generalist predation, was not associated with the loss of cyclicity. Strengthening of delayed density dependence coincided with strengthening small mustelid influence on the summer population growth rates of voles. In conclusion, a strong impact of small mustelids during summers appears highly influential to vole population dynamics, and deteriorating winter conditions are not a viable explanation for collapsing small mammal population cycles.     

Synchrony, scale and temporal dynamics of rock partridge (Alectoris graeca saxatilis) populations in the Dolomites

1. Harvesting records of rock partridge ( Alectoris graeca saxatilis ) were examined first to identify the presence of cycles in a species with a southern European distribution and then to examine synchrony between populations at a range of scales.
2. Hunting records from 1965 to 1994 were obtained from 210 hunting areas and analysed at three spatial scales: subpopulation, population and metapopulation. Rock partridge exhibited cyclic fluctuations in about 40% of the time series with a period of 4–7 years. The results did not change with spatial scale. The density-dependent structure of the populations showed that most populations exhibited damped oscillations.
3. The proportion of populations that were in synchrony increased with scale from the population to metapopulation level. There was no decline in synchrony with distance but a large variation between populations irrespective of distance.
4. The populations clustered into dry and wet habitats, with those in the dry habitat being more cyclic. We suggest the lack of spatial synchrony with distance but greater synchrony within habitats may reflect the influence of stochastic events operating on populations with different density dependence structures.