Evan G. Cooch

Individual Covariation in Life‐History Traits: Seeing the Trees Despite the Forest

We investigated the influence of age on survival and breeding rates in a long‐lived species Rissa tridactyla using models with individual random effects permitting variation and covariation in fitness components among individuals. Differences in survival or breeding probabilities among individuals are substantial, and there was positive covariation between survival and breeding probability; birds that were more likely to survive were also more likely to breed, given that they survived. The pattern of age‐related variation in these rates detected at the individual level differed from that observed at the population level. Our results provided confirmation of what has been suggested by other investigators: within‐cohort phenotypic selection can mask senescence. Although this phenomenon has been extensively studied in humans and captive animals, conclusive evidence of the discrepancy between population‐level and individual‐level patterns of age‐related variation in life‐history traits is extremely rare in wild animal populations. Evolutionary studies of the influence of age on life‐history traits should use approaches differentiating population level from the genuine influence of age: only the latter is relevant to theories of life‐history evolution. The development of models permitting access to individual variation in fitness is a promising advance for the study of senescence and evolutionary processes.

Of bugs and birds: Markov Chain Monte Carlo for hierarchical modeling in wildlife research

Markov chain Monte Carlo (MCMC) is a statistical innovation that allows researchers to fit far more com- plex models to data than is feasible using conventional methods. Despite its widespread use in a variety of scien- tific fields, MCMC appears to be underutilized in wildlife applications. This may be due to a misconception that MCMC requires the adoption of a subjective Bayesian analysis, or perhaps simply to its lack of familiarity among wildlife researchers. We introduce the basic ideas of MCMC and software BUGS (Bayesian inference using Gibbs sampling), stressing that a simple and satisfactory intuition for MCMC does not require extraordinary mathemat- ical sophistication. We illustrate the use of MCMC with an analysis of the association between latent factors gov- erning individual heterogeneity in breeding and survival rates of kittiwakes (Rissa tridactyla). We conclude with a discussion of the importance of individual heterogeneity for understanding population dynamics and designing management plans.

BODY SIZE VARIATION IN LESSER SNOW GEESE: ENVIRONMENTAL PLASTICITY IN GOSLING GROWTH RATES'

We examined the influence of timing of reproduction and brood size on growth rates of goslings of nidifugous Lesser Snow Geese (Anser caerulescens caerulescens) using data collected at La Perouse Bay, Manitoba from 1978 to the present. Gosling growth rates declined significantly during the season, and the declines were independent of the parental genotype. Goslings from larger broods grew significantly faster, but there was no significant annual variation in the seasonal growth rate patterns with either hatch date or brood size. The hatch date and brood size effects could not be accounted for by systematic differences in either egg size or body size of the female parent. Goslings that grew more slowly due to the effects of hatch date or brood size were significantly smaller as adults. Because gosling growth rates influence final adult size, they may have a significant effect on various life history traits in this species.

Long-Term Decline in Body Size in a Snow Goose Population: Evidence of Environmental Degradation?

(1) The effect of environmental factors on annual body size variation was investigated in a breeding population of lesser snow geese (Anser caerulescens caerulescens L.) La Perouse Bay, Manitoba, using data from 1969 to the present. The population size has increased from approximately 2000 to 10 000 pairs during this period (Cooch et al. 1989). (2) Annual mean gosling body mass, tarsus and culmen length (measured at fledging) declined significantly by approximately 16% (240 g), 4% (3.4 mm), and 2% (0.93 mm) respectively in cohorts hatching between 1976 and 1988. The decline in gosling size has led to a significant decline in size of locally bred adults. Annual mean adult female body mass, tarsus and culmen lengths declined by approximately 15% (270 g), 6% (5.4 mm), and 4% (2.3 mm), respectively, in cohorts hatching between 1969 and 1986. (3) Gosling growth and subsequent adult size was also reduced in years with cold, wet weather after hatch. (4) The decline in gosling size was not dependent on the pattern of annual variation in egg or hatch mass, nor systematic changes in annual mean hatch date or post-hatch weather. Goslings reared by individual adult females showed a decline in size over time, suggesting that the general decline reflects a non-genetic change in gosling growth rates during the fledging period. The most probable non-genetic factor is a long-term reduction in food available to the geese at La Perouse Bay. This reduction in food availability may reflect over-exploitation of primary food plants by the geese.

RETROSPECTIVE ANALYSIS OF DEMOGRAPHIC RESPONSES TO ENVIRONMENTAL CHANGE: A LESSER SNOW GOOSE EXAMPLE

In general, analysis of population dynamics can proceed either prospectively or retrospectively. In the case of the former, asymptotic expectations are generally derived (analytically or numerically) from analysis of the potential effects of perturbation of the elements of the life table. However, the vital rates that are indicated by prospective analysis to contribute the most to projected growth rate are not necessarily those that have contributed to observed variation in growth rate over time. We used a retrospective analysis to analyze the life table responses of a population of long-lived herbivorous geese to a systematic reduction in food abundance within the traditional breeding colony. Typical of long-lived species, adult survival rate has been shown previously in a prospective perturbation analysis to have the largest potential impact on projected growth of the population. However, despite a significant long-term increase in adult survival over the course of the study, there has been a long-term decline in growth rates of the population inhabiting the traditional sampling areas, although absolute numbers of individuals in both populations increased over time. Retrospective assessment of the relative contributions of variation in underlying vital rates (adult and juvenile survival, in situ recruitment, emigration and immigration rates into the population) to projected growth showed that the long-term dynamics of the nesting population primarily reflected the combined effects of changes in postfledging juvenile survival, while changes in the postnesting population were most influenced by variation in juvenile survival and adult fidelity rate to the traditional brood-rearing areas. Decreases in both juvenile survival and fidelity reflect systematic reductions in food abundance over the course of the study. Our results confirm previous suggestions that philopatry to the brood-rearing areas may be a significantly more plastic trait than fidelity to nesting areas.

Environmental change and the cost of philopatry: an example in the lesser snow goose

The consequences of philopatric and dispersal behaviours under changing environmental conditions were examined using data from the colony of Lesser Snow Geese (Anser caerulescens caerulescens) breeding at La Pérouse Bay, Manitoba, Canada. In response to increased population size and decreased food abundance over time, increasing numbers of family groups have been dispersing from the traditional feeding areas. Goslings from dispersed broods were significantly heavier (7.3%), and had longer culmens (3.1%), head lengths (2.6%) and marginally longer tarsi (1.9%) on average than goslings that remained within La Pérouse Bay itself. These differences were consistent in each of 5 years. There was no evidence that the larger size of dispersed goslings was due to either a tendency for larger adults to disperse to alternative sites, or increased mortality of smaller goslings among dispersed broods. The most likely cause for the larger size of goslings from dispersed broods was the significantly greater per capita availability of the preferred salt-marsh forage species at non-traditional brood-rearing areas. The larger goslings in non-traditional feeding areas showed significantly higher firstyear survival, suggesting that the use of deteriorating traditional feeding areas may currently be maladaptive in this population.

Environmental degradation, food limitation and reproductive output: juvenile survival in lesser snow geese

Long-term changes in pre-fledging gosling survival, in relation to food availability, were investigated in a population of lesser snow geese (Anser caerulescens caerulescens) at La Perouse Bay, Manitoba. Both colony size and the density of birds using brood-rearing areas increased over the duration of the study (the former from 2000 to 8-9000 pairs). Annual mean size of broods using traditional brood-rearing areas (as a proportion of initial brood size) declined significantly between 1979 and 1991, from 0.9-1.0 to 0.6-0.8. Mean proportion of marked goslings in a brood that survived from hatch to ringing (5-6 weeks of age) declined from approximately 65% prior to 1980 to 35-40% in recent years

When can efforts to control nuisance and invasive species backfire

Population control through harvest has the potential to reduce the abundance of nuisance and invasive species. However, demographic structure and density-dependent processes can confound removal efforts and lead to undesirable consequences, such as overcompensation (an increase in abundance in response to harvest) and instability (population cycling or chaos). Recent empirical studies have demonstrated the potential for increased mortality (such as that caused by harvest) to lead to overcompensation and instability in plant, insect, and fish populations. We developed a general population model with juvenile and adult stages to help determine the conditions under which control harvest efforts can produce unintended outcomes. Analytical and simulation analyses of the model demonstrated that the potential for overcompensation as a result of harvest was significant for species with high fecundity, even when annual stage-specific survivorship values were fairly low. Population instability as a result of harvest occurred less frequently and was only possible with harvest strategies that targeted adults when both fecundity and adult survivorship were high. We considered these results in conjunction with current literature on nuisance and invasive species to propose general guidelines for assessing the risks associated with control harvest based on life history characteristics of target populations. Our results suggest that species with high per capita fecundity (over discrete breeding periods), short juvenile stages, and fairly constant survivorship rates are most likely to respond undesirably to harvest. It is difficult to determine the extent to which overcompensation and instability could occur during real-world removal efforts, and more empirical removal studies should be undertaken to evaluate population-level responses to control harvests. Nevertheless, our results identify key issues that have been seldom acknowledged and are potentially generic across taxa.

Age and reproductive success in female lesser snow geese: experience, senescence and the cost of philopatry

We studied age-related reproductive success in female lesser snow geese Chen caerulescens caerulescens, using data from the breeding colony at La Perouse Bay, Manitoba, 1973-88. We assessed age-specific variation in both individual components of reproductive success and their cumulative product using individuals aged 2-19 years. The composite measure of reproductive success increased among birds 2-6 years of age. Increased reproductive success over these ages was due to significant increases in clutch size and gosling survival and significant reductions in probabilities of total nest and total brood failure. After age 6, the composite measure of reproductive success declined significantly

Long-term decline in fecundity in a snow goose population: evidence for density dependence?

(1) A population of lesser snow geese (Chen caerulescens caerulescens L.) has been studied at the breeding colony at La Perouse Bay, Manitoba, from 1968 to the present. (2) Annual mean clutch size, when adjusted for annual variation in mean laying date and intraspecific nest parasitism, has declined significantly by 0.72 eggs, or 16% of the initial annual mean, over the period of the study. Annual rates of egg loss due to predation, egg hatchability, and fledging success have remained unchanged. (3) The rate of the long-term decline in mean clutch size was independent of female age and breeding experience, and was not related to systematic changes in the age-structure of the breeding population. Individual females had clutch size declines parallel to the decline of the population mean. (4) Mean clutch size is negatively correlated with the size of both the breeding colony and the total flyway population, both of which have increased significantly. The decline in clutch size may reflect increased intraspecific competition for food during spring migration.