Edward P. Caswell-Chen

Demographic window to aging in the wild: constructing life tables and estimating survival functions from marked individuals of unknown age

We address the problem of establishing a survival schedule for wild populations. A demographic key identity is established, leading to a method whereby age‐specific survival and mortality can be deduced from a marked cohort life table established for individuals that are randomly sampled at unknown age and marked, with subsequent recording of time‐to‐death. This identity permits the construction of life tables from data where the birth date of subjects is unknown. An analogous key identity is established for the continuous case in which the survival schedule of the wild population is related to the density of the survival distribution in the marked cohort. These identities are explored for both life tables and continuous lifetime data. For the continuous case, they are implemented with statistical methods using non‐parametric density estimation methods to obtain flexible estimates for the unknown survival distribution of the wild population. The analytical model provided here serves as a starting point to develop more complex models for residual demography, i.e. models for estimating survival of wild populations in which age‐at‐entry is unknown and using remaining information in randomly encountered individuals. This is a first step towards a broad new concept of ‘expressed demographic information content of marked or captured individuals’.

RECONSTRUCTING THE SPREAD OF DIROFILARIA IMMITIS IN CALIFORNIA COYOTES

Dirofilaria immitis is a filarial nematode parasite that is currently widely enzootic in dog and coyote (Canis latrans) populations of California. Weak historical evidence suggests that the initial focus of D. immitis in California occurred 3 decades ago in the Sierra Nevada foothills (SNF) and spread to other parts of California thereafter. However, this hypothesis is difficult to evaluate because of the lack of epidemiological studies on heartworm in California before 1970. We investigated this hypothesis by comparing D. immitis prevalence in coyotes between initial (1975–1985) and current (2000–2002) surveys in the SNF and 2 coastal regions. In the SNF, prevalence of heartworm was not significantly different between initial (35%, n = 169) and current (42%, n = 60) surveys (P = 0.17), suggesting the existence of a stable enzootic focus in the initial survey period. In contrast, current prevalence was 4 times higher than initial prevalence in the northern Coast Range foothills (44 vs. 10%; n = 119, 107; P < 0.001) and in the south San Francisco Bay foothills (32 vs. 8%; n = 31, 59; P = 0.005), suggesting that initial surveys were made during the early stages of colonization. Dirofilaria immitis prevalence, intensity, and abundance was similar in a coastal location in Mendocino County between 1994–1996 and 1999–2002, suggesting some degree of stability in this enzootic focus. Collectively, these findings support the hypothesis that D. immitis established itself initially in California coyotes living in the SNF and subsequently expanded its range of enzootic foci in central California.

The ecology and biodemography of Caenorhabditis elegans.

The nematode Caenorhabditis elegans is a well-known model organism for research on aging and life span, but very little is known about its ecology and natural history. The strain N2 is the standard wild-type C. elegans and arose from the progeny of a single hermaphrodite. Since N2 has passed through laboratory culture, the influence of inadvertent selection and genetic drift on C. elegans strains kept in culture is unclear. Because it seems that other wild-type strains have also been subject to lengthy laboratory culture, the life span and biodemography of wild-caught C. elegans is of interest. We recovered C. elegans from snails (Helix aspersa) in ca. 50% of the California locations where we made collections. In experiments with one of the wild-caught isolates, it differed in important demographic properties, mortality, fertility, fitness, and activity patterns, from the standard N2 strain, when both strains were evaluated in a common laboratory environment. The differences were not only statistically significant; they were also large enough to be biologically important. The differences are consistent with the hypothesis that N2 has adapted to laboratory conditions.

Modeling demographic processes in an endangered population of bighorn sheep

We developed a demographic simulation model to explore how population vital rates, initial size, and the addition of animals influenced the viability of an endangered population of bighorn sheep (Ovis canadensis) consisting of 8 subpopulations. Perturbation analyses indicated that quasi-extinction risk was more sensitive to changes in adult female survival than to changes in reproduction or survival of young animals. This pattern was similar in 8 subpopulations that had different initial sizes, survival rates, and recruitment rates. Subpopulation viability was related to the initial number of females and to adult female survival, but not reproduction. Management actions that increase adult survival may be most effective when implemented in the largest subpopulations, whereas actions involving the addition of animals may be most effective if implemented in subpopulations with high survival rates. Subpopulation augmentation in yearly increments was more effective at reducing quasi-extinction risk than was adding the same total number of animals at the beginning of the simulation. The level of augmentation needed to substantially reduce quasi-extinction risk exceeded reported levels of female movement among populations or subpopulations. This finding led us to speculate that rescue effects, which are uncommon events for bighorn sheep, may be too rare or of inadequate magnitude under current conditions to effectively reverse bighorn sheep population declines.