Pod‐Specific Demography of Killer Whales (Orcinus Orca)

Abstract

Killer whales live in stable social groups, called "pods." It has been suggested that the structure of such groups may influence the vital rates, and hence the fitness, of their members. To test this suggestion we used data from a long-term study of killer whales in the Pacific Northwest ). We constructed stage-classified matrix population models for the entire population, two sub-populations, and individuals pods. The population growth rate for the entire population is A = 1.0254, with 90% bootstrap confidence interval from 1.0178 to 1.0322. The mean female population stage distribution is not significantly different from the predicted stable stage distribution. Population growth rate is most sensitive to changes in adult and juvenile survival, followed by fertility. Factors that cause even small changes in survival will thus have a large impact on population growth. Pod-specific growth rates range from A= 0.9949 to A= 1.0498. Most of the interpod variance in growth rate is due to variance in adult reproductive output. Randomization tests show that this variance is not significantly greater than expected on the basis of variation in individual life histories within the population. We conclude that there is no evidence for an effect of social structure on pod-specific population growth rate. The restriction of population growth rates to such a narrow range suggests, but does not prove, a possible role for density-dependent processes.

Equivalence relationships between stage-structured population models

“…Other studies have shown that simplified models can capture the essential details of a full matrix model [32,33]. Heppel et al [34] found that use of simple age-classified models compared favourably with the use of full Leslie matrix models in terms of comparing patterns of elasticity, suggesting that complete life-history information is not important when looking at the elasticity patterns of a model.…”

Effects of bycatch on the population viability of the narrow-ridged finless porpoises in Ariake Sound and Tachibana Bay, Japan

“…It is possible that a significant number of adult females were bycaught with their calves, because it may be difficult for fishermen to carry heavier individuals back to port. The annual rate of population increase is most sensitive to the calving interval and the adult survival rate (Reilly & Barlow 1986, Brault & Caswell 1993 Fig. 1 Therefore, encouraging the collection of by caught porpoises and investigating their age composition is necessary to provide additional information on the current bycatch status and to plan for the future.…”

What life cycle graphs can tell about the evolution of life histories

“…4b shows a size-structured life cycle with three size classes with only individuals in the largest one capable of reproduction. The fitness proxy Q can be written as Figure 4c shows a stage structured life cycle with one post-reproductive class, similar to the life cycle suggested by Brault and Caswell (1993) for the killer whale Orcinus orca. The fitness proxy Q can be written as…”