![]() ![]() ![]() ![]() An at-sea index of chinook salmon ( Oncorhynchus tshawytscha) abundance is highly correlated with survival rates of fish-eating killer whales in British Columbia ( Ford et al., 2005). ![]() Studies collecting information about interannual changes in survival or fecundity have led to several other examples of correlations between population dynamics of cetaceans and environmental factors. One of the first examples was that of pregnancy rates of fin whales off Iceland which were correlated with changes in food abundance. While some believe these declines are due to oceanographic regime shifts on that timescale, others believe that factors such as direct human-caused mortality or depletion of prey species by commercial fisheries may be at least partially to blame.Ĭetaceans probably have similar, though less dramatic, responses to environmental change, but these changes are harder to detect. Several populations of pinnipeds have experienced long-term (20–30 years) declines for which the cause has been debated. Similarly, fluctuations in cohort strength of crabeater seals ( Lobodon carcinophaga) have been attributed to environmental factors such as sea ice extent and krill production and availability ( Boveng and Bengtson, 1997), and survival of juvenile Hawaiian monk seals has been correlated with the location of the chlorophyll front in the North Pacific transition zone ( Baker et al., 2007). In some cases, such as for the northern fur seal at San Miguel Island, conditions have been bad enough to result in nearly 100% mortality of pups in a given year. These changes include lower fecundity, lower pup survival, and even lower adult survival during extreme events. El Niño oceanographic events, through reductions in prey availability, have led to dramatic changes in survival and reproduction of several species of otariids in places such as California and the Galapagos Islands ( Trillmich and Ono, 1991). Studies of pinnipeds provide the best evidence of the effect of changing oceanographic conditions, particularly because of the ability to closely monitor numbers of pups or adults at rookeries from one year to the next. Cetaceans, in particular, should be less subject to large fluctuations in survival and fecundity from year to year than would be sea otters or pinnipeds.ĭata sufficient to examine such patterns are relatively rare for marine mammals. Species with these traits are often referred to as “K-selected species,” meaning they have evolved to maintain relatively stable population sizes at or near the carrying capacity (typically represented by the letter “ K”) of the environment (rather than fluctuating wildly as seen in small mammals such as the lemming, Dicrostonyx spp., or snowshoe hare, Lepus americanus). Therefore, marine mammals have evolved life-history strategies that keep them relatively buffered from interannual variability in environmental conditions, at least compared to other animals such as small terrestrial mammals. Consequently, such species cannot decline too often or too rapidly when conditions are bad, or they would not have persisted on an evolutionary timescale. Long-lived animals with relatively older ages at sexual maturation and relatively slow population growths cannot respond quickly to favorable environmental conditions. However, at least a few conclusions can be made. The difficulty in precisely estimating population size and life-history parameters has made the study of variation in population growth rates of marine mammals difficult. Wade, in Encyclopedia of Marine Mammals (Third Edition), 2018 A Environmental VarianceĪnother aspect of population dynamics is the study of the effects of extrinsic factors on population growth. Only catastrophic events tilt the balance.Paul R. These extended times allow for careful tailoring of the populations. Space, food, resources: you name it they all contribute to keeping the populations down to manageable numbers, and with a reason: the characteristic times of population growth are centuries, if not more, generally outliving the lifespan of a single individual. The real world is a world of limitations. A temporary increase would necessarily result in a decrease that would bring the number back toward the carrying capacity: we say that the population is controlled by a limiting factor. The population plateaus because the environment can't support more than that number. When a population reaches the carrying capacity, the net growth rate is 0 0 0: the number of births equals the number of deaths (and the other factors affecting the number of individuals balance each other). The carrying capacity definition is the maximum size of a population sustainable by a specific environment. ![]()
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