Magazine article Oceanus

Life Cycles and Population Dynamics: What Role Do Life Cycles Play?

Magazine article Oceanus

Life Cycles and Population Dynamics: What Role Do Life Cycles Play?

Article excerpt

Populations change because of the birth and death of individual organisms, and the probabilities of birth and death change as an individual develops through its life cycle. This means that mathematical models of population dynamics must include some description of the life cycle in order to capture the mechanisms responsible for population growth or decline. Situations where the environment impinges directly on the life cycle are especially interesting. For example, suppose that a population is exposed to a pollutant. The exposure will produce a complex set of cellular and biochemical responses that affect individual survival, growth, maturation, and reproduction - and, eventually, the growth of the population, causing either a decrease or, in the case of some pollution-tolerant species, an increase.

In a project undertaken with Lisa Levin (a former WHOI Postdoctoral Fellow now at Scripps Institution of Oceanography), we analyzed an experiment on two estuarine polychaetes (Streblospio benedicti and Capitella sp. 1) exposed to fuel oil, sewage sludge, and blue-green algae (all common hazards for species living in polluted or eutrophied estuaries). Exposure to these pollutants had dramatic effects on population growth rate, but the effects and mechanisms producing them differed between species. For example, exposure to blue-green algae reduced the population growth rate of S. benedicti (from 1.43 to 1.10), but increased the growth rate of Capitella (from 1.79 to 2.55). The biggest contributor to the decline in S. benedicti growth rate was a reduction in fecundity, while the increased growth rate in Capitella was mostly due to more rapid development. Pinpointing the physiological mechanisms responsible for population-level responses to pollutants is one of the major benefits of this kind of mathematical modelling.

Although polychaetes and other multicellular animals are known to have interesting life cycles, what about the single-celled organisms of the phytoplankton? …

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