Trends and cohort resonant effects in age-structured populations
Journal of Animal Ecology (2004): 73: 1157-1167
- Trends and fluctuations in populations are determined by complex interactions between extrinsic forcing and intrinsic dynamics. As an example, the dynamics of many marine fish are characterized by age-structured dynamics forced by stochastic recruitment.
- In this study we develop stochastic age-structured models for two case studies -- the Atlantic bluefin tuna and the Atlantic cod. The former exemplifies intracohort interactions and density-dependent reproduction, the latter exemplifies density-dependent survival and intercohort interactions.
- We use transfer functions and delay-coordinate models to study how the combination of age-structured interactions and stochastic recruitment can induce low-frequency variability. `Cohort resonance' -- as we dub this effect -- can induce apparent trends in abundance and may be common in age-structured populations.
- Our study complements the theory of structured populations that focuses on cycles and chaos (high-frequency dynamics).
- The innate low-frequency fluctuations we describe can potentially mimic or cloak critical variation in abundance linked to environmental change, over-exploitation, or other types of anthropogenic forcing. 6. From a management and conservation viewpoint, it will be important to find ways to separate anthropogenic forcing from cohort resonant effects and/or to understand the way they interact.
Key words: environmental stochasticity, fish populations, stochastic age-structured dynamics, stock-recruitment, transfer functions, trends.