Effects of offspring size variations on population dynamics in cannibalistic species
In a recent study, we investigated the impact of size variations within cohorts and how this may affect the stability of cannibalistic populations. We found that large variations in size of the offspring tend to stabilize the population dynamics.
Figure 1: shows the variance in size of pike at age 1. The line indicates the mean decrease over time and the blue and orange dots indicate years above and below the long-term mean respectively.
In seasonal environments offspring size variation within each cohort can be large. In the pike population in Windermere in northern England, for example, the offspring at age 1 typically have a length between 15 and 25 cm. However, the observed length interval at this age (often quantified in terms of its variance) changes between years and has in general decreased over time (Figure 1) – in other words, offspring are becoming more similar in size. A similar decrease in offspring size variation has been observed in cod (Olsen et al. 2009). We do not know the causes of this decrease, but whatever they are, another important question is: How could this decrease in offspring size variation affect the population dynamics?
Both pike and cod are known to be cannibalistic species, and this may have quite some influence on the population dynamics (Ohlberger et al. 2014). Cannibalism is an example of a strongly size-dependent interaction among individuals that may induce feedbacks in the population dynamics through shifts in the size distribution of individuals. In a recent study published in Oikos (Ohlberger et al. 2019), we investigated how offspring size variation may affect the stability of population dynamics in cannibalistic species. We did this by simulation analysis of the dynamics in a length-based population model, where we varied the offspring size variance. Our results indicate that a narrowing offspring size distribution tends to destabilize the dynamics, leading to larger fluctuations. The result of the analysis is presented in Figure 2. Hence, a possible consequence of a further decrease in offspring size variation in pike and cod could more variable population dynamics.
Hence, a possible consequence of a further decrease in offspring size variation in pike and cod could more variable population dynamics.
Olsen, E. M., Carlson, S. M., Gjosaeter, J. & Stenseth, N. C. 2009. Nine decades of decreasing phenotypic variability in Atlantic cod. Ecol. Lett. 12, 622–631.
Ohlberger, J., Rogers, L. A. & Stenseth, N. C. 2014. Stochasticity and Determinism: How Density-Independent and Density-Dependent Processes Affect Population Variability. PLoS ONE 9, e98940.
Ohlberger, J., Langangen, Ø., Winfield, I. J. & Vindenes, Y. 2019. The importance of variation in offspring body size for stability in cannibalistic populations. Oikos 19, 268–11.