Populations of animals can be quite complex… they can be composed of many different age classes with individuals seeking out different strategies with the common goal of maximizing reproductive fitness over a variable environment. An excellent example of an organism with such complex age-structured populations are Steelhead salmon.
In this paper that I co-authored with my post-doc advisor Jon Moore as well as Doug Peard, Jeff Lough and Mark Beere, we took advantage of an incredible dataset of life history information compiled from growth information in fish scales for two Steelhead populations in British Columbia watersheds. These scales tell us whether an individual fish was living in fresh vs. ocean water while the scale was growing, and permit reconstruction of individual life-histories. In other words, the scale data gives us an idea of the sequence of freshwater to ocean vs. ocean to freshwater transitions, and how many times individual Steelhead returned to their natal freshwater rivers to spawn.
We found that the complex and variable life histories exhibited by Steelhead salmon can protect their populations from environmental variability. In fact, populations that have a large percentage of individuals coming back to their natal streams to spawn multiple times lowers the Coefficient of Variation of their population sizes over time (the Standard Deviation of the population trajectory divided by the mean), thus stabilizing population fluctuations. This has important conservation implications because fish hatcheries, the building of dams, and other human-induced disturbances within river systems tend to ‘homogenize’ the life histories of fish populations, thus eroding the stabilizing influence of life-history diversity.
So the moral of the story is: it’s good to be different.
Check out the full paper here:
Moore JW, Yeakel JD, Peard D, Lough J, Beere M. Life-history diversity and its importance to population stability and persistence of a migratory fish: steelhead in two large North American watersheds. Journal of Animal Ecology. DOI: 10.1111/1365-2656.12212 [link]