Population: Structure and Principles
Sidney Holt
Sunday, 17 May 2009 16:48 UTC
I have been corresponding with Alan Berryman by e-mail for a week and this has led me to wish to comment on general matters, arising from what I understand Alan’s position to be on all this.
I think there are three general matters on what we need to agree if we are to make progress. They are as follows
1. Our discussion must seek to bring together the findings and ideas that arise from studying terrestrial, aerial and aquatic populations and ecosystems.
2. A population cannot be adequately defined by one feature – numerical size. At the very least the definition must allow discussion of biomass and possibly other overall properties.
3 Whether considered as number of organisms or biomass (or other) it is essential to recognise the importance of structure. Length and body weight distributions come to mind, obviously, and also sex and maturity-immaturity structure.
4 It seems to me that to Alan’s list of a few basic principles must be added body growth. This might not be so important in insects, for example, where one is more likely to be considering metamorphic stages, but in fishes and marine mammals (my speciality) it is difficult to think about population dynamics without the body scale/change feature.
I am also struggling to see how to fit in a general schedule of principles Alan Longhurst’s idea that cannibalism of adults on their young is a near universal feature of teleost evolution, and a means of success in a medium in which the predator and prey organisms are either large or small, with little in between.Sidney Holt
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Replies
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I will try and answer these difficult questions:
1. I agree that we should attempt to integrate all animal and plant populations into a general theory. But how do we do this? Do we accept all the differences between species (i.e., long lived versus short lived) and thereby end up with a huge theory that is basically impossible to work with, or do we only focus on the similarities?
2. Population dynamics means change in numbers, as I see it. Thus, I think we should talk about things that cause numerical changes, like per-capita births and deaths, that’s it; no migration, no weights, no ages. This makes it a lot simpler but, possibly, too simple. But we have, at least in the beginning, to make it simple in order to deal with the complications that will arrive when we try to derive the mathematics. How will we ever formulate a unified mathematical theory if we are continually quibbling about differences? Let’s discuss these later, after the mathematics, which will be difficult enough as it is.
3. I have discussed sex and age structure in the pages under “Should we include age and sex structure?”
4. See 2 above.
5. Cannibalism (simplified) is part of the competitive process. Competition for food (or whatever) results in death, either by starvation or cannibalism. Thus, cannibalism is an adaptation in organisms that suffer frequently from competition. It is common in some insects, particularly those that are limited by a rare or temporary food supply. -
I feel we may be reaching an impasse in this Forum. For me it is impracticable, and undesirable to try, to formulate a general theory that defines a population by one dimension or feature, and particularly by its numerical size. And if I were to be constrained to a single dimension I would chose biomass, not number. But in fact any such restraint is unnecessary, and a population is best defined by a limited array of features, namely numerical size, biomass, age distribution and sex composition.
With respect to cannibalism I differ from Alan. The dynamics of a population are in its birth and death rates and in the growth of individuals, that determines bionmass. Cannibalism in teleosts determines growth rates of young, and they determine both the reproductive rate and the mortality rate of the cohort. I would equally argue that if cannibalism is to be assimilated into another population process it could equally well be into co-operation as into competition. Sidney Holt
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