Seems like a simple question, but how straightforward is it to provide a general definition of what actually constitutes a population of biological organisms?

Apparently, not so straightforward. Yet, it remains a critical question for much of the way we think about biology. From ecology, through genetics to evolution, we (humans¹) seek to describe the patterns we observe throughout the universe. We rely very heavily on mathematical (and statistical) models to provide us with a framework to investigate different hypotheses that can describe the mechanisms that drive the patters we see.

We² have set up a new Forum on Nature Network to discuss the concept, aimed at finding a consensus among researchers for the most appropriate way to think about what a natural population is. For example, can the density fluctuations we observe across taxa, in the field and lab, be sensibly described by simple mathematical models that assume that the population is well mixed? Or, that the variation among individuals’ reproduction and survival can be adequately described by a single mean value? Or, that population structure that arises from organisms behaving differently according to their age or life-history stage should (or shouldn’t) be explicitly incorporated in a model that will allow us to understand why populations behave as they do over time, and predict how they will behave in the future?

In other words, how much information do we need to incorporate from genetic, spatial, environmental, intra- and interspecific structure to be able to generate a useful, general understanding of natural population fluctuations?

Therefore, I humbly introduce the Theoretical Population Dynamics forum to all interested readers and, more importantly, contributors. We have already invited a group of people who are probably not familiar with NN, and now extend the invitation to all those regulars on the Blogs section who may be interested in this topic. I’m glad to say there is already some active discussion going on, but we encourage and welcome new input from those with different experience and views.

Your science philosophy needs you

We hope there will be much open, fruitful discussion of the important ideas from a diverse group of researchers. Young, old, middle-aged, ecologists, evolutionary biologists, geneticists, be they what they may, but hopefully they be not dogmatic! Any new insight from people outside the usual suspects could be of use in furthering the maturation of population dynamics theory.

I feel that Nature Network offers an excellent platform to develop a constructive conversation that can hopefully develop more rapidly than the more traditional peer-reviewed journal platforms. Asking for clarification of certain points, summarising issues in less technical language, embedding images and figures or directing readers to other sources of literature (using doi links where possible) are all benefits of this Forum format.

I’m hoping that at least I can learn a variety of useful viewpoints without having to dredge carefully through the last 100 years of literature on this topic. Piffle to 99% perspiration and all that.

My own personal view is that mathematical modelling of populations represents an inherent trade-off between mechanistic understanding of population behaviour through mathematical (e.g. local stability analyses ³) or simulation analyses of simple model structures (which are not necessarily realistic) and a more phenomenological observation of population fluctuations of organisms in their natural environment.

To do this, we rely on a clear definition of what a population is, that will allow us to relate the mathematical methods to relevant biological processes.

So, what’s the current thinking? Briefly, ecologists often model populations by assuming that a group of individuals (generally of the same species) are freely mixing — competing or co-operating equally or randomly — with all others in the same habitat. This leads us to easily tractable population models that can be extended in a variety of ways to incorporate more of the complexity we know is inherent in biological systems. But are the most basic underlying assumptions accurate or even useful? Hopefully, this is what our discussion will uncover.

Some history: I was contacted a few months ago by eminent forest insect entomologist and population biologist Alan Berryman, who has had a long-standing interest in identifying and crystallising the important philosophical concepts behind theoretical population biology. He’s aimed to explicitly contrast theory and data, to allow us to understand why populations show the patterns of change that we record.

Alan contacted me on the back of his web search for population biologists running blogs, where he found mine. At least someone’s read it… He contacted me by mail, and along with another interested ecologist (Salvador Herrando-Perez), we discussed various formats to start a discussion group. I suggested the Nature Blogs, but as much of the discussion here is less technical, realised the Forums could provide a more focused alternative.

I chose the lynx as our Forum logo for a couple of reasons.

Puck off!

• The Canada lynx represents a classic, historical case study of the investigation of natural population fluctuations. Charles Elton is considered to be a father of modern population dynamics by many, and his work with Mary Nicholson on the regularly cycling faunal populations of North America⁴ in the early half of the 20th Century ignited a wider general interest in uncovering the mechanisms behind the behaviour of populations, generating hypotheses that ranged from sunspots to supper species interactions. They also ignited my interest in Finland, through my initial collaboration with my sadly departed, terribly missed boss, Esa Ranta.

Lynx and hare cycles in North America. Reinforcing interspecific stereotypes since 1845!

• The particular logo above comes from a Finnish ice hockey team – Ilves of Tampere – the most successful Finnish national champions, and favoured by a local friend of mine.

But enough of the details. Please do go and visit our Forum, check out the comments and add your own. We look forward to hearing from the NN regulars about something other than favourite concerts, how to keep your tea warm and unicycling girrafes. Actually, the girrafe’s might have something helpful to say, and my tea mug often offers valuable insight into bacterial population growth and diversification.

Thanks for your attention, and in the classic words of Radar that is all. Except the footnotes, Hotlips.

¹ Herein lies a particular conundrum. As humans, we tend to view, observe and record the universe based on our own experience. This may not always be the same way our study organisms experience life. They might not use the same sort of algebra, for example.

² Alan Berryman, Salvador Herrando-Perez and your bumbling narrator

³ I have recently published a paper that I’m pretty pleased about in this respect, not being trained in anything beyond high school maths: Fowler (2009) Increasing community size and connectance can increase stability in competitive communities. Journal of Theoretical Biology 258: 179-188. doi link. Do have a gander and let me know if you have any questions.

⁴ See also Animal Ecology – 1st edn 1927, Sidgwick and Jackson, London.