Tuesday, February 10, 2015

Charting Our Progress: Evolving Thoughts on Population Dynamics

By: Sarah Solomon

I have always been fascinated by the natural world – by the species that I encounter on a daily basis, and by those that exist on faraway lands. In thinking of how complicated and diverse human population dynamics can be, I’ve always sought to understand how other species’ populations are regulated. Why do some species go extinct, and what prevents others from meeting this same fate?
With ever-increasing human activity around the globe, some species are actually beginning to flourish beyond their natural ranges. From Asian carp to Dog-strangling Vine, the increased abundance and distribution of native and introduced species can be detrimental to the survival of others.
Louis Charles Birch, 1918-2009
University of Sydney

In the early 1940s, Australian insect ecologist Louis Charles Birch began to study how certain species of Dacinae fruit flies were becoming pests as a result of the expansion of cultivated fruit crops. Birch and his supervisor (and would-be lifelong colleague) Herbert Andrewartha were fascinated by the relationship of evolution to ecology, with a particular interest in how evolution can be charted in dynamic species of insects.

At the time of Birch and Andrewartha’s original research on the five most abundant species of fruit flies in eastern Australia, there was a prevailing hypothesis suggesting that all animal populations were self-regulating. This meant that populations of animals would increase under favourable conditions, and that eventually the population would grow so crowded that the birth rate would drop and the death rate would increase – hence the notion of self-regulation. It was believed that once a population reached a low enough density, the pressures on it would decrease and the population would be spared from extinction. Now in a laboratory setting, there was compelling evidence to support this hypothesis, but Birch had a different idea.

What actually happens to populations in nature? Why don’t they become extinct? How might we regulate populations of species that have extended beyond their natural ranges? Birch was part of a generation that started to wonder about these notions as they related to changes in natural environments due to human activities. He decided to look both within and outside of different fruit fly species in order to explore how external factors affect species density and changes in distribution.
In particular, D. tryoni had become a pest to fruit crops spanning the eastern coastal portion of Australia, with a significant increase in its range, adapting to cooler southern temperatures. He was also interested in charting this change – are species actively adapting to environmental changes, and are these changes observable? As it turns out, D. tryoni was even hybridizing with D. neohumeralis – another endemic fruit fly species of eastern Australia – to produce a population of flies with a 15 percent higher survival rate of immature eggs than D. tryoni. And all of this as a result of cultivating more fruit crops across the country!

Thus it seems that Birch was on to something – as it turns out, something very important indeed. It is unclear whether Birch could have anticipated just how much of an impact human activity would have on the environment, and in turn, just how much this change would affect species population dynamics. Today, conservation is at the forefront of science and policy, and the notion of studying the effects of abiotic factors on species population dynamics is imperative.

With scientists like Birch paving the way for thinking beyond the “self-regulating” hypothesis, research groups like that of Australian ecologist Euan Ritchie are committed to producing population models that can help inform conservation policies, and protect at-risk species from extinction.

In a 2009 study, Ritchie and colleagues uncovered how competition between the antilopine wallaroo and its wide-spreading counterpart, the Eastern grey kangaroo, is a threat to the survival of the former species. They also found that habitat – especially changes to landscape and the introduction of cattle ranching – contributed greatly to the viability of these species, and that of the common wallaroo (http://www.ncbi.nlm.nih.gov/pubmed/19175695). This type of modeling research is commonplace in the field of ecology today, as the notion of abiotic factors playing a significant role in species' survival is a widely accepted school of thought. With the growing impacts of climate change becoming more and more evident each day, it seems we have come a long way from the era when Birch’s ideas were considered a minority view.

More than fifty years later, the need to produce sound science with which to inform conservation policy is critical. Since Birch’s kick-start to understanding population dynamics, significant advancements have been made in genetics, and in the technologies for analyzing genetic diversity. Such techniques are helping to further highlight the types of genetic adaptations that Birch started to chart in the 1950s, producing fascinating insights into how populations are disappearing, appearing and adapting to external changes.

Overall, have we made all that much progress since Birch?

I would like to think that in many ways we have, but we still have yet to bridge the gap that exists between sound science and policy enforcement. It seems that despite strides being made on the scientific forefront, useful data are often not used or are discounted by policy decision-makers to suit the goals of various stakeholders. Research, like that of Birch and Andrewartha, and more currently Ritchie and colleagues, has major implications for conservation issues, and particularly for the growing concerns of harmful invasive species (see The Genetics of Colonizing Species).
Share your views, and leave a comment below!

Baker, H. G., and Stebbins, G. L. (1965). The genetics of colonizing species: proceedings. Academic Press Inc.

Ritchie, E. G., Martin, J. K., Johnson, C. N., and Fox, B. J. (2009). Separating the    influences of environment and species interactions on patterns of distribution and abundance: competition between large herbivores. Journal of Animal Ecology, 78,    724-731. doi: 10.1111/j.1365-2656.2008.01520.x

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