I had a few interesting questions from my previous blog post, so I thought I’d answer them here by clarifying a few things.
Dangers of small populations – not just mutations
“How long before the products of a reduced genetic spread results in local extinction of the flora species through mutations?”
The rate that mutations occur in a species is generally constant and does not increase when population size decreases. In a small population, you do not get more mutations occurring, but the mutations that do occur can spread more rapidly through the population due both to random chance and breeding between close relatives. If these mutations are ‘unhealthy’ and build up in the population, this is called inbreeding depression and results in a reduction in fitness (see this and this for more information). For plant species, signs of inbreeding depression can include poor growth, less flowers per plant and less viable seed production.
The time to local extinction for a species depends on a wide range of factors, including the size of the population, the types of unhealthy mutations in the population, pollination and dispersal methods, and other threatening processes.
Definition of a plant population
A plant population is defined as a group of individuals of a particular species that are capable of interbreeding. The area that the population occupies depends on the natural density of the species and the distance over which the species can interbreed, which depends on the distance pollinators can travel and the distance that pollen or offspring can disperse (which can be affected by wind, waterflows etc.).
Before European settlement, most of Australia’s plant species would have had large populations. But now some our native plant populations are small and fragmented, and as a result pollen and seed dispersal is limited. Depending on the species’ pollination methods, some species can still exist as ‘populations’ across a fragmented landscape (see this for an example) but others are confined to the fragment in which they occur.
“Should a reserve of a few hectares with half a dozen blackwoods (the only blackwoods in the area) be subjected to what Jeff Yugovic terms “ecological gardening” by planting another 495 and ideally, 4995 trees, or any other tree or shrub species that are perceived to be scarce, at the expense of other flora species crowded or shaded out by those trees?”
I am not suggesting that we should start cramming our reserves with plants to suit current theories and disregard all other factors such as reserve size, natural plant densities or the presence of other species. Even if you did plant 495 or 4995 plants they would self-thin to their natural density.
“If an area historically has scattered terrestrial orchids, do we need to propagate and plant 500 to 5000 of each species?”
Before European settlement, scattered terrestrial orchids would have occurred as large populations spread across numerous clumps. Orchid pollinators would have moved between these clumps and so facilitated gene flow across the landscape. Now that our orchid populations are fragmented and isolated, the clumps are so distant that they form small separate populations. Orchids should only be propagated if the reintroduction sites have the appropriate pollinators and fungi.
Small reserves – possibly not viable, but important
“If a reserve is only one or two hectares aren’t you limited by area?”
Yes, that true. Many remnant sites that we manage are not large enough to sustain viable population sizes over the long term. They often consist of small, inbred populations with low fitness. This is something that land managers need to understand and manage.
Despite these shortcomings, small reserves still have important ecological and social functions in the landscape.
A long bow, or research in progress?
“Extrapolating formulas suggested by genetic research on fruit flies in 1980 to plants in a varied and dynamic ecosystem is a very long bow indeed.”
Since the work on fruit flies in the 1980s, there has been another 30 years of research into minimum viable population size . If you’d like to read further about the current research, I’ve put in some links at the end of this blog.
Is a small population a threatening process?
“Anyhow, shouldn’t we be concentrating on removing threatening processes before we begin sculpting the population dynamics in remnant ecosystems to suit our current theories?”
In writing this blog, my intention is not to force current theories onto our remnant ecosystems, but to help bushland managers understand the complexities of how ecosystems work (and don’t work), so that we can make better decisions for conservation.
I think the inbreeding depression that occurs in small populations is a serious threatening process. Reduced fitness due to inbreeding can also compound other threatening processes. A bigger, more genetically-diverse population is a healthier population, and while population ‘rules of thumb’ are not perfect, they’re better than nothing.
References and further reading
Bacles, C.F.E., Lowe, A.J. and Ennos, R. 2006. Effective seed dispersal across a fragmented landscape. Science 311, p.628.
Bradshaw, C.J.A. 2009. Classics: Ecological Triage. Conservation Bytes Blog.
Bradshaw, C.J.A. 2009. Managing for extinction. Conservation Bytes Blog.
Bradshaw, C.J.A. 2009. We’re sorry, but 50/500 is still too few. Conservation Bytes Blog.
Flather, C.H., Hayward, G.D., Beissinger, S.R. and Stephens, P.A. 2011. Minimum viable populations: is there a ‘magic number’ for conservation practitioners? Trends in ecology and evolution 26, pp.307-316.
Markert, J.A., Champlin, D.M., Gutjahr-Gobell, R., Grear, J.S., Kuhn, A., McGreevy, T.J., Roth, A., Bagley, M.J. and Nacci, D. 2010. Population genetic diversity and fitness in multiple environments. BMC Evolutionary Biology10:205.
Newman, D. and Pilson, D. 1997. Increased probability of extinction due to decreased genetic effective population size. Evolution 51, pp.354-362.
Traill, L.W., Bradshaw, C.J.A. and Brook, B.W. 2007. Minimum viable population size: A meta-analysis of 30 years of published estimates. Biological conservation. 139, 159-166.
Traill, L.W., Brook, B.W., Frankham, R.R. and Bradshaw, C.J.A. 2010. Pragmatic population viability targets in a rapidly changing world. Biological Conservation. 143, 28-34.