How big is a healthy plant population?
Bushland managers deal with plant populations on a daily basis – they are the base units with which we work. We strive to reduce the population of weed species whilst increasing the size of our native plant populations. We want to make sure that the native plant populations we manage are healthy – so that they can withstand the pressures of being knocked about by threats and chance events, as well as having enough genetic diversity to avoid inbreeding.
But how big is a healthy plant population?
Scientists have tackled this question over the years and can now predict the size of a healthy population for a particular species. This type of analysis has mostly been conducted for endangered species and is used for recovery programs. Because each species is quite specific in terms of its ecological niche, analysis would need to be conducted for each species to work out the optimum healthy population size. Currently, we do not have the funding, expertise or time to analyse all the species in all the reserves we manage. The question from bushland managers is – given we look after not just one species, but hundreds, how are we to know what is a healthy population size?
Rules of thumb
In the 1980s two scientists by the name of Ian Franklin and Michael Soule came up with the 50/500 rule. This rule states that you need a minimum of 50 individuals to avoid inbreeding in the short-term (several generations), and 500 individuals to ensure evolutionary potential over the long-term. Conservation managers have taken this on board and have used it to guide management decisions. However, some scientists have been sceptical of the rule as it has been shown to be too low a target for some species to maintain genetic diversity.
With advances in scientific methods, researchers have been working on a new rule of thumb for healthy population size. They now suggest a population size of 5000 to be healthy. Bushland managers may baulk at this suggestion, given the current state of constrained budgets and lack of resources that are needed to bulk up population sizes. But what happens when populations fall below 5000? Extinction.
The extinction vortex is a concept that describes the process that occurs when a population is faced with extinction. When a large population declines due to over grazing, land-clearing etc., it becomes a small population, which leads to a loss of genetic diversity and an increase in inbreeding, which in turn leads to less reproduction and higher mortality and this results in an even smaller population. And so on and so forth until the population becomes extinct.
Extinction is still happening
We already know this is still happening – we have lists and lists of these threatened species. But it is even happening in the reserves we manage.
John Morgan and colleagues from La Trobe University have been studying native grasslands in Melbourne. From a recent survey of 79 urban grassland reserves (1393ha) they found that of the 250 native species they observed, 80% were confined to 16 or fewer sites. This shows that a lot of the diversity in these reserves occur in small populations and are subject to the extinction vortex.
In addition, Ben Zeeman and colleagues from La Trobe University have compared species lists of 29 grassland reserves in Melbourne of a 2013 survey with surveys conducted before 1994. They found that 65% of native species were in decline, while 27% were increasing. Melbourne’s grassland reserves are becoming more homogeneous as we lose our rare and isolated species, and our common species become even more common.
What can bushland managers do to create healthy plant populations?
If you are one of the fortunate few that have species in your reserves with specific management guidelines about population size, then use them! For all the other species you can eyeball the current population size.
This can be done quite easily by walking through the reserve and estimating the number of individuals for each species. It may be easier to do this in spring/summer when most species are flowering.
While some species may have populations that are confined to the reserve, other species may span across an entire region (e.g. trees along roadsides, private properties and riparian corridors). This can be recorded to provide some landscape context of population size.
With this information you can work out how many plant species in your reserves that have healthy populations. There are a few choices for those species that have unhealthy population sizes:
- Triage the species
- Connect the population to another reserve, or
- Increase the population size within the reserve
While it may sound harsh to triage the species we manage, we are inadvertently doing this by relying on natural recruitment to increase population size – small populations have lost their ‘genetic oompf’ and need new genetic material (i.e. plants or seeds from other populations) to ensure their long-term survival. We simply do not have the funding and resources to save all species from extinction, and we need to make the hard decisions of what we will, and won’t save.
While it may be possible to connect populations across some landscapes by using roadside or riparian reserves, it may not feasible in others, such as urban landscapes. This leaves the last choice – increase population sizes in your reserves.
Many species that are managed by bushland managers have unhealthily small population sizes and need bulking up. With the information gained by eye-balling population sizes, you can start to make some goals to work towards. Maybe it is to get a specific species up to 5000 individuals across all your reserves? Or maybe it is to get 5, 10, or 50 species in one reserve up to 5000 individuals each?
Bigger populations have a better chance of survival.
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References and further reading
Frankham, R., Bradshaw, C.J.A. & Brook, B.W. 2014. Genetics in conservation management: Revised recommendations for the 50/500 rules, Red List criteria and population viability analyses. Biological Conservation. 170, 56-63.
Jamieson, I.G. & Allendorf, F.W. 2012. How does the 50/500 rule apply to MVPs? Trends in Ecology and Evolution. 27, 578-584.
Morgan, J.W. 2014. ‘Beyond the extinction debt’. Morgan Plant Ecology Blog. http://morganvegdynamics.blogspot.com.au/2014/03/beyond-extinction-debt.html
Soule, M.E., Wilcox, B. 1980. Conservation Biology: An Evolutionary-Ecological Perspective.
Traill, L.W., Bradshaw, C.J.A. & 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. & Bradshaw, C.J.A. 2010. Pragmatic population viability targets in a rapidly changing world. Biological Conservation. 143, 28-34.
Zeeman, B.J., McDonnel, M.J., Kendal, D. & Morgan, J.W. 2015. Vegetation change in an increasingly urbanised grassland reserve system. 10th Australian Plant Conservation Conference, Hobart. http://www.researchgate.net/publication/270512071_Vegetation_change_in_an_increasingly_urbanised_grassland_reserve_system