Bushland health check – are your populations large enough?

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 dreaded extinction vortex
The dreaded extinction vortex. Source.

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.

Eyeball your population sizes

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.


Did you like what you just read? Hate it? Either way, share it with your friends and/or comment below.

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


8 thoughts on “Bushland health check – are your populations large enough?

  1. Good blog, John but I choked a bit on this one…

    ‘The US Forest Service’s first spotted owl protection plan, circa 1980, called for protecting 500 pairs of owls each with 1000 acres of old-growth forest, My inquiry started with this question: why 500 pairs? The Forest Service cited a “personal communication” with Dr. Michael Soule (the spiritual founder of Conservation Biology). I called Soule and asked if the FS had cited his views accurately. “No.” Soule never said 500 pairs would prove sufficient to maintain the owl’s viability. Instead, he told the FS that a colleague (Ian Franklin) had observed that 500 fruit flies randomly mating in a jar were sufficient numbers to avoid fixation of a bristle hair mutant through inbreeding. Soule told me that inbreeding depression was unlikely the most significant risk faced by the owl’ – KENDRIC C. SMITH’S blog.

    Fruit flies! To transfer a formula like that to plants in a bushland reserve is seriously problematic. Grasslands managers may be able to use that framework to keep species numbers above the level where a catastrophe could eliminate the whole population, but it’s still a wide application of the research. How long before the products of a reduced genetic spread results in local extinction of the flora species through mutations?

    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. http://www.spiffa.org/why-natural-regeneration-is-better-than-planting.html

    If an area historically has scattered terrestrial orchids, do we need to propagate and plant 500 to 5000 of each species? Colonial orchids as cryptic species, have been shown to be notoriously difficult to judge tuber numbers by rosette numbers. What if the pollinators are not present anymore and they won’t reproduce without intervention by an orchid fancier with a pair of tweezers.

    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.
    If there are very very low (and perceived to be diminishing) numbers of a certain plant with no possibility of natural regeneration, planting is an option but direct seeding would be better. It’s still gardening…
    Conditions have changed so much that we maybe should be giving up efforts to preserve certain species in certain areas where the climate, hydrology, poor pollination potential, missing dispersal vectors or other factors has caused a change in species mix and abundance.

    Maybe time and funds should go somewhere else and the attempt to retain certain species is doomed no matter how much money gets thrown at them, funds that could go elsewhere. There are people who refer to the “Helmeted Moneyeater” and point out the lower profile/priority species undergoing local extinction in the same region and no money to arrest that and that there could be much more “bang for for the same bucks” (personally, I see that as a problem not with these birdies, good luck to ’em, but that biodiversity and in particular threatened species preservation resourcing is a national disgrace). 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?


  2. Hi John, or should I say Dr John? Perhaps I’m being presumptuous. That’s some in-depth stuff ya got there John, but. There’s that ‘but’. As everyone who has been in this game knows extinction vortexes/spirals are nothing new. They have been part of the discussion for decades. What caught my eye were the numbers that you were quoting. 79 urban grassland reserves totalling 1,393 hectares is pretty impressive. What I’d like to know is where are all these grassland reserves? That’s an average of 17.63291139240506 hectares per reserve. What particular urban area are we talking about here? Is this ‘urban area’ Melbourne? No mention of how small the smallest one is or how large the largest one is, either.

    My question is, with regard to size, if a reserve is only one or two hectares aren’t you limited by area? Surely simple mathematics would tell you that you wouldn’t have the room to increase the numbers. As a real estate acquaintance of mine once said “They aren’t making any more land”.

    Anyone who has studied statistics will tell you that you can use them to your advantage. Many a weak argument has been bolstered by their use. As Mark Twain has been erroneously quoted as saying “There are lies, damned lies and statistics”. You can’t even trust the source of quotes.

    I’m also a bit disappointed in your grammar etc. You have a degree but I’m pretty sure the ‘of’ in this little snippet is sort of unnecessary – “too low of a target for some species”. Plus you need to insert a couple of apostrophes in a couple of your ‘its’. No big deal, but. There’s a ‘be’ missing, too.

    Keep up the good work, whatever it is you may be doing.


    1. Hi Combat Wombat, thanks for taking the time to write your comment.

      With regards to the grassland reserve size, as this is not my research or data you would have to ask the researchers this question. The two main researchers I quoted are Ben Zeeman (bjzeeman@students.latrobe.edu.au) and John Morgan (J.Morgan@latrobe.edu.au).

      In regard to you question about reserve size, I will answer this in my next blog post.

      Also, thanks for picking up my grammar mistakes. I’ve made the appropriate changes.

      Many thanks


  3. In addition it seems that my error went through to the keeper. “Vortexes” should read “vortices”. Anyway, getting back to the new rule of thumb…that being a population of 5000 is now the new black. Isn’t that a rather arbitrary figure?


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