Assisted migration can be an expensive and risky way to save a species that is threatened by climate change – but what if we could use it to save whole ecosystems by replacing lost keystone species?

As climatic regions shift with climate change, animals and plants are being forced to migrate into new areas as their old habitat becomes unsuitable. Some species will be able to keep up with the rate of change, but others will be left behind; especially those that move slowly or already have limited distributions. Natural migration may also be limited by natural barriers such as a mountain range, an ocean, or a different soil type, or by artificial barriers like roads or cities.

Another consequence of shifting climatic regions is that as species start to move they will disappear from some areas, leaving a ‘gap’. This is particularly concerning for dominant or ‘keystone’ species that play an important role in ecosystem functioning. Although the particular species is not at risk of extinction, a localised extinction may have many negative consequences for other species and the ecosystem as a whole.

Push and Pull – risks and benefits

In certain cases human intervention may be required to create new populations, in order to ensure the survival of threatened species or ecosystems. This is known variously as assisted migration, assisted colonisation, managed relocation, etc. In a recent review, Lunt et al. (2013) recognised two different methods which they named ‘push’ and ‘pull’ migration.

Capture
Two different types of assisted migration, described by Lunt et al. (2013).

‘Push’ migration is well established as a method of saving threatened species that may be unable to keep pace with the rate of climate change, and would most likely become extinct without human intervention. It involves removing some individuals and creating new populations in more suitable habitat. This method is also used to create ‘insurance’ populations – for example by establishing a population of threatened birds on a predator-free island, or to reduce the risk of catastrophic events like a fire or disease wiping out a whole species in one go. However these relocations are expensive and risky –  for small populations failure could mean not only losing a few individuals but also reducing the viability of the original population, or there could be negative effects on the ecosystem they are being moved to.

ginninderra peppercress
An example of ‘push’ migration – establishing a new population of the rare Ginninderra Peppercress (Lepidium ginninderrense) known from only two locations in the ACT. This method can be very expensive, and carries a significant risk of failure and damage to the receiving ecosystem which is a threatened grassland community.

On the other hand, ‘pull’ migration refers to introducing one or more species to fill the ‘gap’ created by a missing keystone species. In this case, while there is still a risk of negative effects, the benefit of maintaining an entire ecosystem (rather than just one species) may far outweigh the risks.

The Monaro dieback

In a previous post, I suggested that assisted migration could be used to replace a species of eucalypt (Eucalyptus viminalis or Manna Gum) that is disappearing from the Monaro region in south east NSW where it was previously dominant. This may be one of the first examples where a keystone species has been lost due to climate change, and we are faced with the question of what to do about it (for the whole story, check out my previous posts on the Monaro dieback).

Figure 1 dieback on Jindabyne Rd
Manna Gums (Eucalyptus viminalis) are the dominant overstorey species in the Monaro region, but changes in climate have caused widespread decline. Could we use assisted migration to replace this keystone species?

Some people I have spoken to have suggested that we should do nothing – simply let nature take its course and something will naturally fill in the gap*. Well, something almost certainly will fill the gap, but will it really be a replacement? More likely the loss of this keystone species will allow weeds to invade and spread – particularly grassy weeds like African Lovegrass which is already a huge problem and likely to cause further damage to the ecosystem.

Manna Gums are widespread across eastern Australia so they are not at risk of extinction, but they do play an important role as the dominant overstorey species. The Monaro is a harsh landscape, and trees provide a range of benefits for the environment and production, such as shade and shelter for stock, reducing erosion, and habitat for native species. Landholders and organisations such as Landcare and Greening Australia invest large amounts of money in planting trees; if the Manna Gums are no longer going to survive, what species should we use to replace it?

dieback planting trial
This property near Berridale has been planted by the local Landcare group as part of a project to trial different species that could replace the dying Manna Gums.
Which species should we use?

If we were to try to replace the missing Manna gums with another species of eucalypt, there are a few questions we need to ask when making our selection:

  • Which species are likely to survive in the current climate, and also the predicted climate in 50 or 100 years’ time?
  • Which species are unlikely to become pests or disrupt ecosystem processes in their new location?
  • Which species will replace the ecosystem services and values that have been lost?

To answer the first question, we can look at models predicting the future climate for the area. We can then look for a region that has a similar climate now and see what species grow well there. We might also look at what other species already growing in the area but are displaying more resilience – perhaps they are more tolerant of high temperatures, or less susceptible to insect attack?

Unfortunately, many of the characteristics that make species successful at surviving and reproducing under harsh conditions also make them prone to becoming invasive. History has taught us that species introductions are always risky, and we need to be very careful that our interventions don’t have unintended consequences. In a highly modified landscape like the Monaro, the risks of introducing an exotic species are far less than if we were proposing to introduce a threatened species into a relatively intact ecosystem.

The last question is more complicated than it seems, because it’s not always clear what has been lost, or whether we are aiming for conservation of the ecosystem that was there originally, or focussing more on function. Are we looking for a  tree that provides habitat for a certain species e.g. Koala (for which Manna Gums are one of very few preferred hosts); or would we be happy with any tree species that will provide services like shade, hollows, or soil stabilisation.

A positive note

Assisted migration is a contentious issue, and given our limited resources it is very important to accurately weigh up the risks and benefits. While ‘push’ migration may be a useful tool in many cases, the focus on one method means we could be underestimating the possible benefits, or overstating the risks. Unfortunately, this is no longer a hypothetical argument; the Monaro dieback is a scary reminder that climate change is happening now, but also presents an opportunity to explore possible options that could help us deal with similar situations in the future.

*An afterthought…on playing God

It is all too easy to shy away from actions or interventions that feel like ‘playing god’ – after all, what right do we have to decide where species should live, or whether they should survive at all? But it seems to me that inaction – letting nature ‘take its course’ – is not really an option. The reality is that our actions have and will lead to species extinctions and loss of ecosystem functions, and maintaining these values will require significant human intervention.

References:

Lunt I. D., Byrne M., Hellmann J. J. et al. (2013) Using assisted colonisation to conserve biodiversity and restore ecosystemm function under climate change. Biol. Conserv. 157, 172–7.

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