Posted by: Andrew Guerin | July 18, 2008

Assisted Colonisation?

I came across a short paper in the ‘Policy forum’ section of Science this morning, which caught my interest due to a mention of artificial reefs (I’m not using the BPR3 system for this one, because this isn’t a research paper).

The paper (Hoegh-Guldberg et al. 2008) is a discussion of something the authors term “Assisted Colonisation”; a proposed conservation methodology which they suggest might well be applicable for the amelioration of some of the ecological impacts of climate change.

Most species can only persist within a relatively narrow range of physical conditions, often constrained within a certain geographical area. As physical conditions (for example, temperature) shift as a result of climate change, this area can shrink, or move (generally moving towards the poles). While some species may be able to physically move themselves to stay within their optimum conditions, others cannot (this might lead to ecosystems being ‘torn apart’ as different species in an ecosystem will be able to move at different rates). In any case, organisms will only survive if they are able to expand/shift their range into areas of otherwise suitable habitat. Furthermore, as the species move, they may meet areas of unsuitable habitat, at which point their range will begin to shrink.

The authors of the paper start by making the case that conventional conservation practices may not be sufficient to preserve species that are susceptible to this problem, and they offer up for consideration the possibility of ‘assisted colonisation’: in other words physically relocating species to suitable habitat which will be at a suitable temperature for them to survive. They recognise that deliberate species relocations have a long and dismal history, but maintain that it may be possible to identify species for which it is a suitable option. They propose a framework for assessing the suitability of this controversial idea, which I have included below.

It all seems very feasible and pragmatic, put like that.

However, there are some problems beyond the obvious ones concerning introducing species into habitat where they were not originally present. I’m going to focus on the marine examples the authors cite, because they’re right in my area. But first, it is worth mentioning what looks like an obvious flaw in their thinking. Theirs is very much a single species-oriented approach….. which seems to me to be a problem. Species do not exist in an ecological vacuum. Unless you’re very lucky, and find habitat elsewhere that is perfectly suitable but lacks the target species (which might occur in situations where a species range has fragmented and other areas of suitable habitat exist), then that’s all very well. But it seems likely that populations transposed in isolation might not fare well. By extracting them from their natural range you’re causing the very problem that we want to avoid: tearing apart an ecosystem. Really, you’d need to transplant a whole slice of ecosystem, somehow preserving all the linkages between species. Excuse me if I find that impractical.

They also make a suggestion about using artificial reefs to facilitate the shift in range of rocky reef species (which would otherwise be blocked by the lack of suitable hard substrate): “It might be practical, at small scales, to establish artificial, three-dimensional reef structures ahead of migrating coral, fish, and invertebrate species”. Well, “small scales” is exactly right! The logistical requirements of provision of sufficient artificial hard habitat for this to be effective boggle the mind. The best hope would be to use decommissioned oil platforms (I’ll be getting to that in later posts!) to provide new habitat, but even that (a huge endeavour) is likely to make a hopelessly small contribution to the requirement for new habitat. And of course the other problem, paradoxically, is that it might work too well! It is becoming recognised that artificial structures (such as seawalls) can act as invasion corridors for alien species (Bulleri and Airoldi 2005), so by providing a handy way for invasive species to expand their range we might be making things worse in other habitats!

Another possible technique the authors mention is transplanting widespread species ‘within their range’. The idea behind this is that where you have locally adapted populations, you could transplant individuals from lower latitudes (with better heat tolerance) to higher latitudes, where the existing populations are threatened by rising temperatures. Well, for starters you’ve got to be sure that the differences between the heat tolerances of the two populations are inherited, and not due to phenotypic plasticity (where individuals with no genetic difference for a particular characteristic develop differently when they grow in different conditions). The example they use is a curious one: Acropora corals. However, the paper they cite in support (Berkelmans and van Oppen 2006) shows that the variation in heat tolerance is not due to genetic differences between populations, but due to differences in the composition of the zoozanthellae within their tissues. Perhaps this is a minor quibble, as the differences may still technically be inherited, but I can’t help but feel it illustrates the point I made above: we need to be really sure that the differences are genuinely heritable, or your costly and complex transplant endeavour is going to be a complete waste of time.

These are some of the reasons that I am wary of this approach. In order for it to be successful, and to avoid any unwanted negative effects, we would need an extremely detailed understanding of the ecology of the species involved, so that we can with relative certainty predict how they will respond to transplant, and how other species might be effected. I suspect that there are few, if any, species or ecosystems for which we have that level of understanding.

Nevertheless, the authors do acknowledge some of these problems, and we can’t necessarily afford to dismiss this approach out of hand. Climate change will prevent numerous challenges to ecosystems, and we need as many different tools as possible to ameliorate any impacts we are unable (or unwilling) to prevent. In any case, any attempt at ‘assisted colonisation’ will have to be very carefully considered: will the possibility of saving a crucial species ever prove worth the risk of making things worse in existing habitats?


Hoegh-Guldberg, O., Hughes, L., McIntyre, S., Lindenmayer, D. B., Parmesan, C., Possingham, H. P. & Thomas, C. D. 2008. ECOLOGY: Assisted Colonization and Rapid Climate Change. Science, 321, 345-346. DOI:

Bulleri, F. & Airoldi, L. 2005. Artificial marine structures facilitate the spread of a non-indigenous green alga, Codium fragile ssp tomentosoides, in the north Adriatic Sea. Journal of Applied Ecology, 42, 1063-1072. DOI: 10.1111/j.1365-2664.2005.01096.x

Berkelmans, R. & van Oppen, M. J. H. 2006. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change. Proceedings of the Royal Society B: Biological Sciences, 273, 2305-2312. DOI: 10.1098/rspb.2006.3567



  1. Yes, “assisted colonisation” is definitely how a lot of introduced species become “invasive.” Of course, the flip side is studying the species literally to death before anything is done.

    I think half our legislators here in California are waiting for the Delta smelt to go extinct while the studies are being done so they can then say that there is no impediment to pumping water out of the Sacramento-San Joaquin Delta for irrigation and metropolitan purposes.

  2. […] – bookmarked by 4 members originally found by owensad on 2008-09-08 Assisted Colonisation? – bookmarked by 6 members […]

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