Sunday, August 10, 2008

The impact of the greenhouse effect on natural terrestrial environments:

The public is increasingly aware of the significance Grenhouse effect picof the greenhouse effect. There is an expectation that Governments will take preventative and remedial action urgently. However, in the ecological arena there is as yet no consensus about how this should be done.

For example, ecological theory suggests that big changes in climate will, over sufficient time, result in major changes in the species composition and relative abundance of species within ecological communities. But these changes could arise because of species adaptation and evolution in situ, or because of migration to 'track' appropriate climatic conditions. Or, of course, a mixture of the two.

In south eastern Australia there has been a great deal of movement of ecological communities as the climate has gradually changed. The very gradual drying and warming as the continent has drifted,north has resulted in the replacement of most of the rainforest communities with dry-environment communities. Very few species that were once part of the rainforest seem to have evolved into new varieties or species adapted to hotter, drier conditions.

This history suggests that the correct conservation strategy for our flora and fauna is for them to be allowed or assisted to migrate as the climatic bands move. Otherwise it appears that they will perish. The Bio-Clim models showing climatic zones moving hundreds of kilometres, in some cases, reinforce the notion that species or ecological community mobility is important. People even talk of whole nature reserves or Parks being 'in the wrong place' and imply that they should somehow be 'moved'. So the strategy that comes to mind as being the most significant is the maintenance or indeed the re-establishment of corridors to provide for the necessary movement.


However this 'evidence' does not capture all of what is going on. An alternative history has played itself out in special circumstances. For example, on permanent islands, temporary islands (like the Grampians) and in quasi-islands caused by sharply differing geological zones, species have often been trapped in situ, with nowhere to go despite the climatic changes. In such cases many (most?) of the species survive despite the physiological stress, and go on to adapt in-situ. This often leads to accelerated speciation. A good example is the south west of Western Australia where the underlying geology is very broken up and diverse. The region has been subjected to far more extreme changes in climate than south eastern Australia because of the sea current switching effect caused by temperature changes and yet, unlike the south east of Australia, the species have clung on in place. The fact they have nowhere to go because they cannot migrate out is not the critical issue. What is important is that many of the plants from outside the zone, that might otherwise have been better adapted and could therefore have displaced the original species, have not been able to invade because of the barrier created by the different soil types.

So it seems that it is not so much physiological stress, but competition that decides whether most species will persist in the face of climate change. The enormous adaptability of most plants and animals is demonstrated by agriculture, domestic gardens, botanic gardens and zoos where species from extraordinarily different original environments can often survive so long as competition is kept at bay.

These special cases suggest that maintaining the stability of the species composition, that is controlling inputs of weeds and pest animals, is the critical strategy. In fact now that human land use changes have changed most natural areas into ?islands? it seems appropriate to use the example of natural ?islands? to guide us in choosing a species survival strategy.

If we adopt the basic ?island? model, then we have to resolve a paradox about stability and change, especially in the case of small ?islands?. Small ?islands? risk the loss of species due to stochastic shocks - fires, droughts, disease, etc. It is important to have corridors or some other means of allowing species to re-colonise any ?de-stocked? habitat. This implies that species mobility is important. However small ?islands? are more prone to invasion by weeds and pests from the surrounding zone. So minimising species mobility is important. (It seems that the ideal solution would be to surround small native habitat ?islands? with a semi-permeable membrane! - to allow in the local natives but prevent the invasion of exotics.) Larger bush areas, being able to provide for their own restocking, do not raise this dilemma so severely.

So it seems as a universal principle we should not encourage the maximum movement of species in the face of the greenhouse effect. Instead, the conservation of native species is generally best served by maintaining as high a level of stability as the normal short run fluctuations of the environment will allow.

No comments: