I've handed the reins of my blog over to Mark Avery for most of June. Mark's sharing the successes and challenges of saving nature around the world in the run up to the Rio+20 Earth Summit.

The American ecologist Jared Diamond wrote of introduced alien species as one of the ecological horsemen of the apocalypse – a major cause of global extinctions. All over the world, including within the UK, our species’s carelessness or foolishness in moving species about the globe has caused declines and disappearances of native species.

I would guess that most people in the UK have seen a squirrel, but that very few of them have seen a wild red squirrel. The familiar grey squirrel is a native of North America and was deliberately released into the UK in the nineteenth century because it is cute. It is indeed cute, but it outcompetes the red squirrel and moreover transmits a squirrel disease which wipes out the red squirrel and so these days red squirrels are found in decent numbers only in those parts of the UK which grey squirrels have not reached: Northern Ireland, much of Scotland, the remoter parts of Wales including Anglesey and small pockets of England particularly islands such as the Isle of Wight and Brownsea Island in Dorset. Everywhere else now supports only the alien grey squirrel and none of our native red squirrel.

Hawaiian honeycreepers illustrate the potentially devastating impacts of introducing species and the diseases they carry with them. Of the 32 species found on Hawaii at the beginning of the 20th century 11 are already extinct and another nine are critically endangered with only two of the rest being assessed as being safe from extinction. Introduced bird species and introduced mosquitos carrying avian malaria have wiped out large parts of Hawaii’s avifauna with refuges being provided by the tops of mountains where the mosquitos cannot currently live (though climate change may well alter that).

Mammals such as cats, rats and goats (but not bats – even though they also end with ‘-ats’) have been potent sources of biodiversity loss when they have arrived on islands and eaten their way through the native fauna and/or flora. Rats, accidentally transported by ships, are the most potent threat to many island species, particularly birds which have evolved to be flightless.

There have been several successful projects to eradicate non-native rats from islands (see here, here, here and here). These mostly involve the blanket deployment of poison baits in the hope of wiping out the rats completely in a short period of time. 

The RSPB is in the middle of a rat eradication programme on Henderson Island in the Pacific (one of the UK’s Overseas Territories) where non-native rats are threatening its endemic fauna including one of the world’s most truly oceanic of birds – the Henderson petrel. After dropping rat baits in August 2011 rats seemed to disappear from the island and the petrel breeding success soared but in March 2012 a few rats were seen, showing that the initial phase of the project had been almost completely successful but there was still more work to do.

Increased global trade and travel mean that few places are now really cut off from everywhere else. And that means that an accidental introduction of a species is only a plane journey or a boat trip away. Are accidental, but sometimes very damaging, species introductions an unavoidable consequence of modern life? And are there ways in which we could reduce the number of damaging cases and reduce the need for difficult and expensive projects to restore damaged species communities?

Our resident wildlife gardener, Adrian Thomas provides loads of tips on gardening for wildlife every week on his blog. Here's an example of one such recent post, highlighting the beauty and value of the Snake's-head Fritillary. Whilst not strictly native, it was certainly growing in the English meadows as far back as the 16th Century as Adrain points out.

 Dr Mark Avery is a former Conservation Director of the RSPB and now is a writer on environmental matters. We’ve asked Mark to write these 20 essays on the run up to the Rio+20 conference.  His views are not necessarily those of the RSPB.  Mark writes a daily blog about UK nature conservation issues.