Management Approach 1: Eradication

As promised a few days ago, time to get stuck into some invasive species eradication, defined by Parks and Panetta (2009) as 'the permanent removal of entire discrete populations'. See Fig.1 below for a concise summary.

Fig. 1: The principle of invasive species eradication in a fun meme.

Sounds serious don't it? It's also much easier said than done, especially if you want to do a decent job of it.

Starting with the fundamentals, determining eradication feasibility is essential. However, as each case is unique this is often problematic. Despite this, attempts at setting criteria include:

1. The average rate of removal in populations must be > the annual intrinsic rate of increase.
2. There is no immigration of individuals that can breed.
3. And there must be no adverse effects . For example, on Sarigan Island feral goat eradication caused the eruption of an exotic vine (Kessler, 2002).

Another large factor towards feasibility of eradication is scale. For example, when trying to eradicate an aquatic invasive from a small pond, it can simply be drained, but this obviously cannot be done to larger water bodies (Parks and Panetta, 2009).

So yeah, Captain Obvious called and he said he wants his manual back... but even though these rules should clearly be applied, that of course doesn't guarantee that they are...

An example of a less than ideal eradication attempt has recently taken place within the Rondegat River, South Africa, regarding the smallmouth bass (Micropterus dolomieu)  (Jordaan and Weyl, 2013). The area has high fish diversity and endemism (Linder et al., 2010) but the threat of the invasive bass was perceived to be large, hence their removal was made a conservation priority.

A (slightly grumpy looking) smallmouth bass. Mind you, I'd be grumpy if someone tried to eradicate me.

To eradicate the bothersome bass, the pesticide rotenone was put into the river. This substance is highly toxic to most fish but non toxic to plants, birds and mammals (at low concentrations) and does not persist in the environment. Happy days, right?

Wrong. Despite tests being carried out, twice the recommended lowest effective rotenone dose was applied. This caused mortality in non target fish species and macroinvertebrates. Woops... and so much for point number 3 in our feasibility criteria...

The appliance of rotenone could also raise debates over animal cruelty as it blocks fish respiration and causes erratic swimming. An unpleasant experience for them, I'm sure you'd agree.

Moving from the specific to the general, there are arguments that eradication is often a human preference rather than a prerogative grounded in science (Marris, 2005). In the U.S.A they spend over $1 billion annually on invasive species 'control', featuring strike teams to 'pull, poison or burn' anything out of place, in the name of 'making an area liveable for native plants'.

A park ranger applying blue herbicide to invasive weeds.

But is this large expenditure necessary? Most new plant species manage to co-exist with what's already there; only a small percentage prove to be truly invasive. Perhaps instead of blindly eradicating non-native species, we should identify and ameliorate problems, using past cases to show whether or not a problem will even emerge?

However, where eradication has been easier and more successful is on islands, which harbour nearly half of the world's endangered biodiversity. Howald et al., (2007) report that concerning eradication of island rodents, there have been 332 successes Vs only 35 failures, mostly thanks to successful use of rodenticide. 

Location and size of islands where successful rodent eradications have taken place, note the majority are for islands <100ha (Howald et al., 2007)

For example, the Juan Fernández Archipelago, approximately 700km west of Chile, is a biodiversity hotspot threatened by invasive species. A multi-species program was undertaken as this lowers the cost associated with treating species individually. In this location, eradication of rodents also aided eradication of feral cats (Glen et al., 2013).

Eradicating rodents can assist when it comes to getting rid of feral cats.

On the other hand, even island eradication can be complex, particularly on those which are large (see graph above) and human inhabited. For example, toxic baits that are often used to eradicate invertebrates can't be used in areas where they would provide a risk to humans or domestic animals.

There are also ecological complexities associated with the process, with secondary outcomes including release of other invaders, decline of natives (as happened in South Africa) or environmental changes that can make it harder to eradicate other invaders (Morrison, 2007).

And we must remember the word 'permanent' in the definition. That's very hard to achieve, particularly when trying to eradicate weeds which can have extensive and long lasting seed banks (Gloria et al., 2012). It also requires long-term funding and effort, which is often not provided.

Before the 1980s many were sceptical about eradication (Parks and Panetta, 2009), but it is now considered a primary option. However, as we've seen, the process is often fraught with complexity.

Overall, it is important to plan the attack and allocate resources with great care, taking account of species interactions and providing a contingency plan if something goes awry. Research is ongoing to develop species-selective toxins and control devices to target pests without endangering non-targeted species. For now eradication remains a dangerous procedure, but perhaps in future it will become a safer bet.

Over and out

The Invader Inspector

P.S. Thanks to Isabela DePedro and Daniel Hamilton for linking me to the Parks and Panetta and Marris papers. I highly recommend checking out both of their blogs too.