The importance of the early control of invaders

Many pests begin life as exotic invaders. The best way to deal with the problem of potential invaders is to understand their immigration potential (see Section 7.4.2) and prevent their arrival by careful biosecurity processes at a nation's point of entry, or elsewhere on trade routes (Wittenberg & Cock, 2001). However, there are so many potential invaders that it is unrealistic to expect that they all will be prevented from arriving. Moreover, many arrivals will not establish, and many of those that do establish will do so without dramatic ecological consequences. Managers need to focus on the really problematic cases. Thus, the next step in an invader management strategy is to prioritize those that might arrive (or that have recently arrived) according to their likelihood of persisting, establishing large populations, spreading through the new area and causing significant problems. This is not an easy matter, but particular life history traits provide useful pointers (dealt with in Section 7.3.2). We will see in Chapter 22 that assessment of the potential to do harm at higher ecological levels (community/ecosystem) can also be helpful in prioritizing invaders for special attention (see Section 22.3.1).

The arrival of an exotic species with a high likelihood of becoming a significant invasive species should be a matter for urgent action, because this is the stage at which eradication is both feasible and easy to justify economically. Such campaigns sometimes rely on fundamental knowledge of population ecology. An example is the eradication of the South African sabellid polychaete worm, Terebrasabella heterouncinata, a parasite of abalone and other gastropods that became established near the outflow of an abalone aquaculture facility in California (Culver & Kuris, 2000).

Its population biology was understood sufficiently to know it was specific to gastropods, that two species of Tegula were its principal hosts in the area, and that large snails were most susceptible to the parasite. Volunteers removed 1.6 million large hosts, thereby reducing the density of susceptible hosts below that needed for parasite transmission (see Chapter 12), which became extinct.

However, in the words of Simberloff (2003), rapid responses to recent invaders will often 'resemble a blunderbuss attack rather than a surgical strike'. He notes, for example, that a string of successful eradications of small populations of weeds such as pampas grass (Cortaderia selloana) and ragwort (Senecio jacobaea) on New Zealand's offshore islands (Timmins & Braithwaite, 2002) were effective because of early action using brute-force methods. Similarly, the white-spotted tussock moth (Orygyia thyellina), discovered in a suburban region of Auckland, New Zealand, was eradicated (at a cost of US$5 million) using Bacillus thuringiensis spray (Clearwater, 2001). The only population biological information to hand was that females attracted males by pheromone, knowledge that was used to trap males and determine areas that needed respraying. Eradication of a recently established species known to be invasive elsewhere usually cannot and should not wait for new population studies to be performed.

Once invaders have established and spread through a new area and are determined to be pests, they are just another species at which the pest manager's armory must be directed.

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