The allee effect: an effective ally to achieve eradication of invasive species?

Posted by Hester Williams @HesterW123

The rise in biological invasion, strongly related to increasing international trade and travel, is creating global ecological and economical challenges.

The process by which biological invasions occur can be divided into three phases: arrival, establishment, and spread. Early intervention in the form of detection and eradication can be one of the most cost-efficient approaches. Eradication is the deliberate elimination of an invading species from an area, and is greatly assisted by prompt detection when the newly established population is still small and not widely spread.

Given the perceived difficulty of eliminating all individuals of a species, the practicality of eradication has often been questioned. However, recent population studies indicate that low density populations of a variety of species are governed by Allee effects and this may facilitate eradication. Allee effects may arise from a variety of mechanisms (e.g. mate-location failure, failure to overcome host defences, failure to satiate predators) and create a population threshold, below which population growth rate is negative. Consequently, eradication may not require directly eliminating all individuals in a population; instead, it may only be necessary to reduce the population below the Allee threshold, and extinction will proceed without further intervention.

The loss of habitat and fragmentation, which are detrimental to rare and endangered species, are complementary in attempts to eradicate an invasive species from an area. Although habitat loss is not a cause of an Allee effect, it can reduce population size such that the population could then become succeptible to an Allee effect. Fragmentation of an invasive species population (through management actions such as host removal /fragmentation) could result in reduced patch-to-patch dispersal as well as reducing the population densities in each fragmented patch to below the Allee threshold. Thus, sufficiently small and distant patches could lead to extinction of the population.

My studies use Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis, as proxy for an invasive insect pest species (Fig 1).

Fig1

Fig 1: The leaf beetle, Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis, with typical adult damage.

Experiments completed last summer have indicated that at small population sizes, establishment of this beetle is moderated by an Allee effect. This summer I will test the effectiveness of host removal as a management tool to achieve eradication by exploiting the Allee effect. I will remove a selected number of host patches within a meta-population of Neolema ogloblini, thereby fragmenting the remaining population and in turn subjecting it to Allee effects to achieve eradication (Fig 2).

Fig2

Fig 2: Host removal as management tool to achieve eradication through exploitation of the Allee effect. A selected number of host patches within a meta-population of Neolema ogloblini will be removed (denoted by white patches), fragmenting the remaining population and in turn subjecting it to Allee effects to achieve eradication.

Results of this experiment will ultimately give guidance on what eradication approaches are more or less promising for particular invasive species.

 

Hester Williams is a PhD candidate in the School of Biological Sciences, University of Auckland and is stationed with the Landcare Research Biocontrol team in Lincoln, Canterbury. She is interested in invasion processes of both insect and plant species. Hester is supervised by Darren Ward (Landcare Research/University of Auckland) and Eckehard Brockerhoff (Scion), with Mandy Barron (Landcare Research) as an advisor. Her studies are supported by a joint Ministry for Primary Industries – University of Auckland scholarship. The project is an integral part of an MBIE program “A Toolkit for the Urban Battlefield” led by Scion.

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