Pest Potentials
Tealeaves and Tumbling Dice: Anticipating Animal Invasions - Dr Mark Hutchinson
For the great majority of herp species, the answer to this question is no - most species are confined to (diminishing) natural habitat. Behaviour in natural range is not necessarily predictive of pest potential...
The brown tree snake Boiga irregularis, from Australia and Papua New Guinea introduced onto Guam (established about 1945) has decimated the native bird fauna.... but islands are particularly prone to the establishment of feral populations. Factors that correlate well with establishment in new environments by exotic mammals and birds include: 1. High fecundity or otherwise a high potential rate of population growth 2. Climate matching between species' overseas geographic ranges and Australian environments 3. Dietary generalists may be more successful invaders than dietary specialists 4. Human commensals may be more successful invaders than species which can only live in undisturbed habitats. 5. Introduction effort - numbers of individuals or number of releases Of these, the VPC model considers only the biological factors, the first four items on this list. My position is that item 5 is the ONLY strong predictor of feral establishment, and that its omission from the pest assessment process is a serious flaw in the process. 1. Species has high fecundity or otherwise a high potential rate of population growth In practice this equates to negatively weighting species with high fecundity. But potential for population growth is tempered by juvenile and adult mortality Species-specific causes of such mortality - the agents of population control - are among the most difficult ecological parameters to discover and are not known in detail for the majority of the world's animals. So birth rate is NOT predictive unless the causes of mortality are understood and can be factored in. 2. Species has broad ecological tolerances matching Australian environments Can be assessed using BIOCLIM and similar analyses Such predictions have some merit in making assessments more objective. They can predict the most compatible area for an exotic's survival. They do not predict that it will survive, nor that it will not establish in 'unfavourable' areas.
Modelling like BIOCLIM cannot take into account the non-biological factors ('accidents of history') that lead to animal distributions. That is, animals rarely inhabit all of the areas that they could. Past events may have caused extinctions from parts of the range, or there may have been past barriers to their dispersal into climatically suitable regions. Predictions based solely on biology and climate are incomplete... Example: discovery of the Coober Pedy population of the worlds most dangerous snake, the Australian native snake species the Inland Taipan, Oxyuranus microlepidotus.
3. Dietary generalists may be more successful invaders than dietary specialists 4. Human commensals may be more successful invaders than species which can only live in undisturbed habitats. However, as few species that are not human commensals or dietary generalists have been introduced to Australia, these hypotheses are largely untested Factors influencing feasibility of eradication Rate of removal exceeds rate of increase at all population densities
Eradication of a feral population after it has established is clearly a very difficult - perhaps impractical - task. The use of biology to predict a successful feral establishment or spread is limited by our knowledge. Some case histories show that species can have the biological attributes of an invasive species, yet not behave invasively, or can change from non-invasive to invasive for reasons that are not understood. Invasiveness may not be obvious at first. The Asian House Gecko Hemidactylus frenatus is one of the most successful human introductions throughout the Indo-Pacific. Yet spent over a century in Australia confined to a few local footholds. Range Expansion of the Asian House Gecko
Predicting success - the case of Lampropholis L. delicata the delicate skink L. guichenoti the garden skink These two species are genetically, physically and ecologically very similar to one another. In their natural habitat (Australia), both are frequent colonisers of human habitats where they persist in spite of much disturbance Both therefore appear to be good candidates as invasive species. SUCCESSFUL INVASIONS: Lampropholis delicata Oahu, Hawaiian Islands, approx. 1900 North Island, New Zealand, after 1955 Lord Howe Island, after 1972 Lampropholis guichenoti Nil NB: Like Hemidactylus in Australia, L. delicata in Hawaii spent a long initial period as an apparently localised species, then abruptly expanded.
There are good theoretical ecological models for explaining after the fact, why species have established feral populations. But absence of hard ecological data on most reptile and amphibian species renders the ecological approach unconvincing when trying to predict the behaviour of particular species. From the Draft review: "One of the factors most strongly influencing establishment success is introduction effort : the number sites where a species is introduced, the number of times introductions occur and the number of individuals released. Introduction effort is not included as a risk factor in assessing establishment risk in the model. This is because the introduction effort is determined by the management of a species, which is addressed by risk management Standing Committee on Agriculture and Resource Management 2001." Introduction effort The release of large numbers of animals at different times and places enhances the chance of successful establishment For birds [in translocation studies], the success of establishment dropped sharply below a release propagule size of 20 individuals. Above a release propagule size of 40, success rate was asymptotic, as predicted by MacArthur and Wilson (1967). Griffith et al. (1989). Only 3 out of 11 (27%) mammal species that were reported released at one or two sites on mainland Australia established wild populations compared to 18 of 24 (75%) species establishing that were released at three or more sites. J. Long's unpublished data on mammal introductions to Australia. The VPC model used for assessing the level of threat posed by a particular species is valuable, and its expanding data base will make it increasingly useful but - the crucial questions involve management, security and numbers, not ecology. The effect a species has on its native ecology may not be a reliable guide to how it will behave in a new environment. Apparently 'harmless species' may be ecologically damaging. Likewise, seemingly dangerous species may fail to thrive. Successful establishment of feral populations IS highly correlated with numbers; large founding populations or multiple invasions. The preceding examples have been intended to show that predicting the ecological behaviour in a new environment based purely on what is known of its biology is a very imprecise undertaking. In the terms of the title: Predicting ('tealeaves') future feral establishments should focus on numbers, of animals introduced or of distinct introductions ('tumbling dice'). Human management actions, not animal biology, provide the most readily available, practical guide for averting the establishment of any further feral herp species in Australia. Suggestions for action Controlled admission of exotic species into Australia does not in itself pose a high risk of feral establishment. Proliferation of such species does. Given that existing exotics are tolerated under a suite of monitoring protocols, and that the list of currently accepted exotics is largely arbitrary (based on species present prior to legislation) , total bans on any entry of an exotic species do not seem justified provided that the security of exotics can be monitored. Educate reptile lovers. Enthusiasm for the unusual risks the survival of the unique Australian herpetofauna. Rights carry responsibility - the right to possess a desirable exotic animal brings the responsibility to ensure that it is managed to minimise the possibility of escape or disease transmission. Control the keepers not the kept - It is reasonable to expect that scientists, businesses and herpetoculturists agree to monitored holdings of exotics. Self-sustaining captive populations of an exotic species should be kept to a minimum. Those wishing to hold exotic species must accept that ANY of these species has the potential to become an ecological pest. The fact that they have not yet become pests does not make them "safe". There are 75 species of free-living exotic birds in the United States, of which 38% [29 species] are pet bird species that established following escapes from captivity. Temple (1992). Return to conference overview « home |
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