Aspects of Science

Invasion biology is generally taken to mean the study of the establishment, massive proliferation and spread of species in a new range – detriment to the new environment and its species is usually implied.

Though he is highly critical of the way it is practiced, Professor Mark Davis states that, ‘Invasion Biology is a scientific discipline that studies the human transport and introduction of species throughout the world, as well as the subsequent spread of these species and their health, economic and environmental impacts‘. (1)

This mirrors former president Bill Clinton’s 1999 executive order on invasive species, where they were defined as non-native species whose introduction causes, or is likely to cause, harm to the economy, the environment, or human health. (2)

Is Invasion Biology a pseudo-science?

David Theodoropoulos does not mince words. He calls Invasion Biology a ‘pseudo-science’ and implicates Monsanto and other pesticide corporations in promulgating misinformation, fear, and loathing and finds many researchers have an economic interest in promoting weed extermination.

Since the early 1990s he has explored a number of areas – evolutionary theory and ‘nativism’, ‘invasion biology’, biodiversity, extinction. As with most scientists, he is not saying that there are not some species that cause ecological problems but that there should be clear differentiation between those which threaten and those that do not. He cites numerous instances where this is not the case and where species considered threatening (usually because they are common or obvious) either do no harm or indicate underlying changes to environmental conditions which cause native species to be uncompetitive. In some cases they may be beneficial.

He says that “Intact ecosystems are highly resistant to invasion, and there are also many cases of ‘exotics’ acting as nurse-plants and revegetators, helping the native ecosystem to reclaim its man-destroyed habitat. Again and again, we find “invaders” to be disturbance indicators only, symptoms of industrial abuse of the land, integrating ecologically, and with many beneficial effects that are carefully ignored by those promoting a “crisis”. ” (3)

So is there any truth in these allegations?

How does Invasion Biology relate to evolutionary theory? The rise of ‘nativism’.

There has been a lively debate about whether native plant ideology is inconsistent with the basic principles of evolutionary theory and some concern about dangerous political implications when the same concepts are applied to people. In particular, are ‘native plants’ necessarily the best to enhance biodiversity in ecological restoration?

Professor Stephen Jay Gould, was an evolutionary biologist based at Harvard University and New York University. He examined the concept of ‘native plants’ within an evolutionary framework and found “a remarkable mixture of sound biology, invalid ideas, false extensions, ethical implications and political usages both intended and unanticipated.” (4)

David Theordoropoulos (3) and Dr Ken Thompson (5) outline the chaotic course of evolution – continental movements and massive climate changes. Species are ever on the move and adapting, sometimes within a relatively short time frame. Evolutionary theory thus belies the belief in the inherent superiority of native plants based on an assumption that native plants ‘belong’ in a particular place and that their presence in the proper location represents an ‘optimal’ landscape for that place.

Many people also believe that native plants have ‘co-evolved’ with other species of plants and animals, implying that if a species disappears, other species will also disappear because they are dependent upon the plants. While this is the case for some species, it is not the case for most. As Gould explains, “natural systems [do not] always yield rich associations of numerous, well-balanced species.”(4) Others agree (e.g. 3, 6, 7).

A further difficulty with the ‘non-native’ argument relates to the baseline. Its underlying assumption is that the plant and animal communities that existed prior to the arrival of Europeans (or 60,000 years ago, of indigenous Australians) can and should be preserved. This environment no longer exists—and cannot be recreated— the original conditions that produced these assemblages of plants and animals have long since disappeared.

Does Invasion Biology comply with Ecological Theory

David Theodoropoulos finds a number of problems with this field as it is practised including: that essential terms are not defined, that there is selective data mining, that attributed causes are unsupported, that many hypotheses are non-falsifiable, that there are low standards of evidence. He gives many examples where species considered highly threatening are in fact beneficial. (3)

New (2000) warns that “…conservation biology……continues to generate a vast literature. Much of this arises from short-term research funded by bodies with a vested interest in the recommendations that may result. In many cases, opinion, hypotheses or theory, and proven facts are difficult to distinguish’. ‘The presence of many exotic species or predators in a community or habitat may suggest competition or predation as threats or causes of loss. However, simple correlation is not proof of causation.” (8)

Prof. Mark Davis, in the exhaustive book Invasion Biology argues that the entire field has been impeded by its name and that it should focus on the ecology of contemporary species redistribution independently of where they come from (he suggests renaming the discipline SPRED ecology – SPecies REDistribution) (9)

He stresses that “The problem of preliminary conclusions and tentative statements being transformed into invasion gospel is a challenge the field of invasion biology needs to work hard at preventing. If invasion biology is to be a highly regarded scientific discipline, its primary assertions and conclusions need to be based on comprehensive and thoroughly vetted data sets.” (10)

In a recent Australian work Invasion Biology and Ecological Theory, Insights from a Continent in Transformation (11) the term has been used confusingly for both species redistribution and radiation over millions of years and historical movement of non-natives into Australia and of Australian species elsewhere.

The editors (Prins & Gordon) sought to test 10 main hypotheses which have a long and respected pedigree’ in ecological theory. These hypotheses were forwarded to experts in 18 plant and animal fields for assessment. There is a qualitative framework for assessment ranging from ‘strong support’ to ‘strong reject’. On the whole, there was ‘weak support’ for the hypotheses though for many case studies there was insufficient data to assess each hypothesis. In the main, this study bore out the confusing use of terms in Invasion Biology and, in many cases, the qualitative nature of assertions.

“Even after 100 years of research by ecologists and natural historians in Australasia, the authors of the chapters in this book time and again point out that they do not have enough data to draw proper conclusions concerning the hypotheses we asked them to evaluate.”

The diversity of responses from each field of study also indicates that many factors may be important for colonisation success and that it is the interaction between the nature of disturbance and the characteristics of the species which is important. They suggest promoting an evidence-based process similar to that in the medical Cochrane Review. There may, however, be a problem in relying on ‘expert opinion’ in a discipline with such strongly held emotive beliefs and assumptions.

Davis et al. (12) outline the difficulties the field faces because of “the elusive nature of the invasion process (arising) from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules”. They suggest that one testable hypothesis is that invasibility is based on fluctuations in resource availability and thus environments with more temporally or spatially variable resources may be most invasible. This concurs with our view that we need to focus on characteristics of the environment and disturbance as well as the biology of species.

Heger et al. argue that “explanation, prediction, and management of biological invasions remain difficult”. They propose “three key measures” to overcome them: “(1) a check list for definitions to encourage explicit definitions; (2) implementation of a hierarchy of hypotheses (HoH), where general hypotheses branch into specific and precisely testable hypotheses; and (3) platforms for improved communication.” (13)

Biodiversity and The Extinction Quandry

They have names like Devil’s Claw, Patterson’s Curse, Mother of Millions, Tree of Heaven, Morning Glory. Their reputation is poor: they run rampant through our ecosystems, drive native species to extinction, they cost billions to control each year. Alien species are often cited as one of the big threats to biodiversity. What is the truth?

There is no doubt that at the global scale there has been a significant and ongoing loss of species. But is it from invading plant species?

A much quoted study by David Wilcove et al. published in 1998 “Quantifying Threats to Imperiled Species in the United States” claimed that “Invasive species are the second leading cause of extinctions” (14). Several authors have reassessed the data and found the link between invasive species and species extinction lacking:

“The link between species invasions and the extinction of natives is widely accepted by scientists as well as conservationists, but available data supporting invasion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observation.
” (15).

A study in Canada analyzing extinction threats (16) found introduced species to be the least important of the six categories analyzed: habitat loss, over-exploitation, pollution, native-species interaction, introduced species, and natural causes such as stochastic events like storms and limited dispersal ability. Habitat loss, caused primarily by agricultural and urban land conversion, accounted for 94% of threats facing endangered vascular plants.

Sax and Gaines point out that out of 22,000 non-native plants growing in New Zealand, 2,069 have become naturalized but there has been no mass extinction of native plants. They argue that competition from exotic plant species shows little sign of causing extinctions. Dov Sax argues that extinctions are far more likely to be due to predators than competitors. They also maintain that as real ecosystems take on exotic species, they do not show any sign of being saturated (17)

In most analyses, there is no differentiation between plant species threatened by invasive animals, including sheep, cattle, horses etc. and those threatened by alien plants. According the Prof. Mark Davis “How many species of plants in the U.S. have gone extinct because of the thousands of non-native plants that have been introduced? Zero!” Davis maintains that the appearance of exotics increases species richness. Dov Sax, Assistant Professor of Ecology and Evolutionary Biology at Brown University and James Brown, Professor of Biology at the University of New Mexico, are among a growing number of ecologists in agreement (6).

So let’s look at these changes in Australia.

“European settlement has significantly altered Australia’s natural landscape, and with it, Australia’s biodiversity. About 90% of native vegetation in the eastern temperate zone has been removed for agriculture, industry, transport and human habitation. About 50% of Australia’s rainforests have been cleared and the proportion of Australia covered by forest or woodland has been reduced by more than one third (Source: Creating Markets for Biodiversity, Productivity Commission, Canberra, April 2001). The effect of these changes has been considerable. Around 5% of Australia’s higher plants, 7% of reptiles, 9% of birds, 9% of freshwater fish, 16% of amphibians and 23% of mammals are listed as Extinct, Endangered or Vulnerable.”

“Figures from the National Land and Water Resources Audit suggest that, since 1788, over 700,000 km2 (about 20%) of woodland and forest have been cleared or thinned, primarily for crops and grazing. A further 130,000 km2 (35%) of mallee have been cleared since 1788, along with 20,000 km 2 of heath (45%), over 60,000 km2 of tussock grassland and smaller areas of other grasslands. Since European settlement, land clearance has been concentrated in certain areas and ecosystems. Generally those ecosystems found on the most fertile soil have suffered the highest levels of clearance, and about 90% of vegetation in the eastern temperate zone has been removed. Relatively little land clearance has occurred outside of the high rainfall and semi-arid zones, although in these areas other pressures such as grazing (both from domestic stock and introduced herbivores), weeds and changed patterns of fire are having an impact on the land. More than 90% of land clearance has occurred in 25 of Australia’s 85 bioregions (areas of land that contain linked ecosystems). These bioregions occur across south-west Western Australia, southern South Australia, most of Victoria and New South Wales, and central and southern Queensland.
” (19)

There has been a great deal more clearing since then, particularly in Queensland and we now have the new threats of large-scale mining and its infrastructure and its water usage.

In Australia, non-native animals have certainly caused considerable changes to plant communities through increased herbivory and changes to soils and hydrology (horses, cattle, sheep, camels, pigs, donkeys) but also through predation of native herbivores (cats, foxes, dogs).

The Invasive Species Council website states that: “Australia has the worst mammal extinction record in the world, due mainly to invasive species like foxes and feral cats.” However, even here, there is increasing evidence that it is destruction of dingos which is contributing greatly to the proliferation of foxes and cats and thus to mammal extinctions (20). Predation balance has a significant role to play in vegetation management.

Similarly, flying foxes, as a keystone species, are integral to rainforest maintenance and rehabilitation:

“As long-range pollinators and seed dispersers, their activities are essential to the health of native ecosystems. Indeed, as climate change forces species to adapt rapidly, flying foxes will become increasingly important in maintaining gene flow and thus facilitating adaptation. As the populations of flying foxes are in rapid decline, there is the possibility that some species in some areas may become functionally extinct within a few decades.Functional extinction precedes actual extinction; it is a loss of connectivity and mutuality. Lose the flying foxes and there’s no way of knowing just how far the unravelling of life systems will go.” (21).

Let’s put this into a bit of Perspective: Threatening or Beneficial?

Some ecologists argue for a more nuanced approach to identifying problem species. In their article ‘Don’t judge species on their origins’ (22) Prof. Mark Davis and 18 other leading ecologists state “..we urge conservationists and land managers to organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services and economies. Nearly two centuries on from the introduction of the concept of nativeness, it is time for conservationists to focus much more on the functions of species, and much less on where they originated.

They believe that “Natural-resource agencies and organizations should base their management plans on sound empirical evidence and not on unfounded claims of harm caused by non-natives.

In Where do Camels Belong? The story and science of invasive species Dr Ken Thompson (5) argues that species aren’t necessarily ecologically harmful just because they are introduced, nor are native species necessarily good for biodiversity. Thompson argues that most alien species – even some topping the eco-horror lists – cause little or no lasting damage and aren’t worth the angst, effort or money we devote to controlling them.

Nor are introduced species the financial burden they are often made out to be. For one thing, says Thompson, hardly anyone bothers to count the economic benefits of “aliens” such as wheat and cows – a sum that runs to $800 billion per year in the US alone. Moreover, much of the cost of the invaders turns out to be the money spent controlling them.

Davis and Thompson have attempted to classify colonisation after disturbance to stress the ecological processes of succession and to provide a focus for identifying novel plants which may cause problems from those that won’t. (23)

Schlaepfer et al. believe that: “A fraction of non-native species will continue to cause biological and economic damage, and substantial uncertainty surrounds the potential future effects of all non-native species. Nevertheless, we predict the proportion of non-native species that are viewed as benign or even desirable will slowly increase over time as their potential contributions to society and to achieving conservation objectives become well recognised and realised.” (24)

Overselling the threat exotic species pose is bound to lose credibility as exotics make up ever more of the biota around us, says Davis. And it will lead to misguided spending on projects as fruitless as ripping out buckthorn from thousands of acres of parks, when we should focus instead on disease organisms, agricultural pests, and other more pressing threats, Davis believes. “It’s very important,” he says, “to distinguish harm from change.” (6)

We don’t know how serious the threats of alien invaders are to our native flora and fauna; these are scientific questions. And, as is often the case in science, the answer is less clear than reports in the popular press about the devastating impacts of gypsy moth, zebra mussels, and purple loosestrife might imply…. Although we often accuse invasive species of damage to the structure and function of ecosystems, there is usually little hard scientific evidence of such negative impacts.” (25)

Why is a weed there? The role of disturbance

Wherever weeds grow they indicate a disturbance: the disturbance is chronic in forestry, mining, urbanisation or agriculture but may be episodic in natural ecosystems. Weeds are Nature’s way of covering and stabilising soil that has become exposed creating empty ecological niches or of using changed conditions, like the nutrient enrichment of waterways.

Our belief is that we should pay more attention to the changes that are being made to our environments which are stimulating ‘invasions’ rather than focusing on the ‘invading species’.

Human activities which either provide new ecological niches through some disturbance or by importing new plant species are major causes of increasing abundance and diversity of introduced plants. There are nearly always weed seeds in the soil but many will not germinate until the soil is disturbed. Repeated herbicide spraying will interfere with soil micro-organisms and provide ideal conditions for weeds to grow.

“We destroy self-healing landscapes and try to impose native-only species using military-industrial machinery and toxins. In fact, conditions have changed: pollutants now enter the system, the soil has been washed away, the climate is changing rapidly, the people that used to live in and manage the system were destroyed by genocide, and the megafauna were lost only a few millennia ago — but we think we can force nature back to an imagined and non-existent past. If instead we can see weeds as part of nature, and value their vigour and productivity, we can continue to identify with them, but change our philosophy about our potential ecological relationship with this damaged country. We are indeed like weeds, and we can heal the land too.”

For adequate assessment for weed control, we must consider all environmental factors. The elephant in the room is the lack of research into the nature, types, and outcomes of disturbance in different ecological communities.

More Questions than Answers

Jono Neiger worked for The Nature Conservancy restoring riparian forest on the Sacramento River in California in the early 1990s, fighting ‘invasive species’. He recognised that the strategies used were very expensive and energy intensive. He says “I have since come to have more questions than answers about invasive species”. Questions like:

  • Are naturalized species the drivers or passengers of change?
  • Are the long-term effects negative?
  • Do human-caused invasions differ significantly than “natural” invasions?
  • How do we separate invasive species effects from climate change, habitat loss, development, sprawl and human population impacts?
  • What objective ecological criteria identify “alien species” or “invaders”?
  • What are the temporal and spatial scales included in the definitions?
  • What protocols will determine the conservation value of new species populations that have moved outside their “historical ranges”?
  • Should we consider the mere existence of an allegedly non-native species sufficient evidence for control or extermination? On what factual basis can we take this position?
  • How do we measure harm and how is harm different from change?

He says:

“Scientists are grappling with these questions. Notably, Dr. James Brown, Distinguished Professor of Biology at the University of New Mexico and Dr. Dov Sax, Assistant Professor at the University of Georgia’s Institute for Ecology, have wrestled with the relationship of invasive species and ecosystems.” He concludes: “Close observation and good research is required to understand the ecological implications of the changed ecosystems around us. I’ve also looked closely at the emotional reactions of myself and others to the changes in species and ecosystems. The changes we humans are witnessing (and causing) are vast.”


  1. Davis, M. A. 2011. Invasion Biology. Pp. 364-369, in D. Simberloff and M Rejmanek, eds. Encyclopedia of Biological Invasions. University of California Press.
  2. Davis, M. A. 2013. Invasive Species. In, N MacLeod, ed. Grzimek’s Animal Life Encyclopedia: Extinction, 1st Edition. Gale, 779-787
  3. Theodoropoulos, David.
  4. Stephen Jay Gould, 1997. “An Evolutionary Perspective on Strengths, Fallacies, and Confusions in the Concept of Native Plants”), in Nature and Ideology, ed. J.Wolschke-Bulmahn, pp11-19, Dumbarton Oaks, Trustees for Harvard University, Washington DC.
  5. Thompson, K., 2014. Where Do Camels Belong? The Story and Science of Invasive Species. Profile Books Ltd, London.
  6. Breining, G., 2009. Courting Controversy with a New View on Exotic Species, Report, Nov 19.
  7. Chew M.K., and Hamilton A.L. 2011. The Rise and Fall of Biotic Nativeness: a Historical Perspective. Chapter 4 in Fifty Years of Invasion Ecology: The Legacy of Charles Elton, (ed. David M. Richardson, Blackwell Publishing Ltd.
  8. New, T.R., 2000. Conservation Biology: an Introduction for Southern Australia. Oxford Uni. Press, S.Melbourne.
  9. Davis M.A. 2009. Invasion Biology. Oxford: Oxford University Press. Invasion Biology
  10. Davis M.A, 2011. Researching Invasive Species 50 Years After Elton: A Cautionary Tale. In Fifty Years of Invasion Ecology: The Legacy of Charles Elton. (ed.) David M. Richardson, Chapter 20, Blackwell Publishing Ltd (PDF)
  11. Prins H.H.T. and I.J. Gordon (eds), 2014. Invasion Biology and Ecological Theory, Insights from a Continent in Transformation, Cambridge University Press, Sydney.
  12. Davis M.A., Grime J.P., Thompson K.,2000. Fluctuating resources in plant communities: a general theory of invisibility, Journal of Ecology, 88, 528-534)
  13. Heger T., Pahl A.T., Botta-Duka´ t Z., Gherardi F., Hoppe C., Hoste I., Jax K., Lindstro¨m L., Boets P., Haider S., Kollmann J., Wittmann M.J., Jeschke J.M., 2013. Conceptual Frameworks and Methods for Advancing Invasion Ecology, AMBIO42:527–540
  14. Wilcove D., Rothstein, D., Dubow J., Phillips A., Losos E., 1998. Quantifying Threats to Imperiled Species in the United States. BioScience, Vol. 48, No. 8, pp. 607-615.
  15. Gurevitch J. and D.Padilla 2004. Are invasive species a major cause of extinctions? Trends in Ecology and Evolution, Vol 19.
  16. Venter, O., Brodeur N.N., Nemiroff, L., Balland, B., Dolinsek, I., Grant, J.A., 2006. Threats to Endangered Species in Canada, Bioscience, 6, No.11.
  17. Sax, D. F. and Gaines, S. 2008. Species invasions and extinction: The future of native biodiversity on islands. Proceedings of the National Academy of Science. (PDF)
  18. Bush Heritage Australia, 2016. Australia is still clearing too much vegetation.
  19. Australian Bureau of Statistics 2002
  20. Dickman C.R., Glen, A.S., Jones, M.E. Soule, M.E., Ritchie, E.G., Wallach, A.D., 2014. Strongly interactive carnivore species: maintaining and restoring ecosystem function. Ch 13, In Carnivores of Australia, (eds) Glen, A.S. And Dickman, C.R., C.S.I.R.O. Publishing.
  21. Rose, Deborah Bird, 2011. Flying Fox: Kin, Keystone, Kontaminant. Australian Humanities Review, Issue 50.
  22. Davis, Prof. Mark, Chew, MK, Hobbs, R.J., Lugo, A.E. Ewel, J.J., Vermeij, G.J., Rosenzweig, M.L., Gardener, M.R. , Carroll, S.P., Thompson, K. Pickett, T.A., Stromberg, J.C., del Tredici , P., Suding, K.N., Ehrenfeld, J.G., Grime, P.J., Mascaro, J., Briggs, J.C., 2011. Don’t judge species on their origins. Nature, June 8.
  23. Davis, M & K.Thompson, 2000. Eight Ways to be a Colonizer; Two Ways to be an Invader: A Proposed Nomenclature Scheme for Invasion Ecology. Bulletin of the Ecological Society of America, July.
  24. Schlaepfer, M.A., Sax, D.F., and J.D.Olden, 2011. The Potential Conservation Value of Non-Native Species, Conservation Biology, 25, 3, pp428-437
  25. Brown, J.H. & Sax, D.F.(2007). Do biological invasions decrease biodiversity? Conservation Magazine, 8, 16-17
  26. Fenderson, Adam.
  27. Neiger, Jono, 2011. Invasive Species: Some Science and Some Questions. Ecological Landscape Alliance.

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