Icon Books 2015

In 1910  New Zealand's great botanist Leonard Cochayne described the dramatic change in  plant communities which had occurred since the first visit of Captain Cook to the country in 1769 (1). Some 560 new species from Europe, Africa and elsewhere had by become established by then, with half of them common throughout the country from the coasts to the highest mountains:

At first thought, the idea of 560 different sorts of plants- some of them the most aggressive weeds in Europe- having not only been loosed to do their will, but also having established a secure footing, would lead to the conclusion that, if not the flora of New Zealand, at any rate the primitive vegetation was doomed. No conclusion could be more incorrect. Were it not that man has changed, and is changing, the face of nature by means of his farming operations, his grazing animals, his fires, his drains, and his intensive exploitation of rain forest and flax swamp, the host of foreign plant invaders would be powerless- the indigenous plants, attuned to the special life conditions f their native land, would laugh these aliens to scorn. Why, even now, when the introduced plants have man as their potent ally, 66 percent of the species are rare or local, 40 percent being so rare as to be negligible, while merely 34 percent can be classed as extremely common, common, or fairly common, these being taken together. But these percentages do not emphasise the real state of affairs, for many of the commoner plants are confined to sides of toads, neglected building sites, and rubbish heaps- in short, to "waste ground" as it is called- and there are many other species restricted to cultivated land. In fact, probably only about 100 species are established on land where the vegetation would be exposed to modification only by grazing, fire, and other causes due to the indirect action of man.

The warfare, indeed, between the plant inhabitants of primitive New Zealand and the alien invaders is waged almost entirely under conditions where man takes a powerful hand, for, except for certain rock, stony debris, and water-plant formations, no primitive plant community has been desecrated by a single foreign invader. This is a very different version of the story from that even yet current in biological literature, where it is affirmed ad nauseum that the New Zealand vegetation is powerless when it comes into competition with the European plants, which by natural selection have become the very elite of the weed world.

Cochayne's observations made over a hundred years ago are almost identical to those made forcefully in Fred Pearce's provocative new book which takes to task invasion biology- the view that non-native species are generally "invasive", constituting one of the greatest threats to biodiversity and ecosystem health, and need to be controlled and where possible eradicated completely- almost at any cost.

The European Commission on the Environment describe Alien Invasive Species (AILs) as "a major threat to native plants and animals in Europe, causing damage worth billions of euros to the European economy every year." Bird Life International call for a far more extensive policy than that currently proposed, listing over 200 invasive species as of "high priority for urgent risk assessment" in addition to the 37 that are currently listed for control; while the WWF quote the World Conservation Union as saying

the impacts of alien invasive species are immense, insidious, and usually irreversible. They may be as damaging to native species and ecosystems on a global scale as the loss and degradation of habitats.

Hundreds of extinctions have been caused by invasive alien species. The ecological cost is the irretrievable loss of native species and ecosystems.

Pearce, winner the UK's environment journalist of the year in 2001 and author of other books on climate change, population and sustainability, comprehensively rejects these assessments. Exactly as  Cochayne  describes above, "invasives" are more accurately thought of as opportunists which generally only move into ecosystems that have already been severely degraded by other human activities, and are able to thrive on our pollution where nothing else can. For example, the infamous Zebra mussels that spread through Lake Erie at such a rate in the 1980s were moving into an ecological desert so atrophied that everything else had already died- and apparently did a fairly good job of cleaning it up. In time, the previously endangered lake sturgeon, bass and migrating ducks moved back  to feed on the mussels.

This is a typical pattern with so-called "alien invasions" which are generally the consequence rather than cause of previous disturbance. Another infamous "invasive exotic", Kudzu, from Japan- which became known as "the vine that ate the South" because of its rapacious spread through the southern US- had previously been widely planted as an ornamental, and for animal fodder and erosion control. It only got out of hand as a result of other land-use changes:

The vine hasn’t changed. It is still revered in Japan. What has changed in America is the land and people’s expectations of the land. Kudzu’s foliage is no longer needed to feed grazing farm animals, which now live in feedlots. The pastures are abandoned. No longer kept in check by grazing, kudzu now grows where it is not wanted, spreading unchecked almost anywhere south of the Mason–Dixon line. It is the enemy. The pastures are being turned into woodland, where kudzu is a problem.

The reality is that out of tens of thousands of introduced species- including most of our food plants and garden ornamentals- only a tiny minority ever become problematic or a threat to "native" vegetation in this way. Often, for all the trouble they cause, they also can do a lot of good; the costs they are claimed to incur are often wildly exaggerated based on simplistic extrapolations (2), and these could well be exceeded in some cases by the costs of control, which mainly are doomed to failure anyway. Nor is it true that they are generally likely to cause extinctions- Pearce concludes that this is based more of the assumption that  "exotics are bad" than supported by rigorous evidence. While there have certainly been cases of loss of biodiversity on remote islands, where local species have little options to extend their range, there are plenty of counter examples where introduced species have increased biodiversity. The problem is that "invaders" are simply not valued in the same way that "natives" are:

In fact we seem to have gone a long way from any interest in biodiversity. The interest is entirely to do with protecting natives and avoiding change.

...alien species don’t count and are not counted. They do not exist as part of nature. They have no place. They are un-nature, if not anti-nature. They should be gone. Under this definition, biodiversity in the 21st century can only go down. Extinction could cut the number of species, but introductions could never increase it. Thus the inconvenient fact that alien species actually increase real biodiversity in many places is simply defined away. Big Brother in Nineteen Eighty-four would be proud. Franz Kafka would be proud. Joseph Heller would have added an ecological chapter to Catch-22, if he had known. It sounded more like an ideology than good science.

A lot of the problem has to do with conflating "invasive" with "exotic/non-native", but just as most "exotic" novel species are not invasive, so there are also "native invasives". Bracken fern Pteridium aquilinum,  brambles Rubus fruticosus, and gorse Ulex europaeus,  all "native" to the UK but invasive as vigorous weeds or early colonisers of disturbed ground if they come across the right conditions.

More than that, and fundamentally to the whole debate, the line between what is considered "native" and "non-native" is not just blurry but scientifically meaningless. "Everything is visiting. Nothing is native" observes Pearce. What specific combination of plants have ended up living together in different areas at any given time is largely a function of chance. Some species made it across to Britain when there was a land bridge, some got stuck across the water when the sea levels rose again. In trying to maintain "native" vegetation we are necessarily picking a particular period in time from which to judge what can stay and what must go.

Rhododendron ponticum, a major target of conservation control in Britain because of its ability to prevent regeneration of woodland, had been "native" here before the last ice-age. Does this discount it on the basis that plant communities here have since evolved without it? The difficulty is that many other species are in the same category, but are not persecuted as invasive in the same way. Ken Thompson, author of another recent book on the same topic (2) points out that Fritillaria meleagris is generally considered a full-blooded British native, but was first recorded in the wild here only in 1736; while R. ponticum  first introduced to Britain in 1753, is still an "invasive exotic." To further complicate matters, it is really a hybrid of three other Rhododendron species, which has evolved uniquely in the UK and is not found anywhere else! What, then, can it mean to call it anything but a native? Rhododendron  probably only became so widely spread anyway as a direct result of being extensively sown throughout British woodlands to provide cover for game.

(Conversely, Thompson gives the example of the quintessentially "English" oak  which,  having spent more than 99% of the last 2million years in Iberia, might more properly called Spanish.)

While there are specific cases of co-evolution between two species, they tend to be more exceptions than rules, and it is relatively rare for any group of species to be entirely dependent on their specific co-evolutionary companions- it turns out that ecosystems can usually function perfectly well, and often with increased diversity, with a mixture of old friends alongside new neighbours.  Pearce points out that Darwin did not see co-evolution as a principle driver of evolution, and quotes ecologist and invasion biology critic Mark Davis who states 'nativeness is not a sign of evolutionary fitness’. Darwin was clear that the individual species is the primary unit of natural selection, not the "ecosystem"- a nebulous and controversial concept in itself. Much of the ideology surrounding the desire to keep nature in an idealized state of pristine "natural balance", frozen at an arbitrary period of time (generally pre-Columbus) has less to do with Darwin than, as Daniel Botkin has argued, with the much older Judeo-Christian belief in the Great Chain of Being and guilt over the Fall from Eden (4).

Nature is always changing, and is proving in many respects far more resilient and adaptive than it is often portrayed. While most conservation efforts focus on extermination of plants that happen to make it onto the "invasive species" lists,  entire new ecosystems, and often highly diverse regenerating secondary forests are emerging all around us.  Perversely, such habitats are not deemed worthy of conservationists' efforts to protect them- despite increasing evidence that even most so-called "pristine" habitats were subject to significant anthropogenic change in pre-history (5). Pearce sees this as a blinkered and narrow view and a sad lost opportunity. It is this "New Wild" that he feels we should now turn our attention to, since in the rapidly changing world of the Anthropocene,  there is very little, if any, "pristine" wild left. With more ecologists like Davis speaking out from this perspective of a "new ecology"(6), science has already moved on to a large degree from earlier conceptions of ecosystem balance. Now it is time for public understanding of this science, and conservation policy, to catch up.

Pearce has written an engaging book that should be a valuable contribution to this often confusing debate. If he falls short in any area, it is perhaps the cursory passing over of the threat posed by novel pathogens, which are increasing due to global trade of plant material with potentially devastating consequences to trees and shrubs (7).  In general though he is careful not to fall into the trap of claiming there is no issue at all with introducing new species which might be at the very least weedy or have unintended consequences. Some newcomers can cause serious problems, more for humans than for "nature" though,and noone wants rampant weeds in their gardens. Plants should be investigated on a case-by-case basis, not assumed to be inherently bad or noxious on the basis of the largely spurious notion of nativeness: the vast majority are benign. This raises another issue, as pointed out by Thompson: the public's goodwill, without which even the most favourable eradication or control efforts cannot in any case succeed. The public do not intuitively divide plants into native or non-native, but assess them on their characteristics of usefulness or aesthetics, which is exactly why plants are frequently assumed to have been native for a long time when, like the fritillary they are relative recent arrivals, and vice-a-versa. It is not just environmental issues at stake here, but the public's trust in the integrity of science.

More than just a critique of conservation, Pearce also presents a damning indictment of science. With honorable exceptions, few scientists have spoken up against the tendency of NGOs and policy makers to rely heavily on just one or two studies which have either been misrepresented or have little real evidence to back them up, and costly and often damaging alien eradication programs have been allowed to continue unnecessarily, often for decades.

As is clear from Cochayne's book written at the beginning of the last century, there was never any very solid science behind invasion biology, and it is time for this to be more widely understood and debated in the public realm.

Also appeared on on theculturalwilderness

References

1. Cochayne, L.  1910 New Zealand Plants and their Story
2. see Pimentel, D. et al 2000 Agriculture, Ecosystems and Environment 84 (2001) 1–20
   
   Economic and environmental threats of alien plant,

   animal, and microbe invasions Agriculture, Ecosystems and Environment 84 (2001) 1–20.  Pearce claims the widely cited figure of $1.4 trillion being the annual coast to the global economy from invasive species is an extrapolation to the entire world from just six major economies, and the biggest three causes of these costs coming from rats, weeds and plant pathogens in agriculture; among other problems Pearce points out,  no accounting is allowed for possible benefits to the economy from aliens (such as cleaning water pollution in the Lake Erie by the Zebra mussel mentioned above).
3. Thompson, K. Where do Camels Belong?
4. Botkin, D. The Moon in the Nautilus Shell
5. Bowman, David MJS, et al. "The human dimension of fire regimes on Earth." Journal of biogeography 38.12 (2011): 2223-2236.
6. see for example Brown, J. and Sax, D. An Essay on Some Topics Concerning Invasive Species Austral Ecology (2004) 29 , 530–536:
   

   The rare, restricted species are disappearing and the common widespread species are becoming even more abundant and widely dispersed. This hasbeen referred to as the homogenization or cosmopolitanization of the world’s biota (Brown 1995; McKinney&Lockwood 1999).Is this decrease in global biodiversity a bad thing? Is the net increase in local species richness due to invasions a good thing? Is high species richness desirable? We do not believe that these are scientific questions.Science can elucidate the causes and consequences of these changes in biodiversity, but ultimately deciding what is good or bad is a moral and social issue. Few people would question whether the dozens of exotic flower and vegetable species in their gardens are desirable. The value judgements may change, however, if some of those same species were to become naturalized and spread into wild areas or to become serious weeds in agricultural fields.

7. Rackham, O. 2014 The Ash Tree Little Toller Books, Dorset