Dr. Scott Carroll: Ecological communities rapidly adapt to new species

We are republishing an article from the San Francisco Forest Alliance with permission.  This excellent summary of Dr. Carroll’s presentation was approved by Dr. Carroll.  Dr. Carroll is a leader in the scientific community in identifying rapid evolution as a mechanism which enables introduced species to rapidly adapt to their new environment as well as the ability of their new neighbors to adapt to them.


Dr. Scott Carroll of UC Davis, and the founder of the Institute for Contemporary Evolution in Davis, spoke at the Commonwealth Club as part of the series “The Science of Conservation and Biodiversity in the 21st Century.” His main message:

Mixed communities, consisting of non-native and native species of plants, animals and other organisms, are here to stay. We need to find ways to live with these new neighbors. Once they are introduced, they will evolve, and so will the species that were already there. Trying to eradicate “invasive” and non-native species is expensive, likely to cause unforeseen problems, and have uncertain success – in part because evolution will make a moving target of an introduced species.

Read on for notes from Dr. Carroll’s talk. (There are also links to his Powerpoint presentation, and to the audio recording from the Commonwealth Club.)



(Notes from a talk by Dr Scott Carroll)

Plants and animals have always moved around the planet, but gradually enough that the world had distinct bio-regions with their own indigenous species. About 500 years ago, shipping greatly increased the pace – people deliberately or inadvertently introduced species into new places. It’s what people do.

historical bioregions - dr scott carroll talk at commonwealth club 2014Invasion biology” as a discipline originated with Charles Elton’s 1958 book. The response to Invasion Biology is a deeply emotional one, coming from a sense of how an ecosystem should look and how species should interact. The transfer of species around the globe has been called the greatest ecological spasm since the extinction of the dinosaurs. But is it?

brown earthIn fact, there’s been a much more important change. The amount of wilderness on the earth’s surface has fallen sharply, from around 50% in the 1700s, to around 20% by the year 2000. The rest is cultivated or range lands or built-up. We need to look at invasion biology – permanently mixed communities of native and introduced organisms – in the context of that land use change.

These land use changes drastically altered the environment for all species, with a major impact on all species and ecological relationships. Natural selection picks new winners: Changed environments have different fitness criteria, so plants or animals that were successful before may become losers. If populations decline, it reduces their chances of evolving to meet the new environment: fewer individuals mean a smaller gene pool, fewer potentially beneficial random mutations, and fewer offspring. Some species go extinct.

But others don’t. They adapt and evolve and use the resources the new environment or new introduced species provide. Some players in these novel interactions have the capacity to solve their own problems, restoring more balanced kinds of ecological interactions than one would expect from the terms “invasion” and “takeover” and “destruction.”


How do we define an “Invasive Species”?

It’s a species not native to a bio-region that are:

  • Introduced
  • Reproducing independent of our assistance (naturalized, in the case of plants) and
  • Very specifically, they are doing something that we do not like.

This means that it has to be defined with reference to who “we” are. The definition of “invasive” must include who is doing the defining.

In some cases there’s broad agreement. Nearly everyone agrees on fighting invasive disease-causing insects, for instance. In the case of plants and trees and animals, people may diverge sharply in their opinions. Eucalyptus is an example; those who dislike it make consider it invasive; others would disagree vehemently.

The ‘eradication’ arm of Invasion Biology – i.e. those looking to destroy introduced species, perhaps 90% of invasion biologists – is fighting a very difficult battle. It’s extremely expensive, and risks doing much more harm than good.


What are the problems of Eradication policies? Here are 8 issues:

1. It’s extremely expensive, both in time and effort.
Even in cases that seem possible – eliminating rats on an island, for instance, it may be an uphill battle. The first 75% are easy to kill. The next 20% are more difficult. By the time you’re down to the last 5%, your team is exhausted and you’ve “spent $3.2 mn of your $2.7 mn budget.” You haven’t seen a rat recently, so you leave. And then – the 2% of the rats that remain reproduce and repopulate the entire island in five years.

2. It doesn’t necessarily solve the problem.
Even if you succeed in killing off the invader, it won’t necessarily bring back the ecosystem that existed before. For instance, soil conditions may have changed so instead of native plants returning, other non-native plants – or nothing at all – grows.

3. It can disrupt ecological systems.
For instance, an introduced predator may have been keeping an introduced plant-feeding prey species in check. Once the predator is eliminated, the prey may destroy vegetation and the ecosystem as it exists.

4. It can disrupt replacement ecological relationships that existing plants and animals may have developed with the “invaders.” They may be providing food, seed dispersal, pollination, cover and other ecosystem services. For instance, if a native plant species is declining because of climate change, an introduced species can provide food for birds, animals and insects.

Loggerhead_sea_turtle_hatchlings_caretta_caretta public domain5. Sometimes, the new species provide a new ecosystem service to existing species, and destroying them would hurt the native species too. For instance, beachside non-native trees may protect turtle hatchlings from artificial lights that can disorient them, and so improve their survival rates. Or the non-native species may be controlling a different non-native species that might otherwise become a pest.

6. Trying to kill off non-natives can drive them to evolve resistance to the agent used to kill them. This is a common problem when herbicides are widely used to kill “invasive” plants.

7. Killing non-natives may reduce biodiversity of the area by reducing the pressure on native species to diversify and become new species.

8. The new species may directly increase biodiversity in the area, and eradicating them reduces this biodiversity.


rabbit public domain clipartA story that illustrates many of these problems is that of the myxoma virus, used to control Australia’s huge rabbit population.

The Iberian (or European) rabbit was introduced into Australia by Europeans in the 19th century, and eventually bred so prolifically that it started to destroy the environment. In the 1950s, the Australian government introduced the myxoma virus, a New World rabbit disease lethal to European rabbits. Initially, that killed 99.5% of infected rabbits, and the population plummeted.

But the surviving rabbits continued to breed until the next time the disease went around. With each successive outbreak, the mortality declined. Frank Fenner, the scientist overseeing the project, found that the virus was attenuating – becoming less lethal – while the rabbits were developing resistance to it. Eventually, each outbreak killed only 20% of the rabbits in the area.

In 1952, a landowner in France introduced the virus on his farm to control rabbits. Soon it spread across that country, and then to Britain, where it killed 95% of the rabbit population.

Large Blue Butterfly - wikimedia commons cca3 - PJC&coThis led to the extinction of the British population of the Large Blue Butterfly. The butterfly is an unusual species whose caterpillars mimic larvae of the ant species Myrmica sabuleti, so they get carried into the ant’s nests where they eat the larvae. Rabbits cropping meadow-grasses had kept them short, providing ideal conditions for the ants. With the rabbits gone, the grass grew, ants declined, and the Large Blue Butterflies vanished.

iberian lynx wikimedia commons cca25-klia Meanwhile, the myxoma virus also reached the Iberian peninsula, where it devastated the native rabbit population. The rare Iberian lynx, which depends solely on rabbits as a food source, became critically endangered, and the Iberian eagle – which also preyed on rabbits – declined sharply. Aquila_adalberti wikimedia commons cca3Officials are looking to vaccinate the wild rabbit population against myxomatosis.

Another unforeseen consequence occurred on Macquarie Island. This desolate Australian island was a breeding place for seals and sea-birds. Human introductions of rats (inadvertently), rabbits (for food), and cats (to combat the rats) impacted the sea-bird populations. The eradicators first introduced fleas to the island as a vector for the myxoma virus, and then the actual virus in 1978. Then they eradicated the cats. However, the cats had been hunting the rabbits, and now the rabbits multiplied out of control reversing years of conservation efforts. The myxoma virus had likely attenuated, and failed to control the rabbit numbers. The rabbits grazing destroyed the hillsides where the penguins nested, causing landslides that harmed their breeding success.


The story of the soapberry bug is more encouraging.

Soapberries are a plant family with a number of separate species, two of which are invasive vines in Australia. They invade along water-courses, and grow over trees in those areas. One vine species reached Northern Australia around 1680; the other, much taller species arrived in Eastern Australia around in the 1920s and has become particularly damaging to the forests there.

soapberry bug smSoapberry plants have fruit of varying sizes with nutritious (to insects) seeds at the center. Soapberry bugs are specialized soapberry eaters, with long beaks to pierce the fruit and reach the seed. The beak-lengths of these bugs are evolved to fit the particular species of soapberry they prey on.

When the introduced soapberry plants arrived in Australia, the native soapberry bugs had beaks too small to use the new food source. But with time, they started to evolve.

In Eastern Australia, it took 30 years for the soapberry bug’s beak to evolve from 7 mm to 7.5 mm. That doesn’t sound like much, but an increase of 0.5 mm doubles the number of seeds the bug can reach.

In Northern Australia, where the bugs have had over 300 years to evolve, their beaks have grown from about 5.5 mm in length to around 8 mm – exactly the length they need to attack the introduced soapberry plant. They match as well as if the soapberry plant was native.

It’s the same species of bug.

One interesting experiment would be to see if breeding the two strains would help the Eastern bugs grow a longer beak and control the soapberry vines better. Dr. Carroll recommended stopping the plant eradication program in Northern Australia to protect the long-beaked soapberry bugs there while evaluating whether interbreeding the two bug strains could accelerate the evolution to slow the spread of the large vine in Eastern Australia.


Dr. Carroll stopped his presentation there because time ran out. But if you would like to see his PowerPoint slides, they are here (in ppt and pptx formats). The Commonwealth Club’s Audio recording of his talk is also linked here. (There’s a lively question and answer session at the end, which isn’t included in these notes.)

Powerpoint presentation in ppt format: S Carroll Commonwealth Club Jan 2014

Powerpoint presentation in pptx format:S Carroll Commonwealth Club Jan 2014 (2)

Audio recording from Dr Carroll’s Commonwealth Club talk: http://www.commonwealthclub.org/events/archive/podcast/scott-carroll-conciliation-biology-13014

Conciliation Biology: Revising Conservation Biology

Our interest in invasion biology is primarily in its application, specifically to “restoration” projects.  Therefore, as science revises the assumptions of invasion biology we are equally interested in the implications for ecological restorations.

Professor Scott Carroll (UC Davis) is a particularly good candidate to lead the way in revising ecological restoration practices, as informed by current scientific theories of invasion biology.  His study of rapid evolution of the native soapberry bug to accommodate use of non-native vegetation puts him in the forefront of the effort to integrate evolutionary theory into invasion biology.

And so we introduce to our readers, Professor Carroll’s proposal that we turn from efforts to eradicate non-native species in favor of a new approach which manages the co-existence of native and non-native species.  He calls this approach Conciliation Biology.*

Conciliation Biology is based on these premises:

  • The environment has been radically altered by the activities of humans
  • The environment will continue to change in the future.
  • It is not feasible to eradicate non-native species.
  • The cost of attempting to do so is prohibitive.

These are familiar themes on Million Trees and we will not belabor them in this post.  Rather we will focus on those aspects of Professor Carroll’s proposal that are new to us.

Rapid evolution can resolve apparent ecological problems

Garlic mustard. GNU Free

Garlic mustard is an invasive non-native plant which tolerates shade and emits a powerful root toxin known to inhibit the germination of other plants, notably forest trees.  This chemical tool to reduce competition is known as allelopathy,  a weapon used by many plant species, both native and non-native.

Since garlic mustard arrived first in the eastern US and spread slowly west, scientists compared the allelopathic toxicity of a population of garlic mustard known to have arrived 50 or more years ago with a population which arrived only 10 years ago.  The toxicity of the recently arrived garlic mustard was significantly greater than that of the older population.  In fact, the understory and seedling germination were rebounding in the forest with the older population of garlic mustard.

In other words, science informs us that ecological problems caused by the arrival of new exotic species can resolve themselves over time.

New exotic species are sometimes better adapted to the changed environment

Professor Carroll cites a study of two aquatic species (Phragmite and Hydrilla) which provide superior ecological services than their native counterparts because of changes in the environment.  The extreme weather events associated with climate change are subjecting our coasts to unprecedented storm surges.  Native species of marsh grass are not as successful in protecting the coast against the ravages of these storm surges.

We have our own local example of the same phenomenon.  Non-native Spartina marsh grass is being eradicated along the entire west coast of the country.  It grows taller and thicker than native Spartina and it does not die back during the winter months as the native species does.  Since storm surges occur during the winter months, surely the non-native Spartina provides superior protection to our coast.  We have yet to see a scientific experiment which proves this point, but common sense tells us that it is a study that needs to be done, particularly since ornithologists have reported that the eradication of non-native Spartina has been harmful to our dwindling population of endangered California Clapper Rail.

The harmful effects of eradication efforts

Iberian lynx. Creative Commons

We have seen many such harmful consequences of eradication efforts, but Professor Carroll provides his own example.  Iberian rabbits are native to Spain.  They were intentionally imported to Australia where they quickly became a problem.  The Australians imported a virus from South America that killed the rabbits.  The virus was also introduced to Britain for the same purpose.  The virus has spread back to Spain where it is killing the rabbits in their native range.  The rabbits are prey of several rare species of animals in Spain, including the Iberian lynx.  The absence of their prey is now decimating those native predator populations as well.

Biological controls are one of many dangerous games being played by those who share in the fantasy that it is possible to eradicate non-native species without paying a price.  Sometimes that price is greater than whatever cost may be associated with the non-native species.

Simply eradicating non-native species will not necessarily result in the return of natives

Professor Carroll tells us the story of the failed attempt to save the Large Blue butterfly in Britain from extinction to illustrate this point.  This was apparently a spectacularly beautiful butterfly, and so the British spent 50 years trying to bring it back from extinction.  They failed because they figured out too late that the butterfly is dependent upon an ant which lives only in heavily grazed vegetation.  The ant population no longer existed within the range of the butterfly because grazing had long ago been abandoned.

How many other pointless efforts to reintroduce endangered species are there?  We recently told our readers about the effort to reintroduce the endangered Mission Blue butterfly to Twin Peaks in San Francisco.  This is a radically altered environment with high levels of nitrogen and carbon dioxide associated with the urban environment.  The annual brush fires of pre-settlement San Francisco are no longer capable of sustaining the scrub required by the butterfly and the prescribed burns, which are the artificial equivalent, are not allowed in San Francisco.  The scrub is therefore maintained with repeated applications of pesticides which are unlikely to benefit the endangered butterfly.

What is Conciliation Biology

Conservation biology has been “constrained by often futile efforts to restore historical communities, and [does] not appreciate the unavoidable and dynamic contributions of ongoing adaptive evolution.” * Conciliation biology proposes to address these shortcomings by:

  • Taking a longer-term view of the chronic effects of changes in the environment.
  • Making greater use of evolutionary theory
  • Fostering ongoing adaptation by accepting the hybridization that increases genetic variability
  • Identifying and supporting community mechanisms that increase resiliency
  • Improving the effectiveness of the science of invasion biology by using a multidisciplinary approach

How long will It take for this new approach to filter into the minds of those who are busily destroying non-native vegetation and damaging the environment in the process?  How much damage will be done before these destructive methods are abandoned in favor of an approach that accommodates the reality, inevitability, and often the advantages of change?


*Carroll, Scott, “Conciliation biology:  the eco-evolutionary management of permanently invaded biotic systems,”  Evolutionary Applications, 2011, 184-199.