Early detection, early action key to incursion response
Daniel Simberloff: Early intervention
key to dealing with invasive
species.
Human attempts to deal with invasive species have resulted in everything from glorious success to unmitigated disaster. The consequences of our attempts to deal with exotic intruders – usually introduced courtesy of human activity – contain some salutary lessons for our own biosecurity systems. Dr Daniel Simberloff, Nancy Gore Professor of Environmental Studies and Director of the Institute for Biological Invasions at the University of Tennessee, Knoxville, is a world expert on the threats posed by invasive species and has witnessed his share of both successes and failures in attempts to deal with them.
He was in New Zealand in July as keynote speaker at the Conserv-Vision conference in Hamilton. Biosecurity editor Phil Stewart spoke to Professor Simberloff about dealing with invasive species.
Biosecurity: Globally, what types of organisms and environments present the greatest challenges?
Simberloff: I would rate insects, aquatic pests and terrestrial plants as the most troublesome, in that order. As far as environments are concerned, the dogma is that islands and disturbed areas are the most vulnerable. I would qualify that by saying that environmental disturbance from natural processes like fire or wave action doesn't usually open the way for invasive species.
In this context it's interesting to look at the species interactions between the Old World and the New World after Europeans started crossing the Atlantic. Seventy species of ground beetle were introduced to North America from Europe, but only four North American beetles established in Europe. A great number of human diseases were brought to the Americas, but only one – syphilis – crossed to Europe in return.
Biosecurity: Without human intervention, can species invasions sometimes follow the same pattern as a disease epidemic, where there is an explosive growth in pest numbers followed by a fall as competition, predators and disease take their toll? In other words, do some invasions resolve by themselves?
Simberloff: A lot of invasive species populations explode and then decline somewhat, but not a great deal. The cane toad in Australia [Bufo marinus] is an example of this. One instance of a species collapse was the case of Elodea canadensis, Canadian waterweed.
It was introduced to Britain in the 1840s and quickly became a huge problem, clogging rivers and canals. By the late nineteenth century the weed population collapsed. Unfortunately it is very unusual for this to happen with an invasive species. Sitting back and waiting for nature to establish a new equilibrium isn't a good option – passive management doesn't work!
Biosecurity: We're good at detecting and intercepting problem insects like Asian gypsy moth and painted apple moth, but what about plants? Is there scope for improving early detection?
Simberloff: Yes there is. After all, plants don't move around – but new plants do have a way of fitting in with their green surroundings. The secret with plant invasions is to have well-trained surveillance personnel and a well-informed public. One example of the need for vigilance and acting early is the 'killer algae', Caulerpa taxifolia. [See '8 least wanted' Biosecurity 70:12.] This seaweed found its way into the Mediterranean in the 1980s. When it was first discovered, there was a patch of just one square yard. Nothing was done, and it now covers more than 4,000 hectares of the coasts of France, Spain, Italy and Croatia. It's devastated the Mediterranean ecosystem, but there is also some good news about Caulerpa taxifolia. In 2000 it was discovered in two places in California, near San Diego and Los Angeles. A Southern California Caulerpa Action Team was set up and they have managed to wipe it out by covering the weed beds in tarpaulins and pumping in chlorine or bleach. The weed hasn't been seen there since 2002, and last year it was declared eradicated.
Sea lamprey, Petromyzon marinus:
Encouraging progress in the battle to
contain this pest in the North
One thing we need in the United States is an emergency pest line, like your exotic disease and pest hotline. Unfortunately we don't have an effective national framework there for surveillance.
Biosecurity: Do you have other examples of successful eradication or pest management programmes that others could learn from?
Simberloff: Huge numbers. Unfortunately we only tend to hear about the failures, but there are many examples to show it can be done. Interestingly, the tools used for successful pest eradication are often quite blunt instruments – not necessarily a 'scorched earth' policy, but labour intensive. The key is to get started early. A lot of agencies in the United States spend too long studying a problem, losing valuable time. Sometimes the best strategy is 'just do it'!
The 'Alberta Rat Patrol' is one of the most famous examples of how an invasive pest can be stopped in its tracks. Even today, 50 years after the Norwegian rat [Rattus norvegicus] first started crossing into Alberta on its westward march, it's frontpage news if a rat is found. [See sidebar.]
In the United States the sand burr [Cenchrus echinatus] has been successfully eradicated after it got into the Hawaiian Islands in the 1960s. That was done by hard work on the ground – a combination of weed pulling and spraying with glyphosate.
In Western Australia they've managed to eradicate Kochia scoparia, which was infesting thousands of hectares over a linear distance of 800 kilometres. It was deliberately introduced in the early 1990s as a forage plant for livestock, because it flourishes in irrigated saline soils. It quickly spread, and an eradication programme was started in 1992, using herbicides and hand pulling. The last specimen was seen in 2006.
Biosecurity: You say that some fairly basic pest control methods can be effective, but are there any new technologies under development that really excite you?
Simberloff: Certainly it's not always possible to use a scorched earth approach, and there is some very good science being applied now. A great example of this is the control of the sea lamprey [Petromyzon marinus] in North America's Great Lakes. They colonised all the upper Great Lakes from the 1920s after they got in through shipping canals. They're a very aggressive parasite, responsible for three global species extinctions. They have been controlled quite well with lampricides and by weirs preventing access to spawning streams, and numbers are well down from their peaks in the 1950s. However, there are some non-target impacts and fears that current methods may not retain their effectiveness. There has been some exciting new work led by Professor Peter Sorensen at the University of Minnesota, where they have isolated and synthesised a 'migratory' pheromone released by the lamprey's larvae. They have also discovered a second 'sex' pheromone. There's good potential for these to be used to disrupt the lampreys' spawning or help attract them into traps.
Another example of good science at work can be seen in the Galapagos Islands. Island Conservation, a California-based NGO, and an Australian doctoral student, Karl Campbell, showed that using hormone implants in sterilised female 'Judas' goats to prolong their oestrus could increase their efficiency in attracting bucks. This helps with locating and clearing remnant feral herds when population densities are low. Eradicating populations of introduced mammals is very important to these island conservation programmes.
Biosecurity: We often hear encouraging reports about success with biological controls at a localised level, but this doesn't often seem to translate to major pest population collapses. Are we expecting too much of biological control?
Simberloff: There are some successful examples, usually in rangeland or agriculture. But only about 10 percent of well-funded biological control projects give some degree of economically significant control. And as we know, biological control measures can have some very negative outcomes. Cactus moth [Cactoblastis cactorum] is a prime example. It was successfully introduced from its native Argentina to control prickly pear cactus in Australia, but there have been disastrous consequences from its use for the same purpose in the Caribbean. It island-hopped to the mainland United States where it now threatens many native and rare Opuntia cactus species.
Adult cactus moth, Cactoblastis
cactorum: Now threatens rare
species in North America
Biosecurity: Didymo is a major invasive species issue for us in New Zealand, yet it appears to be also turning into a nuisance in its native territory in the northern hemisphere. How common is it for native species to suddenly turn bad?
Simberloff: The instances of this are minuscule compared with problems cause by introduced species. It does happen, though, usually through some environmental change. Elymus athericus, a European shore plant, used to be limited to a range in a band less than a metre wide, but it has suddenly started to invade seaward areas. It is now overwhelming other native plants such as the salt marsh species traditionally grazed by sheep. An increase in atmospheric nitrogen is thought to be the cause.
In the United States some conifers occasionally invade grasslands and prairies, probably as a consequence of forest fire suppression.
Biosecurity: In your experience, can public opposition to spray programmes constrain successful pest eradication or management?
Simberloff: No. Malathion gets used for spray programmes in California, and Foray 48B [also used in New Zealand moth eradication programmes] is used extensively in British Columbia and Washington state. There is always a certain level of complaint – for example, that it kills other Lepidoptera, or that the spray causes allergic reactions – but it has never been to the extent that a programme is stopped. In the United States, especially, the economic stakes are very high, because of the impacts of these pests on forestry or agriculture.
Biosecurity: What's your assessment of New Zealand's biosecurity system for border control, monitoring and incursion response?
Simberloff: It's better than anywhere else! Of course even with the best system there are always better ways to do things, and I think your Parliamentary Commissioner for the Environment pointed that out.
Biosecurity: What do you see as New Zealand's biggest biosecurity challenges?
Simberloff: Probably the pests that don't have a direct effect on agriculture or tourism. Because their impacts aren't understood doesn't mean they won't be harmful. Early warning of new incursions, good training and a well-informed citizenry are key. New Zealand has publicised biosecurity issues far better than places like Europe, where it is hardly discussed, so you are well set up to keep improving your biosecurity system.
Small indian mongoose,
Herpestes auropunctatus:
Responsible for 30 extinctions.
Photo: USDA Forest Service
Biosecurity: If you had to name the world's worst invasive species, what would it be?
Simberloff: I'd actually name four!* The two worst plants are Miconia calvescens, a huge problem particularly in Hawaii and Tahiti, and Brazilian pepper (Schinus terebinthifolius). Brazilian pepper now covers hundreds of thousands of hectares in Florida, and is a problem in many subtropical regions.
For animals I'd start with the rosy wolf snail, Euglandina rosea. It eats other snails and was introduced in the Pacific to control the introduced giant African snail, Achatina fulica. Unfortunately it prefers the smaller native species. The second animal is the small Indian mongoose, Herpestes auropunctatus. It was introduced from India to Jamaica, Hawaiian Islands, Mauritius, Fiji, Okinawa and its offshore islands and other places in the nineteenth and early twentieth centuries to help control rats and snakes. It has killed off 30 to 40 endemic species and has proven almost impossible to eradicate.
I hope we can learn from these examples.
- *Of these four, one (Brazilian pepper) is present in New Zealand.
- www.issg.org/database/species/search.asp?st=100ss&fr=1&sts=
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Page last updated: 30 April 2008