Download Sarah Goodspeed Alien Invaders The problems with invasive

Sarah Goodspeed
Alien Invaders
The problems with invasive species are widespread both ecologically and economically. A species
is considered invasive if it is introduced to a new region and spreads widely. While most introduced
species pose no problems, neither ecologically nor economically, a few do become invasive and damaging
to their new environment, disrupting ecosystems by using resources like space or nutrients, or impacting
keystone species, or reducing agricultural yield, grazing land, or ecotourism. The costs of invasion can be
very high, the US government spending over $1 billion annually to control problematic species (Marris,
2005). Control can be mechanical (manually removing organisms, like trapping rats on many islands or
pulling milfoil and zebra mussels in Minnesota), chemical (using herbicides and pesticides to kill off the
species), or biological control (using other organisms to beat back invaders by predation or competition).
Studying the behavior of invasive species, the facilitation of invasion, and the effects of control methods
used thus far can shed much light on how to better respond to alien invaders in our ecosystems.
Double the predator, double the fun?
The many small islands of the Caribbean offer an opportunity for experimental research on the
population phenomena that are the basis for island biogeography. Thorpe (2005) asserts that familiar small
tree lizards offer high population density in various habitats on the islands and are particularly ideal for
experimentation in evolution and biogeography. Largely free from disturbing the natural habitat, having a
significant yet manageable population size, and other challenges of field testing are overcome on these
assorted islands.
In order to examine the survival rates of the lizard Anolis sagrei on these islands, Schoener et al.
(2005) introduced the predatory lizard Leiocephalus carinatus to six islands in the Bahamas and compared
them with six unmanipulated islands. Since logically A. sagrei would either face predation by the
introduced lizards near the ground (where they both generally reside) or by more birds if they climbed
higher up the vegetation, survival was expected to decrease across the board against the new predator.
Instead, the scientists found that while survival decreased significantly in low undergrowth areas, little to
no difference was found on islands with taller vegetation.
To explain these results, the authors reversed their initial hypothesis that bird predation would
increase where taller vegetation was available. A short investigation revealed that birds did indeed hunt
more in shorter vegetation. Even so, the islands with taller vegetation didn’t see any increase in fatalities
from the increased predator population itself. The researchers rationalized this data with the hypothesis that
the birds passed on the now scavenged islands with the introduced predators to seek islands where prey
density remained high.
Supporting evidence already existed for this hypothesis, in which control islands with taller
vegetation had higher A. sagrei population density, attracting more birds despite the tall vegetation, and
thus saw population declines. When the predators decreased the population density, the birds moved on to
an island with more abundant prey. On the other hand, islands with shorter vegetation (with lower A. sagrei
density and little bird predation in the first place) saw the most significant decline in fraction of survival
upon invasion by L. carinatus. Ultimately the birds killed the prey down to the same density they would
have left without the other predators before moving on, bypassing an island entirely if introduced predators
killed a large percentage of prey. When predators were added to the islands, many previous predators
moved away, leaving prey survival rates unchanged.
The biogeographical implications of this study extend to swiftly changing vegetation structures
around the globe and suggest how species could respond to those changes. Similar experiments could be
used to provide insight into adaptation in communities in other climates as well, in which species
introduction might similarly balance out in the end as well.
Zero to hero
Years of evolution in a stable environment generally produce populations with little genetic
variation, and thus little ability to adapt to ecological changes. Yet a paradox exists in invasion biology in
which such homogeneous life endures well in new environments. Despite the increased risk of extinction of
an invasive species, due to small population size as well as little variation, many alien species indeed
survive and evolve rapidly.
Ecological approaches have identified many factors contributing to invasion success, but genetic
approaches have until now received less attention even though the phenomena suggests an ability to get
around loss of genetic variation. Taking a closer look at this inconsistency, Kolbe et al. (2004) studied the
lizard Anolis sagrei populations throughout the world, from its origin in Cuba and its spread to Florida, the
continental US, Hawaii, and even as far as Taiwan. Genetic analyses indicate at least eight original,
geographically distinct populations which proved to be the source for all introductions beyond Cuba.
Over two-thirds of the introduced populations in Florida contained genes from more than one
native (Cuban) population, leading to the conclusion that Florida experienced multiple introductions, thus
circumventing the genetic “bottleneck.” This hypothesis also supports the recorded time lag between initial
introduction and subjugation, in Florida beginning in the late 1800’s and spreading mid- to late-1900’s.
These more diverse populations, combining the limited genetic variation from multiple populations into a
wider, more disparate population could then more easily evolve in the new environment. The wider-ranging
new population would then spread more easily than the originals, resulting in the extensive incidence of A.
sagrei populations. By mixing genetic material from multiple source populations, these invaders
demonstrate how to persist despite limited variation by actually increasing variation through combination.
This study reveals two frightening potential consequences concerning invasive species control.
Firstly, that the ability to adapt to a new environment may not be impaired due to genetic homogeny.
Secondly, that established alien species can act as a gateway for further introductions due to greater
diversity. Such possibilities make invasive species an even bigger threat to ecosystem management and
biological control.
Beware all ye who enter here
Alien species notoriously escape their natural predators and parasites that live in their native
ranges when relocating in foreign spaces. Natural resistance to alien species, stemming from community
diversity, is the best way to prevent damaging invasions in any ecosystem. Many studies have demonstrated
that native predators can limit local population size of invaders, yet few have shown how they limit the
geographic range of the intruders. DeRivera et al. (2005) examined the crab population on the New
England coast for its ability to block expansion of certain nonnative crabs.
The introduced European green crab Carcinus maenas has colonized nearly the entire globe,
including the cold waters of the northwestern Atlantic along the New England coast. Population expansion
of the northern limit coincides with temperature changes, yet the southern limit remains stable and more
restricted than temperature would allow. Enter the native blue crab Callinectes sapidus, occupying the
same shallow waters in bays along the coast but slightly south. The blue crab grows to nearly twice the size
of the green, and preys on other small crabs, suggesting that it may be an important predator of this
introduced species.
The researchers thoroughly tested the hypothesis that predation by the native crab prevents the
southern expansion of the green crab on the east coast. Checking that environmental variables are
stabilized, population size of both species and rate of predation was compared at 64 sites from Maine to
Virginia. Ultimately, the blue crab abundance alone explained a significant amount of the green crab
abundance throughout the overlapping ranges of these species along the coast.
Predation rates were sufficient to keep green crab populations low or wiped out in the south where
the blue crab was more plentiful. The green crab was demonstrated to be a preferred prey of the blue crab,
so that in the southernmost Chesapeake Bay with the highest population of blue crab, no green crabs
survived, whereas a large population thrived in Casco Bay, Maine, where no blue crabs reside. Although
temperature did have an effect on populations, it was not alone sufficient to explain the stable southern
boundary of the green crab population, but rather contributed to the blue crab population which then
determined green crab population.
This is the only extensive study demonstrating how biotic resistance of a native species limits the
geographic range of an introduced species. It indicates that an increase in temperature and thus blue crab
range would result in a further restriction of green crab range, and in contrast that a decrease in blue crabs
would allow range expansion of the green crab. The Chesapeake Bay recently has been documented with
temperature fluctuation and may therefore be at risk of a surge in green crab population should the blue
crabs move further south. Meanwhile, the blue crabs will feast upon the green crabs and restrict them from
continuing their expansion further down the US coast.
The grass that plays together stays together
Where biodiversity is declining, the importance of the order of species loss is rarely examined.
Because functional groups of species use resources together, Zavaleta and Hulvey (2004) assert that
biodiversity losses disproportionately involve species with similar traits, but most studies overlook the
functional order in which species are lost. Community disassembly entailing losses within similar
functional groups, they argue, results in significantly faster invasion of alien species.
To test this theory, the scientists constructed communities of grasslands based on four years of
observed natural variations to test invasion resistance of a more realistic, nested order of species loss. These
variations arise from widespread effects on grasslands from farming, technology, and endemic natural
occurrences likely affecting species richness similarly. The researchers planted seeds of typical California
grassland communities at different levels of species richness and diversity by functional group to reflect
natural variations. Half of the plots at each richness level were intentionally invaded with a familiar
damaging exotic thistle Centaurea soltitialis.
Thistle production proved stronger in plots with declining richness of functional groups than in
plots of random species richness decline. By reducing richness by functional groups, different balances of
soil content allowed for the thistle to fill the gaps and invade more swiftly. For example, if early annuals
are dominant among the remaining species, the thistle will have ample nitrogen resources late in the season
without strong competition from perennials. The more complete use of resources in the balanced
communities kept thistle production limited.
As should be no surprise, from this article or from recent history, we understand that those species
which have evolved together in an ecosystem live in efficiency and harmony. When actions, oftentimes
directly from humans, disrupt these ecosystems it can open the door to invasion and the destruction of these
balanced communities. But as long as the areas can remain more or less intact, or allowed to slowly adapt
together, the scale can be readjusted to evolve to the next generation.
United we stand
Although most introductions of exotic species fail to become established, the classic invasive
species reproduces rapidly upon a successful invasion. Previous researchers analyzed nearly five hundred
intentional introductions of 79 bird species to New Zealand, discovering the strongest predictor of
establishment to be repeated introduction. Only 20 percent of the birds became established, and most of
those had been introduced more than four separate times. This seeming inconsistency between repeated
failure of introductions and rapid growth once established motivated Korniss and Caraco (2005) to
investigate the phenomenon.
Rather than using typical ecological models based on random variation, the scientists approximate
the competition for space to Avrami’s law of nucleation using a lattice-based model of competition
between one resident and one invading plant species. The simulation tracked propagation of empty sites
and invader survival within a dense resident population. Nucleation theory suggests a critical radius value
for clusters above which a population will be more likely grow than to decline. The simulation confirmed
this, with either higher introduction rates (also influenced by neighboring site population, since a larger
neighborhood with more resident species decreases invader rate of introduction) or greater site size leading
to more multi-cluster invasions. Alternately, a small introduction rate or system size leading to single
cluster invasions. The established species then grows exponentially until maximum potential size is
reached.
The analysis compares random variation in the spatial dynamics of an introduced species with the
contrast between the high probability of introduction failure and the dominance of exotics once established.
The resident species can resist small rare clusters of invaders because of the preemptive nature of
competition, such that the residents have the advantage of preexisting in the area another species will try to
invade and can overpower small clusters in the search for resources. When invaders have large or multiple
population clusters however, they are more difficult to restrain, as the resident species no longer has an
overwhelming percentage of resources.
This research complements earlier research by having a more detailed description of invasion
spatial dynamics. Rather than merely observing uniform dispersal of two species in which both species
cluster and one eventually dominates, Korniss and Caraco simulated an invasion (from zero population)
and observed subsequent challenges in propagation, clustering, and eventual establishment after reaching
critical size. Though using different biological assumptions, mainly that invasive species generally
reproduce rapidly, the study highlights the importance of spatial geometry for understanding invasion
relationships. Under these propagation conditions, invasive species will be more likely to succeed when
striking in a large cluster or as multiple small clusters.
Lost: parasites - please return.
By now it’s clear that demographic release, or escape from natural enemies, is a common
hypothesis for the success of introduced species. The role of parasites as such natural enemies is often
overlooked, though parasites can both decrease population size and body size. When an exotic species is
free of its native parasites and encounters few new ones, it can swiftly expand and endanger the local
ecosystem.
Torchin, et al (2003) performed an analysis of the number of parasitic species of native and alien
origin found on similar native and alien species of many taxological orders. The scientists reported that the
number of parasites found in native populations is twice that found in exotic populations, and that among
alien species there is a lower prevalence of infection than among native species. Furthermore, they also
observed that parasites introduced together with their hosts were equally prevalent in native and exotic
populations, and the parasites left behind were less prevalent in native populations.
This supports the general demographic release theory, but to complicate the hypothesis, native
parasites that did infect exotic species achieved levels equal to the introduced exotic ones. There is
apparently no difference between the susceptibility to native parasites of introduced versus native host
populations. But parasites are often lost exotic invaders arrive. The probability that parasites will be
introduced together with their host a function of the prevalence in their populations, and this may be small.
Secondly, many parasites require more than one host for their complex life cycles, and not all are likely to
invade simultaneously. Thirdly, host population bottlenecks after possible variation loss may end
transmission of the parasites, being limited to specific hosts. On the other hand, repeated introductions after
an exotic population becomes established experience higher transmission.
The importance of the role of parasites in analyzing the problems of invasion can have
implications for biological control and for the study of invasive species in general. Invasions also provide
the opportunity to study the role of parasites in regulating host populations. The enemy release hypothesis
is shown here to be more complicated than it appears, with parasites shown to inhibit invasion success.
Home Field Advantage
In determining interactions between native and invasive species, we again are met with the
popular enemy release hypothesis – that exotic species do well in a new range where their native enemies
don’t live. It is often proposed to use biotic resistance as a method of controlling invasive species by
introducing their natural enemies where the invader has occupied, but this method has the danger of further
disrupting the invaded community. These theories overlook the threat new enemies in the resident
ecosystem may pose, ones that the invaders are not adapted to deter. Exotic plants in particular are often
assumed to be overlooked by herbivores, but this idea primarily applies to specialist enemies and not to the
generalist herbivores which have larger impacts on the plant community.
Assuming that generalist (often vertebrate) herbivores would naturally have a larger effect on
plant survival than specialist (often invertebrate) herbivores due to their size, range, and diets, Parker,
Berkepile, and Hay (2005) used these larger herbivores to obtain very strong results observing the
differences between native and introduced herbivores in the success of plant invasions. Analysis of over
one hundred species showed that in fact, native and exotic herbivores had opposite effects on native and
exotic plants. Native herbivores hindered the invading plant spread, while introducing alien herbivores
(from the same original location as the plant) assisted the success of the invaders, these exotic herbivores
instead consuming many native plants. Though traditionally exotic species have been assumed to become
successful by avoiding their natural predators, in fact this shows that exotic plants display a weakness in
repelling native herbivores.
Invasive plants can thus succeed not necessarily by escaping their natural predators, but by
following them, replacing the native plants that are depleted by the exotic herbivores. For example, large
exotic herbivores from Eurasia were rapidly introduced to the Americas during colonization, decimating
native plants and paving the way for invasions of Eurasian plants. The best means of combating invasive
plants is shown not to be through introduction of their own natural enemies, potentially triggering an
invasional meltdown, but rather by restoration of native generalist herbivores to mitigate exotic invasions.
You Win Some, You Lose Some
It is further assumed that agents of biological control effective in one location will work in
another, but many characteristics of the invaded community contribute to the success of control methods.
Though effective in restraining invasive species under certain circumstances, the same biological control
methods may fail in another situation. Observing that the efficacy of biocontrol varied dramatically on the
same species across its invaded range, Shea et al. (2005) studied the release of natural insect enemies of an
invasive thistle in New Zealand and Australia, revealing some insights on the context-dependent
effectiveness of such biological control.
No clear consensus on the success of the insects in controlling thistles was reached, since the
results were varied for each location, with different species of insects performing better in each country.
The major differences between the thistle populations appeared to be life history, with thistle populations
apparently evolving different strategies during their invasion. The plants exhibit shorter lives in New
Zealand but higher fecundity and thus higher population growth, whereas in Australia the thistles survive
longer and have lower growth rates. Recognizing these differences can lead to better selection for methods
of biological control, choosing insects specifically to target seeds or rosette survival for the greatest results.
Although more complicated and context-dependent, this research makes method selection
ultimately more clear, knowing more fully what type of species could have the biggest impact in restraining
the invaders. Predicting the efficacy of biological control agents will be more effective when taking into
account life histories of the alien species. In situations where invasive species are rapidly expanding in a
new environment, this knowledge would aid management to more efficiently combat the alien spread.
The Elusive Rattus
Invasive populations are generally easier to eradicate in small numbers, yet Norway rats (Rattus
norvegicus) have demonstrated that such is not necessarily the case. To find out why it’s so difficult to
eliminate rats in the early stages of invasion, Russell et al. (2005) released a single rat wearing a radio
collar on an isolated island off northeastern New Zealand, and tracked it’s behavior against a range of
standard techniques employed to detain it.
Four weeks into the study, the rat stayed within about a hectare range, yet all attempts to capture
it, using extensive snap traps, waxed devices, tracking tunnels, and trained dogs, failed. After ten weeks
the radio signal was lost and signs of the rat were found on another island 400 meters away. Five weeks of
further attempts to capture the rat on the second island, this time using peanut butter bait, buried traps,
poison bait, permanent grid, and more trained dogs, also failed. After 18 weeks the rat was caught in a trap
baited with fresh penguin.
The first island had ample, uncontested resources for the rat, which could explain why the bait was
ineffective, yet it still chose to swim to the next island, possibly in search of a mate. The atypical behavior
of the lone invader makes it disproportionately difficult to capture, compared with dense populations which
have already been eliminated from certain island ecosystems.
Successful eradications have already occured, with seabirds wiped out by the rats returning to
New Zealand’s Campbell Island, hit with 120 tons of poison in 2001, soon to be confirmed as the world’s
largest rat eradication. Already seabirds wiped out by the rats are quickly returning. In Mexico, the rare
Xantus has increased nesting by eighty percent since the rats were poisoned on Anacupa Island.
While strategies for eliminating small numbers of rats may be more difficult, it is comforting that
success has already been demonstrated for larger populations. Invasive rats occupy nearly 80 percent of
islands, and attack plants, insects, birds, and small animals, leading in part to about half of recorded bird
and reptile extinctions. As long as this destruction can be controlled on a large scale, the smaller
populations of rats escaping capture are more bearable.
Turning the tide
While defeating exotic species on the mainland is nearly a laughable idea, where organisms can
freely travel between communities, eradication of harmful invaders is proving itself on islands around the
world. As generally the most vulnerable ecosystems, housing disproportionately high numbers of species
for their small size, with those species having evolved without constant bombardment of other competitive
or predatory species, exotic invaders are particularly destructive in island communities. However, with the
exits naturally sealed, professional terminators can wipe out different species using various methods. The
key to eradication is to attack fast and completely, before the invaders can adapt, escape, or continue to
reproduce and rebound their population. New studies have demonstrated that threatened species recover
very well after eradication of the invaders.
New Zealand led the counterattack by hunting the large mammals, the fewest and often most
destructive of introduced species, from land and air with traps and dogs, switching to more traps and
poisons for the smaller mammals as success continued. Even rats, notoriously difficult to eliminate, have
been all but wiped out by dropping poison pellets from helicopters on Campbell Island. On the Galapagos
Islands, introduced goats are tracked using radio collars and lead hunters to the herds.
As a result of eradication efforts, native species have the opportunity to bounce back in dramatic
numbers, with dozens of successes encouraging further efforts. However, with little formal research done,
some complicated results go unexplained. On Santa Cruz Island off southern California for example, earlier
eradication of sheep may have helped pigs overmultiply, making their eradication more urgent, also
drawing golden eagles that also attacked native foxes. Furthermore, some native plant species have even
been demonstrated to decline, possibly because eradicated livestock had been keeping foreign plants in
check as well, like Santa Cruz’s foreign fennel and star thistle, and exotic vines on Sarigan Island.
Ecological kickback can also come from mammals, for example rat expansion after eradicating cats, or
weasels attacking native crops and birds after eradicating foreign rats or rabbits.
Overall, eradication can never return ecosystems completely to their previous states, even after
hundreds of years, but eradication is nonetheless an effective method to protect native species against
extinction. Complexities arise from the altered state of the communities so that rare species don’t
necessarily return, and even other invaders may become rampant, but nonetheless there is overwhelming
confirmation that many threatened species do recover from eradication efforts, proven successful on
hundreds of islands already.
In essence, the characteristics of invasive species are much more complex than anyone previously knew,
but understanding more thoroughly how species become widespread and problematic offers insight into
how best to manage them. Mechanical control has been effective at the grassroots level, the hunting of
large mammals in New Zealand saving the islands from these widely destructive creatures. Chemical
control is also effective, demonstrated by successfully eradicating rats from various islands using poison
pellets. Biological control is evidently the most complex method of control, but possibly the most reliable
if done properly. New research performed examining the activity of invasive species and the effectiveness
of control methods is constantly changing how we can best protect our ecosystems from damaging alien
invaders.
Works Cited
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abundance and distribution of an introduced crab. Ecology 86: 3364-3376.
Kolbe, J., R. Glor, L. Schettino, A. Lara, A. Larson, J. Losos. 2004. Genetic variation increases during
biological invasion by a Cuban lizard. Nature 431: 177-181.
Korniss, Gyorgy and Thomas Caraco. 2005. Spatial dynamics of invasion: the geometry of introduced
species. Journal of Theoretical Biology 233: 137-150.
Krajick, Kevin. 2005. Winning the war against island invaders. Science 310. 1410-1413.
Marris, Emma. 2005. Shoot to Kill. Nature 438: 272-273.
Parker, J., D. Burkepile, M. Hay. 2006. Opposing Effects of native and Exotic Herbivores on Plant
Invasions. Science 311: 1459-1461.
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Shea, Katriona, et al. 2005. Context-dependent biological control of an invasive thistle. Ecology 86: 31743181.
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Torchin, M., K. Lafferty, A. Dobson, V. McKenzie, and A. Kuris. 2003. Introduced species and their
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Zavaleta, E. S. and K. B. Hulvey. 2004. Realistic species losses disproportionately reduce grassland
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