Visit "Leo the Gecko" while he stalks crickets. Compliments of Jen Germano, Western student (Quicktime movie)
|
|
Invasion of the Bahamas:
A Tale of Two Snails and Other Stories
Invasive species, which have members in all five of the major biological kingdoms, are a major economic and environmental problem throughout the world. Non-native species that become established in new areas where they are not controlled can threaten the health of ecosystems, cause decrease in agricultural yields, out-compete or predate native species to local or global extinction, and homogenize the landscape. Island ecosystems, because they are small and enclosed, are most at risk and the most likely to be irrevocable altered by these invaders. This paper will examine the ecological foundations of biotic invasions, the damage that can be caused by invasions, and the introduced species that are currently threatening the health of the Bahamian ecosystem.
What is an Invasive Species?
Before a discussion of the fundamental biology behind biological invasions can be undertaken, it is crucial to define the terminology that is used in this field. It must be noted at the beginning of this essay that there are no fully agreed upon definitions for what constitutes an invasive species. Many have argued for what constitutes an invasive species, often disagreeing on the method of introduction, native range, and other factors that constitute a species being non-native. For this essay, the broadest definition of invasion will be used. An invasion is the geographical expansion of a species into an area not previously occupied by that species [1]. An invasive species thus is a species that expands into a new geographical territory. Under this definition, a species is invasive both if introduced through human activities or through other phenomenon such as tectonic activity, climatic change, and physical transport by another species [1]. Some scientists in the literature would prefer to restrict invasive species only to those introductions that occurred by anthropogenic means, as humans are able to transport biotic species faster and farther than any other species on earth [2]. I have chosen the broader definition for this discussion due to the fact that the narrow definition seems to assume that humans and human actions are not natural. Synonyms that are often seen in the literature for invasive species are alien, non-native, and non-indigenous.
An invasion occurs from a donor biota or region to a recipient biota or rgion. The invasion, or biotic interchange, occurs over a barrier [1]. Barriers can include any number of constraints including water, deserts, mountains, climatic and temperature differences, and predation and competition.
A successful invasion depends on a balance between resource opportunity and enemy escape opportunity. Resource opportunity occurs when there is a large abundance of a resource that an invader is dependent on in the recipient biota. This is a result of either high resource supply, high resource availability due to lack of use by the native organisms, or a combination of both. Enemy escape opportunities occur when there are a low level of natural enemies that would serve as predators to the invader in the recipient biota [3].
In order to successfully invade, a species must find an ecological niche. A niche is defined as the summation of the effects and responses that an organism has in a defined biotic region. The responses include survival, individual growth, and most importantly, per-capita rate of increase. An effect includes consumption of resources, interfering in the ability of other organisms to obtain resources, support of natural enemies, and using space. A niche opportunity is the potential provided by a new biotic region for an invading organism to increase from low density to high density [3]. A niche opportunity is the result of resource opportunities, enemy escape opportunities, or a combination of the two.
The Process of Invasions
Now that the general terminology has been discussed, the actual process of invasions will be covered. Invasions generally occur in the following phases [1,3,4]:
1. Introduction to the new region
2. Establishment of invasive species
3. Population growth
4. Out-competition of native biota
Introduction to the New Region
There are many ways that invasive species can be introduced, or dispersed, to a new biotic region. These include introduction by human aided and human-unaided means. Through anthropogenic activities such as agriculture, forestry, horticulture, soil stabilization, trade, and travel. Non-human introductions are achieved through ocean and water currents, winds, transport via other organisms, climatic and tectonic activity, and their own locomotion over boundaries that are no longer effective [1]. In this manner, organisms have been widely spread outside their natural geographical and climatic boundaries. These introductions may be either intentional or inadvertent but the results are the same; the organisms are able to overcome boundaries that previously prevented their spread to other locations. Some of these species begin to establish themselves, spread rapidly, and become invasive [4].
The introduction of species depends on three main factors: 1. Location and number of donor regions, 2. the modes and routes of transport from the donor region to the recipient region, and 3. the ability of the species in the donor region to take advantage of and use the routes of transport that are available to them [1].
Establishment of Invasive Species
The second phase of invasion is the establishment of the introduced species in the recipient region. For this to occur, the species must be able to increase in population in the invaded area [3]. This ability is dependent upon the establishment of an ecological niche by the invader. The establishment of a niche is determined by the resource and enemy-escape opportunities available in the invaded territory [3]. The resource opportunities may depend on the ability of an invader to procure a resource in a superior manner than the native species. More often, a resource opportunity occurs when an invader successfully uses a resource that the other native species do not [3]. Enemy escape opportunities occur because when a new species invades an area, there are often few or no natural enemies to inhibit their growth.
The complex interactions between resource and enemy escape opportunities will determine the success or failure of an invader. A species that uses a little regarded resource in the recipient region will be able to increase its growth rate to the point where it effects the ability of other species to obtain common resources. For instance, if a plant invades that has the ability to fix nitrogen; will not be dependent on levels of soil nitrogen as other plants in the region are. It can then grow and propagate to where it begins to interfere with other plants' abilities to draw sufficient water for the soil. This is due to its increased population. As for enemy escape opportunity, this allows for an invasive species to propagate without having the predation constraints that other species in the recipient region have. For instance, if a rabbit is introduced to an island where it has no predators, it will be able to out-compete other mammals for food and habitat resources by sheer numbers alone.
Population Growth
Once an invasive species established, it begins a population growth phase. This is generally marked by a period of slow growth, or lag phase, followed by a period of near exponential growth. The reason for this is that it takes time for the invader to adjust to the new region, to gain access to resources, and possibly to wait for natural disturbances such as floods and fires to reduce native populations [4].
Out-competition of Native Biota
Finally, a period of out-competing native flora species marks the successful invasion of the introduced species. It competes for resources and displaces the native plants. The invasive species becomes well established and permits the further invasion into nearby areas through vegetative spread or the dispersal of seeds by agents such as wind, water, humans, and other organisms [4].
Factors that Promote Invasion
It becomes evident in a search of the literature that some species seem to be better at invading than others. The reason for this is not clear, but much work has been done in attempting to create a list of factors that predict a species' potential to invade. These factors are divided into two categories: Factors that increase a species ability to invade and factors that increase potential for a recipient region to be invaded.
Factors that increase the likelihood of a species to invade include physiological tolerance, plasticity, history of invasion, vegetative reproduction, large number of progeny, and, among birds, migration [1,5]. This is an intense field of argument and some scientists might include more characteristics on this list, while others would further pare it down. Unfortunately, most of the studies looking for traits have been performed on plants and birds [5]. More studies need to be performed to further elucidate the characteristics that make a species invasive.
More and more, studies are beginning to indicate that it may be more of a function of the recipient region that promotes invasion. Under this theory, almost any species can be invasive, it is the invaded region that determines or promotes the invasion. Such characteristics might be similarity in terms of climate to the host range of the invasive species, frequent disturbance of the ecosystem (i.e. construction, floods, fires), and low biodiversity [1,6]. These characteristics open a region up for invasion.
The most likely scenario is that it is an interaction between the factors that support invasion in the invader and the ecosystem. The disturbance regime of the recipient region, the amount of biodiversity which correlates with the resistance the region will show to invasion, and similarity in climate all create opportunities for an invader to establish itself. Then, the characteristics such as large number of progeny, fast reproduction, and others allow for the
invader to grow and out-compete native flora and fauna. The field of invasion biology is in its infancy at this point. More work is needed to further discern the complex processes that lead to successful invasion.
The Threat of Invasive Species
While the complexity of the interactions that lead to plant invasions is not well known, the consequences of these invasions are. Unfortunately, they are all to well known. There have been between 2,000 to 50,000 species introduced to the United States, United Kingdom, Australia, South Africa, India, and Brazil. This results in an estimated 480,000 introduced species worldwide [7]. Some of these species have actually been beneficial as corn, wheat, rice, chicken, and cattle now make up approximately 98% of the world's food supply [7]. However, a large number of the introductions have negatively impacted the recipient region's landscape. In the U.S., 400 species on the endangered or threatened list are being listed primarily due to the competition they receive from invasive species [7]. Introduced species cause the loss of between 55 billion and 248 billion dollars in agricultural revenue worldwide.
Island ecosystems are often the hardest hit by invasive species because of the enclosed nature of their ecosystem and the relative lack of diversity that is often found on an island[8]. The Galapagos and Hawaiian ecosystems have both been devastated in recent times due to the introduction of alien species [9,10]. Feral goats, cattle, and pigs have decimated the native vegetation on some of the Galapagos Islands [9]. Introduced plants such as Guava, Quinine, and Avocado have decreased the native plants in the Asteracea family. Similar effects have been seen in Hawaii with the tropical shrub, Lantana camara [9].
The Bahamas Experience
Having established that island ecosystems are particularly threatened and that invasive species can be seriously disruptive, it is time to focus on the Bahamas ecosystem. The country known as the Bahamas is an archipelago of over 700 islands that cover approximately 100,000 square miles. The Bahamas are located 24 15 N and 76 00 W in the eastern Caribbean Sea and Northern Atlantic Ocean. The islands are composed of the plateaus of oceanic mountains that have accumulated coral reefs over time [11]. The islands are rather sparsely populated by humans as the entire population is only 250,000 people [12].
Currently, the Bahamas are being threatened by four main terrestrial invasive species: the
Giant African Snail (Achatina fulinca), the rosy wolf snail (Euglandia rosea), the brown mongoose (Herpestes javanicus), and the red imported fire ant (Solenopsis invicta). These will be discussed separately in the following paragraphs.
The Giant African Snail (Achatina fulinca)
The Giant African snail is a native of east Africa. It is a very large snail, between 7 and 20 cm tall. The adult snail has a brown, conical shell with irregular darker streaks on it. It is a cross fertilizing hermaphrodite that lays eggs. The snail has a very high reproductive output, laying 200 eggs 5-6 times a year with a 90% viability rate. The Giant African Snail was accidentally introduced to the Bahamas through trade and the transport of soil for horticultural and agricultural uses [13]. Once the introduction occurred, the snail turned out to be an agricultural pest and a nuisance. Due to its incredibly large reproductive rate and success, the snail poses road hazards, walking hazards, consume native vegetation, and compete with other native snails for resources [13].
The Rosy Wolf Snail (Euglandia rosea)
The Rosy Wolf Snail, also known as the cannibal snail, is a native of the southeastern United States. The adult snail is approximately 6 cm tall and 2 cm wide. It has a pinkish shell and is a cross-fertilizing hermaphrodite that lays eggs [13]. This snail is a predatory snail that feasts on other snails.
The Rosy Wolf Snail was introduced to the Bahamas as a biological control agent for the Giant African Snail. It was thought that the cannibalistic nature of the snail would make it an ideal agent in the control of the Giant African Snail. Unfortunately, no one had any conclusive evidence that this snail would feed only upon the Giant African Snail. In an effort to appease the people of the Bahamas and to attempt to rid the islands of the Giant African snail, the government of the Bahamas introduced the Rosy Wolf Snail. This turned out to be a spectacularly bad idea.
As is often the case in the introduction of biological control agents, the Rosy Wolf Snail did not feed only on the Giant African Snail. In fact, it seems that it prefers the native snail population much more. This is clearly borne out in other well-documented intentional introductions of the Rosy Wolf Snail. The Rosy Wolf Snail was purposefully introduced to the Hawaiian Islands for the same reasons that it was introduced to the Bahamas: control of the Giant African Snail. The results were a total lack of effect on the Giant African Snail and total devastation of the native Hawaiian snail populations [10]. The Hawaiian Islands once boasted over 750 species of snails, 99% of which were endemic or only found there [10]. A majority of these species have gone extinct or are severely threatened due to the introduction of the Rosy Wolf Snail. The exact same story can be told on the Island of American Samoa where the Rosy Wolf Snail was introduced to control the Giant African Snail. This introduction corresponded to a decrease in native snail populations [14].
While no studies on the Bahamas themselves could be located, a similar effect is possible and, based on the evidence of other island ecosystems, probable.
The Brown Mongoose (Herpestes javanicus)
The Brown Mongoose is a native of Asia. It is a small animal, resembling a weasel, which has pale to dark brown fur. The adults weigh approximately two pounds. The Mongoose was introduced to the Bahamas as an agent to control rats and snakes. Unfortunately, it, like the Rosy Wolf Snail, found native birds, mammals, and reptiles more to its liking. It is also a voracious stealer of eggs including those of sea turtles that nest upon the coast of the islands [13]. The Mongoose is known to have caused extinctions of seven reptiles and amphibians in Puerto Rico. It is also indicated in the extinction of a lizard, snake, two birds, and a rodent in Jamaica [14]. Again, it is also a threat to native Bahamian fauna, although not directly linked to any extinctions yet.
The Red Imported Fire Ant (Solenopsis invicta)
The Red Imported Fire Ant is a native of South America. They are small, reddish-brown ants that often construct large mounds above the ground. They reproduce rapidly and have a formidable bite that is similar to a wasp sting [13]. The Red Fire Ant was accidentally introduced to the Bahamas.
The Fire Ant is a major agricultural and environmental pest. It can interfere with agriculture via eating the young stands of plants, interfering with the root systems of plants, building mounds that inhibit harvesting with combines, and discourage hand harvesting via their sting and fierce nature [15]. Fire Ants also interfere with electrical equipment and can cause short circuits and fires. Furthermore, they are a major threat to native ants, reptiles, amphibians, ground nesting birds, and small mammals [13,15].
Conclusions
Clearly, the ecosystem of the Bahamas is under attack via alien introduced species. Since much of the economy in the Bahamas is devoted to ecotourism and since the island ecosystem is a fragile one that depends on biodiversity to function, it is crucial that these species are brought under control and no new species are introduced. This can be done through proper screening and tightening regulations as far as what is allowed to be imported into the country. Clearly, experience has shown that the use of one introduced species to control another rarely works. Any future use of biological control agents must be highly studied to insure that another problem will not be caused. The Bahamas is a beautiful place, with many endemic species that are unique to the islands. It would be a shame for these species to be wiped out by an introduced species.
Bibliography
1. Vermeij, Geerat J. (1996). An Agenda for Invasion Biology. Biological
Conservation. Vol. 78. Pgs. 3-9.
2. Lee, William G. (2001). Negative Effects of Introduced Plants. Encyclopedia of
Biodiversity, vol 3, Academic Press.
3. Shea, Katriona and Peter Chesson. (2002). Community ecology theory as a framework for biological invasions. TRENDS in Ecology & Evolution. Vol.17.
No. 4 Pgs. 170-176. Apr. 4th
4. Richardson, D.M. 2001. Plant Invasions. Encyclopedia of Biodiversity, vol 4,
Academic Press.
5. Kolar, Cynthia S. and David M. Lodge. (2001). Progress in invasion biology:
predicting invaders. TRENDS in Ecology& Evolution. Vol.16. No.4. Pgs. 199-204.
Apr. 11th
6. Curnutt, John L. (2000). Host-area specific climatic-matching: similarity breeds
exotics. Biological Conservation. Vol. 94. Pgs. 341-351.
7. Pimentel, David et al. (2001). Economic and environmental threats of alien plant,
animal, and microbe invasions. Agriculture, Ecosystems and Environment. Vol. 84.
Pgs. 1-20.
8. Rodriguez, Jon Paul. (2001). Exotic species introductions into South America: an
underestimated threat? Biodiversity and Conservation. Vol. 10. Pgs. 1983-1996.
9. Schofield, Eileen K. (1989). Effects of Introduced Plants and Animals on Island
Vegetation: Examples from the Galapagos Archipelago. Conservation Biology.
ol. 3. Iss. 3. Pgs. 227-238. Sept. 1989.
10. Cowie, Robert H. (1998). Patterns of introduction of no-indigenous non-marine snails
and slugs in the Hawaiian Islands. Biodiversity and Conservation. Vol. 7. Pgs. 349-368.
11. www.odci.gov. (2002). The Bahamas. Accessed via the intenet on May 12, 2002.
12. www.thebahamas.com. (2002). Facts on the Bahamas. Accessed via the internet on
May 12, 2002.
13. www.issg.org. (2002). Invasive Species of the Bahamas. Accessed via the internet on
May 12, 2002.
14. Cowie, Robert H. (2001). Decline and homogenization of Pacific faunas: the land
snails of American Samoa. Biological Conservation. Vol. 99. Pgs. 207-222.
15. www.columbia.edu. (2002). The Small Indian Mongoose. Accessed via the web on
May 12, 2002.
16. creatures.ufl.edu. (2002) The red imported fire ant. Accessed via the web on May 12,
2002.
Next Article
Previous Article
Return to Topic Menu
DOWNLOAD the Paper Posting HTML Formating HELP SHEET!
We also have a GUIDE for depositing articles, images, data, etc in your research folders.
Article complete. Click HERE to return to the Pre-Course Presentation Outline and Paper Posting Menu. Or, you can return to the course syllabus
WEATHER & EARTH SCIENCE RESOURCES |
|
OTHER ACADEMIC COURSES, STUDENT RESEARCH, OTHER STUFF
|
|
TEACHING TOOLS & OTHER STUFF
 |
It is 9:07:36 AM on Saturday, March 13, 2010. Last Update: Wednesday, December 10, 2008
Listen to a "Voice Navigation" Intro!
(Quicktime or