Introduction
Biodiversity refers to the sum total of species, ecosystems and genes in a certain location. The term encompasses all spheres of biological systems such as the genetic or molecular sphere, the population sphere, the species, ecosystem and organ spheres, as well. Conversely, an introduced species is one that exists in an area outside of its native region. Usually, humans may bring them into a certain location either intentionally or accidentally.
Sometimes introduced species may turn invasive and thus cause harm to the native population. The question of introduced species’ effects on biodiversity is a complex one as there is no single consequence of these organisms. Consequently, one must analyse both sides of the debate thoroughly before one can assert that introduced species either are harmful to biodiversity.
How introduced species harm biodiversity
Conservationists are deeply concerned about introduced species because they threaten the existence of native species. At times, this may occur through direct interactions with the introduced species or through the dependence of other species on the threatened one. When other species depend on the replaced one, then they may loose their habitat, source of food, or mode of reproduction. Eventually, they may also become extinct or endangered.
The danger posed by introduced species may occur through various pathways: competition, predation or parasitism, disease, hybridisation, modification of the habitat as well as herbivory. When these phenomena manifest, then they eventually lead to the loss of biodiversity. Studies indicate that introduced species come second to habitat destruction with regard to their threat on native biodiversity. Usually, certain barriers exist in natural environments to allow species to evolve on their own.
Few of these barriers include mountains, deserts, rivers and ocean. If these barriers did not exist, then it would be quite easy for species to enter new niches and minimise biodiversity in that area. Human beings have contributed adversely to circumvention of these barriers by bringing species, from other areas, for food or commercial purposes. Introduction of species, therefore, accelerates the amount of time needed to broaden a species’ range (Cohen & Carlton 1998).
The most direct and serious way of reducing biodiversity by introduced species is competition. Non native populations will compete for the same space, nutrients, polluting insects or light as the native ones. In certain circumstances, the introduced species may be better equipped to fight for these resources than the local ones thus outcompeting them. This may lead to the decline of species in such locations.
One scenario was the Australian paperback tree, which invaded the US in Florida. The tree had a spongy bark and flammable leaves that made it better able to utilise resources in its foreign location; this caused adverse competition for numerous native plants. As the Australian paperback kept spreading, more local plants kept reducing. Even other mammals and birds that had adapted to the native plants started declining.
Therefore, competition causes biodiversity loss by eliminating organisms that directly compete with the invasive species, and reducing the ones that depend on the first variety. Other examples of introduced plants that have outcompeted local ones include the Australian Caulerpa, which steeled in the Mediterranean Sea, as well as the South American water hyacinth.
The North American gray squirrel, which came from Italy and Britain, has a better way of getting nuts than local ones. These indigenous squirrels have less access to resources and thus fewer chances of survival (Moritz 1999).
Sometimes introduced species can become predators or parasites on native populations. These organisms may be too strong for the local ones thus perpetuating their extinction. One such instance was the case of the pacific black rat. It was introduced into Hawaii and attacked the eggs of honeycreepers, which were local Hawaiian birds (Johnson 2003).
In fact, several bird species that live in islands have no way of protecting themselves against introduced hunters like the Pacific black rat. The brown tree snake is also a commonly cited example. This species emanated from Admiralty Islands, and was responsible for the eradication of 11 bird types in Guam. The Nile perch has predated upon cichlid fish in East Africa’s Lake Victoria.
Approximately 100 species of these organisms have disappeared as a direct result of the Nile Perch’s predatory activities. An example of how introduced species can become parasites is the case of the Asian chestnut fungus.
This species used American chestnut trees as hosts and eventually led to their elimination from most parts of eastern US. Approximately 180 million acres of land lost this species biodiversity. Additionally, almost a dozen moth species depended on the American chestnut trees, so they also became extinct or endangered.
Certain introduced species can breed with native organisms and alter the gene pool in the species. Hybridisation affects indigenous populations by creating an offspring that is better adapted to its environment than the local one. As such, the introduced species as well as the hybrid displace the local species into extinction because they have a genetic advantage.
This form of invasion occurs over a relatively long period of time thus making them seem less dangerous. Rhymer and Simberloff (1996) explain that the seriousness of the phenomenon may not be very evident from direct observation of the morphological traits of the species. Therefore, the scientists used molecular technology in order to study the extent of hybridisation among various species.
They argued that gene flow occurs naturally in various settings. However, when hybridisation takes place, then more genes are lost, and at a faster rate than in cases where no introduced species exist. Such foreign species threaten rare species particularly because their gene pools are delicate. This is a process called genetic pollution; other experts called it genetic erosion.
When human beings manipulate organisms’ genes for increased yields in agriculture, they eventually, make wild and domestic varieties less common (Wilson 1992). Genetic erosion is a danger to environmental diversity because it minimises the prevalence of rare genotypes. The native Hawaiian duck is adversely affected by introduced species such as mallards because it has bred with them.
Now the duck’s gene pool is bearing more resemblance to the invader; in other words, it has evolved. Another case was hybridisation of the Texas fish with genes from the mosquito fish. Now the Texas fish is regarded as an extinct species whose contribution to biodiversity in the area has been undermined (Naylor et al. 2001). Even the Gula and Apache trout have hybridized with several species in their areas of introduction.
In Europe, the white-headed duck has become an endangered species because of hybridisation with the ruddy duck from North America. The white-headed duck is regarded as one of the rarest ducks in the world, yet the ruddy duck’s continual expansions into Europe ruined this (Eildredge 1998).
Introduced species can modify habitats as well. They often make their new environments unfavourable to indigenous species, which struggle to adapt and eventually die out. One classic case is the zebra mussel; the organism had a way of filtering large quantities of water that led to the reduction of planktons in the water. Additionally, because the zebra mussel spreads so quickly, it became so difficult for indigenous populations to survive in these environments.
Almost thirty species of freshwater species face extinction in areas inhabited by the zebra mussel. The introduction of Prunus spinosa into Western Europe has substantially altered habitats for the Thecla betulae butterfly. The introduced species came from Eastern Europe, and leafs much faster than the local plants. Not only did the Prunus spinosa endanger the local plants, but also modified the indigenous butterfly’s habitat thus threatening its contribution to biodiversity too (Pimentel et al. 1999).
Non native species can also spread diseases that harm indigenous populations. A typical scenario was the introduction of a European parasite that led to the prevalence of whirling disease among rainbow trout in the US, Pennsylvania. They disease spread to other regions of the country and diminished rainbow trout in most parts.
On occasion, introduced species may not be parasitic or predatory; they may be herbivorous. In this regard, these animals will consume native plant species and thus lead to their elimination. Although many individuals rarely think about these particular animals, the goat has been one of the most threatening organisms to native plant species.
When goats came to the Atlantic Island during the 16th century, they ate plenty of local plant species. As a result, 50% of these organisms were lost. Perhaps it is the beneficiary effects of the goats to the human being that make it appear non invasive. However, when one studies this issue from the perspective of the plant species, then one realises that herbivory can indeed be detrimental to biodiversity (Spicer 2006).
How introduced species may not harm biodiversity
Bartomeus et al. (2008) carried out research on the effects of invasive plants on the pollinating patterns of the population. Their results contradicted what most conservationists say about introduced species. The scientists found that it was not just the invading population that benefitted, but the local population, as well. Invasive plants may sometimes increase the number of pollinating agents that visit a particular ecosystem.
The agents under consideration in this research were insects. It was affirmed that, unlike what floral market analysts claim, more insects increased in that population. Since the introduction of foreign plants in the area of study did not occur in a simple manner, then it was necessary to analyse the competition structure of the respective ecosystem. In this analysis, there were two invasive plants: the prickly pear type called Opuntia stricata and the balsam or Carpobrotus affine acinaciformis.
They both have very attractive flowers that attract more insects than the native populations. Furthermore, because those flowers are also rich in pollen, then more pollinating agents can visit them. The prickly pear type received 30.9% visits from insects while the balsam received 43.4%. The researchers concluded that whenever pollinating resources increase in a community of plants, then more insects will visit the area. Approximately 23 pollinating insects visited the Carpobrotus.
Unlike the other invading species, which was an exclusive feeder, this particular insect visited other plant types, including the local ones. Balsma plants can alter reproduction processes in native species because they change reproduction patters in these new areas.
Local plants have a better chance of increasing their population when such species enter their niches. This study indicates that not all invasive species are detrimental to the existence of local populations. Some of them can increase the rate at which indigenous organisms reproduce and exist.
Some introduced species can also fight pollution when the concerned location becomes unfavourable for other organisms; the Asian oyster is one such example. When introduced into water bodies, it was able to filter out more water pollutants than the native oysters. This was quite beneficial because the water quality in Chesapeake Bay, US had deteriorated adversely. The oyster made the water more habitable for other species by minimising pollution.
Even the notorious zebra mussel has some positive effects on water quality. Studies illustrate that water clarity has increase by five or six times in Lake Erie, where the zebra mussel exists. Greater water clarity has emanated from the zebra mussel’s feeding habits; it consumes fertiliser runoff as well as algae. As a result, more light penetrates into the lake thus providing the underground plants with access to ultraviolet rays needed for survival.
Therefore, microphyte beds have returned to the lake in times when they had previously been missing. Part of the reason for their scarcity was the pollution in the lake. The microphyte beds also provide new species of fish with nurseries, which nurture their re-emergence.
In line with the above argument is the issue of modifying environment in favour of local populations. The common gorse came into New Zealand from Scotland. It easily adapted to its environment, and fitted in well with the rest of the population. However, it threatened many native plants through competition. In the long run, after eradicating some native plant species, it started providing nurseries for their reestablishment.
Introduced species may not necessarily lead to the minimisation of biodiversity if other species enter the ecosystems too (Newcomb 2001). In this regard, when local species are destroyed, then other species may come in to fill the void. This interchange of species can sometimes lead to a greater level of biodiversity in some areas than had existed prior to the extinction of certain species.
Nonetheless, the occurrence of this phenomenon has not been explored in as much detail as the reverse phenomenon; that it, extinction of species. In fact some individuals argue that researchers have a tendency to stop at the immediate losses that occur in a certain location. Many of them do not look into the long term implications of invasive species on diversity.
Sometimes a certain population may be burdened by invasive species. Consequently, humans may introduce other species as a means of biological control. Here, introduced species may remedy the effects of other harmful species in the area. One such instance is the use of the South American caterpillar moth to control prickly pear cactus in Australia. The latter species came from the Americas, and invaded numerous Australian lands.
They seriously affected the ability of local plants to grow and thrive in these locations. It was necessary to introduce an enemy to the invasive species in order to restore biodiversity in the area. An example of such an experiment was the alligator weed in the US, which had come from South America.
The weed was a danger to aquatic diversity in the state of Florida owing to its high rate of expansions. Scientists introduced the alligator weed flea beetle to control this population. As a result, the population of the weed radically reduced, and this enhanced biodiversity for other organisms that had been crowded out (Wiedenmann 2000).
Nonetheless, for biological control to work, then the natural enemy of the invasive species needs to have a high degree of specificity. The conservationists must do a thorough research of the exclusivity of the species under consideration (Geerat 1991). If it is a general feeder, then it may become dangerous to the local population.
The cane toad in Australia was one such example; it was introduced in the continent as a method of biological control against the Greyback and French cane beetle. Unfortunately, the parties responsible for it did not know that it would feed on other insects in the area. They did not predict that the cane toad would also threaten other local frogs through competition and disease introduction.
A number of other carnivorous animals also died from the poison in the cane toads’ parotid glands. The population of tiger snakes, crocodiles, quolls and dogs reduced dramatically when they ate this amphibian. A domino effect can occur when a biological agent’s full effect on the ecosystem s not well understood. Another scenario of biological control gone wrong is the Rhinocyllus conicus. This weevil was supposed to minimise the Canadian and Musk thistles.
The latter introduced species had threatened local plant species extensively. The weevil turned on native plants that were an important source of food to local insects. Consequently, not only did the local plants face the threat of extinction, but so did other insects that depended on the thistles. Eventually, this has a negative effect on the biodiversity of the place.
Introduced species can also improve biodiversity in an area when they are brought for conservation purposes. In such scenarios, a species may be in danger of extinction. Therefore, environmentalists may take it upon themselves to introduce those new species to the same locations that they existed before. Although introduction of the invasive species may have caused the problem in the past, it can also contribute to the prevalence of a higher native population of species.
Cases of native restoration include the introduction of the Red Kite in Scotland and England. Wolves were also introduced in Yellowstone National Park within the US after their numbers were reduced dramatically. Additionally, some individuals take it upon themselves to plant trees in their personal properties that match the local population.
Reintroducing endangered species works by expanding the gene pool of threatened animals. Many of these genes may be highly concentrated in one area but not prevalent in another. It thus becomes necessary to translocate them to the depleted gene pool in order to restore their numbers.
In other instances, the elimination of introduced species may be more of a political battle than an environmental one. In this case, it mirrors xenophobia or ethnic hatred. The attitude towards all things foreign is sometimes transplanted in botany or other areas of nature conservation. Well known writers, such as Nellie Doubleday, have reflected these ideologies in their writings about natural landscapes.
He asserted that American plant life needed to reflect the racial temperament of the country. Therefore, styles of gardening inevitably demonstrate this perspective. Another landscaper called Jens Jensen explained that the landscape environment he was creating needed to reflect the racial traits of the US.
They needed to be devoid of non American influences such a Latin or Oriental mixes. He claimed that plant types that borrowed from these influences were ruining the American character. Furthermore, he added that Latin elements were always spoilers (Simberloff 2003).
While such attitudes were quite strong in the early twentieth century, their influences still exist today. Journalists, researchers and other stakeholders appear to be interested in causing mass hysteria regarding introduced species. For instance algae may be described as ‘killer algae’ while rats may be called ‘giants’. All these words create fear in the masses and propel the government to make laws that support xenophobic tendencies (Gobster 2005).
Sometimes the focus is always on the commercial industries and few arguments dwell on indigenous biodiversity. Several stakeholders may assume that all introduced species are harmful unless proven others. Consequently, one can realise that in certain circumstances intolerance to all things foreign determines people’s reaction to invasive species rather than their actual harm
Conclusion
An analysis of the issue illustrates that although introduced species have certain benefits, they appear to cause more harm than good to biodiversity. They cause competition and modify habitats for local species. Additionally, they may become parasitic or predatory on certain organisms. On top of that, some hybridize and cause diseases among indigenous species.
The purported benefits of introduced species are questionable because when used as biological agents, they may turn on other vulnerable organisms. Additionally, those introduced species that fight pollution or modify habitats for the benefit of other organisms, also destroy it for others; the net outcome is negative. Therefore, introduced species may have a lot of commercial value, but their harmful effects on biodiversity are too much to ignore.
References
Bartomeus, I Santamaria, L & Vila, M 2008, ‘Contrasting effects of invasive plants in plant pollinator networks’, Oecologia Journal, vol. 155 no.4, pp. 761-770.
Cohen, A & Carlton, J 1998, ‘Accelerating invasion rate in a highly invaded estuary’, Science Journal, vol. 279 pp. 555-558.
Geerat, V 1991, ‘When Biotas meet: Understanding biotic interchange’, Science Journal, vol. 253 no. 5024, pp. 69-99.
Eildredge, N 1998, Life in the balance: Humanity and biodiversity crisis, Princeton University Press, Princeton.
Gobster, P 2005, ‘Invasive species as ecological threat: Is restoration an alternative to fear-based resource management?’, Ecological Restoration Journal, vol. 23 no. 4, pp. 262-270.
Johnson, T 2003, Invasive species, Burlington Free Press, New York.
Moritz, C 1999, Conservation units and translocations: Strategies for conserving evolutionary processes’, Hereditas Journal, vol. 130, pp. 45-52.
Naylor, R Williams, S & Strong, D 2001, ‘Aquaculture: A gateway for exotic species’, Science Journal, vol. 294, pp. 1655-1656.
Newcomb, J 2001, ‘Alien species often fit in fine, some scientists contend’, The New York Times, p. A8.
Pimentel, D, Lach, L, Zuniga, R, & Morrison, D. 1999. Environmental and economic costs associated with non-indigenous species in the United States, Cornell University Press, New York.
Rhymer, J & Simberloff, D 1996, ‘Extinction by hybridisation and introgression’, Annual Review of Ecology and Systematics, vol. 27, pp. 83-109.
Simberloff, D 2003, ‘Confronting introduced species: a form of xenophobia?’, Biological Invasions, vol. 5, pp. 179-192.
Spicer. J 2006, Biodiversity: Oneworld Publications, Chicago.
Wilson, E 1992, The Diversity of Life, Harvard University Press, Harvard.
Wiedenmann, R 2000, Introduction to biological control. Web.