Positive Effects of NNS
There are widespread indications of how introduced species have ameliorated the standard of living of humans throughout the historic ages, and propelled agricultural, social and even technological advancement of the human race. Without proliferating the domesticated food species that are transferred widely around the world to suitable regions for cultivation, the human population could not have risen exponentially, and as our population is still exploding the marginal drawbacks of this advancement is becoming increasingly alarming. In the historical times, wars would not have been fought in such a large scale if horses were not introduced through to Southern Asia and China from Europe; in the colonial ages of America, horses were introduced to North America and have since played large part in the daily life of people there as had been the case in Europe and beyond. Most ornamental plants found in temperate developed countries originated from tropical countries, and apart from bringing their beauty to close within reach, they can be sold for a fair price, and some are economically important crops, as are many cash crops, including rubber, as an example. Including Chrysanthemum, Tulips and Orchids, ornamentals are more or less controlled so that they cannot spread much away from where they are nurtured. Emancipated NNS are another matter.
Negavitve Effects of NNS
1. Ecological Impact
The single most important negative effect of NNS is their impact on the natural environment. It may not sound a great deal, but it is tantamount to leaving a daughter to a strange man's care. The capability of these introduced organisms does not seem to receive full appreciation even nowadays, and people keep shuffling the inhabitants of the world around at will, and sadly in many cases inadvertently, without taking heed of the damage they are inflicting upon the environment potentially.
Resources are limited, and bounteous as they look, natural resources are more limited in supply than you may think. To maximize the use of a resource, organisms have evolved special strategies. Different species usually have marginally discrepant niches if they are closely related, just like, say, your taking to chocolate milk, and your brother to strawberry-flavoured milk only, so that resources are neatly divided between species, and the productivity of an ecosystem is fully utilized. However if an elder brother takes both flavours at large plus skimmed milk, he is always well fed enough to compete with his brothers for the remaining milk. Generalist NNS (See Whats and Whys) either compete with the native species that share similar food sources, or explore other sources as well so that their numbers are sustained at higher levels, thus taking a larger percentage of resources available particular to the survival of some specialist natives, plunging the communities into a vicious cycle from which the native species could not easily liberate themselves in succumbing gradually to the NNS. Introduced trees and noxious weeds that threaten native vegetation crowd out other plants through growing leaves earlier in the season and many have a faster growth rate. That way the available light early in the season will be absorbed by the non-native species rather than the natives, and by the time the maximum growth period of the natives is reached the NNS have reached such a large size, with advanced foliage systems to catch sunlight, that the natives are outcompeted. In America as many as 500 exotic plants have managed to sustain themselves and a few have reached such population sizes that an area exceeding that of Texas is covered by introduced plants. And animals can be worse. Intricate food webs link the organisms dependent on green plants as food source to one another, and many animals cannot withstand large tracts of unfamiliar vegetations, which may not fill the ecological roles some native plants represent. Many fire ant species are unpalatable to Horned Lizards, which feed on native Ants that are displaced on a large scale by their vicious cousins. Species counting on the displaced natives for food perish as a result, and the cascade effect ensures that not just a single species is affected when the NNS directly competes with that species. Whole food webs can be disrupted by such invasions, and the supporting links of the ecosystem can be severed as a result, leading to ecosystem collapse.
(Fire Ants are unpalable to Horned Lizards)
Resources are much more than just nutrients. Birds must have a site for reproduction, and many American birds are hole-nesting species. Introduced birds may compete with them for nest sites and reduce breeding success in many native species. Air may be a luxury on land but in badly oxygenated water, any reduction to the oxygen level can spell disaster to native fish that may not be so tolerant of such an unprecedented environmental change. Introduced water weeds may reduce the oxygen level in water and threaten native fish in this way. Any NNS that knocks on the environment for more natural resources can threaten the neighbours' livelihood.
Direct predation falls squarely into this discussion. In an isolated environments such as islands and notably Australia, the modes of predation on the "mega" fauna is limited and native herbivorous Mammals and Birds are not prepared in any way for the radical changes brought about with the introduction of a single Cat, Fox, or several Rats that may not look anything more significant than being filthy and repulsive. They are repulsive to nesting songbirds, land birds and sea birds that lay their eggs conspicuously on the ground.
(Fox)
Previously free of potent natural predators, many birds of the Pacific Islands and Hawaii have been eradicated by introduced Rats as they feed on adult and young birds alike. The last few centuries have witnessed just fewer than 70% of all terrestrial vertebrate species of Hawaii perish, and more than 90% of land birds endemic to Hawaii have embarked upon the pathway of extinction. The Po'ouli (Melamprosops phaeosoma), one of the thirty species of Hawaiian Honeycreepers that survived into the historical times, have only 3 existing individuals known and is following the footpaths of its relatives that had gone extinct over the last few centuries --- the main culprit are Pigs that uproot and degrade its understorey habitat on Maui, where once the understorey was lush and impenetrable. Mauritius has lost the Dodo back in the 15 th century to Rats, and many native birds are still highly endangered, as in many of the islands of the Western Indian Ocean. Australia have had many of their native Marsupials decimated from predation by Foxes, Cats, and competition for food with Rabbits; the latest, in particular, have drastically reduced the number of burrows available for small subterranean Marsupials such as Bilbies and Bandicoots. Foxes hunt medium-sized Marsupials such as Hare-wallabies and Rock Wallabies that cannot escape into burrows, and have significantly reduced their numbers. Up till now 19 of the small marsupials native to Australia have been wiped out mainly by the NNS. The Australian fauna is under siege, but the flora fares no better, as more than 10% of the plant species in Australia are NNS; this unique ecosystem is doing well not being vanquished at all, but many of the endemics would not have survived without conservation efforts.
(Po'ouli)
Guam and many of the South Pacific Islands have introduced Rats, and the former has had its whole endemic forest bird populations wiped out by a single species, the Brown Tree Snake. The Guam Broadbill (Myiagra freycineti) was one of the unique species that have evolved in Guam's tropical rainforests, and was lost around 1974 to snake predation. Numerous examples as such can be cited. Due to the fragile nature of island ecosystems, in which many small vertebrates have become accustomed to predator-free environments, and birds have turned to ground nesting or lost the defensive mechanism towards nest-raiders, any introduction of potential predators end up as ecological disasters.
(Guam Broadbill)
2. Habitat Degradation
When a species has its population above astronomical levels, it is destined to have an effect on the outlook of the environment in which it lives, and the effect is most likely to be negative as a monopoly of the resources offered means that the necessary gradations and trophic levels to a healthy ecosystem are absent, and the original functioning of the ecosystem is bound to suffer disruptions and distortions. Noxious weeds can overgrow and turn healthy pastures into practical wastelands that are nevertheless not the required habitats for the natives; some water plants and Mollusks clog up waterways and canals, and make rivers and streams too cramped to accommodate certain fish species that require more open water, whilst water transport is also deterred. Algae overgrowing in favourable areas can foul fishnets and block water discharge chutes, damaging fisheries and creating unsightly patches along the shore, deterring wildlife and people all the same from using the area. Rabbits, Goats and Cattle are renowned for stripping pastures bare in places where the grass species are not hardy enough to stand such grazing pressure as what the large mammals presented, and Australian plains have progressed into a dust bowl of desert patches in addition to the extensive Great Victorian due to the NNS; the Big Island of Hawaii, much to its chagrin, have hosted goats and other grazing livestock that were relentless in feeding efforts, and many native plants are under the dual pressure from human clearance and overgrazing. The landscape is changed forever as the grass dies and the land turns derelict, not suitable for pastures and farmland alike as soil erosion drains away the nutrients needed for life and turns the land barren, with little recovery possible as long as the NNS persist. More direct effects of animals on the environment includes population explosions that infest cities and villages, such as the Cane Toad that grew completely out of control on the feast of defenseless native insects.
(Goats are renowned for stripping pastures bare)
As a tolerably stable habitat is important for the survival for all organisms, radical changes to the basic physical conditions can be lethal to some of the more sensitive species. Often the ecosystem is traumatized from the bottom up, and the effect shows up most rapidly in the top predators as the number of preys available drops disproportionately more as we go up the energy pyramid. The top predators my then be called indicator species, meaning that their decline is indicative of underlying problems in the ecosystem that may not be apparent at first sight.
3. Economic Loss and Diseases
Figures speak for themselves. The United States suffer an economic loss of a minimal $137 million annually due to NNS. Within that $80 million are spent in dealing with noxious weeds and exotic plants that threaten the local ecosystems, which is 60% more than the annual income from agricultural exports, at around $50 million. In Australia, the agricultural damage caused by Rabbits alone amount to $20 million annually, and the controlling measures combined cost far more than this. It is estimated that pests eat almost 40% of all the crops grown worldwide, and amongst those pests many are NNS that have found no natural predators in the abnormal havens abound with food. They are allowed to multiple with the superb food source provided often by monoculture, which excludes all but the preferred food source of the pests --- care has been taken to remove weeds, pesticides have been sprayed to kill off large predators and deter any predators that may immigrate in search of food, but small insect pests do not have much difficulties escaping from the toxic substances by hiding in the foliage, and with the further reduction of enemies they are literally thrown into heaven, with unlimited food supply and no population control. Disastrous loss of crops is bound to ensue, and culprits include insects such as scale insects, aphids and other sap-feeding ones, and plant viruses that infect and kill large cash crop species such as the Banana Bunch Top Virus that attack banana trees.
Pathogenic outbreaks that have caused important socioeconomic mayhem are to a certain extent related to the non-native species problem. Many pathogens are themselves NNS, with viruses causing West Nile Fever, Hepatitis, Smallpox, Rabies, and other infamous diseases having been spread so thoroughly amongst the human population worldwide that infections occur quite frequently in places and outbreaks are not uncommon. Evidently a recent example of introduced viral pathogen is the Corona Virus causing the Severe Acute Respiratory Syndrome (SARS), which has clearly been demonstrated to have taken advantage of the air traffic and has been transported accidentally from Southern China, the original source of the outbreak, to remote places as Canada and other countries in the Oriental Region. In a similar way many of the less harmful viruses have already been spread as far as the human concentration stays above certain levels that the viral population is sustained. Viruses that affect wildlife are also encouraged with increasing pet trade and introduction of all sorts of NNS that may have been infected with the virus already. Bacteria are dispersed in the same way, and invariably these consist of more that have an effect on humans than on other animals in general, as the transport system approaches consummation within the bounds of prevalent technology. Bacteria that affect livestock are also commuted around the world where farms are situated, and losses from epidemics can be appreciated from the anthrax outbreaks earlier; food-and-mouth disease, caused by viruses, can kill hundreds of cattle in one go, and the approximate cost of vaccinating all the livestock may exceed $1 billion. In the outbreak in 1968 more than 430000 animals are slaughtered. Recent Avian Influenza outbreaks, which have spread from South-western China to Vietnam and beyond, have claimed the lives of tens of thousands of domestic fowl, and the supply of their meat is severely disrupted, and farmers have to bear the loss square in the head --- the estimates have yet to be publicized, but the effects are apparent.
(Many chickens in the recent Avian Influenza outbreaks)
NNS vectors have facilitated largely the killing spree of such diseases. Rats and Mosquitoes rank amongst the most efficient pathogen-transmitting organisms as many viruses and bacteria have life stages within these specific hosts, and their introduction has allowed the pathogens to roam free and wide. In the Hawaiian Islands, Mosquitoes have spread the Avian Malaria wherever they went and as native birds have zero resistance against this new disease whose efficiency in killing is astounding, epidemics have been maintained as one of the most important causes of decline of Hawaiian Birds. Malaria itself takes its toll on human circumtropically, and introduced Malaria Mosquitoes are keen distributors of this disease. The effect of novel pathogens can be extremely acute on humans that have never been brought into contact with the disease, but certain complications exist --- the idea holds, regardless, that NNS have caused huge threat to public health and the general well-being of wildlife as well.
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