What is a Non-native Speices?
Non-native Species are not found naturally and are "foreign". You may ask what it is meant by that, for it simply sounds esoteric the first time the phrase is came across --- for when due consideration is given to our definition, we find that individuals can be peculiar to a place but it is hard to imagine a whole species of organism being so --- at least, not for widespread species like the Barn Swallow, House Mouse and Mackerel, which seem to be so abundant that their range cannot be definitively stated. What makes a non-native species non-native? The question is certainly complex, but as we attempt to delineate the key criteria here, it will become apparent that the ambiguities must be taken into consideration when the problems associated with non-native species are to be dealt with.
This is exactly why we must consider two important concepts to clarify the definition of non-native species. These are natural range (and how this is defined) and means of dispersal --- which leads directly into our central area of concern, the unconscious generation of the non-native species concept. More of this will be discussed in the ensuing section.
Ecologically, there are two extreme roles an organism could adopt in their struggle towards survival, and the human society could be seen as working on a similar principle. In our society we have professionals on one hand, which specializes in one job and usually focuses on perfecting their professional skills. Some people will have changed several jobs before they eventually settle, and some keep on shifting their occupations, widening their scope of adaptations and gaining certain levels of proficiency in many professions. In nature, a graduation of ecological specialization exists, though the range is far more radical and dramatic, while the forms are subtler than that in our society, so obscure that biologists are still a long way from understanding some of the basic mechanisms of how these stratifications come into existence. Specialization and generalization determine the natural range of many species and correlates with the topic of NNS rather intimately. The diversification of organisms in the world in adaptation to their environment, which includes both the physical conditions such as temperature and rainfall, and biotic factors --- by which is encompassed all the other organisms coevolving with that in concern, is termed niche specificity. Generalists are the constant job switchers, straddling a number of available niches --- though not reliably attributed to a specific role, the catholic choice of habitats and food sources characterize these species, and often proffer them auspicious opportunities in times of severe crisis owing to their adaptability. They are not confined to narrow niches but are more open to radical changes in the environment, and are more likely to survive in new environs and alien territories, in many cases establishing themselves --- outcompeting the established tenants as introduced species.
Specialists by contrast have confined niches, which can be effectively seen as analogous to professional fields in human terms. They tend to show unique adaptations that are not easily transcended by other latecomers to the habitat that possesses equally special conditions, either due to the seclusion of their environment from the outside world, or that the presence of many distinct niches have enabled the development of specialist organisms that fill these niches. For example, some hummingbirds of South American rainforests have coevolved with heliconia (a flowering plant) that produce their favourite nectar, and many pairs of these have become so mutually specific in that the plant relies solely on the hummer for pollination, as special shapes of petal tubes allow only certain species of hummers with a complementary bill shape to exploit their nectar while acting as pollen vectors simultaneously. When the food source of specialists is removed, the chances of their survival are drastically reduced. Therefore specialists are generally less adaptable to changes in the biotic composition of communities in their habitat and are vulnerable to non-native species invasion.
The difference between the two broad roles is exemplified by the difference in natural ranges, or distribution, of the organisms. We can infer from the last example that the hummingbird species that live particularly on the nectar of one species of heliconia occur only with the flower, and it follows the range of the hummer is restricted by that of the flower. There are organisms that live nowhere else but in narrow habitats in which particularly special physical conditions prevail, like the blennies and bacteria that live around the hydrothermal vents in the depths of the Pacific. Range restrictions can therefore be so extreme that in the case of some, like the Stephen Island Wren in New Zealand, from an island that has an area of only 1 square mile, the range is too tiny to allow breathing space for the species when alien invasions occur, as indeed transpired --- the species was wiped out by a single cat.
Too much about the victims; in general the range of all species are defined roughly through mainly geographical barriers, control and competition from other organisms, and such physical factors as climate, elevation, salinity, temperature, and the like. The world is divided into several belts, or biomes, in which the dominant vegetation prevails. This determines to a great extent the fauna found in that biome, both on land and in the sea. The vegetation type is mainly determined by the mean temperature and climate of the region concerned: the cold, dry polar areas are denuded, while further towards the equator, as the temperature rises, the vegetation changes from tundra, savanna, taiga forests, deciduous broadleaf forests and tropical rainforests, roughly. The ocean also varies by region in productivity, species diversity and type of vegetation (algae) and this is ultimately linked to the temperature and other conditions of the environment. Organisms are more or less suited to a few of these regions individually, and they have different ranges according to their life requirements. Geographical barriers are more easily envisaged as fences that mark the viable territory of a species as an entity. Barriers could be ill-defined, such as patches of unsuitable habitats shunned as a rule, considering rainforest specialists that find it hard to cross exposed roads that bisect some of their home ranges Amazonian Marmosets (small ornate primates that live in the canopy) cannot cross some of the larger tributaries and have especially narrow ranges, as they are so dependent on their homeland that they could not bear the stress of breaching their limits to seek out other possible haunts. The Pacific is the end of the world for many endemic islanders, apparently, and it is not hard to see that though birds and insects are endowed with the power of flight, many island species have lost the aptitude in this respect as flight became fairly redundant in predator-free habitats. Mountains and deserts are apparent blockades to the pioneers.
What this all lead to is that every species on earth has a natural range. This is not a static boundary restricting physically the movements of organisms as is sometimes thought, but rather a dynamic region in which the organism could normally be found in its natural state, which is defined by factors briefed above. Organisms are supposed to thrive only in their natural range due to the presence of favourable habitats, regular food supply, or the fact that in many cases other similar havens are inaccessible beyond the barricades that impede a species from emigrating and establishing themselves in these foreign lands to which there already are a set of rightful inhabitants. City-dwellers may find it difficult to cope with the featureless, uneventful life in the countryside; those living in the rural areas could be daunted by the stress and clamour of city life, and adaptation to such may be unfeasible for them; more radically speaking, the indigenous people of remote African tribes would be doing the exceptional to be successfully tackling with the pace, style and demands of life in modern civilizations. Yet the cream of the crop do --- adaptive organisms, and let us say NNS, stand a much better chance in the race of evolution, but it is not without the instigative efforts of humans that they could put other organisms out of existence. It is the presence of order that allows disorder to take its toll --- and this is what the NNS problem entails.
Means of Dispersal
The diverse plethora of life on earth has been interacting with the environment they are living in for billions of years. Today we are fortunate to witness this dynamic diversity of life at play and it is particularly miraculous to play a part in this dramatic panorama. However, the very differences, exhaustive to name, amongst these organisms serve to constrain most of them to specific habitats. There are less obscure criteria than habitat preferences and biotic interactions that determine the natural range. Effective physical barriers often delineate the range of some species by the direct blockage of dispersing individuals, for example deserts, mountain ranges and rivers. Species that have a vast natural range either have fortuitously crossed from continent to continent in times of severe gales, utilized drop in sea level that exposes land bridges, or in times of climatic change happened to possess certain characteristics that enabled it to outcompete many of its congeners, allowing it to proliferate. For oceanic species it is even less obscure as the continuity of the seas leads to free dispersal, and the range of oceanic species is determined mostly by their physical tolerance of water temperature. Circumtropical and circumglobal distributions of many Whales and some Sharks illustrate this point. Introduced species seldom applies to the vast ocean, but local shorelines can be prone to infestation, as many organisms thrive in estuaries and tidal zones, and are restricted largely to these areas that are comparatively tiny in area to the ocean.
How do land species disperse? We may note that naturally organisms disperse, and their ranges are ever changing. To colonize new sites the fluctuations in environmental conditions must favour the spread of the organism to previously unpalatable places, and such has happened with Crows that seem to be spreading in the Malay peninsula, presumably due to more scavenging opportunities with the increase in human population, and thus, rubbish dumps; the end of the last glacial period 10000 years ago have enabled trees to recolonize the boreal lands after a period of coldness that saw ice sheets expanding down to Central Europe. For migratory birds and insects the ranges are ill defined --- their extreme mobility and propensity to travelling often causes vagrants to turn up in unexpected places, but these do not normally turn sedentary afterwards and is hardly called dispersal. The idea is that every species, plant and animal alike, have their own levels of mobility and interactions with the environment, or rather dependence on certain habitats, that natural dispersal is limited. Under normal conditions no species could occupy every single spot in the world (with inconsequential exceptions) though some achieve cosmopolitan range --- not meaning that they have penetrated every ecosystem available. There remains inaccessible environments thus for every species of organism, which either are unsuitable to their survival, or not so, but nevertheless physically inaccessible. Plants in the Pacific Islands can disperse with seeds that float on water and travel around the island groups, yet not many succeed in germinating, for the soil conditions where they bump into solid ground may be deterrent. The limitations spelt in the previous section thwarts natural dispersal.
Humans are special. It can scarce be denied that for the single ability of them to transport goods on a global scale merits the claim. We have sighted ourselves as distinct from other animals, and have viewed them as goods. With technology from steam-propelled ships to aeroplanes, we have taken the flow of necessities and amenities to such a scale that could not find rivals across the globe. These transport vessels breach almost all barriers, and previously inaccessible, rich habitats are made available to the travellers that used us, or those which are use by us. Human-aided dispersal defies all natural boundaries, and may be direct or indirect. Direct transport explains itself; but in the process of altering the environment through agricultural development, changing forests into plantations, constructing concrete banks around the rivers, and other activities as such, man also indirectly cause habitat changes --- forests are cleared into huge tracts of arable land, heaths are burnt down to create farmland, wetlands are drained to give pastures, and so on. This enabled some organisms to expand their range exponentially and push others to the brink of extinction. The eviction of natives by generating habitats favourable to others can augment the population of the protégées to pest proportions --- this is particularly true in introduced agricultural pests in places where monoculture has taken place. There are also instances where introduced animals change the landscape so much that natives are driven into isolated patches of relatively untouched habitats, and the remaining is colonized by other introduced species as a result. Grazing animals such as Goats can easily denude islands of their native vegetation, reducing habitats for native species while enabling other introduced species, such as Rabbits and Weasels, to infest upon the plains that have been opened up. The insidious effects never cease until the introduced species are eradicated.
Causes of NNS Establishment and Spread
It is wise to learn from history, and the extent of non-native species problem cannot be underestimated as before. Humans may not be aware of this centuries ago when widespread introductions began with the explorers that poured out to Oceania and the New World --- in fact Europe has already been under tremendous pressure from introduced species then, but the effect was not as apparent as that of habitat destruction, attributed again to an introduced species homo sapiens. We have two main focuses here: one is why NNS have turned up where they should not have in the first place; the second is why have they have spread, and not been eradicated by other organisms in the introduced range or degenerated to oblivion on themselves.
We have seen the advancement of transportation in the past centuries, and in the packs and freights of our crafts have been found many hitchhikers indeed. This is what accidental transfer of organisms are all about --- many of the hardiest invaders have gained access to the remotest island and other continents through human transportation for no reason, and amongst all the Rats must be mentioned, which have been spread throughout the Pacific Islands in this fashion, especially the Black Rat, alternatively named Ship Rat, for its ability to cling to ropes and hide under decks in ships. A researcher in Massachusetts accidentally released Gypsy Moths that he transported there to conduct studies upon, and we may safely assume that it would not be a single incident. Accidental introduction have spread the most pernicious NNS around, as any organism likely to survive the long journeys, scarcity of food and physiological stress of the alien environment are likely to be the most adaptive of all, and when they parachute upon pristine habitats, they and their avaricious descendants can tip the balance of the local ecosystems with stunning swiftness.
Most introductions, however, are not accidental.
1. Affection/Nostalgia
It is surprising that how many species are introduced from the affection of humans towards animals. As it turns out, many of the pets are not kept at home, and the nostalgic urge of many colonizers to the Australasian area have brought along European birds and mammals with them, resulting in effects as serious as transformation of some parts of the landscape. Cats, Dogs, Rabbits, Starlings, Goldfinches and Slider Turtles have been introduced for this reason, and when let loose, have presented dire challenges to the lives of many of the fragile ecosystems whose collapse we have witnessed.
(Goldfinch is introduced die to human affection for it)
2. Food
Many plants such as orange, banana, potato, rubber tree and the like originate from the tropical countries. Plantations in those countries are simply insufficient to satisfy the world demand for these important crops and some of the food crops have been introduced to the developed world and intensive farming methods are adopted to wring as much yield as possible. Livestock such as goats, sheep, cattle and pigs are also widely introduced into ranches created out of prairies in the Americas and tropical forests in the Pacific Islands, so that a few are virtually denuded. The benefits of such introductions gave rise to the pillars of modern economy, and who could say what the world would still be like without the widespread cultivation of potatoes, wheat, rice, and the rearing of livestock as cattle and sheep?
(Orange is introduced for food)
3. Control of pests
Biological control agents are organisms that are deliberately introduced to control pests. They are highly predatory, usually specializing in feeding only on a narrow range of prey species, and are preferably efficient in keeping the population of the pest species at a low level through natural predation.
(Ladybird is introduced for pest control)
Ignorance about the biology of certain introduced controls have unleashed some of the most invasive exotic species as these predators, if non-discriminatory in taking prey with generalist feeding habits, can reduce the numbers of many keystone members of the ecosystem as these may not be accustomed to this novel source and mode of predation. Rats are known to kill Albatrosses, which are large seabirds growing to 90cm in length for medium-sized species, alive when the birds are incubating on islands; native marsupials of Australia have little defense against the introduced foxes that prey on them, while the foxes had been expected to control the burgeoning rabbit populations; the list can be expanded for sure. Successful attempts of biological control are scant, but the method is currently widely explored and researched as a potentially far more effective alternative to applying pesticides to crops badly affected by uncontrollable pest infestations. Good biological controls only eat the pest naturally, and can be effective in preventing large-scale crop loss to pests and use of toxic pesticides that pollute the environment.
4. Environmental Improvement
It is awkward that this should become a category as often this encompasses the rectification of problems caused mostly by humans, and the appeal of many plants to our aesthetic sense. Many tropical plant species like orchids are introduced as pot plants, and the sausage tree (native to Africa) and pencil tree (native to Asia) have been introduced widely as ornamentals. Some widespread ornamentals have turned out to be a great nuisance as they begin to displace native plants by competition. The Brazilian Pepper Tree (Schninus terebinthifolius) has been introduced to South Florida for use as a landscaping plant, and since have been dispersed so widely by birds that feed on their berries, they have jostled many native species out of place. Australian Ironwood (Casuarina sp.), mistakenly purported to be capable of draining flooded swamps to yield available farmland, have also been introduced to Southern Florida and they turn out to be shallow-rooted and despite having been spread widely they collapse easily around coastlines and expose the fragile shores to cyclone, causing similar degrees of trouble as many other introduced plant species.
Present agricultural methods have caused soil erosion in many places as after harvesting, and the stabilizing ability of their roots on the soil is sporadically deprived of. Kudzu trees (Pueraria lobata) and many others have been grown around such agricultural areas, reputedly to alleviate the effects of soil erosion, as their roots are perhaps more efficient in soil retention than most other species --- but in terms of survival skills they are certainly superior. The deep-rooted Tamarisk tree (Tamarix mannifera) helps combat soil erosion, but its displacing effect on local plants is also great, causing radical changes in the species composition of the new habitat, and with the local plants are gone the animals that depend on its fruits and various other ecological services for survival.
(Kudzu Trees)
(Tamarisk)
Having seen the main causes for introducing new species into an environment, our second question would be how the NNS are allowed to spread. Certainly to an organism's survival some factors must hold true; the physical conditions of the new habitat must be allowed within the range of physical tolerance exhibited by the species, or rather, extremely competitive individuals. Once the invaders have gained a toehold, it is usually not difficult for the more adaptable species to continue spreading. The habitat type available determines the success of introduced species; if there are large tracts of disturbed tropical forests, vines can find their way in easily if natural predators are not present (some mites and thrips specialize in feeding on the vines, but these are found in the native rainforests only), and the freshwater systems that are in essence equivalent to counterparts across the Northern Hemisphere have allowed many Eurasian freshwater fish to infest upon the Great Lakes, where similar habitats occur; the North American Largemouth Bass (Micropterus salmoides), in turn, have found the habitats of Lake Titicaca in the highlands of Bolivia and Lake Atitlán in Guatemala quite idyllic --- these lakes are notorious for the loss of biodiversity through habitat destruction and the introduction of freshwater species that endangers the other inhabitants, mainly predatory fish as the Bass We must mention some of the failed introductions to exemplify the fact that not all attempts as such are successful. The Chinese Bamboo Partridge has been introduced to Hong Kong but efforts did not produce viable populations that can sustain themselves.
Disturbed ecosystems are prone to invasion.This is a general rule, and may be too general, but it can be imagined that with the increase of pollution and disturbance to the New World and some remote tropical countries or islands that picked up speed in the last century, species not used to the dominance of men may find it exceedingly difficult to maintain original population sizes as the resources and space are reduced from competition with humans. They can be more prone to NNS that have considerably adapted towards lifestyles that allow them to coexist with humans. They are more open to human disturbance and have evolved corresponding measures against decimation but the natives have not, having also been weakened by direct human influence. This is why the NNS problems often come hand in hand with alteration of the habitat, clearance of forests, and various human activities that degrade the environment. However, conventional wisdom about introduced organisms has proved tenuous in a way, as researchers have found that even healthy ecosystems are prone to invasion. The traits that certain ecosystems exhibit may be encouraging to invaders, with plenty of excess resources for the introduced species to utilize, and not many competitors to curb the spree of the raiding alien species.
Even if predators and competitors are present, those NNS that we see with self-sustaining, or ceaselessly multiplying, populations tend to possess certain weapons that help them parry assaults or even launch powerful counterattacks. One of them is high fecundity, which is the ability to reproduce profusely or rapidly, or both. Mice are fast breeders; their populations can expand tenfold in several weeks, given a surplus of food and enough shelter, which habitats altered by humans often offer. In many cases the NNS that have established themselves are those who can grasp any opportunities of reaching food sources, and are, as mentioned, generalist feeders, meaning that the total amount of food (to put it simplistically) available for the species within a given period of time exceeds that of its competitors, giving the NNS a comparative advantage. As a result they are hard to eradicate due to the sheer numbers present. Resistance against predators is, last but not least, one of the common characteristics of successful invaders, as they either produce toxins, like some Toads and Plants, or are unpalatable with inaccessible nutritious parts, perhaps exploited back where their origin lies but not in the heavenly colonies into which they have poured, where the new neighbours have yet to learn how to tackle them.
The reasons that some non-native species become established in certain areas only are still largely in mystery, but certain factors must be limiting their range in the foreign lands under the same set of natural rules that govern the range of species in their natural conditions.
