Introduced Species

In this final section we shall look at how phylogeography can be used in the ongoing fight against introduced species (also known as invasive or alien species). This is a particularly topical application because, although the colonization of new habitats is sometimes a natural process, in recent years the scale of introductions has accelerated dramatically as a result of human activities that have introduced countless species into habitats that they would not otherwise have encountered. Examples of deliberate human introductions include agricultural plants and animals, pets, decorative plants for gardening, and animals for fishing or hunting. Unintentional introductions include parasites and pests associated with the deliberately introduced species, species transported in the ballast water of ocean-going ships, organisms that hitch-hike aboard shipped goods, plus species such as rats and cockroaches that often accompany human settlements.

Introduced species are a major problem because they often outcompete or prey upon native species (Figure 5.17), and in fact are such a threat to biodiversity that they have been ranked as the second greatest problem in global conservation today, exceeded only by habitat loss. A recent survey of extinct animal species revealed that, of those for which an extinction cause could be identified, 54 per cent (n = 91) of species had been eradicated at least partially by invasive species (Clavero and Garcia-Berthou, 2005). In the Hawaiian archipelago, for example, introduced species such as the black rat have contributed to an alarming reduction in Hawaiian biodiversity over the past 200 years: nearly 40 per cent of birds unique to Hawaii are now extinct, 40 per cent of native plants either have been designated as endangered or are candidates for such classification, and approximately 900 species (71 per cent) of around 1263 historically described species of Hawaiian land snails are extinct, partly because of increased predation, disease and

Figure 5.17 A North American red squirrel (Tamiasciurus hudsonicus). The interactions between red and grey squirrels provide a good example of how introduced species can adversely affect native species. In North America, native red and grey squirrels have a long, shared history and co-exist in many areas. In Britain, however, the only native squirrel is the European red squirrel (Sciurus vulgaris). Grey squirrels were introduced onto this island in 1876. Since that time the red squirrel has been declining, and one important reason for this is competition from the grey squirrel. In addition, grey squirrels carry the parapox virus, which is often lethal to European red squirrels. As a result, although habitat loss is also implicated, invasive grey squirrels are playing an important role in the decline of native red squirrels in Britain (Yalden, 1999). Photograph provided by Kelvin Conrad and reproduced with permission

Figure 5.17 A North American red squirrel (Tamiasciurus hudsonicus). The interactions between red and grey squirrels provide a good example of how introduced species can adversely affect native species. In North America, native red and grey squirrels have a long, shared history and co-exist in many areas. In Britain, however, the only native squirrel is the European red squirrel (Sciurus vulgaris). Grey squirrels were introduced onto this island in 1876. Since that time the red squirrel has been declining, and one important reason for this is competition from the grey squirrel. In addition, grey squirrels carry the parapox virus, which is often lethal to European red squirrels. As a result, although habitat loss is also implicated, invasive grey squirrels are playing an important role in the decline of native red squirrels in Britain (Yalden, 1999). Photograph provided by Kelvin Conrad and reproduced with permission competition from alien species. This loss of biodiversity is expected to continue, because each year more and more species are being introduced; by 1995 an estimated 2600 insect species and 861 plant species had been introduced into Hawaii (Howarth, Nishida and Asquith, 1995).

There are also enormous economic costs associated with invasive species. In the USA, for example, billions of dollars are spent every year on controlling the damage inflicted by invaders. This includes keeping waterways clear of such plants as Sri Lankan hydrilla (Hydrilla verticillata) and Central American water hyacinth (Eichhornia crassipes), controlling or eradicating outbreaks of European and Asian gypsy moths (Lymantria dispar) in forests, and managing populations of the virulent Asian tiger mosquito (Aedes albopictus). The tremendous biological and economic costs of invasive species around the world highlight the importance of detecting and controlling invaders and possibly preventing future outbreaks.

One way in which phylogeographic analyses can help us to control invasive species is through the identification of cryptic invaders. Differentiating between invasive taxa and local populations on the basis of morphology may be problematic, particularly if there is substantial morphological variation within the native populations. When this occurs, molecular comparisons between endemic populations, putative invaders and possible source populations can be invaluable. This was the situation found in the gastropod Melanoides tuberculata in Lake Malawi in East Africa (Genner et al., 2004). Melanoides tuberculata is native to most of tropical Africa, Asia and Oceania and is also an introduced species in much of the tropical and subtropical New World. When a non-native morph of M. tuberculata was found within Lake Malawi, researchers were unable to determine from shell morphology whether this had been introduced from a neighbouring population or from another continent. The latter is potentially more serious, because the introduction of a foreign genotype is more likely to have disruptive effects on an ecosystem.

Mitochondrial sequences from 38 M. tuberculata individuals sampled from 20 populations in Africa (including Lake Malawi), Israel, Sri Lanka and South-East Asia were used to reconstruct the phylogenetic relationships of the various populations. Samples from Lake Malawi fell into two distinct groups. Sequence divergence within each of these groups was 0 -- 0.86 per cent, whereas sequence divergence between the two groups was 13.28-14.31 per cent. One group contained the native Lake Malawi samples plus all other African populations and samples from Israel and Sri Lanka. The other group contained the non-native Lake Malawi samples plus the South-East Asian samples. These molecular data strongly suggest that the non-native morph of M. tuberculata did not come from an allopatric African population, but instead was introduced from South-East Asia. It is not known how this introduction could have occurred, although one possibility is via discarded ornamental aquarium contents.

A cryptic invasion of genotypes also has been blamed for the explosive increase in the distribution and density of the common reed (Phragmites australis) in the USA over the past century (Saltonstall, 2002). Chloroplast sequences were obtained from 345 plants around the world, including herbarium samples that were collected before and after the range expansion of 1910. These revealed 27 haplotypes, one of which occurs at a high frequency throughout Europe and continental Asia. This same haplotype was very rare in the USA before 1910 but is now the most widespread haplotype in that country. It is likely that this invasive haplotype was first introduced to North America in the early 19th century, probably through several ports along the Atlantic coast. Since then it has dramatically expanded its range and now dominates many North American wetland areas at the expense of native P australis genotypes and numerous other species.

Phylogeography may also help us to retrace the intercontinental dispersal routes of invasive species by using techniques similar to those that allowed researchers to identify postglacial recolonization routes. This was the approach taken in a study of the amphipod Echinogammarus ischnus (Cristescu et al., 2004). This species is endemic to the Ponto-Caspian region, an area that encompasses the Black, Azov, Caspian and Aral Seas. Sometime during the past century, a single genotype of E. ischnus spread from the Black Sea to Western Europe and also to the Great Lakes of North America. By comparing DNA sequences from multiple sites, the authors of this study concluded that the most likely sequence of events involved E. ischnus first colonizing Western Europe and then invading the Great Lakes. Interestingly, the colonization route of E. ischnus appears very similar to that of Cercopagis pengoi, another Ponto-Caspian crustacean that recently invaded the Great Lakes (MacIsaac et al., 1999). The two species have markedly different life histories and dispersal capacities, and their common colonization route suggests that invasions from the Ponto-Caspian region into Western Europe and North America may be more common than was previously believed.

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