It usually takes years sometimes even centuries before an exotic species becomes a problem The speed with which this happens depends on how fast the population can grow Species that produce many young in a short period of time can multiply much more quickly than those that reproduce more slowly Thus fast reproducers such as rabbits are much more likely to become invasive than slow reproducers such as elephants It’s hard to pinpoint exactly what makes an organism become invasive Scientists have proposed several different explanations One of these is known as the enemy-release hypothesis This hypothesis is based on the idea that in an ecosystem nature has evolved to keep itself in balance Predators diseases and competitors keep any one species from multiplying too quickly and taking over However when a species moves to a new ecosystem it may not face such challenges With nothing to impede its growth it can quickly spread and outcompete native species for food and habitat It disrupts the balance of the food web that is already in place harming the native plants and animals that live there 10 SPECIES SPOTLIGHT THE NILE PERCH The Nile perch was introduced to Lake Victoria in Africa in 1962 for commercial fishing This large fish is native to the Ethiopian region It quickly began to dominate the ecosystem in Lake Victoria causing hundreds of native fish species to go extinct 8 It is a voracious predator and also carries parasites that can spread disease to other fish The perch changed the ecosystem in other ways as well A large fishing industry quickly sprang up on the banks of Lake Victoria displacing local people who traditionally fished in the lake This affected both the economy and the environment of the area
However some invasive species do face predation and disease in their new habitats And there must be some reason why only a small percentage of introduced species become invasive The answer lies in the concept of fitness Fitness is the ability of an organism to maintain or increase its numbers in the next generation This ability depends on how well suited an organism is to its environment whether it is good at finding resources defending itself against predators and diseases surviving the climate conditions and so on The fittest organism in an environment is best able to pass on its genes Whatever the reason invasive species are simply more fit than their native competitors NATURAL INVADERS In 1883 a massive volcanic eruption obliterated all life on Krakatoa an island near Java and Sumatra in the Indian Ocean But it didn’t take long for plants and animals to recolonize the island Spiders were the first animals to arrive likely carried on the wind Soon grasses and other plants sprouted from seeds carried by the wind or in the droppings of passing birds Today birds nest and sing in the lush rain forest on Krakatoa Introduced species are most likely to become invasive in habitats that are already disturbed For instance the Nile perch's introduction is blamed for causing approximately 200 native fish species to go extinct in Lake Victoria 9 However the populations of these species had already been shrinking from overfishing pollution erosion and excess nutrients They were not well adapted to these changing conditions The more-adaptable perch may have simply finished them off 11
THE RIPPLE EFFECT In any ecosystem there is a delicate balance of nature Just like pulling a single thread can unravel a whole sweater introducing a new organism can have drastic and complex effects on all the other organisms in an ecosystem Such a ripple effect also called a trophic cascade took place on Macquarie Island near Australia after several different species were introduced First rats and mice arrived accidentally on ships When they became a problem sailors brought cats to the island to control the rodents In 1878 seal hunters introduced rabbits to the island to provide a food source for their long journeys Although some of the rabbits were eaten by cats they still devastated the native vegetation So in 1968 scientists introduced the rabbit myxoma virus to the island Feral cats can quickly take over islands as they have done on Aoshima Island in Japan 12
to control the rabbits Slowly the vegetation began to grow back But now the cats had nothing to eat They started eating birds instead In 1985 scientists started a program to remove the cats By 2000 the cats were gone But with no cats to keep it in check the rabbit population skyrocketed Again they began to destroy the vegetation In some cases the changes brought about by an invasive species can even change the environment itself For instance a South American tree known as the velvet tree has invaded mountainous areas of Hawaii It grows so thickly that it has almost completely replaced the native understory in some areas Its shallow root system does not hold the soil in place as securely as the species it replaces This has led to numerous landslides on the steep mountain slopes Cheatgrass quickly grows large root systems that often use up groundwater before native plant seedlings grow their own root systems An annual grass called cheatgrass encourages wildfires in the areas it invades Before cheatgrass invaded Utah’s Great Basin fires came along every 60 to 110 years Today they happen every 3 to 5 years 10 While the native vegetation tends to grow in small clumps highly flammable cheatgrass fills in the spaces allowing fire to spread rapidly In addition cheatgrass seeds spring up quickly after a fire while native plants may be killed off Animals such as the sage grouse and pygmy rabbit rely on these native plants for food and habitat so their populations suffer when cheatgrass takes over 13
