Oceanic islands are extremely susceptible to destructive invasions by non-native plants and animals. Geographic isolation allows those few organisms that find their way to an island or are swept ashore by wind or waves to colonize and evolve without the stiff competition for resources that continental organisms face. Without the constant pressure of predation and resource competition, many island organisms lose or never develop biological or behavioral defense mechanisms. When humans colonize islands, they often deliberately or accidentally introduce new species of plants and animals into the carefully balanced island ecosystems. The new organisms are frequently hardier, more aggressive and produce more offspring more often. They can quickly overwhelm the native species, which are not designed to deal with that level of environmental pressure.
In the Hawaiian Islands there have been two major periods of new species introductions. The first occurred when the Polynesians discovered and colonized the islands. They brought pigs, rats, cats, and dogs, as well as a variety of plants, such as taro, which was a staple food crop. The second wave of introductions is on-going. It started with the European discovery of the Hawaiian Islands. The Europeans also brought pigs, rats, cats, and dogs, some of which supplanted the Hawaiian introductions, as well as deer, cows, goats, horses, mongoose, and a large number of food and ornamental plants.
As is often the case, the main problem with introductions isn't the fact of them, it's the rate. The rate of organism introduction has increased tremendously since human discovery of the islands. Before humans, on average one new species of plant or animal came to the Hawaiian Islands every 35,000 years. After the Polynesians arrived that rate increased to an average of 3 or 4 new species every 100 years. After the Europeans arrived the rate increased further to an average of nearly 40 new species introduced every year1. Plants and animals that evolved in the early, stable environment have had a difficult time responding to the influx of new organisms and the resulting fluctuations in the environment.
1Hawaii and the Pacific Islands--Overview of the Pacific from Status and Trends of the Nations Biological Resources. USGS. 1999.
