Understanding the concept of invasive species and what they involve might not be as easy as we would like it to be. On this page you will find a general introduction to the dynamics of nature, definitions and concepts of alien and invasive species, and examples of consequences of biological invasions.
Nature
What comes to your mind when you hear the word "nature"? During a quick survey we did among our staff, we got answers like "trees", "green", "rivers" and "peace". Indeed, nature is generally conceived as an idyllic place, where people can go to get some deserved rest.
From an ecological point of view, however, nature is far from being peaceful. In a natural setting, plants and animals are constantly competing among each other. Seedlings try to outgrow their neighbors in order to get the best and sunniest places in the forest canopy. Other plants cling to trees, killing them and making room for other, faster growing species.
Native species battle with invasives, such as Quinine, Cinchona pubescens (right foreground).
An ant colony invades the territory of another colony and displaces it; the displaced ones go out and in turn invade somebody else's home. After a time, this fierce competition develops rules, thereby preventing any of the players from becoming too strong and so providing an opportunity for everybody. At that point it becomes what we call the "balance of nature", and it happens everyday, everywhere in the world, and with every living species as part of their instinctive way of being.
Natives, aliens and invasives
In a balanced environment, plants and animals basically get used to each other. They compete, but everybody has a chance of survival because no one is allowed to become stronger than the others, and every species knows how to defend itself from the known enemies. However, what would happen if an unknown plant or animal turns up in a balanced environment? First, of course, the other species don't know how to react, since their defense mechanisms are set to keep away things they have known for thousands of years. Second, the intruder (or alien species, as we call them) will begin following its instincts and do some competition of its own. If the already present species (the native species) are not strong enough to resist this new attack, the alien species will certainly succeed. If this alien organism is aggressive, that is, capable of spreading fast while at the same time displacing the native species, that's when we call it an invasive species.
Island species have a way of being quite helpless when confronted with new threats. Since islands are usually isolated from external influences, plants and animals evolve with very few defense mechanisms. In this way, if a new organism arrives on an island, it is very likely that it will take over rather soon and sometimes with devastating consequences.
Isabela Island
Isabela, the largest island in Galapagos, has the highest concentration of endemic species in the archipelago, and approximately 50% of the population of giant tortoises lives in the northern part of the island. Although southern Isabela has a permanent human settlement, the northern side had remained relatively pristine until the introduction of feral goats in the early 1970's.
© CDFHerd of goatsBy 1998 the population was estimated to be between 75,000 and 125,000 goats. The large number of goats caused land erosion, a vast consumption of vegetation, and grazing pressure. The threat to endemic plants and the organisms that depend on them (including the tortoises) was very high and could have led to the extinction of species and subspecies, as well as changes in microclimate and water retention, especially on the crater rim of Alcedo volcano.
The Charles Darwin Research Station and the Galapagos National Park Service set up a major campaign to remove the goats on Alcedo, in an effort that involved the use of helicopters, trained dogs, satellite systems, high-tech tracking technology, lots of people and, of course, lots of money. That's a huge effort for just one species on just one spot of one island! Now, Galapagos has over 500 alien species spread over more than 120 islands of various sizes.