When I say the words "invasive species" chances are you can quickly come up with such an animal even if you lack a strong scientific background. The cane toads of Australia, the Burmese pythons of the Everglades, snakehead fish, lionfish, European starlings; there are thousands of invasive species around the world that are spreading beyond their native regions...almost all of them with the assistance of humans. Regardless of where you are from, there is an invasive species invading your own local ecosystem, even if you aren't aware of it.
Invasive species are a nightmare for biologists and the local community in which they establish. Once a breeding population has successfully established, there is almost no chance of eliminating it from the ecosystem. These animals cause untold direct damage to the local environment, preying upon or competing with native species that have not evolved alongside their invaders, leaving them vulnerable. With no native predators to keep them in check, these populations flourish and begin snuffing out native species. When this happens, the best that biologists can do is attempt to give local species some sort of edge to survive (such as the newest plan for the cane toad invasion).
However, many people do not fully appreciate the damage that these species can bring to the environment. There is this widely-believed misconception that invasive species will eventually find their niche in the ecosystem, or that the inhabitants of that ecosystem will evolve to coexist with that species. We might lose a couple species, but the ecosystem will find a balance and eventually recover. While this is one potential outcome following an invasion (though it doesn't happen very frequently), the devastation brought by these invaders can not only wipe out native species of wildlife, they can actually destroy the ecosystem itself. The invasive brown tree snake of Guam is a perfect example of just how much one species can catastrophically alter the ecosystem it invades.
In the mid-1940's, bird species across the Pacific island of Guam began experiencing a mysterious decline. For years, it wasn't understood exactly what was causing the population to crash until 1987, when a US ecologist named Julie Savidge found the culprit: the brown tree snake. Following its introduction, the brown tree snake rapidly spread across the island, feeding on a bird population that was completely unprepared to deal with such a voracious predator. The pressure the snakes put on the bird populations was too much; since the snakes' arrival, ten of Guam's twelve native bird species have been completely wiped out, and the remaining two are considered to be functionally extinct.
As if the snake's effect on the native bird population wasn't catastrophic enough, it appears to only be the tip of the iceberg. We are only just starting to recognize the drastic indirect effect these brown tree snakes are having on the island through their predatory behavior. The loss of the native bird species is having a profound effect on Guam's forests: the establishment of new trees and the diversity of those trees is rapidly declining. The trees rely heavily on birds to spread their seeds across the island; the birds feed on the trees' fruits and defecate the undigested seeds in a different location where competition for light, water and nutrients is less intense (if they simply fell below the parent tree, the competition would likely be too much to survive). In the absence of the birds, it would found that 95% of seeds dropped by two common trees on Guam land directly beneath their parent tree. When compared to the nearby islands of Saipan, Tinian and Rota (islands with no brown tree snake presence), less than 40% of seeds land near the parent trees thanks to the native bird populations. Sees that escape their parent trees are five times more likely to survive, and seeds that pass through the gut of an animal actually increase the chances that the seed will be able to germinate...up to four times more likely. Around 70% of Guam's native trees rely on birds to spread their seeds; with the elimination of birds by the brown tree snake, the establishment of new seedlings has decreased by 61-92% depending on the species [source].
The forests are also shrinking and growing more clumped. On other islands, when a mature tree falls, new saplings spring up to take it's place in the sun. However, on Guam, because 90% of seeds are dropped directly beneath their parents, they are unable to fill these newly vacant areas. "Pioneer" species or plants generally colonize these gaps, but because they too rely on native birds, these gaps have been growing in size and quantity. Biologists worry that the forests of Guam may not survive the invasion of the brown tree snake.
The brown tree snake invasion might seem like an extreme example, but it highlights the threat posed by invasive species. Without a natural means of keeping their numbers in check, they can easily grow far beyond the normal constraints of their ecosystem and being applying catastrophic pressure to the species around them. Because everything is connected in the web of life, the consequences of their introduction will cascade through the ecosystem in a chain effect; it is simply impossible to remove a link in the web without dire consequences. Invasive species require far more awareness than we often afford them; though they may only be a single species, the can drastically alter entire systems and drive countless species to extinction.
Globally, invasive predators are responsible for the extinction of 142 species of birds, mammals and reptiles; another 596 are considered vulnerable, endangered or critically endangered as a result of species introduction.
