Uninvited Guests: How Invasive Animal Species Reshape Terrestrial Ecosystems

We often forget that, during the Age of Exploration, as humanity took sail to colonize every undiscovered corner of the planet, we were not alone. Animals came along with us on our journeys, whether by choice, in the case of domestic species, or as stowaways, hiding in cargo holds. Those species inevitably settled, just as we did, and just as we made our own mark on the unprepared lands with our unceremonious arrival…so did they.

The problem with all this being, that the mark left by invasive species is often as indelible as it is destructive. Those species which have mistakenly or purposefully introduced into a foreign ecosystems by humans often become ecological bullies, outcompeting, preying upon, or altering the lives and habitats of endemic species in unforeseen ways. And while modern globalization is an excellent thing for humanity, the benefits that our species reaps from it often come at the expense of the natural world.

In this article, we will discuss how invasive species have negatively affected terrestrial systems all over the planet. We will go over a few different examples, most of which we’re willing to bet that readers are at least partially familiar with. By the time you’re done reading, perhaps you too will see how invasive animals may be one of the most pressing and unrecognized ecological issues affecting our world today.

What is an Invasive Species?

Before we go over the textbook definition of invasive species, it’s important that readers understand not all introduced species are considered to be invasive. As a matter of fact, many species have the capacity to live in harmony with the endemic natives of their new ecosystems. They might be different, an odd sight to see, but by and large those somewhat innocuous invaders cause little to no disruption.

However, a truly invasive species is one that, once transplanted, vastly upsets the ecological balance and/or causes harm to the environment. Invasive species can, in certain circumstances, be so detrimental to a region that they disrupt human health or the local economy. Typically, the most disruptive invasive species are overly aggressive, are poisonous to native plants or animals, have diverse diets, or reproduce rapidly. In these instances, the invaders will outcompete endemic animals and will oftentimes become so prevalent within an ecosystem that they become incredibly difficult, if not impossible, to eradicate.

As we begin our examination of invasive species as a global issue, it makes sense that we give readers a few examples of invasive critters they might already know about.

Domestic Cats – Everywhere

We begin with a species that needs no introduction; the beloved house cat. These feline companions of many a lonesome bachelor or bachelorette might be beloved pets, but they are also one of the most destructive invasive species in human history. When they are allowed to roam freely, endemic species invariably suffer from their voracious appetites and aggressive tendencies. Island ecosystems are particularly sensitive to feral cats and it is believed that cats have contributed to the extinction of at least 63 animal species, including birds, reptiles, and mammals.

Most of the time, this occurs because the animals in question have no natural predators on their island homes and cats are nothing if not excellent predators. On islands such as New Zealand, the Galápagos, and Hawaii, cats have had a devastating impact on ground-nesting birds. These days, most island governments take great effort to control feral cat populations through neutering and removal programs, though it’s safe to say the damage cats have wrought has already been done.

Brown Tree Snake – Guam

A more subtle example of invasive introduction of a species is the brown tree snake, a reptile which is normally a native of Australia and Papua New Guinea. Brown tree snakes were accidentally introduced to Guam in the mid-20th century, likely via military cargo ships, and before long, their population exploded. The snakes quickly rose to the top of the food chain, resulting in the extinction of 10 of the 12 native bird species on the island. The snakes also fed copiously on small mammals as well, though did not drive nearly as many to extinction.

As one might expect, the loss of birds didn’t just result in a quieter Guam, it also caused a trophic cascade of destruction. Several species of native trees stopped reproducing efficiently because their seeds were no longer dispersed by the jungle birds. Other plants and animals suffered as well. Though some efforts to control the snake population have managed to curb numbers, the damage the brown tree snake has already dealt, has been enough to completely alter the island ecosystem for all time.

European Starling – North America

In the late 19th century, about 100 European starlings were brought to New York City and released in Central Park. The stunt was performed by a misguided group that hoped to introduce every bird mentioned in Shakespeare’s plays to America. Unfortunately, these short-sighted individuals had no idea that these hundred birds would eventually number in the hundreds of millions within a century or so. Today, starlings are outcompeting native bluebirds and woodpeckers all over the continent. Their massive flocks have also been known to damage crops and create issues near airports, so their effect isn’t just ecological, but economical as well.

Cane Toad – Australia

The tale of the cane toad’s introduction to Australia is well-known to those of us who examine invasive species. First brought there in 1935, the toad was meant to control beetles in sugarcane fields, which it did. Unfortunately, the toad’s toxic skin and eggs proved lethal to Australian predators that were not used to such defenses. Native species like goannas, quolls, and even crocodiles that preyed on the cane toads died, leading to population crashes and disruptions all across the food web. To this day, the interloping toads remain a major ecological problem for Australians, who have still found no definitive way to get rid of them for good.

Burmese Pythons and Boa Constrictors – Southeastern United States

Snakes crop up not once, but twice in the history of invasive animals. Once brought to United States as pets, many species of constrictor snakes have been callously released into the wild by ill-equipped pet owners and frightened wildlife smugglers. Once they made their way into the Everglades, the bayou, and many other southeastern, swamp ecosystems, the snakes began to breed and eat without surcease. The problem has resulted in slow destruction of many native species of birds and mammals and it is only getting worse with each passing year. Today, these former Asian and South American species have become such a problem that the State of Florida will even pay individuals who bring them dead snakes!

Our Role in All This

As one might expect from the tone of articles like this, the fault for all these invasive animals lies with us. It should come as no surprise that the movement of invasive species is so closely tied to human activity. Global trade, travel, and even some conservation efforts have inadvertently introduced species to new environments. We can look at the pet trade, cargo trade, agricultural trade, and even the practice of intentional stocking as primary ways that invasive species have ended up on foreign shores.

How do We Manage This Problem?

Dealing with invasive species is incredibly complex and most efforts to do so have been stop-gaps at best. Proper solutions elude many nations, especially when invasive species have gained a reproductive foothold in a region. Nature is nothing if not resilient and once a species becomes established, complete eradication is often impossible. Thus management of invasive species usually relies on early detection, rapid response, control, containment, and though brutal in many ways, total eradication.

The trick is always to catch species before they can spread, which is why so many species still cause havoc in certain wild places; we simply didn’t get to them in time. That said, public education about these invasive plants and animals can help, as it can allow individuals to report sightings or even get rid of the offenders themselves. If you live in the USA, for example, you might have seen warnings about the invasive Spotted Lantern Fly and calls to action about squashing the invaders whenever you encounter them.

True Investigator Says…

As you can see, the spread of invasive animal species is something that will only become more prevalent in the coming years. In the past, the fate of the Dodo and other such native animals have acted as a stark reminder of how dangerous interloping animals can be. As the planet becomes more and more interconnected in the coming centuries, we must be vigilant to ensure that no unforeseen stowaways end up up-ending the already much-damaged natural balance.

The present has already shown us that mounting challenges like climate change and habitat loss are only getting worse and the threat posed by invasive species only compounds those issues. Thus, it falls to us to be more responsible than our forbearers. We can do that by being educated, taking steps to remove offending species before they become problems, and cooperating with one another to ensure a brighter, healthier future for our world.