Buzz
Nonnative or invasive species are often perceived as nothing but destructive forces within the ecosystems they invade. While many of these species do indeed cause significant harm, there are also cases where invasive species have been introduced and made positive contributions, even helping to protect endangered species.
10. European Green Crabs and Their Role in New England's Salt Marshes
The European green crab is often regarded as one of the most despised invasive species due to its aggressive nature, devouring almost anything in its path. These crabs have spread across coasts worldwide, but they have had an unexpectedly beneficial effect on the New England region. In areas where overfishing had left salt marshes vulnerable, native marsh crabs, which once played a key role in managing the marsh ecosystem, were overpopulated and consuming too much cordgrass (Spartina alterniflora), leading to significant soil erosion due to their burrowing. The introduction of European green crabs has helped restore balance in these marshes.
With the green crab's introduction, the marshes began to show signs of recovery. The cordgrass began to regrow in areas dominated by green crabs, as these crabs displaced the native marsh crabs from their burrows. Although European green crabs have caused damage in various regions, in New England they have unexpectedly played a role in helping to revive an ecosystem that had been disrupted by native species.
9. Spartina Grass and the California Clapper Rail
The endangered California clapper rail, a bird species found only in the San Francisco Bay, has taken to nesting in Spartina grass, an invasive species partly responsible for the destruction of their original habitat. While urban development has also contributed to the loss of their natural home, the birds have not adapted to nesting in the urban sprawl. Instead, they have adapted to using the invasive grass as a nesting ground.
This adaptation has posed challenges for local researchers, who must now figure out how to manage the endangered clapper rail's recovery while addressing the invasive species that has become crucial to the bird's survival. It also raises the question of whether other endangered species may have similarly become dependent on an invasive species in ways not yet fully understood, complicating efforts to eradicate the invader.
8. Japanese White-Eye and Hawaiian Flowering Plants
The Japanese white-eye, a bird species introduced to Hawaii in the late 1920s to help control the insect population, quickly adapted and became the most common bird in the Hawaiian Islands. As other native bird species have gone extinct—often due to the overwhelming presence of the Japanese white-eye—many flowering plants have been left without their pollinators. These plants, which are found nowhere else in the world, now face the threat of extinction as well due to the disappearance of the birds that once helped pollinate them.
However, researchers have discovered that the Japanese white-eye is stepping in to fill the pollination gap left by the extinct native birds. It has been found to pollinate two species of flowering plants, the Mauna Loa clermontia and the small-flowered clermontia. Though previously thought to offer no real benefit, the Japanese white-eye's role in pollination suggests it may have a valuable, albeit complex, impact—solving a problem it may have also contributed to.
7. Tamarisk Shrubs and the Southwestern Willow Flycatcher's Nesting Sites
The struggle of the southwestern willow flycatcher is a result of numerous unintended consequences brought about by the repeated actions of the US government, creating a situation that almost seems absurd. Tamarisk trees, also known as salt-cedar shrubs, were deliberately introduced into the United States to combat soil erosion due to their significant ability to absorb water. Over time, however, it became evident that the water consumption of the tamarisk was draining riverbanks and destroying the willow trees that the southwestern willow flycatcher depended on for nesting.
With the loss of its natural nesting sites caused by the spread of tamarisk trees, the endangered southwestern willow flycatcher adapted by using the invasive tamarisk trees for nesting. Despite this adaptation, the United States Department of Agriculture (USDA) sought to eradicate the tamarisk trees by introducing yet another invasive species: leaf-eating beetles from Central Asia.
Aware of the southwestern willow flycatcher’s reliance on the tamarisk for nesting, the Department of Agriculture pledged not to release the beetles within 320 kilometers (200 miles) of the known bird nesting locations. The USDA also claimed that these beetles would not survive in the areas where the endangered birds resided. Naturally, the beetles survived in these areas and began destroying the birds’ nests in the tamarisk trees. As a result, the use of beetles as a biological control measure was ultimately discontinued due to this unforeseen consequence.
6. Aldabra Tortoises and the Ebony Trees of Mauritius
Introducing an invasive species is often more successful if it closely resembles the species it is replacing. This was true for the Aldabra tortoises, which were brought to the island of Ile aux Aigrettes in 2000 on a small scale. These large tortoises—often weighing 300 kilograms (660 lb)—were introduced with the goal of aiding in the restoration of the island's endangered ebony forest, where many of the ebony trees had been overharvested for firewood, limiting new growth.
The failure of the ebony trees to recover stemmed from the arrival of humans on the island, which led to the extinction of not just the native giant tortoises but also the giant skinks and, most likely, flightless dodo birds. With these animals gone, there were no longer any creatures to consume the fruit of the ebony trees, preventing the seeds from spreading beyond the base of the mature trees.
Though an invasive species, the Aldabra tortoises are closely related to the giant tortoises that once inhabited Ile aux Aigrettes, and they have successfully taken over the role once held by their extinct relatives. These tortoises consume the fruit of the ebony trees and disperse the seeds throughout the island. More than ten years after their introduction, researchers are observing that the endangered ebony forests are starting to recover. In addition to aiding seed dispersal, the tortoises also contribute to seed germination, as the seeds pass through their digestive systems.
5. Yellow-Crowned Night Herons in Bermuda
When Bermuda was first colonized hundreds of years ago, Bermuda night herons were among its native species. However, they quickly went extinct due to the introduction of new predators that preyed on the birds, along with humans finding the herons so tame and approachable that they were easily killed with “stones and staves.”
With the Bermuda night herons gone, the land crab population (Gecarcinus lateralis) began to grow uncontrollably. The herons, which had relied on the crabs as a primary food source, were no longer there to keep their numbers in check. Concerned by the imbalance, the Bermuda government sought to introduce a close relative of the extinct heron to help restore balance in the ecosystem.
In the late 1970s, the yellow-crowned night heron, a relative of the extinct Bermuda night heron, was brought to the island from Florida as a biological control for the overpopulated land crabs. The introduction was successful, and the invasive yellow-crowned night herons now have a stable population on Bermuda, with land crabs making up 95 percent of their diet.
4. Zebra Mussels and the Great Lakes
The invasive zebra mussels in the Great Lakes have attracted considerable criticism, and much of it is well-founded. These mussels can spread quickly and in large numbers, often covering anything they can attach to. They have disrupted the ecosystem by competing with native mussel species for food, some of which are already classified as endangered.
Beyond their ecological impact, zebra mussels are also believed to be the cause of Type E botulism poisoning, which led to the deaths of thousands of birds on Lake Michigan in 2008. Researchers speculate that zebra mussels filter out botulism from the water, and when predators consume these mussels, the toxin travels up the food chain, eventually reaching and poisoning birds that eat the infected fish.
While the zebra mussels’ presence in the Great Lakes is mostly harmful, there are some positive effects. The water where zebra mussels reside has been linked to healthier salmon populations and clearer water. In particular, Lake Ontario has seen a significant improvement in water clarity due to the filtering abilities of zebra mussels.
This clearer water has benefited both the salmon and the alewife that they prey upon, as they rely on sight to hunt. With the once-turbid waters of Lake Ontario now clearer, alewives can easily find and feed on an abundance of shrimp. The alewife population’s health in turn positively influences the health of the salmon. However, it’s important to note that these benefits do not outweigh the severe environmental and economic damage caused by zebra mussels.
3. Honeybees In North America
Today, with the honeybee population in danger due to colony collapse disorder, industrial farming practices, mites, and deforestation, there is growing concern about how its potential extinction will impact an ecosystem that has become heavily dependent on the bees for pollination of a vast array of plants. Despite being an invasive species, the honeybee’s beneficial role in the ecosystem highlights the positive contributions some nonnative species can make once they are introduced.
Now that honeybees face various threats to their survival in North America, including colony collapse syndrome, industrial agriculture, mites, and deforestation, there are increasing concerns over how the disappearance of the honeybee could affect the ecosystem. This is particularly troubling given the critical role honeybees play in pollinating a diverse range of plant species.
2. California Butterflies, Yellow Star Thistle & Eucalyptus Trees
California is home to many invasive plant species, many of which were introduced with good intentions, often for decorative purposes. While some of these plants have proven toxic to certain butterfly species, there are several examples where butterflies are thriving thanks to the presence of these nonnative plants.
As native trees along California’s Central Coast have been cut down, monarch butterflies are increasingly turning to the nonnative eucalyptus trees as their primary overwintering site. Although eucalyptus trees may not always be strong enough to endure the harsher winter storms along the coast, they have become a vital habitat for the butterflies, allowing them to survive in these trees.
The yellow star thistle, an invasive plant that is widely disliked throughout California, has become an important source of nectar for many of the state’s native butterflies, especially in the Central Valley. The thistle offers an abundant and accessible food source for these butterflies.
1. Multiple Invasive Tree Species And Abandoned Agricultural Land In Puerto Rico
The impacts of over-farming are well understood, with soil erosion and compaction frequently resulting from unsustainable agricultural practices. In Puerto Rico, many of these overused lands were abandoned, and the native tree species, once native to the region, struggled to return. However, a number of invasive tree species, such as the white siris, African tulip trees, rose apples, and others, have shown an ability to thrive in these former agricultural lands, even with poor soil quality.
The survival of these nonnative trees in the degraded agricultural fields is not the most significant aspect—what truly matters is that these trees play a critical role in allowing the eventual return of native tree species. It appears that the early success of the invasive species creates a suitable environment for native species to grow beneath them. For the African tulip, this process takes about 40 years. After that period, the native species can take over, regrow in the shade of the tulips, and eventually recolonize the area.
