The Island Naturalist True Stories from Catalina's Wildlands

Issue #44: iSland invasives

Alien organisms

By Alexa Johnson

The foundation of life on Earth is made up of innumerable plants, animals, and microbes. Maintaining this biological diversity- or biodiversity- is crucial to maintaining functional ecosystems and their associated services (see issue #25). The Center for Biological Diversity estimates that every day, dozens of species are going extinct. This is primarily due to human activities.

One of the greatest threats to biodiversity worldwide is non-native invasive species. Invasive species are plants or animals that have been introduced to an environment by people and whose introduction causes economic or environmental harm or harm to human health. In the United States, invasive species cost an estimated $138 billion in damages annually. They reduce crop yield, increase fire risk, clog waterways, and reduce recreational value. In addition, invasive species displace native species, altering ecosystem structure and function. Their impact is so severe that these alien species have been declared a threat to national security by the Department of Homeland Security.

Normally found in the tropical waters of the Pacific, lionfish were first discovered in the U.S. Atlantic in the late 1980's. Voracious eaters, it is believed the species established themselves after being dumped by aquarium owners. With their poisonous spines, lionfish are as deadly as they are beautiful. With no natural predators, the species has spread across the eastern seaboard and into the Caribbean. Photo: NOAA

The Channel Islands are referred as the “Galapagos of North America” because they harbor a tremendous diversity of plants and animals, including a disproportionately high value of species that are unique to the islands, called endemic species.

Here on Catalina, invasive plants and animals are the number one immediate threat to maintaining the Island’s biodiversity. Hundreds of non-native plants and animals have been introduced to the Island. Some intentionally, like with landscaping plants, game species, livestock, or feed to support livestock, and others unintentionally when they get caught on someone’s shoes or in a pet’s fur, through construction materials, or as a stowaway on a boat. Many of these don’t pose a threat, but some wreak havoc on the Island’s native flora and fauna.

This image illustrates the damage that bark beetles, an invasive insect from Asia, can inflict on forests. Photo: John Frank

In their natural range, these plants and animals support healthy, functional ecosystems. However here on Catalina, there aren’t the necessary controls, like competition or predation, to keep their numbers in check. Having evolved without this added pressure, island species aren’t equipped to defend themselves or compete for scarce resources.

Mule deer were first introduced to Catalina Island over 80 years ago. Because Catalina's endemic plants evolved without the presence of deer or other large plant-eating animals, most developed without spines, sharp leaves or other protective mechanisms that would normally stave off browsing. With no natural predators on the island to keep their population in check, Mule deer pose a serious threat to Catalina's native plants. Photo: C de la Rosa / Catalina Island Conservancy

Pervading plants

By Alexa Johnson, Amy Catalano, and Jeff Gilmour

Catalina Island is home to approximately 700 different species of plants. Of these, nearly 2/3 are native and the remaining species were introduced via landscaping, ranching, or contaminated (muddy/dirty/unclean) construction equipment or vehicles.

Invasive plants tend to be fast-growing and prolific seed producers that disperse seeds over great distances. They also usually are unpalatable to animals, difficult to manually remove and halt natural ecosystem succession once they disturb an area. Invasive plants reduce the abundance and diversity of native plants and their availability to associated wildlife, in turn, reducing the abundance of native wildlife.

Fennel (Foeniculum vulgate) is one of two notoriously invasive plant species on Catalina Island - the other being Genista linfolia. Photo: Catalina Island Conservancy

One of the ways invasive plants displace native species is through negative allelopathy. Allelopathy is when a plant produces a chemical that influences the success of another organism. Many introduced species on the Island, including fennel and eucalyptus, leach chemicals into the soil that impede the growth of other plants.

The Catalina Habitat Improvement and Restoration Program, CHIRP for short, manages over 60 invasive plant species on Catalina Island. Because more than 220 non-native plant species exist on Catalina, CHIRP must carefully prioritize invasive plant species for treatment. To do this, they rank each species for ecological impact, invasive potential, and distribution on the island. The highest ranked species cause severe ecological impacts, have high invasive potential, have limited population distribution and are located in highly sensitive areas. CHIRP focuses on top ranked species to “cash in” on the biggest ecological bang for their buck.

CHIRP spends approximately 95 percent of its effort each year treating nine species that are considered to be highly invasive on Catalina Island. These are:

Flax-leaf broom (Genista linifolia)

Fennel (Foeniculum vulgare)

Harding Grass (Phalaria aquatic)

Mock orange (Pittosporum undulatum)

Milk Thistle (Silibum marianum)

Tamarisk (Tamarix ramosissima)

Common fig (Ficus carica)

Periwinkle (Vinca major)

Pampas Grass (Cortaderia selloana)

Each project area and plant species poses unique challenges, such as being located in steep terrain or plants that ooze irritating sap. CHIRP uses a variety of management strategies and treatments to effectively control invasive plant species and ensure the island’s unique biodiversity stays intact.

Boaters may recognize this specimen as Sargassum horneri - an invasive seaweed common in the waters off Catalina Island and Southern California. Native to Asia, S. horneri grows quickly and often chokes out endemic kelp and marine algae. Photo: Niko Kaplanis

Horehound (Marrubium vulgare), shown above, is can often be found along Catalina's trails and roadsides, where it pushes out native wildflowers and other endemic plants. Photo: WikiCommons

The Island Bush Poppy (Dendromecon harfordii), a California Islands endemic, is one of the many native species whose distribution and abundance has been restricted by the presence of non-indigenous invaders. Photo: Catalina Island Conservancy
Clockwise from top right: zebra mussels, Burmese python, citrus long-horned beetle, feral pig, nutria, European starling.
"Globally, invasive non-native species have contributed to 40% of the animal extinctions that have occurred in the last 400 years" - United Nations Environmental Programme, 2010 Convention on Biological Diversity 

ForEign fauna

By Alexa Johnson

For as long as humans have inhabited Catalina, exotic animals have been translocated to this island sanctuary. Some were brought for utility, like ranching animals including sheep and cattle, some were introduced for recreation, like mule deer, pigs and goats, others arrived as people’s pets, and some were accidently transported aboard vessels or in gear.

This raccoon nearly made it to Catalina by stowing away on one of the Catalina Express boats. Fortunately it was spotted before it made it to the island and was captured by animal control officers. Photo courtesy of Ciara Virdan

Generally speaking, introduced animals pose an enormous threat to Catalina’s native species and delicate ecosystems. Before modern humans arrived on Catalina, the largest grazer that any plant had to defend itself against was the endemic Catalina California ground squirrel (Spermphilus beecheyi nesioticus), an animal with a head the size of a walnut.

When tens of thousands of ranching and hunting era herbivores were introduced in the 19th and 20th centuries, the Island’s flora was unequipped to withstand such a deluge and became severely overgrazed. Many native plants were significantly reduced on the island and others were driven to near extinction, surviving only within protective cages, on isolated rocks, or on inaccessible cliffs. Some were eliminated completely, like the Trask’s monkeyflower (Mimulus traskiae), which hasn’t been observed on Catalina since 1901 and is presumed extinct.

Aside from their impact on the Island’s native flora, introduced animals can reduce the success of native wildlife through predation and competition. In issue #18 we talked about the devastating effects feral cats (Felis catus) are having on bird, rodent, and reptile populations worldwide. Here on Catalina, feral cats are also competition for the endemic Catalina Island fox (Urocyon littoralis cataline), reducing food resources and inflicting injuries to the smaller, more timid island fox.

Feral cats threaten biodiversity worldwide, killing billions of song birds, small mammals, and reptiles each year. Photo: Karl E. Huggins

Managing the influence of introduced animals is not easy. Before taking action, impacts are researched and thoroughly assessed. If control is warranted, outside experts and partner organizations are consulted to ensure control actions are undertaken using the latest information and techniques.

Top animal invaders on Catalina Island include black rats (Rattus rattus), Norwegian rats (Rattus norvegicus), mule deer (Odocoileus hemionus), European starlings (Sturnus vulgaris), bullfrogs (Rana catesbeiana), raccoons (Procyon lotor) and Argentine ants (Linepithema humile).

True or false:

Invasive plants and animals increase biodiversity?

This one is a bit complicated. Invasive plants and animals may increase total biodiversity in the short term. For example, if you introduce a new plant to your garden, then obviously you have increased the number of species that exist in that area. However, invasive organisms push out native species, reducing the region’s “uniqueness.” This makes the most sense when you think of biodiversity on a global scale. Organisms that are considered to be invasive on Catalina are already represented in their normal ranges. Once introduced to a new environment, they have a severe impact on "unique" biodiversity. Unique biodiversity is the subset of native and especially endemic organisms that are found on Catalina. When introduced species are allowed to invade new areas, what is lost cannot be found anywhere else in the world and global biodiversity decreases.

Flax-leaf broom (Genista linifolia), the yellow flowering plant in the image above, was introduced for landscaping, but now blankets Avalon and Descanso Canyons. Photo: Catalina Island Conservancy

St. Catherine's Lace (Eriogonum giganteum giganteum) can only be found growing in the wild on Catalina Island and is the largest member of the buckwheat family. Photo: Denise Knapp


The Catalina Island Conservancy has successfully removed a poisonous weed from the Island?

Native to southern Europe, Yellow starthistle (Centaurea solstitialis) is a widespread weed throughout California that thrives in disturbed areas (mostly along roads and in developed regions). While an uncommon choice for human consumption, yellow starthistle is extremely toxic to horses. It leads to a disease known as “chewing disease.” Horses that ingest large amounts of this plant develop lesions on the brain that cause unusual behavior. The animal is unable to swallow and may continue chewing the same mouthful of food for several hours, leading to depression, starvation, and thirst.

The Conservancy’s Catalina Habitat Improvement and Restoration Program (CHIRP) team successfully eradicated yellow starthistle from the Catalina, once again making the Island safe for our equine buddies and protecting Catalina’s rich biodiversity from the competition of this non-native invasive.


Created By
Catalina Island Conservancy

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