The Island Naturalist True Stories from Catalina's Wildlands
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.
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.
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.
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.
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.
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.
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.
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.
CATALINA'S MOST WANTED
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.
DID YOU KNOW...
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.