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Invasive Threats of Hawaii

By Emily Hauck - Environmental Outreach Specialist | Marine Corps Base Hawaii | April 2, 2019

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While Hawaii is typically known for its Aloha spirit, there are a number of species that make there residency here, but are unwelcome. Invasive Species are non-native, introduced plants or animals that out-compete or harm native species and disrupt their habitat. Typically, invasive species are a biological entity that exists in a particular environment because it was either purposefully or accidentally brought to the new location by a human agent, and they cause significant harm to the economy, environment, and/or human health and wellbeing.  Everyone should understand both the threats posed by invasive species and the tools required to eradicate them. Thousands of invasive species have been introduced throughout the US both accidentally and, in many cases, deliberately. Executive Order (E.O.) 13112 on Invasive Species requires that DoD prevent the introduction of invasive species; detect and respond rapidly to and control populations of such species; monitor invasive species populations; promote native species restoration, and promotes public education on invasive species.  MCBH currently works in partnership with other groups, such as the USDA and the Oahu Invasive Species Committee (OISC), to more effectively combat invasive species and to educate installation personnel and other stakeholders about invasive species’ impacts and actions that can be taken to contain them.

Invasive species are specifically a hazard for military bases as they have the potential to take over training lands, injure people, damage equipment, overwhelm listed and at-risk endemic species, and significantly erode natural resources and training budgets. One example of an impactful species found in Hawai‘i is the Little Fire Ant (LFA). LFAs are ground and tree dwellers. They are naturally aggressive and swarm over anyone or anything that disturbs their nest. They sting people and pets, short-out electrical equipment, and attack ground- and tree-nesting birds. Left uncontrolled, fire ants can displace native species and upset the ecological balance of nature. Another example of an impactful, invasive species is the Coconut Rhinoceros Beetle (CRB). In Hawaii they are considered a major pest of coconut and other palms, as well as the indigenous hala trees.  Left unchecked, they could decimate palm populations across Oahu.  Introduced slugs and African snails spread rat lung disease that affects the brain and spinal cord resulting in paralyses.  Hawai‘i has no natural predators to control most of these harmful introduced species.  

The constant influx of invasive species is one of the most serious problems facing Hawai‘i ecosystems.  Of these species, one stands out as the most disruptive ecological threat: cats.  Last year, cats were added to the Hawai‘i list of most impactful invasive species. Feral cats were noted as one of the most detrimental, introduced species along with fire ants, pigs, and opiuma. “Cats at large” are known to be a threat to terrestrial and marine animals and tend to prey on native species. They are responsible for killing endangered birds and small mammals, reptiles, and amphibians across the United States. Feral cat predation in the US, on birds alone, leads to billions of dollars in economic loss each year. This predation is a particularly problematic issue in Hawai‘i, where a large proportion of native bird species are federally listed as threatened and endangered. Because Hawaiian native birds evolved in a habitat free from mammalian predators, many have evolved to be ground-nesting birds; this behavior makes them especially susceptible to mammalian predation.

There are a lot of reasons why people allow their domesticated cats to roam around freely. Some of these Common Cat Myths are listed below:

“If I feed my cat well, then my cat won’t have a desire to hunt birds.”

Across the globe, extensive studies of the feeding habits of free-roaming domestic cats have been conducted over the last 50+ years. These studies show that the number and types of animals killed by cats vary greatly depending on the individual cats, age of the cat, the time of year, and availability of prey. One well-fed cat that roamed a wildlife experiment station was recorded to have killed more than 1,600 wild animals over 18 months. The data collected, from multiple studies, suggests that well-fed cats will still kill birds and other wildlife because the hunting instinct is independent of the urge to eat.

“I put a bell on my cat’s collar so the birds will know where it is and fly away.”

Additional scientific studies have shown that bells on collars are not effective in preventing cats from killing birds or other wildlife. Birds do not necessarily associate the sound of a bell with danger, and cats with bells can learn to silently stalk their prey. Bells offer no protection for helpless nestlings and fledglings.  Birds that nest or feed on the ground, such as the federally-protected Wedge-tailed Shearwater, are the most susceptible to cat predation, as are nestlings and fledglings of many other bird species.

“My cat is healthy and neutered! It’s not a threat to anyone!”

Cats are the only known reproductive host of the Toxoplasma gondii parasite. It reproduces in the feline digestive system. A single cat can excrete 145 billion eggs per year in its feces. Once released into the environment, these eggs can infect marine mammals and wildlife (such as the endangered Hawaiian Monk Seal), as well as humans, both on land and in the ocean, with toxoplasmosis. Toxoplasmosis is a disease that causes cysts in muscle and organ tissues and can cause inflammation of the heart, liver and brain. It is known to cause serious problems for pregnant women and their unborn children. Feeding cats anywhere in Hawai‘i is likely contributing to the problem, due to the nature of the islands’ watershed systems. The parasite can live for months in soil and can wash into streams and runoff and be carried into the ocean from almost anywhere. According to the National Oceanographic and Atmospheric Administration (NOAA), Toxoplasma has been identified as the cause of death for eleven endangered Hawaiian monk seals since 2001.  With only an estimated 1,500 Hawaiian monk seals still in existence, we simply cannot afford to lose even one of these critically endangered mammals to a disease that is preventable.

“My cat was born to be outdoors and it’s unhappy if I keep it inside.”

Outdoors, cats are a non-native and invasive species that threaten birds and other wildlife, disrupt ecosystems, and spread diseases. It is a common misconception that going outside is a requirement for feline happiness. While letting cats outdoors may seem like the natural thing to do, the hazards that cats face when they leave home are numerous. The best way to keep cats healthy, and to protect wildlife and human health, is to keep cats indoors. Regularly playing with your cat, walking it on-leash, or installing a “cat-tower” in your home will help acclimate them to a life indoors.

According to Hawai‘i Invasive Species Council, multiple studies have found that free-roaming cats reduce wildlife abundance and diversity, and cause dramatic reductions of native wildlife populations. Often, the mere presence of a predator, such as a cat or dog, can deter native bird species from nesting in their natural habitat. Cats do not need to physically attack a bird to cause death, simply causing a bird to abandon a nest of eggs will lead to the death of unhatched chicks; this is especially detrimental to our ground-nesting waterbirds, seabirds, and raptor populations.

 Misguided policies encourage the trap, neuter, and re-release of feral cats onto the landscape, often on public property or on property not owned by the individual providing care for the released cats. Trap-Neuter-Release (TNR) as a viable program has been debunked in a number of scientific, peer-reviewed studies and summarized in a meta-analysis titled “Critical Assessment of Claims Regarding Management of Feral Cats by Trap-Neuter-Return” by Longcore et al. (2009) in the journal Conservation Biology. This analysis has shown TNR to be ineffective at its primary goal of reducing the number of cats on the landscape. TNR also does not mitigate the primary impacts of feral cats on native wildlife or humans. TNR cats can continue to prey on native wildlife and continue to spread disease that is harmful to wildlife and humans. Wildlife veterinarians, public health officials, natural resources agencies, and other animal-focused organizations oppose TNR.

“Managed cat colonies are humane.”

If a feral cat survives kitten-hood, their average lifespan is less than two years.  While people may provide food, most colonies do not have access to adequate shelter or medical care; many will die from disease, exposure, or parasites. “Cat colonies” often form around an artificial food source, which will attract other pest species, e.g., rats, mongoose. Artificial feeding may seem humane; nevertheless, it exacerbates the overpopulation problem because the cat colony will grow. Additionally, cat colonies often serve as dumping grounds for unwanted or abandoned pets.

Nobody knows how many “cats-at-large” roam free on Oahu, but estimates from 2016 suggest well over 350,000. A female cat can have up to three litters per year, with four to six kittens per litter. Whether domesticated, stray (abandoned or lost), or feral (descendants of strays living in the wild), these non-native predators often lead short, miserable lives, and wreak havoc on populations of birds and other wildlife already under siege from many other threats. Unchecked, cat populations have skyrocketed. Unaltered, roaming domesticated cats also contribute to the problem. Please do your part as a responsible pet owner and keep your cats indoors, neuter your cat, and never abandon them.  

Invasive species, can increase the rate of native species extinction; others can cause loss of biological diversity, spread disease, push a species to extinction, and reduce water quality. Less directly, they can impact trade and commerce, undermine operations, impact recreational activities, and decrease ecosystem services vital to our nation’s economy. It does not take long for native landscapes to become overrun with invasive species. Preventing and controlling invasive species requires a unique approach that considers the environmental implications for both military installations and neighboring communities.


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