Non-native species, also referred to as alien, non-indigenous, foreign, or exotic, are plants, animals, or other organisms (such as microbes) that have been deliberately or unintentionally introduced to an area beyond their native range or from another site of introduction, usually by human actions (Kairo et al. 2003). Invasive species are those species (native or non-native) whose establishment causes economic damage, environmental harm to ecosystems, habitats, or species, or harm to human health (Presidential Executive Order 13112 Feb. 1999). Not all invasive species are non-native; native species can become invasive in response to an environmental change, such as habitat destruction or the removal of predator, and likewise, not all non-native species are invasive: these include livestock, agricultural, and ornamental species that confer benefit to human communities. This website focuses primarily on non-native invasive species that have become problematic to habitats and species within the USVI and are having negative environmental, economic, and cultural impacts.
Almost all goods and raw materials used in the USVI have been imported via ship or air, providing ample opportunity for the movement of non-native organisms. While some problematic species have arrived in the U.S.V.I through colonization events such as natural range expansions (e.g., cattle egrets (Bulbicus ibis), although this species has expanded its range in response to widespread anthropogenic habitat change and animal husbandry practices (Arendt 1988)), most non-natives are the result of deliberate or accidental introduction by human mechanisms.
Caribbean islands have a long history of species introductions. Early species of invaders include rats (Rattus spp.) transported on European ships, and sailors also introduced goats and other livestock to oceanic islands to ensure the availability of fresh meat on subsequent journeys. More recent unintentional introductions include Pacific Lionfish (Pterois spp.) presumably transported in ballast water of cargo ships, the Common Ground Lizard (Ameiva exsul) suspected to have been introduced to St. Croix in a tire shipment from Puerto Rico (Treglia et al. 2013), and Cuban Treefrogs (Osteopilus septentrionalis) and Puerto Rican Coquis (Eleutherodactylus coqui) as hitch-hikers in landscaping or construction material (Platenberg 2007).
The Small Indian Mongoose (Herpestes auropunctatus) was deliberately released in an unsuccessful effort to control rats in agricultural fields (Nellis & Everard 1983). Cane Toads (Rhinella marina) were also released as a biocontrol for white grub (beetle larvae) agricultural pests. Both guinea grass (Megathyrsus maximus) and tan-tan (Leucaena leucocephala) were brought in as fodder for livestock. The presence of Red-eared Sliders (Trachemys scripta elegans) and Boa constrictors is the result of the pet trade: they were either intentionally released when owners were no longer willing or able to care for them, or escaped. The ornamental landscape trade has inadvertently introduced numerous invasive non-native species including: coral vine (Antigonon leptopus), rubber vine (Cryptostegia grandiflora), and sweet lime (Triphasia trifolia).
While there is a good understanding of how organisms are arriving in the USVI, there is generally a poor understanding of the transport-enabling factors. These factors are species-specific, and are influenced by the behavior and biological requirements of these species. And while the scientific community is receptive to addressing the issue, the public may be resistant to the knowledge that common practices may have become a threat.
Invasives are distinguished from other non-native species by their negative impact on native ecosystems. They may be predators of native species, compete with natives for limited resources, or introduce diseases to which the local population has not been exposed. They can also have significant economic impacts by infesting or invading human food systems, or otherwise affecting agriculture, fisheries, forestry, and tourism.
Non-native invasive species have been identified as being a major cause of global biodiversity declines. More than 400 of the over 1300 species currently protected under the US Endangered Species Act are considered to be at risk at least in part due to influences from invasive species (USFWS 2012a). The impact of these species on island systems such as the U.S.V.I. and other U.S. Territories is particularly severe, as these systems contain unique species and community assemblages found nowhere else, occurring in small populations and in small isolated locations with little to no opportunity of dispersal to avoid the impacts.
Threats to native species include direct predation, competition for limited resources, introduction of diseases and parasites, and reduction in the availability of essential habitats. Invasives can also cause destruction or alteration of habitats through disruption of ecosystem function and changes to nutrient cycling, water quality, and hydrology. Environmental impacts can be compounded through cumulative or synergistic effects; for example, the effects of climate change (e.g., drought, increased disturbance through storms or flooding) may make species or habitats more vulnerable to impacts from invasives. Many of these impacts are difficult to quantify.
Historically, the U.S.V.I has experienced losses and declines in biodiversity due to invasives, particularly on St. Croix, with the endemic St. Croix Racer (Borikenophis sanctaecrucis) having been driven to extinction and the St. Croix Ground Lizard (Ameiva polops) extirpated from the island (Platenberg & Boulon 2006) sometime during the 20th Century. More recently, lack of response to invasive threats has led to severe impacts on endemics, including widespread mortality of the Puerto Rican endemic century plant (Agave missionum) on St. Thomas and St. John by the Agave Snout Weevil (Scyphophorus acupunctatus), an insect pest introduced via imported ornamental agaves that also has the potential to threaten the federally protected St. Croix Agave (Agave eggersiana) if not managed (Chamorro et al. 2016). Another federally protected species, the Virgin Islands Tree Boa (Chilabothrus granti) continues to suffer ongoing mortality from predation by feral cats (Platenberg & Boulon 2006, Platenberg & Harvey 2010).
Some species have rapid and measureable impacts, while for others, the impacts are difficult to understand and quantify. Indo-Pacific Lionfish (Pterois spp.) are showing rapid and significant deleterious impacts on commercially important reef fish communities; these impacts are exacerbated by the existing stressors of overfishing, habitat damage from land-based sources of pollution and runoff, and climate change effects of increasing sea surface temperature and ocean acidification (Schofield 2009, Côté & Maljković 2010). The lionfish poses a potential threat for severe ecosystem degradation (Albins & Hixon 2013). However, for another rapid marine colonizer, the seagrass Halophila stipulacea, the impacts are as yet undetermined (unpublished University of the Virgin Islands Masters in Marine and Environmental Science data). Impacts of other organisms that have been long established, such as the Mediterranean House Gecko (Hemidactylus mabouia), Red-footed Tortoise (Chelonoidis carbonaria), and Malaysian Trumpet Snail (Melanoides tuberculata), have yet to be evaluated. There is virtually no understanding of the impacts of non-native invertebrates, fungi, or microbes within the USVI.
Invasive species are expensive to island economies. They can damage infrastructure, disrupt agriculture, fisheries, and tourism, and compromise ecosystem services provided by native species (Kairo et al. 2003). The cost of response can overwhelm budgets of small agencies tasked with response. In 2011 alone, the US Department of Interior is estimated to have spent $100 million on invasive species prevention and response, and other related activities including research, outreach, international cooperation, and habitat restoration (USFWS 2012a). Cost estimates of damage caused by invasives run to more than $120 billion in the US each year (Pimental et al. 2005). Deer and iguanas in the USVI are causing damage to local crops that significantly affects small-scale farmers. The extent of this impact is unknown, but one farmer claimed he lost an entire crop of lettuce in one afternoon of iguana grazing (Wade, pers. comm., 2016).
The USVI depends on a tourism economy to which invasive species pose a major threat. Tourist visitation to the Caribbean plummeted in 2016 due to fears of Zika, a virus transmitted by the non-native Aedes mosquito (World Bank Group 2016). Other potential disruptions to tourism include coral reefs becoming less attractive due to over-predation of colorful reef fish by lionfish. Even iguanas at resorts can have a negative influence when they become too numerous.
The costs of control rise precipitously once species are established. In Florida, over $6 million has been spent since 2005 in attempts to control and eradicate large invasive constrictor snakes (USFWS 2012b); feral Boa constrictors are currently posing a threat to ecosystems on St. Croix (Akin 2015). The US Fish and Wildlife Service (USFWS) spends around $15,000 annually just to control mongoose during the turtle nesting season at Sandy Point National Wildlife Refuge on St. Croix (N. Angeli, pers. comm., 2016).
The loss of ecosystem services as a result of ecological disruption by invasives is difficult to quantify, especially since there have been few efforts to value ecosystem services in the USVI. The overgrowth of algae on reef systems is likely to be exacerbated by predation of herbivores by lionfish, but economic values of those impacts are unknown.
Socio-economic and cultural impacts of invasive species include introduction of diseases or disease vectors (mosquitos and dengue, chikungunya, and zika; cholera in Haiti), disruption of traditional livelihoods, and economic losses from damage to crops or homes. Invasive species can also cause damage to historic structures, by undermining soils or destroying original building materials (Su et al. 2003).
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