Invasive species

Introductions of fauna are part of human history and legacy.  The reasons for the introductions vary from involuntary to deliberate.  The effects on the ecosystems have in some cases been drastic and, thus, have long been objects of study.  Several animal species have been introduced, from vertebrates to invertebrates.  With respect to herpetofauna, introduced Reptile and Amphibian species can have various deleterious impacts on the ecosystems they are introduced to.  According to these include:

• Decrease in abundance
• Increase in abundance
• Extinction of species
• Change in spatial ecology
• Extinction of populations/range contraction
• Reproductive interference
• Genetic contamination
• Evolution of defenses
• Loss of pollination services
• Change in behavior
• Loss of seed-dispersal services
• Change in plant community
• Reorganization of trophic web
• Increase in nutrient flows
• Formation of nutrient sinks

The Bipartite network plots shown below illustrate the biogeographic origin of species introduced to the Bahamas (BA), Greater Antilles (GA) and Lesser Antilles (LA).  They are  color coded by time of introduction .

Mammals

Asian Mongoose (Herpestes javanicus):  Introduced to combat the black rat and Norwegian rat damage to sugar cane on Jamaica by W.B. Espeut in 1872; it apparently worked for several years after introduction (Espeut, 1882).  Unfortunately the mongoose is diurnal and the nocturnal rats took to the trees.  The mongoose took advantage of the endemic ground dwelling vertebrates as well as the invertebrates.  They not only ate boa prey, such as anoles, they ate the boas themselves.  Afterwards the mongoose was introduced to the rest of the Caribbean where sugar cane production took place .  If one map was made of sugar cane and crop fields, mongoose,  and herpetofauna extirpations, respectively, and then the maps over-layed one atop another it would practically appear as one map .

Peters (1917) found the mongoose “excessively abundant”  on Tortola, St. John and St. Thomas.   At the time he claimed Jost van Dyke and Water Island, Ginger Island, Mosquito Island and others in the outlying Keys were mongoose free .  Currently the Asian Mongoose occurs on all islands of the Greater Antilles, 20 islands of the Lesser Antilles, and five small islands near larger islands that support mongoose populations .

Williams (1918) conducted a three month study on the stomach contents of the mongoose.  He found one mongoose would prey upon, approximately,  18 snakes, 17 lizards and 29 anurans in a three month period.  Frogs were preyed upon in sugar cane fields and snakes in cacoa plantations .

Stejneger, while collecting on Puerto Rico in 1904 laments the lack of ground dwelling vertebrates due to the ravages of the mongoose:  “This ferocious little animal is also responsible for the present comparative infrequency of snakes on the island.  Formerly snakes were common enough, for as late as 1835 Dr. Moritz found them so numerous that in places one could hardly make a step without seeing several.  At present one may traverse the island from one end to the other without getting a glimpse of one”  .

Cats:  Initially used for the same purpose as the mongoose, feral cats have become one of the single most uncontrolled threats to herpetofauna.  They have become a problem on a global scale, causing unimaginable damage to wildlife populations.  Lynn and Grant (1940) commented on the decline of Chilabothrus subflavus in the Portland Point area of Jamaica after the introduction of house cats.  (Tolson 1996) and (Tolson & Henderson 1993) blame cats for predation on and injuries to C. granti on St. Thomas, USVI.  At least 175 vertebrate taxa (25 reptiles, 123 birds, and 27 mammals) are threatened by or were driven to extinction by feral cats on at least 120 islands .

Father Du Tertre writes in 1667, “There are a large number of cats by all these Isles which, indeed, were brought there by the Spaniards. Most of them are inlaid with red, white & black: & have flat & very shiny hair. Several of our François after having eaten the flesh, take the skins to France to sell them. These cats at the beginning that we knew in Guadeloupe, were so accustomed to eating Partridge, Doves, Thrushes, & other little birds, that as I said, they did not deign to look at the rats; & I saw a cat in one of our houses, who every day brought to the little ones several good pieces of game, who fervent us very much to feed the sick that we had at the time at the Convent”  .

On Dominica, in 1879, Ober writes of the feral cat:  “It was not a wild cat in the true sense of the word —not a Lynx rufus — being only a “— a cat of the domesticated species run wild. It was “chat maron” gray in color, striped with black, and larger and more strongly made than the cats of the coast, who do not have to forage for a living ; showing how, in time, a new species might be possibly the result of this change of life. It lives in the deep woods, preying upon small birds, lizards and crabs, and is as savage and untamable as any specimen of the genus to be found in American back-woods. My men skinned it at my request and wrapped the skin in a plantain leaf, to be hung up upon our return”  .

Dogs:  Populations of feral dogs on the islands have become a real issue over the last century.  The dogs have had a detrimental effect on many of the iguana populations and other ground dwelling reptiles throughout the Caribbean.  Du Tertre again writes, “The Dogs do not look natural in these places, if there are certain small dogs that I have seen on a few Savages: they had their heads and very long ears, and approached the form of foxes.  They bark much more clearly than other dogs.  All the others were there bring by the Hunters.  Many have left it in the woods, which by succession of time have multiplied, that we sometimes meet bands of ten or twelve together, & which do a lot of damage to the hunt; they are called maroon dogs.  Although the greater part of these brown dogs flee from men, & a few others are content to bark, some furious ones are met with, who boldly cast anger at men, and some would have been devoured if they had not been rescued”  .

The large Iguana, Cyclura carinata, was nearly extirpated on Pine Cay in the Caicos islands  during the construction of a tourist facility/hotel and for 3 years thereafter (Iverson 1978). Dog and cat predation during the hotel construction resulted in the decline of iguana numbers from about 5,500 adults to only around five.  Evidence suggested that population declines of Cyclura elsewhere in the Turks and Caicos Banks could be attributed to dog and cat predation as well .

Rats and mice:  Rats have been described as being ‘preadapted to pestilence’, with  ecological traits such as small body size, omnivorous diet and high reproductive capacity .  Brought to the New World with the early explorers, the rodents have wrought nothing but destruction in their wake.  Crops, food prey and herpetofauna all make up the diet the of the rodents.  It is impossible to accurately attribute the number of extinctions and extirpations caused by rodents and the mongoose.  We do know, however, that number is ever growing.  Tolson found that Chilabothrus granti was absent in all sites surveyed on the Puerto Rico Bank that had an introduced rat predator, and, when C. granti was reintroduced to cays where rats had been eradicated, the boa populations flourished .

John Davies (1666) writes of the rat pestilence in the Caribbean, “Mice and Rats were creatures heretofore unknown to the Caribbians; but now since the coming in of so many Ships to those Islands, and the casting away of divers of them in the very Roads, where they afterwards rot, they have got to land, and are so muitiply’d, that in some places they do abundance of mischief among- the Potatoes, Pease, Beans, and particularly that kind of Wheat which is called Turkey-wheat : Nay did not the Snakes destroy them, and search for them in their holes under ground, in the clefts of rocks, nay even in the coverings of houses, which consist of Palm-leaves, or Sugar-canes, it would no doubt be a very hard matter to secure Provisions from them. Now indeed there are Cats in these Islands, which give them no quarter; nay, Dogs are taught to hunt them, and it is no small diversion to see how subtle they are to find them out, and expert in the hunting and killing of them.
Nor is this inconvenience particular to the Caribbies ; nay it is much worse in Peru; for Garcilasso, in his Royal Commentary , affirms, that these pestilent creatures being extreamly numerous in those parts commit very great spoils, ransacking the places through which they pass, making the Fields desolate, and gnawing the Fruits even to the stalks, and roots of the Trees.
The Inhabitants of the Islands have an invention which they call Balan, to keep the Rats from eating their Cassava, and other Provisions. This Balan is a kind of round hurdle, or haply square, consisting of several stakes, on which they place the Cassava after it hath been dried in the Sun: It is fasten’d at the top of the Hut, hanging down by a Witth or Cord; and that the Rats may not come down along the Cord, and so get to the Balan, they put the Cord through a smooth gourd which hangs loose in the midst of it, so that the Rats being come to that place, being not able to fasten their feet in it, and fearing the motion of the Gourd, are afraid to venture any further : Were it not for this secret, the Inhabitants would find it a hard task to keep their provisions.”  .

There are records of brown rats colonizing tropical islands wholly or largely uninhabited by humans and becoming established there i.e., Dennery Island, St Lucia .  A mark & recapture study on Rat Island, St. Lucia indicated a density of 22.8 rats per hectare (ha) on a 1.4 ha island .  The arrival of the black rat on New Zealand’s Big South Cape Island is believed to be the worst ecological event following a rat invasion.  Eight land bird species declined or went extinct (Atkinson 1985), a species of weevil went extinct and a population of bats disappeared (Towns et al., 2006).

Hayes et al.(2004) recorded significantly lower density of iguanas (Cyclura spp.) on four rat infested cays in the Bahamas than on six rat-free cays and low recruitment in the San Salvador Island iguana C. rileyi rileyi on rat-infested islands.  Populations of reptiles have been recorded to increase rapidly after rat eradication on infested islands. The Antiguan racer (Alsophis antiguae) population doubled in 18 months after black rat eradication on Great Bird Island (Daltry 2006) .

One of the primary issues with determining the rat damage and predation is most publications list birds (because they are so conspicuous), mammals, plants and “invertabrates.”  The common practice of listing reptiles and amphibians as invertabrates has, for the most part, prevented the detailed collection of data for those attempting to quantify or qualify rat predation of herpetofauna and their prey.

Racoons:  In 1997 a single racoon (Procyon lotor) was responsible for decimating the Sandy Cay Rock Iguana (Cyclura rileyi cristata) and brought it to the brink of extinction.  It was estimated only 112-168 iguanas remained by the time the racoon and Black Rats (Rattus rattus) were eradicated .

Cattle, pigs, sheep, donkeys, goats, rabbits & monkeys:  Introduced on the islands as a food staple they quickly became feral, destroying dry tropical habitat that was home to many boa species and their prey.  As a result, thorny scrub plants quickly replaced ideal habitat.  Consequently, much of the boas’ food prey items disappeared along with that habitat.  Some prey have moved, adapted and are no longer found within the habitat of the boas.  Green monkeys (Chlorocebus sabaeus) were introduced to Barbados.

Amphibians

Cane toads (Bufonidae):  The toad is widely established across the Caribbean and will eat almost anything within its reach.  The extent of its ability to maintain populations on the islands is not yet fully understood.  Time will determine whether it is able to establish viable populations where introduced and what effect they will have on the local herpetofauna.  The cane toad has been documented as the cause of death in the CITES I Jamaican boa, C. subflavus (Wilson et al. 2010).  Barbour documents Bufo marinis has been carried to almost all the Antilles .

Rainfrogs (Eleutherodactylidae):  These frogs have become widely distributed throughout the region.  Species are now found on many of the islands as a result of stowaways in cargo and nursery plants.  At least 6 species are now established across the islands.

Cuban Tree Frog (Hylidae):  The frogs have become widely established throughout the Caribbean.  The frogs and their tadpoles can out compete endemic frogs, causing declines in natural populations.  They feed on insects, small lizards and snakes, effecting those  populations as well.

Frogs (Ranidae):  Five species have been reported from the region.  The majority of species were imported for the food industry.  Some species were intentionally introduced on several islands as a potential food source for the population.

Frogs (Leptodactylidae):  Endemic to the Caribbean, the “Mountain Chicken” has been introduced to other islands as a food source.  All attempts at introduction on those islands have failed.  It is now in decline across its original range.  Introduced to Puerto Rico in 1929 and 1932; both attemps failed (Grant

Frogs (Microhylidae):  Introduced from North America via ornamental plants.  It is now established on Grand Cayman and the Bahamas.

Frogs (Leiuperidae):  Originally from South America, it has been found on Aruba, Curacao and, recently, Bonaire.

Reptiles

Boa constrictors (Boidae):  Introduced accidentally, as hitch-hikers on imported materials or as pet escapes, these snakes are now established on several islands.  They compete for the same resources as the endemic boas and other herpetofauna.

Tortoises (Testudinidae):  Whether brought in from South America or North Africa, many were imported for use as ornamentals or a food source.  Attempts at establishing them on many islands has met with failure.

Crocodilians (Alligatoridae & Crocodilidae):  Introduced to Isla de la Juventud, Cuba for  food and hides.  All other sightings were one-off reports and none have become established in the region.  One record exists of an individual (C. intermedius) rafting to Grenada from the Orinoco Basin on 6 September, 1910 .

Geckos (Gekkonidae):  Gekko gecko can be found on Guadeloupe and Martinique.  House geckos are continuing their invasion of the islands and can be found on walls and buildings.  Other gecko arrivals point to the slave trade as points of introduction.  Many introductions and arrival times of other species simply can’t be determined.

Dwarf geckos (Sphaerodactylidae):  These diurnal species arrived across a broad timeline, from pre-human times to relatively recent introductions.  Most have been attributed to cargo stowaways.

Iguanas (Iguanidae):  Zoos, tourists and the “pet trade” have been credited with the translocation of iguanas.  Some were intentionally relocated to unpopulated cays, others moved by tourists to several unoccupied cays (Hines and Iverson, 2006a, 2006b).  Conservation concerns are behind the movements of iguanas in the Turks/Caicos, Bahamas and BVIs.  There are valid concerns about I. iguana hybridizing with endemic species.

Anoles (Polychrotidae):  Several species of anoles from within the region have been found outside their normal range; this is primarily due to nursery plants and as stowaways in building materials, etc.  Anolis sagrei was establish in Jamaica as early as mid-19th century as recorded by Gosse in 1855.

Corn snakes (Colubridae):  Primarily the result of ornamental plants in the nursery trade, the snakes have been found in the USVI and the Bahamas.  The pet trade has been implicated in the introductions on the Virgin and Cayman Islands.

Insects

Snails, crazy ants, fire ants, bees and flat worms are now found on several islands.  The effects of these invasive insects has yet to be determined.

Flora

Much of the invasive flora has been introduced as ornamentals or as travelers with commercial gardening mediums.  The invasives can squeeze out the endemic growth by volume or ability to adapt to new environs.

Birds

Rock Dove, House Sparrow, Collared Dove, Chicken, Muschovy Duck, Chukar Partridge, Common Myna, Wood Duck, Yellow-Crowned Parrot, Eurasian Collared Dove, Red Masked Parakeet,  Zebra Dove, Yellow Canary, Common Waxbill, Madagascar Fody, Java Sparrow.

All of these exotic or invasive species and predators have contributed to region-wide displacement, hybridization, loss of food prey, mistaken identity (for eradication and control purposes), species competition and genetic swamping.  We listed invasive fauna using the information provided by Hailey et al .  Additionally, Tolson lists effective methods for the control of many invasive species that have an impact on West Indian herpetofauna and their habitat (Tolson 2000).

Eradication programs

Of the top 100 worst invasive species on islands, feral cats are among the 4 predators on that list.  Feral cats are probably the most harmful of the four, causing numerous extinctions of endemic vertebrates or, at the very least, threatening them with extinction.  Feral cats have been completely eliminated from 83 islands globally.

At least 38 IUCN ­listed CR species (10 reptiles, 25 birds and 3 mammals) are still threatened by feral cats on 41 islands .  Feral cats on islands contributed to at least 14% (33 species: 2 reptiles, 22 birds, and 9 mammals) of all 238 vertebrate extinctions recorded globally by the IUCN. Furthermore, feral cats threaten 8% (38) of the 464 species listed as critically endangered (Medina et al. 2011).  Five IUCN ­listed CR species have already benefited from feral cat eradications (1 reptile [Cyclura carinata on Long Cay, Caicos Bank]. .  Crabs on Long Cay initially prevented the first attempt at rat eradication as they crowded around the bait traps and prevented the rats from reaching them (Hayes et al 2004).

Antigua and Babuda have had seven successful (and one unsuccessful) rat eradication projects;  Bahamas has had 2 successful (and one unsuccessful) rat eradication projects and St. Lucia has had five successful rat eradication projects .  There have also been rat eradication projects in Martinique (4), Guadeloupe (1), Turks and Caicos (4), British Virgin Islands (1) , U.S. Virgin Islands (3) and Puerto Rico (3).

Depending on the target type for eradication, the costs can run anywhere from $4 to $431 per hectare-these programs should be run in conjunction with other targeted problem invasives to realize the full benefit of the program and reduce costs (Nogales, Vidal, Medina, Bonnaud, Tershy, Campbell & Zavaleta, 2013).  Even when the program is used for saving one species, there are added benefits of possibly saving others in the process of threat eradication.  If feasible,  rodents, rabbits, feral cats and mongoose should be targeted simultaneously for  eradication, which will provide substantial cost savings versus single­ species eradication attempts (Griffiths 2011).  Given the costs associated with invasive species eradication, Return on Investment (ROI) is now factored into the equation when planning eradication programs.

Citations