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Non-Native and Invasive Species

Non-native species are species that have been introduced into new areas that have not historically been part of their native range. 'Exotic,' 'alien,' 'nonindigenous,' and 'introduced' are all synonymous terms referring to non-native species. In contrast, native species are species that have historically occurred as part of an ecosystem in a specific location. Native species are also called indigenous species. Introduction is defined by Krueger and May (1991) as the transfer by humans of an organism outside its native range.

Plants and animals have been transported by humans and introduced into new areas for many centuries. Some of these introductions have been intentional, although a great many have not. Intentional introductions are often for purposes of livestock or agricultural production, e.g., introduction of domesticated cattle, goats, swine, and honeybees from Europe to the New World in centuries past.

The Australian spotted jellyfish, Phyllorhiza punctata, is a non-native organism that has been collected from the Indian River Lagoon

Many intentional introductions of cultivated stocks and crops have, of course, been of great economic benefit. Hundreds of nonindigenous plants and animals have been introduced to Florida in the five centuries since the arrival of the first European settlers and many of these have greatly benefited man. For example, the cultivated plants of genus Citrus originated in tropical to subtropical southeast Asia, India, and the Near East and were introduced to Florida and the New World by Spanish explorer Ponce de Leon in 1513. The various modern hybrid citrus cultivars-including the world-famous "Indian River Citrus" varieties-now represent a billion-dollar industry in the state.

The exotic zebra mussel, Dreissema polymorpha, is a highly invasive species that has caused problems in the U.S. Great Lakes and elsewhere.
Photo courtesy USGS.

Unfortunately, not all species introductions have proven beneficial. Whether intentionally released to the environment, intentionally introduced and then accidentally released, or introduced entirely by accident, the negative ecological and economic impacts of exotic species introduction are substantial. Very often, the invaders go largely unnoticed until populations have grown and spread to a point where eradication is difficult and costly, or even impossible. The most infamous such invasion in recent memory in North America is that of the zebra mussel (Dreissena polymorpha) in the Great Lakes. In Florida, the introduction and spread of the green mussel (Perna viridis) in Tampa Bay is a prime example of an exotic introduction that has had substantial ecological and economic impacts.

What Makes a Successful Invader?

Exotic species have the potential to become invasive when they are released into a new environment. Invasive nonindigenous species are those species whose introduction causes, or is likely to cause, significant economic or environmental harm. Not all non-native species become invasive when released to novel systems. In fact, the majority of individual introduction events are believed to be of little consequence, with the introduced individuals failing to lead to established populations. In other cases, the introduced species does become established but, for reasons not entirely understood, its presence causes little or no ecological or economic impact.

Freed from natural competitors, Old World climbing fern, Lygodium microphyllum, quickly invades and overgrows a native Florida plant community.
Photo courtesy SFWMD.

Nevertheless, any species removed from its native range and introduced to a new area has the potential to become an invasive species. The newly-introduced organisms now exist free from the natural forces that keep populations in check within their native range. Such natural forces include predators, competitors, parasites, and diseases that the species co-evolved with, as well as environmental factors and other natural control mechanisms (Torchin et al. 2003, Wolfe 2002). When an introduced species is released from these controls, populations may become established and to grow unchecked to the point where they achieve invasive species status. When this occurs, unchecked invasive populations may compete with or otherwise negatively impact native populations, displacing them in some instances.

There are certain biological traits that together determine the capacity for a non-native organism to become invasive. Organisms with wide habitat and/or dietary preferences, broad physiological tolerances across a range of environmental conditions, and relatively short generation times may be predisposed to become successful invasives. Organisms that are abundant (but not rampant) over large portions of their native ranges and those with a high degree of genetic variability may also become successful invasives on release to novel environments (Erlich 1989, Williams and Meffe 1999).

The ability to act as a pioneering species-among the earliest to occupy available ecological niches and to establish 'founder' populations-is another trait shared by several successful invasive species. These organisms typically exhibit high fecundity such that release of only a few individuals can give rise to a population sizeable enough to establish itself in its new setting.

It is important to bear in mind that although we can recognize many of the traits that allow introduced species to become invasive, predicting which species actually will become invasive in a new setting remains difficult. This is a key reason why the intentional introduction of non-native species, either for commercial purposes or as a means of biocontrol to manage other ecological invaders, is undertaken only after years of careful study has shown the invasive potential for the new species to be minimal.

Although predicting which introduced species will eventually become invasive is difficult, identifying the sorts of habitats most likely to be invaded is somewhat less so. Habitats most susceptible to invasion tend to have low natural biological diversity, simple food webs, and a degree of vulnerability to natural or man-made disturbance (Williams and Meffe 1999, Ray 2005).

Agricultural lands, managed forests, and variously urbanized habitats all share the above traits, and all are susceptible to exotic invasion. But sheltered coastlines and estuaries are also vulnerable to invasion, and these systems also tend to have a relatively low diversity and usually suffer from anthropogenic disturbance (Ray 2005).

Florida and the Indian River Lagoon

Florida is one of the two states most affected by invasive species, second only to Hawaii. Both states share features that make them particularly susceptible to bioinvasion.

Florida continues to be urbanized and altered as the human population steadily increases.
Photo courtesy NOAA

Simberloff (1986, 1997) notes that two sorts of sites particularly prone to invasion impacts are islands and habitats created or greatly disturbed by humans. Hawaii, an oceanic island archipelago whose lowlands are heavily impacted by human disturbance and development, contains an abundance of both types of sites. The situation in Florida is strikingly similar. South Florida is effectively a habitat island, bounded to the east, south, and west by ocean, and on to the north by winter low temperatures. Like true oceanic islands, the habitats of south Florida are typified by an impoverished native flora and fauna which leaves them vulnerable to negative impacts from invasive populations (Myers and Ewel 1990). With a resident population of 17 million people and growing, and several times that number visiting each year, much of Florida is dominated by habitats created or extensively modified by humans.

Also similar to Hawaii, large expanses of tropical to subtropical habitats add to Florida's vulnerability to invasive species impact. The subtropical to tropical climate and the high seasonal rainfall are two climatological factors that foster the establishment of tropical nonindigenous species, particularly terrestrial plants (McCann et al. 1996).

First introduced to Florida as an ornamental plant, Brazillian pepper (Schinus terebinthifolius) is now among the most highly invasive exotics in the state.
Photo courtesy USGS

Large numbers of tropical exotic species were first introduced to the state as ornamental plants or pets. Accidental or intentional release of these organisms to adjacent ecologically accommodating natural areas in south Florida gives them a foot in the door toward establishing self-sustaining populations. South Florida is more susceptible to invasion than north Florida, and the effects of invasion are more apparent. In fact, many south Florida invasive populations have a distribution whose northern extent terminates abruptly at approximately the latitude where winter freeze events are commonly recorded (Simberloff 1997).

Another factor contributing to the invasive exotic crisis in Florida is the fact that the state is an important transportation and shipping hub. Fully 85% of the hundreds of millions of live plants imported to the U.S. utilized Miami as the port of entry (Simberloff 1997). Most of the exotic animals shipped to the U.S. for the pet trade also pass through Florida.

Certainly, the vast Florida coastline, at almost 3,000 km, offers marine invaders ample opportunity to gain a foothold in coastal and inshore habitats. Finally, seasonal tropical storms common to Florida can facilite the spread of exotics in the state by physically dispersing individuals or propagules (McCann et al. 1996).

Noted entomologist and conservation biologist E. O. Wilson, in his foreword to the book, Strangers in Paradise: Impact and Management of Nonindigenous Species in Florida, eloquently summed up the invasion status of the state and what is at stake:

What is being lost? The answer is easy. A precious and irreplaceable part of Florida's, and the nation's, heritage is disappearing. Plants, animals, and entire ecosystems that took tens of thousands to millions of years to evolve are at risk. What is being gained in their place? A hodgepodge of species found in other parts of the world, in some cases all around the world in tropical and subtropical environments. . . . Florida is being homogenized, and everyone, for all time to come, will be the poorer for it.

Large portions of the IRL watershed have been greatly altered through human activity.

Turning attention specifically to the Indian River Lagoon (IRL) region of east-central Florida, a number of features leave the system vulnerable to exotic invasion. The long and narrow coastal IRL straddles climatic provinces (subtropical to warm-temperate) and also straddles land and sea. Open communication with the Atlantic Ocean occurs via five inlets that cut through the barrier island, while an extensive network of canals drains upland portions of the watershed into the lagoon. At a glance, then, it is apparent that the IRL and its watershed offer would-be invaders a broad range of environmental conditions as well as means of entry, whether by land, surface water, or sea. Beyond these geographical considerations, the IRL is an urbanized estuary situated on the densely populated Florida east coast. Recall that human disturbance typically predisposes an area to bioinvasion.

We need also consider the mix of regional commercial land uses (e.g., agriculture, ornamental aquaculture), and outdoor recreational activities (e.g., fishing and boating) that have the potential to act as vectors for the introduction of new species. Consider the ease with which recreational boaters can transport marine invaders large distances when they trailer boats up and down the coast or between Florida's Gulf and Atlantic coasts.

With the above considerations in mind, it can only be assumed that unintentional new species introductions are occurring all the time in the IRL watershed and are a very real and ongoing threat to ecosystem structure and function.

By Land and By Sea: Introduction Pathways

Several known or suspected non-native species introduction pathways are described here, and some of the species likely to have been transported via each pathway are noted. The actual invasion pathways for many nonindigenous species remain unknown and that species are likely to have arrived via a number of different pathways and multiple introduction events. Additionally, the original native range of many species is not known with certainty. Species with unknown geographical origins are often referred to as 'cryptogenic' species.

Intentional release for human benefit

A number of nonindigenous plants and animals have been directly introduced into new locations in order to derive some benefit resulting from the introduction. The widespread introduction of the Asian oyster (Crassostrea gigas) throughout much of the world in an effort to augment or replace depleted native species fisheries is a well-known marine example of such an introduction. Introduction of European brown trout (Salmo trutta) to new locations in order to establish sport fisheries is a freshwater example. The honeybees (Apis mellifera) that are of nearly incalculable economic and agricultural value in Florida and elsewhere in the U.S. are descended from European honeybees brought to the New World some 300 ago.

Dense stands of non-native Australian pine, Casuarina equisetifolia, displace native vegetation and alter community structure.
Photo courtesy Collier Dept. of Environmental Services.

Several examples from the Indian River Lagoon region of Florida are of note as well. The introduction of torpedo grass (Panicum repens) in the late 19th century was as a cattle forage grass. Australian pine (Casuarina equisetifolia) was introduced in the late 19th century partly for use as a windbreak to border agricultural groves. Melaleuca (Melaleuca quinquenervia) was introduced to Florida on several occasions and for multiple purposes, including wood production, soil stabilization, and to form forests intended to dry up the Everglades and other wetland ecosystems. In Florida, as elsewhere, release of the marine toad (Bufo marinus) was intended as a means of controlling populations of crop-damaging insects.

Accidental escape from captive cultivation

Often, non-native organisms are intentionally introduced to new regions so they may be cultivated under captive conditions and subsequent release to the environment is due to accident or carelessness.

The bulk of this fish sample collected from an IRL mosquito impoundment is made up of the blackchin tilapia, Sarotherodon melanotheron, a non-native species.
Phot courtesy Craig Faunce

For example, a number of exotic species now established in the IRL region of Florida were accidentally released by the aquarium trade. This list includes walking catfish (Clarias batrachus), blackchin tilapia (Sarotherodon melanotheron), and the aquatic plant hydrilla (Hydrilla verticillata). Pet escapes are likewise responsible for the presence of feral house cats (Felis catus) and green iguanas (Iguana iguana) in Florida.

Brazilian pepper (Schinus terebinthifolius) is possibly the best-known Florida example of an ornamental plant whose escape from controlled cultivation has had devastating ecological and economic effects in the state. Among the most ecologically damaging accidental domesticated animal releases in the southern U.S. is the release and subsequent establishment of feral pigs (Sus scrofa). The species was also released in the U.S. in the form of European wild boars intended for sport hunting, and today's U.S. feral pig populations are a combination of both descendant lines.

Accidental transport in agricultural/commercial products

A great many non-native species are transported to new locations as undetected infestations or stowaways in commercial agriculture shipments and subsequently released to the environment.

Cuban treefrogs (Osteopilus septentrionalis), for example, were most likely introduced to Florida as undetected stowaways in vegetables imported from Cuba (Behler 1979) while the cactus moth (Cactoblastis cactorum) may have been brought to Florida unnoticed in infected prickly pear nursery stock. Red imported fire ants (Solenopsis invicta), a species that has become a scourge in the southern U.S., may have been accidentally transported here in the 1920s in shipments of potted live South American plants.

Analogous introductions in marine environments can be found as well. The non-native ranges of animals like green porcelain crabs, serrated swimming crabs (Petrolisthes armatus, Scylla serrata) and other exotic marine species have very likely been expanded as these organisms have been accidentally transported in live shipments of oysters, mussels, and other aquacultured or wild-harvested marine products.

Unintentional transport as ship hull fouling

The majority of marine and estuarine introductions are related to overseas transport and shipping, and this has likely been the case since the dawn of maritime navigation.

The non-native pleated tunicate, Styela plicata, probably first came to U.S. waters as part of the hull-fouling marine community.
Photo courtesy Thomas Stach

The current circumglobal distributions of the surface-fouling pleated sea squirt (Styela plicata) and wood-boring species like shipworms (e.g., Teredo navalis) and boring isopods (e.g., Sphaeroma terebrans) are almost certainly reflective of a long human history of aiding-and-abetting marine exotic introductions that includes several hundred years of wooden sailing fleets moving goods, people, and accidental hitchhikers across vast ocean basins.

Historically then, ship surface fouling has been a significant vector for non-native marine species introduction. Today as well, species like barnacles (e.g., Balanus amphitrite) and sea squirts that are capable of attaching to hard surfaces are still transported on ship hulls, and also on buoys, service barges, anchors, chains, ropes, etc. (Carlton and Hodder 1995, Carlton 2001).

Unintentional transport in ship ballast

Ballast refers to any heavy material used to improve stability and control of a vessel. In a cargo vessel laden with goods, most of the ballast is provided by the cargo stowed in the hold. When the cargo is offloaded, however, an equivalent mass of some other material must be taken on to maintain stability. Prior to the 1880s, rocks, sand, dirt, or other forms of 'dry ballast' were typically employed, but for well over a century seawater has been primarily used as ballast.

Several centuries of transporting dry ballast across and between oceans spread innumerable species of insects, wharf roaches (Ligia exotica), desiccation-resistant marine invertebrates like barnacles, plants, and other organisms between the world's harbors and surrounding environs (Carlton and Hodder 1995).

Ballast water pumped into a ship's hold at one location will contain adult-, larval-, and propagule forms of a variety of water column and sediment-associated organisms occurring in that location. When that ballast water is pumped out at a new location perhaps several-thousand miles away, an undetermined number of viable exotic organisms are pumped out as well. Ballast water is the most important man-made, long range transport vector for marine plankton, including the free-swimming planktonic larval stages of organisms that as adults inhabit hardbottom and soft sediment intertidal and subtidal communities.

The Green Porcelain Crab, Petrolisthes arnatus, produces planktonic larvae capable of surviving transport in ballast water.
Photo courtesy SERTC/CSDNR

The majority of new shipping-related marine exotic introductions are believed to come from ballast water (Lavoie et al. 1999, Ray 2005). Of the marine and estuarine bioinvaders now found in the IRL, species such as green mussels (Perna viridis), charru mussels (Mytella charruana), striped barnacles, green porcelain crabs (Petrolisthes armatus), serrated swimming crabs (Scylla serrata), and several others were likely introduced to U.S. waters as larval forms transported in ballast water.

The Eurasian zebra mussel (Dreissena polymorpha), noted previously, appears to have been introduced into the Great Lakes in the late 1980s as larvae transported in ballast water and released from a Black Sea commercial cargo vessel. D. polymorpha has flourished in the U.S. since its initial release, spreading southward down the Mississippi River and in all of the major river drainages east of the Rocky Mountains.

Natural range extension

Occasionally, an exotic species comes to occur in a new area not through any of the human-facilitated introduction pathways described above, but through natural range extension. The initial appearance in Florida of the highly migratory cattle egret (Bubulcus ibis), native to Africa, appears to be a largely natural occurrence unassisted by humans (Weber 1972, Kaufman 1996). Similarly, the initial Florida panhandle population of nine-banded armadillos (Dasypus novemcinctus) represented a natural westward range expansion of the Texas armadillo population.

Although the initial occurrence of such animals in new areas may have taken place in the absence of human helping hands, subsequent spread of these populations may still be dependent on human activity. In the case of the cattle egret, for example, range expansion in the U.S. subsequent to the arrival of the species appears related to widespread landscape conversion to pasturelands (Telfair 1994).

Impacts and Consequences of Invasion

The ecological consequences of exotic species invasion are many and varied. They include potential displacement of native species, modification of community structure, alteration of food webs, and possible reduction in habitat value. Invasive terrestrial plants may alter habitat hydrology and soil make-up, and can affect natural fire regimes in fire-mediated ecosystems (Mazzotti et al. 1997). Of utmost concern to conservation biologists is the potential for loss of biological diversity when native species are outcompeted by invasives and displaced from the ecosystems to which they may be uniquely adapted.

Biological invasions carry serious economic and ecological consequences. Nationwide, bioinvasions over the past two centuries have resulted in the establishment of 50,000 foreign plant and animal species, with one of every seven of these species becoming invasive. The United States Department of Agriculture Animal and Plant Health Inspection Service has estimated the national cost due to invasive species, including damage to crops and other industries and the cost of management and control, at nearly $140 billion annually (APHIS 2001).

Assessing the Invasion Status of the IRL Watershed

The efforts of the Indian River Lagoon Species Inventory Project authors during the 2006-2007 funding cycle were primarily directed toward assessing the status of the IRL region with regard to nonindigenous species. Each of the fifty new species reports added to the inventory during this period summarizes the life history, invasion history, and potential ecological and economic impacts of a variety of non-native species occurring here and elsewhere in Florida.

Additionally, an initial atttempt was made to assess the native/non-native status of the biota of the IRL region via examination of the relevant literature and collection records. Though by no means exhaustive, this first examination of the invasion status of the region identified approximately 240 species present in the region as either exotic or cryptogenic. Of these, approximately 170 species represent organisms new to the Species Inventory. These species are denoted in the Inventory index pages as being either "Non-Native" or "Cryptogenic."


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