Kartesz J (2010) A
synonymized checklist and atlas with biological attributes for the
vascular flora of the United States, Canada, and Greenland. Floristic
synthesis of North America. Second Edition. CD-ROM Version 2.0. North
Carolina Botanical Garden, Chapel Hill.
NeoBiota 9: 71–73, doi: 10.3897/neobiota.9.1316
Counting "exotics"
Qinfeng Guo
Eastern Forest Environmental Threat Assessment Center, USDA-Forest Service, 200 W.T. Weaver Blvd Asheville, NC 28804, USA
Corresponding authors: Qinfeng Guo (qguo@fs.fed.us).
Academic editor: Michael McKinney
An introduced or exotic species is commonly defined as an organism accidentally or intentionally introduced to a new location by human activity (Williamson 1996; Richardson et al. 2000; Guo and Ricklefs 2010). However, the counting of exotics is often inconsistent. For example, in the US, previously published plant richness data for each state are only those either native or exotic to the US (USDA and NRCS 2004), not actually to the state. Yet, within-country (e.g., among states, counties) species introductions which form “homegrown exotics” (Cox 1999) or “native invaders” (Simberloff 2011) are undoubtedly numerous. The growing human population and associated activity increase species introductions at all levels, both international and internal but to date intercontinental species introductions have always been the focus. Those species introduced among neighboring areas are often unnoticed but they are actually far more frequent due to the proximity and environmental similarities. Many domestic exotic plant species exhibit high invasiveness such as Spartina alterniflora (smooth cordgrass; introduced from the east coast to California) and Molothrus ater (brown-headed cowbird; introduced from the Great Plains to California).
How widespread is the mismatch between definition and
practice? It varies among countries and taxonomic groups. Based on the
published records of plant introductions, virtually all the large
countries in the world such as China, Canada, Russia, India,
Australia, and the United States have not included domestic exotics in
counting exotic richness in internal units (e.g., states, provinces,
counties; but see Kartesz 2010).
In other words, the national boundaries are used to count exotics.
This is because, for many taxonomic groups, species introductions
among internal administrative units are rarely monitored. The effect of
using national boundary to count exotic species becomes greater when the
internal units become smaller. For example, within California,
detailed records show many native species (to some parts of the state)
have been naturalized in other parts of the state (e.g., Hickman 1993;
S. Norman, pers. commun.). In contrast, as the focal area becomes
larger, the exotic species pool becomes smaller (at the global scale,
there are no exotic species).
Using boundaries larger than the concerned internal units
could drastically underestimate the exotic richness (and overestimate
native species richness). The native species richness automatically
changes when the “exotic” species richness has been correctly estimated.
After the correction is done, it is critical to re-examine and
evaluate previously reported diversity patterns, the relationships
between the natives and exotics, and the relationships of species
richness in different categories with other biotic, social, and
physical variables.
Also, there are discrepancies in domestic species
introductions among taxonomic groups. For example, domestic species
introduction of fishes within the US but among drainages (not states)
usually have good records (e.g., Fuller et al. 1999). Therefore, comparing invasion patterns for different groups needs to take this into account.
In short, counting exotics correctly is critical in invasion
biology, conservation, and biogeography. Similar to international
species invasions, internal species introductions also have serious
ecological and environmental consequences thus pose significant
social-economic problems (Cox 1999). For example, as with foreign exotics (Lockwood and McKinney 2001),
internal species introductions also homogenize local, state, and
national floras and thus lead to similar ecological/social consequences.
Indeed, internal species introductions apparently have a greater
homogenizing effect than external introductions for plants and fishes (McKinney 2005) and mammals (Spear and Chown 2008).
To provide quality information for more effective management,
close monitoring of internal species introductions among states (or
provinces especially in large countries) is urgently needed. Public
education and policy/management decisions should be in place. Domestic
species introductions, for whatever reason (e.g., market-based trade
and travel or accidental), should be minimized, especially if some of
the species may be highly invasive. Other actions may include: (1)
checking local seed/plant companies and nurseries regarding their
sources, (2) monitoring major transportation ports/hubs, and (3) using
truly local species (not those native only to the US but those native
to the immediate vicinity) for restoration. At present, such practice
and/or regulations either do not exist or are minimal. Indeed,
identifying species introduced across states or provincial borders is
challenging as we need to separate them from natural migrants (not
“exotics” by definition) for various reasons. However, because of the
proximity and unprecedented and increasing within-country traffic,
human induced species introductions should be reduced to the minimum
level possible.
ReferencesCox G (1999) Alien Species in North America and Hawaii. Island Press, Washington, DC.
Fuller PL, Nico LG, Williams JD (1999)
Nonindigenous fishes introduced into inland waters of the United States.
American Fisheries Society, Special Publication 27, Bethesda,
Maryland.
Hickman JC (Ed) (1993) The Jepson Manual -
higher plants of California. University of California Press, Berkeley
and Los Angeles.
Kartesz J (2010) A synonymized checklist and
atlas with biological attributes for the vascular flora of the United
States, Canada, and Greenland. Floristic synthesis of North America.
Second Edition. CD-ROM Version 2.0. North Carolina Botanical Garden,
Chapel Hill.
McKinney ML (2005) Species introduced from
nearby sources have a more homogenizing effect than species from distant
sources: evidence from plants and fishes in the USA. Diversity and
Distributions 11: 367-374.
doi: 10.1111/j.1366-9516.2005.00181.x
Richardson DM, Pyšek P, Rejmánek M, Barbour
MG, Panetta FD, West CJ (2000) Naturalization and invasion of alien
plants: concepts and definitions. Diversity and Distributions 6:
93-107.
doi: 10.1046/j.1472-4642.2000.00083.x
Simberloff D (2011) “Native invaders.” In:
Simberloff D, Rejmánek M (Eds) Encyclopedia of Biological Invasions.
University of California Press, Berkeley and Los Angeles: 472-474.
Spear D, Chown SL (2008) Taxonomic
homogenization in ungulates: patterns and mechanisms at local and global
scales. Journal of Biogeography 35: 1962-1975.
doi: 10.1111/j.1365-2699.2008.01926.x
USDA & NRCS (2004) The PLANTS Database, Version 3.5. (
http://plants.usda.gov). National Plant Data Center, Greensboro, North Carolina.
Williamson MH (1996) Biological Invasions. Chapman & Hall, London.