Abstracted from a manuscript published in 2009.
Nesom, G.L. 2009.
Assessment of invasiveness and ecological impact in non-native plants of
Texas. J. Bot. Res. Inst. Tex. 3(2):
971–991. [pdf]
Non-native species in Texas ––list and documentation.
A list of
naturalized non-native species in Texas has been developed. Documentation for the occurrence of these
species in Texas is an essential and critical part of both this process and
this report –– it begins with literature and other records that have been
published mostly after 1970, the date of publication of Correll and Johnston’s
“Manual of the Vascular Plants of Texas,” which remains the primary floristic
resource for the state.
Invasiveness and ecological impact of Texas
non-native species.
The system
outlined here for use in Texas emphasizes simplicity, allowing assessment of
the large number of non-native species (all that are known to occur in the
state). Because essential elements of
biology and geography are included in the assessments, the system is termed the
Fundamental Invasiveness Index.
F1 Invasive in
both disturbed and natural habitats.
Negatively affecting native species or natural biodiversity by altering
native vegetation and habitats or by outcompeting or hybridizing with native species; or, invasive into agricultural
habitats and causing significant economic damage. Aquatic species known to occur in 10 or more counties. Woody, herbaceous, and aquatic species. Examples: Arundo donax, Bothriochloa
ischaemum var. songarica, Centaurea
melitensis, Eichhornia crassipes, Ligustrum sinense, Lonicera japonica, Lygodium japonicum, Nandina domestica, Salvinia molesta, Sorghum halepense, Triadica (Sapium) sebifera, Ulmus pumila.
F2
Abundant in number and widespread, commonly invasive in disturbed
habitats, much less commonly in natural habitats.
F2-Woody
Trees, shrubs, subshrubs, and woody vines. Abundant in number and widespread, commonly invasive in disturbed
habitats such as roadsides, fencerows, woods edges, and others, sometimes
potentially or incipiently damaging in natural habitats (as F1). Examples: Albizia julibrissin, Morus
alba, Nicotiana glauca, Poncirus trifoliata, Pyrus calleryana, Vitex agnus-castus.
F2-Herbaceous
Annual and perennial herbs and grasses.
Known invasive and/or clearly expanding in geographic range, primarily
in lawns, roadsides, and other open, disturbed habitats, sometimes in
croplands, widespread and usually abundant but often producing relatively
little biomass. Not significantly
affecting native species or natural biodiversity or otherwise strongly altering
native vegetation and habitats.
Examples: Avena sativa, Bellardia trixago, Coronilla
varia, Cuscuta polygonorum, Daucus carota, Duchesnia indica, Erodium cicutarium, Galium aparine, Gamochaeta coarctata, Hypochaeris microcephala, Lamium amplexicaule, Lolium perenne, Medicago
lupulina, Melilotus officinalis, Stachys
floridana, Stellaria media, Taraxacum officinale, Torilis nodosa, Trifolium repens, Veronica arvensis, Vicia sativa.
F2-Aquatic Aquatic species
known to occur in 1 to 9 counties.
F3
Relatively few in number, known from relatively few localities, usually
in disturbed habitats.
F3-Woody
Trees, shrubs, subshrubs, and woody vines. Relatively few in number, known from relatively few localities,
usually in disturbed habitats, repeatedly introduced or perhaps merely
long-persisting at some localities, not showing aggresively invasive
tendencies, or perhaps incipiently invasive.
Examples: Ardisia crenata, Buddleja
lindleyana, Cinnamomum camphora, Hibiscus syriacus, Koelreuteria
elegans, Manihot esculenta, Photinia serratifolia, Pistacia chinensis, Pyracantha koidzumii, Pyrus
communis, Rosmarinus officinalis.
F3-Herbaceous
Annual and perennial herbs and grasses.
Occurrence outside of cultivation known from only one or a few
populations, usually in disturbed habitats.
Apparently showing little or no increase of abundance or geographic
range since the initial report.
Examples: Cichorium intybus, Gomphrena globosa, Phyllanthus fraternus, Plantago
coronopus. Or in some species, sporadically appearing
from repeated introductions and not reproducing. Examples: Citrullus lanatus, Consolida
orientalis, Ipomoea batatas, Luffa aegyptiaca, Narcissus pseudonarcissus, Tagetes erecta.
F4
Status unknown.
For accurate evaluation of an individual species, knowledge
is required of the following.
* Nativity. Is the
species native or non-native?
* Approximate date of introduction in Texas (e.g., pre-1970,
1970s, 1980s, 1990s). Documentation in
the current system does not provide specific information for species introduced
before 1970 (those included in Correll & Johnston’s Manual of the Vascular
Plants of Texas), but for species recorded since that time, dates are evident,
as much as possible, in a review posted and periodically updated on the
internet (Nesom 2009a).
* Current geographic distribution. See distribution maps in Turner et al. (2003), distribution maps
generated by data from Invaders of Texas (2009), and records from herbaria
(primarily those in Texas).
* Ecological/reproductive behavior in Texas and in other
regions. Based on field experience of
the author and others, published literature, information from herbarium
collections.
* Basic habitat and growth form (aquatic or terrestrial,
herbaceous or woody). Based on field
experience, published literature, information from herbarium collections.
Watch list: non-native species in Texas potentially ranked
as F1.
Many of the
non-native species in Texas are known from relatively few populations. Among these are a significant number that
have been recently recorded for the state and that are known to be both
invasive and ecologically destructive in other regions of the United States or
other parts of the world. These
features characterize the species included on the “Watch List” –– the ones with
high potential to rapidly become destructive in Texas. A number of additional F3 species,
especially the woody ones, and some of those ranked as F4 (“status unknown”) probably
belong on the Watch List.
The Watch List
account probably is the most significant part of the overview of Texas
non-native native plants provided here.
The F1 species and many of the F2s are well-known invaders and already
so widespread that it is unlikely that they can be eradicated or even
controlled except by sustained efforts on local levels. Research toward the possibility of
biological control will be ongoing.
Further, there probably is little hope even of eradicating many of the
Watch List species –– a number of the
woody species are widely cultivated, providing abundant seed sources close to
natural areas, and it is unlikely that plants will be removed from the
cultivated landscape. Some measure of
control might be gained by limiting further planting.
From among the
Watch List species, a subset is indicated as a “Super
Watch List” –– those species that perhaps can be removed from the
Texas landscape before they become impossible to control. These are the
ones to which immediate attention should go. All of the F2 aquatics are included because
of their potential for extremely rapid dispersal and growth.
ACKNOWLEDGEMENTS
The authenticity of this
report, especially in the F1 species and Watch List but also in basic concepts,
owes to contributions from Jed Aplaca, Bill Carr, Norma Fowler, Laura Hansen, Stephan Hatch, Bruce
Hoagland, Eric Keith, Barney Lipscomb, Andy McDonald, Michael McRoberts, Tom
Patterson, Jackie Poole, Mike Powell, Nelson Rich, Monique Reed, David Rosen,
Jason Singhurst, Bruce Sorrie, Billie Turner, Damon Waitt, and Justin Williams.
10
December 2009