There are 394 species known to be naturalized in North Central Texas from outside the United States since the time of Columbus. They make up 17.7 % of the North Central Texas flora (the percentage would be slightly higher if species that have invaded Texas from elsewhere in the United States were included). These introduced taxa are variously referred to as alien, exotic, or foreign species. They are also sometimes called “weeds,” but that word can have different meanings (Randall 1997). From the sociological standpoint a weed is a plant growing where it is not wanted or a “plant-out-of-place” (Stuckey & Barkley 1993); if defined in this way, introduced species are indeed often weeds. Biologically, weeds (sometimes termed colonizing or invasive plants) are species that “have the genetic endowment to inhabit and thrive in places of continual disturbance, most especially in areas that are repeatedly affected by the activities of humankind” (Stuckey & Barkley 1993). Again, many introduced plants fall within this definition of weedy species.
While introduced species include some of our most beautiful ornamentals (e.g., Iris, Narcissus, and Wisteria species), some are also extremely invasive taxa capable of becoming serious agricultural pests or of destroying native habitats. Luken and Thieret (1997) examined the assessment and management of plant invasions and gave a selected list of species interfering with resource management goals in North America. Particularly problematic are those that aggressively invade native ecosystems, reproduce extensively, and occupy the habitat of indigenous species. In some cases, single invasive species can come to dominate communities and occur in near monocultures, completely changing the species composition, structure, and aspect of an ecosystem. After habitat destruction, invasion by exotics may be the most serious threat facing native plants in North Central Texas and it is a common but underestimated problem in many ecosystems around the world (Cronk & Fuller 1995). It is also a potentially lasting and pervasive threat (Coblentz 1990). According to Cronk and Fuller (1995), “It is a lasting threat because when exploitation or pollution stops, ecosystems often begin to recover. However, when the introduction of alien organisms stops the existing aliens do not disappear; in contrast they sometimes continue to spread and consolidate, and so may be called a more pervasive threat.” Invasive exotics are an example of the phenomenon of ecological release—an introduced species is released from the ecological constraints of its native area (e.g., diseases, parasites, pests, predators, nutrient deficiencies, etc.) and is consequently able to undergo explosive population growth. There are numerous examples in North Central Texas, some of the most serious including Bothriochloa ischaemum var. songarica (King Ranch bluestem), Festuca arundinacea (tall fescue), Hydrilla verticillata (hydrilla), Lespedeza cuneata (sericea lespedeza or Chinese bush-clover), Ligustrum sinense (Chinese privet), Lonicera japonica (Japanese honeysuckle), Pueraria montana var. lobata (kudzu), and Sorghum halapense (Johnson grass). For example, kudzu, an aggressive vine, can completely cover native forests (e.g., in the southeastern United States) and, unfortunately, it is well established in a number of North Central Texas counties (Grayson, Lamar, and Tarrant). Festuca arundinacea is capable of invading intact native tallgrass prairies and is considered by some (e.g., Fred Smiens, pers. comm.) to be the most serious invasive threat to tallgrass Blackland Prairie remnants (such as the Nature Conservancy’s Clymer Meadow in Hunt County).
Some exotic species are currently spreading in North Central Texas. For example, the offensive Carduus nutans subsp. macrocephalus (musk-thistle or nodding-thistle) is each year becoming more abundant in the northern part of North Central Texas (e.g., Grayson Co.). A possibly even more serious threat, Scabiosa atropurpurea (pincushions or sweet scabious) is currently taking over roadsides and adjacent areas in the northern part of North Central Texas (e.g., Collin Co.) and has the potential of becoming one of the most destructive invasive exotics in the area. From the aquatic standpoint, Hydrilla verticillata is a serious pest which can completely dominate aquatic habitats eliminating native species, clogging waterways, and severely curtailing recreational use (Steward et al. 1984; Flack & Furlow 1996). It is rapidly spreading at present in North Central Texas (M. Smart, pers. comm.), probably from lake to lake by boats or boat trailers and also intentionally by fishermen (L. Hartman, pers. comm.) to “improve” the habitat. This activity is both illegal and ill-advised since it ultimately degrades the fishery. In fact, because of their potential as problematic invaders, five aquatic species that occur in North Central Texas, Alternanthera philoxeroides (alligator-weed; Amaranthaceae), Eichhornia crassipes (common water-hyacinth; Pontederiaceae), Hydrilla verticillata (Hydrocharitaceae), Myriophyllum spicatum (Eurasian water-milfoil; Haloragaceae), and Pistia stratiotes (water-lettuce; Araceae), are considered “harmful or potentially harmful exotic plants” and it is illegal to release, import, sell, purchase, propagate, or possess them in the state (Harvey 1998).
These alien taxa are from nearly all parts of the world (e.g., Bromus catharticus, rescue grass, from South America; Chenopodium pumilo, ridged goosefoot, from Australia; Eragrostis curvula, weeping love grass, from Africa; Bothriochloa ischaemum var. songarica, King Ranch bluestem, from Asia; Stellaria media, common chickweed, from Europe) and have gotten to North Central Texas in assorted ways. However, most weeds in eastern North America, including many in North Central Texas, are from central and western Europe. It is thought that many
weedy colonizing species evolved in Europe over thousands of years as humans disturbed and modified the environment for agricultural purposes; these same species do well in the disturbed habitats of the eastern United States (Stuckey & Barkley 1993). Numerous such European species entered North America at seaport cities along the Atlantic coast and spread westward across the continent (Stuckey & Barkley 1993). An excellent example of this phenomenon can be seen with Chaenorrhinum minus (dwarf snapdragon), which was first observed growing in North America in New Jersey in 1874 (Martindale 1876) and has since spread to over 30 states and nine Canadian provinces (Widrlechner 1983). In some cases, seeds were introduced with soil, sand, or rocks being used as ballast in seagoing ships; Mühlenbach (1979) discussed the role of maritime commerce in dispersal. Other currently problematic taxa were intentionally introduced as ornamentals (e.g., Ligustrum species, privets), as windbreaks (e.g., Tamarix species, saltcedars), or in misguided attempts at habitat improvement, erosion control, soil stabilization, etc. In yet other cases, exotics are thought to have been accidentally introduced with crop seeds (e.g., Myagrum perfoliatum), hay (e.g., Carduus nutans subsp. macrocephalus), cotton or wool, or are associated with livestock yards (e.g., Onopordum acanthium, Scotch-thistle). Still others are transported by trains (e.g., Chaenorrhinum minus—Widrlechner 1983); Mühlenbach (1979) discussed the importance of railroads as a means of dispersal. A particularly unusual dispersal mechanism is suspected for Soliva pterosperma, lawn burweed or stickers, which is thought to have been introduced into North Central Texas at a soccer field by the spinulose achenes sticking in athletic shoes.
The percentage of exotics in the North Central Texas flora—17.7% as stated above—is approximately what would be expected based on data from other parts of the United States. Elias (1977) estimated the level of exotics at 22% in the northeastern United States and more recently Stuckey and Barkley (1993) indicated that in northeastern states the percentage of foreign species ranged from 20% to over 30%. Their data, compiled from a number of sources, showed that there are higher percentages of foreign species in those states that have been occupied the longest by non-native inhabitants and in those that have been most extensively involved in agriculture. Some northern and western states, with less human influence and disturbance, have figures below 20%. While rather recently colonized by European settlers, North Central Texas, particularly the Blackland Prairie portion, has been extensively cultivated and numerous exotic species have arrived and become naturalized. Comparable percentages of foreign species are seen in the floras of California (17.5%) (Rejmánek & Randall 1994), Colorado (16%), Iowa (22.3%), Kansas (17.4%), and North Dakota (15%) (Stuckey & Barkley 1993).
Several introduced species have only recently been reported in North Central Texas, including Chaenorrhinum minus (dwarf snapdragon) (Diggs et al. 1997), Cerastium pumilum (dwarf mouse-ear chickweed), and Stellaria pallida (lesser chickweed) (Rabeler & Reznicek 1997). As this book was nearing completion, another European species, Agrostemma githago (corn-cockle), was discovered in the area (O’Kennon s.n., Parker Co.) as were two exotics new to Texas, Cerastium brachypetalum (gray chickweed) (Rabeler 1333, Red River Co.) and Plantago coronopus (buck-horn plantago) (O’Kennon 14221, Tarrant Co.) (O’Kennon et al. 1998). Additional exotics can be expected to become part of the North Central Texas flora in the future, many with serious negative consequences to the remnant native flora.
From the introduction of Shinners & Mahler’s Illustrated Flora of North Central Texas; copyright 1999 BRIT and Austin College