Economic and Ecological Effects

Invasive nonnative pests cost the United States an estimated $137 billion per year (Pimentel and others 2000). This figure does not include the costs of species extinctions. Of the 958 listed threatened and endangered species, 57 percent are at risk primarily because of competition with and predation by invasive nonnatives (Reichard and White 2001). It is difficult or impossible to accurately and objectively determine the cost of species extinctions or of less severe damage to species and habitats. For this reason, natural resource losses are more difficult to estimate than agricultural losses. Forest product industries, although they represent only a small part of total forest value, are easier to evaluate economically. National losses in traditional forest products due to nonnative invasive insects and pathogens were estimated at $4.2 billion per year (Pimentel and others 2000). It has been estimated that 360 nonnative insects have become established in American forests (Liebhold and others 1995).

Data specific to southern forests are scarce, especially for invasive nonnative weeds. Although no comprehensive figures specific to forestry losses due to nonnative weeds are available, the State of Florida has compiled some impressive statistics for invasive nonnative weeds in wetlands. Their control costs for melaleuca (Melaleuca spp. L.) alone are $3 to $6 million per year and for purple loosestrife (Lythrum salicaria L.) $45 million per year. Florida spends $14.5 million per year to control Hydrilla spp. L.C. Rich., and still estimates losses in recreation values for just two lakes at $10 million per year (Pimentel and others 2000).

Since European settlement, nonnative forest pests have changed the composition and function of eastern forests in important ways. For example, as early as 1864, American chestnut [Castanea dentata (Marsh.) Borkh.] trees were being eliminated from the Southern Appalachian Mountains, although the cause was not discovered until 1932. Ink disease, caused by the nonnative pathogen Phytophthora cinnamomi Rands, virtually eliminated American chestnut in valleys and coves and gradually was extending upslope when chestnut blight [Cryphonectria parasitica (Murrill) Barr] arrived and removed the remaining trees, which occupied drier ridges (Crandall and others 1945, Hansen 1999). P. cinnamomi continues to impact southern forests, causing littleleaf disease of shortleaf pine (Pinus echinata Mill.), root rot on Fraser fir [Abies fraseri (Pursh) Poir.] Christmas trees, a decline syndrome in loblolly pine (P. taeda L.), and hundreds of other hosts. This same fungus killed 79 percent of the flora in the forests of Western Australia (Weste and Marks 1987) and was recently cited as causing an oak (Quercus spp. L.) mortality epicenter in Mexico (Tainter and others 1999).

The oak component in Kentucky, Virginia, and North Carolina is under attack from the advancing front of gypsy moth [Lymantria dispar (L.)]. The same forests may soon be threatened by a new species of Phytophthora now causing sudden oak death (Phytophthora ramorum Werres, de Cock & Man in’t Veld) in parts of California. An outbreak of this disease in Oregon is being eradicated, but pathologists are conducting surveys to determine whether other, undetected outbreaks may exist. Beech bark disease (Neonectria galligena), dogwood anthracnose (Discula destructiva Redlin), and butternut canker (Sirococcus clavigignenti-juglandacearum) have reduced host populations as they spread through the understory. Adelgids [Adelges piceae (Ratzeburg)] attacking balsam fir [Abies balsamea (L.) Mill.] are causing losses of rare and threatened species dependent upon the special habitat associated with the fir (Alsop and Laughlin 1991). Similar losses are anticipated in hemlock forest types (Tsuga spp. Carr.) as the hemlock woolly adelgid [Adelges tsugae (Annand)] spreads south.

The threats posed by diseases and insect pests have long been recognized by the forestry community. In contrast, invasive nonnative forest plants are more insidious and have received far less attention from foresters. Although weeds cause losses roughly equivalent to those caused by insects and diseases in agricultural systems (Pimentel 1993), the frequent reliance of nonnative plants on disturbance as an entrée to invasion has led to the expectation that such invasions, therefore, are less significant in forests. However, this expectation has proven to be false for two reasons. First, a number of invasive weeds establish successfully without disturbance. Among them are garlic mustard [Alliaria petiolata (Bieb.) Cavara & Grande], oriental bittersweet (Celastrus orbiculatus Thunb.), and melaleuca. Second, forests are subject to frequent disturbances of various origins. Invasive nonnative plants often proliferate after harvests, fire, windthrow, or hurricanes, which create gaps of disturbed habitat. The increasing occupation of forests by nonnative plants has also been linked to increasing anthropogenic disturbance (Stapanian and others 1998). Such plants inhibit regeneration of native plants and reduce forest growth and yield. Invasive nonnative weeds can alter ecosystems by changing nutrient cycling, geomorphology and physical structure of the site, drainage patterns and water flow, sedimentation rates, and disturbance regimes. They displace native flora by competition, and thus alter wildlife habitat (D’Antonio 2001, Reichard and White 2001).