Environmental Issues > Wildlands Main Page > All Wildlands Documents

Top of Report


Chapter 4

INVASION BY HARMFUL EXOTIC (NON-NATIVE) PLANTS AND ANIMALS

CHAPTER CONTENTS
Invasion by non-native species: Roads, soil disturbance, and reduced forest cover facilitate invasion by exotic (non-native) species.

Spread into undisturbed areas: Exotics can spread into adjacent undisturbed areas from roads and other disturbed sites.

Damage to ecosystem processes: Exotic species disrupt essential ecosystem processes, including natural succession, nutrient cycling, erosion rates, disturbance regimes, and community composition and dynamics.



Invasion by non-native species: Roads, soil disturbance, and reduced forest cover facilitate invasion by exotic (non-native) species.


Key Finding: Non-native plant species occurred on high-use, low-use, and abandoned forest roads, with the greatest frequency on roads with the highest level of disturbance and lowest percentage of canopy cover.

Source: Parendes, L. A. and J. A. Jones. In press. Light availability, dispersal, and exotic plant invasion along roads and streams in the H. J. Andrews Experimental Forest, Oregon. Conservation Biology.

The authors surveyed exotic (non-native) plant species along road segments and streams in the western Cascades and their association with different levels of light, disturbance, and dispersal mechanisms. Three types of forest roads were studied: high-use roads, low-use roads, and abandoned roads. Five transects were placed in each of the habitat types. The presence or absence of 21 exotic species was recorded, along with light levels (as measured by percentage canopy cover). Nearly three hundred 50 x 2 m sampling units were surveyed.

All of the sample sites on high-use and low-use roads had at least one exotic species present. Roads abandoned for 20 to 40 years varied in terms of exotic species being present or absent, but had up to eight species on some sample units. Exotic species were more frequent along high-use and low-use roads than on abandoned roads. These roads also had higher light levels and a greater frequency of disturbance due to traffic and maintenance. The six most frequently occurring exotic species (occurring in more than 50% of the sample units) were clearly correlated with higher light levels and had a higher frequency on roads that had a greater use. The relationship with plant dispersal ability was relatively complex.

The authors concluded with a discussion of the role roads play in facilitating exotic invasions, by providing suitable habitat of higher light levels due to reduced canopy cover and frequent disturbances, and through transport of seeds on vehicle tires.


Key Finding: Exotic annual plants invaded an ecological reserve in California along a pipeline corridor and were still dominant in the corridor 10 years after the disturbance occurred.

Source: Zink, T. A., M. F. Allen, B. Heindl-Tenhunen and E. B. Allen. 1995. The effect of a disturbance corridor on an ecological reserve. Restoration Ecology 3: 304-310.

Sample plots were established in the Santa Margarita Ecological Reserve in California to investigate plant species composition and soil properties on a pipeline corridor and in four adjacent undisturbed habitats - oak woodland, coastal sage, grassland, and chaparral. Exotic annual plants were found to dominate the entire length of the corridor, with little reestablishment of native plant species, although more than 10 years had passed since the last disturbance. Exotics also were invading adjacent grassland (19% exotic species coverage), coastal sage (16% exotic species coverage) and oak woodland (13% exotic species coverage) communities, but were not present in undisturbed chaparral.

Significantly less organic matter was present in soil layers of the disturbed corridor than in the adjacent undisturbed areas. Available nitrogen and extractable phosphorus were higher in soils of the disturbed corridor. The authors believe that the more rapidly decomposing leaf litter of exotics and higher nutrient mineralization rates in the pipeline corridor will continue to favor dominance by the exotic plant species.


Key Finding: Oriental bittersweet, an exotic vine of the eastern United States, responded vigorously to increased light intensity after disturbances such as road construction, logging, or windthrow.

Source: McNab, W. H. and M. Meeker. 1987. Oriental bittersweet: a growing threat to hardwood silviculture in the Appalachians. Northern Journal of Applied Forestry 4: 174-177.

The authors review the threats posed by the exotic vine oriental bittersweet (Celastrus orbiculatus) and present information collected from foresters' first-hand experiences in the Appalachian mountains. The vine is considered a pest in forested areas of the northeastern and southern United States. It forms impenetrable thickets and maintains dominance through heavy shading of the ground layer. It is a problem for both regeneration and pole-sized timber, rapidly overgrowing seedlings in forest openings and shading out crown foliage in young stands.

Much of the information collected for this article was from the Bent Creek Experimental Forest in North Carolina. Bittersweet seedlings, which can persist in the shade, exhibited rapid growth with increased light from disturbances, such as construction of logging roads, logging, or windthrow. Construction of the Blue Ridge Parkway in the 1960s apparently provided a new, sunny corridor, and oriental bittersweet was observed to expand along the highway and other roads in the Bent Creek Experimental Forest. Oriental bittersweet was present on 79% of road segments sampled. The vine also rapidly overgrew a small clearcut in mature oak forest in the Experimental Forest within seven years.

Once established, oriental bittersweet expands easily into shaded forest. Birds help disperse the seeds, and seedlings of bittersweet can reportedly survive in dense shade. A 15-year-old sapling stand in Bent Creek Experimental Forest was invaded by the vine, which girdled many of the trees and caused stem and crown deformities. These trees were later more susceptible to ice storm damage, as well.

The authors review efforts to control the vine, noting that it is very difficult to control mechanically, although herbicides may be effective.


Key Finding: Spotted knapweed invaded new areas along roadsides.

Key Finding: Spotted knapweed preferred open canopies and disturbed areas.

Source: Marcus, W. A., G. Milner and B. Maxwell. 1998. Spotted knapweed distribution in stock camps and trails of the Selway-Bitterroot Wilderness. Great Basin Naturalist 58: 156-166.

The authors review the history and spread of spotted knapweed (Centaurea maculosa), a non-native plant. The plant was introduced in the early twentieth century and now reportedly covers almost 3 million ha in the northwestern United States. It inhabits areas with a broad range of elevations, soil types, and moisture levels. In forested areas, spotted knapweed poses the greatest threat to low-elevation ponderosa pine and Douglas-fir habitat. The weed is reported to spread rapidly along roads by dispersing from the undercarriage of vehicles. It can also be carried into new forest areas in hay or on camping equipment.

The authors surveyed spotted knapweed in campsites and trails on the Selway-Bitterroot Wilderness (Montana/Idaho). There was no spotted knapweed in camp sites with closed tree canopies. Spotted knapweed was restricted to camps with an open canopy and with high disturbance levels. Along trails, most of the spotted knapweed was within 0.5 km of the trailhead, and abundance decreased with increasing distance (and correspondingly lower disturbance) from the trail.


Key Finding: Spotted knapweed and diffuse knapweed, two exotic species, preferred open, disturbed habitat, including roads, over shaded areas.

Source: Watson, A. K. and A. J. Renney. 1974. The biology of Canadian weeds. Centaurea diffusa and C. maculosa. Canadian Journal of Plant Science 54: 687-701.

The authors summarize information on the distribution and ecology of two exotic plant species in Canada. Spotted knapweed (Centaurea maculosa) is present in British Columbia, Southern Alberta, Ontario, Quebec, and the Maritimes, while diffuse knapweed (Centaurea diffusa) occurs in southern British Columbia and Alberta. Both knapweeds are reported to prefer open habitat and to invade roads, railroad right-of-ways, and overgrazed rangelands. They usually are not found in shaded areas. Both knapweeds colonize a variety of soil types, with density significantly correlated with degree of soil disturbance.

The two weeds outcompete native plant species. Their dense, spiny growth prevents the growth of other species. They are also reported to be allelopathic (chemically inhibiting growth of other species).


Key Finding: Exotic weeds spread along logging roads in forests at all elevations in western Montana.

Key Finding: Exotic weeds invaded clearcuts in mid-elevation forests.

Source: Forcella, F. and S. J. Harvey. 1983. Eurasian weed infestation in western Montana in relation to vegetation and disturbance. Madroño 30: 102-109.

Exotic plant species were surveyed along roadsides and along an elevational gradient in western Montana. Exotic plants occurred along roadsides at all elevations. In grasslands and the low-montane zone (ponderosa pine), exotic weeds had also invaded adjacent communities of native vegetation. In mid-montane forests, exotic plants occupied up to 60% of the plant cover in areas that had been clearcut, but were not present in undisturbed areas. Exotic plants were not present in undisturbed areas of the subalpine zone.

The authors suggest that the amount of light available plays an important role in exotic plant establishment and that loss of canopy cover encourages invasion by weeds. They also note that climate may play a role, with higher weed coverage in lower elevation communities, which have more frost-free months and higher mean July temperatures.


Key Finding: In a regional survey, a greater proportion of anthropogenically disturbed plots in the southeastern and northeastern United States contained at least one exotic species compared to undisturbed plots.

Source: Stapanian, M. A., S. D. Sundberg, G. A. Baumgardner and A. Liston. 1998. Alien plant species composition and associations with anthropogenic disturbance in North American forests. Plant Ecology 139: 49-62.

As part of the Forest Health Monitoring Program (administered by the USDA Forest Service), exotic species were measured on 279 forest plots during the summer of 1994. Plots were distributed in seven regions of the United States - the Northeast, the Mid-Atlantic, the Southeast, California, Minnesota, Colorado, and the Pacific Northwest. However, only four regions * the Southeast, the Northeast, the Mid-Atlantic, and California * had a large enough sample size for statistical analysis. Types of anthropogenic disturbances were recorded for each plot, although more undisturbed plots than disturbed were available, so limiting the information available on effects of disturbance.

A total of 139 alien plant species were recorded. The proportion of plots with at least one exotic species was significantly higher in disturbed areas than undisturbed areas in the Southeast. In the Northeast, a slightly higher proportion of disturbed plots had exotic species, compared to undisturbed plots. Although exotic plant occurrence in the Mid-Atlantic and California did not have a significant relationship with disturbance, exotic species occupied the highest proportion of ground cover in these regions (20% and 25% respectively).


Key Finding: The red imported fire ant, an exotic pest in the southeastern United States, colonized roads, power lines, and forest gaps created by logging.

Key Finding: The density of red imported fire ant mounds was correlated with the degree of soil disturbance and direct sunlight exposure.

Source: Stiles, J. H. and R. H. Jones. 1998. Distribution of the red imported fire ant, Solenopsis invicta, in road and powerline habitats. Landscape Ecology 335: 335-346.

The red imported fire ant (Solenopsis invicta), an exotic pest species, colonizes roads, power lines, and gaps in forests created by logging, displacing native ants, reducing native arthropod diversity and abundance, and increasing predation of young birds, small mammals, and reptiles. The authors report that in South Carolina, roads and power line cuts may provide a source population for dispersal into adjacent forest gaps, where they have observed fire ants.

The authors studied the density and volume of fire ant mounds along four road types with varying canopy cover and disturbance frequency, as well as in power line cuts. Mound density was correlated with light levels and degree of soil disturbance. Mound density was higher along open canopy roads with intermediate to frequent disturbance than roads or power lines with infrequent disturbance. Mean size of mounds was inversely related to frequency of disturbance, with lowest volume in most disturbed habitat. The authors believe that mound density rather than volume is the key factor when evaluating the impact of fire ants.



Spread into undisturbed areas: Exotics can spread into adjacent undisturbed areas from roads and other disturbed sites.


Key Finding: Exotic annual plant species invaded adjacent undisturbed oak woodland, coastal sage, and grassland communities from a pipeline corridor in an ecological reserve in California.

Source: Zink, T. A., M. F. Allen, B. Heindl-Tenhunen and E. B. Allen. 1995. The effect of a disturbance corridor on an ecological reserve. Restoration Ecology 3: 304-310.*

Sample plots were established in the Santa Margarita Ecological Reserve in California to investigate plant species composition and soil properties on a pipeline corridor and in four adjacent undisturbed habitats - oak woodland, coastal sage, grassland, and chaparral. Exotic annual plants were found to dominate the entire length of the corridor, with little reestablishment of native plant species, although more than 10 years had passed since the last disturbance. Exotics also were invading adjacent grassland (19% exotic species coverage), coastal sage (16% exotic species coverage), and oak woodland (13% exotic species coverage) communities, but were not present in undisturbed chaparral.

Significantly less organic matter was present in soil layers of the disturbed corridor than in the adjacent undisturbed areas. Available nitrogen and extractable phosphorus were higher in soils of the disturbed corridor. The authors believe that the more rapidly decomposing leaf litter of exotics and higher nutrient mineralization rates in the pipeline corridor will continue to favor dominance by the exotic plant species.

* See also key finding above.


Key Finding: Exotic weeds spread outward from roadsides in lowland forest and rangeland in Montana, invading relatively undisturbed areas.

Source: Forcella, F. and S. J. Harvey. 1983. Eurasian weed infestation in western Montana in relation to vegetation and disturbance. Madroño 30: 102-109.*

Exotic plant species were surveyed along roadsides and along an elevational gradient in western Montana. Exotic plants occurred along roadsides at all elevations. In grasslands and the low-montane zone (ponderosa pine), exotic weeds had also invaded adjacent communities of native vegetation. In mid-montane forests, exotic plants occupied up to 60% of the plant cover in areas that had been clearcut, but were not present in undisturbed areas. Exotic plants were not present in undisturbed areas of the subalpine zone.

The authors suggest that the amount of light available plays an important role in exotic plant establishment and that loss of canopy cover encourages invasion by weeds. They also note that climate may play a role, with higher weed coverage in lower elevation communities, which have more frost-free months and higher mean July temperatures.

* See also key finding above.


Key Finding: Originally confined to roadways and abandoned farmland, cheatgrass now invades shrub, ponderosa pine, and pinyon-juniper ecosystems.

Source: Monsen, S. B. 1994. The competitive influences of cheatgrass (Bromus tectorum) on site restoration. pp. 43-50 in Proceedings ( Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen, eds. INT-GTR-313. USDA Forest Service. Intermountain Research Station.

The author reviews the spread of cheatgrass (Bromus tectorum) in the Intermountain Basin and the Columbia Basin and describes the resulting cycle of increasing fire frequency and cheatgrass abundance. Initially reported on roadways and abandoned croplands in the early 1900s, cheatgrass has now invaded shrub systems as well as ponderosa pine forests and pinyon-juniper woodlands. Fire frequency has increased on all these sites as a result. An aggressive species, cheatgrass outcompetes many native species and is quick to reestablish after a fire.

The author also reviews restoration attempts in cheatgrass-dominated areas. On treated areas, few native species planted for restoration can compete successfully, and cheatgrass from untreated areas can rapidly reoccupy treated sites. Mechanical tilling and herbicides are necessary for any reseeding attempts to be successful. It is also difficult to prevent cheatgrass from spreading. However, cheatgrass has been documented to naturally disappear from some areas after periods of drought.


Key Finding: The red imported fire ant, an exotic pest in the southeastern United States, is believed to disperse into forest gaps from adjacent roads and power lines.

Source: Stiles, J. H. and R. H. Jones. 1998. Distribution of the red imported fire ant, Solenopsis invicta, in road and powerline habitats. Landscape Ecology 335: 335-346.*

The red imported fire ant (Solenopsis invicta), an exotic pest species, colonizes roads, power lines, and gaps in forests created by logging, displacing native ants, reducing native arthropod diversity and abundance, and increasing predation of young birds, small mammals, and reptiles. The authors report that in South Carolina, roads and power line cuts may provide a source population for dispersal into adjacent forest gaps, where they have observed fire ants.

The authors studied the density and volume of fire ant mounds along four road types with varying canopy cover and disturbance frequency, as well as in power line cuts. Mound density was correlated with light levels and degree of soil disturbance. Mound density was higher along open canopy roads with intermediate to frequent disturbance than roads or power lines with infrequent disturbance. Mean size of mounds was inversely related to frequency of disturbance, with lowest volume in most disturbed habitat. The authors believe that mound density rather than volume is the key factor when evaluating the impact of fire ants.

* See also key finding above.



Damage to ecosystem processes: Exotic species disrupt essential ecosystem processes, including natural succession, nutrient cycling, erosion rates, disturbance regimes, and community composition and dynamics.


Key Finding: Oriental bittersweet, an exotic vine in the eastern United States, inhibited seedling regeneration and damaged young hardwood stands through stem girdling.

Source: McNab, W. H. and M. Meeker. 1987. Oriental bittersweet: a growing threat to hardwood silviculture in the Appalachians. Northern Journal of Applied Forestry 4: 174-177.*

The authors review the threats posed by the exotic vine oriental bittersweet (Celastrus orbiculatus) and present information collected from foresters' first-hand experiences in the Appalachian mountains. The vine is considered a pest in forested areas of the northeastern and southern United States. It forms impenetrable thickets and maintains dominance through heavy shading of the ground layer. It is a problem for both regeneration and pole-sized timber, rapidly overgrowing seedlings in forest openings and shading out crown foliage in young stands.

Much of the information collected for this article was from the Bent Creek Experimental Forest in North Carolina. Bittersweet seedlings, which can persist in the shade, exhibited rapid growth with increased light from disturbances, such as construction of logging roads, logging, or windthrow. Construction of the Blue Ridge Parkway in the 1960s apparently provided a new, sunny corridor, and oriental bittersweet was observed to expand along the highway and other roads in the Bent Creek Experimental Forest. Oriental bittersweet was present on 79% of road segments sampled. The vine also rapidly overgrew a small clearcut in mature oak forest in the Experimental Forest within seven years.

Once established, oriental bittersweet expands easily into shaded forest. Birds help disperse the seeds, and seedlings of bittersweet can reportedly survive in dense shade. A 15-year-old sapling stand in Bent Creek Experimental Forest was invaded by the vine, which girdled many of the trees and caused stem and crown deformities. These trees were later more susceptible to ice storm damage, as well.

The authors review efforts to control the vine, noting that it is very difficult to control mechanically, although herbicides may be effective.

* See also key finding above.


Key Finding: Tree seedling density decreased with increasing cover of an exotic honeysuckle, Lonicera tatarica.

Key Finding: The diversity and density of herbaceous species declined as honeysuckle cover increased in three of four northeastern forest stands.

Source: Woods, K. D. 1993. Effects of invasion by Lonicera tatarica L. on herbs and tree seedlings in four New England forests. American Midland Naturalist 130: 62-74.

The impacts of an exotic honeysuckle species, Tatarian honeysuckle (Lonicera tatarica), on herb species richness, density, and tree seedling success was studied in four stands in southwestern Vermont and northwestern Massachusetts. Sample plots were 2 x 2 m, further divided into four 1 m2 quadrats.

Herb richness and cover decreased as honeysuckle cover increased in all three Vermont stands. No effect of competition was apparent in the Massachusetts stand, a site with poor soils. The authors believe that moisture may have been the primary variable driving both herb and honeysuckle cover at this site, where honeysuckle was confined to moist microsites. The effects of honeysuckle on native herbaceous species varied by species, probably due to varying life-history traits.

Density of tree seedlings (less than 1 m tall) declined significantly with increasing honeysuckle cover in all stands. Seedlings established prior to the honeysuckle invasion (1-2 m tall) were not affected by the invading honeysuckle.

The study did not evaluate causal mechanisms, but the authors hypothesize that honeysuckle may outcompete other species by leafing out earlier in the spring than trees and other woody species, thereby shading the forest floor at a time critical for the development of many herbs.


Key Finding: In shrub-steppe ecosystems, invading weed species, which were usually non-mycorrhizal, disrupted succession by native species, 99% of which were mycorrhizae-dependent.

Source: Wicklow-Howard, M. C. 1994. Mycorrhizal ecology of shrub-steppe habitat. pp. 207-210 in Proceedings ( Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen eds. INT-GTR-313. USDA Forest Service. Intermountain Research Station.

The author reviews the importance of mycorrhizal fungi in shrub-steppe habitat in southwestern Idaho, including their crucial role in acquiring nutrients and water. Studies show that 99% of native plants on undisturbed sites were mycorrhizal, while only 1% of successful exotics were mycorrhizal. The author discussed the long-term impact this may have on succession by native plants in disturbed sites. Sites disturbed and then invaded by exotics reportedly had no vesicular-arbuscular mycorrhizae for up to 10 years. With no host plants to support the mycorrhizae, the fungal propagules may not be able to survive. As a result, establishment of mycorrhizal-dependent plants is expected to be difficult.


Key Finding: An exotic weed, bull thistle, reduced growth rates of ponderosa pine seedlings by up to 33% in a forest plantation.

Source: Randall, J. M. and M. Rejmanek. 1993. Interference of bull thistle (Cirsium vulgare) with growth of ponderosa pine (Pinus ponderosa) seedlings in a forest plantation. Canadian Journal of Forest Research 23: 1507-1513.

In the absence of other vegetation, the exotic bull thistle Cirsium vulgare was found to suppress the growth of ponderosa pine seedlings in a plantation in the western Sierra Nevada. The relative growth rate of ponderosa pine seedlings was negatively correlated with the density of thistles growing within 2 m of seedlings. Growth was reduced by 25-33 % in the treatments having the highest density of thistles. The authors did not determine the causal mechanism. They note that soil moisture, nitrogen, phosphorus, and potassium limitations were not a factor. Competition for nutrients not examined may have been a factor limiting seedling growth or bull thistle may be allelopathic.


Key Finding: Forest litter depth and soil organic layers were lower and pH was higher in sites invaded by two exotic plant species (Japanese barberry and a Japanese grass species), when compared to adjacent uninvaded forest sites.

Key Finding: Native oaks and shrubs occurred at a lower density in forested sites invaded by Japanese barberry and a Japanese grass species than in uninvaded sites.

Source: Kourtev, P. S., J. G. Ehrenfeld and W. Z. Huang. 1998. Effects of exotic plant species on soil properties in hardwood forests of New Jersey. Water, Air and Soil Pollution 105: 493-501.

The effect of invasion by two exotic plant species, Japanese barberry (Berberis thunbergii) and a Japanese grass (Microstegium vimineum) was examined in three deciduous hardwood forests of northern New Jersey. The authors established transects extending through invaded sites and adjacent uninvaded sites and sampled vegetation, woody debris and soils at 50-m intervals.

For all three sites, significant differences in soil and vegetation characteristics existed between invaded and uninvaded plots. The pH of soils in the invaded plots was significantly higher than in the uninvaded plots. Soil organic layers and litter were thinner in invaded plots than in uninvaded plots. Invaded areas had fewer oaks (Quercus spp.) in the canopy and lacked native understory shrubs (Vaccinium spp.).


Key Finding: Surface runoff and soil erosion were greater from spotted knapweed-dominated sites than natural bunchgrass-dominated sites.

Source: Lacey, J. R., C. B. Marlow and J. R. Lane. 1989. Influence of spotted knapweed (Centaurea maculosa) on surface runoff and sediment yield. Weed Technology 3: 627-631.

The authors compared 12 paired plots, dominated either by the exotic, spotted knapweed (Centaurea maculosa), or by natural bunchgrass communities. A 30-minute simulated rainfall was conducted on each plot to measure surface runoff and sediment yield. Spotted knapweed plots had lower average infiltration rates, higher surface runoff, and greater sediment yield than bunchgrass plots. However, there was great variability in volume of runoff and sediment. Slope and percent vegetation cover explained much of the variation in sediment yield, and slope was a significant factor in predicting surface runoff.


Key Finding: Fires have become more common and extensive in pinyon-juniper woodlands and sagebrush ecosystems invaded by cheatgrass, an exotic grass.

Source: Billings, W. D. 1994. Ecological impacts of cheatgrass and resultant fire on ecosystems in the western Great Basin. pp. 22-30 in Proceedings - Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen eds. INT-GTR-313. USDA Forest Service. Intermountain Research Station.

The author reviews research on cheatgrass (Bromus tectorum), an exotic grass in the western United States. Cheatgrass is highly competitive and has replaced most of the natural bunchgrasses in the sagebrush ecosystem of the Great Basin. Growing more densely than the naturally scattered bunchgrasses, it utilizes much of the open space between shrubs. During the dry summers of this region, cheatgrass rapidly dries out and becomes an abundant and pervasive source of fuel. Extensive and disastrous fires became common in the sagebrush ecosystem of the western Great Basin in the mid-1930s.

By the 1950s, cheatgrass had also invaded pinyon-juniper woodlands. Fires became more common. Trees were slow to return, with little cover reestablished even after 60 years. Cheatgrass, however, was quick to reestablish, and these woodlands have been replaced by extensive areas of annual grasslands.

The author monitored the effects of cheatgrass on permanent plots established in a Great Basin sagebrush community for 47 years in Nevada. After a large wildfire of 260 ha, little above-ground vegetation remained. More than 40 years after the fire, some shrub species had still not been able to reestablish, while cheatgrass had rapidly returned and increased in abundance.


Key Finding: The incidence of fire has increased in ponderosa pine forests and pinyon-juniper woodlands where cheatgrass, an exotic annual, has invaded. This grass has proven difficult to control.

Source: Monsen, S. B. 1994. The competitive influences of cheatgrass (Bromus tectorum) on site restoration. pp. 43-50 in Proceedings ( Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen, eds. INT-GTR-313. USDA Forest Service. Intermountain Research Station.*

The author reviews the spread of cheatgrass (Bromus tectorum) in the Intermountain Basin and the Columbia Basin and describes the resulting cycle of increasing fire frequency and cheatgrass abundance. Initially reported on roadways and abandoned croplands in the early 1900s, cheatgrass has now invaded shrub systems as well as ponderosa pine forests and pinyon-juniper woodlands. Fire frequency has increased on all these sites as a result. An aggressive species, cheatgrass outcompetes many native species and is quick to reestablish after a fire.

The author also reviews restoration attempts in cheatgrass-dominated areas. On treated areas, few native species planted for restoration can compete successfully, and cheatgrass from untreated areas can rapidly reoccupy treated sites. Mechanical tilling and herbicides are necessary for any reseeding attempts to be successful. It is also difficult to prevent cheatgrass from spreading. However, cheatgrass has been documented to naturally disappear from some areas after periods of drought.

* See also key finding above.


Key Finding: Invasion in Texas by the red imported fire ant resulted in a 90% decrease in native ant abundance and a 70% decrease in ant species richness.

Source: Porter, S. D. and D. A. Savignano. 1990. Invasion of polygyne fire ants decimates native ants and disrupts arthropod community. Ecology 71: 2095-2106.

The authors studied the ecological impact of invasion by the exotic fire ant Solenopsis invicta in central Texas. Multiple-queen (polygyne) colonies, which have five to 10 times higher nest densities than single-queen colonies, are reported to be increasing in frequency. The authors used pitfall and bait traps to measure ant abundance and diversity in a 32-ha tract of woods and grassy fields. Density of fire ant mounds ranged from 60 mounds/ha in wooded areas to 400 mounds/ha in open areas.

The fire ant populations had a significant adverse effect on the native ant population, which declined in number of individuals by 90%. Species richness of ants decreased by 70%. The abundance and diversity of other arthropods were also affected, declining by 75% and 30% respectively in fire ant infested areas.

Fire ants were 10 to 30 times more numerous than the native ants had been before invasion. Competition for food and nesting sites is the most likely explanation for fire ant success over native ant species.


Key Finding: Native ants had a lower abundance and diversity in areas invaded by the Argentine ant, an exotic ant.

Key Finding: The trophic structure of invertebrate communities changed in areas invaded by Argentine ants, with higher numbers of scavengers at the expense of herbivores, predators, and parasites.

Source: Human, K. G. and D. M. Gordon. 1997. Effects of Argentine ants on invertebrate biodiversity in northern California. Conservation Biology 11: 1242-1248.

A pitfall trapping study was conducted in Jasper Ridge Biological Preserve, northern California, to study the impact of the Argentine ant (Lipepithema humile) on native ant species. Approximately 30% of the preserve had been invaded by Argentine ants, primarily along edges. Sampling was done in grassland, open oak woodland, closed canopy oak forest, and chaparral sites. Captured non-ant invertebrates were classified according to their diet - predators, herbivores, parasites, and scavengers (which included saprophages and mycophages).

Native ant abundance was lower in invaded than uninvaded areas: 19 native ants were trapped in the invaded areas compared to 1,994 in uninvaded areas. Ant communities were also less diverse. Below-ground foraging ants seemed to suffer less displacement by the Argentine ant than above-ground species and composed a much greater percentage of native ants in invaded areas. The total number of ants (including Argentine ants) was higher in invaded areas than in uninvaded areas.

Non-ant invertebrate communities were also less diverse in invaded areas, and the trophic structure had changed. Scavengers were overrepresented in invaded areas, and herbivores, predators, and parasites were underrepresented. Changes in populations were attributed to competition and some predation, although Argentine ants are primarily scavengers.


Key Finding: Northern bobwhite populations in Texas decreased after invasion by the non-native red imported fire ant.

Key Finding: Densities of northern bobwhites increased after treatment to reduce infestation by the red imported fire ant.

Source: Allen, C. R., R. S. Lutz and S. Demarais. 1995. Red imported fire ant impacts on northern bobwhite populations. Ecological Applications 5: 632-638.

The vulnerability of northern bobwhite (Colinus virginianus) populations to red imported fire ants (Solenopsis invicta), a non-native ant species, was studied in 15 Texas counties. Christmas Bird Count data were used for determining bobwhite abundances. No trend in bobwhite abundance was detected during the 12 years prior to fire ant infestation. The species abundance declined, however, after fire ant infestation, while uninvaded counties showed no decline.

Field experiments were also conducted to determine the efficacy of treating ant-infested sites. Densities of northern bobwhites increased on treated areas. The mechanism through which red fire ants affect northern bobwhites was not determined. The birds' eggs may be vulnerable to predation, or declines in the native insect communities may reduce their food source.

Sign up for NRDC's online newsletter

See the latest issue >

Give the Gift That Will Make a Difference: Leader of the Pack

NRDC Gets Top Ratings from the Charity Watchdogs

Charity Navigator awards NRDC its 4-star top rating.
Worth magazine named NRDC one of America's 100 best charities.
NRDC meets the highest standards of the Wise Giving Alliance of the Better Business Bureau.


Donate now >

Related Stories

Q&A: Documentary Filmmaker Ken Burns on National Parks
Ken Burn spoke to OnEarth about his motivation for his new documentary series on America's national parks.
In the Canadian Boreal Forest, a Conservation Ethic at Work
After fighting successfully for years to keep destructive logging, hydropower and mining projects out of their traditional territory, the people of Poplar River are now working to secure permanent protection for their boreal forest homeland.
Shop Smart, Save Forests
Share | |
Find NRDC on
YouTube