Ecological Impact of Invasive Species: Why We Should Care

Ecological Impact of Invasive Species: Why We Should Care

As this blog comes to a close, it is time to reiterate the impact of invasive species on ecosystems.  The regional distinctiveness of the earth’s biota developed over millions of years of evolution.  Biodiversity was maintained by the isolation of the continents.  While invasions may have occurred occasionally, human activities have greatly accelerated introduction of non-native species.  As a single phenomenon, biological invasions have had a greater impact on the world’s biota than many of the other aspects of global environmental change such as rising CO₂ levels, climate change, and decreasing ozone layer.  As seen in previous blogs, non-native species have displaced or replaced native plants and animals, disrupted nutrient and fire cycles, and changed the pattern of plant succession (Bhowmik 2005).  Invasions by non-native plants, animals, fungi, and microbes are believed to be responsible for greater losses of biological diversity than any other factor except habitat loss and direct exploitation of organisms by humans (Bhowmik 2005).

Invasive plants tend to have many similar biological attributes relating to high reproductive potential and stress tolerance.  These traits include: 1) rapid seedling growth and maturation, 2) ability to reproduce at an early stage, 3) ability to reproduce by vegetative propagules as well as by seeds, 4) ability to produce viable seeds, 5) seed dormancy ensuring periodic germination, 6) diverse dispersal mechanisms and high dispersal rate, 7) high rates of photosynthesis, respiration, transpiration, and growth, 8) high acclimation capability, 9) tolerance of high habitat disturbance, and 10) rapid and generalistic response to resource availability (Bazzaz 1986).

                                                                          

In previous blogs, I discussed the mechanisms that invasive species use to gain a survival advantage over competition.  The diagram in figure 1 summarizes some of these mechanisms (Wolfe 2005).

Figure 1

 

As is seen above in Figure 1: One, greater litter production can lead to greater fire potential.  Two, root exudate can alter resource availability for belowground communities.  Three, invasive species can release novel chemicals with antimicrobial activities.  Four, novel nutrient acquisition strategies, such as nitrogen fixation, can alter biogeochemical processes.  Five, invasive plant roots can induce differences in the local soil environment.

 

Figure 2

 

As previously discussed in blog 9 and seen in Figure 2, an altered fire regime with the grass-fire positive feedback is another mechanism used by invasive plants to alter ecosystems and replace normal woody plant succession with permanent grasslands (Vitousek 1996).

While some advocate control of invasive plants with herbicides and physical removal, biological control with natural enemies (herbivorous insects and fungal pathogens) from the plants’ native range has been more successful.  In order to be used safely, the agents should be genus or species specific for the targeted plant and proper risk analysis procedures should be followed.  The advantages are that these organisms are self-producing and self-sustaining and can be used against weeds that are difficult to control or occur in areas that are ecologically sensitive and prone to harm if chemical or mechanical means are used for their control.  The best results are seen with weeds that are not closely related to native species.

Figure 3

While it would be desirable to test the potential efficacy of the agent before release (Figure3, Step3), how biological control agents perform in nature will depend on a complex set of population processes that cannot be tested in the laboratory and include the effects of parasitoids, predators, competitors, and climate (Van Driesche 2013).  Ecosystem management must be determined individually for each type of habitat. 

The consequences of biological invasions on ecosystems demonstrate the importance of species properties in controlling ecosystem properties.  Plants are most capable of altering ecosystem characteristics when they differ in life form from native plants.  Invasive plants can establish positive feedbacks that magnify the ecosystem-level impact of biological invasions (Vitousek 1986).

Why should we care about the impact of invasive species?  Almost half of the native species in the US are endangered due to invasive species, which is the second greatest threat to biodiversity.  They alter ecosystem processes, as is seen in the change in the frequency and severity of a fire regime caused by invasive grasses.  It is estimated that invasive species cause $120 billion per year in economic losses in the US as a result of loss of crop and infestation management.   Human activities (intentional and inadvertent) are responsible for the spread of invasive species.  The longer the problem is ignored, the greater the severity.  Collectively we have to be much more cautious when introducing anything non-native into an ecosystem; they do not recognize manmade boundaries.  We must be informed; most states have an exotic or invasive species council. The bottom line: invasive species change ecosystems, and these changes make it difficult or impossible for native plants and animals to survive in these affected ecosystems (What Are Invasive Species and Why Should We Be Concerned About Them 2013).

References

1. Bazzaz, F. A. (1986). Life History of Colonizing Plants: Some Demographic, Genetic, and Physiologic Features. Ecology of Biological Invasions of North America and Hawaii. Springer-Verlag, New York.

2. Bhowmik, Prasanta C. (2005). Characteristics, significance, and human dimension of global

invasive weeds. Invasive plants : ecological and agricultural aspects. Birkhäuser Verlag, Berlin.

3. Van Driesche, Roy and Center, Ted. (2013).Biological Control of Invasive Plants in Protected Areas. Plant Invasions in Protected Areas. Springer, New York

4. Vitousek, Peter M. (1986). Biological Invasions and Ecosystem Properties. Ecology of Biological Invasions of North America and Hawaii. Springer-Verlag, New York.

5. Vitousek, Peter M., et al. (1996). Biological Invasions as Global Environmental Change. American Scientist. 84:468-478.

6. What Are Invasive Species and Why Should We Be Concerned About Them. (2013). Extension. http://www.extension.org/pages/62270/what-are-invasive-species-and-why-should-we-be-concerned-about-them#.VHkBTzHF8YE.

7. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.

Grass Fire/Altered Disturbance Regime

Grass Fire/Altered Disturbance Regime

The most significant ecosystem level effect of invasive grasses is that caused by fire.  Fire is a type of disturbance, and ecosystems are partly defined on the basis of disturbance regimes.  In the case of fire, we describe the regime by frequency, intensity, extent, type (ground, surface, crown), and seasonality (Brooks 2004). Disturbance patterns affect ecosystem properties such as the rates of soil erosion or formation and the pathways and temporal patterns of nutrient cycling and energy flow. Disturbances act as a selective force affecting the life history traits of individual species and the composition, structure, and properties of entire groups of organisms.  In the past, the fire regime promoted coexistence of plant species with different life forms dominating at different stages of post-fire succession (Brooks 2004).

Grasses, in general, are more flammable because of their supply of standing dead material, their large surface to volume ratio that easily desiccates, their tissue chemistry, low moisture content, and fine size (Mooney 1981).  Alien grasses tend to also replace the discontinuous woody shrubs with a horizontaly continuous fuel and create a fuel packing ratio that facilitates ignition (Brooks 2004).  As a result of the thin, uniform canopy, the grassland microclimate tends to be hotter and less humid than forests and woodlands.  A grass’s life cycle allows quick recovery following a fire because there is little above ground structural tissue, so all new tissue fixes carbon and grows.  Following grass-fueled fires, invasion and extension of alien grasses leads to greater susceptibility to fire. Grassland fires are a part of their natural history, and to prevent their occurrence serves to increase their severity later (D’Antonio 1992).  However, invasive grasses have increased the frequency of fires from one in 60-100 years to 3 in 3-5 year return cycles converting native shrub lands to alien dominated grasslands (Foxcroft 2013).

                Brush fire due to cheatgrass                                 Altered disturbance regime - cheatgrass                                                                                                     replacing native sagebrush following fire  

Ecosystem nutrient stores are altered by fire.  Carbon and nitrogen are volatilized but others become biologically available.  Atmospheric nitrogen loss leads to nitrogen limitation further exacerbated by losses to erosion, ground water, and streams (D’Antonio 1992).  The large African grass, Andropogon gayanus, or gamba grass, was introduced into Australia as pasture grass but later spread out into the native vegetation.  It also has altered the fire regime in the ecosystems it has invaded.  It has a number of advantages over native grasses including higher rates of photosynthesis, transpiration, and soil water uptake plus drought resistance, and a longer growing period.  In the nitrogen-depleted soils of Australia, its rapid growth seems paradoxical.  Gamba grass has developed a mechanism for conserving soil nitrogen by inhibiting nitrifying soil microorganisms with secondary allelopathic compounds.   Nitrate, which would have been used by native plants or leached away, is not produced.  Nitrogen is maintained as the relatively immobile ammonium ion in soil.  Gamba grass roots can take up ammonium six times faster than native grasses and prefer it as a source of nitrogen, giving the plant an additional advantage over native grasses (Rossiter-Rachor 2009).

                                                            Andropogon gayanus

Globally, the effects of alien grasses on fire and ecosystems are compounded by anthropomorphic land use changes.  Humans clear wooded lands to create grassland for domestic animals, often using fire to clear and maintain the land as grassland.  Even by selective logging, the amount of combustible material increases.  The probability of fires increases, and the fires increase the rate of conversion of wooded areas and forests to grassland.  Fire and grasses are increased separately, but both are increased synergistically by the grass-fire positive feedback cycle.  Land use change in the Americas and Australia has increased fire and grazing to the highest levels ever; the population of selected grasses, often the Eurasian and African varieties that can tolerated fire and grazing best, has also increased.  Fire frequency has delayed or prevented the succession to woody plants (D’Antonio 1992).  Invasive grasses cause permanent degradation of ecosystems and prevent successional changes.

References

1. D’Antonio, Carla M., Vitousek, Peter M., (1992). Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecology and Systematics.

 2. Brooks, Matthew L., et al. (2004). Effects of Invasive Alien Plants on Fire Regimes. BioScience. 54:677-688.

3. Foxcroft, L. C., et al., (2013). The Bottom Line: Impacts of Alien Plant Invasions in Protected Areas. Plant Invasions in Protected Areas: Patterns, Problems and Challenges. (Springer)

4. Mooney, H. A., Bonnicksen, T. M., Christensen, N. L., Lotan, J. E., Reiners, W. A., (1981). Fire regimes and ecosystem properties. USDA Forest Service General Technical Report. WO-28 Washington, DC.

5. Rossiter-Rachor, N. A., et al. (2009). Invasive Andropogon gayanus (gamba grass) is an ecosystem transformer of nitrogen relations in Australian savanna. Ecological Applications. 19:1546-1560.

Invasion of Exotic Grasses/Changes in Land Use

Invasion of Exotic Grasses/Changes in Land Use

Invasive exotic grasses have occurred on every continent and can alter ecosystems by replacing native species with a dominant single species.  They compete by altering environmental conditions or resource availability.  Many of these species tolerate and enhance fires following which they are able to extend dominance by rapid growth.  While grasses can invade undisturbed native vegetation areas as well, exploiting disturbed areas is their strength.

In North America, invasions are most common in the arid west and Great Basin.  Introduction of European annuals may not have been planned but occurred with introduction of sheep and cattle.  One of the worst grasses is the European annual, Bromus tectorum, or cheatgrass that has widely replaced native bunchgrasses (Wolfe 2005). It is a winter annual that germinates in fall, grows in winter and spring, produces lots of seeds for its soil seed bank, and dies in early summer.

                                                    Bromus tectorum

Perennial grasses may have been introduced as vegetation for erosion control or for grazing land because the grasses tolerated the defoliation by ungulates better than native grasses.  Now that grazing no longer occurs over much of this land, the grasses have spread widely.  Crested wheat grass (Agropyron desertorm) and bufflegrass (Cenchrus ciliarus) are examples (Rogler 1983).

      

                                                         Agropyron desertorm

Invasive grasses have effects at multiple levels of ecological organization from population to ecosystem.  At the population level, they absorb light in their canopy and reduce water and nutrient availability to other species (Vasquez 2008).  At the ecosystem level, they alter the boundary humidity, reduce nutrient mineralization, and alter the fire regime.  Invasion of woody perennials in dense grasslands is unusual unless there is a disturbance.  By drawing down soil moisture with their dense shallow roots, invasive grasses are more adept at suppressing water-sensitive oak seedlings and preventing successional change (D’Antonio 1992).  However, saplings and adult woody species with larger roots can reach and obtain water and nutrients.  Another difference favoring the invasive grass A. desertorum over native grasses is its high seed output, lower seed predation, and large seedbank store.  Loss of plant diversity by grass invasions is accompanied by a loss of animal population diversity due to alterations of food supply or habitat.

                  Ammophila arenaria                                                 Eragrostis lehmanniana

European beachgrass (Ammophila arenaria) was introduced for erosion control but has eliminated native plant species along the coast of California and Oregon; furthermore, a few rare insect species that depended on these native plants have also been eliminated (Slobodchikoff 1977).  Lehmann lovegrass (Eragrostis lehmanniana) from South Africa has replaced native shrubs and herbs in the Sonoran desert, and reduced the population of native birds (scaled quail) and insects in the ecosystem (Medina 1987).  In Idaho and Wyoming, cheatgrass and other invasive plants have replaced sagebrush with the resulting loss of the sage grouse, jackrabbit, prairie ground squirrel, and their predators, the prairie falcon and golden eagle (Knick 2003).  Alteration of ecosystem effects (both increases and decreases) such as nitrogen fixation, litter quality, and decomposition rate, as well as allelopathic suppression of nitrifying bacteria, have been documented with invasive alien grasses.   Invasive grasses can affect the microclimate of an ecosystem by their production of dense litter that holds onto moisture and allows germination of seeds and sapling growth.  On the other hand, their shallow canopy and smooth aerodynamics, compared to forests or woodlands, leads to higher canopy and surface temperatures and lower relative humidity.  These conditions favor the growth of plants with C₄ photosynthesis (usually grasses) and fires to be discussed in the next blog (D’Antonio 1992). The invasive grasses result in loss of diversity and habitat, and cause regression of successional changes. 

References

1. D’Antonio, Carla M., Vitousek, Peter M., (1992). Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecology and Systematics.

2.  Steven T. Knick, et al. (2003). Teetering on the Edge or Too Late? Conservation and Research Issues for Avifauna ofSagebrush Habitats.  The Condor. 105:611-634

3. Medina, A. L., Diets of Scaled Quail in Southern Arizona. The Journal of Wildlife Management. 52:753-757.

4.  Rogler, G. A., Lorenz, R.J., (1983). Crested Wheatgrass– Early History in the United States. Journal of Range Management. Vol. 36:91-93.

5.  Slobodchikoff. C. N. and Doyen, John T. (1977). Effects of Ammophila Arenaria on Sand Dune Arthropod Communities. Ecology. 58:1171-1175. 

6. Vasquez, Edward, Sheley, Roger, Svejcar, Tony, (2008). Nitrogen Enhances the Competitive Ability of Cheatgrass (Bromus tectorum) Relative to Native Grasses. Invasive Plant Science and Management.

7. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.

Soil Fertility and Invasive Species

Soil Fertility and Invasive Species

Exotic plants can alter ecosystems by altering the soil community, soil structure, and soil fertility. 

After invasion, exotic plants can alter the soil community composition and the relationship between soil and plant community indirectly by altering plant-derived soil inputs. The timing, quality, and quantity of litter production affects nutrient inputs to the soil.  At one extreme, an invasive plant could add so much litter that the potential for more frequent and more intense fires can occur and effect the above-ground community, as well as the soil community structure, function, and soil fertility.  Alternatively, invasive plants can alter the soil community directly by releasing secondary compounds from their roots into the soil (Wolfe 2005).  Allelopathic exudates are highly inhibitory to plants and/or soil microbes in invaded communities, but ineffective against natural neighbors that had adapted over time.  While there are many thousands of different LMW (low molecular weight) products, there are only a relatively few for which a function has been identified.  These jobs include soil nutrient acquisition, defense against herbivory, root communication, and antimicrobial protection (Calloway 2004).

Diffuse knapweed (Centaurea diffusa) is a weed that was imported from Eurasia and has spread widely in the semiarid grasslands of western North America.  It is a short-lived perennial that produces a rosette in the first year, and in the second year, flowers, sets seeds, and dies.  The dead plant can act like a tumbleweed and spread the seeds.  Its spread has resulted in loss of native plants for grazing animals (Invasive Plants, Seastedt 2005). 

 Centaurea diffusa  rosette                          mature plant                                    flower

                                                                Centaurea maculosa 

Diffuse knapweed is known to produce a root exudate, 8-hydroxyquinoline, that alters soil fertility by chelating phosphorus and suppressing the ability of North American native plants to acquire phosphorus.  Curiously, plants that had evolved with knapweed in Asia were less affected.  A related species that has successfully invaded North America is spotted knapweed (Centaurea maculosa). 

It produces a root exudate, (+/-)-catechin, a racemic mixture of which one enantiomer suppresses plant pathogens, and the other has phytotoxic activity. The latter activity is caused by triggering intracellular reactive oxygen, eventually leading to cell death.  In addition, this plant is able to alter the soil biota, arbuscular mycorrhizal fungi, so that it can parasitize carbon (up to 15% of plant carbon) from neighboring native bunchgrass (Invasive Plants, Weidenhamer 2005).

               

Garlic mustard (Alliaria petiolata) was introduced to the U.S. in 1868 from Eurasia and is now widely distributed.  It is an obligate biennial that produces first year rosettes that over-winter under snow, bolt early in the spring, set seed, and die by mid-summer.  It is unusual as an invasive herbaceous plant because it is shade tolerant, non-mycorrhizal, and can invade forest understory, even when there is no disturbance.  Like knapweed, it produces a number of secondary compounds.  Among these are glucosinolates, sulfur and nitrogen containing compounds that degrade into volatile cyanide molecules.  These compounds then inhibit herbivory, plant growth, and fungal growth (Rodgers 2008). 

                                                                       Alliaria petiolata

Experimental findings suggest that garlic mustard suppresses growth of arbuscular and ectomycorrhizal fungi.  While garlic mustard is non-mycorrhizal, most plant species do form symbiotic relationships with these fungi so an advantage is gained over competitors by suppressing this mutualism.  Garlic mustard does not appear to affect soil bacteria and the non-mycorrhizal fungi, and was found to increase litter decomposition, increase soil pH, and increase N, P, and base cation availability.  Phosphatase activity is low, so increased phosphate is felt to be due to the garlic root exudates that increased pH and decreased sorption of inorganic phosphorus by aluminum and iron.  Finally, positive feedback is seen by the fact that garlic mustard grows better in soils it has previously occupied, compared to soils occupied by native plants.  Changes in nutrient availability caused by garlic mustard are detrimental to native plants but beneficial for its own growth (Rodgers 2008).

           

Non-indigenous plants have developed weapons in the lands where they evolved that are less effective there because other species have adapted to those characteristics.  In new lands, these weapons affecting soil characteristics and mutualisms are used to their advantage to suppress competition, expand the invasive species abundance and territorial size, and alter the ecosystems.

References

1. Calloway, Ragan M., Ridenour, Wendy M. (2004). Novel Weapons: Invasive Success and the Evolution of Increased Competitive Ability. Frontiers in Ecology and the Environment, 2:436-443.

2. Rodgers, Vikki L., et al. (2008). The invasive species Alliaria petiolata (garlic mustard) increases soil nutrient availability in northern hardwood-conifer forests. Oecologia, 157:459-471.

3. Seastedt, Timothy R. et al. (2005). Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America. Invasive Plants: Ecological and Agricultural Aspects (Birkhauser Verlag, Boston) pp 129-139.

4. Weidenhamer, Jeffrey D., Romeo, John T. (2005). Allelopathy as a mechanism for resisting invasion: the case of Polygonella myriophylla. Invasive Plants: Ecological and Agricultural Aspects (Birkhauser Verlag, Boston) pp 167-177.

5. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.

Nutrient Cycling and Invasive Species

Nutrient Cycling and Invasive Species

               

               Ecosystem changes in nutrient cycling by invasive species are quite numerous and complex.  Although invasive plants often do succeed because they possess some trait that allows them to better exploit available carbon, nitrogen, and phosphorous, it is an oversimplification to think that these are independent of the other ways that these plants can alter ecosystems.  Advantages in nutrient cycling can sometimes be gained by large over small plant size, perennial over annual tissues, seasonal growth timing, herbaceous over woody, or symbionts (Ehrenfelt 2003).

                Regarding carbon cycling, in a previous blog, I discussed productivity and how an invasive plant can succeed by having a larger biomass, larger root stores, higher quality litter, and faster growth rates.  An exotic plant in Australia owes part of its success to its different photosynthetic process, crassulacean acid metabolism (CAM).  Prickly pear cactus (Opuntia) is a succulent plant that closes its stomata during the day and is very efficient at conserving water and carbon in arid environments.  Some cacti can survive in temperatures up to 63⁰C, among the highest tissue temperatures of any vascular plants.  In eastern Australia, the plants have exploited overgrazed and drought-damaged grasslands where they had no competition (DeFelice 2004).

Prickly pear cactus (O. stricta)

 Hawaiian fire tree (M. faya)

                The often cited example of an invasive plant that has exploited and changed nutrient cycling in an ecosystem is the Hawaiian fire tree, Morella faya.  It is an actinorrhizal nitrogen fixer that has invaded nitrogen-deficient, young volcanic areas where there were no native species with similar capabilities.  This tree doubles the available nitrogen and water available under the canopy.  It has gradually replaced the native Metrosideros tree mentioned in an earlier blog.  Experimental studies have shown that M. faya has altered primary successional ecosystems by increasing the amount and biological availability of fixed nitrogen (Vitousek 1990).

                

                Increased nitrification, without nitrogen-fixation, is part of the successful invasion of Japanese barberry (Berberis thunbergii) and Japanese stiltgrass (Microstegium vimineum).  When compared to the soil under native species, there were differences in the soil under the two non-indigenous plants.  There was a decrease in fungal abundance and an increase in European earthworms under the barberry tree; under the stiltgrass, there was, along with the increase European earthworms, an increased abundance of the arbuscular mycorrhizal fungi (AMF) (Wolfe 2005).

Japanese barberry (B. thungergii)                                                            Japanese stiltgrass (M. vimineum)

                                                     Bridal creeper (A. asparagoides)

                An example of an invasive plant that excels in phosphorus cycling is the bridal creeper (Asparagus asparagoides).  In the acidic, nutrient-poor, weathered soils of Australia, available phosphorus is quickly adsorbed to soil particles.  The low soil phosphorus, maintained by native plants that absorb phosphorus before litter falls, prevents many exotic plants with high phosphorus requirements from competing.  Bridal creeper is efficient at extracting both phosphorus and nitrogen from soil.  It produces high quality nitrogen and phosphorus-rich litter that decomposes rapidly and leaches the phosphorus.  Plentiful shoots catch litterfall, and a thick tuberous root mat traps nutrients from the soil.  Soil phosphorus pools increase which improves soil fertility and allows other invasive plants to thrive (Simberloff 2011).

                Nutrient cycling differences between native species and non-indigenous species have been well documented.  Photosynthesis and carbon fixation differences are reflected in increased net primary production on the ecosystem level.  Higher carbon input rates due to differences in carbon physiology, nutrient use efficiency, and leaf carbon cost have been described.  Advantages in nitrogen and phosphorus acquisition, either due to plant physiology or symbiotic relations, can explain the successful invasion by other plants that lead to overall changes in the ecosystem.

References

1. DeFelice, Michael S., (2004). Prickly Pear Cactus, Opuntia spp.: A Spine-Tingling Tale. Weed Technology. 18:869-877.

2. Ehrenfelt, Joan G., (2003). Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes. Ecosystems. 6:503-523.

3. Simberloff, Daniel.,( 2011). How common are invasion-induced ecosystem impacts?.  Biological Invasions. 13:1255-1268.

4. Vitousek, Peter M., (1990). Biological invasions and ecosystem processes: towards an integration of population biology and ecosystem studies. Oikos. 57:7-13.

5. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.

Water Flux/Soil Hydrology and Invasive Species

Water Flux/Soil Hydrology and Invasive Species

            

Water flux and soil hydrology refers to the means by which water in all three forms, liquid, solid, and vapor, circulate through the biosphere.

Saltcedar (Tamarix) is a woody plant native to Eurasia that has had a dramatic effect on the ecosystems of southwestern riparian habitats because of changes in water flux.  Introduced in the 1800’s for bank stabilization and windbreaks, it has spread rapidly into not only all wetland and river system of the southwest, but also northward to Utah and Montana.  Human activity, such as river damming, has altered the natural flooding regime of the ecosystem.  Past clearing of woodlands for agriculture also disturbed the ecosystem.  Saltcedar is much better adapted to exploit the new abiotic characteristics of the areas than native species (Lovich 1998).

 Saltcedar is an aggressive invasive species because of several traits: it is prolific (500,000 seeds/year); it can reproduce vegetatively by resprouting when damaged; it is resistant to drought, fire, flood, and high salinity; its salty leaf litter prevents germination of its competitors like Cottonwoods; its high water uptake and evapotranspiration rates make the environment xeric and unfavorable to native species; its dense groves shade out native species (Lovich 1998).  A natural spring with marshland in Death Valley became devoid of surface water after it was invaded by saltcedar.  After the trees were removed, the surface water reappeared (Vitousek 1986).  By trapping and stabilizing alluvial sediment, saltcedar can reduce channel width and impede a channel’s ability to adjust to increased flow, and leads to more frequent floods.  The drought-tolerant deciduous saltcedar produces a high fuel load leading to more frequent fires, following which saltcedar is more resistant, and quicker to recover than native shrubs (Lovich 1998).

                                                                             Saltcedar

Another invading plant that alters soil hydrology is the Yellow starthistle (Centaurea solstitialis) of California’s grasslands.  This plant has displaced annual and perennial native grasses.  Yellow starthistle has a deep taproot and longevity throughout the summer.  Its high evapotranspiration rate significantly lowers the total soil water content and inhibits competitors with more shallow roots (Enloe 2004).

                                                                    Yellow Starthistle

Invasion of South African native shrublands (fynbos) by non-indigenous plants has reduced watershed runoff and caused rivers to dry up.  Native plants on these lands provide an ecosystem service necessary to insure proper function of the watershed.  These plants are adapted to summer droughts, nutrient poor soil, and periodic brushfires that are of moderate intensity due to the low biomass. With their low biomass, these shrubs are able to prevent erosion and consume relatively little water, so there is always runoff for the rivers and streams that benefit habitats below the watershed. Woody plants introduced for lumber such as eucalyptus, Pinus pinaster, Hakea sericea, and Australian Acacias have invaded these lands to the detriment of water resources.  These plants have high biomasses and consume much more water than the native shrubs.  Runoff from the watershed is reduced or eliminated and wildfires tend to be much more intense because of the greater biomass (VanWilgen 1996).

                   P. pinaster                                                  Hakea sericea

                                                Acacia                                                            Eucalyptus

Invasive plants change ecosystems by their effect on hydrology. Changes of rate, timing of evapotranspiration, or runoff of the region, are due to differences between invasive and native plants with respect to the transpiration rate, phrenology, biomass of photosynthesis, or rooting depth (Levine 2003).

References

1. Ecology of Biological Invasions of North America and Hawaii. (1984). Springer-Verlag: New York.  p 163-173.

2. Enloe, Stephen R., et al (2004). Soil Water Dynamics Differ among Rangeland Plant Communities Dominated by Yellow Starthistle (Centaurea solstitialis), Annual Grasses, or Perennial Grasses. Weed Science. 52:929-935.

3. Levine, Jonathan M. et al (2003). Mechanisms Underlying the Impacts of Exotic Plant Invasions. Proceedings of the Royal Society of London. 270:775-781.

4. Lovich, Jeffrey E. and Roland C. DeGouvenain. (1998). Saltcedar Invasion in Desert Wetlands of the Southwestern United States: Ecological and Political Implications. Ecology of Wetlands and Associated Systems. (The Pennsylvania Academy of Science). Chap 30. P. 447-467

5. VanWilgen, Brian W., Richard M. Cowling and Chris J. Burgers. Valuation of Ecosystem Services. BioScience. 46:184-189.