It's Complicated . . . Part 2

A few weeks ago, I wrote about the complicated nature of invasive species. I mentioned some of my studies at the University of Liverpool helped me to understand the complexity that exists between native and non-native species and the management plans that we as humans have developed. Today I decided to post a short essay I wrote on this particular subject while attending University. For my American readers, since my school of higher learning was in the UK, some of the phrases or spelling may seem wrong, but are in fact correct within Britain. Links will be provided to describe unfamiliar scientific words and scientific papers for those who want to research the topic. I hope you enjoy and gain a deeper understanding of the invasive species debate.

The Importance of Considering Non-native Species When Making

Conservation Management Decisions

Introduction 

Biodiversity loss is becoming a major concern in present-day ecological, cultural, and economic

circles. The loss of biological diversity within ecosystems is attributed to many different factors

and management regimes must take these factors into account to find success in conservation.

One of the top 5 factors of biodiversity loss in the United Kingdom and the United States (U.S.)

is attributed to invasive, non-native species (EPA, 2012; Natural England, 2013). Non-native

species that move into an ecosystem can cause widespread changes that significantly impact

biodiversity. 

Exotic species have the potential to change predator-prey relationships, out-compete native 

species, change ecosystem regimes, and impact agricultural and historic culturally important

species (Schlaepfer et al, 2011:429). Many conservation managers see all invasive, non-native

species as a negative intrusion on local ecosystems and set goals of complete removal

(Shackelford et al, 2013:55). Though many non-native species do impact systems in negative

ways, management schemes must take into account the complicated manner in which these

species interact with their new environment. In this essay I will explore the importance of

understanding interactions between non-native species and their new habitat in order to develop

management techniques that address the complex nature of the problem. I will first examine

cheatgrass invasion in the Great Basin of the western U.S. and how managers are learning the

multifaceted issues in preventing its spread. Next, I will investigate the unique interactions of 

tamarisk plants with local species in the western U.S. and how that may affect conservation and 

management plans. 

Cheatgrass 

Cheatgrass, Bromus tectorum, has become a major invasive, non-native species in the Great

Basin. First introduced to the U.S. in the 1890s through wheat supplies, it has established itself as

an opportunistic plant of the dry, high deserts of the Great Basin region (Pellant, 1996:1). Its

spread has been exacerbated due to heavy grazing by domestic animals and fire disturbance

(Fuhrman et al, 2009:2; Pellant, 1996:1). B. tectorum is an annual grass that is able to out-

compete many native species by growing and germinating earlier than other annual and perennial

grasses (Goergen et al, 2011:2). The grass completes its lifecycle quickly and then dies back,

making it a prime fuel source for wildfires (Balch et al, 2012:174). Because the grass is opportunistic, it has made its home in several different ecosystems including halophytic shrublands, sagebrush steppe, pinyon-juniper stands, and even ponderosa pine stands just outside the Great Basin (Balch et al, 2012:174; Furhman et al, 2009:2). Its vulnerability to fire has increased fire returns in all of these regions. Ecosystems that have seen B. tectorum become established have had fire returns reduced by half than those parts where cheatgrass is not growing (Balch et al, 2012:178). Fires in cheatgrass regions are more likely to burn larger areas and for longer periods (Balch et al:178-179). Increased fire returns threaten whole ecosystems with regime change from woody plants to herbaceous ones, especially in sagebrush steppes which are quite sensitive to fire (Balch et al:178-179; Furhman et al, 2009:2). 

Zion National Park in southwest Utah has prescribed burns from time to time to help manage the 

ecosystems within its boundaries (Fuhrman et al, 2009:1). Though fire was suppressed in the

U.S. for many decades, its usefulness to ecosystems is now understood (Fuhrman et al, 2009:1). 

However, managers have found that cheatgrass is establishing itself in areas disturbed by fires,

especially in areas where fire has historically been supressed (Fuhrman et al, 2009:3-4). This has

the reverse effect that managers are looking for when prescribing burns – reduction in easy fuel

sources (Fuhrman et al, 2009:4). Conservation managers have concluded in this particular case

that because cheatgrass interrupts a return to historical fire schedules, control of this invasive

species should not include burning (Fuhrman et al, 2009:4). 

 

Although fire is not a good control for this exotic species, some researchers have found that there 

may be other ways to combat its spread and help valuable ecosystem species retake their

dominance. For example, in sagebrush steppes the limiting factor of restoration is the re-

establishment of native grasses (Ray-Mukherjee et al, 2011:359). Research by Ray-Mukherjee et

al found that one of the characteristics that make B. tectorum so successful is its focus on

establishing a large root network to help the plant access constrained resources (Ray-Mukherjee

et al, 2011:363). When compared with native grasses, it was found that indigenous Snake River

wheatgrass, Elymus wawawaiensis, has similar characteristics which could make it a good local

competitor of cheatgrass (Ray-Mukherjee et al, 2011:364-365). 

 

Likewise, Goergen et al studied the effects of cheatgrass on native annual and perennial grasses. 

Their research revealed that native grass populations which had been invaded by B. tectorum

were responding phenologically to help them compete (Goergen et al, 2011:4). The growth and

flowering of the native grasses occurred earlier in communities with cheatgrass than

communities without it, which allowed many of these systems to be more tolerant of cheatgrass

competition (Goergen et al, 2011:5-6). What is clear from this research is that native grasses

found in colonised areas may actually act as resource material in restoration efforts (Goergen et

al, 2011:7). 

 

Conservation managers must account for the dynamism of communities to an exotic species such 

as cheatgrass, as well as for its habits when implementing controls. B. tectorum increases fire

return and establishes itself after an area is burned. Other native plants have responded by

starting growth and flowering earlier than they would otherwise. Some control may be

implemented by seeding areas with competitive native grass. The effects of fire, earlier

phenological responses, and competitive native species require management of cheatgrass which

will not result in overall harm to native plant populations if indigenous species are to be

protected from substantial biodiversity loss. 

Tamarisk 

Tamarisk (Tamarix spp.), or saltcedar, is another plant which has moved into the western U.S. 

and outcompeted native trees and large shrubs. It has become a dominant plant in riparian

communities and is thought to be negatively impacting water availability, biodiversity, and

overall habitat quality (Hultine et al, 2010:467). Many ecologists and land managers have

previously taken an approach that aggressively removes tamarisk through various means. This

has included chemical spraying and the release of the parasitic tamarisk beetle (Hultine et al,

2010:467; Paxton et al, 2011:255). These tactics were undertaken before understanding how

native plant and animal species were responding to the exotic saltcedar (Belote et al, 2010:449-

450; Shackelford et al, 2013:58-59). 

 

Recent studies are establishing the complex nature of tamarisk and local, native species.

Tamarisk is thought to alter soil salinity and out-compete similar plants for the limited water

supply, which in turn has led conservationists to believe there has been a reduction in

biodiversity in areas dominated by this plant (Belote et al, 2010:450). Whilst there has been an

overall reduction in native trees such as cottonwood, willows, and box elder in riparian zones,

many other species have adapted to the monotypic stands of saltcedar (Paxton et al, 2011:255-

256). For instance, the endangered SouthwesternWillow Flycatcher nests in tamarisk trees and

depends on the shade they provide to successfully reproduce and rear their young (Hultine et al,

2010:471; Shackelford et al, 2013:58). Many other native and migrant birds rest in and feed on

insects found on tamarisk (Hultine et al, 2010:471; Paxton et al, 2011:256-261). The danger for

these birds, especially for the endangered Flycatcher, is that the parasitic beetle defoliates 

the tamarisk in late spring over successive years until the plant eventually dies (Paxton et al,

2011:256-261). This makes Flycatcher eggs vulnerable to excessive heat and reduces insect

numbers for other insectivorous birds (Paxton et al, 2011:256-261). 

 

Birds may have other woody plant species they can nest in and feed off of; however, their ratio is 

small, increasing pressure on all bird species (Hultine et al, 2010:471). A study in Grand Canyon

National Park also found that saltcedar stands had similar species richness to non-invaded areas;

in fact, richness and diversity was higher because there were many other non-native species

found in tamarisk stands (Belote et al, 2010:453). In addition, areas where tamarisk was

proactively removed did not undergo a return to native plants over 1-3 years after removal

(Belote et al, 2010:455). These results suggest that either native species are in such low numbers

that proactive seeding and planting must take place to help revive the native tree populations, or

that tamarisk has already altered the soil salinity, making it difficult for native populations to

recover their historic spread (Belote et al, 2010:456-457). 

 

Finally, research has found that tamarisk has actually contributed to the stabilising of river and 

stream banks, as well as making the flow of the channels narrower and deeper (Hultine et al,

2010:469). This has caused a change in sediment supply and flooding patterns that native tree

populations have historically relied on (Hultine et al, 2010:469). Control and removal of

saltcedar will most likely change flooding patterns again, resulting in significant erosion during a

flood event because of the narrower, deeper flow of water channels (Hultine et al, 2010:469).

Such a scenario has the potential to impact ecosystems downstream, as well as hindering the

ability of native plant populations to move in and keep the banks stable. 

 

Tamarisk is indeed an exotic, invasive species which has already altered arid riparian ecosystems 

in the western U.S. Native populations of riparian woody plants may have declined, yet many

species of plants and animals have adapted to the changes. Aggressive control techniques have

the potential to disrupt or extirpate species which have adapted to use the tamarisk stands for

their own benefit. In addition, because saltcedar stands are found to harbour other invasive

species, the removal of tamarisk opens up the possibility of invasion by another exotic species,

without the return of large native plant populations (Hultine et al, 2010:470-471). Again, it is

important for conservationists and land managers to consider the implications of control

measures in regards to non-native species, as they may actually negatively impact biodiversity. 

Conclusion 

Non-native species that are introduced into a new location can have very negative influences on 

native species and the overall health of an ecosystem. However, as I have explained in this essay,

non-native species interact with the ecosystem in very complex ways. Controlling or removing

the spread of an exotic species must be undertaken only when these interactions are understood.

If the interactions are not fully understood by managers, attempts to extract an exotic species

from a habitat may actually contribute to its spread, such as fire and cheatgrass. Other times, it

may actually result in threatening many other endemic species which have come to rely on the

invading plant, as is the case with the Flycatcher and tamarisk. In order to preserve native

biodiversity, careful steps and consideration need to be taken when addressing non-native

species. 

References

Balch, J.K., Bradley, B.A., D’Antonio, C.M.,Gómez-Dans, J., 2013. Introduced annual grass 

increases regional fire activity across the arid western USA (1980-2009). Global Change   Biology  19, 173–183. 

Belote, R.T., Makarick, L.J., Kearsley, M.J.C.,Lauver, C.L., 2010. Tamarisk Removal in Grand 

Canyon National Park: Changing the Native - Non-native Relationship as a Restoration Goal. 

Ecological Restoration 28, 449–459. 

Environmental Protection Agency, 2012. Invasive Species. Water: Habitat Protection. 

http://water.epa.gov/type/oceb/habitat/invasive_species_index.cfm. Accessed 5 March 2013. 

Fuhrmann, K., Decker, C., Johnson, K.A., 2009. Prescribed fire and nonnative plant spread in Zion 

National Park. Park Science 26, 1–8. 

Goergen, E.M., Leger, E.A., Espeland, E.K., 2011. Native Perennial Grasses Show Evolutionary 

Response to Bromus tectorum (Cheatgrass) Invasion. PLoS One 6, 1–8. 

Hultine, K.R., Belnap, J., Van Riper, C., Ehleringer, J.R., Dennison, P.E., Lee, M.E., Nagler, P.L., 

Snyder, K.A., Uselman, S.M.,West, J.B., 2010. Tamarisk biocontrol in the western United 

States: ecological and societal implications. Frontiers in Ecology and the Environment 8, 467–474. 

Natural England, 2013. Non-native species. Our Work. 

http://www.naturalengland.org.uk/ourwork/conservation/biodiversity/threats/nonnativespecies.aspx. Accessed 5 March 2013.  

Paxton, E.H., Theimer, T.C., Sogge, M.K., 2011. Tamarisk Biocontrol using Tamarisk Beetles: 

Potential Consequences for Riparian Birds in the Southwestern United States. The Condor 113, 255–265. 

Pellant, M., 1996. Cheatgrass: The Invader That Won The West. Bureau of Land Management. 22p. 

Ray-Mukherjee, J., Jones, T.A., Adler, P.B., Monaco,T.A., 2011. Immature Seedling Growth of Two 

North American Native Perennial Bunchgrasses and the Invasive Grass Bromus tectorum. 

Rangeland Ecology & Management 64, 358–365. 

Schlaepfer, M.A., Sax, D.F., Olden, J.D., 2011. The Potential Conservation Value of Non-Native 

Species. Conservation Biology  25, 428-437. 

Shackelford, N., Hobbs, R.J., Heller, N.E., Hallett,L.M., Seastedt, T.R., 2013. Finding a middle-

ground: The native/non-native debate. Biological Conservation 158, 55–62.