discussion

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Abstract

Introduced plants used in urban locations have many beneficial uses, including

reducing erosion, sequestering carbon, and providing a colorful and beautiful

landscape. A small portion of them become aggressively invasive, however, and have a

number of impacts, including competition for resources, changes in ecosystem

properties, and an increase in wildfire frequency. Urban areas are sources for wildland

invaders for several reasons. The large number of landscape species used, and the large

number of individuals of each species, increases the probability of invasion through

propagule pressure. Seeds travel outside of urban areas to wildlands by several

vectors, including wind, animals (especially birds), and motor vehicles. Fortunately, not

all introduced species are invasive and using “Codes of Conduct,” or best management

practices, may help reduce the number of invasive species used in urban areas,

preventing escape to surrounding areas.

Scientists divide the epochs that make up the geologic timescales using terms like the

Holocene and the Pleistocene. Some have suggested that future humans will term our

current epoch the Homogocene because of the blending of not only cultures and

economies, but also plant and other species. Our world is beginning to look the same

everywhere—the same plants dot the roadsides and wild landscapes. These “invasive”

species are impacting native plants and animals in ways we are only beginning to

understand. Urban systems are the source of many problems in wildlands, and the

urban systems themselves are also impacted.

Invasive plants are nonnative species that are capable of colonizing everything from

your backyard to wildlands, develop self-sustaining populations, and become

disruptive and even dominant. Other terms used include aliens or exotics, two terms

meaning nonnative. Not all countries have the same concerns about invasive species.

Those that were sett led more recently by people of European heritage, such as the

United States and Australia, tend to be more worried about their impact. The native

indigenous people rarely moved species across long distances, so regional floras were

mostly intact when the Europeans arrived, bringing familiar species.Places such as

Europe, however, have had centuries of people moving long distances across the

continent. In those areas, biologists tend to make a distinction between archeophytes,

or those plants that are not native but were present before explorers brought plants

back from the “new world,” and neophytes, or those introduced after Columbus and

subsequent explorers (Pyšek, 1998).

Why Is There Concern about Invasive Plants?

Introduced invasive plants are now recognized as a major environmental issue,

responsible at least in part for nearly 60% of the imperiled plant species in the United

States (Wilcove et al., 1998) and costing several billion dollars annually in damages and

control costs (Pimentel et al., 2005). The ecological impacts are wide ranging (Mack et

al., 2000) and include everything from simple competition for resources such as water

and light to complex changes in the food web. Vines are eff ective competitors for light

when they cover trees during the summer growing months and prevent sunlight from

reaching either the trees or the forest floor. Some invaders, such as American

southwest invader saltcedar (Tamarix ramosissima Ledeb.), also have higher water use

than native species (Sala et al., 1996). Their uptake of water can lower the water table

below the root zone of native species, increasing competition for this valuable

resource.

Replacement of a native species by an aggressive nonnative species can lead to a

number of changes in plant and animal communities and ecosystems. For instance, a

study of giant knotweed [Fallopia sachalinensis (F. Schmidt) Ronse Decr., syn.

Polygonum sachalinense F. Schmidt] in Washington State found that it dominated the

banks of rivers and streams, replacing native trees (Urgenson et al., 2009). It produced

70% less leaf litt er than the trees, and the amount of nitrogen in the leaves was very

different. The knotweed reabsorbed 76% of the nitrogen into the rhizomes and roots,

while the native species—willows (Salix spp.), alders [Alnus glutinosa (L.) Gaertn.], and

cottonwoods (Populus spp.)—only resorbed 5 to 33%, depending on the species. Fewer

nitrogen-rich leaves falling onto the ground and into the water resulted in less food for

insects, which in turn meant less food for fish and other animals.

Invasive plants may alter natural disturbance regimes, especially the frequency of fire

in ecosystems maintained by fire. For instance, in the shrub-steppe communities in the

Great Basin of the United States, fires have historically occurred every 60 to 100 yr,

but following the invasive of cheatgrass (Bromus japonicus Thunb.), fires are occurring

every 3 to 5 yr (Whisenant, 1990). Native shrubs cannot regenerate in that timeframe,

and the invasion has resulted in millions of hectares that are essentially a monoculture.

Some of the effects can be long-lasting, well aft er the plants are removed. While a lack

of nitrogen in the Urgenson et al. (2009) example was a problem, some plants can add

nitrogen to the soil, which is not always good. Some soils, such as sandy dunes, igneous

soils derived from volcanic activity, and prairie soils, are naturally low in nitrogen and

the plants native to them are adapted to this. When plants that “fix,” nitrogen (convert

nitrogen to the forms useable by the plants) invade these types of areas they may

increase the soil nitrogen. Vitousek and Walker (1989) documented a number of

effects of Morella faya (Aiton) Wilbur, a shrub invading Hawaii, and found that it

substantially altered nitrogen when it invaded relatively fresh lava flows. Such a

change can push the ecosystem beyond the tolerances of native species and facilitate

invasion of other nonnatives. Nitrogen may remain elevated for an extended period of

time, making restoration after removal more difficult. For instance, the grassland soils

around nitrogen-fixing Scotch broom [Cytisus scoparius (L.) Link] remained high in

total nitrogen and nitrate and lower in pH than nearby soils several months aft er the

plants were removed (Dougherty and Reichard, 2004). Plants other than native species

are therefore better able to establish, and the Scotch broom facilitates a community of

invasive plants. Restoration of these areas will therefore require waiting for the

nitrogen levels to slowly drop naturally or actively trying to change soil chemistry.

There is some debate about whether invasive species are “passengers” or “drivers” of

environmental change (MacDougall and Turkington, 2005). Many people think that

invasive species are causing, or driving, the alterations of systems, while others think

that they are the result of changes in hydrology, disturbance, or other differences in

how systems function, acting as passengers to the change. In the examples above, the

invasions are clearly driving change, but they may be present because of other

changes, acting as passengers as well. For instance, changes such as increased

impermeable surfaces in urban areas may increase flooding in rivers, which may then in

turn increase invasion along the banks. Unfortunately, determining whether species

are responding to or causing environmental change can be very difficult for many

reasons. It would be best to know if they were passengers before beginning control

work, because if underlying problems remain, the species will likely return, but it is

usually unrealistic. It is also possible that the same species may be a driver or a

passenger under different circumstances. In urban areas where there is a highly

altered landscape, many invasive plants are likely passengers, but the same species in a

wildland may be affecting change as a driver.

Why Are There so Many Invasive Species in Urban

Locations?

Urban areas tend to have very high levels of invasive plants. A study of the urban

vegetation of Berlin found the outskirts of the city were about 30% nonnative

invasives, but the urban center was closer to 50% (Kowarik, 1995). This may occur for

many reasons. First, urban areas are highly disturbed—soil is removed or compacted,

streets are paved, curbs alter surface water movement, and so on. Disturbance and

invasive species are oft en closely linked (D’Antonio et al., 2000; Hobbs 2000). Invaders

have traits that allow them to exploit disturbance better than most native species,

including a short time from seed germination to fruit production, vegetative

reproduction, and high seed production. These traits allow the plants to grow and

reproduce quickly following a reduction in competition from other species because of

the disturbance.

Urban ecosystems have perhaps the optimal disturbance conditions for invasion.

Continually disturbed areas, such as agricultural fields that are tilled regularly, may

have invasion from herbaceous species, while wildlands that see minimal disturbance

may not have enough reduction in competition to allow establishment. Plants and

animals already established may provide a form of biological resistance to invasion

(Simberloff, 1986). Urban locations, however, have moderate levels of disturbance

along roadways, railroads, greenbelts, and parks. The intermediate disturbance theory

(Connell, 1978) suggests some removal of competitors because of disturbed

conditions, but not continual disturbance, are ideal quickly adapt to disturbance, and

nearby introduced species can capitalize on the reduction in competition.

There has been the suggestion that forests are less invaded than open areas, including

urban zones, because the multilayered canopies typical of forests capture most

sunlight before it filters to the forest floor, impairing seed germination of most species,

and providing a buffering against disturbances (Corlett , 1992). Communities with high

species richness (i.e., the number of species found there) may also be resistant to

invasions (Elton, 1958), but terrestrial plants do not conform to this theory as well as

other organisms (Mack et al., 2000).

Another reason there are so many invasive species in urban locations is the large

amount of source material. Many introduced plants are used to landscape homes,

businesses, and parks, and some may become invasive (Chapter 9, Volder, 2010, this

volume). Probably about 60% of all invasive plants are introduced for landscape

purposes, and the total is higher for woody plants, perhaps as much as 82% (Reichard,

1997). If the plants have some of the types of traits already discussed, they may be able

to invade outside gardens. There is oft en an assumption that federal governments

screen newly introduced plants for their invasiveness before they allow introduction.

In general, this is not true. In recent years Australia and New Zealand have required

these assessments, but other countries have not. The United States and the European

Union are investigating ways to do this, but are not currently assessing the risk of

invasiveness of the plants, only whether they have insects or pathogens. In the future

we may see more regulation. Some importing nurseries are concerned about these

environmental issues and are selective in their inventory, but many are not.

Spread Following Introduction

Invasions tend to have several phases (Fig. 12–1). Species first arrive, either through

accidental methods, such as seed contamination, or intentionally, for food, forage,

landscape, or other use. Increasingly, most species are intentional introductions,

especially for use in urban gardens. Accidentally introduced species may not be

climatically suitable for the introduction site, but intentionally introduced species are

usually matched for the climate and may receive supplemental water and nutrition to

allow them to establish. A small percentage of these species may become casual

invasives, self-sowing in the garden or establishing small populations, often in

disturbed areas near the growing sites. Of these, a small percentage may become a

problem either in the urban area, or by dispersing out of it through various methods

into surrounding wildlands.

There is a correlation between the number of plants in a species found in an area,

either in a location or across time, and the probability of invasion. This is often termed

propagule or inoculation pressure, and the concept is simple: the more plants that are

used, the greater the likelihood that the seeds will land in places that are suitable for

germination and growth. For instance, a British analysis of catalogs from several

nurseries found that the frequency of sale in the 19th century

predicted naturalization today (Dehnen-Schmutz et al., 2007a). Additional study also

found that species whose seeds were sold then at less expensive prices were more

likely to be invasive in current years, suggesting they may have been purchased more

because of their reasonable price, which meant more propagules for dispersal

(Dehnen-Schmutz et al., 2007b). Mack (2000) also suggested that the link between

cultivation for horticulture and agriculture and invasion may also be due to the nature

of cultivation. By reducing environmental stochasticity through protection from

predators, parasites, drought, frost, and so on, a seed bank develops in the soil from

which invasions can occur.

The minimum resident time, the time from introduction until recorded invasion, is also

correlated with invasion. In an analysis of a longtime nursery in Florida, the probability

of invasion increased the longer species were sold, with 70% of the species sold more

than 30 yr becoming naturalized, if not fully invasive (Pemberton and Liu, 2009). In

Chile, species with a shorter minimum resident time had a more limited geographical

spread than those that had been there longer (Castro et al., 2005). Similarly, species

that were recorded as naturalized early in New Zealand were more likely to be

widespread in the country now (Gatehouse, 2008), and vines that had been

documented earlier in Australia were more widely distributed (Harris et al., 2007). This

phenomenon may be correlated with propagule pressure because the longer a species

is available, the more individuals are likely to be found in gardens. It may also allow for

greater opportunities for dispersal along roads or other vectors.

Many plants may be present in gardens for a substantial period of time before they

begin to move outside of cultivation. These “sleeper weeds” may be used in gardens for

some time before they begin to spread. The time between when they are introduced

and when they begin to invade is the lag phase. There are many reasons this may occur.

Some are extrinsic to the biology of the species. For instance, it may just be a matter of

a species increasing in popularity or some other factor related to propagule pressure.

In other instances, it may be a change in conditions, such as an episodic disturbance

such as a hurricane, or a change in nutrient enrichment such as from nitrogen-fixing

species that alter the soil chemistry to facilitate additional invasions. There may also be

intrinsic, biological reasons for a lag phase. When a species is introduced many times

independently, the initial genetic types might have been less invasive, but subsequent

introductions are more aggressive. In some cases, however, the lag phase may be a

matter of perception. Knowledgeable people may simply fail to observe that a species

is moving from gardens, especially if it does not have colorful flowers or fruits. For

instance, it is believed that serrated tussock grass [Nassella trichotoma (Nees) Hack. ex

Arechav.], a serious problem in pastures, was introduced into Australia in the early

1900s, but it was not reported until 1935. By then, it was already widespread.

Species that are not initially invasive may become so aft er repeated introductions of

the same species causes hybrid genotypes to develop. For instance, the ornamental

pear (Pyrus calleryana Decne.), a native of China, has long been a popular ornamental

in commercial and residential areas, especially in the eastern United States. It was

considered noninvasive due to self-incompatibility, but repeated introductions of new

cultivated varieties from different regions of China introduced sufficiently different

genotypes for reproduction to occur. Fruit production is now possible, and it is in the

early stages of invasion (Culley and Hardiman, 2009).

What Difference Does It Make if They Invade Urban

Areas?

Having more plants in cities is generally a good thing—they help slow falling rainwater

and reduce erosion, they provide cooling for buildings by shading, and aid in reducing

global climate change by absorbing carbon (Chapter 18, Aitkenhead-Peterson et al.,

2010, this volume). So what is wrong with having a few more of them spreading along

roadsides and into urban greenbelts and parks?

Many, if not most, urbanites derive their “sense of place” or impression of the natural

surroundings of their regions, by exposure to small patches of nature in the city. A

relatively small number of people find their way to wildlands. Environmental

psychologists think the recognition of place helps to develop concern and connect us

with the history of our surroundings. The philosopher and writer Wendell Berry

summarized this idea: “You can’t know who you are until you know where you are”

(Berry, 1994). He identified 17 rules for developing a sustainable community, the

second of which is that nature, including air, water, land, and native organisms, should

be considered members of the community. Therefore, by keeping some patches of

nature in cities similar to wild

areas, we are helping people achieve that sense of place they need to move us toward

greater sustainability.

Urban dwellers oft en do not see the connection between the cities and the areas that

surround them. They think of the geography as distinct, rather than continuous. Plants

in urban areas spread out into wildland areas, and wild areas surrounding cities may be

the most invaded. For instance, in New Zealand, invasive species in wildlands were

more common in regions with higher population

density (Gatehouse, 2008).

Invasive species oft en move from cities by natural mechanisms such as bird

consumption. Many plant species have carbohydrate- and protein-rich fruits to attract

avian dispersers. The seeds survive passage through the digestive tract and are

defecated or regurgitated in a new location. While bird dispersal of seeds is found

mostly among woody invasive plants and only about 50% of

them overall, it is common in forest invaders (Reichard and Hamilton, 1997). Most

frugivorous birds are generalists and will forage on a number of species, increasing the

likelihood of visitation and dispersal of horticultural introductions (Reichard et al.,

2001). The distribution of perch sites along the urban to wildland gradient also may

factor into spread from urban areas. A study in Belgium found that the spread of an

invasive trees species from urban areas depended on the presence of trees where birds

could rest and defecate (Deckers et al., 2005). The length of time seeds can remain in

the bird’s digestive tract is variable, but can be as high as 100 h (Proctor, 1968), and

some species may travel considerable distances between foraging and roosting sites

(Moulton, 1993; Nakamura and Miyazawa, 1997; Waterhouse, 1997) and while

migrating (White and Stiles, 1992). Models have shown that bird dispersal is

theoretically an effective vector for plant invasion (Moody and Mack, 1988), leading to

numerous small populations that escape detection because of the scattered

distribution. Seeds moving through the digestive system of birds oft en have the

hard seed coat ruptured, facilitating germination.

Natural waterways may also be responsible for the movement of invasive plants. The

previously discussed Fallopia sachalinense is known to spread downstream by

fractured rhizomes during floods. Seeds may also be transported. One study

documented that all populations of giant hogweed (Heracleum mantegazzianum

Sommier & Levier) in the drainage of the Auschnippe River in Germany were

descended from a single individual plant that was probably cultivated in 1982 in a

garden next to the river (Schepker, 1998, as related in Kowarik, 2003). Those living

near rivers and streams have a particular need to carefully manage their land.

Of greater concern for spreading invasive plants out of cities is movement by vehicles

and recreation. A number of studies have documented that vehicles may carry a

number of seeds (Clifford, 1959; Wace, 1977; Schmidt, 1989). Being on a vehicle,

however, does not mean that it would necessarily be dislodged from it. A study in

Berlin documented this using a motorway tunnel with a high wall dividing the lanes

going out of the city with those going in (von der Lippe and Kowarik, 2007). Seed traps

were put at ground level 150 m into the tunnels and periodically emptied. The contents

were put on pots of soil and germinated in a greenhouse. The study found that 11,818

plants in 204 species were dislodged from the cars and landed in the traps during 1 yr.

About one half were not native to Berlin and 39 were known to be problematic

somewhere in the world. Most of the species were not found near the tunnel and seed

traps placed near the mouths of the tunnels at slightly higher levels did not have these

species, so they were not likely entering by wind. This study nicely documented that

cars moving from urban to wild areas are major vectors of invasive plants. As the seeds

fall off vehicles along roadsides, they may establish populations if required growing

conditions are met. Densities of invasive species are usually highest adjacent to the

roads, but in a study in Indiana, individuals were found even 30 m into adjacent forests

(Flory and Clay, 2006). Paved roads may have higher levels of invasive species, perhaps

because of increased vehicle use leading to increased seed numbers or because water

shed from the pavement increased soil moisture levels and improved germination

(Gelbard and Belnap, 2003). Roadside vegetation management may control adjacent

invasive plants, but unless it is done on a regular basis, plants will likely establish

farther from the road and increase invasion into wildlands. Vehicles are not the only

such vector moving urban plants out of cities. We oft en use recreational equipment

such as mountain bikes and even ice chests in urban areas and use them again outside

cities. Even small amounts of soil may contain many seeds. For instance, prairie

grasslands may contain 300 to 800 seeds per square meter of soil and coniferous

forests up to 1000 seeds (Silvertown et al., 1987).

What Can People in Urban Areas Do?

Urban locations can be considered “staging areas” for invasion into wildlands. The

abundant landscaping, nurturing of plants in gardens, disturbance of soils, and

movement of wind, birds, and vehicles from urban to wilder areas all increase the

likelihood of invasion. As propagule pressure grows, the likelihood of seed dispersal

into surrounding areas by cars or natural dispersal agents such as birds increases. It is

important that people living in urban areas take responsibility for the plants they grow.

The problem may seem overwhelming, but there are several simple steps that people in

cities can do to prevent or control invasions in their gardens, city parks and greenbelts,

and surrounding areas. First, because so many invasive plants are introduced as garden

plants, gardeners can make careful choices in selecting garden additions. Plant

selection is a value-laden activity—we select our choices based on how the plants look

and what they need to grow. Including potential for invasion is one more factor to

consider when making the selection. Increasingly, government agencies, nonprofits,

and even the garden centers are making lists of regional invasive species available.

Because, as previously stated, few governments assess risk of invasion before a new

species is introduced, it is especially important that urban gardeners consider it.

In 2001 a group of people got together in St. Louis to establish “codes of conduct,” or

best management practices, about invasive species for gardeners (Baskin, 2002).

Known as the St. Louis Declaration, the codes of conduct for gardeners are

summarized below.

● Plant only environmentally safe species in your gardens. Work toward and promote new landscape design that is friendly to regional ecosystems.

● Seek information on which species are invasive in your area. Sources could include botanical gardens, horticulturists, conservationists, and government

agencies. Remove invasive species from your land and replace them with

non-invasive species suited to your site and needs.

● Ask for only noninvasive species when you acquire plants. Do not trade plants with other gardeners if you know they are species with invasive

characteristics.

● Request that botanical gardens and nurseries promote, display, and sell only noninvasive species. Volunteer at botanical gardens and natural areas to

assist ongoing efforts to diminish the threat of invasive plants.

● Ask garden writers and other media to emphasize the problem of invasive species and provide information. Request that garden writers promote only

noninvasive species.

● Help educate your community and other gardeners in your area through personal contact and in such settings as garden clubs and other civic groups.

Invite speakers knowledgeable on the invasive species issue to speak to

garden clubs, master gardeners, schools, and

other community groups.

● Seek the best information on control of invasive plant species and organize neighborhood work groups to remove invasive plant species under the

guidance of knowledgeable professionals.

● Participate in early warning systems by reporting invasive species you observe in your area. Determine which group or agency should be

responsible for reports emanating from your area.

It should also be noted that many invasive garden plants have a number of cultivated

varieties, or cultivars. Cultivars are selected genotypes that may have a unique growth

habit, leaf shape, or color or flower or fruit characteristics from others in the species.

These are given special names at the end of the species names, usually set apart with

single quotation marks. Cultivars may be less

invasive than the wild-type species, but this is usually not assured. In general, cultivars

with variegated leaves grow more slowly and may be safer.

Because most invasions start in the urban staging areas, informed urbanites should

note if they see a species growing wild that they have not previously detected and

bring it to the att ention of knowledgeable people. These can be agencies involved in

noxious weed control, parks employees, botanical garden staff, university employees,

or others. Several studies have shown that when control work begins when the

invasions are small, it is possible to eradicate the invasion completely. If, however, the

populations become large and numerous, the species will likely continue to spread.

Municipal agencies, park departments, and nonprofits all assist with removal of

invasive plants in urban parks and greenbelts. They usually welcome citizen volunteers.

It can be very rewarding to return a park to greater native species composition and

help to achieve Wendell Berry’s rule of considering native organisms as members of

our larger community.

We may be inevitably entering the Homogocene, but awareness about invasive plants

and a few actions taken now can help us maintain our “sense of place.” Through careful

management, urban areas can maintain pockets of native vegetation in parks and

greenbelts. Through careful plant selection, urban landscaping need not become a

danger to the preservation of wildland plant and

animal communities.