discussion
Yanicole
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.