Download CHALLENGES FOR THE BUILT ENVIRONMENT When Native

C O N T I N U I N G E D U C AT I O N U N I T
CHALLENGES
FOR
THE
When Native Species Meet Their Alien Relatives
BUILT ENVIRONMENT
By William M. Fountain and James R. Lempke
The arborist will be able to
h differentiate among terms that describe species based
on their area of origin and current location
h describe characteristics of non-native plants that may
be suitable for the built environment
h explain conditions that may cause a non-native plant
to become invasive
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The new, the different, the exotic.
initially arrived on foreign shores. Some, like the zebra mussel, emerald
ash borer, and common rat would sneak across oceans on ships as
people traveled to distant shores and traded with other cultures. Some
would come as welcomed friends—at least initially. Johnsongrass
and crabgrass were initially introduced as summer forages for North
American livestock. Kudzu, a well-mannered roadside plant in Japan,
escaped the ornamental garden and rapidly “ate” its way across hundreds of thousands of acres in the southeastern United States. On the
other hand, the tulip, native to Asia Minor, remains relatively harmless
and requires considerable effort to perpetuate in our gardens. What
is the difference? Why are some species devastating, while others are
relatively benign? That answer is complicated and lies in geologic history, ecosystem balance, and the botanical characteristics of each species.
Species Characteristics for Native/Non-native Plants
The statement, “All natives are always good, and all non-natives are
Mention any of these ear-catching words and you are sure to attract
always bad,” is indefensible. Before we can discuss questions of why
attention. There is a natural human attraction to things that we think
some plants are invasive while others are not, several terms must
are uniquely different or better than what we currently possess. The
be defined.
lust for new plant species is one facet of this that has both enriched
our lives and provided life-altering
challenges—and is a commonality
across generations and cultures.
Through the centuries, the return
of sailors and travelers was eagerly
awaited by people who wanted to
see the exotic treasures and oddities
they brought home from distant
places. Early European plant explorers
rushed into areas marked as Terra
Incognita (unknown land) on maps of
the Americas and Asia. Their primary
objective was to return with unique
additions for the herbarium, ornamental garden, and dinner table. Along
the way, a very limited number of
these species would become problematic and remain so for generations to
come.
Where people have traveled, diseases and pests have always followed.
These exotic invaders have rarely
Kudzu (Pueraria montana var. lobata), native to Japan, is a climbing, coiling vine that smothers and kills
other plant species, and it now covers hundreds of thousands of acres in the United States.
appeared to be threatening as they
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JAMES R. ALLISON, GEORGIA DEPARTMENT OF NATURAL RESOURCES, BUGWOOD.ORG
LEARNING OBJECTIVES
C O N T I N U I N G E D U C AT I O N U N I T
A native or indigenous organism is any
species (plant, animal, or microorganism)
that was part of a natural community
prior to the intervention of humans.
Just because a species is native does
not mean that it will always remain
a part of that specific biological community. All species that do not evolve
will eventually become extinct. For example,
dinosaurs were once native to every continent.
Their environment changed, they could not adapt,
and they became extinct. Birds are believed to be
their only living descendants.
Conversely, a non-native is an organism that is not originally from
an area. Non-native species do not have to be living and reproducing in an area to be classified as a non-native. Quarantine laws seek
to restrict many non-native species from areas where they are not
currently living and reproducing.
An alien species is a non-native that has been moved into a new
location by humans. This movement may be intentional or unintentional, but it is always associated with human activities. It does
not include the natural migration of an organism by its own efforts
or by the forces of nature. Some of these alien species are so closely
related to native species that they will interbreed with native species,
producing hybrids, or species with a mixture of characteristics from
the two different parents.
An invasive species is a non-native or an alien species that has
begun to reproduce and replace native species. The native species
being replaced may be closely related to the invasive species. Invasive
species may replace native species by encroaching on habitat currently occupied by native species, leaving the native species with
a diminishing area in which to grow and reproduce. It may also
involve a disease pathogen or parasite actually attacking a native
species. Invasive species affect the web of life in other ways as well—
pollination, herbivory, nesting, feeding, migration, reproduction, and
predation of native species all may be affected by the change in species
composition following such invasion. Most invasive species are problematic under limited circumstances involving climate or habit. For
example, a terrible weed of warmer wetlands might be a benign garden plant, or not even able to survive in cooler or drier habitats. It
is important to know this in advance so that any restrictions imposed
in a given region will be reasonable and not ignored as overzealous
environmentalism.
Animals and plants that have migrated on their own or by natural
efforts are not considered invasive actions even if they are problematic for humans. A species that is termed “invasive” is attributed to
human action in transporting these organisms, be the actions intentional or unintentional. The armadillo and opossum are examples of
animals considered problematic that were originally native to South
America. When the isthmus (Panama) joined the two Americas a
short three million years ago, these strange-looking, slow-moving
animals ambled their way northward into North America.
An exotic is something that is recognized as new, different, and
appealing to humans. The perceived beauty or interest in an exotic
species often results in their introduction without regard for their potential to become an invasive species. A recent survey (Hulme) of the 100
most problematic invasive species of plants in the world attributed
more than half to “escapes” from cultivation in botanical gardens.
An escape is an exotic species that was or is cultivated but has been able to reproduce from cultivated
sources. If escapes become part of wild populations,
the species is said to be naturalized.
A naturalized species is a non-native that has been introduced into a new environment, where it is now able to grow and
reproduce in its new location without human assistance. Naturalized
species are always non-native. For various reasons, only a relatively small percentage of the introduced species are capable of
becoming naturalized, and only a small percentage of this group is
capable of reaching population levels or has characteristics that are
considered problematic to humans or native species. Unless there
is an overriding issue, naturalized species that are not producing a
significant negative impact on their environment are generally not
considered to be invasive.
Like invasive alien species, aggressive native species have a natural tendency to spread, forming dense thickets. These natives may
approach a true monoculture and cover extensive areas. They may
or may not be offensive to humans. If these species are in their natural (native) habitat, they are not considered to be invasive even if
they are viewed as problematic to humans. By definition they are
native, and this is where and how they grow in their natural systems. In eastern North America, poison ivy (Toxicodendron radicans),
black locust (Robinia pseudoacacia), and native cane (Arundinaria
gigantea) are all known for forming dense thickets, making them
unsuitable for small residential properties. Early travelers avoided
areas where these were the dominant species. Nevertheless, they are
native to eastern North America. This is their natural habitat, and the
JOSEPH O'BRIEN, USDA FOREST SERVICE, BUGWOOD.ORG
Invasivespecies
affect the web
of life.
Chestnut blight canker on American chestnut (Castanea dentata).
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AUGUST 2011
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New Species in the Urban Environment
Biological populations of plants and animals growing in natural
systems are constantly in a balancing act. Although it is usually too
slow to see in a human life span, evolution continues, climates change,
and continents drift, permanently altering the environment. Species
go extinct and new species evolve as a result of biotic and abiotic
pressures on populations. In order to be able to recognize what is
native, we must look at a specific point in time. Anything that appears
as a result of human activity after that point in time is non-native.
If a species arrives unassisted by human activity, it is also considered
native even though it is recognized as a recent arrival.
It is unrealistic to expect to find all local native species in a limited
area; because not only do we have to look at a specific point in time,
we must also look at relatively large geographic areas. The least confusing designation is to cite plants native to one or multiple states,
provinces, or geographic regions of a country.
Very little in life is absolute. Before we label all non-natives as bad,
remember what you ate for your last meal. Even if grown locally,
the odds are that the majority of the plant products you consume
and wear are non-natives. Non-natives can be beneficial, even essential to our lives. Most notable are our food crops. Our diets would
be both meager and limited in variety if we were forced to live exclusively on edible plants native to our specific regions. Agricultural
production of non-native foods allows most of us to live in cities.
These high-population centers help preserve natural areas in a more
natural state than if human populations were uniformly spread around
the world.
Cities, or the built environment, only remotely resemble their
pre-human character. Soils have been altered and hydrology changed.
Reflected light and drying winds make it an alien environment for
most native species. Human stresses ensure that plants do not survive as long in the built environment as they would in their native
homes. Species that survive in altered urban and suburban environments are often species associated with flood plains or less hospitable
climates. Some urban-tolerant species are native species selected
IAN TRUEMAN, UNIVERSITY OF WOLVERHAMPTON,
BUGWOOD.ORG
C O N T I N U I N G E D U C AT I O N U N I T
web of life that has evolved in such areas over millennia depends
on them. The small, suburban lots and the residents now reside
where these species once formed dense stands, and are now the
ones who are out of their element (e.g., non-native).
The tree of heaven (Ailanthus altissima), native to China, is an invasive
species in Europe and the United States that grows quickly. The tree
also releases allelopathic chemicals that can suppress the growth of
other species. Photo location: Poland.
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from the extremes of their natural range. Others tolerant of urban
pressures are from distant continents. An important criterion for
inclusion in the urban landscape is that introduced species do not
become invasive or problematic in the human or natural environment.
Why do some non-native plant species become invasive while
others do not? There is no single reason. Rather, there are complex
environmental and biological causes that are compounded with
the anatomical characteristics of the introduced plant.
Some non-native species are not readily adapted to their new
environment. While they may be able to become naturalized, environmental conditions (e.g., moisture, soil drainage, temperature,
length of the growing season, pollinators) may limit growth rates or
ability to reproduce and become a dominant species. These species
just have difficulty surviving.
Non-native species with the potential to reproduce rapidly are
often introduced without the diseases and insects that have traditionally kept populations under control. Host plants and the diseases
and insects that feed on them always live in a delicate balance. A
pathogen that kills its host may die. The absence of primary predators
in its new home sometimes results in a host plant growing and
reproducing with wild abandon; that is, until the local pathogens
and pests adapt to being able to consume the recent invasive species
as a new food source. Plants develop new defenses against existing
pathogens and pests with time. This delicate balancing act occurs
rather gradually as viewed through human eyes.
Some species are able to out-compete native species by production of allelopathic chemicals, which suppress the growth of other
species. Bush honeysuckle (Lonicera maackii and L. tatarica), introduced from Asia in the late 19th century, reduces competition from
natives by producing chemicals in its foliage. The bush honeysuckle
also gains an upper hand in the competition by leafing out very early
in the spring and retaining green foliage later into autumn than its competitors. This helps it gradually outgrow and shade out other species.
Invasive Insects and Diseases
It is important to understand that it is not just plants that become
invasive. Insects and diseases can also become invasive when they
are moved to areas where they are not native. The hemlock woolly
adelgid (Adelges tsugae) was unintentionally introduced into eastern North America from southern Japan. The Canadian hemlock
(Tsuga canadensis) is highly susceptible (has no resistance) to this
insect. Resistance differs from immunity in that a species that is
resistant to predation can sustain some feeding. A species that is
immune will not be susceptible to any feeding from the pest. In its
native Japan, the adelgid population has been kept under control
by a combination of host plant resistance and natural predators.
Disaster struck the Appalachian forests of North America in the
early 1900s when a fungus, later named chestnut blight
(Cryphonectria parasitica), is thought to have been accidently introduced into the northeastern United States on either timber or live
trees brought from Asia. Within a few decades, 4 billion native chestnut trees (Castanea dentata), a quarter of the canopy cover in the
Appalachian range, were dead or dying. Species of chestnuts from
Japan and eastern Asia are resistant to the fungal disease.
Another fungal disease, later named Dutch elm disease (Ophiostoma
ulmi), began killing elms in Europe by 1910. By 1928, it had traveled
on logs from Europe to North America. This fungal disease is thought
to have originally come from Asia given how Asian species of elm
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Ecosystem Balance
slightly different environmental conditions. Insects and diseases
began to feed on their host plants. By chance, some of these host
plants had naturally occurring defensive compounds that resulted
in slightly less predation. This gave minimally resistant host plants
a slight edge in reproduction over others in the same species.
Eventually, resistance to feeding increased. At the same time that the
host plant was increasing its ability to defend itself, the insect or
disease was also evolving an ability to tolerate the plant’s defensive
strategy. While this may sound wasteful, a host that does not suffer
some predation is going to become less vigorous. Over time,
insects, diseases, and host plants develop a delicate ecological balance where each is able to tolerate (and benefit) from the presence
of the other.
This balance between a host and its pathogens and predatory
insects worked well until inquisitive humans began traveling the
globe, moving different species from one ecosystem to another. The
most significant of these disasters have occurred since the mid 19th
century and can only be expected to continue with globalization of
world economies. These problems stemming from invasive species
are of our own making. Solutions are rarely simple and vary with
the characteristics of the host and pest:
• Quarantines work when their need is understood and when
they are reasonable (and thus respected) and are fairly but
consistently enforced.
• Infestations discovered before they become widespread can
often be eradicated.
• Selective breeding for resistance or introduction of resistant,
non-native species is another management technique.
• Programs involving prescribed use of pesticides and/or cultural
techniques have contained some pests at manageable levels.
• Over time, doing nothing is often the most expensive option.
Provenance—the reference to where an organism evolved or is
considered to be native—is important in determining the potential
for a species to become problematic to native species. References to
where a species is native must reference geographic place names.
Researchers recognize that many organisms are not bound by artificial lines, be they state/province or the national boundaries found
on our maps. Birds and other animals migrate with predictable regularity between continents. They carry pests, pathogens, and seeds on
their bodies and in their waste products. Seeds are also blown long
distances and across borders by wind.
These means of travel are recognized as natural methods of dispersion. They differ from human dissemination in that they are more
gradual and have occurred over millennia, giving natural systems a
chance to adapt. It also frequently
differs in that chance introductions
might be extremely limited in scope,
and thus unlikely to succeed, whereas plants or animals introduced by
humans will be more numerous.
This vastly increases the odds that
one of these individuals will find the
perfect niche in which to reproduce
and begin the process of invasion. It
also increases the odds that natural
variation within the large population
of individuals will include the equivalent of the perfect genetic storm of
site compatibility and vigor. Such has
been the case with many of the plants
introduced for conservation purposes.
Species that are almost identical,
but are separated geographically, are
referred to as disjunct species. Disjunct
Cities, or the built environment, only remotely resemble their pre-human character. Soils have been altered
species of plants began to co-evolve
and hydrology changed. Reflected light and drying winds make it an alien environment for most native
with similar, but slightly different,
species. Human stresses ensure that plants do not survive as long in the built environment as they would
in their native homes.
insects and pathogens and under
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15
C O N T I N U I N G E D U C AT I O N U N I T
Species
lost are
lost forever.
are resistant to the disease. Because
species in North America and Europe
had never developed a tolerance for this
disease, they had no resistance and
died by the thousands. Later, two other closely
related species of the fungus were introduced.
One was from western Himalaya (O. himalulmi) and another (O. novo-ulmi) was thought
to be a hybrid of the other two fungal species or—some believe—from
China.
While North American customs officials look vigilantly for any
biological organism that may be coming in from another country,
most of the problematic disease and insect pests of North America
are the result of introductions from Asia and Europe. The flow of
invasive diseases and insects has moved from each of these continents
to the other two. Relatively few plant pests have come from South
America, Africa, and Australia. The reason is provenance.
C O N T I N U I N G E D U C AT I O N U N I T
While the problems created by invasive species and species
extinction will always find a natural balance, this occurs more in a
geologic time frame rather than over the simple course of multiple
human life spans. The loss of a single species sends ripples through
the ecosystem as other life forms dependant upon it are forced to
adapt or enter the fossil record. Species lost are lost forever.
What are our obligations as green industry professionals?
1. Recommend and install only plant species known to present
no ecological problems in the area and habitat in question.
The acquisition of this knowledge is an ongoing process.
2. Work to rectify problems of the past by practicing environmental stewardship and restoration. The green industry is
responsible for the majority of the most significant invasive
species causing problems today.
3. Learn and practice the principals of Plant Health Care. The
urban environment is a biologically dynamic web of life.
4. Educate the public on the value of species diversity.
5. Match the plant to the site.
6. Install and manage plants in accordance with industryrecognized Best Management Practices.
7. Work with engineers and design professionals for sustainable
green infrastructure. Trees and other urban plants only provide
maximum benefits when they are capable of becoming a
mature portion of a safe and sustainable urban environment.
Selected References
Borrell, B. 2011. A Friend to Aliens. Scientific American (February
2011) 304(2):74–77.
Burdick, A. 2005. Out of Eden: An Odyssey of Ecological Invasion.
Farrah, Straus, & Giroux: New York, NY.
Davis, M.A. 2009. Invasion Biology. Oxford University Press.
Dawson, W., A.S. Mndolwa, D. F.R.P. Burslem, P.E. Hulme. 2008.
Assessing the risks of plant invasions arising from collections
in tropical botanical gardens. Biodiversity and Conservation
17(8):1979–1995.
Marris, E. Rambunctious Garden: Saving Nature in a Post-Wild
World. Bloomsbury (in press).
Pollan, M. 1994. How To Make a Pond. New York Times Magazine
July 24, 1994. <http://michaelpollan.com/articles-archive/
how-to-make-a-pond/>
Sternberg, G., and J. Wilson. 2004. Native Trees of North American
Landscapes. Timber Press.
William M. Fountain is Extension Professor of Arboriculture and
Landscape Management, University of Kentucky. James R. Lempke
is Curator of Native Plants and Natural Ecosystems (retired),
The State Botanic Garden of Kentucky.
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Challenges for the Built Environment: When Native Species Meet Their Alien Relatives,
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1. Which of the following is most likely to
become an exotic invasive species in
Europe?
a. a native species that produces suckers
b. a native species propagated by grafting
c. species introduced from Asia
d. species recognized as wildlife cover
2. The majority of invasive species introduced
over the last century can be traced to
a. climate change
b. the green industry
c. landscape architects
d. the migration of animals
16
3. An exotic species is one noted for
a. aesthetic attractiveness
b. ease of cultivation
c. growing only on another continent
d. unusual growth habit
4. An alien species is one that has been
a. crossed with another species
b. cultivated for less than 100 years
c. able to reproduce from cultivated sources
d. introduced from another area by human
activity
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5. A naturalized species is one that is
a. approved by government plant quarantine
officials
b. a native species that has been introduced
c. an introduced species that reproduces
naturally
d. a species with no disease or insect
problems
6. All non-native species are always bad,
and all native species are always good.
a. True
b. False
7. Non-native species may be more suitable
for the built environment than natives
because non-natives are often
a. easier to transplant and grow
b. more attractive and easy to propagate
c. more disease and insect resistant
d. more adaptable to heat and compaction
8. An American chestnut (Castanea dentata) that
has no resistance to infection by chestnut
blight is said to be
a. disjunct
b. immune
c. impervious
d. susceptible
9. Which of the following might be responsible
for keeping a naturalized exotic under control?
a. climatic temperature extremes
b. lack of appropriate pollinators
c. native diseases and/or insects
d. all of the above
ARBORIST • NEWS
11. Using natives exclusively will guarantee
landscapes free of insects and diseases.
a. True
b. False
12. The task of determining what is native to a
specific area necessitates that one consider a
specific reference point in time, because
a. it determines what was present before
people began relocating species
b. recent climatic change has eliminated
many native species
c. significant numbers of species move
naturally over long distances each year
d. none of the above
13. The difference between an alien species and
an exotic species is that exotic species are
always
a. cultivated
b. invasive
c. non-native
d. similar to a disjunct species
14. In order to be classified as being naturalized,
an organism must be
a. growing in an area before people were there
b. growing and reproducing without
human assistance
c. missing from the earliest taxonomic
records for the area
d. native to the site for at least 100 years
15. When invasive pests from other continents
are introduced, the prudent long-term
approach for management of the pest is to
a. apply pesticides to all plants in the area
in order to control the pest
b. do nothing—natural systems will rapidly
acclimate
c. eradicate all of the host plants in a 30 mi
(50 km) radius
d. investigate other organisms and conditions
that keep the pest under control where it
is native
16. The urban environment differs from natural
adjacent systems, because urban plants
experience
a. confined spaces
b. disturbed soils
c. higher temperatures
d. all of the above
17. When a native is cross-pollinated with a
closely-related alien species, the resulting
seedlings are called
a. cultivars
b. disjunct species
c. hybrid
d. new species
18. Species that are native do not have the potential to become problematic in landscapes.
a. True
b. False
19. Allelopathic conditions occur when one
plant suppresses the growth of another by
a. producing chemicals that are toxic
b. competing for water and mineral elements
c. shading sunlight exposure
d. all of the above
20. You should be suspicious that a new, nonnative species might be a potential invasive
species if it characteristically
a. begins to produce numerous seed at an
early age
b. leafs out early in spring and retains its
leaves late into the fall
c. reproduces by both suckers and seed
d. all of the above
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C O N T I N U I N G E D U C AT I O N U N I T
10. When comparing the close genetic relationship between the American beech (Fagus
grandifolia) of eastern North America and
the European beech (F. sylvatica), you may
describe them as
a. co-evolved species
b. disjunct species
c. exotic species
d. potentially invasive species