The Weedy Facts
By Sarah Hanson
Have you ever experienced the disappointment of a dying garden? We identify with the
nature that we choose to have in our environment. Beautiful and exotic flowers are planted in our
gardens, set in our homes, and line the sidewalks of our towns. The problem is some of these
non-native species have unintended effects on the natural ones, which occur in places without
human intervention. Non-native or alien species are those that have been relocated far from their
native habitat, and once they spread they become “invasive.” Many invasive plants found in the
United States are introduced through the horticultural industry, commonly referred to as the
nursery trade. The ability of a non-native species to thrive in an environment foreign to its own
depends first on the ecology of the plant, and second on the invasibility of the environment.
Because of certain ecological characteristics, invasive plants can out-compete native
plants. Think of this competition like an army taking over a country. Slowly the army will
infiltrate, trying to get the people to adopt their cultural beliefs and customs. Eventually the
original customs of the country can be lost. This is how invasive species work, too. An invasive
plant can be introduced as just a single individual, but with time the native plants can no longer
be found. Some of the tactics used by invasives to compete with native species include crowding,
hoarding water resources, altering the soil composition, lacking natural predators, or being
favored by dispersers.
A local example of an invasive plant is Golden bamboo. The grass causes harm to native
plants by crowding around and absorbing the available sunlight, making it difficult for
neighboring plants to photosynthesize and grow. Other invasive plants are extremely thirsty, and
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manage to drink most of the water. A great example of this type of invasive is the eucalyptus tree
found throughout South Africa.
Phyllostachys aurea (Golden bamboo)
Eucalyptus grandis (Flooded gum)
In the late nineteenth century, the trees were planted by the thousands to be used by the booming
gold mining industry as walls to their mines. Two centuries after their introduction, an initiative
was launched by the Working for Water foundation to have the trees removed: not only did they
deplete all of the ground water, but they were also allelopathic, which means that the trees were
able to inject a chemical into the soil that caused native plants to become ill. Natural regulatory
controls, such as herbivores or pests, that would keep the growth of these invasive species in
check in a native setting, are often absent in the introduced environment, so plants can spread at
an extremely fast pace in comparison to their natural environment. Unfortunately, while pests
and insects commonly avoid feeding on non-native plants, birds and mammals often do consume
their fruit. Fleshy fruited invasive plants have the highest potential to spread and outcompete
native ones because in addition to wind and water, they have animals to spread their seeds. If an
animal or pollinator favors the invasive flower or fruit, a native plant becomes ignored. When the
animals change their diet, from native to invasive, often the native species is eradicated
altogether.
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But what makes an environment so unfit for native species and so welcoming for invasive
species? Simply put, some environments are more invasible than others. Generally, environments
that lack human activities such as roadways are not heavily overturned or harvested and are least
at risk for biological invasions. The most invasible environments are those that have an altered
state of physical landform, vegetation, and hydrology, creating bare soil, erosion, or
sedimentation. Once land is disturbed, soil, sunlight, water, and other nutrients become readily
available to the invasive that can enjoys freedom from competition with native species and can
take root rather quickly. Another factor that makes an environment invasible is a gap. Often the
species that are most successful to filling these open niches are those that are non-native, as
native plants are less successful in exploiting the open areas.
Like grilled stickies and vanilla ice cream, sometimes an invasive’s ecology and the
foreign environment are extremely compatible. In South Africa, for example, there is a small
Nature Reserve called Dwesa-Cwebe. The coastal reserve is extremely diverse in plant and
animal species, but can be considered an extremely disturbed place, because located within its
boundaries is a hotel. To better suit the guests’ needs in the early nineteenth century, a garden
was planted near the shore and was planted with bananas, avocado, and guava fruit. Today, the
garden is overgrown with grass and no longer produces bananas or avocado, but the guava
remains abundant. The guava’s success in comparison to the banana and avocado trees can be
attributed to the production of viable seeds as well as its tolerance to various environmental
conditions. The guava is now listed in the Global Invasive Species Database as a fierce invader
in regions of Hawaii and the Galapagos Islands, in addition to southern Africa. The small
baseball-sized fruits have an extremely seedy center, as seen in Figure 1.
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Figure 1: Psidium guajava (Common guava)
It takes only three years for them to mature from a seedling to a fruit-producing adult. The shrub
is widely tolerant of drought, sea spray, and can grow in nearly any soil type. Monkeys, birds,
cattle, and humans can be seen munching on the fruit, and the seeds have been excreted nearly
everywhere.
Roadways are particularly important in the discussion of invasive species like the guava
for several reasons. First, tire treads are great at carrying and depositing seeds. Second, roadways
must be laid on top of the earth’s surface, causing sediment to be removed and regularly tossed
around. Third, birds and small mammals like the vervet monkey, which are abundant in DwesaCwebe, prefer to hangout along forest edges. The presence of a roadway divides a forest into
parts and causes animals to come to the outskirts. Finally, roadways aid in the establishment of
invasive plants because of the power lines that often trace their course. Power lines are a
preferred perching location for birds, and as a result thousands of guava seeds have been
excreted by birds directly above the overturned soil causing the shrub to spread like wildfire
along the forest edges. Like citrus fruits, the guava shrubs require sunlight to grow and will not
be found growing in forest interiors. Figure 2 shows the distribution of the guava shrub along the
roadways in the Dwesa-Cwebe Nature Reserve.
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Hotel
Historic
Garden
Guava
Figure 2: Visualization of invasive guava distribution at the intersection of two roadways in the Dwesa-Cwebe Nature Reserve.
Credit: Sarah L. Hanson, Department of Geography, The Pennsylvania State University. Google Earth.
You can notice that the guava is most abundant near the hotel and the intersection of two
roadways, and becomes less prevalent as distance increases from the site of introduction, the
historic garden. Not much is understood about the affects the shrub has on native species, and no
action is being taken by conservationists against the shrub to date. Like many weeds, when the
shrub is cut several grow back in its place, making it extremely difficult to control. One thing is
for sure though: the shrub is spreading as fast as water runs rapid, and will inevitably cause the
decline of natural plant species.
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Environments are constantly changing as new species are introduced and others are
removed. Plants and animals are continually adapting to avoid being outcompeted, but many
times invasive species have the upper hand and can affect the ecology of an area before actions
are made to prevent the change. So the next time you are at the local nursery, you should make
informed decisions about the types of plants you are choosing for your garden, and recognize
that your desire to shape your environment can have a broader impact.
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Special Thanks to:
Dr. Erica Smithwick, Director of Landscape Ecology at Penn State (LEAPS)
Ms. Del Bright, Giles-in-Residence Writer
About the Author
Sarah Hanson is a senior majoring in Geography pursuing two minors in Geographic
Information Science (GIS) and the Science, Society, and Environment of Africa. Sarah studied in
South Africa on a faculty-led program called Parks and People in 2011, and was able to return
the following year to Dr. Erica Smithwick with her research. In June of 2012, Sarah was able to
assist Dr. Blair Hedges in surveying the remaining biodiversity of Haiti’s reptiles and
amphibians and she currently works with Dr. Hedges on the molecular timescale and
evolutionary history of life database known as TimeTree.org. After graduation, Sarah plans to
earn an advanced degree in geography, forestry, or photo journalism.
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Works Cited
Bargeron, Chuck. “Golden Bamboo.” Photo. Ipmimages.org 17 Nov. 2003. 15 Feb. 2013.
< http://www.ipmimages.org/browse/detail.cfm?imgnum=1237039>.
“Fazenda em Agudos.” Photo. Flickr.com 04 May 2010. 15 Feb. 2013.
< http://www.flickr.com/photos/psicodrops/4609098972/in/photostream/>.
Henderson, Scott. "Ecology of Psidium Guajava." Global Invasive Species Database. 10 Aug.
2010. Web. 17 Jan. 2013.
<http://www.issg.org/database/species/ecology.asp?si=211>.
“Mighty Guava.” Photo. Super Human Foods.org 16 June 2012. 10 Oct. 2012.
<http://superhumanfoods.org>.