Russell D. Hunt, Department of Earth Sciences,
accelerating problem of exotic species naturalization along coastal areas of
Keyword: exotic species, biodiversity, competition
An exotic or alien species is one that is introduced to one region from another geographical region (Spellerberg and Sawyer, 1999). There is a myriad of examples of exotic species that have established themselves throughout the Dog River Watershed including terrestrial, marine, and freshwater plants and animals. When these invaders establish large populations they take up space and use up resources that native species need to survive and successfully reproduce. Eventually such competition may lead to the extinctions of countless native species in the affected areas.
Exotic species are invasive to the areas where they multiply rapidly because they do not have natural predators like their native counterparts. Exotic species thrive on exploitation opportunities via disturbance events, whether they are naturally induced or a result of human activity. Furthermore, exotics usually disperse themselves in the same way that a common weed would claim territory. Weeds will make their way into an area that has been disturbed, grow very quickly, produce offspring, and the offspring will, in turn, claim territory in very widespread areas, often in a single season.
The focus of this project is on
exotic terrestrial plants, although projects need to be done on other types of
invasive species within the watershed as well.
One species of particular interest to the Dog River Watershed is the
Chinese tallow or popcorn tree (Fig. 1), which seems to
have taken up residence in every conceivable niche. Benjamin Franklin brought it to the
An interesting aspect
that probably contributes to the tree’s invincibility is the fact that its
seeds float. Also, a recent study
Within the Dog River Watershed invasive species are common in areas that have been altered by humans. These species reproduce quickly and in large numbers leading to the eventual invasion and degradation of biodiversity in largely unaltered habitats. What is the biodiversity of typical undisturbed bottomland hardwood forests near the boundary of freshwater and saltwater influxes? Are invasive species abundant? Where do more exotic species exist? Are native species abundant? Where do more native species exist? What exotic and native species are represented?
To determine how large the terrestrial exotic plant species population is and how many species, on average, there are within the Dog River Watershed, two general locations are studied. Transect sites were located on a small peninsula near the mouth of Halls Mill Creek before it empties into Dog River and at the opposite side of the creek from the peninsula (Fig. 2 & Fig. 3). The sites on the opposite side of the creek from the peninsula were reached via canoe while the peninsula sites were accessible by foot. All study locations have been largely unaltered and are considered pristine in this study. All study sites are extremely muddy and usually waterlogged as well, even when the tidal influence is low. When the tidal influence is high all study sites are six to twenty-four inches underwater. Regular waterproof boots were worn in the field. However, most of the time the water was too high for them to be effective so hip boots would have been more appropriate.
Transect 2 was the only one not near the shoreline but it was still very wet and muddy. Transect 6 was, by far, the wettest of all the transects. The forest canopy was thickest at Transect 1 while the others were intermittently open and canopied. The transect location descriptions are as follows:
1st Transect: Upstream
3rd Transect: Downstream
5th Transect: Southwestern part of North Island Shoreline
6th Transect: Shoreline Flooded
All transect sites where species sampling occurred were measured from start to finish with a thirty-meter tape measure (Fig. 4). The starting and ending points were recorded with a GPS unit. A variety of six transects were laid out, ranging from shoreline points to points inland to sites that were well away from the shoreline. At each designated transect all plant species were identified and each representative individual was tallied in a field book. In order for an individual plant to be counted it must have some sort of identifying characteristic and leaves must be old enough to key out taxonomically.
By far, the most difficult part of this project was the identification of unfamiliar plants in the field and using collected specimens to key them out taxonomically. Many plants, especially at the first three transects, were unfamiliar to me so I had to collect stem and leaf samples and put them in quart-sized zip-lock plastic bags to keep them organized and from drying out. Then I used a variety of plant taxonomy books to identify them. All species with fifty or more individuals were given a “50+” designation and were dominant plants within their associated transect.
Table 1 shows which species were found and the associated tallies for each of the six transects.. The most dominant plants which earned “50+” designations in the field were changed to “50” in this table to allow for statistical analysis.
Some plants in the above table are designated with question marks around a makeshift name because of identification difficulty. This difficulty was probably due to a combination of factors, including young plants, the absence of mature leaves, the absence of flowers, and unfamiliarity with many of the small plants encountered. It is important to note that Transect 5, which had the highest population of invasive exotic species, the highest total population, and the highest species richness. The transect also did not have the highest percentage of dominant individuals; that title goes to Transect 4, which had 80%.
A recurrent theme when counting individuals in the field is the clustering of species. Clustering also occurs across species lines. There is a correlation between cluster locations and abiotic and aquatic variables such as topography or water depth. The distribution of cluster sizes exhibits equilibrium and nonequilibrium behavior depending on species life history (Plotkin et al., 2002).
Aquatic plants seem to be the only ones able to really take root within the study transects. The only invasive species found abundantly throughout all transects was alligator weed (Fig. 5), which was one of the dominant plants in each transect. This plant is always found in great numbers and effectively covers large tracts of wetland ground. The only other exotic invasive species found was wild taro, which was only near the shoreline of Transect 5 and was represented by 25 individuals (Fig 6). In a disturbed environment, selection seems to favor fast growth to maturity and reproduction. Although high relative growth rates may confer an initial competitive advantage to invasive species, short juvenile periods (minimal generation times) are also very important for the spread of an invasive species. Species with this fast growth strategy, in terms of both biomass and numbers, are the potential invaders when introduced to an area (Grotkopp et al., 2002).
Disturbances have unanimously been
shown to favor plant invasions. They
simultaneously increase resource availability and decrease competition from
resident species, thus facilitating the colonization of species with greater
competitive abilities than the natives (Prieur-Richard and Lavorel, 2000). From this we can gather that shifting tides
represent the disturbances that have allowed alligator weed to establish itself
so successfully. One could assume that
the study area’s largely unaltered habitat has too much biodiversity and
therefore native competition present for other invasive species to successfully
become established and reproduce. This
does not, however, mean that invasive species are not a problem in the
One reason for preserving habitat
and native species is because biodiversity is related to ecosystem function, so
that ecosystem services like water purification and pollination depend on a
very broad array of species. This
argument for the preservation of biodiversity would be strengthened if native
species, rather than introduced species, were the key contributors to ecosystem
function. This argument basically
centers on the role of rare species in ecosystems (Krebs, 2001). Experts consider invasive species second only
to habitat loss as a threat to native biological diversity, and they are a
leading factor in listings under the Endangered Species Act. Whether they arrive as unseen stowaways on
planes or ships or are intentionally introduced as kudzu or the popcorn tree,
they all have the potential to become serious problems (Jenkins, 2002). Citizens living within the watershed have a
vested interest in the health of the surrounding habitat, which ultimately
contributes to water quality and health problems in the population. Biodiversity preservation must begin while
there is still something worth preserving and before exotic species have the
opportunity to completely destroy the fragile
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