Russell D. Hunt, Department of Earth Sciences, University of South Alabama, Mobile, Alabama  36688.  E-MAIL:

            The accelerating problem of exotic species naturalization along coastal areas of the southeastern United States poses a serious threat to native species and biodiversity.  The focus of this study is limited to terrestrial land plants.  Biodiversity is a good indicator of the overall health of the surrounding environment.  Areas that have low species diversity are generally thought to be more likely to have higher densities of exotic species present.  Habitat preservation provides conservation of native species, overall biodiversity, and ecosystem functions such as water purification and plant pollination.  In order to find out typical biodiversity of lowland areas within the Dog River Watershed, six thirty meter transects are measured in swampy bottomland forests near the confluence of Halls Mill Creek and Dog River.  All plant species and the associated number of individuals are counted from a distance of 1.85 meters on both sides of the transect line.  After this, a species analysis is conducted and the densities of natives and exotics is compared.  The most and least problematic exotic species can also be determined.  Exotics are characteristically opportunistic competitors and tend to be found in clusters in areas with lower biodiversity.  Thus, they are generally poor competitors in areas of high biodiversity.  Habitat preservation, the usage of native species in landscape decoration and gardens, as well as diligent, systematic eradication programs for exotics will help preserve biodiversity for future generations.  Alligator-weed is a problematic exotic in all transects but biodiversity is relatively good.

            Keyword:  exotic species, biodiversity, competition



            Biodiversity within the Dog River watershed is adversely affected by many factors that range from large-scale issues such as urban sprawl and exploitation of the surrounding environment all the way to something as seemingly insignificant as the introduction of a single exotic plant into an area where it multiplies like wildfire throughout the surrounding natural habitat.  Species diversity is the measure of the relative abundance of individuals of different species in an area or a sample (a measure of the evenness).  The species diversity of an area is a function of both species richness and relative abundance.  Species richness, the simplest measure of biodiversity, is the number of species found in an area.  High species diversity occurs when a large number of species are present and all species are represented by relatively the same number of individuals.  Low species diversity occurs when one species or a few species are represented by large numbers of individuals and the other species by few individuals (Spellerberg and Sawyer, 1999).

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 United States in the late 1700s when he shipped some tallows to his friend in South Carolina to cultivate for candle wax.  For more than a thousand years the trees had been prized in their native range in eastern Asia for their oily seeds.  Franklin’s actions made sense in the 18th century; the word “ecology” had not even been invented yet.  Unfortunately, Chinese tallow went on to invade nine southern states where it overruns bottomland forests and wet prairies (Devine, 1998).

An interesting aspect that probably contributes to the tree’s invincibility is the fact that its seeds float.  Also, a recent study conducted at Clemson University showed that at least sixteen bird species eat and transport the seeds, contributing to the tree’s invasiveness as well (Renne et al., 2002).  So, in a sense, the popcorn tree has naturalized itself and has displaced native species that the birds would normally feed upon.  From this, scientists are left with several questions that need to be answered.  Are the tallow seeds as nutritious as native seeds that the birds were originally eating?  What effects do the seeds have on the birds that like, or perhaps have developed a preference for, the seeds?


Research Question


            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 Peninsula Shoreline of Halls Mill Creek

2nd Transect:  Middle Peninsula (extremely muddy)

3rd Transect:  Downstream Peninsula Shoreline of Halls Mill Creek

4th Transect:  North Island, Southernmost Point Parallel to Halls Mill Creek Shoreline

5th Transect:  Southwestern part of North Island Shoreline

6th Transect:  Shoreline Flooded Forest Strip across from Peninsula Tip

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.


Results and Discussion

            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 Mobile Bay area.  It does mean that this particular area has excellent potential for conservation efforts.

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 Mobile Bay estuarine ecosystem that the Dog River watershed contributes to and is, therefore, a part of.




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