What is an alien species?

No, it is not some large green slimy being with a big head and weird eyes traveling down from Mars to destroy our planet. It is, however, just as frightening. An alien species or invasive species, is a "species that has been moved into an area and reproduced so aggressively that it has replaced some of the original species." (www.speciesatrisk.gc.ca/glossary e.cfm) Alien species come from our very own planet and attack our very own species. They can be in our lakes and streams, parks and nurseries, or even our own backyards.

Why do we care?

Alien species have the overwhelming potential to destroy habitats, convert them into completely new ecosystems, or cause mass extinctions of native species. Next to habitat destruction, invasive species are the most important cause of species endangerment and extinction. Historically, we have associated invasive species as only being significantly detrimental in island communities. In island communities, invasive species out compete the native species for resources. There are no natural predators that have been able to evolve to suppress their numbers, thus they take over. On islands or island like areas, native species have no where to escape to and no new possible niches to occupy, thus they go extinct. The effects of alien species are often times not even seen, however. Now, we are discovering the invisible "bottom-up" invasions of alien species in non-island ecosystems, such as the effects of invasive species in the soils of deciduous forests.

Invasive Species in Deciduous Forests: (A Hypothesis)

In 1990, scientist Vitousek proposed the possibility of a positive feedback loop caused by invasive species changes in soil composition. He suggested that by changing the natural soil ecology, invasive species could not only cause the acceleration of their own growth, but perhaps even promote further growth by other invasive species.

An experiment was constructed to prove how invasive plants could effect soil composition and how these changes in soil characteristics could affect the natural ecological community. In the experiment, two dominant alien species, Berberis Thunbergii and Microstegium Vimineum, were studied in the under story of three highly infested New Jersey parks, Morristown Natural Historical Park, Worthington State Forest, and Allumuchy State Park. The ancient soils of these three deciduous forests are considered a peinobiome. The lands were never covered by the ocean since they were part of the old Gondwana shield and thus they have suffered from long term leaching and are extremely nutrient poor.

The Experiment:

An experiment was conducted to prove how invasive plants could effect soil composition and how these changes in soil characteristics could effect the natural ecological community. In the experiment, two dominant alien species, Berberis Thunbergii and Microstegium Vimineum, were studied in the under story of three highly infested New Jersey parks, Morristown Natural Historical Park, Worthington State Forest, and Allumuchy State Park. Berberis Thunbergii is a woody shrub with dense surface roots. It was used often as an ornamental plant in the late 1800s and thus is found frequently in abandoned farm lands. Microstegium Vimineum is a C4 grass that forms continuous lawns in the under story.

The results of the experiment revealed that the soil below the invasive plants had significantly higher pH than the soil found below native species. The NH4 concentration studied between March and June under Berberis had increased probably due to the early leaf growth of Berberis. The NO3 soil concentrations below both invasive species were also increased for the entire year studied. As the season progressed, the net nitrification rates had increased, thus by midsummer most of the mineralization in the soil below the exotics, was due to nitrification. This high level of nitrification is unusual and detrimental to native plants, like Vaccinium, who have become adept to peinobiome soils. By midsummer, the soils below Vaccinium are found to have higher nitrification rates, however, ammonification also increases. The ammonification increase contributes the largest portion of the soils net mineralization and keeps the soil acidic unlike nitrification.

Berberis Thunbergii (www.habitas.org.uk)

Microstegium Vimineum (www.dnr.wi.gov)

The increased root density of Berberis might contribute to its higher pH.

Soil respiration in the soils below the exotics also increased, adding further nutrients to the soil. Root biomass probably is partially responsible for this as well.

The litter decomposition under Berberis is extremely fast. In fact, up to 90% of the initial plant material is decomposed every year. 17% of the nitrogen remains in the litter after decomposition, thus again increasing the nitrogen nutrient concentration in the normally nutrient poor soil. Microstegium litter decomposes about 40%-60% in the first, but very little after that. The decomposed litter, however, contains almost all of its original nitrogen.

Conclusion:

Deciduous Forest plant species are perfectly evolved and adept to growing in highly nutrient poor soils. Invasive species, like Berberis Thunbergii and Microstegium Vimineum are altering the natural characteristics of deciduous soils, making them more prone to further, successful invasions. The result could mean the loss of the entire deciduous forest ecosystem.