Nutrient Cycling and Invasive Species

Nutrient Cycling and Invasive Species

               

               Ecosystem changes in nutrient cycling by invasive species are quite numerous and complex.  Although invasive plants often do succeed because they possess some trait that allows them to better exploit available carbon, nitrogen, and phosphorous, it is an oversimplification to think that these are independent of the other ways that these plants can alter ecosystems.  Advantages in nutrient cycling can sometimes be gained by large over small plant size, perennial over annual tissues, seasonal growth timing, herbaceous over woody, or symbionts (Ehrenfelt 2003).

                Regarding carbon cycling, in a previous blog, I discussed productivity and how an invasive plant can succeed by having a larger biomass, larger root stores, higher quality litter, and faster growth rates.  An exotic plant in Australia owes part of its success to its different photosynthetic process, crassulacean acid metabolism (CAM).  Prickly pear cactus (Opuntia) is a succulent plant that closes its stomata during the day and is very efficient at conserving water and carbon in arid environments.  Some cacti can survive in temperatures up to 63⁰C, among the highest tissue temperatures of any vascular plants.  In eastern Australia, the plants have exploited overgrazed and drought-damaged grasslands where they had no competition (DeFelice 2004).

Prickly pear cactus (O. stricta)

 Hawaiian fire tree (M. faya)

                The often cited example of an invasive plant that has exploited and changed nutrient cycling in an ecosystem is the Hawaiian fire tree, Morella faya.  It is an actinorrhizal nitrogen fixer that has invaded nitrogen-deficient, young volcanic areas where there were no native species with similar capabilities.  This tree doubles the available nitrogen and water available under the canopy.  It has gradually replaced the native Metrosideros tree mentioned in an earlier blog.  Experimental studies have shown that M. faya has altered primary successional ecosystems by increasing the amount and biological availability of fixed nitrogen (Vitousek 1990).

                

                Increased nitrification, without nitrogen-fixation, is part of the successful invasion of Japanese barberry (Berberis thunbergii) and Japanese stiltgrass (Microstegium vimineum).  When compared to the soil under native species, there were differences in the soil under the two non-indigenous plants.  There was a decrease in fungal abundance and an increase in European earthworms under the barberry tree; under the stiltgrass, there was, along with the increase European earthworms, an increased abundance of the arbuscular mycorrhizal fungi (AMF) (Wolfe 2005).

Japanese barberry (B. thungergii)                                                            Japanese stiltgrass (M. vimineum)

                                                     Bridal creeper (A. asparagoides)

                An example of an invasive plant that excels in phosphorus cycling is the bridal creeper (Asparagus asparagoides).  In the acidic, nutrient-poor, weathered soils of Australia, available phosphorus is quickly adsorbed to soil particles.  The low soil phosphorus, maintained by native plants that absorb phosphorus before litter falls, prevents many exotic plants with high phosphorus requirements from competing.  Bridal creeper is efficient at extracting both phosphorus and nitrogen from soil.  It produces high quality nitrogen and phosphorus-rich litter that decomposes rapidly and leaches the phosphorus.  Plentiful shoots catch litterfall, and a thick tuberous root mat traps nutrients from the soil.  Soil phosphorus pools increase which improves soil fertility and allows other invasive plants to thrive (Simberloff 2011).

                Nutrient cycling differences between native species and non-indigenous species have been well documented.  Photosynthesis and carbon fixation differences are reflected in increased net primary production on the ecosystem level.  Higher carbon input rates due to differences in carbon physiology, nutrient use efficiency, and leaf carbon cost have been described.  Advantages in nitrogen and phosphorus acquisition, either due to plant physiology or symbiotic relations, can explain the successful invasion by other plants that lead to overall changes in the ecosystem.

References

1. DeFelice, Michael S., (2004). Prickly Pear Cactus, Opuntia spp.: A Spine-Tingling Tale. Weed Technology. 18:869-877.

2. Ehrenfelt, Joan G., (2003). Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes. Ecosystems. 6:503-523.

3. Simberloff, Daniel.,( 2011). How common are invasion-induced ecosystem impacts?.  Biological Invasions. 13:1255-1268.

4. Vitousek, Peter M., (1990). Biological invasions and ecosystem processes: towards an integration of population biology and ecosystem studies. Oikos. 57:7-13.

5. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.