The Ecology of Energy and Nutrient Fluxes - Article Example

Critique and Key Learnings from the Article THE ECOLOGY OF ENERGY AND NUTRIENT FLUXES IN HEMLOCK FORESTS INVADED BY HEMLOCK WOOLLY ADELGID, published by the Journal ‘Ecology’ HWA (Hemlock Woolly Adelgid) infestation has been causing a severe reduction in the survival rates of eastern hemlock in the North American forests and is currently reported from 15 states in eastern USA. As a result of this infestation, there are enormous changes in the nutrient and energy fluxes in the forest ecology.

Vitousek (1990) described three effects of the infestation of exotic insect species on a forest ecosystem, in terms of changes in the nutrient availability in biogeochemical cycles, trophic relationships within food webs, and physical (structural) alterations of the biotic and abiotic environment. Insect herbivores affect the foliage of the plants that they infest, causing alterations in nutrient cycling in the canopy and leading to changes in the decomposition dynamics by altering plant tissues and chemical properties of litter produced by the infested plants.

Such changes alter the carbon, nitrogen, and phosphorous content in living foliage and also affect the production of secondary compounds such as polyphenols and tannins (Choudhury 1988, Karban and Baldwin 1997, Belovsky and Slade 2000, Hattenschwiler and Vitousek 2000, Chapman et al. 2003). Furthermore, the excreta and protective structures, such as wax wool, of these insects change the quantities of energy and nutrients available in the canopy (Seastedt and Crossley 1984, Schowalter et al.1986, Lovett and Ruesink 1995, Stadler et al.

1998, Schowalter 2000, Christenson et al. 2002). These effects manifest themselves by altering the throughfall chemistry and litter properties. This study was conducted to evaluate the actual changes occurring in the nutrient cycling across the forest ecology because of an HWA infestation. In order to proceed with the study, the authors first examined the life cycle of HWA and evaluated the process of infestation in a stepwise manner. The canopy throughfall, litter samples beneath the trees, and leachate obtained from infested and non-infested trees were examined to estimate their chemistry, i.e. the amount of dissolved organic nitrogen (DON), dissolved organic carbon (DOC), inorganic nitrogen [ammonium-N (NH4-N), nitrate-N (NO3-N)], potassium (K+), total nitrogen (N total), and particulate organic matter (POM).

Their effect on the soil microflora was also assessed. The samples were obtained from field experiments at the Mount Tom State Reservation, located in the town of Holyoke, Hampden County, in south-central Massachusetts, USA This study successfully evaluated the effects of a Hemlock woolly adelgid (HWA) infestation on the “vertical energy and nutrient fluxes” in the eastern hemlock trees. The litter and throughfall collected beneath the infested trees were found to have higher dissolved organic carbon, organic nitrogen, and potassium flux, and a lower inorganic nitrogen flux when compared to the litter and throughfall collected from non-infested trees.

The needle and birch litter beneath the trees were also compared. Upon examination, the needle litter collected from beneath the infested hemlock trees had a higher nitrogen concentration than that collected beneath the non-infested trees. However, there was no difference in nitrogen concentrations of birch litter from infested and non-infested trees. The birch litter micro-flora in response to the HWA-infested and non-infested throughfall was also examined. There was no significant difference in the titer of yeasts and filamentous fungi, whereas, bacteria were found to be more abundant in the litter beneath infested trees.

Furthermore, studies on litter microcosms showed that there was less dissolved organic carbon in the birch leachate when compared to the hemlock needles, even though both were exposed to HWA-affected throughfall. The inorganic and dissolved organic nitrogen concentrations were however found to be higher in birch leachate. Significance This study investigates the effect of an exotic insect pest in terms of its overall impact on forest ecology. Such studies are very rare as most often the effect of insect herbivores on forest ecologies is considered to be linear, and their overall impact on nutrient and energy flow on the forest ecology is seldom studied.

This study brings out the relationship between HWA-affected throughfall and the “qualitative and quantitative differences in nitrogen, carbon and potassium export from the litter layer”. The significance of this study lies in the fact that it took the biology and the physiological effects of the pest and its effect on the hosts and their immediate ecosystem into consideration, for understanding the change in dynamics caused by them on the overall ecology. It is a common assumption that insect herbivores and pests affect the ecosystem functions in a linear way.

Yet, considering the complex food web relationships occurring in such ecosystems, alternative scenarios were taken into account while establishing this study. Recommendations Although this study successfully examines the effect of insect infestation on nutrient and energy flux, there is a need to examine their effect on the magnitude and direction of the matter and energy flow in such an ecosystem. Further studies that consider population ecology while analyzing the nutrient and energy flow are also recommended.

Definitions of key terms 1. Litter: Litter is defined as the layer of non-living biomass over the mineral soil layer, comprising dead and decomposed wood, leaves, and roots. The litter is rich in organic matter and its composition is greatly affected by variations in throughfall and nutrient fluxes in plants. 2. Throughfall: The excess water, resulting from precipitation on the surface of leaves, shed onto the surface of the soil below the vegetation is called throughfall. 3. Nutrient cycling: It is defined as the flow of nutrients through various trophic levels of an ecosystem. 4. Leachate: It is defined as the water that carries inorganic and organic nutrients and dissolved salts that are collected while it percolates through minerals and rocks. 5. Nutrient flux/Energy flux: It is defined as a constant change or fluctuation in nutrient or energy flow across the ecosystem.