Botkin and Kellar (2014) describe an ecosystem as “communities of organisms and their nonliving environment in which nutrients and other chemicals cycle and energy flow.” I have chosen the forests of Pennsylvania as a holistic ecosystem to examine.
Pennsylvania has approximately 17 million acres of forest, making up 59% of this State. The predominant species associations are oak/hickory in the central and southern areas and Northern hardwood (including black cherry, maple and birch) in the north and north-west (Sustaining Penn’s Woods, 2001). The vast forest area contains mature ecosystems with the aforementioned tree species as the climax species. Natural succession means that some mid-tier species such as white ash are out-competed by the oak, and the black walnut emits a natural root-chemical that inhibits tree growth nearby. Light and chemical conditions that prevent other species from becoming dominant are examples of abiotic features. The white ash and other species are, however, common regeneration species and occur widely in younger forests, along with numerous other species. Native fauna species include Soricidae (shrews) and Talpidae (moles) as well as bats, rabbits, hares and rodents. There are also several carnivores present at the top of the typical forest food chain – the bobcat (although rare) and several species of coyote.
As Botkin and Kellar (2014) describe, the forest ecosystem, like most other ecosystems, is constantly subject to change through natural cycles and human-induced events - therefore a ‘steady state’ cannot be maintained for any long periods. Catastrophic events such as fire, floods and storms are necessary for the maintenance of those systems because they promote regeneration.
However, large-scale human-induced events such as logging and clearing of vast areas of old forest can often irreversibly affect the forest ecosystem because they are not a natural process. Pennsylvania has been subject to such human influences (both in recent history and currently) and this negatively affects the forest ecosystem in a catastrophic way. Clearing for housing development and timber is ongoing, as much of Pennsylvania’s forest is in private ownership rather than in national parks.
Such clearing for logging etc can interrupt the natural nutrient cycle, as has been the case in Pennsylvania forests where high-value timbers such as oak and black cherry have been decimated. Soil can no longer regenerate and the nutrient cycle has in many cases been irreversibly damaged. Once soil is eroded, the forest cycle cannot continue.
There are numerous examples of food chains within the forest ecosystem and Pennsylvania forest is certainly no exception. Senescent trees provide roosting sites for native bats, woodpeckers and raptors. Habitat features such as these are critical for the food chain. If large trees are lost, the habitat for certain species is reduced and there may become an abundance of predated species lower down the food chain which in turn are not kept in check by natural predators. A loss of any part of the food chain due to man-made or natural factors results in imbalance.
In terms of this imbalance, Pennsylvania forests have a number of introduced species which have become invasive (“invasive” is when an introduced animal or plant or other organism adversely affects an indigenous ecosystem). An example is the white-tailed deer. Efforts are being made to control and reduce numbers of deer. Deer browse saplings, bark and shrubbery which has a long-term effect on the growth and regeneration of wildlife habitat and species biodiversity (Sustaining Penn’s Woods, 2001). Abrams and Johnson (2012) studied the white-tailed deer population in Valley Forge Historical National Park where deer numbers in the early 2000’s were between 70-93 animals per square kilometre. They found that in areas which specifically excluded deer by fencing them out, shrub cover after 18 years was 51% compared to 5% in the un-fenced plots where deer were still able to browse. Fenced plots also had higher species diversity, although lacked significant regeneration of oak species (Abrams and Johnson, 2012), indicating perhaps that this species is most susceptible to long-term browsing and may take a considerably much longer time to regenerate than other primary forest species.
References
Abrams, Marc D., and Johnson, Sarah E., 2012. Long-term impacts of deer exclosures on mixed-oak forest composition at the Valley Forge National Historical Park, Pennsylvania, USA. The Journal of the Torrey Botanical Society. 139(2), pp.167-180.
Botkin, Daniel B., and Keller, Edward A., 2014. Environmental Science: Earth as a Living Planet. 9th Edn.
Sustaining Penn’s Woods. Pennsylvania’s Forest Ecosystem. 2001. H.L.M.A Education Inc. Web, 9 April, 2016. http://www.hlma.org/pennswoods/overview/ecosystem.htm
