Natural systems (and what I mean by “system” is a collection of parts that interact and affect each other through the exchange of energy or material) are dynamic – they are always changing. On a global scale these systems can be large-scale or small-scale. However, they are almost always very complex and hard to measure.
An ecosystem, for instance is a complex set of living (biotic) and non-living (abiotic) components that occur within a particular place. Ecosystem characteristics are influenced by the underlying bedrock, regional climate, landforms, surface water, soil types, previous and current vegetation, disturbance patterns, animals present, etc…. As any of these factors change, the other factors shift to compensate for those changes. Small ecosystems are part of larger ecosystems, and larger ecosystems are, ultimately, part of a global system that actively uses, exchanges, distributes, and recycles resources that maintain life on the planet.
Humans change the environment more than any other single species. We place pavement and cement, build buildings, grow crops, suppress fires, set fires, bulldoze and plow the soil, cut down trees, mow grass, reroute rivers, add fertilizers, spread pesticides and generate pollution. Most of these things we do deliberately to make our environment suitable to our desires.
The changes we make are hurtful to some species and beneficial to others. When we set fires to areas, those fires prevent trees from growing and prevents species of animals that prefer trees from inhabiting that area. Consequently, the repeated setting of fires promotes the growth of grasses, which in turn benefits those species that prefer grasses. When we suppress fires, the opposite takes place.
Take for instance the historic red pine (Pinus resinosa) / white pine (Pinus strobus) forest ecosystem of the area which now makes up the Seney National Wildlife Refuge near Seney, Michigan. The best estimates of pre-European settlement indicate that this particular ecosystem comprised between 5% and 6% of the total cover types. In some reports, the prevalence of this particular ecosystem was described as historically dominating the area.[7, 8] These stands of pine were apparently regulated by both abiotic and biotic factors. About every 25-35 years low-intensity fires would remove potential competitor species from establishing themselves, while also providing the conditions needed for pine seedlings to establish themselves. The fires may have been started by either lightning or pre-European humans. However, other biotic conditions may have also affected the amount of fuel available, thus helping to regulate the fire intensity. The occurrence of jack pine budworm (Choristoneura pinus) and spruce budworm (Choristoneura fumiferana) may have induced higher rates of mortality, thus reducing the available fuel for future fires.
Following European settlement of the area, timber cutting initially favored white pine. When the white pine quality was diminished, cutting shifted to the best and largest (a harvest style called “high-grading”) of the red pine. This heavy harvest of the trees in the red pine/white pine ecosystem was combined with the impacts of fires which occurred among the “slash” – the debris which remained after logging. These types of fires occurred repeatedly, preventing suitable regeneration of the timber types.
As a result, the red pine/white pine ecosystem now only exists in about ⅙ to ⅕ of the historic quantities.[2, 3, 4, 6] It appears that it was replaced by second growth Quaking Aspen (Populus tremuloides) and Jack Pine (Pinus banksiana). Such severe changes in vegetation likely results in just as severe of change in the species that depend on the historic vegetation for survival. In addition to changing the biotic conditions of the region, it is also conceivable that the change in vegetation could, over the long term, also change abiotic conditions (e.g., soil types, moisture regimes, and frequency of fires).
With this particular example, humans altered a natural environment, yet it remains largely vegetated and still supports the dynamic nature of ecosystems. In urban settings, where high human population densities, concrete, and asphalt predominate, the natural ecosystems have been altogether removed and additional consequences (e.g., water, soil, and air pollution) arise.
Albert, D. 1995. Regional landscape ecosystems of Michigan, Minnesota, and Wisconsin: a working map and classification. United States Forest Service, North Central Forest Experiment Station., St. Paul, MN.
Casselman, T., M.D. Sprenger, N.M. Fuller, C.M Wooley, and T.O. Melius. 2009. Comprehensive Conservation Plan. Seney National Wildlife Refuge. 198pp.
Drobyshev, I., P.C. Goebel, D.M. Hix, R.G. Corace, III and M. Semko-Duncan. 2008. Pre- and post-European settlement fire history of red pine-dominated forest ecosystems of Seney National Wildlife Refuge, Upper Michigan. Can. J. For. Res. 38:2497-2514.
Frehlich, L.E. and P.B. Reich. 1996. Old growth in the Great Lakes region. Pp. 144-160 in M.B. Davis, ed., Eastern old-growth forests: prospects for rediscovery and recovery. Island Press, Washington, D.C.
Karamanski, T.J. 1989. Deep woods frontier: a history of logging in northern Michigan. Wayne State University Press, Detroit, MI. 305p.
Thompson, I.D., Simard, J.H. and R.D. Titman. 2006. Historic changes in white pine (Pinus strobus L.) density in Algonquin Park, Ontario, during the 19th century. Natural Areas Journal 26:61-71.
Vogl, R.J. 1970. Fire and the northern Wisconsin pine barrens. Proceedings Tall Timbers Fire Ecology Conference 10:175-209.
Whitney, G.G. 1986. Relation of Michigan’s pre-settlement forests to substrate and disturbance history. Ecology 67:1548-1559.