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Introduction
In the following discussion, important terms, phrases and concepts related to this field trip are shown in light blue. Items you need to take notes on (from the table in your lab manual) are italicized in maroon.
An ecosystem is a community of plants and animals, and their associated non-living physical environment (sunlight, nutrients, air, soil, water). Although we can consider Schmeeckle Reserve one large ecosystem, we will observe several smaller ecosystems within Schmeeckle Reserve. Together, these smaller ecosystems make up the larger ecosystem of Schmeeckle itself. Each ecosystem, regardless of scale, provides habitat for various species. An organisms habitat is the physical location within a community where that organism lives, or its "home." The environmental conditions at any given site will favor its habitation by a particular species or a group of species. On this field trip, we will observe the plant species at several sites, and we will assess the environmental situation at those sites that results in the habitation of those sites by those species.
Functions within ecosystems include interactions between living organisms, such as flows of energy through the ecosystem. Sunlight (insolation) is the initial source of energy for all ecosystems, and the availability of sunlight is a major factor affecting ecosystem composition. Insolation is determined primarily by incidence angle and daylength (hours of sunlight). Thus, latitude and time of year both affect insolation. Producers, or green plants, convert sunlight to organic energy through the process of photosynthesis, making this energy available for consumers. Consumers include herbivores (plant eaters), carnivores (meat eaters) and omnivores (plant and meat eaters), and top carnivores. Decomposers do their work of returning nutrients to the soil, water and air at every step in this sequence of consumption, or food web, that defines the flow of energy through an ecosystem. Other ecosystem functions include flows of water (the hydrologic cycle) and flows of nutrients, such as carbon, oxygen, nitrogen, and major soil nutrients (e.g. calcium, potassium, magnesium and sodium). All organisms within an ecosystem perform some function with respect to the flow of energy or other elements through the ecosystem.Interactions between living organisms and their non-living physical environment also help define ecosystems. Every species has certain tolerance limits which define the environmental conditions under which that species can survive. Availability of sunlight and moisture are two major factors affecting the geographic distribution of organisms and ecosystem composition. Thus, all plants have specific temperature and moisture requirements. Some species are able to tolerate a wide range of sunlight conditions, while other species have a very limited range of conditions under which they can survive. Soil provides a necessary medium for plant roots, and from the soil plants get water and necessary nutrients. Again, some species are able to tolerate a wide range of soil moisture conditions, while others are not. Given the local environmental situation, therefore, certain species will grow and reproduce while other species will be excluded from that location. Knowing something about the tolerance limits of various species, and identifying the dominant plant type or species at a particular site, will provide clues on the environmental conditions at that site. The dominant plant type may exert a controlling influence on the nature and composition of the rest of the community. Thus, we will identify the dominant plant type, and when possible, the dominant plant species at each site. We will also observe plant heights. Plant height may provide an indication of the environmental situation; for example, very wet or very dry sites may tend to have shorter plants than moderate sites. Plant height also provides an indication of the age of the vegetation, a useful clue for assessing the successional stage (defined below) of the site.
In addition to observing the dominant species at each site, we will also observe other plant types and species at the sites we visit. The composition of each community is affected by available habitat, and also affects habitat available for animals (which we are not specifically focusing on). Some species have very specific habitat requirements. For example, young jack pine require high sunlight intensity; they are intolerant of shade. Some species of birds will nest in only one specific species of tree. Other organisms will tolerate a wider range of habitats. Thus the dominant plant type or species at a particular site within Schmeeckle provide a variety of habitats for other organisms, including other plants as well as animals.
Several of the sites we will visit are forest communities. Within forest ecosystems, the composition of the understory is affected by the dominant species and also provides habitat for certain animals. Thus we will attempt to assess the density of the understory, and when possible, identify some of the understory species. This information will provide clues primarily on the availability of sunlight, but also on microscale temperature and moisture variations, hence we will look for microclimatic indicators. The canopy, or leaf coverage, provided by the dominant species is the main factor determining the availability of sunlight for the understory. The moisture availability, or surface hydrology, at a site is affected by soil type and drainage patterns.
Disturbances also affect the composition of ecological communities. A disturbance is generally defined as an event that alters the composition of an ecosystem. Some examples of natural disturbances include floods, volcanic eruptions, landslides and wildfires, while some examples of human disturbances include urbanization, agricultural expansion and deforestation. Once a disturbance occurs, ecological succession follows. Ecological succession involves the repeated replacement of one kind of community of organisms by other, usually more diverse, communities of organisms over a period of time. The successional stage of an ecosystem depends on the severity of the disturbance and the length of time since that disturbance occurred. Pioneer communities are the first communities to appear after a disturbance. As time progresses, succession results in communities that are more mature, and more stable given the environmental situation of that community. The most mature, most stable community given the environmental situation is called a climax community. For example, wildfire greatly alters the composition of a community. Over time, the community composition changes as vegetation tolerant of high sunlight is slowly replaced by vegetation tolerant of shadier conditions. We will look for evidence of disturbances, and when we see evidence of disturbances, we well attempt to determine the type of disturbance. Many of the ecological communities present within Schmeeckle Reserve are artificial communities created by human activities, thus one of the most common types of disturbances we will observe is that of human activity.
Finally, we want to judge the overall community type. Within Schmeeckle Reserve we find several different types of forest communities, including broadleaf forests, needleleaf forests and mixed forests. We also find wetland and grassland (prairie) communities.
In summary, the composition of an ecosystem is affected by the environmental situation of that ecosystem, the functions that may be filled within the ecosystem, and the available habitat. On this field trip, we will focus primarily on the environmental situation of the ecosystems within Schmeeckle Reserve that result in distinctive habitats.
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Created by Karen A. Lemke (klemke@uwsp.edu). Last modified July 13, 2000.