Note: We skipped Chap 4, and will pick it up on Oct. 1. These notes cover two classes, Wed. Sept 24 and Monday Sept. 29.

The Biosphere is divided into various types of units for detailed analysis. As we saw last Chapter, the large vegetation/climate complexes (like forests, tundra, and deserts) are called biomes. The flows of matter and energy within biotic systems are called food chains, and this was the first example of an ecosystem offered in Chapter 6. But we have to recognize that the term "ecosystem" has various definitions. This chapter examines ecosystems as flows of energy and mass (food chains) and as assemblages of plant and animal species that interact in a given geographical area. I will stick mostly to that second idea of ecosystem, although they build on each other (e.g., the assemblages of species often interact as food chain; that is, members play specific roles and eat each other!).

Ecosystems defined as systems that cycle energy and mass among species (food chains) are defined by the roles that species play (consumers and producers) of organic energy. The system is driven by primary production that is then partitioned among the members, from primary consumers on up to the top predators. You do not need to memorize the material about food chains, but do know that:

Food chains and other forms of ecosystems are shaped by limiting factors, usually emanating form the abiotic environment: like sunshine, climate (temperature and precipitation), and nutrients from the solid earth (rocks and soil). Any species has a certain ranger of tolerance to these factors (tolerance thresholds) which set that species’ range and potential population.

We’ll focus in this class only on terrestrial ecosystems.

The climate limits mentioned above are chiefly responsible for the distribution of ecosystems on the earth’s surface, so the biomes (Fig. 6.8) reflect the climate limits mapped in Fig.6.4. Biomes are large-scale ecosystems defined by mixtures of major components, like forests, and climate and/or terrain manifestations, like desert and mountains. The main biomes are listed on Fig. 6.8.

The structure of biomass (total weight of biotic material) varies among biomes and ecosystems. Some, like forests, maintain sizeable biomass well above ground, while others, like grasslands and tundra maintain a lot of biomass at ground level and in the soil.

Composition: the most common notion of an ecosystem is as an interacting assemblage of species. As such, the ecosystem is defined by those species and their characteristics. The number of species sets its species diversity or biodiversity (you can also speak of the biodiversity of a geographical area, like Boulder County that might not be thought of as an ecosystem per se). The organisms that are members of a species make up that sepcies population. The geographical area occupied by that species pop is its range. Species that have larger tolerance thresholds for sets of limiting factors tend to have larger ranges. Species with overlapping ranges that interact are called a community.

Ecosystem Processes: The text discusses two main models of ecosystem dynamics. The successional model describes the ecosystem community changing over time from pioneer species to a climax community in equilibrium with limiting factors. The disturbance model describes the more realistic situation in which external forces, like fire, storms, drought, etc. causes shifting limiting factors on short time and space scales, thus making sure that ecosystems are not uniform communities that are always in equilibrium. These two models reflect our discussion early in this class of descriptive (disturbance) and prescriptive (succession) ideas of the environment.

We are in this course mostly interested in ecosystems as perturbed by human actions. The spread of human populations, agriculture, cities, etc. across the globe has altered natural ecosystems greatly. These alterations are repetitive, they may be purposeful or inadvertent, and include six major patterns:

Reduction: loss of areal coverage of an ecosystem or community

Fragmentation: (accompanies reduction): breaking ecosystems into spatially-separated units or fragments.

Substitution: replacement of one or more organisms/species with others.

Simplification: reduction in the number of specie sin an ecosystem or community. Often accompanied by substitution and reduction of the species populations.

Contamination: introduction of pollutants, typically man-made chemical constituents. Both words, contamination and pollutants, refer to chemical alterations that are considered bad for ecosystems health.

Overgrowth: this term refers to the negative manifestations o a broader human alteration of ecosystems: fertilization (though it is not described this way in the text). Humans have increased nitrogen fixation in terrestrial ecosystems by adding fertilizers and altering plant species to use the added fertility. This purposeful act allows us to grow more and larger crops, grass, and trees. We have also inadvertently fertilized ecosystems by creating a stream of waste nitrogen and other nutrients into soil and water, which has resulted in eutrophication of freshwater lakes.