Chapter 8

Biogeographic Processes

Biogeography explores the distribution of plants and animals on the Earth. This chapter examines how organisms live in ecosystems and how energy and matter is cycled through those ecosystems.

This chapter also explores ecological biogeography by examining factors that determine the distributions of organisms in time and space. It examines processes such as evolution, dispersal, and extinction of species through time.

  • Ecologyis the study of the interactions between life forms and their environments.
  • An ecosystem is defined as a group of organisms and the environment with which they interact. Both matter and energy are imported into and exported from ecosystems.
  • The food web, or food chain, refers to the flow of food energy between different organisms within an ecosystem.
  • Primary producers are plants and animals that are able to synthesize carbohydrates from carbon dioxide, water and light energy through a process known as photosynthesis.
  • The transfer of food energy to different levels of the food wed is accomplished by consumerswhich feed on the primary producers or on other consumers.
  • Decomposers (microorganisms and bacteria) feed on decaying organic matter at all levels in the food web.
  • Solar energy is absorbed initially by the primary producers and stored as chemical energy which is available for digestion by consumers. Since on average only ten to fifty percent of the energy at any level is passed on to the next level, the amount of organic matter and consumers must therefore decrease with each level.
  • Photosynthesis is a biochemical reaction which results in the production of carbohydrates and oxygen using water, carbon dioxide, and light energy. A simplified version of this biochemical reaction is:

H2O + CO2 + light energy = —CHOH— + O2

  • During the respiration process carbohydrates are broken down and combined with oxygen to create carbon dioxide, water, and chemical energy. A simplified version of this chemical reaction is:

—CHOH— + O2 = CO2 + H2O + chemical energy

  • Photosynthesis is dependent on light and heat. Photosynthesis only occurs when light is available and therefore, longer days result in more plant growth. Photosynthesis also increases steadily with temperature to about 20°C and then levels off.
  • Net photosythesis is the amount of carbohydrate remaining after respiration has broken down a sufficient amount of carbohydrate to feed the plant. Net photosynthesis increases with temperature until approximately 18°C, after which it declines as the rate of respiration increases faster than the rate of photosynthesis.
  • Net primary production is the annual amount of useful energy produced by an ecosystem. It is controlled by light intensity and duration, temperature, and water availability. Net primary production is measured as biomass, the dry weight of organic matter per unit area within an ecosystem.
  • Biomass is an important source of renewable energy. Using biomass as an energy source involves releasing solar energy that has been stored in plant tissue through photosynthesis. Energy can be obtained by burning firewood, or through intermediate products such as charcoal, methane gas, and alcohol.
  • Biogeochemical cyclesdefine the pathways of particular nutrients or materials through the Earth's ecosystem.
  • The macronutrients hydrogen, carbon, and oxygen account for 99.5% of all living matter.
  • The Carbon Cycle
  • most carbon lies in storage pools as carbonate sediments
  • only 0.2% is available as CO2 or as decaying biomass in active pools.
  • carbon exists as carbon dioxide in the atmosphere and oceans, carbohydrate in organic matter, hydrocarbon compounds in rock, and as mineral carbonate compounds.
  • CO2 is added to the Earth’s atmosphere through volcanic eruptions and anthropogenic consumption of fossil fuels. Carbon dioxide is removed from the Earth’s atmosphere through the combined photosynthetic efforts of terrestrial plants and marine phytoplankton.
  • The Oxygen Cycle: oxygen is added to the Earth’s atmosphere by volcanic activity and photosynthesis. Oxygen is removed from the atmosphere through organic respiration, mineral oxidation, industrial and natural combustion, and ocean water solution.
  • The Nitrogen Cycle
  • the atmosphere is a large storage pool of nitrogen.
  • nitrogen can only be utilized through nitrogen fixation and is lost to the biosphere through denitrification.
  • human influence has increased the amount of nitrogen in the biosphere through the use of nitrogen fertilizers and fuel combustion.
  • Sedimentary cycles involve many macronutrients such as calcium, magnesium, iron, potassium, sodium, and phosphorus which move from the land surface to the ocean and subsequently return to land surfaces by tectonic uplift. Storage pools include sea water, sediments, and sedimentary rocks. Eventually these macronutrients are released into the Earth system through weathering.
  • Ecosystems are strongly influenced by landforms and soils.
  • Habitat refers to the preferences and needs of an organism or group of organismswith respect to such factors as conditions of slope, water drainage, and soil type.
  • Ecological nicheof a species describes how it obtains energy and how it influences other species within its own environment. Many species may occupy the same habitat, but only a few will ever share the same ecological niche.
  • A community is an assemblage of interacting organisms that live in a particular habitat.
  • The most important environmental factors influencing the location of species are moisture and temperature.
  • Species have a variety of adaptations to help them cope with the abundance or scarcity of water. Xerophytes are plants adapted to dry conditions.
  • Temperature affects physiological processes in plants. Plants have a temperature range within which they can survive in addition to optimum temperatures for each of their specific functions such as photosynthesis, flowering, fruiting, and seed germination.
  • Climatic factors such as moisture, temperature, light, and wind can limit the distribution of plant and animal species. Boundaries marking the critical level of climatic stress beyond which a plant or animal species cannot survive are called Bioclimatic frontiers.
  • Geomorphic factors influencing ecosystems include slope steepness, slope aspect (the orientation of the sloping ground surface with respect to geographic north) and relief. Since soil types are largely related to climate, Edaphic or soil factors are also important in differentiating habitat.
  • Species interactions also determine the distribution patterns of plants and animals. Interactions may be positive or negative. Examples of species interactions include:
  • Competition between species occurs when two species require the same resource that is in short supply.
  • Negative interactions include predation(on species feeding on another), parasitism(one species gaining nutrition from another), herbivory(animal grazing reduces the plant population viability), and allelopathy(chemical toxins emitted by one plant species inhibits the growth of another).
  • Symbiosis includes three types of positive interactions between species: commensalisms (one species benefits while the other is unaffected), protocooperation(both species benefit from the relationship),and mutualism(one or both species cannot survive without the other).
  • Ecological succession is a development sequence in which plant communities, or seres, succeed one another as they progress to a stable climax which represents the most complex community of organisms possible in an area.
  • Succession starts with pioneer specieswhich are species that can survive in harsh conditions. The pioneer species eventually moderate the harsh conditions allowing the gradual invasion of other species (secondary succession). Disturbancessuch as fires, insects, disease, and disruptive human activities such as clear cutting which remove or alter existing communities results in secondary succession.
  • Four key historical biogeography processes that affect the distribution of species are evolution, speciation, extinction, and dispersal.
  • Patterns of distributions include endemic species, which are found in one region and no where else, cosmopolitan species which are found widely, and disjunction, in which closely related species are found in widely separated regions.
  • Biodiversity is determined by the variety of the Earth’s environments, as well as the processes of evolution, dispersal, and extinction through geologic time. Human activity on Earth is rapidly decreasing biodiversity by contributing to extinctions through dispersing competing organisms, hunting, fire, habitat alteration, and fragmentation.