Unit 26


Unit Overview

This unit examines processes that occur within the biosphere. The main sections are as follows:

  • Dynamics of the biosphere
  • Plant successions
  • Geographic dispersal

The dynamics of the biosphere involve photosynthesis and the subsequent flow of energy within the ecosystems. An ecosystem is a linkage of organisms to their environment. The initial suppliers of the energy are organisms known as autotrophs. Herbivores and carnivores transfer energy. Major changes within ecosystems occur as a result of plant successions, which are initiated by internal and external agents.

Limiting factors in two main categories controls the distribution of plants on the Earth's surface: physical and biotic. The physical factors include temperature, the availability of water, the availability of light, wind, snow cover, the distribution of soils, and landforms. The biotic factors include competition, amensalism, predation, mutualism, and endemism. As can be discerned from above, complex processes involving numerous factors control the assemblage of plants in any given location.

Unit Objectives

  • To discuss the process of photosynthesis and relate it to climate controls
  • To introduce the concept of ecosystems and highlight the important energy flows within ecosystems
  • To outline the factors influencing the geographic dispersal of plant and animal species within the biosphere

Glossary of Key Terms

Allogenic succession Plan succession in which vegetation change is brought about by some external environmental factor, such as disease.
Amensalism Biological interaction in which one species is inhibited by another.
Biomass The total living organic matter, encompassing all plants and animals, produced in a particular geographic area.
Carnivores Animals that eat herbivores and other animals.
Climax community Achieved at the end of a plant succession; the vegetation and its ecosystem are in complete harmony (dynamic equilibrium) with the soil, the climate, and other parts of the environment.
Cyclic autogenic succession Plant succession, in which one type of vegetation replaces another, which in turn is replaced by the first, with other series possibly intermixed.
Dispersal Ancestral species from which modem species evolved arrived in a given area by movement over land, swimming, rafting, or flying.
Ecosystem A linkage of plants or animals to their environment in an open system as far as energy is concerned.
Endemism Tendency of an isolated region to contain significant percentages of species of plants, animals, and other life forms that exist nowhere else on Earth.
Food chain The stages that energy in the form of food goes through within an ecosystem.
Herbivores Animals that live on plants, or more generally the first consumer stage of a food chain.
Linear autogenic succession A plant succession that occurs when the plants themselves initiate changes in the environment that consequently cause vegetation changes.
Mutualism Biological interaction in which there is a coexistence of two or more species because one or more is essential to the survival of the other(s); also called symbiosis.
Photosynthesis The process in which plants convert carbon dioxide and water into carbohydrates and oxygen through the addition of solar energy; carbohydrates are a significant component of the food and tissue of both plants and animals.
Phytomass The total living organic plant matter produced in a given geographic area; often used synonymously with biomass, because biomass is measured by weight (plants overwhelmingly dominate over animals in total weight per unit area).
Plant succession The process in which one type of vegetation is replaced by another.
Species-richness gradient The phenomenon involving the general decline over distance in the number of species per unit area as one proceeds from the equatorial to higher latitudes.
Trophic level Each of the stages along the food chain in which food energy is passed through the ecosystem.
Vicariance Ancestral species from which modem species evolved arrived in a given area by being carried along as landmasses drifted apart over tens of millions of years.

Unit Outline

  • Dynamics of the biosphere
    • Photosynthesis
      • a process rooted in the Earth's evolution
      • chlorophyll and light absorption
      • chemical foundations and environmental implications
      • limitations
        • variations in solar-energy receipt
        • variations in water availability
      • phytomass productivity
        • regional variations in total plant matter produced (see Fig. 26.3)
        • greatest in moist tropical lowlands
        • at its least in desert, upland, and high-latitude zones
    • Ecosystems and energy flows
      • ecosystem: a linkage of plants or animals to their environment in an open system as far as energy is concerned
      • exemplified through food chains
    • Ecological efficiency
      • trophic levels within food chains (see Fig. 26.5)
      • variation in ecosystem efficiencies
  • Plant successions
    • Linear autogenic succession
    • Cyclic autogenic succession
    • Allogenic succession
    • Climax communities
  • Geographic dispersal
    • Physical factors
      • temperature
      • availability of water
      • other climatic factors
        • daylight duration
        • wind action
        • duration of snow cover
      • distribution of soils
      • landform (especially slope) variations
    • Biotic factors
      • competition
      • amensalism (inhibition of one species by another)
      • predation
      • mutualism (symbiosis)

Review Questions

  1. Discuss the connections between photosynthesis and the broad distribution of plant life across the Earth's landmasses.
  2. What are ecosystems and how energy-efficient are they at different trophic levels?
  3. List the biotic and physical factors that shape the distribution of natural vegetation.