ECOSYSTEM ECOLOGY

Communities

• interacting groups of species in which each organism has a specific niche
  

Ecosystem

• a system of interacting organisms and their nonliving surroundings
  
• an ecosystem is a community of organisms interacting within a particular physical environment. Stated another way, an ecosystem is a community plus its abiotic factors, e.g. soil, rain, temperatures, etc.
  
  

Major roles of Organisms

• Producers
  • have the ability to make new, complex, organic material from the atoms in their environment
    
  • Virtually all energy on earth comes from the sun, via photoautotrophs (primarily plants), and it is ultimately distributed throughout ecosystems
    
• Consumers
  • Primary consumers - herbivores (Consumers that eat plants)
    
  • Secondary consumer - carnivores (Consumers that eat animals)
    • consumers that eat both plants and animals are omnivores
      
• Decomposers (fungi and bacteria) obtain their energy by breaking down the remains or products of organisms
  • Detritivores are decomposers which eat detritus - organic wastes and dead organisms.
    
    

Structure of Ecosystems

• Ecosystems are arranged by trophic (feeding) levels between various producers, the autotrophs, and consumers, the heterotrophs
  • First trophic level: Contains the autotrophs which build energy containing molecules. They also absorb nitrogen, phosphorous, sulfur and other molecules necessary for life. They provide both an energy-fixation base as well as the nutrient-concentration base for ecosystems. Two types of autotrophs
    • Photoautotrophs - plants
    • Chemoautotrophs - bacteria.
      
  • Second trophic level: Contains the primary consumers which eat the primary producers. These include herbivores, decomposers and detritivores. E.g. insects, grasshoppers, deer and wildebeest.

• Third trophic level: Contains the secondary consumers, primary carnivores. They eat the herbivores. E.g. mice, spiders and birds
  
• Fourth trophic level: Contains the tertiary consumers, secondary carnivores. They eat the primary carnivores. E.g. weasel, owl, sharks and wolves.
  
• Such linear food chains as described above are probably rare in nature because the same food source may be part of several interwoven food chains and many organisms have several food sources. Because of this energy flows through ecosystems via food webs, intricate pathways of energy flow and material cycling.
  

ENERGY FLOW THROUGH ECOSYSTEMS

• Gross primary productivity is the rate at which the primary producers capture and store energy per unit time. However, since the primary producers expend energy during respiration the net primary productivity is considerably lower than the gross productivity.
  
• Productivity is usually measured as biomass (dry weight of organic matter per unit area per a specified time interval, e.g. kg/m2/yr)
  
• The trophic structure of an ecosystem is often represented by a ecological pyramid, with the primary producers at the base and the other levels above
  
• However, because most of the food eaten by organisms is converted to biomass, or used to maintain metabolic functions, or lost as heat loss, only about 10% of the energy makes it to the next level
  
• This massive energy loss between trophic levels explains why food chains can't contain more than a few levels
  

Example

On the savanna of Africa it takes takes billions of primary producers to support millions of wildebeests, which support a few thousand lions. This also explains why there are so few large carnivores on earth.

• An energy pyramid is a more useful way to depict an ecosystem's trophic structure. Each block of the pyramid is proportional to the amount of energy it contains
  
• The energy pyramid concept helps explain the phenomenon of biological magnification - the tendency for toxic substances to increase in concentration at progressively higher levels of the food chain
  
  

Example

DDT was once a widely used insecticide. However when washed off croplands into streams and lakes it became concentrated in fish that were ultimately eaten by birds such as bald eagles. The DDT caused fragile eggs such that populations of large predator birds rapidly declined. Since DDT was banned in the US in 1968 bird populations have made dramatic comebacks.

SUCCESSION

• ecosystems are dynamic, constantly changing units
• plants grow and die, animals feed on plants and on one another, and decomposers recycle the chemical elements that make up the biotic portion of any ecosystem
• since all organisms are linked together in a community, any change in the community affects many organisms in it
• over long periods of time, it is possible to see trends in the way the structure of a community changes
• this series of regular, predicable changes in the structure of a community over time is succession
  • Primary succession - begins with bare mineral surfaces (rock) and water
  • Secondary succession - begins with the destruction or disturbance of an existing ecosystem
    

Primary Succession: Terrestrial Succession

• bare rock is very inhospitable
  • Why?
    
• The organisms that can live here is called the Pioneer Community
  • Lichens
    • a mutualistic relationship between algae fungi
      
  • acids produced by the lichen tend to break down the rock and produce a Thin Soil
    
  • the thin layer supports fungi, small worms, insects, bacteria, protozoa, and perhaps a few tiny annual plants
    
  • as these organisms grow, reproduce and die, they contribute additional organic matter for the soil-building process
    
• This process eliminates Pioneer Community and is replaced by a small community of perennial plants
  
• Perennial plants are eventually replaced by shrubs
  
• Shrubs are replaced by trees that need lots of light
  
• These trees are replaced by trees that can tolerate shade
  
• Eventually, a relatively stable, long-lasting, more complex, and interrelated community of plants, animals, fungi, and bacteria are produced....this is a CLIMAX COMMUNITY
  
• Each step in this process is called a successional stage or serel stage
  
• The entire process - from pioneer community to climax community - is called a sere
  
• CLIMAX COMMUNITIES have certain characteristic regardless of type
  • They can maintain their mix of species
  • They have an energy balance (successional communities tend to accumulate large amounts of new material (gain energy)
  • They tend to have many kinds of interactions among organisms than does a successional community
  • The general trend in climax communities is toward increasing complexity and energy efficiency
    

Primary Succession: Aquatic Succession

• All aquatic ecosystems receive a continuous input of soil particles and organic matter from surrounding land
  • This tends to fill shallow bodies of water (ponds and lakes)
    
• In deep lakes only floating plants and algae can exit
  
• submerged plants to become established after time, and organic matter accumulates at the bottom
  
• eventually the aquatic system fills in with sediment, the sediment dries out and grasses and other plants take over ..... MEADOW
  
• typical shoreline of a lake example
  
  

Secondary Succession

• the same processes that drive Primary Succession result in secondary succession
• secondary succession occurs when an existing community is destroyed (forest fire, flood, conversion of an area to agriculture)
• the destroyed ecosystem is not returned to bare rock therefore the establishment of this community is much faster
  

BIOMES

• A biome is a large region of land characterized by the climax vegetation of the ecosystems within its boundaries.
  • Climax vegetation -
    
• The distribution and key features of biomes are the outcome of temperatures, soils and moisture levels (which vary with latitude and altitude), and evolutionary history.