Topic 4 - Ecology

• Species


  A group of organisms that can interbreed to produce fertile offspring
• The ability to breed and produce fertile offspring is a useful method of distinguishing species for organisms that reproduce sexually but can be difficult to apply in some situations
• Organisms that reproduce by asexual reproduction, such as bacteria, cannot be classified using this method
• On rare occasions, animals of different species breed together and produce fertile offspring, such as the so-called 'wholphin'; the fertile offspring from a cross between a melon-headed whale and a common bottlenose dolphin
• According to the species rule above the wholphin would be a new species, but while scientists do believe that hybridisation can lead to new species it needs to be a frequent event for this to occur, and wholphins are rare
• Note that the melon-headed whale is actually a species of dolphin, so the name 'wholphin' is a bit inaccurate!
• The imperfect nature of this method of classifying species means that other characteristics are often used at the same time
• Other characteristics used to classify species

  • Organisms of the same species share similar morphology
  • DNA sequences can be compared, with a certain level of similarity indicating that organisms are the same species
• Population


  A group of organisms of the same species living in an area at one time
• A population can be isolated from other populations of the same species due to living in a different area
• This isolation means that members of the separate populations cannot breed together and gene exchange or gene flow cannot take place between them
• As long as these isolated populations could, in theory, interbreed to produce fertile offspring, they are the same species
• If the environmental conditions affecting each population are different, then natural selection could act differently on each population and eventually lead to speciation
• Genetic drift can also lead to speciation
• Once speciation has taken place, the two species can no longer produce fertile offspring; they are reproductively isolated
• Reproductive isolation of two populations of trees can occur when the populations are separated for a long period of time
• Organisms need energy in the form of ATP to survive
• The energy stored in ATP comes from other organic molecules, such as carbohydrates, and is transferred during the process of respiration

  
• Mode of nutrition


  The method by which an organism gains organic molecules to fuel respiration
• Main modes of nutrition

  • Autotrophy
  • Heterotrophy
• Autotroph


  An organism that synthesises, or produces, its own organic molecules from simple inorganic substances in its environment
• Photosynthetic organisms use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates

  
• Some autotrophs use energy from the oxidation of inorganic compounds instead of light energy
• Types of autotrophs


  • Photoautotrophs (use light energy)
  • Chemoautotrophs (use energy from oxidation of chemicals)
• Because autotrophs make their own organic molecules without relying on other organisms, they are known as producers
• Examples of autotrophs


  • Most green plants
  • Algae such as seaweeds
  • Photosynthetic bacteria such as cyanobacteria
• Heterotroph


  An organism that gains its organic molecules by ingesting the tissues of other organisms
• Types of heterotrophs


  • Consumers
  • Detritivores
  • Saprotrophs
• Mixotroph


  An organism that is able to make use of more than one mode of nutrition, such as auto- and heterotrophy
• Euglena is a single-celled eukaryotic organism that makes use of both autotrophy and heterotrophy
• Euglena cells can take in bacterial cells by endocytosis, and then digest them using digestive enzymes stored in lysosomes
• Euglena cells also contain a light-sensitive spot that enables them to position themselves so that maximum light reaches their chloroplasts
• The majority of plants and algae are photosynthetic, meaning that they are autotrophs that rely on energy from the sun to convert carbon dioxide in the air into organic molecules in their tissues

  
• Their photosynthetic cells contain pigments which absorb light energy
• Photosynthetic pigments in plants and algae

  • Chlorophyll (main pigment in green plants)
  • Carotenoid pigments (accessory pigments in green plants)
  • Fucoxanthin (brown pigment in brown algae)
  • Phycobilins (pigments in red and green algae)
• There are some unusual exceptions to the autotrophic mode of nutrition used by most plants and algae
• Examples of non-autotrophic plants and algae

  • Some plants parasitise the roots of other plants
  • Some plants parasitise fungi (mycoheterotrophy)
  • The rare plant Epipogium aphyllum (ghost orchid) has no leaves and no chlorophyll, gaining its organic molecules from fungi
• When exceptions to accepted trends are observed in the natural world, it can sometimes mean that established modes of thinking are incorrect, so it is important to consider discrepancies carefully
• In the case of non-photosynthetic plants and algae: They are rare, They appear to have evolved on multiple occasions from autotrophic ancestors, There is not enough evidence to disprove the mode of thinking that says that plants and algae are autotrophs, but we can say that there are a few exceptions to this rule
• Consumers


  Organisms that gain their organic molecules by ingesting the tissues of other living organisms or recently dead organisms