4.1.6 Interdependence & Competition

• One species may be dependent on another within a community
  • An obvious example is that a predator species is dependent on there being an adequate supply of its prey
• An extension of dependence is the concept of interdependence
  • All organisms in an ecosystem depend upon one another
  • The success or failure of one species can affect the success or failure of the others
• The types of interaction between species can vary
• In order to survive and reproduce (have offspring), organisms need certain resources from their surroundings (from the ecosystem they are living in)
• This means that members of a species will often interact with members of its own species or other species
• Some examples of these interactions include:
  
  • Predators (carnivores) feeding on prey
  • Herbivores eating plants
  • Plant species being pollinated by bees
  • Seed dispersal via animals eating the fruit of certain trees and passing the seed in its faeces
•  If one species is removed it can affect the whole community
  • This is called interdependence
•  A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant

A food web shows the interdependence of organisms

• For example, in the food web above, if the population of earthworms decreased:
  • The population of grass plants would increase as there are now fewer species feeding off them
  • The populations of frogs and mice would decrease significantly as earthworms are their only food source
  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)

Parasitism 

• Parasites live off a host organism
• By taking what they need from the host and giving nothing in return
• The host is harmed, although the parasite would wish to keep the host alive as a dead host is no longer as useful to it
• An example is the tapeworm
  • Tapeworms attach themselves to the insides of the intestines of animals such as cows, pigs, and humans
  • They feed on the host's partly digested food, depriving the host of nutrients
• Mistletoe is a plant parasite, and gains its water and nutrients from a host tree by anchoring to the tree and tapping into its tissues
  • Some mistletoe varieties can photosynthesise to a small extent, but only until they have fully anchored to their host tree and are gaining all their food and nutrients from it
• Parasitism is a win-lose scenario

Mutualism

• Unlike parasitism, two species can interact to mutually benefit
  • Flowering plants produce elaborate, coloured flowers containing sugar-rich nectar
  • Bees are attracted visually and with the scent of the flowers and nectar
  • Bees get a nutritious meal, and in return...
  • The flowers are able to be pollinated to ensure their survival
• Certain species of shark have a mutualistic relationship with a much smaller fish called a cleaner wrasse
  • The shark requires its teeth to be razor-sharp and free of debris from the last kill
  • The wrasse (which might otherwise be part of the shark's normal prey) are allowed into the shark's mouth to pick off the debris and clean the shark's teeth
  • The shark benefits by having its teeth cleaned and made ready for the next kill
  • The wrasse benefits as it feeds by removing debris 

Competition in plants & animals

• If a group of organisms all need the same resource in order to survive and reproduce (have offspring) but there is a limited amount of the resource available, they are said to compete for the resource
• Competition can be between members of the same species (intraspecific competition) or between members of different species (interspecific competition)

• Plants in a community or habitat may compete with each other for certain limited resources

Resources Competed for by Plants Table

• Animals in a community or habitat may compete with each other for certain limited resources

Resources Competed for by Animals Table

Intraspecific competition between two grey squirrels (same species) for a limited resource

Interspecific competition between a grey squirrel and a red squirrel (different species) for a limited resource