Interdependence & Competition (OCR GCSE Biology A (Gateway))

Interdependence

• 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

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