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
Exam Tip
Questions about interdependence in food webs are common and simple to gain marks on if you answer them fully and correctly. Do not say an animal or plant would ‘die out’ as this is unlikely to happen – stick to using the words 'decrease' or 'increase'. If in doubt, always give your reason for the increase or decrease in population.
Intraspecific competition between two grey squirrels (same species) for a limited resource
