Organisms depend on other species for resources such as nutrients. Parasites live in or on another organism, which is called the host. The parasite takes what it needs from the host but the host receives nothing in return and often suffers as a result. An example of parasitism is the relationship between fleas and dogs. Fleas live on dogs and feed on their blood. The dog receives no benefit but the fleas are provided with food and a habitat. Fleas attach themselves to hairs and can spread from one host to another by jumping huge distances.
Tapeworms are parasites that live inside the small intestine of their hosts. Some human tapeworms have measured up to 15 metres in length. These worms have no digestive system and absorb the digested products of digestion from their hosts. They release eggs in the faeces which can infect other hosts. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. Parasites do not usually kill the host because this would cut off their food supply.
Parasites are adapted so that they receive maximum benefit from the host but do not kill them. Tapeworms have many adaptations such as strong suckers and hooks for attachment to the lining of the small intestine. Tapeworms are thin and flattened and have a very large surface area for absorption of nutrients. They have a huge reproductive potential and release lots of eggs because the chances of the parasite finding another host is very small and many eggs will die.
Mutualism is another type of relationship between two species, where both species benefit. For example bees and flowering plants have a mutualistic relationship. Bees obtain nectar for food and spread the flower pollen from one flower to another, which helps reproduction in plants.
Different species of fish can display mutualism. Cleaner fish are smaller fish and swim near the gills of much larger species and eat the parasites around the fish gills of the larger fish. The smaller cleaner fish derive nutrition and protection and the larger fish have the gill parasites removed.
Lichens are another example of mutualism. They are formed by algae and fungi living together. Algae can photosynthesise and make food, which is shared with the fungus. The fungus in turn shelters the algae from a harsh climate.