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INTER-SPECIFIC RELATIONSHIPS – Information and Worksheet In an ecosystem, there are two sorts of interactions between different organisms: 1. INTRA-SPECIFIC interactions, between members of the same species. For instance a pack of 6 hyaenas might all squabble and fight over a dead carcass of an antelope. The hyaenas are all the same species - indeed the other hyaenas with which s/he fights may even be brothers and sisters. 2. INTER-SPECIFIC interactions, between individuals of different species. In this case lions and hyaenas may be fighting over the same dead carcass. Interspecific interactions are interactions among organisms of different species. Typically, these interactions are classified based on whether they BENEFIT one or both of the species involved or whether they HARM one of the species involved. (Sometimes the word SYMBIOSIS is used to describe inter-specific interactions but it is best to avoid using that word because often it is used, in error, when people really mean mutualism.) Types of INTERSPECIFIC interactions Predation When a larger animal eats other, smaller animals. Lions may eat buffalo, and wolves may eat deer. Spiders, like the orbweaver, capture and eat insects in their webs. The predator benefits but obviously the prey is harmed. Mutualism A relationship between two species where both benefit. The yucca moth both pollinates and feeds on the yucca plant; acacia ants live in the thorns of, defend, and are fed by the acacia tree in which they live; and trees can’t get along without mycorrhizae living in/on their roots and absorbing food for them. Many plants and their pollinators have evolved mutualistic relationships. Butterfly-weed provides food for and is pollinated by butterflies like pipevine swallowtails. Commensalism A relationship between two species that is beneficial to one but neither benefits nor harms the other. Cattle egrets follow cattle to feed on the insects stirred up by the grazing cattle. (Might there be a benefit for the cattle?) Parasitism Parasitism occurs when a smaller organism feeds on a larger, weakening or killing it. This is a relationship where one organism benefits and the other is harmed. Often the host is not killed outright. Because a parasite lives in/on the body of its host and needs the host to remain alive, it is usually advantageous for the parasite to not kill its host. Humans and domestic animals are occasionally infected with or bothered by tapeworms and roundworms (endoparasites) and mosquitoes and leeches (ectoparasites). Competition Two species challenging each other for the same resource (often food) is called competition. In this sort of interaction, both species are harmed - one species may lose the competition and be driven away, but the winner has used up a lot of energy and may be injured in the challenge. Lions and hyaenas might challenge for a dead antelope; the hyaenas may be the 'winners' but have probably suffered too. Neutralism In this interaction, neither species involved apparently gains any benefit and neither is harmed. So for instance in a grassland, you may find frogs and deer and rabbits all feeding together in the same place but in no way interacting with each other. This then is the most common of interactions but surprisingly many ecologists argue that this interaction never actually exists. They argue that there will always be some relationship. Maybe the deer create little paths with their feet so that the frogs and rabbits can more easily move around? All three fertilise the grassland with their excrement, which therefore benefits all of them. Maybe the rabbits eat the grass shoots which the deer prefer. Who knows? So .... a summary An inter-specific interaction is a close ecological relationship between the individuals of two (or more) different species. These interactions can always be looked at in terms of benefits and harms. Mutualism -- both species benefit Commensalism -- one species benefits, the other is unaffected Parasitism -- one species benefits, the other is harmed Competition -- neither species benefits, both in a sense, are harmed Neutralism -- neither species is harmed or benefited To simplify things, ecologists often only use the signs +, 0 or - to describe inter-specific interactions. The following table illustrates the correct use of these terms in interactions between two species: Species "A" and Species "B". "+" denotes BENEFIT to the species "0" denotes neither harm nor benefit "-" denotes HARM to the species SPECIES A + Parasitism 0 - Commensalism Mutualism Neutralism Competition - 0 SPECIES B +