Community interactions
Unit 8 · Environmental Biology
Competition, niche and resource partitioning, predation and defence, herbivory, and symbiosis (mutualism, commensalism, parasitism), with the roles of keystone species.
Types of interaction
Organisms of different species in a community interact in ways that may help, harm or have no effect on each partner.
| Interaction |
Species A |
Species B |
| Competition |
− |
− |
| Predation / herbivory |
+ |
− |
| Parasitism |
+ |
− |
| Mutualism |
+ |
+ |
| Commensalism |
+ |
0 |
| Amensalism (allelopathy) |
0 |
− |
Competition
Competition occurs when organisms use the same limited resource (food, light, water, space, mates). Intraspecific competition is within a species and interspecific competition is between species.
- Competitive exclusion (Gause's principle): two species with identical niches cannot coexist; the better competitor eliminates the other. In Gause's experiments, Paramecium aurelia eliminated P. caudatum when grown together.
- Resource partitioning: similar species can coexist if they use the resource differently. In Sri Lankan forests, several species of bulbul, flycatcher and squirrel feed at different heights or on different foods.
- Niches: the fundamental niche is the full range of conditions a species could use; the realised niche is the smaller range it actually uses in the presence of competitors.
- Allelopathy: some plants release chemicals that inhibit others, such as the invasive Parthenium weed and Eucalyptus leaf litter.
Predation and defence
A predator kills and eats its prey. Predators control prey numbers, and prey numbers affect predator numbers, so the two populations tend to cycle. Predation also drives evolution: the fastest prey and the best hunters survive.
- Herbivory is predation on plants. Plants defend with thorns, tough leaves and toxic chemicals (alkaloids, latex).
- Prey defences: camouflage (a stick insect; the leaf-like green vine snake), warning (aposematic) colouration (the black and yellow bands of wasps), mimicry (a harmless species resembling a harmful one, as some butterflies resemble the unpalatable Crow butterflies), spines (porcupine), chemical defence (toxins, foul secretions) and alarm calls.
- Predator adaptations: speed, sharp senses, claws, venom, hunting in groups, and ambush.
Symbiosis
Symbiosis is a long-term close association between organisms of two different species.
- Mutualism (both benefit): lichen (fungus provides water and protection, alga provides food); Rhizobium in root nodules of legumes; mycorrhiza (fungus helps the root absorb phosphate, plant gives sugar); corals and zooxanthellae (algae supply food, coral gives shelter and CO₂); flowering plants and pollinators; gut bacteria and cellulose-digesting microbes in ruminants and termites.
- Commensalism (one benefits, the other unaffected): epiphytic orchids and ferns growing on forest trees for light; cattle egrets feeding on insects disturbed by cattle; the remora (sucker fish) attached to a shark for transport and scraps.
- Parasitism (one benefits, the host is harmed): ectoparasites (leeches, lice) live outside the host; endoparasites (Taenia tapeworm, Ascaris, Plasmodium in human blood) live inside; plant parasites include the stem parasite Cuscuta (dodder) and mistletoe (Loranthus), which grows on and weakens mango, tea-shade and other trees.
Keystone species
A keystone species has a disproportionately large effect on a community. The elephant opens forest gaps and spreads seeds; a top predator such as the leopard keeps herbivore numbers in check; mangrove trees create a habitat for hundreds of species. Removing a keystone species can reduce biodiversity greatly.
Exam tip: To classify an interaction, ask who benefits and who is harmed (+, −, 0) and then name it. A leech on your leg is parasitism, not predation, because the leech does not kill its host.