Ecology (BIO C322) Community Ecology

Habitat and Niche Habitat = The place where an organism lives. Ecological niche = Physical space + Organism’s functional role in the community.

Ecological Niche Includes the physical space occupied by an organism (Habitat niche), trophic position (Trophic niche), and position in environmental gradient of temperature, moisture, etc (Multidimensional or Hypervolume niche).

Example: Both water backswimmer (Notonecta sp.) and water boatman (Corixa sp.) occupy same habitat (shallow vegetation-choked areas of ponds/lakes); Water BackswimmerWater Boatman

However, they occupy different trophic niches: The former feeds on small living organisms whereas, the latter on decaying vegetation.

Organisms in niche 1 tolerate higher pH & temp, whereas those in niche 2 lower pH and temp. pH and temperature  Niche dimensions. The rectangular area(s)  Niche space(s).

Within a species, competition is often reduced when different life stages occupy different niches. Example: Tadpole (herbivore), adult frog (carnivore).

Charles Darwin’s ‘On the Origin of Species’: “ As species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.” This is the concept of Niche Overlap.

Niche Overlap

Darwin’s Finches Galapagos Islands

Darwin studied 14 distinct species of finches (Geospiza sp.) occurring on Galapagos islands. Existing species derived from generalized finch- like birds, evolving into different species occupying different niches.

Six ground-feeders (heavy beaks to crush heavy-coated seeds), Eight insect-feeders, perching on tree branches (beak structure different). Competition reduced due to niche diversification or niche shift. Also, change in morphological features. e.g. beak morphology in the above case.

Fundamental Niche = Hypervolume that a population can fill in absence of competitors. Sometimes in competition, if similar requirements are there along niche dimensions, their niches will overlap. Fundamental & Realized Niches

Realized Niche = When the species constrained by competition & other biotic limiting factors. If the niche of one species completely overlaps that of another, then one species will be eliminated according to the competitive exclusion principle.

Gause Principle - Competitive Exclusion Principle When there is a common environmental requirement between two closely related species, one species outcompetes the other. Closely related organisms having similar habits often do not occur in the same places. If at same places, they use different resources or are active at different times.

This is called ecological separation of closely related or similar species.

Gause’s Study on Paramecium Cultures of Paramecium caudatum and P. aurelia studied. In separate cultures, sigmoid growth curves. When cultured together, P. aurelia alone survived after 16 days. WHY?

P. aurelia had a more rapid growth rate (higher ‘r’ value), and thus outcompeted P. caudatum for the limited amount of food.

If the niches overlap partially, coexistence is possible in different ways. Either one species occupies its own fundamental niche, second species occupies a smaller realized niche. Or both species occupy smaller realized niches. Both these ways help to reduce interspecific competition.

Guilds Guild = Group of species that exploits the same class of environmental resources in similar way. Example: In the case of Darwin’s finches, two guilds – one ground feeders (seeds) and another insect feeders (tree branches).

Example: Different species of nectar-feeding insects constitute a guild.

Ecological Equivalents Ecological equivalents = Species that occupy similar niches in different geographical regions. Example: Grassland ecosystem develops wherever there is a grassland climate, but the species of grass and grazers may be different.

Example: The kangaroos of the Australian grassland are ecological equivalents of the bison of the North American grassland. KangarooBison

Types of Interactions in Community Combinations of neutral (0), positive (+) & negative (-). Neutralism (00): Neither population is affected by association with the other. Competition, Direct interference type (--): Both populations actively inhibit each other (fighting/defending territory).

Competition, Resource use type (--): Indirect inhibition, common resource in short supply. Amensalism (-0): Population 1 inhibited, 2 not affected. Commensalism (+0): Population 1, the commensal, benefits; 2, the host, not affected.

Parasitism (+-): Population 1, the parasite, generally smaller than the host (2). Predation (+-): Population 1, the predator, generally larger than the prey (2).

Protocooperation or facultative cooperation (++): Interaction favourable to both, not obligatory. Mutualism or symbiosis (++): Obligatory interaction; neither can survive without the other.