Community Ecology Community - a group of organisms, of different species, living in the same area Community ecology is the study of the interactions between species The presence of one species may affect the ability of another species to live in the same area positively, negatively, or have no effect. 1 Classification of types of interspecific interactions: Type of interaction Effect of Species 1 on Species 2 Effect of Species 2 on Species 1 Competition - - Predator-Prey + - Parasite-Host + - Consumer- + - Consumee Mutualism + + Commensalism + 0 2
Competition among individuals of a single species is called intraspecific competition - this results in density dependent effects on population growth between members of different species is called interspecific competition Two species can have negative effects on each other through mutual consumption of a limited resource A resource is anything that if decreased will decrease the numbers or growth rate of a species in the area. Competition for resources occurs if consumption by one individual reduces availability of the resource to others. 3 Two types of interspecific competition: Contest or Interference competition - one or both species actively defend a resource against consumption by the other species Exploitative competition - both species use the resource but neither defends it Contest competition - involves active defense of a resource examples: Red cockaded woodpeckers defend nest holes from occupation by squirrels Damselfishes defend mats of algae from consumption by other fishes Hummingbirds in tropics defend nectar producing trees from other nectar feeding birds and insects 4
Exploitative competition - competition through consuming activities of an animal or plant examples: Canopy trees consume light and limit its availability to understory plants Many fish species feed on the same invertebrates Exploitative competitors may not even encounter each other because they feed at different times or on different life stages of prey Example: in the deserts the principal consumers of seeds are ants and birds 5 The limiting resource concept - In natural systems it is common to find that one or at most a few resources are in limited supply for a species In plants, it is commonly just one soil nutrient that is in limited supply. Plant nutrients: N, P, K and micronutrients - all are required for plant growth. If any one becomes reduced below a critical value then growth of the plant or the plant population is limited. Leibig s Law of the Minimum - each population increases until the supply of some resource no longer satisfies the population s requirement for it. Which nutrient becomes limited first often depends up the rate of consumption of the nutrient by the species using it. 6
P dn/dt > 0 Resource utilization will reduce both N and P, but N will be reduced to limiting level before P. P* limiting level of P dn/dt < 0 N N* limiting level of N 7 Competition Experiments - exclusion and coexistence Early experiments (by Gause) with cultures of more than species of Paramecium showed that one species can cause the extinction of another Each species had logistic growth when cultured alone When cultured in pairs either exclusion or coexistence occurred 8
Gause s Competitive Exclusion Principle - when two competitors utilize the same required resource that is available in only limited quantities, one species will cause the extinction of the other - complete competitors cannot coexist Exclusion doesn t happen if the two species are able to exploit portions of their niche that don t overlap with the other s Gause found P. caudatum was able to feed on bacteria at the top layers of the culture tube and excluded P. bursaria from that zone. P. bursaria fed on yeast in the bottom layers of the culture and excluded P. caudatum from that zone. 9 The outcome of competition often depends on the environment If one environment is more favorable to one species - allowing faster growth or a higher carrying capacity - it may be able to out-compete a species that would replace it in another environmental situation. 10
Competition in nature - many field experiments have shown that competition is an important process in natural communities. Fenced enclosures constructed to exclude Kangaroo rats but not exclude smaller rodents A larger number of smaller rodents could live inside the enclosure when Kangaroo rats were excluded - They were each exploiting some of the same resources - after removal, the small rodents did not have Kangaroo rats as competitors 11 Field experiments have demonstrated that the effects of competition are often not symmetrical. Sceloporus merriami has a negative effect on Urosaurus ornatus but the reverse is not true. S. merriami is limited more by abiotic factors. U. ornatus is limited by biotic factors (competition). 12
Classic example of competition in nature - interaction between barnacles - Chthamalus and Semibalanus Chthamalus lives in the intertidal zone Semibalanus lives in the subtidal zone If Semibalanus is removed Chthamalus also utilizes the subtidal zone Semibalanus is unable to live in the intertidal zone due to desiccation Semibalanus excludes Chthamalus from the subtidal zone through its rapid growth - it undercuts or crushes Chthamalus The realized niche is smaller than the fundamental niche for 13 Chthamalus Mathematics of Competition - the Lotka-Volterra equation The logistic equation describes the growth of a single species when there is a limited supply of resource. dn K N = rn( ) dt K This equation can be modified to incorporate the effect of a second species: dn K N α N = rn ( ) dt 1 1 1 2 1 K1 14
The Lotka-Volterra equation incorporates the effect of a second species on species 1. α represents the effect of an individual of species 2 on species 1 relative to the effect of an individual of species 1 on species 1. 1 1 1 1 2 2 2 K Here the effect of species 2 on species 1 is less than the effect of species 1 on species 1: α < 1. 15 dn K N α N = rn ( ) dt 1 1 1 2 1 K1 K 1 /α N 2 dn 1 /dt > 0 dn 1 /dt < 0 dn 1 /dt = 0 Any combination of N 1 and N 2 that falls on the green line represents a point where species 1 will not grow or decline N 1 K 1 16
dn K N β N = rn ( ) dt 2 2 2 1 2 K2 The effect of species 1 on species 2 can also be expressed by the Lotka-Volterra Equation Here, β represents the effect of species 1 on species 2 relative to the effect of species 2 on species 2 17 dn K N β N = rn ( ) dt 2 2 2 1 2 K2 K 2 dn 2 /dt < 0 N 2 dn 2 /dt = 0 dn 2 /dt > 0 N 1 K 2 /β 18
When species 1 and species 2 are considered together 4 different outcomes are possible Case 1: Species 2 wins K 2 K 1 /α N 2 dn 2 /dt = 0 dn 1 /dt = 0 N 1 K 1 K 2 /β 19 Case 2: Species 1 wins K 1 /α K 2 N 2 dn 2 /dt = 0 dn 1 /dt = 0 N 1 K 2 /β K 1 20
Case 3: Stable coexistence K 1 /α Equilibrium population sizes K 2 N 2 dn 2 /dt = 0 dn 1 /dt = 0 N 1 K 1 K 2 /β 21 Case 4: Unstable coexistence K 2 Unstable equilibrium K 1 /α N 2 dn 2 /dt = 0 dn 1 /dt = 0 N 1 K 2 /β K 1 22
Predation is an important factor in community organization and can reduce the effects of competition. Predators can keep each species below its carrying capacity - allowing coexistence, so long as the predator is present. Environmental disturbances and disease can have similar effects - keeping species below the limiting level of resources. 23 When species coexist through utilization of different types of resources or different ranges of a limiting resources it is called Resource Partitioning or Niche Partitioning Anolis lizards in Cuba live in the same area but exploit different microhabitats 24
Species under continual competition will sometimes evolve differences because of selection against those individuals that suffer the most severe competition Finches in the Galapagos Islands feed on seeds and their beak size determines the size seeds they can eat When found on different islands (allopatric) the beak sizes are similar When found on the same island (sympatric) they have divergent beak sizes - called Character Displacement Competition for seeds is highest on intermediate sized seeds - one species specializes on large seeds, the other on small seeds 25