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Community Ecology Community - a group of species that live and interact within a relatively well defined geographic area Typical communities Oak-Hickory forest Nearshore Cypress-Tupelo Desert Spartina marsh Prairie Grassland Mangrove Serpentine soil Coral reef Peat Bog Sometimes a community is defined by the range of one or a few species - usually plant species (Oak-Hickory), other times it is defined by a distinct geographic region (nearshore), and other times it is defined by a physical parameter (e.g. desert, serpentine soil) Questions addressed by community ecologists: What organizes communities? What determines what kind of species live together in an area? Why don’t other species live among them? How do species interact? What are the results of their interactions? Properties of individual species determine where each species can live. Interactions between species may restrict where each species is found. The full range of resources and habitat space that a species might potentially use is its “fundamental niche.” Species can only exist for long periods of time within their niche. A niche is more than just a place or habitat. It is a description of how and where the species exists. Species commonly do not exploit their entire niche. They utilize only a portion of their niche. The portion they utilize is called their “realized niche.” Species are often restricted from utilizing their entire niche due to the presence of other species. Interspecific Competition is an important type of interaction between species that organizes communities and limits the size of a niche Interference competition occurs when two species engage physically while trying to secure a resource Exploitative competition occurs when two species both utilize the same resource but do not interact directly Classic example - 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 - undercuts or crushes Chthamalus - interference The realized niche is smaller than the fundamental niche for Chthamalus Competition - 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 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. 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 Species under continual competition will sometimes evolve 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 - greater difference when sympatric than when allopatric is called “Character Displacement” Competition for seeds is highest on intermediate sized seeds - one species specializes on large seeds, the other on small seeds The effects of competition can be demonstrated with field experiments 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 Predation is an important factor in community organization Predators exploit prey and can cause extinction of prey - Didinium preys on Paramecium But often prey have refuges where predators can’t exploit them The presence of a predator feeding on one or more competing species will allow competitors to coexist because prey numbers are kept low and they can’t completely exploit a resource that limits another species Predation selects for behaviors or physical features in prey that deter predation In plants, physical characteristics include spines, thorns, irritating hairs, chemical defenses such as irritants or toxins Chemical defenses include ortho-quinone in many vegetables, mustard oils in cabbage and relatives, opiates in poppies, THC in marijuana, digitalis in foxgloves, cardiac glycosides in milkweed, caffeine in many plants, ephredrine in Ephedra Chemicals that have no metabolic function and are effective deterrents of herbivores are called “Plant Secondary Compounds” The presence of secondary compounds selects for characteristics in herbivores that reduce the effects of the toxin Cabbage butterflies can tolerate mustard oils and their caterpillars are the only important herbivores on many species in the group Monarch butterfly caterpillars feed on milkweed and incorporate the cardiac glycosides into their bodies. They become toxic to any vertebrates that might eat them. The plant evolutionary response to effective herbivores can be to increase the concentration of toxin in their tissues - can result in a “coevolutionary arms race” Animals also have strategies to avoid predators Crypsis (camouflage) is used by many species Chemical defenses are also used by animals and they are often associated with aposematic (warning) coloration Some species evolve to look like toxic or otherwise noxious species and gain reduced predation - “Mimicry” Batesian mimicry - a toxic model and a nontoxic mimic the Monarch and the Viceroy Mullerian mimicry two toxic species evolve to look alike both benefit from the other’s presence Other mimics A beetle A fly Coral and King snakes Many species only exist due to the beneficial actions or presence of another species Symbiosis - “living together” Symbiotic interactions can be defined through their effects N am e of interaction Com m ensalism M utualism Parasitism Effect on species 1 Effect on species 2 + + + 0 + - Commensalism Many examples - humans (0) eyelash mites (+), cattle (0) and egrets (+), anemones (0) and clownfish (+) Mutualism Many more examples - corals and dinoflagellates, sea squirts and photosynthetic bacteria, trees and mycorhizzal fungi, ants and aphids, acacias and ants, plants and pollinators and seed dispersers Acacia provides food from extra-floral nectaries and Beltian bodies, and nurseries inside enlarged thorns Ants provide protection from herbivores and clear area of competitors Parasitism Even more examples Ectoparasites - lice, ticks, fleas, leeches on humans and others, fungi like ringworm, athletes foot, plants that parasitize others: dodder, mistletoe Parasitoids - insects larvae feed on living host - screw-worm fly, bot fly, many wasp larvae feed on living but stunned caterpillars or spiders Endoparasites - protistans include trypanosomes, Plasmodium, Entamoeba histolytica, worms include many nematodes and flatworms, many fungi parasitize plants Brood parasitism - the eggs of one species are left in the nest of another - seen in several birds, and many fish Cuckoos leave an egg in another bird species nest Young hatch early and expel eggs of parent Parent cares for cuckoo until fledged Some bird species have developed the ability to detect cuckoo eggs when they don’t look like their own - In some areas cuckoos have eggs that match the color of their favorite host - another “arms race” Some species make conditions right for the coexistence of many species - these are called “Keystone Species” Predators can be keystone species because they reduce competition and promote coexistence of prey species Beavers create a habitat for many species that would have no habitat otherwise Coral polyps create a habitat for thousands of species Not all species interactions are direct one species may have an effect on another through its interaction with a third species - these effects are “indirect effects” Rodents and ants both eats seeds Ants prefer small seeds Rodents prefer large seeds Ants and rodents compete for seeds - direct effect But the presence of rodents benefits ants because rodents each large seeds, leaving small seeds to germinate and benefiting future ant generations Disturbance can play an important role in communities Fires, floods, freezes, droughts, tree-falls, etc. all reduce population sizes of competitors and promote their coexistence Very frequent and severe disturbance can cause species extinctions No disturbance can result in competitive exclusion Intermediate levels of disturbance reduces competition and maintains high diversity This is the “intermediate disturbance hypothesis” Communities change and develop by passing through a series of stages - each stage is called a “sere” - the whole process is called “succession” The first type of succession described was on cleared farmland that was allowed to return to forest naturally. In such situations the seres are often - grasses and weedy dicots (good dispersers), followed by shrubs and larger herbs (good competitors), followed by small trees like pines (better competitors), and finally followed by hardwoods (the best competitors) This type of succession - on reclaimed land - is called “secondary succession” The last sere is maintained until a major disturbance - the last sere is called the “climax community” Primary succession is the development of each sere starting from sterile rock or soil - e.g. after glacial retreat Lichens are the first colonizers. They help weather rock into patches of soil. Mosses colonize the soil and improve it further by adding organic matter. Shrubby alders colonize and make the soil acidic, which makes it favorable for Spruce. Spruce grow and their shade eliminates the alders. The overall trend in succession is from r-selected to K-selected species. The early species are tolerant, and facilitate the entry of intermediate sere species, climax species inhibit the entry of other species