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Transcript
Community Ecology: Community Ecology I. Community structure & properties II. Community assembly Populations and species do not exist in a vacuum… III. Factors that influence community structure Definitions • species interact! I. Community structure & properties 1. Diversity Community: a group of interacting species that occupy the same area 2. Growth Form & Structure 3. Dominance (Which species do you ignore?) 4. Trophic structure 5. Stability Assemblage: a group of taxonomically related species that occupy the same area low evenness A B C D E F species relative abundance relative abundance 1. Diversity • richness = number of species • evenness = relative abundance of species high evenness 1. Diversity Metrics of Diversity 1) Species richness: number of species in a community 2) Species composition: identity of species that constitute a community 3) Species diversity: species richness and relative abundance (evenness) Shannon-Weiner index of diversity: A B C D E F species H' = -Σ pi (ln pi) Where pi is the proportion of individuals in the community that are species i 1 Scales of species diversity 1. Diversity 1) Alpha (α): within habitat diversity Illustration of Shannon-Weiner index species diversity 100 H'= 0.87 E = 0.63 75 No. of indiv.s 50 100 50 25 25 0 0 A B C H'= 1.39 E = 1.00 75 D 100 2) Beta (β): between habitat diversity 3) Gamma (γ): total diversity (α + β) H'= 1.10 E = 0.79 75 50 25 0 A B C A D B C D Evenness can be calculated as: Species Rocky Intertidal habitat A x Sandy Beach habitat B x C x x D x x E x F x G x Bay habitat x H E = H'/(ln S) x I where, S is species richness Max evenness = 1; Min evenness = 0 1. Diversity x α diversity 4 5 3 β diversity rocky vs. sandy: 5 sandy vs. bay: 6 bay vs. rocky: 7 9 γ diversity 2. Growth form & structure Questions about diversity: • What creates and maintains diversity? • What causes loss or gain of diversity? • How does diversity influence influence ecosystem function? • Is there a particular form that characterizes the community? (e.g., massive corals vs. branching corals) How does altering the growth form & structure affect the members of the community? consumer (secondary carnivore) 4. Trophic structure 3. Dominance • Is there one or a few species that control the community? (e.g., Pisaster sea star in the intertidal) • who eats whom? Producers Herbivores Carnivores (primary) Consumers Decomposers Carnivores (secondary) consumer (primary carnivore) What happens when a dominant is removed? consumer (herbivore) producer What happens if the abundance of one trophic level changes? (trophic cascades?) 2 5. Stability 4. Trophic structure • food webs competition reef shark piscivore guild herbivore guild striped parrot Sargassum queen trigger Diadema Sp. A Sp. B Sp. C time Tripneustes stoplight parrot Lobophora Dictyota resilience total abundance predation barracuda resistance abundance - guilds - competition - predation total time Halimeda 5. Stability • Are communities stable? – are they resistant to change? II. Community Assembly • random collections or integrated, co-evolved units? • What attributes of communities contribute to stability? (e.g., their diversity?) Goal of community ecology To understand how communities are assembled and how they function Are there rules that determine how communities are assembled? abundance – do resilient communities follow a fixed pattern of succession? abundance – are they resilient after a perturbation? distance or environmental gradient distance or environmental gradient III. Factors that influence community structure • abiotic (e.g., temperature, salinity, O2, ocean currents) • biotic (e.g., competition, predation, disease) 3 Five fundamental types of species interactions: Niche: how a species fits into a community Effect on species A B – biotic and abiotic requirements and tolerances Competition – niches do not exist without a species to define them Predation/ Parasitism Mutualism Commensalism Amensalism Concept of the Niche 3) Two types of niche 1) Best known definition of niche is Hutchinson s (e.g., 1957) a) role organism plays in environment b) role can be determined by measuring all of an organism s activities and requirements 2) Examples: high 2-factors 3-factors Height in intertidal low low Wave exposure high er ov e t c ga en oal c r r Pe ac m 3) By extension… niche defined as an N-dimensional hyperspace a) fundamental: niche space determined by physical factors ( potential ) and resource requirements. Manifest in the absence of other organisms. b) realized: niche space determined by combined physical ( actual ) and biological factors. Realized in presence of other organisms. fundamental realized • fundamental niche always bigger (or at least as large) - biological interactions can (usually do) limit realized niche (encompasses all effects and requirements of a species) More about a few key biological interactions: • Competition Competition: the common use of a resource that is in limited supply 1) Occurs within and between species • Predation a) Intraspecific - among individuals of the same species source of density dependence discussed last time • Mutualism b) Interspecific - among individuals of two or more species 2) Two types of competition a) Interference b) Exploitative 4 3) Competitive exclusion principle: 2) Two types of competition (Gause s Law, 1934) a) Interference - direct competition A Two identical species cannot coexist on the same limited resource B aggression, territoriality (e.g., in fishes, birds, limpets) a) Species occupying the same niche cannot coexist. b) Exploitation - indirect competition b) The more similar the species (the greater the niche overlap ), the greater the likelihood of competitive exclusion, leading to local extinction of one species. — compete through a resource (R) (e.g., sessile spp. -- space) B A barnacles R mussels c) Over evolutionary time scales leads to resource partitioning . space 5) Spatial patterns caused by competition 4) Resource partitioning number of individuals a) non-overlapping spatial (or temporal) distribution adaptation A B C DE (evolution) resource gradient* A * e.g., species packing B C D E B rock llow & ye k c fish bla rock her gop resource gradient fish resource gradient 5) Spatial patterns caused by competition 6) Competitive release a) Change in distribution (or some other response such as growth) when separate and together a) negative (inverse) relationship in abundance i) gradient in density AA A B A B AB AB A B B BBA sympatry (together) tidal height s acle barn sels mus absence of mussels barn ii) patchy / clumped Abundance sp. B s acle barn sels mus tidal height reef depth - food size - depth - height in intertidal Abundance sp. A number of individuals A A AA A AA B B B B BB A A A A A A acle s absence of barns sels mus Could test for competitive release observationally or experimentally. Which is better? 5 7) Competitive symmetry 8) Responses to competition a) relative competitive strength a) Individual responses: b) superior vs. inferior (or) dominant vs. subordinate Symmetrical A = B A B A > B A B A < B A B • • • Behavioral (feeding rates, foraging distribution) Physiological (growth rate, reproductive rate) Morphological (body size, biomass) b) Population responses: • • • Asymmetrical Abundance (density) Distribution (zonation) Demographic rates (population growth) How would you assess this? Predation Consumption of one organism (prey) by another (predator), which by definition, occurs between organisms on different trophic levels (vs. competition: within same trophic level) diagrammatically: food web A Herbivore Primary producer (plant / alga) • death of individuals B C Indirect effects : influence of predator on variable other than death or mortality A C B D E 3) Effects on prey (individual and population): Individual responses: behavioral (feeding rates, foraging distribution) physiological (growth rate, reproductive rate) • behavioral (feeding rates, foraging distribution) • physiological (growth rate, reproductive rate) • morphological (body size, biomass) F 4) Complex interactions (with other processes) E.g., competition: e.g., predator that specializes on barnacles and is restricted to the mid and lower intertidal morphological (body size, biomass) With predators Without predators … and you can get completely or partially eaten Population responses: • • • • Direct effects : direct losses (removal of individuals) (mortality rate of population) food chain Predator • • • • 2) Effects on prey (direct and indirect): abundance, density tidal height barn s acle sels mus s acle barn sels mus distribution (habitat use) structure (e.g., size, age, sex ratio, genetic, spatial) dynamics and persistence (regulation) In absence of predator, barnacle out-competes mussels and expands distribution down into the mid intertidal 6 4) More complex predation interactions: Apparent competition Trophic cascades A linear food chain Effect on species A B Strong top-down effects that produce downward rippling effects through a food chain. C C Where A and B are prey but A is favored and C is a common predator. Presence of either prey increases overall predation rates, leading to negative indirect effect on one another. A B C Trophic cascade Where, A is primary producer, B is an herbivore, and C is a predator. Higher tropic level predators indirectly affect plant biomass via their impacts on herbivore populations. Predator Herbivore B Effect of species on adjacent trophic level has net positive indirect effect on next trophic level. Plants A Oksanen/Fretwell Model: Productivity and Food Chain Length Apex predators • Depending on productivity of community, food chains can have fewer or more than three trophic levels. Herbivores • As primary productivity increases, trophic levels will be sequentially added. • Food chains that have an odd number of trophic levels should be filled with lush vegetation, because herbivores are kept in check by predators. Plants • Food chains that have an even number of trophic levels should have low plant abundance because plants are herbivore limited. Abiotic Resources Mutualism / commensalism Oksanen/Fretwell Model secondary Carnivores Occurs within or between trophic levels, more often between trophic levels a) mutualisms obligate – required for each others existence (e.g., pollinators) facultative – not required (e.g., cleaner fish and parasitized hosts) Biomass Carnivores Herbivores b) commensalism: (also called facilitation) Plants Abundance sp. A Abundance sp. B Low Environmental Productivity High mutualism (symmetrical) A = B A B commensalism (asymmetrical) A < B A B What tests could you do to distinguish mutualism from commensalism? 7