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Transcript
Ecological Communities Chapter 45 Characteristics of Communities • Community: all species that coexist and interact with one another in a given location • Characteristics: – Species richness and composition • Communities are assembled thru the gain and loss of individual species – Species compositions do change over time and space!!! Communities Change Over Space and Time • Species composition changes along environmental gradients – Remember the range or dispersal of a species is due to abiotic and biotic factors • Some species of organisms only live in specific temperature, precipitation, soil type, light levels, etc. • Presence of one species may have a direct affect on the presence or absence of another!! – Often certain animal species are only associated with certain vegetation – which in turn is only associated with certain biomes or geographical locations! Several Processes Cause Communities to Change Over Time • Three major processes contribute to changes in community composition: – Extinction and colonization – Disturbance • Ex: sudden environmental change – earthquake, volcanic eruption; fire • Succession – characteristic pattern of change that occurs in a community after an environmental disturbance – Primary vs secondary succession – Climate Change Dung Beetle Species Composition Changes Over Times Each species directly affects/contributes to changes in various abiotic or biotic factors that can then promote or inhibit other species!! Species Composition Changes with Climate Change Succession • Characteristic pattern of species change after a disturbance – Each species alters abiotic/biotic factors that promote/inhibit other species • Primary – No soil present; often associated with NEW islands, etc. • Secondary – Soil still present, but serious damage to species on environment has occurred • Ex: clear cutting forest; forest fire • Ecological transition – distinctly new community recolonizes after a disturbance Succession • Change involving a series of species replacements in a community following a disturbance • Primary succession – Occurs in area where there is no soil - no preexisting ecosystem • Lichens (pioneer - break down rock to soil) • Secondary succession – Soil present - prior community present • Old-field • Fire • Pioneer species – First species to take hold - usually a producer – Climax community - mature, established community Secondary Succession (a) Soon after fire (b) One year after fire Primary Succession 1941 1907 2 Dryas stage 1 Pioneer stage, with fireweed dominant 1860 0 5 10 15 Kilometers Glacier Bay Alaska 1760 4 Spruce stage 3 Alder stage Soil nitrogen (g/m2) 60 50 40 30 20 10 0 Pioneer Dryas Alder Successional stage Spruce Trophic Interactions In Communities • Energy flow and nutrient cycling in ecosystems – Nutrients and materials cycle – Energy is a one-way street • Ecological efficiency • Trophic interactions can change the species composition of communities – Ex: trophic cascade – wolves!! • Energy flow and Chemical cycling – While nutrients (C, N, P, water) are recycled - Energy is NOT recycled in an ecosystem • Relationships – Trophic levels • Primary producers (input energy into system; convert inorganic to organic compounds) • Primary consumers • Secondary and tertiary consumers • Detrivores/detritus (release nutrients back as inorganic substances) • Food chain A Food Web in Yellowstone The Major Trophic Levels Trophic Cascade removal of one species (keystone species) dramatically alters trophic levels or species diversity/richness Diversity = species richness + evenness Energy Flow: Primary Productivity • Primary productivity - amount of light energy converted to chemical energy by autotrophs – Gross (GPP): total energy converted – Net (NPP): represents the storage of energy available to consumers – R: respiration (amount of energy used by the primary producers for own energy needs) • • • • NPP = GPP - R Biomass: primary productivity reflected as DRY weight of organic material Secondary productivity: rate at which an ecosystem’s consumers convert chemical energy of the food they eat into their own new biomass Overall, terrestrial ecosystems contribute about 2/3 of global NPP – Temperature and moisture influence PP – PP generally lowest in high-latitude tundras and deserts and highest at equator in tropical rain forests • High productivity provides varied niches and increased biodiversity • Marine ecosystems contribute 1/3 – In aquatic ecosystems, both light and nutrients control primary productivity • Depth of light penetration affects primary production in the photic zone – Often more limiting factor in photic zone is nutrient level (N, P) • Pyramid of net production • Pyramid of biomass (dry weight) • Pyramid of numbers Energy Transfers Between Trophic Levels Less than 10% Efficient – These all represent the loss of energy with each transfer in a food chain!! • Ecosystems are limited to number of trophic levels they can support!!! • Limits the number of individuals at any particular trophic level!! – Remember Malthus - while reproduction rates exponential, resources are limited!!! Tertiary consumers 10 J Secondary consumers 100 J Primary consumers 1,000 J Primary producers 10,000 J 1,000,000 J of sunlight Trophic level Tertiary consumers Dry weight (g/m2) 1.5 Secondary consumers 11 Primary consumers 37 Primary producers 809 Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida. Trophic level Tertiary consumers Number of individual organisms 3 Secondary consumers 354,904 Primary consumers 708,624 Primary producers 5,842,424 Energy Transfers Between Trophic Levels Less than 10% Efficient -WHY?? Species diversity affects community function • Species diversity = species richness + species evenness • NPP and GPP can vary greatly with species richness – Diverse plant communities may use soil nutrients more efficiently – Output of seeds and fruit by plants is a function of insect diversity – Retention of nutrients in ecosystem increases with more diversity of decomposers – Insert fig 45.10 Species Richness Increases Toward Equator Structurally Complex Habitats Support Greater Diversity NPP Varies Among Ecosystem Types Which ecosystems have highest net primary productivity and why? Why is open ocean so low? Often limiting factor in photic zone is nutrient level (N,P) Marine NPP is Highest around Coastlines Aquatic Ecosystem NPP Generally Limited by N or P Availability • Too much of a good thing may cause problems • High nutrient levels can lead to eutrophication and dead zones. Terrestrial NPP Corresponds to Climate Terrestrial NPP Varies with Temperature and Precipitation Energy is a One-Way Street But Nutrients Cycle • Biogeochemical cycles - nutrients cycle between both biotic and abiotic components in an ecosystem – – – – Water Carbon Nitrogen Phosphorus • In studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors: 1. Each chemical’s biological importance 2. Forms in which each chemical is available or used by organisms 3. Major reservoirs for each chemical 4. Key processes driving movement of each chemical through its cycle Chemical Elements Cycle among Compartments of the Biosphere Global Water Cycle Global Carbon Cycle Global Nitrogen Cycle N2 in atmosphere Assimilation Nitrogen-fixing bacteria in root nodules of legumes Decomposers Ammonification NH3 Nitrogen-fixing soil bacteria NO3– Nitrifying bacteria Nitrification NO2– NH4+ Nitrifying bacteria Denitrifying bacteria