Ecosystems: Energy Flow and Food Webs

Transcription

1 Ecosystems: Energy Flow and Food Webs MODULE 21: ECOSYSTEMS: ENERGY FLOW AND FOOD WEBS UNIT 3: COMMUNITIES AND ECOSYSTEMS Objectives At the end of this series of lectures, you should be able to: Define terms. Discuss the different types of controls of ecosystem function. Describe and interpret ecological or trophic pyramids. Describe energy flow through an ecosystem. Explain trophic efficiency. 1

2 Energy flows. Nutrients cycle. Energy Sources Autochthonous Energy captured by autotrophs within the system. Allochthonous Energy captured by autotrophs outside the system. Aquatic systems Rivers and streams 2

3 Trophic Levels The position in a food web Determined by the number of transfers of energy from primary producers to a particular level. Primary producers Autotrophs Primary consumers Herbivores Secondary consumers Carnivores Tertiary consumers Carnivores Detritivores Decomposers 3

4 Trophic Pyramids A graphical representation of the biomass or energy content of the all of the members of a trophic level within an ecosystem/community. Less energy available at each higher trophic level. Limits the number of trophic levels Less NPP means fewer trophic levels and less diversity within each level. More NPP means more trophic levels and more diversity within each level. 4

5 Trophic Pyramids Inverted Pyramids Occasionally in systems maintained by high reproductive rates. Producers have high reproductive rates, short lives, and typically small Consumers have lower reproductive rates, longer lives, and often are much larger Few levels Problems Difficulty defining area Occasionally counts of individuals rather than energy or biomass 5

6 Energy Transfer Efficiency Trophic efficiency The amount of energy at one trophic level divided by the amount of energy at the tropic level immediately below it. Consumption efficiency Assimilation efficiency Production efficiency Energy Transfer Efficiency Consumption Efficiency Energy transfer between trophic levels About 13% of terrestrial NPP is consumed Range % About 35% of aquatic NPP is consumed Range % 6

7 Energy Transfer Efficiency Assimilation efficiency Influenced by the quality of the food and the physiology of the consumer. Plant material varies considerably in quality 20-50% assimilation efficiency for herbivores 80% assimilation efficiency for carnivores Related to thermal physiology Energy Transfer Efficiency Production efficiency Related to thermal physiology and size of consumer Endotherm Higher assimilation efficiency, but lower production efficiency. Smaller endotherm organisms Lower production efficiency 7

8 Energy Transfer Efficiency 10% rule Ray Lindeman 8

9 Trophic Cascades Changes in the biomass and species composition of trophic levels due to interactions between species. Typical in species-poor systems Trophic Cascades Bottom-up control Influence of physical and chemical factors on ecosystem function. Temperature Nutrients Top-down control Influence of consumers on ecosystem function. 9

10 Trophic Cascades Number of Trophic levels Addition of a top predator Insertion or loss of predator in middle of food chain Changes in omnivore feeding perference Food Webs A diagram showing the connections between organisms and the food they consume. Food webs are complex Trophic interaction strength varies 10

11 11

12 Energy flows. Nutrients cycle. 12