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Food Chains and Webs Ecosystem Interactions Organisms in an ecosystem constantly interact causing: 1. Stability within ecosystems 2. Growing or declining populations 3. Changes in abiotic and biotic factors Stable Ecosystem A stable ecosystem is one where each of the following occurs: 1. Population numbers of each organism fluctuates at a predictable rate. 2. Supply of resources in the physical environment fluctuates at a predictable rate. 3. Energy flows through the ecosystem at a fairly constant rate over time. Stable Ecosystems Predator-Prey relationships help to stabilize ecosystems. As the prey population increases, so does the predator population. As the prey population decreases, so does the predator population. Limited Resources When resources are limited, species with compete for resources. Can be among same or different species, but usually within the same *niche* Niche is the role an organism in its environment (includes type of food it eats, how it gets food and how it interacts with other organisms). Flow of Energy in Ecosystems 1. A food chain is the simplest path that energy takes through an ecosystem. 2. Energy enters an ecosystem from the sun. 3. Each level in the transfer of energy in an ecosystem is called a trophic level. Flow of Energy in Ecosystems Organisms in each trophic level use some of the energy in the process of cellular respiration, lose energy from heat loss and store the rest. Trophic Levels 1st Trophic Level: Producers/Autotrophs 2nd Trophic Level: Primary Consumers (herbivores-eat plants only) and Heterotrophs 3rd Trophic Level (or any higher): Secondary/Tertiary Consumers-can be carnivores (eat only primary consumers)or omnivores (eat both primary consumers and producers) A heterotroph that breaks down organic material returns it to the soil is a decomposer. Food Webs Represent many interconnected food chains describing the various paths that energy takes through an ecosystem.