Community Ecology II: Direct and Indirect Effects in Food Webs PDF

Community Ecology II: Direct and indirect effects in food webs That’s disgusting! Need to know Define and be able to describe a food chain and food web Understand and be able to describe a trophic cascade, and its relevance to ecological management and restoration Understand an...

Community Ecology II: Direct and indirect effects in food webs That’s disgusting! Need to know Define and be able to describe a food chain and food web Understand and be able to describe a trophic cascade, and its relevance to ecological management and restoration Understand and be able to identify direct vs. indirect effects in food webs Be able to describe the phenomenon of apparent competition Understand the processes by which wolves have transformed the Yellowstone ecosystem, and be able to describe ecological networks Be able to describe how ecological networks depict species interactions within communities Structure and dynamics of communities depend in part on feeding relationships heterotrophs autotrophs  Structure and dynamics of communities depend in part on feeding relationships Trophic structure describes: – Who eats whom – Transfer of energy and materials Different food chains link to form food webs Tertiary consumers Secondary consumers Primary consumers Plants Different food chains link to form food webs Arrows show direction of energy flow Real food webs can be Here’s an example a tad bit of a “simplified” Arctic food web… complex…  Problem: You’re managing a game reserve and you want more zebras  Problem: SO DO YOU GET RID OF SOME LIONS?  Problem: OR DO YOU INCREASE ZEBRA FOOD?  What controls the abundance of species at different trophic levels? There are two models  “Bottom-up” vs. “Top-down” population regulation Bottom-Up: availability of energy and nutrients for primary producers controls their abundance, which in turn controls abundance of species at higher trophic levels. Populations at different trophic levels are “food limited”  “Bottom-up” vs. “Top-down” population regulation Bottom-Up: availability of energy and nutrients for primary producers controls their abundance, which in turn controls abundance of species at higher trophic levels. Top-Down: The opposite of Bottom-up. Populations of species in food webs are controlled by their predators (“predator limited”). Does this graph suggest top- down or bottom up control? total algae biomass each dot is a lake nutrients  Biomanipulation:Top-down vs. bottom up control in the real world…. Environmental problem: too much algae in a lake how might we manipulate algal levels?  Using Top-Down Control Concept for Environmental Management In this “trophic cascade”, there are both direct and indirect effects Indirect effect Direct effect  direct effects indirect effects Example of importance of trophic control: Restoration projects goal: recover ecological communities and their ecosystem functions and services methods: plant out seeds or saplings problem: expensive, and many restoration projects fail! Example of importance of trophic control: Restoration projects goal: recover ecological communities and their ecosystem functions and services methods: plant out seeds or saplings problem: expensive, and many restoration projects fail! Example of importance of trophic control: Restoration projects Plant Plant Higher vegetation abundance à Real-world food webs are often altered. Species introductions Species invasions Species extinctions or extirpations Changes in nutrient or resource levels Case study of direct and indirect effects: The story of foxes in the Aleutian islands Fur traders introduced foxes to some islands in the 1800s …and later, ecologists noticed something interesting Why were fox-free islands greener?  foxes seabirds vegetation Indirect effects can generate misleading patterns. Example: “Apparent Competition” + - - zebra (grass eater) eland (shrub eater) Learning alytics! In an aquatic system with strong top-down control that has 5 trophic levels (large top predator fishes, medium predatory fishes, small predatory fishes, herbivorous fishes, and aquatic plants), increasing the population of the top predators would __________ the abundance of the aquatic plant community. a. increase b. decrease c. have no effect on You are a species of squirrel that is preyed upon by foxes, which are preyed upon by mountain lions. A new species of squirrel is introduced to your ecosystem. This squirrel eats entirely different plants than you do, but it shares the same fox predator. What do you predict the population of the mountain lions will do over time, in response to this change in the biotic environment? (note: this system is strongly regulated from “the bottom up”) a. Increase b. Decrease c. Stay the same “Ecological networks” depict different types of species interactions (not just trophic) Species are represented by “nodes” Interactions are represented by lines connecting “nodes” = pollinator = plant Will elephant declines affect plant-pollinator interactions? Large herbivore extinction and pollination webs Large herbivores affect vulnerability of plants and pollinators to extinction Lose this pollinator Less resistant to extinction Lose this pollinator More resistant to extinction Yellowstone National Park Yellowstone 1926 - 1995  Wolves re-introduced in 1995 Stream banks stabilized, streams deeper and cooler Birds and fish recover Coyote numbers decrease Beavers recolonize Have a great weekend!