Transcript Food Webs
Chapter 17 Species Interactions and Community Structure Objectives: 1. Determine the difference between a food chain and a food web and describe each 2. Describe the attributes of a Keystone Species and what it does for the community 3. Describe what changes occur in a community when an exotic invader alters the food web Who eats Whom? Food chains: Show simple the feeding relationships in a community. Chain infers one connected to one other link (food item). Food Chains - Antarctic Killer whales Ross seals Crabeater seals Krill Food Chains - Antarctic Killer whales Ross seals Blue Whale Crabeater seals Krill Flying birds Fig 17.2 Fig 17.3 Food Web Facts 1. Can be very complex and hard to interpret 2. Elimination of “weak” interactions creates easier picture to follow 3. Identify several trophic levels 4. Identify top predators, intermediate predators, down to base of food web Food Web Facts 5. Do not indicate the strength of a food relationship 6. Identify the direction of energy flowing through the system 7. Does not identify the amount (strength) of energy flowing between trophic levels Food Webs Strong/Weak interactions: Not necessarily the amount of energy flowing between the two, but the degree of influence on community structure. Fig. 17.5 Food Webs Fig. 17.5 Food Web Structure Degree of influence varies for each association in the food web Those species that are most influential may be called: What do we know so far? 1. 2. 3. 4. Food webs are complex with many species Some interactions are weak Some interactions are strong Keystone species may be the most influential in the community--- What does this mean? Keystone Species Keystone Fig. 17.14 Keystone Species Paine’s keystone species hypothesis: 1. Predators may keep prey below K 2. Potential for competitive exclusion low 3. Number of species in a community would increase OR------------ Keystone Species Paine’s keystone species hypothesis: Some predators may increase species diversity! Fig. 17.6 Fig. 17.6 Keystone Species Paine’s experiment, page 328 book: 1. Removed the Keystone Predator (starfish) 2. Followed community for two years 3. 3 months - barnacle Balanus occupied from 60-80 % of space 4. One year - Balanus crowded out by mussels and gooseneck barnacles Keystone Species Paine’s experiment, page 328 book: 5. Benthic algae declined - lack of attachment space (taken up by mussels and gooseneck barncles) 6. Herbivorous chitons and limpets left - no food 7. Sponges crowded out Keystone Species Paine’s experiment, page 328 book: 8. After 5 years - community had only two species: mussel and goose-neck barnacle Fig. 14.6 Exotic species Exotic predators can collapse and simplify the structure of the food web. • Fewer #’s species • Simplified food web, loss of species • Loss of one or more trophic layers • Change in ecosystem dynamics Exotic Species Invasion Lake Victoria: a case study, pages 335-337 • 26,000 square miles • Tropical environment • 400 species of fish, 300 endemic • Last three decades, half species extinct • Major changes in water quality AND • Introduced Fish The Nile Perch African Cichlids Nile Perch Nile Perch • 1950’s introduced • Low level until ~ 1980 • Populations exploded, from 20% to 80% of biomass • Loss of 200 species of fish • Complete change in water quality (due to fish change and/or other environmental factors) Fig. 17.15 Question for the Day???? Can the introduction of a species that will reduce the number of species and the number of individuals in the community a good thing? Defend your answer, cite an example if possible. Done for Today 1. Food Chains vs. Food Webs 2. Food webs can be very complex 3. Keystone species and importance to food web 4. Exotic species can collapse a food web 5. Communities infer a complex interaction of species, to think otherwise would be folly