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WF mouse

Resource competition (via shared resources). Cooper’s Hawk. Weasel. WF mouse. Chipmunk. Cooper’s Hawk. Weasel. Veery. Chipmunk. Acorns. (-). Cooper’s Hawk-. Chipmunk. (+). Chipmunk- Weasel. (-) effect. (+) effect. (-) × (+) = (-). (1) A “simple” and simplified

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WF mouse

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  1. Resource competition • (via shared resources) Cooper’s Hawk Weasel WF mouse Chipmunk Cooper’s Hawk Weasel Veery Chipmunk Acorns (-) Cooper’s Hawk- Chipmunk (+) Chipmunk- Weasel (-) effect (+) effect (-) × (+) = (-)

  2. (1) A “simple” and simplified food web in the Antarctic

  3. Indirect effects (> 3 spp) Trait-mediated (via changes in spp traits) Density-mediated (via changes in spp abundance) • e.g., predator facilitation • ecology of FEAR • Resource competition • (via shared resources) • Apparent competition • (via shared predators) • Indirect mutualism

  4. Indeterminacy in food webs or why we often cannot predict the effects of changes in the abundance of a species on other members of the food web – 3 different sources - Complexity, there are many, many links oh, and did I say many links? - Time lags – effects don’t show up until the interactions flow through many intermediate species - Unpredictable behavior – Trait-mediated interactions, ecology of fear, ecosystem engineering

  5. (1) A “simple” and simplified food web in the Antarctic

  6. From your text, a more realistic food web based on 10 fish species and their foods

  7. Owls on Kangaroo rats: • Direct: owls consume K-rats - • Density-mediated: owls kill snakes • that kill K-rats + • (3) Trait-mediated: owls scare k-rats • into the fangs of snakes - • Total = ??? Have we forgotten any??? (4) Rattlesnake reduce their activity in the presence of owls (5) Owls reduce the abundance of K-mice, a competitor

  8. Time lags A release from short- term competition gives way to release from a long-term mutualism

  9. Review Papers: Sih et al. (1985) reviewed >100 predator removal studies - most effects were large … > 2 fold change in prey abund. - 22-46% of studies showed unexpected effects, i.e., prey declined in response to predator removal Mauer (1999) – Few studies have shown significant effects due to competition when analyzed after 3 years.

  10. So what can we do???? • Perhaps we cannot determine with confidence what the • long-term effect will be from variation in the abundance of • community members…so …. • We can determine with much greater confidence what • the effects will be to trophic levels rather than individual spp. • (2) Ignore the weak interactions and concentrate on strongly • interacting species that have disproportionate effects • KEYSTONES

  11. Trophic cascades model: Alternating (-) and (+) effects at each lower trophic level = the top trophic level limits the abundance of the level below itself,which in turn allows 2 levels below to flourish and so on….. + + _ Caterpillars _ Oak trees

  12. Trophic cascades model: Alternating (-) and (+) effects at each lower trophic level = the top trophic level limits the abundance of the level below itself,which in turn allows 2 levels below to flourish and so on….. + + _ “Coyotes reduce cat populations that allow birds to flourish in turning keeping insects to a minimum so that trees flourish” - i.e., the world is GREEN Caterpillars _ Oak trees

  13. “Cat reduce the numbers of birds allowing insects to flourish and defoliate the forest” - i.e., the world is BROWN X + _ Caterpillars _ Oak trees

  14. “Cat reduce the numbers of birds allowing insects to flourish and defoliate the forest” - i.e., the world is BROWN X + _ The difference is in the number of trophic levels: Odd # - GREEN World Even # - BROWN World Caterpillars _ Oak trees

  15. But what if Predators don’t rule the world? Bottom-up control rather than Top-down control + + + + + _ + Caterpillars _ + Oak trees

  16. GREEN BROWN Bottom-up control: Response to an increase in productivity

  17. An increase in Productivity • causes an increase in consumers Zooplankton Biomass (2) Adding fish causes a decrease in consumers and an increase in plants Plant Biomass (chlorophyll) Zooplankton Biomass Plant Biomass (chlorophyll) After Leibold et al. 1997

  18. Ecological Meltdown in Predator-Free Forest Fragments

  19. 4300-km2 hydroelectric impoundment in the Caroni valley of the state of Bolivar, Venezuela • Isolated by rising water in 1986, these • islands range from 0.2 to 4.9 km from the • nearest point on the mainland, and all are situated at least 100 m across open water from any other landmass. • The vegetation is semideciduous, • tropical dry forest.

  20. Area (ha) 0.25 0.7 0.3 0.6 0.5 0.9 No. trees > 10 cm DBH 203 490 290 381 301 403 No. tree species 32 40 33 47 54 55 No. stems > 1 m tall 9 84 147 90 156 301 No. howler monkeys 2 6 3 6 - - Howler monkeys per ha 4.0 8.6 10.0 0.0 - - No. leaf-cutter colonies 1 3 2 4 2 1 No. leaf-cutter colonies/ha 4.0 4.3 6.7 6.7 4.0 1.1 Area (ha) 11 12 350 350 150 ML ML No. tree species per 49 51 50 55 46 51 50 stems > 10 cm DBH 375 236 304 - 379 340 - No. leaf-cutter colonies 2 2 54 4 2 - -No. leaf-cutter colonies/ha 0.18 0.17 0.01 0.01 0.01 0.04 0.04

  21. Jaguar Harpy Eagle Leaf cutter ants Agouti Howler Monkey Coati PLANTS

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