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Beyond keystone predation

Beyond keystone predation. Predation is a pairwise interaction Interference competition is a pairwise interaction Effects on the two species involved There can be effects beyond the pair of species

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Beyond keystone predation

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  1. Beyond keystone predation • Predation is a pairwise interaction • Interference competition is a pairwise interaction • Effects on the two species involved • There can be effects beyond the pair of species • Indirect effect: An effect of one species on another that occurs via an effect on a third species

  2. Predator #2 Predator #1 + + - - Prey A surprisingIndirect effect RESOURCE COMPETITION negative effects caused via a shared victim

  3. Predator - + Herbivore - + Plant Indirect effect Increase predator  Decrease Herbivore  Increase Plant TROPHIC CASCADE effects produced 2 or more trophic levels down from top predator

  4. Predator + + - - Prey #1 Prey #2 Indirect effect Decrease prey #1  Decrease Predator  Increase Prey #2 APPARENT COMPETITION negative effects caused via a shared enemy

  5. Apparent competition • Can play a role in effects of invasions • Novel pathogens can have devastating effects on natives • American Chestnut • Pollen data for eastern forests • White oak 25-65% of stems • Hickory 5-15% • Am. Chestnut 5-15% • Parallel story for American Elm

  6. Invasion effects via native enemies Variegated leaf hopper VLF (Erythroneura elegantula) Grape leaf hopper GLF (Erythroneura variabilis) Feed on grape in California GLF native; VLF invasive 1980s: as VLF spread in San Joaquin Valley, GLF declined Apparent competition Settle & Wilson 1990

  7. Parasitoid • Anagrus epos • Egg parasitoid • Attacks both, prefers GLH • as proportion of VLH increases, proportion of unparasitized eggs that are VLF increases • and therefore proportion parasitism of GLH increases

  8. Reductions of GLF • Interspecific competition detectable, but not particularly strong or asymmetrical • Apparent competition seems to be the main driver of replacement of GLF by VLF

  9. Intraguild Predator - + + Intraguild prey + - - Resource Indirect effect INTRAGUILD PREDATION Preying on your competitor

  10. Intraguild predation (IGP) • Intraguild predator and intraguild prey are competitors • For IGP to be stable, intraguild prey must be better competitors for the shared resource than intraguild predators • otherwise intraguild prey must have access to resources unavailable to intraguild predators • high productivity favors intraguild predators • low productivity favors intraguild prey

  11. Intraguild predation (IGP) Resource + Intraguild prey + Intraguild predator Resource + Intraguild prey Resource + Intraguild predator Resource Productivity (Carrying capacity for resource)

  12. Intraguild predation (IGP) • Diehl & Feissel 2001 • Tested this with: • Bacteria (=resource) • Tetrahymena (=intraguild prey) • Blepharisma(intraguild predator)

  13. Predator #1 Predator #2 + + - - Prey #1 Prey #2 - - Indirect effect Decrease predator #1  Increase Prey #1  Decrease Prey #2  Decrease Predator #2 INDIRECT PREDATOR MUTUALISM positive effects of one predator on another via competing prey

  14. Indirect effects • Possibilities are complex • Become more complex with more species • Two problems: • 1. How do you detect indirect effects? • 2. How important are indirect effects in determining community composition?

  15. Kinds of indirect effects • Up to this point – density mediated effects • direct interactions produce effects that in turn have effects on other species • other possibilities exist

  16. Kinds of indirect effects • Chains of interactions • effects of one species’ population propagate through chains (or networks) of other direct interactions like competition and predation • also called “density mediated interactions” • Interaction modification • the presence of one species alters in some way the direct interaction of two other species • also called “trait mediated interactions”

  17. Density vs. Trait mediated interactions B B A C A C A C increase C, increases B, which indirectly decreases A the presence of B changes something about how A and C affect one another

  18. Examples of trait mediated interactions • Apocephalussp. • phorid fly • parasite of ants • Pheidolediversipilosa • host • Other ant species competing for food • presence of competitors improves Apocephalus ability to find and to parasitize P. diversipilosa • Presence of Apocephalusat food • reduces competitive ability of P. diversipilosa

  19. Detecting indirect effects • You must know something about the pairwise direct interactions within the community • You often must do experiments, typically species removals and additions • If you don’t know which pairwise interactions are present, indirect effects may be interpreted incorrectly even in an experiment

  20. Predator #2 + - Predator #1 + - - Prey Competitor - Misinterpreting an indirect effect in an experiment • Remove predator #2 • Predator #1 increases • Prey decreases • Competitor increases • If you don’t know the interactions, it looks like Predator #2 might prey on Competitor

  21. The importance of indirect effects • Commonly assumed that • direct effects are strong • indirect effects are weak • Relative to any single direct effect, indirect effects may be stronger, more important determinants of species composition and diversity • Data? (Wootton 1994)

  22. + Sea star Leptasterias - Birds (crows, gulls) - + + + - + Predatory snail Nucella + - - + + - Goose N. Barn. Pollicipes - - - - - - Mussel Mytilus Acorn Barnacle Semibalanus - - Intertidal invertebrates (again)

  23. Interactions in intertidal • Observation: Exclude bird predation (cages) • Nucella: decreases relative to control (2 - 4 X) • Pollicipes: increasesrelative to control (~5 X) • Semibalanus: decreases relative to control (3 - 7 X) • Mytilus: decreases relative to control (to 70%) • Excluding predator: • 2 prey species decrease • 1 non-prey species decreases • 1 prey species increases

  24. Understanding this effect • A hypothesis to explain this result • Which direct interactions are strong? • affect numbers of individuals • Which direct interactions are weak? • do not affect numbers of individuals

  25. + Sea star Leptasterias - Birds (crows, gulls) - + + + - + Predatory snail Nucella + - - + + - Goose N. Barn. Pollicipes - - - - - - Mussel Mytilus Acorn Barnacle Semibalanus - - Hypothesis #1: strong & weak interactions

  26. Hypothesis #2: strong & weak interactions + Sea star Leptasterias - Birds (crows, gulls) - + + + - + Predatory snail Nucella + - - + + - Goose N. Barn. Pollicipes - - - - - - Mussel Mytilus Acorn Barnacle Semibalanus - -

  27. + Sea star Leptasterias - Birds (crows, gulls) - + + + - + Predatory snail Nucella + - - + + - Goose N. Barn. Pollicipes - - - - - - Mussel Mytilus Acorn Barnacle Semibalanus - - Hypothesis #3: strong & weak interactions

  28. Hypotheses  new predictions • Remove Pollicipes with birds excluded • H #1: Mytilus, Semibalanus, Nucella all increase • H #2: Mytilus, Semibalanus increase • H #3: Mytilus only increases • vs. birds excluded only

  29. Hypotheses  new predictions • Exclude birds after removing Pollicipes • H #1: no effects • H #2: Nucella decreases, Leptasterias increases • H #3: Semibalanus, Nucella decrease, Leptasterias increase • vs. removing Pollicipes only

  30. Seastar Leptasterias Birds EXCLUDED + + - Predatory snail Nucella - + + - Goose N. Barn. Pollicipes - - - - - Mussel Mytilus Acorn Barnacle Semibalanus - - Experiment 1Manipulate Pollicipes without birds

  31. Seastar Leptasterias Birds (crows, gulls) + - + + - - Predatory snail Nucella + + REMOVE Pollicipes - - - Mussel Mytilus Acorn Barnacle Semibalanus - - Experiment 2.Manipulate birds without Pollicipes

  32. Results of experiment 1 • Remove Pollicipes in cages that exclude birds • Mytilusincreases (2 X) • Semibalanusincreases (7 X) • Nucellaincreases (3.6 x) • compared to cages with Pollicipes • As predicted by hypothesis #1 • Inconsistent with hypotheses #2 & #3

  33. Results of experiment 2 • Exclude birds (cages) after removing Pollicipes • Mytilusunaffected • Semibalanusunaffected • Nucellaunaffected • compared to no exclusion of birds after removing Pollicipes • As predicted by hypothesis #1 • Inconsistent with hypotheses #2 & #3

  34. More... • Experiment 3. Removal of Nucella • no effects on Pollicipes, Semibalanus, Mytilus • As predicted by hypothesis #1 • Inconsistent with hypotheses #2 & #3 • Experiment 4. Removal of Semibalanus • Nucella decreases • As predicted by hypothesis #1 • Inconsistent with hypotheses #2 & #3

  35. Path analysis • Statistical technique for estimating direct and indirect effects among observational variables • Analysis predicts important direct paths are: • birds  Pollicipes • Pollicipes  Mytilus, Semibalanus, Nucella • Semibalanus  Nucella • Mytilus  Semibalanus • Most similar to Hypothesis #1

  36. Overall... • Experiment, alterntive hypotheses, new predictions, new experiments • Sophisticated experiments to test indirect effects • Statistical technique combined with experiments • Hypothesis #1 clearly supported • Indirect effects of primaryimportance in this system

  37. Predator - + Herbivore - + Plant Trophic cascades • Hairston, Smith, Slobodkin, 1960. Am. Nat. • Green earth argument • predators limit herbivorous prey and so enhance production & populations of plants • Examples: Morin pp. 214-221

  38. Trophic cascades • May involve more than trophic interactions • May cross ecosystem bondaries • Ecosystem engineers: species affect others, but the interaction has no effect on their own fitness or population growth • Large herbivores • Burrowing species • Fire-prone species • Trophic cascades can work through ecosystem engineers

  39. Foxes on Aelutian IslandsCroll et al. 2005 • Beginning 1900 • Foxes introduced • Absent on some • Effects • Reduced bird density • Vegetation change • Change in nutrient import

  40. Resource subsidy from marine system defecating hunting N, P

  41. Without Foxes Large nesting bird populations Lots of guano input N, P high soil P More grass, less shrub Greater grass biomass With Foxes Bird populations reduced (100x) Reduced guano input low soil P (60x) Less grass (3x), more shrub (10x) Less grass biomass (3x) Effects of foxes as predators

  42. significance • Importance of subsidies from one ecosystem to another • Importance of predation, even predation several trophic levels removed • trophic cascade • Trophic cascades can include nontrophic interaction. • Birds impact via ecosystem engineering, not feeding • This type of effect rarely demonstrated

  43. Trophic cascades across system boundaries(Knight et al. 2005) • Species with complex life cycles • Aquatic larvae – terrestrial adults • Amphibians, Odonates, Mosquitoes, many insects • How do predators in one environment (aquatic) affect trophic systems in the other (terrestrial)?

  44. Fish predation • Dominant factor in freshwater systems • Influences abundances of many invertebrates

  45. Knight et al. • Eight ponds • 4 with fish (Sunfish) • 4 without fish • Not experimental • Dragonflies • Abundances significantly lower in and around fish ponds vs. no fish ponds. • Particularly for medium and large dragonflies

  46. Plants and pollinators • St. John’s Wort • More pollinators near fish ponds • More Diptera, Lepidoptera, & especially Hymenoptera

  47. Knight et al. • Fish • Reduce dragonflies • Increases pollinators • Does this matter to the plants? • Does reduced pollinator density near fishless ponds reduce plant reproductive success?

  48. Knight et al. • Pollen supplementation • St. John’s wort • Supplemental pollen increases seed set near both fish and fishless ponds • Magnitude of increase ~3X greater near fishless ponds (where pollinators are reduced) • Similar for Sagittaria as well

  49. Effects on pollinators • Data suggest that effects of dragonflies on pollinators is both density mediated and trait mediated • Pollinators avoid behaviorally areas with lots of dragonflies

  50. Effects of fish • Solid – direct • Dashed - indirect

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