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Multitrophic interactions. Interactions across trophic levels, or involving more than two “players”. Multitrophic interactions.

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Multitrophic interactions

  • Interactions across trophic levels, or involving more than two “players”

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Multitrophic interactions

  • Interactions in real communities include all interactions discussed so far (competition, herbivory, predation, pollination) plus others (decomposers, mutualistic and antagonistic fungi, predators and parasitoids)

  • How does the outcome of one interaction affect the outcome of another?

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Why the world is green

  • Predators, parasites, and diseases keep herbivores rare (top-down regulation). Hairston et al. 1960




Hairston, Smith, Slobodkin. 1960. Am Nat 44: 421-425

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Why the world is green, part 2

  • Plants are fundamentally poor food (bottom-up regulation) Southwood 1973.

  • Herbivores are “between the devil [predators] and the deep blue sea [inedible food]” Lawton and McNeill 1979.

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Playing Chutes and Ladders

  • Top-down and bottom-up both important

Hunter and Price 1992. Ecology 73: 724-732

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Trophic cascades do not include all interactions

De Deyn and Van Der Putten, 2005. TREE 20 (11) 625-633

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  • Reintroduction of wolves to Yellowstone leads to aspen recovery—through both predation and fear

  • Recovery primarily in riparian areas with downed logs (predation risk higher)

Ripple and Beschta 2007. Biological Conservation 138:514-519

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  • Fish eat dragonfly larvae; dragonflies eat pollinators like bees and flies

Knight et al. 2005. Nature 437:880-883

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  • Fewer dragonfly larvae in and adults near pods with fish (for three size categories)

Knight et al. 2005. Nature 437:880-883

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  • Visit rate higher near ponds with fish; plants near these ponds had lower pollen limitation

Knight et al. 2005. Nature 437:880-883

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  • Scale insects perform differently on different citrus cultivars; performance of scale affects size and sex ratio of offspring of parasitoid wasp

Hare and Luck, 1991. Ecology. 72: 1576-1585

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  • A trait that defends against herbivores reduces time enemies spend hunting prey on Datura

Gassman and Hare, 2005. Oecologia 144:62-71

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  • Herbivore survival depends on both plant defenses and enemies

Gassman and Hare, 2005. Oecologia 144:62-71

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  • Dodder (Cuscuta) in salt marsh changed competitive relationships

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  • Herbivore damage may increase when competitors (that protect the herbivore) are present

J. Facelli, unpublished

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  • Flower size smaller in plants damaged manually or by herbivores

Strauss 1997. Ecology 78:1640-1645

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  • Pollinators discriminate based on flower size

Strauss and Conner 1996. Am Nat 147:1098-1107.

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  • Castilleja (indian paintbrush) is a hemiparasite

  • Experimentally grown on lupine hosts that either do or do not produce chemical defense (alkaloid)

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  • Alkaloids from host plant reduce herbivory, increase pollinator visits in Castilleja

% plants visited

Adler 2000. Am Nat 156:92-99

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  • Smaller flower size, reduced visitation in fireweed without AM fungi (compared to plants with AM fungi)

Wolfe et al 2005. Ecology Letters 8:218-223

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  • Suppression of mychorrizae in rough fescue grassland changed # flowering stems per plot in a species-specific manner

Cahill, Elle, Smith, Shore 2008. Ecology 89:1791-1801

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  • Three species (non-myc) increased when myc suppressed, others no significant change

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  • Myc suppression reduced per-stem visit rate… for species with no change in stem number

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  • Change in plant community led to change in pollinator community



sweat bees

Bumble bees

Cahill, Elle, Smith, Shore 2008. Ecology 89:1791-1801

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Whole community mutualism

  • Tide pools are low N areas, only slow-growing seaweeds usually grow there

  • Fast-growing Cladophora, when associated with invertebrates, can grow even in N-limited tide pools

    (ostracods, nematodes,

    polychaetes, etc)

Bracken et al 2007. Ecology 88(9):2211-2219

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Whole community mutualism

  • Cladophora adds structure to tide pools, allows for greater total biodiversity when present

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Ecosystem-level effects

  • Poplar species hybridize; grow along rivers

  • Genetic variation among parent species (Populus fremontii and P. angustifolia) and their hybrids in leaf tannin content

Many papers by Tom Whitham and others

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Ecosystem-level effects

  • Variation in tannins affects:

    • Population densities of endophytic fungi, aquatic invertebrates, herbivorous insects, insectivorous birds

    • Decomposition rate of leaves, N mineralization in the soil

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Ecosystem-level effects

  • Experimental planting of poplars:

    • Selective felling by beavers (avoid trees high in tannin)

    • Change in relative frequency of high-tannin trees: beavers select for more tannins!

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Multitrophic interactions

  • Some multitrophic effects can be predicted from pairwise interaction effects and trophic cascades

  • Thus far, no emerging theory; we are working towards integrating mutualism, competition into models of trophic cascades to allow for better prediction