Discuss Suggestion That Herbivores Have Significant Impact On Plants (Hartley And Jones 1997)

Front 1

gyr and pace 1993

Back 1

Impact increases with productivity (Gyr and Pace 1993). Higher NPP supports higher trophic level, natural enemies supress herbivore populations and there is compensatory growth.

Front 2

ltd impact on biomass

Back 2

80% from aquatic algae, 18% terrestrial plants, 30% aqutic macrophytes.

Front 3

compensatory growth mechanisms

Back 3

o Plant compensatory growth mechanisms: increased C fixation/ improved light/ water/ nutrient availability for remaining tissue; production of new shoots from dormant buds; inaccessible reserves; delayed senescence/ or rejuvenation of leaves; altered source/ sink relations- resources>>roots>>leaves. Reduces impact

Front 4

some have substantial impact

Back 4

Salvinia molesta and Cyrtobagous salviniae. Beetle destroys part of plant, plant compensates, new plant parts have higher N concentration, enhances beetle reproductive capacity.

Front 5

impacts not sufficient for weed control

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Thomas and Reid 2007- 76% cases no effect on weed abundance. Control of optunia vulgaris by dactylopius ceylavicus- 1st successful use of insect to control noxious plant

Front 6

Marone and crone 2006

Back 6

Mixed picture
Depends on how you measure performance
Type of herbivore- invertebrates vs mammals- no consistent difference
Seed and flower consumers major impact.
Plant functional group- no consistent difference.
Environmental and habitat gradients
•Greater effect on grassland forbs
•Greater impact in disturbed habitats.

Front 7

is herbivory unimportant

Back 7

Biomass removal small
Biomass of weeds rarely altered
Herbivory more than biomass removal- other impacts- plant pops; plant communities; ecosystem effects

Front 8

Plant populations

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 Asymmetry in interactions
 Relatively few examples foliar insects
 Ragwort-cinnabar moth system= typical: insect dynamics are food ltd but plant dynamics aren’t herbivore driven
 Context dependent: Bonsall et al 2003- coastal dunes in Netherlands, dynamics cyclic. Interaction involves direct and delayed density dependence. Difference caused by differences in the importance of seed limitation in plant recruitment.
 Grasslands SE England- insect little impact on plant dynamics, no time lags in density dependence.
 Seed predators can impact more markedly- kangaroo rats and desert annuals- selective predation of large seeded spp by Kangaroo Rats= keystone spp. Maintained diversity of winter annuals.

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o Plant communities

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 Herbivory affects plant spp composition; spp richness; through changes in competitive relationship btw plant spp.
 System 1: chalk grassland- high spp richness-grazing, low nutrient status of soils. Plant spp richness increase if herbivores feed selectively on dominant/ competitive/ fast-growing spp. High spp richness maintained through selective grazing on competitive grasses
 System 2- heather moorland- overgrazing by sheep- reduces diversity, reduces heather cover, increases mat grass. Plant spp richness decreases if herbivores fed selectively on subordinate/ uncompetitive spp. British uplands dominated by mat grass and loss of heather due to overgrazing.

Front 10

o Plant spp composition and diversity

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 Herbivory changes competitive relationships btw plant spp via: selective herbivory; differential palatabilities
 4 possible outcomes: herbivore grazes
• Preferentially on
o Dominant spp
o Subordinate spp
• Neutrally
• Switching- whichever is the most abundant
 Model: herbivory increases plant spp diversity if herbivores feed selectively on dominant/ common spp, preventing competitive exclusion. Key criterion: frequency dependent herbivory- preferred when spp common, switching, herbivory reduced when spp rare.
 Effects vary with habitat, plant spp identity, herbivore identity
 If competitiveness and palatability are correlated (growth defence hypothesis)
 Unimodal response of diversity to grazing intensity
 Connell’s 1978 intermediate disturbance hypothesis

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o In relation to island biogeography theory

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 Local colonisation increases: propagules dispersal; disturbance-gaps for germination
 Local extinction decreases: selective herbivory on competitively dominant spp; palatable spp; tall spp; competitive interaction relaxed so competition decreases; spatial heterogeneity created

Front 12

Bakker et al 2006

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assemblages incl large herbivores increased plant diversity at higher productivity but decreased it at low productivity. small herbivores didn't have consistent effects along productivity gradient

Front 13

Three themes

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the world isnt always green. all that is green isn edible. what is edible isnt necessarily of sufficiently high quality to allow increase of herbivore pop.

Front 14

flower and bud feeding

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coniferous trees- 40% female flower loss btw may and august attributed to insect feeding, 22% of surviving fruit crop showing signs of feeding damage.

Front 15

shrub Gutierrezia microephala and grasshopper Hesperotettix viridis

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80% bushes heavily attacked, failed to produce any flowers at all

Front 16

indirect effects on pollination

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reduced pollination may have little impact on plant pop dynamics- may restrict pollen choice of female parent- hence genetic makeup of the progeny

Front 17

seeds

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leads to reduced seed size, with potential effects on seedling competitiveness. may also influence evolution of seed size