Herbivory review. Defense theory has emphasized probability of being eaten and cost of herbivory.

Transcription

1 Herbivory review Defense theory has emphasized probability of being eaten and cost of herbivory. Investment in defense invokes a trade-off between growth and mortality. Herbivores can therefore influence species distributions across resource gradients. Most herbivore damage occurs on young leaves. Plants development can limit exposure of young leaves to herbivores or limit resource loss. Plants may also engage a third trophic level as a defense against herbivores.

2 Growth and defense characters of tropical trees (from Coley 1983 and subsequent work)

3 Delayed greening Young leaves are white or pink and do no net photosynthesis Only observe delayed greening in tropical forest understories, but is a common trait across evolutionary lineages

4 Rapid leaf expansion Develop whole leaves (or branches in a few days) Brownea claviceps

5 Herbivory and the third trophic level Inviting friends to feast on foe Many ways that plant harness the third trophic level to defend themselves: - Acacia, Cecropia, Macaranga ant mutualisms

6 Sunshine Van Bael (2003) Birds defend trees from herbivores in tropical forest canopies Enclosed branches in mesh to exclude birds/mammals but not insects. Then measured herbivory rates

7 Open bars are for exclosure treatment Hatched bars are for bird-accessible treatment Three tree species Cecropia spp + Anacardium Why no effect in the understory?

8 Quantitative defenses slow down insect feeding and/or digestion rates Quantitative defenses (tannins, fiber and toughness) do not present an absolute barrier against herbivores. Hypothesis: Defensive effectiveness depends on making herbivores more susceptible to predation Slowing herbivore growth rates lengthens the time that larvae are exposed to predators and parasitoids ( slow-growth-highmortality SG-HM hypothesis) Slowing grazing rates is important because most damage occurs in the last instars of insect development

9 Evidence for SG-HM: Benrey and Denno (1997) - Several studies using free-living larvae show higher incidence of mortality from parasitoids for slow vs fast developing larvae. - Not supported in cases where larvae are protected (building shelters out of plant material or inside galls) instar Fast developing larvae are better able to defend themselves against parasitoids or have a lower probability of being found

10 Trophic cascades Alosa (planktivorous fish) selects small zooplankton?? phytoplankton growth?? Alosa selects for smaller-sized zooplankton species

11 Do predation effects cascade down through a food chain? Two viewpoints: Errington s hypothesis (1946) Predators have little impact on total prey abundance because Many species are highly territorial (eg blackbirds, oystercatchers), removing birds does not affect growth rate.

12 Support for Errington s hypothesis: Allan (1982): Studied effect of trout removal on stream invertebrate abundance - Reduced trout abundance to 10 % pre-experiment level by electroshock fishing a 1.2 km stretch of Colorado stream - Stomach contents analysis of fish showed that they fed intensively on a few taxa of invertebrates but that populations of these taxa showed no significant change in abundance in response to trout removal Why no effect in this study?

13 Alternative Hairston, Smith and Slobodkin (1960) HSS Hypothesis or Why the world is green Why don t herbivores become so abundant that they consume all the plants? (world is brown) - Divide terrestrial communities into 4 compartments: primary producers, herbivores, carnivores, and detritivores (decomposers)

14 Trophic levels 3 Carnivores No predators, must be food limited (competition) 2 Herbivores Lots of food?? Must be predation Limited? 1 Producers Resource limited (light, water nutrients). (competition)? Detritivores Food limited? Detritus doesn t accumulate?

15 Why does HSS represent a trophic cascade? Because the third trophic level (carnivores) exerts a significant influence on the abundance of organisms in the first level (producers) by limiting consumption of producers by the second level Also known as top-down regulation Arguments against HSS Availability of plants to herbivores may be illusory if most plants are unsuitable for food (too well defended). Effects of carnivores may not cascade directly (Polis and Strong 1996)

16 2 trophic level systems There are no carnivores to regulate herbivore populations, therefore competition should regulate herbivores and producer populations could be driven down to very low levels Example: Gypsy moth (Lymantria dispar) - introduced from Europe to USA during the Civil war to start a silk industry. Moth lacks natural enemies in introduced range. Outbreaks of moths can completely defoliate their hosts (oak and aspen trees). Sustained defoliation over 4 yr will kill the host.

17 4 trophic level system: occurs when a second carnivore level feeds on the third trophic level. Limited to productive habitats where there is sufficient energy flow to support secondary carnivores.

18 Bottom-up versus top-down! Bottom-up! Top-down! What would happen if you add another level?!

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20 Hairston and Hairston (1993) Some of the differences between terrestrial communities (green) and aquatic communities (blue) might be because freshwater ecosystems tend to have four trophic levels. Terrestrial food chain (mostly): Producers -- herbivores -- carnivores Aquatic food chain (mostly): Producers -- grazing zooplankton -- planktivorous fish (e.g., Alosa) -- piscivores

21 Carpenter et al. (1985) Cascading trophic interactions and lake ecosystems Observation: Lakes that have very similar supply rates of limiting nutrients (P) have up to a thousand fold difference in productivity Carpenter provides a framework linking demography of piscivores (eg pike and salmon) in lakes to the abundance of plankton and primary productivity: How might the effects of high abundance of a top-predator fish cascade through a community?

22 Lake systems often have four trophic levels Phytoplankton - Zooplankton - planktivores - piscivores (little fish) (big fish) Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity Lake Manipulation Phytoplankton response Lillestockelidsvatten Planktivores removed Primary production reduced by 92 % Michigan Tuesday Stocked with piscivores reduced Planktivores to 20 % initial Piscivores stocked to eliminate Planktivorous fish Summer chlorophyll reduced to 20 % of initial level Chlorophyll reduced to 20 % of Initial level Round 70 % of planktivores removed Chlorophyll reduced by 50 % St George 50 % removal of planktivores No change detected

23 Lake systems often have four trophic levels Phytoplankton - Zooplankton - planktivores - piscivores (little fish) (big fish) Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity Lake Manipulation Phytoplankton response Lillestockelidsvatten Planktivores removed Primary production reduced by 92 % Michigan Tuesday Stocked with piscivores reduced Planktivores to 20 % initial Piscivores stocked to eliminate Planktivorous fish Summer chlorophyll reduced to 20 % of initial level Chlorophyll reduced to 20 % of Initial level Round 70 % of planktivores removed Chlorophyll reduced by 50 % St George 50 % removal of planktivores No change detected

24 Solid line : response to increased recruitment of large fish. Note: young piscivorous fish start off as vertebrate planktivores when they are small Dashed line: response to a reduction in piscivorous fish biomass - eg exploitation or high over-winter mortality

25 Wootton and Power (1993) Opposite approach to Carpenter to manipulate bottom-up indirect effects: Manipulated amount of light available to algae by differentially shading small portions of a natural stream Varying light availability either increased or decreased the primary productivity and biomass of algae Predictions: - If the consumption rate of predators is a simple function of prey density (αv) then community structure should be determined by predators at the top of the chain and by producers at the bottom of a 3 level chain.

26 - In a 4 trophic level chain, increasing productivity should increase the biomass of secondary carnivores, but not primary carnivores, allowing biomass of herbivores to increase Results: Productivity did respond to light (but lots of variance)

27 In 3 trophic level chains predator and algal biomass did respond to light; grazer biomass was not affected:

28 3 versus 4 trophic levels showed expected effects...

29 The case of the cownose ray Myers et al. (2007) Cascading effects of loss of predatory shark

30 Upsurge in shark fishing in last 20 yrs (bycatch, meat and finning) has cleaned the oceans of large >2 m shark Loss of top predator has weakened top down control of shark prey: These are elasmobranchs (rays, skates, small sharks). Elasmobranchs are large - even as juveniles. Their only predators are large sharks Meta-analysis of time series data - surveys of shark and elasmobranch populations

31 Shark populations have plummeted in 30 years Populations have declined by 87 % (sand bar shark) to >98 % (the rest) Sharks are now functionally extinct Concurrently, mesopredators (prey of great sharks) have increased:

32 Most elasmobranch predtors have increased in population size by and order of magnitude over the last 30 years Cownose ray: Range SE Florida to New Jersey Population size estimate: 40 million!! (rays and sharks notorious for low fecundity and late maturity, therefore not a population spike )

33 What are all these rays eating? Bay scallops Oysters Soft-shell clams If rays consume 200 g of shell fish per day, and spend 100 days/yr in Chesapeake, then the population will consume 840,000 tonnes/yr Commercial scallop catch Virginia and Maryland in 2003: 300 tonnes

34 Ray exclosure experiments show that rays cause 100 % scallop extinction Commercial fishery is now closed