Herbivory and Mutualism– Chapter 14

How Can The World Be Green?• Some herbivorous populations may evolve self-

regulatory mechanisms that prevent them from destroying their food supply

• Predation may hold herbivore populations in check

• Plants have evolved an array of defenses against herbivores– Has led to a coevolutionary contest betweens plants

and herbivores in evolutionary time

Plant Defense

• Plants may discourage herbivores by structural mechanisms– Rose thorns

• Some plants produce secondary plant substance’s for defense– Not found in all plants– Are by-products of the primary metabolic pathways in

plants

Secondary Plant Substance Evolution

1) Compounds are primarily waste products of metabolism– Adaptation to avoid autointoxication– Plant excretion is different than animal

excretion– Plants can volatize, leach, or chemically alter

toxic chemicals

• Two possible benefits– Releasing chemicals into the environment

may have an allelopathic effect– Plant may become toxic or distasteful to

herbivores

Secondary Plant Substance Evolution

2) Chemicals were specifically evolved by plants to deter herbivores– Assumes that few secondary plant

substances are really useful to the plant as excretory products, and that most are actively produced at a metabolic cost to the plant.

– Plants that produce secondary plant substances are at an advantage

– If all herbivores were removed, plants would not produce secondary chemicals

-The plant defense theory argues that herbivores have a strong effect on plant fitness.

-Then well-defended plants are fitter

-If plant defense characteristics are inherited, then all the elements needed for natural selection are present!

If plant defense has a cost in terms of fitness then:

• Plants evolve more defenses if they are exposed to much damage, and fewer defenses if the cost of defense is too high.

• Plants allocate more defense to valuable tissues that are at risk.

• Defense mechanisms are reduced when enemies are absent, and increase when plants are attacked.

• Defense mechanisms are costly (energy and nutrients are diverted from other needs) and cannot be maintained if plants are severely stressed by environmental factors.

Plant Defense And Life History

• Long-lived plants are more likely than short-lived plants to be found by herbivores.– Long-lived (easy to locate) plants evolve a different

chemical defense than short-lived plants (not easy to locate).

• Quantitative defense – the amount of chemical in the plant can vary.– Tannins and resins in leaves may make up to 60% of the

dry weight of a leaf

• Qualitative defense – the chemical is either present (usually in low quantities) or absent.– Poisons that protect plants from generalized herbivores.

Fast-growers vs. Slow-growersVariable Fast-growers Slow-growers

Growth Characteristics:

Maximum Growth Rates High Low

Maximum Synthetic Rate High Low

Dark Respiration Rates High Low

Leaf Protein Content High Low

Responses to Pulses in Resources Flexible Inflexible

Leaf Lifetimes Short Long

Successional status Often Early Often Late

Antiherbivore Characteristics:

Expected Rates of Herbivory High Low

Amount of Defense Metabolites Low High

Type of Defense Qualitative Quantitative

Turnover of Defense High Low

Flexibility of Defense Expression More Flexible Less Flexible

• Herbivores tend to prefer fast growing plants– Slow-growing plants tend to lose more

investment/leaf, so they invest a lot in chemical defense (resource availability hypothesis)

Arrows indicate levels of defense that maximize growth rate.

• Inducible defense – plants can avoid the cost of producing defense chemicals when they are not needed

– Once a plant is attacked, it will activate its defenses.

• Not a heavily studied area, but if induced responses are occurring then:

1) Do the induced changes affect herbivore foraging or herbivore distributions?

2) Do plants suffer less damage and have greater fitness as a result of induced changes in leaf chemistry?

• Herbivores can either detoxify plant chemicals or alter their life cycle to avoid the noxious chemicals of the plants

Tannins in Oak Trees• Oaks are eaten by winter moth larvae early in

spring, but not in summer.– Leaves become less suitable as they age?

Diet Mean Peak Wt

May 16: ‘young leaves’ 45

May 28 – June 8: ‘old leaves’ 18No adults emerged from the ‘old leaves’ feed

Older leaves are tougher than younger leaves.

• If the larvae are fed ground up leaves, then age doesn’t seem to matter

• Why not evolve mouthparts (Some have)

• Tannins increase and proteins decrease in summer – so less nutrition

• Those that feed on oak leaves late in summer usually have low growth rates

Diet (ground up leaves) Mean Peak Wt

May 16: ‘young leaves’ 37

May 28 – June 8: ‘old leaves’ 35

Spines and Thorns• Actually little evidence that spines and

thorns are used for defense• Resource availability hypothesis – desert

plants

Herbivore Response to Plants

• Irruption – The increase and subsequent decrease of herbivore (usually ungulates) populations.– Has been observed in natural and introduced

populations

– Caused by overpopulation / over grazing

Introduced reindeer populations on a subantarctic island.

Four Stages of Irruption

1) Population Increases and food resources are reduced.

2) Population exceeds carrying capacity, food plants are over utilized and damaged.

3) Population collapses due to food shortage (often correlated with severe weather).

4) Population either collapses to near extinction or stabilizes at a lower population level.

Noninteractive Grazing• European finches feed on seeds of trees and

herbs and do not affect the subsequent production of their food plants.

• Controls operate in one direction:– Food Plant Production Herbivore Density

• One group feeds on herb seeds and their populations are stable.

• Another group feeds on tree seeds and their population fluctuates.– Herbs tend to produce the same amount of seeds each

year, trees do not.– Populations are strongly influenced by year to year seed

production

• Finches that depend on tree seeds can experience great irruptions in population density.

Seed Dispersal• Plants benefit from herbivores via seed

dispersal.• Coevolution between plants and their seed

predators may help to explain some features of plant reproductive biology and animal feeding behavior.– Coevolution is most clear in the evolution of

fruits– Plants advertise their fruits by a ripening

process in which the fruit change color, taste, and odor.