Herbivory

Don White, Jr., Ph.D.

Herbivory

• Herbivory is the consumption of plants and plant parts by animals.

• About ½ the insect species in temperate regions are herbivorous.

• Many mammals and a few birds are also herbivorous.

Herbivory and parasitism differ from predation and parasitoidism

Predators and seed predators

Grazers and browsers

ParasitoidsParasites and arthropod herbivores

Dur

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ass

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Probability of death

Overall the estimated % of new plant material consumed is:

• least in forests (10%)• intermediate in grasslands (40-60%)• highest in aquatic habitats (>80%)

Why is consumption not higher? Plant defenses.

If not mortality, what effects do herbivores have on plants?

• Plant growth can be reduced.• Reproductive output may be reduced or

delayed.• Plant fitness is reduced, but to what extent?

The relationship between intensity of herbivory and plant fitness

Effe

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Fitn

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Death

Ungrazed plants

Intensity of herbivoryLow High

Fitness declines in direct proportion to the intensity of herbivory.

Plants can compensate for the damage done by herbivores—Model 1: Compensation

Effe

ct o

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Fitn

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0

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Death

Ungrazed plants

Intensity of herbivoryLow High

Fitness does not decline through lower levels of herbivory, but declines at higher levels = compensation.

Mechanisms of compensation may include:

• Increase in chlorophyll content.• Shift in root/shoot ratio. • Change from apical to lateral growth.

Plants can compensate for the damage done by herbivores—Model 2: Overcompensation

Effe

ct o

n pl

ant (

Fitn

ess)

0

-

+

Death

Ungrazed plants

Intensity of herbivoryLow High

Fitness increases through lower levels of herbivory before decreasing at higher levels = overcompensation.

The question (and controversy) of “overcompensation”

• Low level herbivory appears to enhance the fitness (i.e., growth rates and reproduction) of plants.

• Examples include:– Increased seed production in herbs browsed by mule

deer and elk.– Rapid recovery of grasslands after grazing by

wildebeest and zebras in the Serengeti.• Caution: other factors could explain these patterns and

evidence of higher fitness is not conclusive.

Grazers and browsers

• Grazers, such as bison and grasshoppers, eat herbaceous plants such as grass and forbs ("graze" and "grass" have the same Anglo-Saxon root).

• Browsers, such as deer and rabbits, eat the leaves and twigs of woody plants, and young shoots.

• The distinction between grazing and browsing is handy, but it just scratches the surface of the way animals make use of plants as food.

Specific types of herbivores

• Fruit, seed, nectar, and pollen eating.

• Leaf mining. • Boring.• Root eating.• Sap sucking.• Gall forming.

Plant defenses against herbivory

• Morphological/structural plant defenses.• Chemical plant defenses.

– Disruptive.– Protective/defense.

• Associational plant defenses.

Morphological plant defenses

• Spines.• Stinging hairs.• Thick seed coats.• Silica in grasses.• Heavy calcification of

certain algae.

Chemical plant defenses

• Disruptive chemical defense.– The plant may accumulate compounds that make tissues hard

to eat, difficult to digest, or unpalatable.– Example: tannins found in the vacuoles of oaks.

Hydrolyzable tannins inactivate digestive enzymes of herbivores.Condensed tannins defend against microbial or fungal attack.

Chemical plant defenses

• Protective/defensive chemical defenses.– The plant may produce chemicals that are strongly

aversive or even toxic.– Examples include:

Mustard oils (glucosinolates) are found in several plant families but most notably in the mustards.Alkaloids in the nightshades and tobacco (nicotine is an alkaloid).Cardiac glycosides (cardenolides) in the milkweeds and dogbanes.

The metabolic fate of pyrrolizidine alkaloids in mammals and lepidopterans

Diagram from Louw, G. 1993. Physiological animal ecology. Longman Group UK Limited, Essex, England. Page 150.

The pyrrolizidine alkaloids are found in Senecio, a genus of weeds found in England. The caterpillars of the tiger moth and the cinnabar moth complete their entire life cycle on these plants.

Chemical plant defenses

• In general, defensive chemicals tend to occur in plants of disturbed areas and early successional stages.

• Protective chemicals are generally referred to as secondary compounds, implying that they are not essential compounds of the plant's basic cellular metabolism but by-products instead.

• Many plants are now known to produce various animal hormones or mimics of them.

– Ecdysone (molting hormone) in many ferns.– Estrogen-like compounds in whitebark pine seeds.

Secondary plant compounds

Attract pollinatorsFlavonoidsMimic animal hormonesSteroids

Fungicide and insecticideTerpenes

Inhibit grow of competing plants

QuinonesBad taste; fungicidePhenolics

Causes cancer and nerve damage

Other N and S compoundsAffect nervous systemAlkaloidsSome rolesClass

Associational plant defenses

• Associational defense means the protection gained by a plant by living in association with another species.

• It may be the simple matter of unpalatable plants happening to form refuge zones for palatable one growing nearby.

Associational plant defenses

• The attraction of ants by certain species of plants, usually having extrafloral nectarines (i.e., nectar-producing organs somewhere on the plant other than the flowers).

Bullhorn acacia

Effects of herbivory on plant distribution and abundance

• The role that selective herbivory plays in determining the species composition of vegetation are substantial but poorly known.

• Many studies have focused on the effects of herbivory through the use of exclosures—plots from which various herbivores are excluded.

• In certain areas where deer are abundant, their browsing can alter the composition of the forest that regenerates after clearcutting or even prevent woody regeneration, maintaining herbaceous openings in the forest.

Exclosures

Boulder Creek, Utah Glen Canyon, Utah

The 3 major influences on the dynamics of an animal population:

• The dynamics of its limiting resource.• The functional response of an animal to the

level of the resource.• The numerical response, in terms of rate of

increase, to the level of the resource.

Definitions

• A resource is something an animal needs; by definition it is beneficial.

• A habitat comprises all the physical attributes of the environment that make an area habitable for a species.

A resource may be:

• Nonconsumptive• Consumptive

The use of a resource may be:

• Preemptive• Consumptive

The relationship between a resource and a population may be:

• Interactive• Reactive

The dynamics of the resource

Plant growth over 3 months as a function of plant biomass at the beginning of the 3 months, for pastures in Kinchega National Park, Australia (After Robertson 1987).

Rainfall amounts for the 3 month period.

The functional response of consumer to resource: the trend of intake per individual against the level of the resource.

Food intake per individual per day for kangaroos and rabbits at varying levels of food availability (After Short 1987).

Functional responses when expressed in terms of metabolic weight.

The numerical response of consumer to resource: the effect of the resource on the animals.

Rate of increase on a yearly basis of a population of red kangaroos at varying levels of food availability. (After Bayliss 1987).

Is this a case of using a resource preemptively or consumptively?