Plant Ecology - Chapter 11 Herbivory & Pathogens

Plant Ecology - Chapter 11

Herbivory & Pathogens

Herbivory

The consumption of all or part of a living plantA predator when it kills and eats an individual

Herbivory

Granivores - eat seeds or grainsGrazers - eat grasses, low-growing plantsBrowsers - eat leaves from trees, shrubsFrugivores - eat fruits

Herbivory

How much do they eat?Estimate - 10% of leaves of forest trees eaten each yearLeast in temperate forest, most in dry tropical forest

Is herbivory good for a plant?

Reduce self-shadingRemove leaves in excess of optimum LAIReduce respiratory “drag” on plant

Herbivory can cause death

Girdling (ring-barking) of young trees by rabbits, squirrels, and rodents


Introduction of disease into plant by grazer

Dutch elm diseaseFungus carried by elm bark beetleClogs “circulatory” system of American elm trees


Grazing on one species may be sufficient to sway competitive interaction in favor of another species


Large populations of fluid-suckers (e.g., aphids) can virtually stop growth and/or kill a plant

Herbivory can affect survival

Repeated defoliation often required to kill mature plantLarge proportion of seedlings killed by single “attack”

But some seedling plants have high tolerance - e.g., 75% survival after 5 defoliations

Herbivory can affect growth

Effects range from none to total cessation of growthDepends on:

Timing of defoliationType of plant involved (grasses most tolerant because of basal meristem rather than apical meristem)

Herbivory can affect fecundity

Grazed plants tend to be smaller and bear fewer seedsHerbivory can delay flowering (move it into inhospitable season), reduce, or totally inhibit floweringSome eat flowers, fruits, and seeds and reduce fecundity

Good herbivores

Some pollen-eaters help pollinateSome fruit-eaters help distribute seedsSome seed-eaters store seeds in ground and forget them

Mutualistic relationships

Compensation for herbivory

Temporarily mobilize stored carbohydrates until regrowth returns photosynthesis to normal


Reroute photosynthetic products to damaged areas to enhance regrowth

To roots, or shoot, or leaves


Increase rate of photosynthesis in remaining leaf surface area


Stimulate dormant buds to grow, or reduce death rate among surviving parts

Despite all these possible mechanisms, compensation is rarely perfect, so plants are harmed in the long-term


Defensive responses to grazers

Grow bigger, sharper spines


Produce more or new defensive chemicals


Reduce palatabilityTougherMore fiberLower nitrogen content

Effect of grazing on whole population of plants

Do they only prey on the weak?Reduction in intraspecific competition

Can reduce high LAI to more optimal levels and improve plant productivityTypically only works in high-density populations; little or no compensation in low-density populations


Controversial and unresolvedTwo explanations on why herbivores are NOT important regulators of plant populationsTop-downBottom-up


Top-down - herbivores usually at such low densities because of their predators, cannot have negative effects on entire plant population


Bottom-up - plant populations are limited by abiotic factors (light, water, nutrients), not by herbivores


On the other hand, there are some various obvious examples of population control by herbivores - e.g., gypsy moths and oaks


Another example - bark beetles and conifers - widespread mortality in N. Amer.

Effect of grazing on plant distribution

Eating can limit distribution in some areas, or rodent/squirrel caches can enhance abundance

Biological control

Moth introduced into Australia to kill invasive prickly pear cactus - good there, but problems elsewhere

Biological control

Beetles introduced to control purple loosestrife

Herbivory & communities

Vertebrate, invertebrate grazers can have dramatic effects on plant communities


E.g., rabbits and grasslands of southern England


Native and introduced grazers can have significant profound effects

Herbivory & communitiesLarge herbivores in Yellowstone

Parasitic Plants

Obligate parasitic plants - obtain energy, nutrients, water from host plantE.g., mistletoes

Parasitic Plants

Hemiparasites - independent and photosynthetic, or parasite on other plants (e.g., roots)

Plant Pathogens

Fungi, water molds, bacteria, viruses cause diseases in plantsIndividual, population, and community effects

Plant Pathogens

Soybean rust - fungus from Asia, infects leavesSurvives only on green tissue (eliminated each fall here, but kudzu in south is infested)

Plant Pathogens

Citrus canker - bacterium causes premature leaf, fruit drop

Plant Pathogens

Smuts - affect flowers, are caused by fungiSexually transmitted

Plant Pathogens

Chestnut blight - fungal canker disease, kills cambium under barkAmerican chestnut formerly dominated plant communities

Plant Pathogens - people

Irish potato famine resulted from potato blight caused by water moldDestroyed Irish potato crop in 1840s


1 million people died from famine and disease1 million emigrated to U.S., Canada (especially New York, Boston)


Population of Ireland has not recoveredRemnants of former potato farms remain today

Plant defense against pathogens

Phytoalexins - secondary chemicals produced at site of infection to kill microbesPhloem plugging - phloem clogs in response to damage, prevents spread of infection through vascular systemLocalized tissue death - barrier to infection

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Plant Ecology - Chapter 11 Herbivory & Pathogens