Plant Ecology - Chapter 11 Herbivory & Pathogens

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

Herbivory Granivores - eat seeds or grains Grazers - eat grasses, low-growing plants Browsers - eat leaves from trees, shrubs Frugivores - eat fruits

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

Is herbivory good for a plant? Reduce self-shading Remove leaves in excess of optimum LAI Reduce respiratory “drag” on plant

Herbivory can cause death Girdling (ring-barking) of young trees by rabbits, squirrels, and rodents

Herbivory can cause death Introduction of disease into plant by grazer Dutch elm disease Fungus carried by elm bark beetle Clogs “circulatory” system of American elm trees

Herbivory can cause death Grazing on one species may be sufficient to sway competitive interaction in favor of another species

Herbivory can cause death 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 plant Large 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 growth Depends on: Timing of defoliation Type 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 seeds Herbivory can delay flowering (move it into inhospitable season), reduce, or totally inhibit flowering Some eat flowers, fruits, and seeds and reduce fecundity

Good herbivores Some pollen-eaters help pollinate Some fruit-eaters help distribute seeds Some seed-eaters store seeds in ground and forget them Mutualistic relationships

Compensation for herbivory Temporarily mobilize stored carbohydrates until regrowth returns photosynthesis to normal

Compensation for herbivory Reroute photosynthetic products to damaged areas to enhance regrowth To roots, or shoot, or leaves

Compensation for herbivory Increase rate of photosynthesis in remaining leaf surface area

Compensation for herbivory 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

Compensation for herbivory

Defensive responses to grazers Grow bigger, sharper spines

Defensive responses to grazers Produce more or new defensive chemicals

Defensive responses to grazers Reduce palatability Tougher More fiber Lower 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 productivity Typically only works in high-density populations; little or no compensation in low-density populations

Effect of grazing on whole population of plants Controversial and unresolved Two explanations on why herbivores are NOT important regulators of plant populations Top-down Bottom-up

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

Effect of grazing on whole population of plants Bottom-up - plant populations are limited by abiotic factors (light, water, nutrients), not by herbivores

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

Effect of grazing on whole population of plants 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

Herbivory & communities E.g., rabbits and grasslands of southern England

Herbivory & communities Native and introduced grazers can have significant profound effects

Herbivory & communities Large herbivores in Yellowstone

Parasitic Plants Obligate parasitic plants - obtain energy, nutrients, water from host plant E.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 plants Individual, population, and community effects

Plant Pathogens Soybean rust - fungus from Asia, infects leaves Survives 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 fungi Sexually transmitted

Plant Pathogens Chestnut blight - fungal canker disease, kills cambium under bark American chestnut formerly dominated plant communities

Plant Pathogens - people Irish potato famine resulted from potato blight caused by water mold Destroyed Irish potato crop in 1840s

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

Plant Pathogens - people Population of Ireland has not recovered Remnants of former potato farms remain today

Plant defense against pathogens Phytoalexins - secondary chemicals produced at site of infection to kill microbes Phloem plugging - phloem clogs in response to damage, prevents spread of infection through vascular system Localized tissue death - barrier to infection