Predation Chapter 8

Predation Consumption of one organism (prey) by another (predator), in which the prey is alive when first attacked by the predator

Functional classification of predators True predators Grazers Parasites Parasitoids

True predators Kill prey immediately after attacking it, attack many

Grazers Attack many prey, but “kill” only part of each individual

Parasites Attack only one or few prey, “kill” only part of it

Parasitoids Attack only one prey, cause no immediate death, but eventually kills prey

Effects of predators on prey True predators and parasitoids kill prey Grazers and parasites do not kill prey Affect both individual prey as well as prey populations

Compensation for herbivory Reduce self-shading Remove leaves in excess of optimum LAI Reduce respiratory “drag” on plant

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

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”

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)

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

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

Effects of grazing on grazer Survival, growth, fecundity dependent on food availability Rises as availability increases over a certain range Must be some minimum availability to keep grazer alive (threshold) Above certain level, grazers become satiated and do not respond to increasing levels

Grazer response Food availability threshold satiation

Trees and grazer satiation All trees of one species within a region produce large crops of seeds at odd intervals - mast years Tied to climatic variables

Trees and grazer satiation Seed predators cannot respond fast enough reproductively, so many seeds survive and sprout Only a predator with short generation time could take advantage of mast years

Importance of food quality Large availability of food not helpful if it is all of poor quality Cannot eat enough to get the required nourishment

Predator behavior Food selection - monophagous to polyphagous Parasites and parasitoids tend to have the most specialized diets True predators generally have broad diets Grazers fit into all groups fairly equally (specialized to unspecialized)

Diet preferences Ranked preferences - foods have similar composition, but vary in size or accessibility Energy gained per unit handling time Balanced preferences - consume mixed diet to meet specific requirements Items appear in constant proportion in diet regardless of their proportional availability

Fixed preferences Specific food items preferred at all levels of food availability Preferred when it makes up majority of foods available Still preferred when it makes up small proportion of foods available

Switching preferences Some predators may switch preferences at different levels of availability Eat disproportionately more when common Ignored disproportionately when rare

Switching can occur when…. Different types of prey occur in different habitats and predators concentrate on most profitable ones Predator becomes more efficient/successful in dealing with more abundant food (learning) Predator develops specific search image for abundant foods

Diets and natural selection Natural selection can act to restrict diets Prey can exert pressures demanding specialized morphological or physiological responses from the predator Selection favors specialization as long as prey species remains abundant, accessible, predictable

Diets and natural selection Natural selection can broaden diets Diets will be broad if individual food items are inaccessible, unpredictable, or lacking in certain nutrients If diet is broad, food is easy to find, search costs are low, and fluctuations in abundance of one prey type are unlikely to cause starvation

Coevolution of predator-prey relations? Improvement in predator ability leads to improvement in prey’s ability to avoid/resist predator leads to improvement in predator ability leads to …. No real supportive evidence, but Asplanchna and Brachionus example

Coevolution of predator-prey relations? Asplanchna and Brachionus: rotifers in lakes

Optimal foraging theory Predict foraging strategy under specified conditions Predictions based on search time and handling time

Optimal foraging theory Predators with short handling times relative to search times should be generalists

Optimal foraging theory Predators with long handling times relative to search times should be specialists

Optimal foraging theory: “decision rules” 1) prefer the more profitable prey 2) feed more selectively when profitable prey are abundant 3) include less profitable prey in the diet when most profitable prey are relatively scarce 4) ignore unprofitable items regardless of their abundance

Problem with optimal foraging in nature? Predator avoidance by the predator Mutual interference reduces efficiency Partial refuges for prey in some habitats Ideal free distribution of predators - balance between attractive and repellent forces

Predator-prey Patches Time Cumulative energy extracted Slope = optimum energy extracted per time spent Search time to find patch Optimum time to spend in patch

Predator-prey Patches Time Cumulative energy extracted Differing productivities

Predator-prey Patches Time Cumulative energy extracted Differing search times

Predator-prey Cycles Prey numbers Predator numbers

Predator-prey Cycles Time Population size PreyPredator Time lag = 1/4 cycle

Predator-prey Cycles

Predator-prey Relations K Population size New individuals added to population per time period Human harvest of wild populations -trees -fish -ducks -deer