Predation Lecture 13

Predation and Herbivory Responses of individuals to predation Overview Chapter in Text: 15, 17 Predation and Herbivory Responses of individuals to predation Responses of populations to predation – refuges Importance of Predators Maintenance of ecosystem diversity as a Keystone species

Ecological definition of predator: What is a predator? Narrow sense Broad sense Ecological definition of predator: Herbivores: Grazers and browsers – consume part of plant/plant not killed Seed eaters + planktinovores: consume entire plant Parasite: generally do not kill host Parasitoid: lay eggs in host – feeding by larvae eventually kill host

Under simple laboratory conditions, the predator often exterminates its prey It then becomes extinct itself having run out of food!

Lotka-Volterra Models and Predator-Prey cycling: Developed during 1920s Assume mutual interaction of predator and prey numbers Predict persistence of both predator and prey populations The Lotka–Volterra equations for predator and prey populations link the two populations Each population functions as a density-dependent regulator on the other Predator as a source of density-dependent regulation on the mortality of the prey population Prey as a source of density-dependent regulation on the birthrate of the predator population

The paired equations, when solved, show that the two populations rise and fall in oscillations The cycle can continue indefinitely — the prey is never quite destroyed; the predator never completely dies out

Change in prey population: dNprey/dt = rNprey-cNpreyNpred Lotka-Volterra Equations: assumes a mutual regulation of predator and prey populations Change in prey population: dNprey/dt = rNprey-cNpreyNpred Change in predator population dNpred/dt = b(cNpreyNpred)-dNpred Linked by cNpreyNpred = mortality of prey due to predation (rate at which prey are captured) d = mortality rate of the predator population b = birthrate = conversion efficiency of captured prey into new predators c = efficiency of predation cNprey = per capita prey consumption

Additional factors that influence predator–prey interactions The Lotka–Volterra model is widely criticized for overemphasizing the mutual regulation of predator and prey populations Additional factors that influence predator–prey interactions Coevolution difficulty in locating prey as it becomes scarcer Choice among multiple prey species

Natural selection (think in terms of fitness)  “Now, here you see, it takes all the running you can do, to keep in the same place” – the Red Queen Coevolution: as prey species evolve ways to avoid being caught, predators evolve more effective means to capture them Natural selection (think in terms of fitness)  “smarter,” more evasive prey “smarter,” more skilled predators

Prey defenses to avoid being detected, selected, and captured by predators: Chemical defense Alarm pheromones Repellants Toxins Cryptic coloration Warning coloration Protective armor Behavioral defense

I’m not eating this again! Chemical Defenses Some animals receive an added benefit from eating plants rich in secondary chemical compounds Caterpillars of monarch butterflies concentrate and store these compounds They then pass them to the adult and even to eggs of next generation Birds that eat the butterflies regurgitate them Blue jay I’m not eating this again!

Plant Responses to Herbivores Physical Thorns Height Heavy seed coat Chemical Toxins Digestion inhibitors Nutritional Low levels of N in older foliage Tough, difficult to masticate foliage

Chemical Responses of Plants to Herbivory Mustard oils protected plants from herbivores at first At some point, however, certain insects evolved the ability to break down mustard oil Adult Green caterpillar These insects were able to use a new resource without competing with other herbivores for it Cabbage butterfly caterpillars

Animal Defenses against Predation Monarch butterfly Physical Behavioral Chemical Toxins Coloration Cryptic Warning coloration – aposmatic Batesian mimicry – harmless mimics After Henry Bates, a 19th century British naturalist Müllerian mimicry – common coloration of toxin bearing spp After Fritz Müller, a 19th century German biologist Viceroy butterfly

Self Mimicry Involves adaptations where one animal body part comes to resemble another This type of mimicry is used by both predator and prey Example “Eye-spots” found in many butterflies, moths and fish

Müllerian Mimicry Two or more unrelated but protected (toxic) species come to resemble one another Yellow jacket Masarid wasp Thus a group defense is achieved Sand wasp Anthidiine bee

Yellow jacket

Predation and Behavior Modification - Refuges Schooling of prey fish – response to predator attack – some survive Alarm calls – Prairie dogs, ground squirrels Song birds mob and harass predator bird species Avoidance – temporal, spatial Refuges

Refuges A mechanism that allows exploited population to escape predation/parasitism – many forms: Place/form of cover, schooling, synchronized reproduction (large numbers at one time), size May not provide absolute sanctuary, enough for species to survive Important for survival of predator too!

Protection in Numbers Living in a large group provides a “refuge.” Predator’s response to increased prey density: Prey consumed x Predators = Prey Consumed Predator Area Area Wide variety of organisms employ predator satiation defense. Prey can reduce individual probability of being eaten by living in dense populations.

Examples of Predator Satiation Synchronous widespread seed and fruit production by plants - masting. Synchronized emergence of Cicadas – 16-17 year cycle Williams estimated 1,063,000 cicadas emerged from 16 ha study site. 50% emerged during four consecutive nights. Losses to birds was only 15% of production

Size As A Refuge If large individuals are ignored by predators, then large size may offer a form of refuge. Peckarsky observed mayflies (Family Ephenerellidae) making themselves look larger in the face of foraging stoneflies. In terms of optimal foraging theory, large size equates to lower profitability.

Is regulation top-down or bottom-up? ie. primary productivity versus limits imposed by predator populations

Ch 18 p 344

Diffuse predator–prey interactions The lynx, coyote, and horned owl are responsible for the periodic cycles in the snowshoe hare population Diffuse mutualism A single plant species may depend on a variety of animal species for successful reproduction

Lynx predates weakened hares – eventually crashes Hare popul crashes as: 1. Reduced forage  weakened hares, high lynx prdation 2. Forage produced after heavy browsing accumulates plant defense chemicals less palatable Lynx predates weakened hares – eventually crashes

Is regulation top-down or bottom-up? ie. primary productivity vs. limits imposed by predator populations

Predators and Diversity – see pages 340-344 Alter competitive balance amongst prey spp. Robert Paine studies: sea star exclusion in intertidal plots  decreased prey diversity (15  8 spp) Selective alteration of competitive relationships Peter Morin studies – altered competitive relationships amongst immature frog spp by predatory newt

Keystone Predator (or Keystone Consumer – p 379 + ) Species essential to maintenance of ecosystem structure/diversity Example (there are many): CA sea otter – kelp forest community

THINGS TO WORRY ABOUT Your Pores — Portals for Invasion? Musty Dankness Fleas & Ticks — Tiny Terrorists What's Embedded in Your Bed? What Your Mother Never Told You About Those Hidden Corners and Cracks Pink Mold — Slime or Scourge? Mildew — Mold's Evil Twin