Predation Lecture 12

Predation and Herbivory Responses of individuals to predation Overview Chapter 14 in Text 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 (see chapter 17)

Predator-Prey Interactions Predation is the consuming of one organism by another, usually of a similar or larger size Under simple laboratory conditions, the predator often exterminates its prey It then becomes extinct itself having run out of food!

Predator-Prey Interactions In nature, predator and prey populations often exhibit cyclic oscillations The North American snowshoe hare (Lepus americanus) follows a “10-year cycle” Two factors involved 1. Food plants Willow and birch twigs 2. Predators Canada lynx (Lynx canadensis)

Population Fluctuations

Snowshoe Hares - Role of Food Supply Live in boreal forests dominated by conifers. Dense growth of understory shrubs. In winter, browse on buds and stems of shrubs and saplings such as aspen and spruce. One population reduced food biomass from 530 kg/ha in late Nov. to 160 kg/ha in late March. Shoots produced after heavy browsing can increase levels of plant chemical defenses. Reducing usable food supplies.

Snowshoe Hares - Role of Predators Lynx (Classic specialist predator) Coyotes may also play a large role. Predation can account for 60-98% of mortality during peak densities. Complementary: Hare populations increase, causing food supplies to decrease. Starvation and weight loss may lead to increased predation, all of which decrease hare populations.

Predator-Prey Interactions and Community Structure Predator-prey interactions are essential in the maintenance of species-diverse communities reduce competitive exclusion by reducing interspecies competition For example, sea stars prevent bivalves from dominating intertidal habitats Other organisms can share their habitat Keystone species are species that play key roles in their communities

Role of Predators Increase diversity reduce competitive exclusion Example: sea stars prevent bivalves from dominating intertidal habitats Act as Keystone species: key role in maintenance of ecosystem structure Example: Sea Otter in maintenance of kelp forest structure Sea urchin grazes on kelp, sea otter preys on sea urchin thus maintaining kelp bed – kelp bed  diverse ecosystem dependent on sea otter predation on urchin

The Evolutionary Response of Herbivores 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

I’m not eating this again! Animal 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!

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!

Gause attempted to produce population cycles with P Gause attempted to produce population cycles with P. caudatum and Didinium nasutum. Didinium quickly consumed all Paramecium and went extinct. (Both populations extinct) Added sediment for Paramecium refuge. Few Paramecium survived after Didinium extinction

Refuges Huffaker studied six-spotted mite Eotetranychus sexmaculatus and predatory mite Typhlodromus occidentalis. Separated oranges and rubber balls with partial barriers to mite dispersal. Typhlodromus crawls while Eotetranychus balloons. Provision of small wooden posts to serve as launching pads maintained population oscillations spanning 6 months.

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.

Various Forms of Mimicry Aposematic Coloration Mullerian mimicry: Look like other poisonous species – advertise poison Batesian mimicry: Pretend to be dangerous or poisonous Self Mimicry

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

Summary Natural conditions may result a cyclic balance between predator and prey populations Keystone species are species that play key roles Many predator-prey relationships have resulted in a process of co-evolution resulting in changes in structure, appearance and behavior