1. Predation Lecture 12

2. 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)

3. 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!

4. 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)

5. Population Fluctuations

6. 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.

7. 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.

8. 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

9. 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

10. 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

11. 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!

12. 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

13. 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!

14. 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

15. 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.

16. 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.

17. Examples of Predator Satiation
Synchronous widespread seed and fruit production by plants - masting.
Synchronized emergence of Cicadas – 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

18. 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.

19. 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

20. 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

21. 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