Predation

Great White Shark and Fur Seal

Effects on populations

• Population regulation refers to the tendency of a population to decrease in size when above a particular level, and to increase in size when below that level. Population regulation can only occur as a result of one or more density dependent processes acting on birth or death rates. 

• Population abundance is determined by the combined effects of all factors and processes that influence population size, whether they are density dependent or density independent.

Predator effect on individual prey

Mink

Muskrat

Arctic Ground Squirrel – Predator population is self-limited

Red Grouse in Heather –Predator population is self-limited

Tawny Owl

Bank vole

Predator Switching Regulates Prey Population

Cinnabar Moth and Caterpillar on Ragwort Tansy

Snowshoe hare and Lynx

Lynx

Ruffed Grouse Snowshoe hare

Sea Otter

Sea Urchin

KelpForest

Sea Otter eating Sea Urchin in Kelp Forest

Comparison of kelp and urchin biomass with and without sea otters

Sea Urchin Barren

Kelp forest ecsystems with and without sea otters

Sea Mink

Plant Resource Defense

• Qualitative defense - highly toxic substances, small doses of which can kill predators

• high nutrient environment/fast growth (high turnover in plants) - use toxins (plant secondary compounds) that often require N, expensive to make (must be replaced often), but can be made rapidly - cyanide compounds, cardiac glycosides, alkaloids - small molecules

Plant Resource Defense

• Quantitative defense - substances that gradually build up inside an herbivore as it eats and prevent digestion of food

• low nutrient environment/slow growth (low turnover in plants) - primarily use carbon structures - wood, cellulose, lignin, tannins - large molecules - makes plant hard or unpleasant to eat (woodiness, silica), but plants are slow to make these defenses

Evolutionary “Arms” Races

Monarch and milkweed

Evolutionary “Arms” Races

Evolutionary “Arms” Races

California garter snake Pacific newt

Other Plant Defenses Include:

• mechanical defenses - plant thorns and spines deter many vertebrate herbivores, but may not help much against invertebrate herbivores

• failure to attract predators - plants somehow avoid making chemicals which attract predators

• reproductive inhibition - some plants such as firs (Abies) have insect hormone derivatives which if digested, prevent successful metamorphosis of insect juveniles

• masting - the synchronous production of very large numbers of progeny (seeds) by trees of one species in certain years

Birches release pheromones that prevent bark beetles from being attracted to them

Abies (fir trees) produce juvabione that mimics insect juvenile hormone

Perpetual juvenile firebugs

Eurasian Jay with Acorn

Masting

Masting

Fagus sylvaticus – European Beech

Dipterocarp distribution

Dipterocarp trees

Beech seeds and boring moth

Lyme’s disease life cycle

Masting and Human Health- Lyme’s Disease

Induced Defenses

• Another aspect of plant defenses is that plants do not always have tissues loaded with defensive chemicals - in many plants, defensive chemicals are only produced when they are needed, usually after the plant has experienced some herbivory - this is an induced defense

Impact of Herbivores Is Not Uniformly Experienced

Aphids attacking Alfalfa

Spotted Alfalfa Aphid

Induced defenses in Birch Trees

Induced defenses in Birch Trees

Induced defenses in Birch Trees

Rubus prickles

Acacia depanolobium

Plant defenses are developed at a cost to fitness when:

1. Organisms evolve more defenses if they are exposed to much damage and fewer defenses if cost of defense is high

2. More defenses are allocated within an organism to valuable tissues that are at risk

3. Defense mechanisms are reduced when enemies are absent and increased when plants are attacked - mostly true for chemicals not structures

4. Defense mechanisms are costly and cannot be maintained if plants are severely stressed by environmental factors