Predation

Great White Shark and Fur Seal

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

Eurasian Jay with Acorn

Masting

Masting

Fagus sylvaticus – European Beech

Often masts in years following hot summer, but not in years following cool summer

Dipterocarp distribution

Dipterocarp trees

Dipterocarps usually mast during El Nino years

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

Pine beetle infestation – British Columbia

Serengeti Grazing System

Serengeti Grazing System

Grazing facilitation

• Grazing facilitation occurs when the feeding activity of one herbivore species improves the food supply for a second species

Opuntia stricta – prickly pear

Prickly pear infestation in Australia

Area infested with prickly pear before biocontrol

Same area after biocontrol

Biocontrol Agent – Cactoblastis cactorum