Chapters 14 and 15
• Exploitation and mutualism involve the use of one organism as a resource by
• Exploitation and mutualism influence the distribution and abundance of both
• Persistence in the face of exploitation may require refuges
• Mutualism develops when benefits exceed costs
Effect on
Species 1
Species 2
Interaction between populations that enhances fitness of one individual while
reducing fitness of the exploited individual.
– Predators kill and consume other organisms.
– Parasites live on host tissue and reduce host fitness, but do not generally
kill the host.
• Parasitoid is an insect larva that consumes the host.
– Pathogens induce disease.
Predators and their Food Supply
Herbivores (predators that eat plants) can have an effect on their prey
– Stream insects and the algal and bacterial populations they consume
– Opuntia and Cactoblastis – there once was a woman who swallowed a
Herbivorous Stream Insect and Its Algal Food (Fig. 14.7, 14.8, 14.9)
Exploitation and Abundance (Fig. 14.11)
Introduced Cactus and Herbivorous Moth
– Mid 1800’s: prickly pear cactus Opuntia stricta was introduced to Australia.
– Established populations in the wild.
– Government asked for assistance in control.
– Moth Cactoblastis cactorum found to be effective predator.
– Reduced by 3 orders of magnitude in 2 years.
– Cyclic stability now in metapopulations
Exploitation and Abundance
Cycles of Abundance in Hares and Lynx (Fig. 14.14)
• Snowshoe Hares (Lepus americanus) and Lynx (Lynx canadensis).
– Extensive trapping records.
Cycles of Abundance in Snowshoe Hares and Their Predators
Elton proposed abundance cycles driven by variation in solar radiation.
Keith suggested overpopulation theories:
– Decimation by disease and parasitism.
– Physiological stress at high density.
– Starvation due to reduced food.
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
– Reduces usable food supplies.
Snowshoe Hares - Role of Predators
Single source theory
– Lynx (Classic specialist predator)
– Predation can account for 60-98% of mortality during peak densities.
Complementary factors
– Hare populations increase, causing food supplies to decrease.
– Starvation and weight loss may lead to increased predation
– Effect is todecrease hare populations.
Experimental Evidence (Fig. 14.15)