11

Exploitation: Predation, Herbivory, Parasitism, and Disease

Chapter 14

22

Introduction

• _________________: Interaction between populations that enhances fitness of one individual while reducing fitness of the exploited individual.– –

• ___________ is an insect larva that consumes the host.–

33

Parasites That Alter Host Behavior

• Spring-Headed Worm (Acanthocephalans) changes behavior of amphipods in ways that make it more likely …

– Infected amphipods swim toward light, which is usually indicative of shallow water, and thus …

44

Parasites That Alter Host Behavior

55

66

Parasites That Alter Host Behavior

77

Parasites That Alter Host Behavior• Rust fungus Puccinia monoica manipulates

growth of host mustard plants (Arabis spp.).– Puccinia infects Arabis rosettes and invades actively

dividing ________________________.• Rosettes rapidly elongate and become topped by a cluster

of bright yellow leaves.– Pseudo-flowers are fungal structures including sugar-containing

spermatial fluids.» Attract pollenators

88

Entangling Exploitation with Competition

• Park found the presence/absence of a protozoan parasite (Adeline tribolii) influences competition in flour beetles (Tribolium).– Adelina lives as an intercellular parasite.• Reduces density of T. castaneum but has little effect on

T. confusum.• T. castaneum is usually the strongest competitor, but

with the presence of Adelina, T. confusum becomes strongest competitor.

99

Exploitation and Abundance

• 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.

1010

Exploitation and Abundance

1111

Cycles of Abundance in Snowshoe Hares and Their Predators

• Snowshoe Hares (Lepus americanus) and Lynx (Lynx canadensis).– Extensive trapping records.– Elton proposed abundance cycles driven by

variation in solar radiation.– Keith suggested overpopulation theories:• Decimation by …• ____________________________ at high density.• _____________ due to reduced food.

1212

Population Fluctuations

1313

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.

1414

Snowshoe Hares - Role of Predators

– Lynx (Classic specialist predator)• Coyotes may also play a large role.

– _______________ 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.

1515

Population Cycles in Mathematical and Laboratory Models

• Lotka Volterra assumes host population grows exponentially, and population size is limited by parasites, pathogens, and predators:

dNh/dt = rhNh – pNhNp

• rhNh = Exponential growth by host population.– Opposed by:• P = • Nh =

• Np =

1616

Model Behavior

• Host exponential growth often opposed by _________________.– Host reproduction immediately translated into …

– Increased predation = – More predators = – Larger predator population eventually reduces

host population, in turn reducing …

1717

Refuges

• To persist in the face of exploitation, hosts and prey need …

• 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.

1818

1919

Other kinds of Refuge?

2020

Predator Satiation by Periodical Cicadas

• Periodical cicadas Magicicada spp. emerge as adults every 13-17 years.– Densities can approach 4x106 ind / ha.

• 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.

2121

Protection in Numbers• • Predator’s response to increased prey density:

Prey consumed x Predators = Prey Consumed

• Wide variety of organisms employ …

– Prey can reduce…

2222

Size As A Refuge• If large individuals are ignored by predators,

then ....– Peckarsky observed mayflies (Family

Ephenerellidae) making themselves look larger in the face of foraging stoneflies.• In terms of optimal foraging theory…

2323