11

Biology 205Ecology and Adaptation

Lecture 11: Exploitation: Predation, Parasitism, and Disease

Dr. Erik D. Davenport

1

2

Outline

• Introduction about exploitation• Complex Interactions• Exploitation and Abundance• Population Fluctuations– Models

• Refuges– Prey Density– Size

2

3

Introduction

• Exploitation: 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.– Pathogens induce disease.

3

4

Concept 1: Complex Interactions

Exploitation weaves populations into a web of relationships that defy easy generalization

• About 10 million species on the earth.• However, the number of exploitive

interactions among these species are far greater.

4

5

Parasites That Alter Host Behavior

5

6

Parasites That Alter Host Behavior

• Spring-Headed Worm (one type of parasites) (Acanthocephalans) changes behavior of amphipods (host) in ways that make it more likely that infected amphipods will be eaten by a suitable vertebrate host (secondary host).

• Infected amphipods swim toward light, which is usually indicative of shallow water, and thus closer to predators.

6

7

Parasites That Alter Host Behavior

• Rust fungus (parasites) Puccinia monoica manipulates growth of host mustard plants (Host) (Arabis spp.).

• Puccinia infects Arabis rosettes and invades actively dividing tissue.

• Rosettes rapidly elongate and become topped by a cluster of bright yellow leaves.

• Pseudo-flowers are fungal structures including sugar-containing fluids.

• Attract pollenators – why? 7

8

Parasites That Alter Host Behavior

8

9

Entangling Exploitation with Competition

• The parasite may make the outcome of a competition hard to predict.

• Park found the presence/absence of a protozoan parasite (Adeline tribolii) influences competition in flour beetles (Tribolium).– 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.

9

10

14_06.jpg

10

11

Concept 2: Exploitation and abundance

Predator, parasites, and pathogens influence the distribution, abundance, and structure of prey

and host populations.

The most important concept in this lecture!How does the prey and predator population

abundance change over time?????

11

12

Herbivorous Stream Insect and Its Algal Food

12

13

Herbivorous Stream Insect and Its Algal Food

• Lamberti and Resh studied influence of caddisfly (Helicopsyche borealis) on algal and bacterial populations on which it feeds.

• Results suggest larvae reduce the abundance of their food supply.

13

14

Exploitation and Abundance

14

15

Exploitation and Abundance

• Relationship between Cactus and 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.

15

16

Predator-prey, host-parasite, and host-pathogen relationships are dynamics

16

The abundance is dynamic!

17

Population Fluctuations

17

18

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:• declined by disease and parasitism.• Physiological stress at high density.• Starvation due to reduced food.

18

19

Snowshoe Hares - Role of Food Supply

• Live in boreal forests dominated by conifers.• Snowshoe hares have the potential reduce the

quantity and quality of their supply.– 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.

19

20

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.

– The decrease of Hare population eventually will also cause the decrease of its predator-lynx population

20

21

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 = rate of parasitism / predation.• Nh = Number of hosts.

• Np = Number of parasites / predators.

21

2222

23

Population Cycles in Mathematical and Laboratory Models

• Lotka Volterra assumes parasite/predator growth rate is determined by rate of conversion of food into offspring minus mortality rate of its own population:

dNp/dt = cpNhNp-dpNp

• cpNhNp = Conversion rate of hosts into offspring.

• pNhNp = Rate at which exploiters destroy hosts.• c = Conversion factor

23

2424

2525

Predator vs. Prey

Without predator, Prey population will grow exponentially.

Without prey, predator population will decrease (die)

25

26

Model Behavior

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

destruction by predator.– Increased predation = more predators.– More predators = higher exploitation rate.– Larger predator population eventually reduces

host population, in turn reducing predator population.

26

27

Model Behavior

• Reciprocal effects produce oscillations in two populations.

• Although the assumptions of eternal oscillations and that neither host nor exploiter populations are subject to carrying capacities are unrealistic, L-V models made valuable contributions to the field.

27

2828

29

Laboratory Models

• Utida found reciprocal interactions in adzuki bean weevils Callosobruchus chinensis over several generations.– Gause found similar patterns in P. aurelia.

• Most laboratory experiments have failed in that most have led to the extinction of one population within a relatively short period. Why?

29

30

14_18.jpg

30

31

Concept 4: Refuges

To persist in the face of exploitation, hosts and prey need refuges.

31

32

Refuges

32

33

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.

33

34

Refuges

34

35

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.

35

36

14_24.jpg

36

37

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.

37