+ chapter 17: predation and herbivory (and a bit of chapter 20) robert e. ricklefs the economy of...

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Chapter 17: Predation and Herbivory (and a bit of Chapter 20)Robert E. Ricklefs

The Economy of Nature, Fifth Edition

The rabbit/myxoma story

Interacting populations evolve in response to each other

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Evolution of Resistance in Rabbits

Decline in lethality of the myxoma virus in Australia resulted from evolutionary responses in both the rabbit and the virus populations: genetic factors conferring resistance to the

disease existed in the rabbit population prior to introduction of the myxoma virus: the myxoma epidemic exerted strong selective

pressure for resistance eventually most of the surviving rabbit population

consisted of resistant animals

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Evolution of Hypovirulence in Myxoma Virus

Decline in lethality of the myxoma virus in Australia resulted from evolutionary responses in both the rabbit and the virus populations:

less virulent strains of virus became more prevalent following initial introduction of the virus to Australia: virus strains that didn’t kill their hosts were more

readily dispersed to new hosts (mosquitoes bite only living rabbits)

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The Rabbit-Myxoma System Today

Left alone, the rabbit-myxoma system in Australia would probably evolve to an equilibrial state of benign, endemic disease, as in South America: pest management specialists continue to

introduce new, virulent strains to control the rabbit population

Contagious diseases spread through the atmosphere or water are less likely to evolve hypovirulence, as they are not dependent on their hosts for dispersal.

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RABBITS AND MYXOMA …

… is an example of a predator (the virus) and prey (the rabbits).

Prickly Pear cactus were also introduced into Australia.Like rabbits, they

quickly spread over the continent.

A predator of the cactus was introduced. The cactus moth. The cactus only

survived in areas where the moth was absent.

Comparing cactus before (a) and after (b) the moth introduction.

The cactus is an example of predator prey interactions. Do predators limit prey population growth?

Do prey limit predator population growth?

The balance between the two depends on their adaptations. Some adaptations were already found in species. Some adaptations are a result of predator/prey interactions.

All life forms are both consumers and victims of consumers.

There are many consumer-resource interactions: Predator-prey Herbivore-plant Parasite-host

Producers

Consumers Predator; Parasite; Parasitoid: Herbivore; Detritivore

+Some Definitions

Predators catch individuals and consume them, removing them from the prey population.

Parasites consume parts of a living prey organism, or host: parasites may be external or internal a parasite may negatively affect the host but does

not directly remove it from the population

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+More Definitions

Parasitoids consume the living tissues of their hosts, eventually killing them: parasitoids combine traits of parasites and

predators

Herbivores eat whole plants or parts of plants: may act as predators (eating whole plants)

or as parasites (eating parts of plants): grazers eat grasses and herbaceous vegetation browsers eat woody vegetation

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+Detritivores occupy a special niche.

Detritivores consume dead organic material, the wastes of other species: have no direct affect on populations that produce

these resources: do not affect the abundance of their food supplies do not influence the evolution of their resources

are important in the recycling of nutrients within ecosystems

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An example of a parasitoid wasp.

This was is laying its egg in the caterpillar. The egg will develop into

larvae. The larvae will consume

the caterpillar as it grows.

A combination of predation, and parasitism.

Predators have adaptations for exploiting their prey.This lion has

adaptations to capture fast prey.

This whale is a filter feeder.

Spiders make webs to subdue prey.

Even predator adaptations take practice!

Predators and prey are different sizes, and this can pose problems.

If a prey item is too small – it may be too hard to handle. Imagine trying to capture mice with your hands.

If a prey item is too large – the predator may not be able to subdue. Imagine trying to tackle a elephant to eat.

Blue whales weigh many tons, but eat tiny shrimp (use of filters).

+Form and Function Match Diet

Form and function of predators are closely tied to diet: vertebrate teeth are adapted to dietary items:

horses have upper and lower incisors used for cutting fibrous stems of grasses, flat-surfaced molars for grinding

deer lack upper incisors, simply grasping and tearing vegetation, but also grinding it

carnivores have well-developed canines and knifelike premolars to secure and cut prey

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+ A predator’s form and function are closely tied to its diet. (a) upper incisors are used to cut plant material; (b) flat-surfaced molars for grinding plant material; (c) knifelike premolars secure prey and tear flesh

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+More Predator Adaptations

The variety of predator adaptations is remarkable: consider grasping and tearing functions:

forelegs for many vertebrates feet and hooked bills in birds distensible jaws in snakes

digestive systems also reflect diet: plant eaters feature elongated digestive tracts with

fermentation chambers to digest long, fibrous molecules comprising plant structural elements

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+Distensible jaws: shift the articulation of the jaw with the skull from the quadrate bone to the supratemporal

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Burmese python (3.9m) vs alligator (1.8m) in Everglades National Park (Florida)

+ Digestive tracts of consumers are adapted to their diets. Digestive organs of herbivores > carnivores

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Large carnivores tend to pursue large prey.Size of prey consumed is related to size of predator.

What about the prey? How much energy do you have available for growth?

If you are predated upon, your growth rates are affected.

Prey have adaptations to avoid being consumed.

Hiding If a predator can’t see you, it can’t eat you. Evolution of cryptic coloration.

Escaping If you can outrun your predator, it can’t eat

you. Evolution of speed or maneuverability.

Active defense mechanisms Animals with poison glands. Plants with thorns, toxic substances.

+Crypsis and Warning Coloration

Through crypsis, animals blend with their backgrounds; such animals: are typically palatable or edible match color, texture of bark, twigs, or leaves are not concealed, but mistaken for inedible

objects by would-be predators

Behaviors of cryptic organisms must correspond to their appearances.

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+Cryptic appearances (a) mantid; (b) stick

insect; (c) lantern fly24

For more…See slideshow – posted on the ecology site

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Chemical defenses.

The production of chemicals which repel potential predators.

Toxin + boiling temp =>

Notice the colors of this bombardier beetle.

+Warning Coloration: aposematism

Why should a prey item evolve bright colors? It definitely brings attention to you. Black and yellow are the most common colors.

Unpalatable animals may acquire noxious chemicals from food or manufacture these chemicals themselves: such animals often warn potential predators with warning

coloration or : certain aposematic colorations occur so widely that predators

may have evolved innate aversions

If an animal eats a brightly colored prey item: It may get sick. It may die. If it lives, it will remember.

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+ Unpalatable organisms 26

+Why aren’t all prey unpalatable?

Chemical defenses are expensive, requiring large investments of energy and nutrients.

Some noxious animals rely on host plants for their noxious defensive chemicals: not all food plants contain such chemicals animals using such chemicals must have their own

means to avoid toxic effects

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+Batesian Mimicry

Certain palatable species mimic unpalatable species (models), benefiting from learning experiences of predators with the models.

This relationship has been named Batesian mimicry in honor of discoverer Henry Bates.

Experimental studies have demonstrated benefits to the mimic: predators quickly learn to recognize color patterns

of unpalatable prey mimics are avoided by such predators

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+Harmless mantid (b) and moth (c) evolved to resemble a wasp (a)

+Müllerian Mimicry

Müllerian mimicry occurs among unpalatable species that come to resemble one another: many species may be involved each species is both model and mimic process is efficient because learning by

predator with any model benefits all other members of the mimicry complex

certain aposematic colors/patterns may be widespread within a particular region

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+ Costa Rican butterflies and moths

EXAM APRIL 22ND (EARTH DAY): 2 TO 3.30 PM, EXAM HALL

Class petition…Any questions?

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MALARIA, MOSQUITOES, EVOLUTION

Latest news…

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For more…See slideshow – posted on the ecology site

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Parasites!

Parasites have adaptations to allow them to live in the host. The host has adaptations to

fight off parasites.

The parasite does not want to kill the host, but disperse its offspring to another host.

+Parasites have adaptations to ensure their dispersal. Parasites are usually much smaller than their hosts and

may live either externally or internally: internal parasites exist in a benign

environment: both food and stable conditions are provided by

host parasites must deal with a number of

challenges: host organisms have mechanisms to detect and

destroy parasites parasites must disperse through hostile

environments, often via complicated life cycles with multiple hosts, as seen in Plasmodium, the parasite that causes malaria

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+Parasite-Host Systems: A Balancing Act

The parasite-host interaction represents a balance between parasite virulence and host defenses: immune system of host can recognize and

disable parasites but parasites may multiply rapidly before an

immune response can be deployed

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+Parasites may defeat a host’s immune response.

Circumventing the host’s immune system is a common parasite strategy: some parasites suppress the host’s immune system

(AIDS virus) other parasites coat themselves with proteins that

mimic the host’s own proteins (Schistosoma) some parasites continually coat their surfaces with

novel proteins (trypanosomes)

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+Cross-Resistance

Some parasites elicit an immune response from the host, then coat themselves with host proteins before the immune response is fully mobilized: initial immune response by host may benefit

the host later when challenged by related parasites in a phenomenon known as cross-resistance

Once an immune response has been elicited, antibodies can persist for a long time, preventing reinfection.

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Many parasites have complex life cycles.Malaria (Plasmodium) parasitic life cycle.

+Plants have antiherbivore defenses.

Plant-herbivore “warfare” is waged primarily through biochemical means.

Full spectrum of plant defenses includes: low nutritional content of plant tissues toxic compounds synthesized by the plants structural defenses:

spines and hairs tough seed coats sticky gums and resins

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Plant adaptations against predation.

Nutritional value?

It could be as simple as a spine. “Ouchy bush!”

It could be as complicated as chemicals. Tannins. Secondary compounds.

+ Spines protect the stems and leaves

(a) cholla cactus and (b) prickly pear cactus

+Digestibility

Animals typically select plant food according to its nutrient content: especially important to young animals,

which have high demands for protein

Some plants deploy compounds that limit the digestibility of their tissues: tannins produced by oaks and other plants

interfere with the digestion of proteins some animals can overcome the effect of

tannins through production of digestive dispersal agents

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+Secondary Compounds

Secondary compounds are produced by plants for purposes (typically defensive) other than metabolism.

Such compounds can be divided into three major classes: nitrogen compounds (lignin, alkaloids,

nonprotein amino acids, cyanogenic glycosides)

terpenoids (essential oils, latex, plant resins) phenolics (simple phenols)

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+Induced and Constitutive Defenses

Constitutive chemical defenses are maintained at high levels in the plant at all times.

Induced chemical defenses increase dramatically following an attack: suggests that some chemicals are too expensive to maintain under

light grazing pressure plant responses to herbivory can reduce subsequent herbivory

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+Herbivores control some plant populations.

Examples of control of introduced plant pests by herbivores provides evidence that herbivory can limit plant populations: prickly pear cactus in Australia

controlled by introduction of a moth, Cactoblastis Klamath weed in California

controlled by introduction of a beetle, Chrysolina

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+Effects of Grazers and Browsers on Vegetation

Herbivores consume 30-60% of aboveground vegetation in grasslands: demonstrated by use of exclosures limiting access to

vegetation by herbivores

Occasional outbreaks of tent caterpillars, gypsy moths, and other insects can result in complete defoliation of forest trees.

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Imagine a plant being eaten, which stimulates plant or chemical production.

Mite growth is inhibited if the plant was previously eaten.

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Look at the impact of herbivores.

Outbreaks of herbivorous insects can defoliate forests.

Tell me please – do not put your name on the paper 3 issues you want to discuss in the remaining class period

Please include at least one topic relevant to Lebanon or the region Now – take 5 minutes

Also tell me please The good The bad The anything else (beautiful or ugly)

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Chapter 18: Dynamics of PredationRobert E. Ricklefs

The Economy of Nature, Fifth Edition

Population Cycles of Canadian Hare and Lynx

Charles Elton’s seminal paper focused on fluctuations of mammals in the Canadian boreal forests. Elton’s analyses were based on trapping records

maintained by the Hudson’s Bay Company of special interest in these records are the

regular and closely linked fluctuations in populations of the lynx and its principal prey, the snowshoe hare

What causes these cycles?

Some Fundamental Questions

The basic question of population biology is: what factors influence the size and stability of populations?

Because most species are both consumers and resources for other consumers, this basic question may be refocused: are populations limited primarily by what they eat or by

what eats them?

More QuestionsDo predators reduce the size of prey

populations substantially below the carrying capacity set by resources for the prey? this question is prompted by interests in

management of crop pests, game populations, and endangered species

Do the dynamics of predator-prey interactions cause populations to oscillate? this question is prompted by observations of

predator-prey cycles in nature, such as Elton’s lynx and hare

Consumers can limit resource populations.

An example: populations of cyclamen mites, a pest of strawberry crops in California, can be regulated by a predatory mite: cyclamen mites typically invade strawberry crops

soon after planting and build to damaging levels in the second year

predatory mites invade these fields in the second year and keep cyclamen mites in check

Experimental plots in which predatory mites were controlled by pesticide had cyclamen mite populations 25 times larger than untreated plots.

What makes an effective predator?

Predatory mites control populations of cyclamen mites in strawberry plantings because, like other effective predators: they have a high reproductive capacity relative to that of

their prey they have excellent dispersal powers they can switch to alternate food resources when their

primary prey are unavailable

Consumer Control in Aquatic Ecosystems

An example: sea urchins exert strong control on populations of algae in rocky shore communities: in urchin removal experiments, the biomass of algae quickly

increases: in the absence of predation, the composition of the algal

community also shifts: large brown algae replace coralline and small green

algae that can persist in the presence of predation

Predator and prey populations often cycle.Population cycles observed in Canada

are present in many species: large herbivores (snowshoe hares, muskrat,

ruffed grouse, ptarmigan) have cycles of 9-10 years: predators of these species (red foxes, lynx, marten,

mink, goshawks, owls) have similar cycles small herbivores (voles and lemmings) have

cycles of 4 years: predators of these species (arctic foxes, rough-

legged hawks, snowy owls) also have similar cycles cycles are longer in forest, shorter in tundra

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Predator-Prey Cycles: A Simple Explanation

Population cycles of predators lag slightly behind population cycles of their prey: predators eat prey and reduce their numbers predators go hungry and their numbers drop with fewer predators, the remaining prey survive better and

prey numbers build with increasing numbers of prey, the predator populations

also build, completing the cycle

Time Lags in Predator-Prey SystemsDelays in responses of births and deaths to an

environmental change produce population cycles: predator-prey interactions have time lags associated

with the time required to produce offspring 4-year and 9- or 10-year cycles in Canadian tundra or

forests suggest that time lags should be 1 or 2 years, respectively: these could be typical lengths of time between birth and

sexual maturity the influence of conditions in one year might not be felt until

young born in that year are old enough to reproduce

Time Lags in Pathogen-Host Systems

Immune responses can create cycles of infection in certain diseases: measles produced epidemics with a 2-year

cycle in pre-vaccine human populations: two years were required for a sufficiently large

population of newly susceptible infants to accumulate

Time Lags in Pathogen-Host Systems other pathogens cycle because they kill sufficient

hosts to reduce host density below the level where the pathogens can spread in the population: such cycling is evident in polyhedrosis virus in tent

caterpillars In many regions, tent caterpillar infestations last about 2

years before the virus brings its host population under control

In other regions, infestations may last up to 9 years Forest fragmentation – which creates abundant forest edge

– tends to prolong outbreaks of the tent caterpillar Why? Increased forest edge exposes caterpillars to more intense sunlight

inactivates the virus thus, habitat manipulation here has secondary effects