+ 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
30
+ Costa Rican butterflies and moths
EXAM APRIL 22ND (EARTH DAY): 2 TO 3.30 PM, EXAM HALL
Class petition…Any questions?
41
MALARIA, MOSQUITOES, EVOLUTION
Latest news…
42
For more…See slideshow – posted on the ecology site
43
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
33
+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)
34
+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.
35
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
41
+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
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