Lecture 11 + 12 Flashcards
True or false: Parasite & host populations can fluctuate in ways that are similar to predator-prey population cycles
True
In the myxomatosis-rabbit system, what are the two vectors?
Mosquitos and fleas
Metapopulation
Refers to patchiness in a population that allows it to function like many connected smaller subpopulations.
If a plant evolves a defensive chemical to ward off herbivory how do we expect an insect herbivore to evolve?
The insect herbivore may evolve resistance to the chemical
True or false: A virus is likely not spreading if R0 (R nought) is much greater than 1
False
Exploitation interactions
An interaction that benefits one individual at the expense of another (ex. interactions with +/- outcome)
Includes: predation and herbivory (grazing); parasitoidism and parasitism this lecture
Parasitoids
- insect that lays one or a few eggs on or in a host organism, which the resulting larvae remain with, consume, and kill in the process
- invasion of the host is similar to a parasite
- functionally equivalent to a predator
Parasites are found in many groups
Viruses, bacteria, protists, fungi, plants, prions, worms (flatworms, roundworms, segmented worms, etc.), arthropods (crustaceans, insects, ticks), and vertebrates (mammals, birds, fishes)
Interaction: common cold and humans
Low lethality, high intimacy
Interaction: flea and dogs
Low lethality, medium-high intimacy
Interaction: vampire bat and cows
Low lethality, medium intimacy
Interaction: deer and bluegrass
Low lethality, low intimacy
Interaction: ebola and humans
Medium lethality, high intimacy
Interaction: raccoon and blackberries
Medium lethality, low intimacy
Interaction: wasp and caterpillars
High lethality, high intimacy
Interaction: trout and small minnows
High lethality, low intimacy
Interaction: predators
High lethality, low intimacy
Interaction: herbivores (grazers)
Low lethality, low intimacy
Interaction: parasites
Low lethality, high intimacy
Interaction: parasitoids
High lethality, high intimacy
Interaction: omnivores
Medium lethality, medium intimacy
How numerous are parasite species?
There are more species of parasites than all non-parasite species combined
Parasite species are abundant
Categorizing parasites: micro vs. macro
Rule of thumb: you see individual macroparasites with a “naked eye”
Parasites
A relationship in which an organism lives on the tissue of its hosts, often reducing the fitness of the host, but not generally killing it
Micro vs. macro: size
Microscopic vs. relatively large
Micro vs. macro: 3 example organisms
Bacteria, viruses, protozoans, some fungi
Parasitic worms, ticks, fleas, some fungi
Micro vs. macro: # per host
Numerous vs. low to intermediate densities
Micro vs. macro: generation time
Short generation time vs. relatively long generation time
Micro vs. macro: generally reproduce inside or outside of host body
Multiply directly in host vs. grow but do not multiply within host; persist by continual reinfection
Micro vs. macro: intracellular or extracellular
Many intracellular vs. live in body cavities or on body
Micro vs. macro: host immunity to second infection
Often induce immunity to reinfection vs. induce short-term immune response
Micro vs. macro: low or high impact on host populations
Moderate to high ability to regulate host population vs. low ability to regulate host population
Advantages of ectoparasitism
- ease of dispersal
- safer from host’s immune system
Disadvantages of ectoparasitism
- feeding more difficult
- exposure to external environment
- vulnerability to natural enemies
Advantages of endoparasitism
- ease of feeding
- protected from external environment
- safer from enemies
Disadvantages of endoparasitism
- dispersal difficult
- vulnerability to host’s immune system
True or false: A parasite should do a great deal of damage to a host, because doing so tends to maximize its own fitness
False. Parasites have evolved to maximize fitness. They do not have an evolutionary interest in causing extra damage to a host. Symptoms of some parasites are often related to their transmission strategies.
Parasites can have complex life histories
Butterfly -> several stages (complex)
Humans -> just get bigger (simple)
What step in the life cycle of Toxoplasmosis gondii might be the most difficult to achieve?
Mouse to cat. It needs to live in a mouse and then a cat. Mice don’t like to be consumed by cats!
Ecological effects of parasites
Parasites can modify
-reproductive success of host
- behaviour of host (ant’s abdomen changing colour and going to top of plants to hold their abdomen up high to attract birds and eat them)
- morphology of host
- the outcome of competition
- host community structure
- geographic range of host
- the physical environment (erosion and silt content)
Parasites can decimate populations and alter ecological communities
ex. American Chestnut and chestnut blight fungus
R0
The basic reproductive number
Represents how many new hosts are infected by the average host
R0 = 1
Each host infects one new host and the disease prevalence remains constant (replacement level)
R0 > 1
Each host infects more than one new host and the disease prevalence is increasing
R0 < 1
Each host infects less than one new host and the disease prevalence is decreasing
Nl = N0 * R0
Nl = population of infected hosts at time l
N0 = initial number of infected hosts
R0 = reproductive number
Disease spread (R0) depends on:
- population density (N)
- how many hosts are available to get the disease? (susceptible individuals)
- how frequently do hosts come into contact with each other? - infectious period (L)
- how long does a host spread the disease for? - Transmission rate (B)
- effective transmission per unit time
- how easy is it for the disease to spread from an infected individual to a susceptible individual?
R0 = N * L * B
Covid disease spread (R0) depends on
Population density (N)
* No big gatherings
* Reduced capacity at locations
* Vaccination
Infectious period (L)
* Quarantine periods
* Testing
Transmission rate (B)
* Masks
* Social distancing
* Cleaning surfaces
All of these actions help R0 decline. Combined with a vaccination, even strong impact on R0
How vaccines work
Initial exposure to antigen, develop antibodies -> primary immune response
Second exposure to antigen, response is faster and larger -> secondary immune response
This effect occurs when a disease is “natural acquired” or with a vaccine
Imaging R0 = 3, vaccines are 100% effective; 25%, 50%, 80% vaccination rate
New R0 = % susceptible individuals * R0
0.75 * 3 = 2.25 (R0)
0.5 * 3 = 1.5 (R0)
0.2 * 3 = 0.6 (R0) <- herd immunity
Many factors could functionally lower R0 in ecological situations
- low density host population (lower population size)
- environmental conditions
- issues with intermediate vectors
- host immune systems
- a population with few susceptible individuals (P = G + E)
- behavioural avoidance
How did evolution by natural selection shape the global COVID-19 pandemic?
Originally lower transmissibility -> emergence of a more transmissible variant -> higher transmissibility, the new variant spreads
Parasitism is most similar to
Predation
Parasites are like predators in that they: (Select one or more answers)
- Can reduce the competitive impacts of otherwise dominant competitors
- Can reduce the fitness of the organism on which they are preying upon or parasitizing
- Typically remove one individual from a population
- There impacts are not at all similar
- Can reduce the competitive impacts of otherwise dominant competitors
- Can reduce the fitness of the organism on which they are preying upon or parasitizing
What reasons explain why Myxomatosis virulence declined in European rabbits outside of their native range? (Select one or more answers)
- Myxoma was not subject to natural selection
- Myxoma virulence was selected against
- Intermediate virulence was selected for
- There was selection for increased transmissibility
- Myxoma virulence was selected against
- Intermediate virulence was selected for
- There was selection for increased transmissibility
You are conducting a biodiversity survey of all the organisms in a field outside of Ithaca. You record 24 species of plants and 6 species of mammals. Given this data, how many parasites can you expect to find?
- Less than 30
- 30
- Slightly more than 30
- Many more than 30
- Zero
Many more than 30
Which of the following are characteristics of microparasites? (you may select one or more answers)
- Relatively large
- Includes: bacteria, viruses, protozoans, some fungi
- Long generation times
- Multiply directly in host
- Many intracellular
- Typically do not induce immunity to reinfection
- Can often regulate host populations
- Includes: bacteria, viruses, protozoans, some fungi
- Multiple directly in host
- Many intracellular
- Can often regulate host populations
True or false: All microparsites are endoparasites
False. Think of a fungal infection on a foot.
True or false: Ectoparasites have an easier time spreading than endoparasites
True. Endoparasites have the added complication of getting out of the hosts body.
True or false: Most parasites are lethal to their hosts
False. A parasite does not want to negatively impact its own fitness if it was very lethal to host species.
If the Ro of a pathogen was 10 the prevalence of this disease in a population would be
Increasing
A pathogen would spread less quickly if… (You may select one or more answers)
- The host population was small
- Many individuals in the population were immune
- Environmental conditions were unfavourable to a vector
- The hosts had strong immune systems
- The host population was small
- Many individuals in the population were immune
- Environmental conditions were unfavourable to a vector
- The hosts had strong immune systems
An optimally foraging organism is trying to maximize it’s_______
Fitness
Which is the best measure of effectiveness of optimal foraging?
Net rate of energy intake
True or false: “Optimally foraging” children, seeking to sell water, should all stand on the same side of the road
False
True or false: Large African herbivores will shift their preferred foraging areas in the presence of predators
True
How should food relate to fitness?
Amount of food (x-axis) v. fitness (y-axis); linear increase
Energy rate maximization
Animals should forage such that they gain the most calories per unit time to maximize their fitness (subject to constraints)
Why do bats go into torpor (hibernate) during the winter?
When considering optimal foraging, we need to consider costs and benefits
- bats in North America are predators that eat insects (insectivores)
- there are no insects flying around in the winter
- energy: simple cost, organisms need to spend energy to get energy
- costs > benefits; it is not worth it to forage
Measuring when to stop foraging with The Giving Up Density (GUD)
- used with animals that have diminishing returns while foraging
- measured as the amount of seed remaining
The Giving Up Density
Diminishing returns while foraging
- quit when the costs = the benefits
Quitting harvest rate
Rate of resource gain where an animal quits foraging because benefits = costs
However, predation is also a cost that influences optimal foraging
Cost benefit analysis with predation
- most organisms are prey for others
- death is bad
- many organisms face a fundamental tradeoff between food and safety
- why would an organism ever risk the huge negative effect of death for the small benefit of food?
Why would an organism ever risk the huge negative effect of death for the small benefit of food?
- organisms will trade off food for safety
- energy rate maximizers would still need to consider predation by incorporating that cost (risk of death) into decisions
- organisms will quit foraging when the benefits = the costs
Predation as a foraging cost
- most organisms are involved in some exploitation interaction where they are the “-“
- organisms must balance costs and benefits in order to maximize fitness
Links between fear, physiology, and behaviour
- fight or flight
- your physiology will impact decisions
The ecology of fear
Predators do not only reduce prey fitness by eating them, they also change prey’s physiology & behaviour, which has an impact on fitness and populations
Optimal foraging (context matters)
Individual differences matter
- not all individuals are the same
Food and safety trade-off (context matters)
ex. There are 4 gerbils of the same species, with the same body length and muscle mass. Which should be “more willing” to risk death for food?
– A) A hungry gerbil who weighs 27g
– B) A full gerbil who weighs 27g
– C) A full gerbil who weighs 24g
– D) A hungry gerbil who weighs 24g
– E) None of them should be willing to risk death
A hungry gerbil who weights 24g
Rules of thumb
Not all optimal foraging decisions are complex
- if you are about to die of starvation go out to get food, no matter what
- forage in the patch with the greatest return
- eat prey items that are closer to you
- eat the prey item that has the most calories (largest)
- quit foraging when the costs = the benefits
- forage in the location with the fewest competitors
Game Theory
Modelling: economics meets ecology
- early ecologists borrowed theories and ideals from economists
- now economists are borrowing from ecologists (biologists)
ex. 75% of being betrayed. Skip or cooperate?
If cooperate: (0.75 * 12 hours) + (0.25 * 4 hours) = 10 hours
If skip: (0.75 * 8 hours)+ (0.25 * 0 hours) = 6 hours
Therefore, you should skip
ex. 50% chance of being betrayed. Skip or cooperate?
If cooperate: (0.50 * 12 hours) + (0.50 * 4 hours) = 8 hours
If skip: (0.50 * 8 hours)+ (0.50 * 0 hours) = 4 hours
Therefore, you should skip
ex. 1% chance of being betrayed. Skip or cooperate?
If cooperate: (0.01 * 12 hours) + (0.99 * 4 hours) = 4.08 hours
If skip: (0.01 * 8 hours)+ (0.99 * 0 hours) = 0.08 hours
Therefore, you should skip
Behavioural ecologists employ Game Theory and Evolutionarily Stable Strategies (ESS)
Evolutionary Stable Strategy
Behavioural strategy that is adopted by a population that cannot be invaded by another strategy
- all members of a population adopt the strategy
- no other strategy will yield a greater benefit to individuals over the long term
Skipping is the ESS
Imagine a population where you will play “partner project” against a random opponent.
- The skipping strategy cannot be invaded
- Think of a population where everybody cooperates. Skipping provides a large advantage. A “skipper” spends zero time. Skipping would evolve to take over.
- Think of a population where everybody skips. Cooperating does not give an advantage. Cooperating means you always spend 12 hours on the project. Everybody stays a “skipper”.
- The best solution is to skip.
- This is true even though, on average, if everybody cooperated there would be less total time on the project.
True or false: Mutualisms are a +,+ interactions so individuals of each species in the interaction should “support each other” to maximize the benefits
False. Consequence of mutual interaction is +/+.
Ex. The flower benefits most by giving as less pollen as possible. Pollinator wants a much pollen as possible without it bogging it down. While the result of the interaction is +/+, each individual is still trying to maximize its own fitness, not maximize the benefit of the other organism
Game theory with frequency dependence
An organism making a decision to maximize its fitness based on what others are doing. The frequency of actions of others.
Ideal free distribution (resource matching)
The idea is that organisms will distribute themselves such that they maximize fitness based on what others have done
The tragedy of the commons
The idea that when individual interests prevail, common resources are likely to be overexploited. Individuals benefit by utilizing more of a resource, meaning there is little incentive for individuals not to use as much as they can.
With regard to global warming, can the tragedy of the commons be overcome?
If we view cooler temperatures as a shared resource, actions taken by individuals, nations, etc. for their own benefit, can degrade that shared resource and increase the global temperature.
For example, a nation might have higher CO2 emissions that benefit its economy, but the costs of that are the shared consequences of the global increase in temperature.
Elinor Ostrom
- argued that basic game theory and the tragedy of the commons is an oversimplification for humans
- trust is important and community members are not “trapped by greed”
- chronicled numerous examples of community success in overcoming the tragedy of the commons
Ostrom’s design principles
- clear boundaries around the common pool resource
- rules for resource use match local needs and conditions
- the people using the resource can make and modify the rules
- outside authorities respect local rules
- a system for monitoring resource users
- graduated punishment for rule-breakers
- a low-cost means of conflict resolution
- build responsibility for governing the common resource in nested tiers from the lowest level up to the entire interconnected system
True or false: When considering the energetic gain of an optimal forager, it is important to consider that there are also energetic costs
True. All foraging activities will typically have both costs and benefits.
True or false: In basic models optimally foraging organisms could maximize fitness through energy rate maximization, however this would not work perfectly for many foraging organisms in the real world, because they must also respond additional factors like predation risk.
True. Think about optimal foraging in lecture. There are important tradeoffs between food and safety. Furthermore, most models will not be perfect because they cannot account for every factor in the environment.
Which of the following behaviors is most likely to be observed in a predator exhibiting optimal foraging: ________
- Leaving a patch of prey when encounter rates get low
- Leaving a patch when prey densities remain constant
- Dropping common food items from its diet
- Dropping high energy food from its diet
Leaving a patch of prey when encounter rates get low
Which of the following gerbils is most likely to have the strongest behavioural response (measured by the amount of time hiding rather than foraging) in response to the risk of predation from a fox?
- A gerbil that is good at outrunning foxes and will die of old age in 5 years (unless it is killed by a fox)
- A gerbil that is good at outrunning foxes and will die of old age in 1 year (unless it is killed by a fox)
- A gerbil that is not good at out running foxes and will die of of old age in 5 years (unless it is killed by a fox)
- A gerbil that is not good at outrunning foxes and will die of old age in 1 year (unless it is killed by a fox)
A gerbil that is not good at outrunning foxes and will die of old age in 5 years (unless it is killed by a fox)
Individual differences matter. A gerbil that is less able to deal with a predator would tend to have a strong response to that risk, because the risk is functionally greater.
A gerbil that can expect to live a long time has higher likely future fitness, as such, it will respond more strongly because it has “more to lose”. Essentially the value or resources is low relative to the value of safety. Likewise, the gerbil that will soon die of natural causes would benefit from harvesting resources to invest in reproduction immediately, essentially the value of resources is high relative to the value of safety.
True or false: Game Theory typically considers games with only one player
False. Game theory applies to situations with more than one player. The optimal decisions depend on what other players are doing.
True or false: An evolutionary stable strategy (ESS) is one that, when adopted by an entire population, cannot be invaded by an alternative strategy
True. Think about the prisoners dilemma. The ESS of “testify” could not be invaded.
When considering ecological models one must keep in mind
- That models typically do not have built in assumptions
- That models typically incorporate 90% of ecological factors
- That ecological interactions in nature are typically more complex than a given model provides for
- None of the above
That ecological interactions in nature are typically more complex than a given model provides for.
Think about the examples of optimal foraging and game theory from lecture. Both had a number of assumptions.
True or false: The tragedy of the commons is a pervasive phenomenon that cannot be overcome
False. Think about the end of lecture. Elinor Ostrom (the first woman to win the Nobel Prize in Economics) document examples of communities overcoming the tragedy of the commons.
