Predator-prey

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Ecology

IV. Community Ecology (Who's who and who's not)

B. Predation (Consumer-Resource Interactions)

Models of Predation

Lotka and Volterra proposed independently:

dN₁/ dt = r₁N₁ - PN₁N₂ where

N₁= density (or population size) of the prey

P = coefficient of predation

N₁N₂= probility of encounter between predator and prey

dN₂/ dt = P₂N₁N₂ - d₂N₂
N₂= density (or population size) of the predator

P₂= coefficient of expressing effectiveness of the predator

d₂= mortality rate of the predator

What do these equations assume about factors that affect growth rate of prey (i.e. do they not consider)?
What do these equations assume about factors that affect reproductive rate of predators (i.e. do they not consider)?

Other models have also been developed.

One outcome- Ossillation of predator and prey (when prey high and predator low, predator increases.....)

Is this modeling an open system or a closed system? Is this equilibrium or non-equilibrium coexistence? What is the 'feedback' that maintains predator-prey coexistence? Are cyclic population changes observed in nature?

Experiments in Predation

Gause, using the ciliates Paramecium and Didinium, could not create a real system that would mimic predator-prey cycles predicted by Lotka-Volterra.

However, the system could be maintained either by occasional re-introduction of prey or by providing a refuge in form of bottom sediment. In other words, an open system required to maintain predator-prey.

How important is predation in ecological systems?

Ecological examples:

Effects on density:

Effects on morphology of prey:
Brooks and Dodson - size selective fish eliminate large-bodied zooplankton species.

Effects on evolution of predators and their prey:

The evolutionary importance of predation is clearly demonstrated in the sophisticated predator and prey adaptions. Co-evolution is very intense in these interactions because improved adaptation of one species will directly affect other, resulting in adaptations that influence the ability of predators to detect prey.

Examples of predator-prey adaptations in coloration:

Camouflage


	disruptive coloration	countershading	bioluminescence

Chemical interactions such as toxic prey, and responses of prey to substances released by predators.

For example, how might length of spines change in the prey rotifer Brachionus in the presence of fitrates from the predacous rotifer Asplanchna?

'Swamping' predators including synchonizing the timing of reproduction and movement.

If competition is prevalent, then why don't predators drive prey extinct?
Predatory-Prey Coexistence

Based on the theory of natural selection, would predators evolve strategies to 'manage' prey populations in such a way as to prevent their elimination?

Spatial refuges Coexistence was achieved in Gause's ciliates by adding sediment that acted as a prey refuge.

In another experimental system using flour beetle, 1 mm capillary tubing provided refuge for prey resulting in coexistence (Crombie, 1945).

Temporal-spatial refuges

e.g. Huffaker (1958) used 2 species of mites (predator Typhlodromus occidentalis and prey Eotetranychus sexmaculatus) on oranges and added barriers to dispersal (rubber balls, paper, vasoline).

In a moderately complex environment (20 orange surfaces) predator drove prey extinct and predators then went extinct.

In a complex environment (252-orange systen with only 1/20 exposed of orange surface exposed, oscillations developed. Prey were able to colonize oranges in a hop-skip-and-jump fashion and keep one step ahead of the predator.

Non-equilibrium models can produce persistence and coexistence over long periods of time.
(Non-equilibrium systems have no mathematically stable points, e.g. predator existence always leads to extinction of prey and itself on a small-scale. In contrast, models like the Lotka-Volterra predation model where predator and prey have negative feedbacks that lead to coexistance)
What is the 'feedback' that maintains predator-prey coexistence?

Coexistence in a non-equilibrium model can occur because "for even very simple components in reasonably small numbers, high levels of connectedness lead to astronomically long delays..."

For example, given:

100 light bulbs - on or off
For each bulb in a second, the probability of turning from on to off is 0.5 (local extinction)
For each bulb in a second, the probability of turning from off to on is 0.5 only if a connected bulb is on (will not go extinct if migration occurs), but is 0.0 if connected bulbs are off

Where bulbs are analogous to patches of habitat and a bulb going off is analogous to extinction of a species from that patch.

Starting with all bulbs on:

If no bulbs are connected (no migration), all are off in 1 s (global extinction)

If all bulbs are connected (migration to any patch possible, all off in 10²² years (universe is only 10¹⁰years old).

If each bulb connected to only 10 other bulbs, all off in 17 minutes.

Caswell (1978) develops a such a model for predator and 3 prey species that compete - no species went extinct even after 1000 generations.

Bottom line: systems that are connected to other systems can maintain proccess longer even though there are no processes that should lead to persistence. Perhaps by conceptualizing systems as closed (percieved to be easier) ecologists spend to much time developing mathematical situations where equilibriums are necessary. What are the implications of this for the study of landscape ecology?

Multiple prey species

Key to understanding: Predators do not always respond to prey with same intensity

Prey taken is a function of prey density (Holling 1959)

Numerical response - predator population size increases (why?) as prey population increases.

Functional response - the number of prey taken per existing predator (i.e. number of predators do not change) increase. At some point, levels off because individuals become satiated.

Type I - linear

Type II - rapid increase and slower increase as satiation approched

Type III - slow increase at low prey, then rapid until some point. Why?

Which of the above predator responses to prey density could explain predator prey coexistence in a system with multiple prey species, and how?

What factors should determine when (at what prey density) should a preadator switch?

What happens if one species of prey is removed from a system where predator switchs to different prey depending on their denisity?

Other types of "predation"

By definition, are the following 'predator-prey' interactions?

Cannibalism - is not common and is often associated with harsh conditions.

Cannibalism could also be considered as interference intraspecific competition with the added benefit of added nutrition. So if such a behavior eliminates competitors and delivers high-quality prey, why isn't cannibalism more common in nature?

parasite and host: What are the differences between parasite - host interactions and 'typical' predator - prey interactions? How is the "habitat" of parasites different than that of a 'typical' predator and what then are the challenges for a parasite?

Some generalizations concerning parasites:

Highly sophisticated despite tendency to think of parasites as degenerate. In fact, parasites display some of the most sophisticated and specialized adaptation. Life cycles are often complex involving several vectors that may include reservoir and intermediate hosts
Highly specific and important to ecology of hosts "An average species has 16 parasites at home, but only about three of those make it over. After it arrives, it picks up another four, but on average, it has less,"
Overlooked in many fields of biology and medicine, but are extremely diverse (may outnumber the number of free-living species 4 to 1).

parasitoid and host: How does a parasitoid differ from a parasite?

Plant and herbivore: Is this more like parasite - host interactions or 'typical' predator - prey interactions?

Compared to carnivore foraging strategies, nutritional value is often more important in herbivore food choice relative to choice based on availability. Why?

Choice both between plant species and within a plant (young leaves tend to be more nutritious).

Do herbivore-plant interactions always fit the general definition of predator-prey interaction?

Moderate grazing can stimulate biomass production, especially in grasses where new tissue from the meristem is close to the ground and not grazed.

Why is fruit so tasty, nutritious, and brightly advertized?

Prey (plant) defense can also be quite sophisticated:

Trees can transfer chemical deterents to damaged leaves (one insect predator counters this by ringing an area to prevent transfer)

Individual trees of the same species can differ in defensive chemicals making it difficult for scale insect pest to evolve consistently effective methods of attack (Edmunds and Alstad 1978 from Connell 1980).

Actual act of heribivory and predation may be relatively rare events. How can these be studied? (And how do we study organism that live in environments that are less hospitable to humans?)

NOAA's Hydrolab habitat

Bottom line on species interactions: There are lots of possible interactions among multiple species with a community and in most communities there are many species. Given this complexity, are there generalizations that can be made in the study of whole communities?