Major force controlling distribution and abundance of both prey and predator.

Question:

What are the effects of predators?

How do the effects vary?

Why do they vary?

Predation: consumption of one organism by another where the prey is alive when the predator attacks.

1. true predators - kill prey immediately, usually consume entire prey, but not always.
2. grazers - attack large numbers of prey over lifetime but often remove parts of prey of each individual rather than the whole – harmful but not lethal.
3. parasites - similar to grazers except usually intimately associated with or few prey (parasites can cause biological death by castration (often parasite reside in the reproductive organs or gonads and consume eggs, etc.
4. parasitoids - like parasites in the sense of having few prey. Always kill prey so more like true predator. Limited mostly to Diptera (flies) and Hymenoptera (bees and wasps). Comprise 25% of all known species.

Can alter allocation of energies, e.g. if defoliated, roots ® leaves. If roots destroyed, leaves ® roots,

Can affect timing and amount of reproduction ® delayed with few seeds,

Can affect transmission of disease – grazers are often vectors of plant viruses.

Production of defense compounds or structures (spines, etc.). True for most sessile organisms such as plants and colonial invertebrates.

e.g. predation induced responses e.g. bryozoans attacked by predators produce defensive spines.

or, toxins induced in may plants serve to thwart grazers.

Predator swamping - mast years in plants, production of very large numbers of offspring that swamp predators - limited ability to consume prey.

Predator numbers limited – cannot consume all prey.

Effects of predation on prey populations not always negative to population.

• elimination of individuals that otherwise are least likely to survive (Wolf/Caribou interaction)
• reduced intraspecific competition (compensatory effects by prey)

Understanding the ecology of predators can help us to understand the evolution of their behavior when we examine it in the context of the prey environment.

This line of investigation leads to models predicting "Optimal foraging"

Secondly, investigating the behavior of predators can tell us how predators influence the population dynamics of both predator and prey.

For example, if both predator and prey are subject to density-dependent increases in mortality or decreases in fecundity, this will tend to regulate population size leading to stability.

If subject to inverse density-dependence, i.e. mortality decreases or fecundity increases with density of predator or prey it can lead to destablizing effects.

Point – behavior of predators can have significant effects beyond individuals.

Most predators have broad diets, particularly grazers and true predators (not so for parasites and parasitoids).

1. energetically most valuable
2. nutritionally necessary to have mixed or balance diet

While the preferences of many species are fixed, others frequently switch prey item.

i.e. feeds on more on item when common than rare.

Switching usually based on:

1. different prey in different habitats – habitat occupied by predator is which is most profitable.
2. Predators become more efficient or successful with more abundant prey.
3. Development of specific "search image" occurs over time.

Examples: Figs.1, 2 and 3

How do predators response to change in prey density?

1. numerical response - increase numbers
2. functional response – changes in consumption rates - 3 forms.

Type 1 Fig.4

- curve asymptote where ingestion rate is limitation

- linear response (handling time=0)

example: herbivores feeding on passive particles such as Daphnia feeding on yeast using filtering apparatus.

Handling time = pursuing, subduing, consuming

Type 2 (most common)

Feeding rate --Fig.5

Food density

Here: gradual increase in consumption rate as prey increase then decelerates– overall handling time is constant as prey density increases, but takes up more time consuming prey.

Feeding

Rate Fig 6

Food density

Upper part of curve similar to type 2

-increase in prey density leads either

to > linear response in efficiency and/or

decrease in handling time (kill and consume only the most profitable parts of prey.

For type 1 and 2 – rate of consumption decelerates as

prey density increases Þ inverse density-dependence

For type 3 – rate of consumption accelerates as prey density increases at intermediate prey densities Þ density-dependence

Implications – in general a functional response alone will not control prey density, requires other factors such as numerical response.

That is to say that predators are not likely to limit the prey since feeding rate will asymptote, unless intra- or interspecific competition is limiting prey or something other than predation.

Let’s consider the factors that influence predator behavior in more detail – what should a predator include in its diet?

Given that a predator consumes a narrower range of resources than that which is available, we can ask what are the factors that determine foraging behavior?

Assumptions of the theory:

1. patterns of foraging observed are due to natural selection.
2. High fitness is obtained by achieving a high rate of consumption.
3. Observations to evaluate theory are made in the context of that which evolution has acted on.

For assumption 2 – degree of risk associated with foraging may constrain fitness payoff (e.g. predators who themselves are prey upon).

Charnov (1976) proposed model to predict optimal foraging patterns of predators. That is what should be the breadth of the diet.

Should the diet be specialized or broad?

Notion is that if predator is specialized the prey will be of high value but may require more time to search and find (high cost) vs. generalized which cost less to find but not all items will be of high value.

So, given that a predator has a certain number of profitable items in the diet, should the predator expand its diet to include the next most profitable item?

For a given diet the average energy input per item can be expressed as: avg.E / avg h

Next most profitable item is defined as E_i/h_i where,

E_i =energy content of item (gain) currency=calories

h_i =handling time (cost)

So, is the new diet more favorable than the old diet?

Need to know energy input the predator would forego

to eat the new item i, because if the predator chose not to eat the item it would have to continue to search for another acceptable item.

The additional search time would be the average search time for the old diet item which is designated as avg. s. Thus the energetic input for this item is given by:

avg E/(avg s + h)

now we have the energetic input for both the new and old diet. Because the optimal diet maximizes the energetic input the diet should expand to include the new item if:

Ei/hi > avg E/(avg s + h),

The relative values of search and handling time are crucial.

In the simplest case say –if handling times for a few items are very small, Ei/hi will be large and will select for a generalist diet. (e.g. gleaning birds)

In contrast – if search time is small, avg E/(avg s + h) become approximately = avg E/avg s. Only new items with smaller handling times will be added to diet. Select for specialized diet. (e.g. lions )

Simply put, if handling a prey item does not take too long, predator might as well eat it – gain for little cost.

If search time is small but handling time determines when a prey is consumed, then predator will be more specialized.

e.g. Hawk (large) vs kestral (small) , hunting for variety of small mammals in field.

For kestral difficult prey are rabbit or squirrel – high handling time. Can only feed on mice.

Search time the same for both predators – result kestral specialist, hawk generalist.

1) Specialization in more productive environments (avg s shorter because prey is more abundant), conversely in lower productive environments predators more generalists (avg s is large, prey less abundant).

2)Abundance of unprofitable prey type is irrevalent. To incorporate ith item depends on profitability of the item, E_i/h_i vs avg E/ ave h. Depends on search time for items in the diet already minus avg s and thus their abundance, but not dependent of s_i.

Summary: predators specialize when profitable prey are common and/or differences in profitability are great; and they should be indiscriminate when profitable prey are rare and/or differences in profitability amongst prey are slight.

But,

They should ignore insufficiently profitable prey irrespective of their abundance.

Examples Figs 7,8