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High Parasitoids Parasite Intimacy Low Predator Grazer Low Lethality High
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There are 4 general categories
“True” predators Herbivores Grazers Browsers Granivores Frugivores Parasites Parasitoids
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“True” predators Kill their victims shortly after attack, most of the time (some get away!) Consume many prey items in a lifetime
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Herbivores Attack many prey items in a lifetime
Consume only a bit of the victim Do not usually kill prey in the short term (but may do so in the long term) Grazer Mainly eats leafy material Browser Mainly eats woody material Granivore Mainly eats seeds Frugivore Mainly eats fruits
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Native vs. Domestic Grazing Systems
Differences in selection & competition among species Grass Forbs & Browse Cacti Bison 100% Horses Cattle 90% 10% Sheep 40% 60% Pronghorn 6% 83% 11%
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Parasites Parasitoids Consume part of their prey
Do not usually kill their prey Attack one or very few prey items in their lifetime Parasitoids
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Parasites Parasitoids
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Predation is important because:
Evolutionary arms race (example of coevolution)
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Predation is important because:
It may restrict the distribution of, or reduce the abundance of the prey species. Predation, along with competition, is a major type of interaction that can influence the organization of communities. Predation is a major selective force, and many adaptations of organisms have their explanation in predator-prey coevolution. Evolutionary arms race Predation drives the movement of energy and nutrients in ecosystems.
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Optimality theory we expect that natural selection yields efficient, economic animals; maximizing benefits or minimizing costs, thus maximizing net energy/time (e/t)
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Predation Rate of increase of prey population dH/dt = rH
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Predation Rate of increase of prey population a' = capture coefficient
dH/dt = rH Predators eat prey dH/dt = rH-a'HP a' = capture coefficient H = Prey pop size P = Predator pop size
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Predation Rate of increase of predator populations dP/dt = -qP
If only predators exist, no prey, so predators die
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Predation Rate of increase of predator populations
dP/dt = -qP If only predators exist, no prey, so predators die dP/dt = fa’HP-qP f = is a predation constant Predator’s efficiency at turning food into predator offspring. a' = capture coefficient q = mortality rate
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Predation Equilibrium population sizes Predator Prey dP/dt = fa’HP -qP
fa’H= q H= q/fa’ Prey dH/dt = rH-a’HP 0= rH-a’HP rH= a’HP r = a’P P = r/ a’
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Predation Graphical Equilibrium
Prey (H) equilibrium (dH/dt=0) is determined by predator population size. If the predator population size is large the prey population will go extinct If the predator population is small the prey population size increases Predator Pop size dH/dt =0 r/a’ Prey pop size
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Predation Graphical Equilibrium q/fa’ dP/dt =0
Predator (P) equilibrium (dP/dt=0) is determined by prey population size. If the prey population size is large the predator population will increase If the prey population is small the predator population goes extinct Predator Pop size q/fa’ Prey pop size
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Predation Predator-Prey interaction q/fa’ dP/dt =0
The stable dynamic of predators and prey is a cycle Predator Pop size r/a’ q/fa’ Prey pop size
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Rosenzweig & MacArthur (1963)
Three possible outcomes of interactions
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i) Prey iscoline N Predator density Prey increase Prey density K N
2 Predator density Prey increase Prey density K N ii) Predator iscoline 1 1 N 2 K 2 Predator decreases Predator increases Predator density Prey density N 1
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Predator-Prey Models Superimpose prey and predator isoclines
One stable point emerges: the intersection of the lines Three general cases Inefficient predators require high densities of prey Damped oscillations Prey isocline Predator a) Predator Density Prey Density Time Population density
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Predator-Prey Models Three general cases (cont.)
A moderately efficient predator leads to stable oscillations of predator and prey populations Stable oscillations Population density Predator equilibrium density b) Prey Density Predator Density Time
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Predator-Prey Models Three general cases (cont.)
A highly efficient predator can exploit a prey nearly down to its limiting rareness Increasing oscillations Predator density Prey Density Time Population density
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All these models make a series of simplifying assumptions
A homogenous world in which there are no refuges for the prey or different habitats. There is one predator species eating one prey species and there are no other species involved in the dynamics of these two populations Relaxing these assumptions leads to more complex, but more realistic models. All predators respond to prey in the same fashion regardless of density Functional Response
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Conclusions form field studies
There is not a clear relationship between predator abundance and prey population size. In some, but not all cases, the abundance of predators does influence the abundance of their prey in field populations.
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What makes predators effective in controlling their prey?
Foraging efficieny Within a patch, the searching efficiency of a predator becomes crucial to its success. But searching efficiency varies with abiotic factors and can also decrease at high predator densities because of interference of other predators.
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What makes predators effective in controlling their prey?
Some systems are Bottom-up regulate. Some systems are Top-down regulated.
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Predation Response of predator to prey density Numerical Aggregative
Functional
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Types of functional responses
Limited by handling time The rate of capture by predator Alters Behavior Type I Type II Type III C. S. Holling (1930–)
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Types of functional responses
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Keystone predator Bob Paine at University of Washington
mussel is a competitive dominant in this system
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Other examples of keystone predators
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The effects of herbivory
Individual plants are affected in the following areas plant defenses plant compensation plant growth plant fecundity
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Chemicals Defenses Qualitative Defenses: Quantitative Defenses:
Prevent digestion as they accumulate in the gut. Usually found in large quantities in the plant parts that are eaten. Most of these compounds are “Carbon Rich” Common defense of plants growing in nutrient poor soils (conifers). Qualitative Defenses: Usually toxic in small quantities. Found in relatively small amounts in the portion of plants that is eaten (leaves). These compounds are “Nitrogen Rich” and therefor expensive to produce by the plant. More common in plants growing on nutrient rich soils.
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