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Exploitation
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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
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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 Browser Granivore Frugivore
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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
which straddle the parasite and true predator categories - they lay eggs inside their host which they eventually kill
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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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Alfred Lotka ( ) Vito Volterra ( )
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Predation start 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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Do these models apply to natural populations?
Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey
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Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey Assumptions?
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Rosenzweig & MacArthur (1963)
Three possible outcomes of interactions The oscillations are stable (classical oscillations of Lotka-Volterra equations). The oscillations are damped (convergent oscillation). The oscillations are divergent and can lead to extinction.
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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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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 Eat all you want Eat all you can Eat all you can, if you can find it! C. S. Holling (1930–)
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Types of functional responses
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Not all predators are created equal
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Keystone predator Bob Paine at University of Washington
mussel is a competitive dominant in this system
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Keystone predator absent
Figure: 49.9 right Caption: When Pisaster is excluded, the same habitats are dominated by beds of California mussels.
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Keystone predator present
Figure: 49.9 left Caption: When the sea star Pisaster ochraceous is present, rocky intertidal habitats have a large diversity of species with varied forms.
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Other examples of keystone predators
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Keystone predator absent
Figure: right Caption: When sea otters are not present, herbivory by sea urchins causes drastic reductions in the size and frequency of kelp forests.
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Keystone predator present
Figure: left Caption: When sea otters are present, kelp forests are common in the nearshore habitats of western North America.
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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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