Predation Vine snake eating a young iguana, Panama Vertebrate predators: lions and jaguars 1
Most predators are insects Parasitoids lay eggs in their hosts, and the larvae consume the host from the inside, eventually killing it (wasp laying egg in aphid) 2
Parasitoids lay eggs in their hosts, and the larvae consume the host from the inside, eventually killing it (flies emerging from caterpillar) Questions concerning the role of predation in population ecology and evolution: A. Do predators limit prey populations below their carrying capacity? (and does this in turn affect competitive outcomes?) 3
Questions concerning the role of predation in population ecology and evolution: A. Do predators limit prey populations below their carrying capacity? (and does this in turn affect competitive outcomes?) e.g. starfish B. To what extent do predators stabilize or cause fluctuations in the prey population sizes? C. Do predators over-eat their prey? D. Do predators influence the evolution of prey characteristics, and vice versa? Mathematical models of Lotka (1925) and Volterra (1926) for the population dynamics of predator-prey interactions PREY (N 1 ) PREDATOR (N 2 ) ALONE: dn 1 /dt = rn 1 dn 2 /dt = -d 2 N 2 (d = death rate w/o prey) TOGETHER: # eaten will depend on the # of encounters and the efficiency of catching prey dn 1 /dt = r 1 N 1 - cn 1 N 2 dn 2 /dt = -d 2 N 2 + b(cn 1 N 2 ) c = efficiency of each predator in catching prey (or ability of prey to escape) b = efficiency of converting captured prey into predators (approx 10%) bc = efficiency of catching and converting prey into predators 4
PREY (N 1 ) PREDATOR (N 2 ) ALONE: dn 1 /dt = rn 1 dn 2 /dt = -d 2 N 2 (d = death rate w/o prey) TOGETHER: dn 1 /dt = r 1 N 1 - cn 1 N 2 dn 2 /dt = -d 2 N 2 + b(cn 1 N 2 ) AT EQUILIBRIUM: (ie dn 1 /dt=0) (ie dn 2 /dt=0) then cn 1 N 2 = r N 1 b(cn 1 N 2 ) = d 2 N 2 and N 2 = r 1 /c N 1 = d 2 /bc + + Prey=yellow Predator= red dn 2 /dt=0 dn 1 /dt=0 Ignore curving lines 5
1. oscillations of predator/prey populations are constant through time (figure c) 2. amplitude (magnitude of population changes) and period (duration of a single cycle) depend on starting population numbers Objections to model 1. there are no density-dependent restrictions on population growth 2. no tendency for populations to return to a stable equilibrium (ie extinction of populations more likely) 6
More realistic graphical models by Rosenzweig and MacArthur (1963) A. Changing the prey isoclines to be more realistic - adding density dependence 1. at high prey densities, the prey population is limited by resources ie it reaches its carrying capacity, K, so that no predators are necessary for zero growth (dn/dt=o at K). 2. at low prey densities, prey isocline could bend down if small populations of prey have a lower realized growth rate ie prey productivity is so low, it takes few predators to keep them at zero populations growth (dn/dt=o) Does changing the shape of the prey isocline affect population dynamics? predator numbers r 1 /c prey numbers K dn/dt=o for prey dn/dt=o for predator predator numbers x dn/dt=o for prey d/bc prey numbers K 7
See board for population consequences (increasing oscillations until extinction of predator or prey) dn/dt=o for predator predator numbers x dn/dt=o for prey d/bc prey numbers K See board for population consequences (decreasing oscillations converging on stable equilibrium) dn/dt=o for predator predator numbers x dn/dt=o for prey prey numbers d/bc K 8
If predator isocline intersects prey isocline at the peak, then stable cycles results. If it intersects the prey isocline to the left of the peak, then there are increasing oscillations until one or both populations go extinct. If it intersects the prey isocline to the right of the peak, then oscillations are dampened and an equilibrium is reached. Moving the predator isocline to the right increases the stability of the system (as bc gets smaller = predators are less efficient at catching prey). dn/dt=o for predator predator numbers x dn/dt=o for prey prey numbers d/bc K Biological control of insect pests goals: reduce pest populations and maintain stable predator populations methods: have inefficient predators poly-cultures of plants make it harder for predators to find prey dn/dt=o for predator predator numbers x dn/dt=o for prey prey numbers d/bc K 9
Predation II: Stability and Productivity 1. Factors which enhance stability of predator/prey interactions A. Density dependence population regulation of predator and of prey (humped prey isocline) B. Less efficient predators (small value for bc) e.g. predators that need large populations of prey to maintain their population (isocline to right of hump) C. Evolution of prey characteristics that enhance their ability to avoid predation (isocline to right of hump) predator numbers x dn/dt=o for predator dn/dt=o for prey prey numbers d/bc K Evolution of prey characteristics that enhance their ability to avoid predation Crypsis: anatomical (caterpillars in the Neotropics) 10
Evolution of prey characteristics that enhance their ability to avoid predation Crypsis: anatomical (tapirs in the Neotropics) Evolution of prey characteristics that enhance their ability to avoid predation Crypsis: behavioral, moths, bats and jamming radar 11
Evolution of prey characteristics that enhance their ability to avoid predation Physical defense: sea urchins Evolution of prey characteristics that enhance their ability to avoid predation Chemical defense: marine sea slugs (nudibranchs) 12
Evolution of prey characteristics that enhance their ability to avoid predation Chemical defense: monarch butterflies sequester milk weed cardiac glycosides Evolution of prey characteristics that enhance their ability to avoid predation Social behavior: group living in chimpanzees (schooling in fish, flocking in birds) 13
Evolution of prey characteristics that enhance their ability to avoid predation Mutualisms: wasps protect ants against anteaters, and ants protect wasps against army ants Wasp nest Ant nests Army ants 14