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biol 275 species int

types of Species interactions predation, herbivory, parasitism, competition, mutualism, commensalism
+/- predation, herbivory, parasitism
-/-- competition
+/+ mutualism
+/0 commensalism
Effects of predators predators can limit the size of prey populations. "top-down" control
Parasitoids Live within tissues of host Feed on and kill host as larvae
Herbivores Can have substantial effects on plant communities
Population cycles Sometimes predators exhibit cycles that correspond with their prey Snowshoe hares and lynx populations from trapping pelts (Hudson’s Bay Co.)
Experimental evidence for predator-prey cycles Huffaker’s predator-prey experiment with mites: Habitat Design: Food sources (oranges) spaced far apart Dispersal barriers (Vaseline) in between Wooden pegs aid prey dispersal
Modelling coupled predator-prey dynamics Lotka-Volterra models developed independently for predator-prey populations in 1925-6 One of the earliest efforts in mathematical ecology
Prey population model see equation
N prey population
r intrinsic growth rate
P predator population
c capture efficiency per encounter
When is prey population stable (𝑑𝑁/𝑑𝑡 = 0)? P=r/c (zero growth isocline)
zero growth isocline number of predators where prey pop is stable)
prey pop model N ↑ when P is below isocline N ↓ when P is above isocline Visualize on a plot of N vs. P (phase space plot)
Predator population model see equation
Captured prey (=c×N×P) increase predator population growth
a specifies how much each captured prey increases predator survival and birth rates (assimilation efficiency)
m is rate of pop decrease when zero prey (mortality rate)
When is predator population stable (𝑑𝑃/𝑑𝑡 = 0)? N=m/ac (zero growth isocline for predators)
zero growth isocline for predators (number of prey where predator pop is stable)
Predator population model P ↑ when N is to the right of isocline P ↓ when N is to the left of isocline
Two-toed sloths typically feed on plant matter, but occasionally feed on lizards. A particular forest contains 10 sloths and 50 lizards. Is each population increasing or decreasing? N=60 (sloths increasing) P=5 (lizards decrease)
Joint population trajectory Together, both isoclines show how populations affect each other Counterclockwise cycle around joint equilibrium
Predator-prey cycles Populations will cycle together over time Predator peak occurs one quarter phase after prey peak
Assumptions of Lotka-Volterra model 1. No prey density-dependence 2. Only one prey species 3. Predator declines exponentially without prey 4. Encounters happen at random 5. Predators cannot be satiated 6. No time lags (Very rare that all are met)
Functional response to prey density Relationship between prey density and an individual predator’s rate of prey consumption each predator consumes more prey when are abundant
3 types (shapes) of functional response: Type I, II, III
Type I functional response: Linear increase in consumption rate (until satiation)
Type II functional response: Consumption rate is rapid at first, levels off as prey density increases Results from handling time
handling time time needed to process (eat ) each prey
Type III functional response: Consumption rate is initially slow, accelerates, then levels off as prey density increases
numerical response Increase in predator density when prey density is high Can occur through population growth or immigration
what is competition? Individuals reduce one another’s fitness through pre-emptive use of shared limiting resources
reduced fitness lower survival or reproduction, lower per capita population growth
examples of shared limited resources for competition sunlight, water, nutrients, food, space
Density-dependent pop growth is an example of competition acting within a species
intraspecific competition competition acting within a species
Competition among species Species that use the same resources may also compete with one another (inter specific competition) -reduces pop growth of both sp -one sp may die out as a result
Competitive Exclusion Principle If two species are limited by same resource, the better competitor will drive the other to extinction “complete competitors cannot co-exist” (lab experiments with Paramecium)
Liebig’s law of the minimum Most limiting resource prevents further population growth Inadequate supply of a limiting resource results in population decline
Resource limitation example Diatoms require silica to grow - Astrionella pop grows until silica is reduced to 1 μM - Synedra pop grows until silica is reduced to 0.4 μM
What will happen when both species grow together? (resource limitation) Silica is reduced to 0.4 μM by Synedra Below minimum for Asterionella Asterionella cannot persist
Niche differentiation Species may specialize on different resources (habitats) Distantly-related species may still compete for resources Lower resource overlap
example of niche differentiation -white bedstraw is a better competitor on alkaline soils -heath bedstraw is a better competitor on acidic soils
what does niche differentiation mean? intraspecific comp > interspecific comp Conspecific individuals have a stronger negative effect than heterospecific individuals -sp can co-exist when they limit themselves more than they limit eachother
Lotka-Volterra competition models We can model interspecific competition as an extension of logistic growth Sp. 2 is a competitor that reduces the pop growth of Sp. 1 (as N2 increases, (𝑑𝑁_1)/𝑑𝑡 decreases)
alpha competition coefficient for Sp. 2’s effect on Sp. 1
Interpretation of competition coefficient α = 0 Sp. 2 is not a competitor to Sp. 1
Interpretation of competition coefficient 0 < α < 1 Sp. 1 has a greater effect on itself than Sp. 2 has on it (intraspecific comp > interspecific comp)
Interpretation of competition coefficient α > 1 Sp. 2 has a stronger effect on Sp. 1 than Sp. 1 has on itself (interspecific comp > intraspecific comp)
zero population growth isocline is: 𝑁_1=𝐾_1−𝛼𝑁_2 (K1 is reduced)
Zero population growth isocline for N1 Phase-space plot of N1 vs. N2 (like the predator-prey models) When N2 = 0, then N1 = K1 When N1 = 0, then N2 = K1/α N1 ↑ when it is left of the isocline, ↓ when to the right
Sp2 Lotka-Volterra competition models : β? a competition coefficient for Sp. 1’s effect on Sp. 2
Sp. 2’s zero growth isocline is: 𝑁_2=𝐾_2−𝛽𝑁_1
Zero population growth isocline for N2 When N1 = 0, then N2 = K2 When N2 = 0, then N1 = K2/β N2 ↑ when it is below the isocline and ↓ when above it
Graphical analysis of competition Overlay isoclines for both species Yellow area: both pops ↑ Green area: both pops ↓ Blue area: one pop ↑, other pop ↓
How to identify the winner using isoclines? Isocline with K1 is for Sp. 1 (left-right movement) Isocline with K2 is for Sp. 2 (up-down movement) Once in blue region, will move towards N1=K1, N2=0
graphical analysis slide 52 When Sp. 2 isocline is on top, moves towards N2=K2, N1=0
When isoclines do not cross, one species will always outcompete the other
when isoclines cross and When K1 and K2 are furthest points, winner depends on initial pop sizes (foudner control)
when isoclines cross and When K1 and K2 are innermost points both species reach stable co-existence. -Intrasp. comp > Intersp. comp (α × β < 1)
Species must each _____ themselves to co-exist limit
Ferrets: K1=10, α=0.8 Owls: K2=20, β=2.1 What is the outcome of competition if N1=2 (ferrets), N2=15 (owls)? founder effect. winners owl
Species can co-exist if they are limited by different ___ resources. they compete more strongly with themselves than each other
Competitive outcomes depend on abiotic conditions example Poli’s barnacle can survive dessication in upper intertidal zone In lower intertidal zone, it is outcompeted by Rock barnacles Rock barnacles cannot survive in upper intertidal zone
types of competition exploitative, interference, apparent
Exploitative competition: reduce shared resource to level that limits other species
Interference competition: competitors do not consume resources, but defend them from others
Apparent competition: Two species have a negative effect on each other because of a common enemy
example interference competition long legged ants plug holes of harvester ants because they compete for seed collecting
apparent competition exmaple Caribou population in northeast AB declined in early 2000s At same time, deer increased 17-fold Deer attracted wolves, which preyed on caribou as well
Created by: abc273
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