Field experiments on competition. Field experiments on competition. Field experiments on competition

Transcription

1 INTERACTIONS BETWEEN SPECIES Type of interaction species 1 species 2 competition consumer-resource (pred, herb, para) mutualism detritivore-detritus (food is dead) Field experiments on competition Example 1: competitive exclusion (Tansley) Two species of bedstraw plants: Species A grows in acidic soils Species B grows in basic soils Common garden experiments: Acidic soil Basic soil A A B B Field experiments on competition Important points for Example 1: 1. Presence or absence (in nature) of each species determined by competition 2. Environmental conditions affected the outcome of competition 3. Competition is a population-level process Field experiments on competition Example 2: Stable coexistence Two species of salamanders in the Appalachian mountains (species C and D) They co-occur and overlap in diet Do they also compete with each other? Removal experiments: Control Removal Treatment 1 Removal Treatment 2 +C, +D +C, -D -C, +D

2 Field experiments on competition Important points about Example 2: 1. Competition is a population-level process - 2. Competition can be asymmetric - 3. Even if two species are competing they can both persist Recipe for Lotka-Volterra competition models 1. Start with 2 species, each with their own carrying capacity (i.e. logistic models are used for each species) 2. Species compete (use part of each other s K) 3. Competition coefficients indicate how much each takes from the other s K 4. K for each species is now reached via a combination of the density of both species Recipe for Lotka-Volterra competition models 5. Population stability for each species can be shown graphically in terms of an isocline (line of zero population growth) 6. Determine the outcome of competition by superimposing the isocline of each species coexistence or competitive exclusion 7. Can identify conditions where both species can stabilize at zero population growth and not drive each other extinct (coexistence) 2. Species compete Frame represents K for species 1 Dark squares are species 1 White squares are species 2 Once frame is full the carrying capacity for species 1 has been used up Species 2 is taking some of K1 The more individuals of species 2, the less room for species 1 Species 2 can affect the population size, growth rate, and equilibrium of species 1 How can we represent this mathematically? Need to convert individuals of species 2 to species 1 Competition coefficients enable this

3 3. Competition coefficients the effect of species 2 on species 1 Each individual of species 2 consumes 4x the resources that an individual of species 1 does with respect to K1 Each individual of species 2 is worth 4x species 1 in terms of their effect on the population growth rate of species 1 Effect of species 2 on species 1 = 4! = 4 3. Competition coefficients the effect of species 2 on species 1 If K 1 = 100, we can fill it up with 100 individuals of species 1 or 25 of species 2 K 1 can be reached with many different combinations of N1 and!n 2 K 1 = +! N 2 3. Competition coefficients the effect of species 2 on species 1 Represent the impact of species 2 on species1 Measures relative importance of intra- vs. interspecific competition: 4. Adding species 2 to species 1 logistic equation Without competition: d /dt = r 1 (1- /K 1 ) Need to include species 2 since it also uses up K 1 Replace with: N1 +!N 2 With competition: d /dt = r 1 (1-[ +!N 2 ]/K 1 ) If! = 1 If! > 1 If! < 1 Intraspecific Interspecific competition Interspecific Intraspecific competition Intraspecific Interspecific competition

4 5. Plotting isoclines on graph species 1 N 2 N 2 = - /! + K 1 When there are no indivs of sp 2, sp 1 is at its own K Without any indivs of sp 1, sp 2 will fill entire K 1 with indivs Can have a stable pop of species 1 with many different combinations of numbers of sp2 and sp1 i.e. where K is saturated This is called the isocline all possible combinations of and N 2 where K 1 is saturated species 1 population is stable 6. Determining the outcome of competition both species combined - In the real world, both species are simultaneously changing in density - We need to consider how changes in both species population sizes affect each other - We have two equations: d /dt = r 1 (1-[ +!N 2 ]/K 1 ) dn 2 /dt = r 2 N 2 (1-[N 2 + " ]/K 2 ) 6. Determining the outcome of competition both species combined - Both populations are at equilibrium when d /dt = 0 = dn 2 /dt - We can superimpose the isoclines of both species onto one graph to determine what the outcome of competition will be stable coexistence or exclusion? - There are 4 different outcomes depending on the values of K 1, K 2,!, and " N 2 6. Determining the outcome of competition outcome possibility #1: species 1 wins K 2 K 2 /" K 1

5 N 2 6. Determining the outcome of competition outcome possibility #2: species 2 wins 6. Determining the outcome of competition outcome possibility #2: Competitive Exclusion Example w/ Paramecium K 2 K 1 K 2 /" 6. Determining the outcome of competition outcome possibility #3: stable coexistence 6. Determining the outcome of competition outcome possibility #3: stable coexistence N 2 K 2 K 1 K 2 /" Stable coexistence requires: Biology: Both species can invade when rare (i.e., increase from N=0 when the other species is at its own carrying capacity) Math: requires >K 2 (sp1 can invade) and K 2 /">K 1 (sp 2) This is hard to get if Ks are very unequal (one species is much more efficient at using resources) and This is hard to get unless! and " are both << 1 (i.e. the species have little overlap in their resource use)

6 6. Determining the outcome of competition outcome possibility #4: unstable exclusion N 2 K 2 When Lotka-Volterra models do not work (i.e. when populations do not reach equilibrium) 1. Disturbance Examples: Rainforests treefalls create gaps w/ light Rocky intertidal waves create bare rock Inferior competitors can persist Fast life histories ( r-selected ) Fugitive species or gap specialists Example: sea palms are inferior competitors to sea mussels but can coexist b/c of disturbance K 2 /" K 1 When Lotka-Volterra models do not work (i.e. when populations do not reach equilibrium) 2. Third species Example: A predator If predators mainly affect a superior competitor, inferior competitors may be able to persist Example: Sea stars as predators in rocky intertidal When sea stars present diverse community When sea stars absent mussels take over How do we test for competition? What is good evidence for competition? e.g. if we remove 1 of 2 competing species, the remaining species increases in density Yes this supports competition hypothesis But, what if there is a third species involved (e.g. a predator, parasite)? Can have Apparent competition but not real competition Predator Prey 1 Prey 2 -

7 How do we test for competition? How can we test for competition if there is a third species involved? Experiment with 3 different treatments: Treatment 1: remove prey 1 and predator Treatment 2: remove prey 1 only Treatment 3: remove predator only Control Estimate how prey 2 density changes in response to each treatment Niche: natural history of a species ranges of conditions and resource qualities in which a species can persist Performance persistence Factor Before competition begins i.e. when 2 species are first brought together in space After competition has been occurring for a long time i.e. after natural selection for differentiation has occurred Frequency Frequency Resource Dimension Resource Dimension

8 Resource partitioning example: Mojave desert plants Resource partitioning example: terns on Christmas Island Freq Can look at where species do and do not cooccur to see if natural selection due to competition has led to divergence Island 1 spp A only Island 2 spp B only Island 3 both spp Can look at where species do and do not co-occur to see if natural selection due to competition has led to divergence Example with Galapagos finches RD RD RD