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Lecture 15

# Lecture 15 - Interspecific competition theory Competitive...

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1 Interspecific competition: theory Competitive exclusion (Gause): Two species cannot coexist if their niches are too similar Competing species can coexist if their niches are different enough " N " t = rN ( K # N K ) Logistic growth equation dN dt = 1.0 N Exponential growth Logistic growth dN dt = 1.0 N 1,500 N 1,500 K = 1,500 0 5 10 15 0 500 1,000 1,500 2,000 Number of generations Population size ( N )

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2 " N 1 " t = rN 1 ( K 1 # N 1 K 1 ) " N 1 " t = r 1 N 1 ( K 1 # N 1 # \$ 12 N 2 K 1 ) " N 2 " t = r 2 N 2 ( K 2 # N 2 # \$ 21 N 1 K 2 ) Single species logistic population growth Now add a competitor (species 2) that lowers population growth Where α 12 is the effect of an individual of species 2 on species 1 N = population size t = time K = carrying capacity r = intrinsic rate of increase α = competition coefficient " N 1 " t = 0 When r 1 = 0, or N 1 = 0 " N 1 " t = r 1 N 1 ( K 1 # N 1 # \$ 12 N 2 K 1 ) N 1 = " # 12 N 2 + K 1 When does ? K 1 " N 1 " # 12 N 2 = 0 Or when: Which can be rearranged as: N 1 N 2 N 1 = " # 12 N 2 + K 1 K 1 K 1 " 12 ( y = mx + b ) Zero growth isocline, species 1
3 N 1 N 2 N 2 = " # 21 N 1 + K 2 K 2 K 2 " 21 Zero growth isocline, sp. 2 N 1 N 2 K 2 K 2 " 21 K 1 K 1 " 12 Scenario 1: species one goes to carrying capacity, competitively excludes species two. N 1 N 2 K 2 K 2 " 21 K 1 K 1 " 12 Scenario 2: species two goes to carrying capacity, competitively excludes species one.

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4 N 1 N 2 K 2 K 2 " 21 K 1 K 1 " 12 Scenario 3: stable coexistence of both species N 1 N 2 K 2 K 2 " 21 K 1 K 1 " 12 Scenario 4: Unstable coexistence. Any perturbation from the equilibrium point leads to competitive exclusion of one species or the other. K 1 > " 12 K 2 and K 2 > " 21 K 1 Conditions for stable coexistence K 2 " 21 > K 1 and K 1 " 12 > K 2 N 1 N 2 K 2 K 2 " 21 K 1 K 1 " 12
5 Animal Behavior Types of behavior Inheritance of behaviors Selection on behaviors – Foraging – Reproduction – Social Behavior Sexual selection Male-male comptetion – Female choice • Altruism Animals behave Do plants behave? http://plantsinmotion.bio.indiana.edu/plantmotion/movements/nastic/mimosa/strongmimosa.html

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6 Proximate vs. Ultimate Causes of a Behavior Behavioral ecologists distinguish between proximate and ultimate causes of behavior – Proximate causes: the immediate stimulus and mechanism for the behavior (proximate) – Ultimate causes: How the behavior contributes to survival and reproduction (ultimate) Some behaviors are hard-wired A fixed action pattern (FAP) Is a sequence of unlearned, innate behaviors that is unchangeable Once initiated, is usually carried to completion A FAP is triggered by an external sensory stimulus Known as a sign stimulus In male stickleback fish, the stimulus for attack behavior – Is the red underside of an intruder Figure 51.3a (a) A male three-spined stickleback fish shows its red underside.