Population Ecology 2: Population Regulation and Fluctuation PDF

Population Ecology 2: Population Regulation and Fluctuation  McCarty Woods Restoration: Next Event: November 17, 9-10:30 am Meet at NE corner by parking lot  McCarty Woods Restoration; Nov. 17 Free t-shirt day (first 100 volunteers)  Need to know Be able t...

Population Ecology 2: Population Regulation and Fluctuation  McCarty Woods Restoration: Next Event: November 17, 9-10:30 am Meet at NE corner by parking lot  McCarty Woods Restoration; Nov. 17 Free t-shirt day (first 100 volunteers)  Need to know Be able to describe both the exponential and logistic model of population growth, and their assumptions. Be able to use population growth equations to calculate population size at a future time Be able to describe and contrast biotic and abiotic factors that control population size, and define and identify density dependent vs. density independent population control. Understand and describe the population cycle between lynx and snowshoe hare. Basic Population Growth Model* *note: ignores migration in and out of population B (birth) M (death) Eq. 1. Change in Pop. Size =∆ N/ ∆T = B - M  Population growth is not additive, but multiplicative If we have a population of 100 individuals, 50 females have 1 baby each (the rest are males), and assuming no deaths… Ist generation – 100 individuals 2nd generation – 150 individuals 3rd generation – 225 individuals 4th generation – 337 individuals  So we need to calculate per capita rates of birth Expressed on a per-individual basis (i.e., per capita): per capita birth rate b = B/N where N = # of individuals in the population So if there are 500 births in a population of 1000 elephants, then b = 500/1000 = 0.5  …and per capita rates of death Expressed on a per-individual basis (i.e., per capita): per capita death rate m =M/N N = # of individuals  We can then combine birth and death rates into a rate of population growth (r) The population growth equation (Eq. 1) on a per capita basis is: Eq. 2. ∆ N/∆T = bN-mN Per capita rate of increase = r = b-m, therefore: Eq. 3. ∆ N/∆T = rN if r > 0, population is growing, if r r)  LOGISTIC GROWTH So how do we know what the actual r is at any given point along the curve of logistic growth? r=? K-N r = rmax * K r=? r = rmax r=? In other words, rrmax =? times the “growth rate modifier” term  LOGISTIC GROWTH So how do we know what the actual r is at any given point along the curve of logistic growth? K-N r = rmax * K In other words, rmax times the “growth rate modifier” term  Logistic Growth Model: Example This is r r dN K-N 1000 - 20 = rmax N = 0.05*20* dt K 1000  Logistic Growth Model: Example r dN K-N 1000 - 250 = rmax N = 0.05*250* dt K 1000 Logistic Growth Model: Example r Population size < K, growth rate is positive, pop. size > K, growth rate negative K Per capita growth rate (r) Population size(N) How do real species’ populations grow?  Simplifying Assumptions of Logistic Model 1: No immigration or emigration 2: All individuals contribute equally (i.e., rmax is constant) 3: Environment, and thus K, is constant  What Regulates Population Size? Biotic factors: examples: parasitism, disease, predation, competition  What Regulates Population Size? Biotic factors: biotic factors often operate in a density- dependent manner, Therefore: birth and/or death rates change with changing density Biotic factors and density dependence Example: negative density dependence and decreased clutch sizes of song sparrows each dot is a different population Biotic factors and density dependence Example: positive density dependence and increased seed production of Sanicula plants ”allee effects” population size note: each dot on graph is a different population  What Regulates Population Size? Biotic factors: examples: parasitism, disease, predation, competition often biotic factors operate in a density- dependent manner; birth and/or death rates change with increasing density Abiotic factors: examples: fire, flood, hurricane, drought often abiotic factors operate in a density- independent manner – these factors affect population size regardless of population’s density. Abiotic factors and density independence Drought and Darwin’s Finches Most populations fluctuate or “cycle” Fluctuations are the rule for natural populations “K” Population Fluctuations and Cycling Populations often have periodic cycles -- definition: regular fluctuations between high and low population sizes Learning Catalytics This experiment indicates that Hare survival rate hare populations are being influenced by A. A combination of predators and food B. Food only C. Predators only D. Neither predators nor food How about human populations…are we fluctuating up and down? Human Population Growth Well-approximated by the exponential model This is unusual (if not unique) among organisms in nature Current population size is about 8 billion Predicted population size in 2025 is 8.2 billion  Human Population Growth Population is still increasing, but the rate of change is decreasing…good news? 2.2 2 1.8 1.6 Percent increase 2003 1.4 1.2 1 0.8 0.6 0.4 0.2 0 1950 1975 2000 2025 2050 Year  Have a great long weekend…see you next Wednesday!  Ecology in the news… Mind-control by parasites! Toxoplasma gondii A common protozoan parasite found in rats.  Bad news… Half of y’all are infected.