Population Ecology

• Basic concepts
  • Population: group of potentially interbreeding organisms of same species (all have potential to change in size over time) 
  • Ecology: study of interactions between living organisms and physical environment
  • Population ecology: considers # individuals of species in area/population dynamics
• Changes in population size
  • N: represent # individuals in population (like 1000 birds) 
  • Each year 100 chicks hatch and 20 die -> yearly change in pop. size = 100 - 20 = 80 per year pop. increases
  • Pop. growth rate expressed mathematically as ((delta N / delta t) = B - D)
  • (Delta N / delta t) = rN
  • Per capita: by head/per individual 
  • b = per capita birth rate, m = per capita mortality rate
  • Per capita birth rate: B = bN (average # chicks per adult bird)
  • Ex: 75 individuals/year and 1000 individuals in population; divide 75 by 1000 to get .075 = b 
  • Population with 200 individuals: multiply .075(200) to get 15 births/year 
  • Per capita mortality rate: D = mN (average likelihood of bird dying each year) 
  • Rate of change r: how change occurs over long time 
  • ROC = birth rate - mortality rate r = b - m 
  • Change in population over time = births - deaths in pop. 
  • If r < 0 population declining (refer to formula below) 
  • If r > 0 population increasing 
  • If r = 0 zero population growth (ZPG)
  • Instantaneous growth rate: dN / dt = r (inst) N where r (inst) = instantaneous per capita rate of increase/decrease 
• Models of population growth (exponential): population increases under idealized conditions 
  • Rate of increase @ maximum = r max > 0 and is constant
  • Equation of exponential population growth = (dN/dt) = r(max) N
  • Limitations of exponential growth: can’t continue indefinitely 
  • Environmental resistance: limits set by environment 
  • Run out of food resources, living space, would build up toxic wastes 
• logistic growth model: takes into account environment resistance
  • carrying capacity (K) = maximum pop. size environment can support
  • Pop. grows more slowly nearing carrying capacity (more realistic) 
  • sigmoid/S-shaped curve: dN/dT = r inst N ((k-n)/k)
  • (k-N)/k = decline in growth as pop. reaches k 
  • If N not close to k -> (k-N)/k = 1 (high growth rate)
  • If N close to k -> (k-N)/k approaches 0 and low growth rate
  • When N - K -> (k-N)/k = 0 there is no growth 
  • Logistic model fits few real populations but better than exponential
  • Curves in nature not perfectly logistic usually; pop. fluctuates around k, environment changes, K may change as well (resources) 
• Life history: many different strategies 
  • Organism’s life history: evolutionary outcomes
  • Main variables: age @ 1st reproduction, how often organism reproduces, how many offspring produced per reproduction (rep. episode) 

• Two extremes 

• Factors influencing population size: Density-independent factors (tend to be r-selected)
  • Environment factors: happen regardless of density
  • Abiotic factors: elements of nonliving world (snow, wind)
  • birth/death rate doesn’t change density 
• Density-dependent factors
  • Factors whose impact on pop. affected by pop. density (more dense, > impact on pop.)
  • Pop. maintains itself @ relative constant size near K
  • Biotic factors: other living things as approach carrying capacity
  • Negative feedback system: territoriality, predation, toxic waste, competition for resources, disease
  • Pop. size likely controlled by many factors 
  • As pop. size increases, DDF slows growth (and opposite cases for when decreases)

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