Ecology BIO C 322 Population Ecology contd Concepts
Ecology (BIO C 322) Population Ecology (cont’d)
Concepts of Rate • Population a changing entity Population dynamics. • N = Number of organisms; t = time • Average growth rate = ΔN / Δt • Specific growth rate (SGR) = ΔN / NΔt • Percentage growth rate = SGR x 100
Instantaneous Rates • • • Rate of change when Δt 0 Instantaneous growth rate = d. N/dt Instantaneous SGR = d. N/Ndt Growth curve vs growth rate curve. Slope at any point on growth curve growth rate. • Point of Inflection: The point where growth rate is maximum.
• In non-limiting environment (w. r. t. food, space, others): • d. N/dt = r. N; • r = rmax = Instantaneous coefficient of population growth or intrinsic rate of natural increase or biotic potential or reproductive potential. • ‘r’ depends on environmental condns.
• Nt = N 0 ert r = (ln Nt - ln N 0) / t; • • b & d = inst. specific birth & death rates i & e = inst. immigration, emigration rates r = b – d + (i – e); If i = e, then r = b – d; • r>0 (b>d) Population increases exponentially; • r<0 (b<d) Exponential decline; • r=0 (b=d) No change in population size.
Read this… • Populations in nature often grow exponentially for short periods when there is ample food & no crowding effects, etc. • e. g. plankton blooms, pest eruptions, bacterial growth in new culture media. • But exp increase can’t continue very long. • Interactions within population & external environmental resistances soon slow down the rate of growth.
Population Growth Forms • J-shaped Exponential increase followed by sudden decline; • d. N/dt = r. N; • Sudden decline due to shortage of food resource, sudden temperature change, end of reproductive season. • Boom-and-bust pattern.
S-shaped growth form • Sigmoid or logistic 4 phases: – Establishment or lag phase; – Logarithmic phase; – Negative acceleration (environ resistance); – Equilibrium. • d. N/dt = r. N [(K - N) / K] = r. N [1 – N/K] • K = (Max) carrying capacity = Max no. of individuals of a population that can survive in a given habitat = 2 x Inflection point.
Practice Concept • The equation for logistic growth patterns is the same as the exponential one, with the exception of the expression [1 – N/K]. • This term indicates the environmental resistance created by the growing population itself, • which brings about an increasing reduction in the potential reproduction rate, as population size approaches carrying capacity.
• Human population is constantly increasing…
But it is predicted that we would reach ‘K’ value in 21 st century!!
• Exponential: Highest growth form; • Logistic: Lowest growth form (-ve feedback); • Intermediate pattern for most populations.
Fluctuations around ‘K’ value • The S-shaped (logistic) growth curve suggests a levelled-off stage in which population and environment are at equilibrium (at the carrying capacity). • However, in biological systems, equilibrium is a dynamic state of fluctuation rather than an unwavering constant.
Irruptive Populations • Insects, some plants, pests. • Explode in numbers unexpectedly in a boom-and-bust pattern. • e. g. At certain time of year, mosquitoes ↑. • Fluctuations in population size: – Due to extrinsic factors (temp, rainfall); – Due to intrinsic factors (disease, predation). • Cyclic species: Show regular variation in population size (irrespective of environ).
9 - to 10 -year Oscillations (Canada) • Prey: Snowshoe hare (Lepus americanus). • Predator: lynx (Felix lynx).
• Lynx population peak 9 -10 years. • Peaks of abundance followed by crashes. • Similar cycle for hare. • Hare cycle precedes lynx cycle by 1 -2 years. • Hare cycle either predator-driven {top-down} or resource (food)-driven {bottom-up} Both.
Pests always harmful? Time to rethink… • Coniferous forests: Older trees suppressing growth of young trees (shade). • But older trees vulnerable to defoliation by caterpillars of budworm. • Older trees killed Nutrient return to soil. • Younger trees released from shadesuppression, grow rapidly.
Coniferous Forest rejuvenated!!
Mechanisms of Population Regulation • Density-independent factor (Extrinsic): If its effect is independent of the population size. – Climate Determinant of J-shaped growth form. • Density-dependent factor (Intrinsic): Its effect on population is a function of population density. – Biotic factors (self-crowding, competition, parasites, pathogens) Determinants of Sshaped growth form.
Density-dependent Regulation in Plants • Log of average plant weight (y-axis; grams) plotted against log of population density (x-axis; plants per m 2) Line with a slope of -3/2; • Self-thinning or -3/2 power law.
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