BIO326 : Population : Population Characteristics : Lesson

Glossary terms that are important in this lesson: Age structure, allele, bottleneck, cline, density, dispersion, ecotype, founder, gene flow, gene pool, genetic drift, habitat, ideal free distribution, inbreeding, inbreeding depression, landscape concept, MHC, modeling, mutation, outbreeding, patchiness, population growth rate, self incompatibility, stable age distribution, survivorship curve

Individuals occupying the same habitat and sharing the same gene pool make up a population. A population is dispersed in its environment, it has a growth rate, it has an age structure and can be identified as an ecological unit.

I. Population

1. Introduction
   • Definition
   • Spatial structure
     • Individuals live in suitable habitat
     • Abundances vary with ecological factors: food, predators, nest sites

2. Population structure
   • Density
   • Spacing of individuals
   • Age classes
   • Mating system
   • Genetic variation; gene pool

3. Habitat and distribution
   • Distribution - geographic range
     • Suitable habitat: sugar maple; startlings
     • Barriers to dispersal
     • Patchwork of occupied and unoccupied areas
     • Microhabitats: Clematis, horned lark
     • Migratory species included habitats in different locations: golden plover, Blackburnian warbler
   • Variety of habitats - ecological range

4. Population density
   • Definition
   • Related to habitat quality
   • Fluctuations in density reflect changing local conditions: chinch bug in Illinois

5. Populations in heterogeneous landscapes
   • Landscape concept - mosaic of habitat patches
   • Patchiness affects the distribution and quality of suitable habitat
   • Forest fragmentation: cowbirds, scallops

Ideal free distributions - apparent quality of patch depends on: Intrinsic patch quality Exploitation Migratory birds, stickleback Sense the environment - know the resources Behavioral flexibility and freedom to choose; dominance Source and sink populations Good habitats produce surplus populations Poor habitats receive immigrants Blue tits in southern European oak forests

8. Dispersion
   • Clumped (aggregation)
     • Social groups - flocks of birds
     • Resources clumped - salamanders
     • Progeny remain with parents - vegetative reproduction, e.g., balsam poplar
   • Spaced (uniform)
     • Individuals antagonistic - sea birds, some plants
   • Random
     • Individuals do not influence each other

9. Spatial coherence
   • Dispersal - movements within populations
   • Emigration/immigration - between populations, e.g., sources and sinks
   • Measurement - Drosophila
   • Modeling dispersal: describe, predict, compare
     • Standard deviation
     • Neighborhood size
     • Comparing birds, snails, lizards
   • Introduced populations - starling

1. Genetic variation (Review from prerequisite classes)
   • Individuals and subpopulations are genetically different
   • Allows response to environmental change
   • Basis of adaptation
   • Introduced by mutation and gene flow
   • Generally negative impact

2. Mating systems
   • Inbreeding/selfing
     • Definition: mating among close relatives
     • Results in expressing deleterious recessive alleles - leads to inbreeding depression: Banksia
     • Advantage in special circumstances, e.g., when few pollinators or when individuals widely spaced
   • Outbreeding (negative assortative mating)
     • Smell genetic differences in MHC genes
     • Self-incompatibility in plants: biochemical mechanisms
     • Temporal separation of male and female function
     • Optimal outcrossing distance - a compromise: Delphinium

Genetic changes in small populations Genetic diversity is less (in small populations) Large genetic differences between populations Chance effects are important: genetic drift in allele frequencies Fixation is inversely related to population size Founder event: colony established from a reduced gene pool Population bottleneck: cheetahs; gene pool restricted Probability of extinction increases Geographic variation in the gene pool Cline: gradual change in allele frequency from differential selection Ecotype: genetic basis; Göte Turesson Achillea, toxic metals, Teleogryllus, Bradybaena, Linanthus

1. Age structure and population growth rates
   • Lambda: ratio of population size from one time period to the next - a growth rate

2. Population age structure: birth rates (fecundity) and death rates (mortality) are dependent on the age of individuals
   • Prereproductives - growing individuals, not reproducing
   • Reproductives - sexually mature, producing offspring
   • Postreproductives - older, not reproducing, mortality rising

3. Stable age distribution
   • Age-specific birth and survival rates are constant
   • Constant population growth rate
   • Constant proportions in each age class

4. The life table - a model
   • Statistics
     • Number of individuals in each age class
     • Survival probabilities: can draw survivorship curves
     • Fecundity rates
     • Predicts expectation of further life
   • Estimate survival in natural populations
     • Survival of individuals to a particular age
     • Survival of individuals in an age class from one time period to the next
     • Age at death
     • Age structure

When you have completed this lesson, go on to Review Questions