Population Ecology Flashcards
Population
- a group of individuals of the same species that live in the same area at the same time
- and potentially interact with each other
- defining borders of a population can be difficult
Metapopulation
a population of populations connected by migration (gene flow)
Range of species
- different scales
- populations are displayed in a patchy way depending on the suitability of the environment
- as you zoom in there are more and more populations
Population Density
the number of individuals per unit area
Population Dispersion
- how individuals are distributed relative to one another
- 3 main types: Random, Clumped, and Uniform
Random Dispersion
- an individual has an equal probability of occurring anywhere in an area
- e.g. a field of dandelions have wind dispersion
Clumped Dispersion
- individuals are closer than expected by chance
- if natural selection has favored being close to another individual
e. g. schools of fish reduce the chance of predators eating an individual
Uniform Dispersion
- relatively equal spacing among individuals
- maximize space
- if natural selection has favored being farthest away from each other
- e.g. plants in the desert are uniformly spaced because their roots are competing for water in the soil
3 Main Ways to Calculate the Size of A Population
- Count all individuals in the population
- Sub-sample and extrapolate
- Mark-recapture
Counting all individuals int he population
-usually not possible, unless there is a small population such as the Northern Spotted Owl
Sub-sample and extrapolate
- estimate of population size
- e.g. quadrant and transect sampling
potential difficulties:
- over what area do you extrapolate
- heterogeneous habitat (not every sample is the same and leads to inaccurate estimate)
- movement of individuals
Mark-Recapture
mark: catch some individuals and mark them
recapture: return later and catch some individuals and tally number marked and unmarked
M/N = m/n M- number marked m- number marked in recapture N- estimated population size n- total in recapture
assumptions:
- individuals not moving in/out of study area
- individuals size between captures
- no catch bias
- no fitness difference between marked and unmarked
Demography
- study of vital statistic of populations
- birth and immigration rates (+)
- death and immigration rates (-)
- the values of vital statistics depend on the traits of the focal organisms
- these factors are part of what determines the population size
Life table
- summarizes age-specific demographic stats
- data gathered for a cohort
Cohort
- group of the same age that can be followed through time
- e.g. group of lizards born the same summer
Survivorship
-the average proportion of a cohort that survives to a particular age class
Fecundity
- the number of females produced by each female
- average births/year/original female
Net-reproductive Rate
- growth rate of a population per generation
- average number of female offspring that each female produces over her lifetime
Fertility
-average number of offspring (male + female) that a female produces during lifetime
r-selection
- Type III survivorship
- high fecundity and low survivorship
includes:
- many offspring
- small offspring
- early maturity
- small body size
- low disease resistance
- low predator resistance
- short life span
K-selection
- Type I survivorship
- low fecundity and high survivorship
includes:
- few offspring
- large offspring
- late maturity
- large body size
- high disease resistance
- high predator resistance
- long life span
Per capita rate of increase (r)
-birth rate minus death rate per individual
r = (births - deaths) / N
Intrinsic rate of increase (r-max)
- maximum value of r
- optimal conditions for growth
- species specific
Exponential growth
- when r is positive and constant
- density independent does not depend on the number of individuals
- adds an increasing number of individuals as the population grows
- happens in nature when there is little intraspecific competition
- including: new population, after a bottleneck, increase in resources, decrease in predation
-dN / dt = rN
Exponential growth formula
dN / dt = rN
Christmas Bird Count
- National Audubon Society
- began in 1900
- bird censuses by volunteers
- helped to provide long-term demographic data for hundreds of species
Logistic growth formula
dN / dt = r N [ (K -N) / K]
Carrying Capacity (K)
-maximum number of individuals that can be supported and sustained in a particular habitat
Limits to population growth
-r and K can fluctuate over time
density independent:
- changes in environment (abiotic and biotic)
- e.g. natural disasters, drought
density dependent:
- function of population size
- increased competition
Density dependent growth
- survival declines at high population density
- fecundity declines at high population density
due to:
- function of population size
- increased competition
Population Dynamics
- changes in populations through time and space
- K, r, births, deaths, immigration, emigration
Metapopulation Dynamics
- metapopulations connected by immigration and emigration
- extinction and re-colonization
Ilkka Hansi
- studied Glanville fritillary (endangered species of butterfly)
- ranged across fragmented landscape
- although some populations go extinct over time, migration can restore or establish populations
- overall balance between population extinctions and re-colonizations
Significant predictors of patch extinction
- small population size
- small size of neighboring population
- low flower abundance
- low heterozygosity
- given enough time, each population within a metapopulation is expected to go extinct
Heterozygosity importance to populations
- a proxy for genetic diversity
- often small population sizes (loss of diversity from drift) possibly with inbreeding
- low genetic diversity is a bad thing for the long term health of population
Population cycles
somewhat regular rise and fall in population size
Factors that cause population cycles
- competition for resources
- predator-prey
- host-parasite
- competition with other species
Lemmings
- order Rodentia
- 4-5 species
- example of population cycles
- Arctic tundra
- herbivores
- jumping off cliffs
- resources (mosses and grasses or prey?
- exclosure experiments supports prey
- but scope and scale was limited
- some models support resources
- but include parameters that are difficult to estimate
- the debate continues
Most populous countries
China, India, US
Population thinking
- holds that variation among individuals in a population is not irrelevant
- rather, understanding this variation is key to understanding how populations change through time in their abiotic and biotic environments
Population Ecology
-the study of how and why the number of individuals in a population changes over time and space
Generation
-the average time between a mother’s first offspring and her daughter’s first offspring
Age class
-a group of individuals of specific age
Age-specific fecundity
-the average number of female offspring produced by a female in each age class
Life history
- traits related to how individuals allocate resources to growth, reproduction, or survival
- including survivorship, age-specific fecundity, age at first reproduction, and growth rate
- often include fitness trade offs
Replacement rate
-the average fertility required for each woman to produce exactly enough offspring to replace herself and her offspring’s father
(slightly above 2.0 to account for the mortality of some women before they bear children)
Zero population growth
when replacement rate is sustained for a generation and r = 0
Long term data on wild populations is…
hard to collect
