ECO 250 Exam 2

Sigmoidal Growth

growth rate decreases as size increases

• usually followed by a population stabilizing at K (carrying capacity)

• breeding slows down as K is reached; environmental feedback built into the system

• usually K-selective species without a high reproductive rate

Survivorship

survival rate of individuals in each age group

1. early loss

2. constant loss

3. late loss

Early loss

a low rate of survival at early age

• type III

• does NOT always mean short lifespan

• Ex. fish

Constant loss

constant survival rate at all ages

• type II

• Ex. birds

Late loss

high survival until certain age, then high rate of mortality

• type I

• common in K selective species

• Ex. big horned rams, people

Density

the total number of organisms within a defined area (x per y²)

• on a practical level density is more often that not a range based on different ways to find the density

• Ex. 10 wolves/km² ; 5,000 paramecia/cm²

Ways to find Density

1. Direct Count

2. Estimate relative density

3. Estimate absolute densityireD

Direct Count

count every individual in a habitat

• only feasible for big conspicuous organisms

• Ex. elephants, moose

Estimate relative density

determine which habitat contains more of an organism; involves comparison of two habitats usually

• cannot usually estimate an actual density

• count organisms and compare them between areas and time - a LOT of data

• count signs of organisms and compare them between areas or times - tracks and scat

Estimate absolute density

sample some of the organisms; extrapolate the numbers caught to a prediction of actual density

• mark - recapture density; tag and release

Sex Ratio

percentage of population that is male/female

• not super simple because it changes over time

Types of Sex Ratios

1. Primary

2. Secondary

3. Tertiary

4. Quaternary

5. Effective

Primary Sex Ratio

sex ratio at fertilization

• there is no primary sex ratio for alligators

Secondary Sex Ratio

sex ratio at birth or hatching

Tertiary Sex Ratio

sex ratio at sexual maturity

• usually becomes skewed toward females

• Ex. deer antlers, peacock feathers - it all comes down to mate

• Ex. Boat tailed grackle - big tails on males attract females, also make it harder to fly

Quaternary Sex Ratio

sex ratio in adult population

• more skewed towards females

• may change every year as males kill each other to attract females

• Ex. fish/amphibians their ratio can change male - female and they can change their gender to accommodate the sex ratio

Effective Sex Ratio

ratio of males and females that actually mate

• alleles of many males never get passed down

• Ex. lion prides; sea lion with sea lionesses; elk with his harem

Dispersion

the distribution of organisms across their habitat

• NOT the same as dispersal

Types of Dispersal

1. Random

2. Aggregated

3. Hyperdispersal

Random dispersal

no apparent pattern; rare in nature

Aggregated Dispersal

individuals tend to group together

• common in nature

• social interaction - individuals of some species always travel in groups throughout different habitats → BEHAVIOR is very important (hyena cackle)

• habitat heterogenity - organisms tend to group together where habitat is best (fish)

Hyperdispersed Dispersion

individuals try to stay as far away from each other as possible

• common in territorial animals (bears, praying mantis, mountain lions,) are all cannabalistic due to their aggressive territorial behavior

Life Tables

quantification of population demographics: life expectancy, sex ratios, survivorship

• a bunch of parameters that allows predictions about demographics

Population Regulation

population density is governed by factors internal or external to the population

• external: prey/predation

• internal: stress, territoriality

Population Control

ecological factors limit density

• density independent

• density dependent

Density Independent

affects population regardless of the population’s density

Ex. floods, fire, severe weather → tend to be abiotic factors

• independent factors have equal opportunity to affect density but are not caused by a population’s density

Density dependent

affects population based on the population’s density

Ex. disease, starvation, predation, stress

Population Cycle

the pattern of change in a population over time

• all things in nature cycle

• 4 basic cycles for population growth

4 Basic Population Cycles

1. Small scale fluctuation

2. Large scale fluctuations

3. Cyclical

4. Irruptive

Small-scale fluctuations

• population is generally stable with small irregular changes over time

• NOT a predictable cycle but generally a stable one

Ex. K-selective species (right around carrying capacity) - large birds, large mammals (orcas, rhinos, zebras, birds)

Large scale fluctuations

• population has many large irregular increases and decreases

• NOT PREDICTABLE

Ex. pest species; R-selective species, boom and bust population species like Aphids

Cyclical

• large population increases and decreases at regular intervals

• rare in nature

Ex. cicadas, lemmings

Irruptive

generally stable populations with occasional large increase, followed by reciprocal decrease

Ex. common in the ultimate pest and disease species → have R-selective potential (locusts)

Population Equilibrium Theory

an “ideal” size (K) exists for every population

• biotic and abiotic factors usually keep that population at this level (ie carrying capacity)

• Equilibrium Theory puts emphasis on these biotic and abiotic factors to keep at K

Extrinsic Biotic

Biotic factors external to the species itself are responsible for population regulation

• emphasizes density-dependent factors to keep a population around K

• extrinsic factors affect the density of a population

Ex. disease, food, predation

“Top-Down”

an example of an extrinsic biotic scenario

• predators regulate populations of its prey

“Bottom Up”

an example of an extrinsic biotic scenario

• prey regulates population of its predators

Intrinsic

population abundance self-regulates regardless of external factors

• NOT about food, disease, or predators

• Self regulation

Stress

An example of an intrinsic factor

• a high density of individuals will cause a lower reproductive rate even with ample food, water, etc.

Ex. lab rats, humans

Territoriality

an example of intrinsic factors

• the competition for space keeps populations regulated, even with ample food, water, etc.

Example of hyperdisperal

Nonequilibrium Theory

carrying capacity is less important than abiotic environmental factors

• emphasizes density independent factors like weather

• K is less important and more dependent on abiotic/environmental factors

Abiotic extrinsic

density-independent factors external to the species regulate the population

• temperature, rainfall, sunlight, etc.

• all populations will increase if environment is good; increases in favorable environmental conditions

Equilibrium v. Nonequilibrium theory

Equilibrium: K-selective species tend more equilibrium theory based and also constant environment temperatures tend towards species favoring equilibrium

Nonequilibrium: R-selective species tend more towards non equilibrium because they are more impacted by abiotic factors like weather and environment; the harsher the environment the more likely there is a lower equilibrium

Species Richness

total number of species within a community or more likely a sample

• does not reflect abundance of individuals

• often samples are just relative value

Eveness

the degree to which the number of individuals is evenly distributed among species

True Diversity

reflects both number of species (species richness) and evenness of individuals

• attempts to incorporate ecosystem functioning

Erwin

There are around 30 million species on Earth

Species Sample Curves

plot the cumulative number of species caught by number of samples collected

• gives the known species richness

• extrapolate to an estimate of total species richness with in a given area

• no negative species richness, it can only go up or level off because it is cumulative

• SSC show a good comparison even if the amount of work/effort is different

Abundance-based coverage estimator (ACE)

predicts the actual number of species based on the proportion of species with <10 specimens total

• rate species = <10 specimens total

• the more non-abundant (<10 specimens) species in a sample, the greater the estimate of species richness

• line on a graph is higher than SSC

Incidence-based coverage estimator (ICE)

predicts the actual number of species based on the proportion of species occurring in <10 samples

• the more rare (<10 samples) species in a sample, the greater the estimate of species richness

• the ICE line on a graph is the only one that goes down and is always higher than SSC

N_t = K/1 + ce^-rt

N_t = population size at any given time; should be very close to K
K = theoretical carrying capacity
r = intrinsic growth rate

t = time elapsed

c = constant determined by conditions

X

age interval usually expressed in years

n_x

number of individuals of the population (N = population) that survive at the beginning of X

l_x

l_x = n_x/ N

proportion/percentage of total surviving to beginning of X

d_x

d_x = n_x - n_x+1

number dying during each age interval

L_x

L_x = (n_x + n_x+1 / 2)

mean number of individuals alive during that age interval

T_x

total number of organism years to be lived by all individuals of the age interval

e_x

e_x = T_x/n_x

Life expectancy for an individual at an age interval

Survivorship Curve

plot l_x against X

Regression Analysis

A helpful tool to show the patterns of population growth by plugging in different variables

y = predicted response variable

x = predictor variable

b = y-intercept

m = slope of line

Ex. Thrips density (y) using 4 variables (x) using regression analysis

x₁ = winter temperatures (population independent variable)

x₂ = spring temperature (population independent variable)

x₃ = spring rainfall (population independent variable)

x₄ = size of overwintering population (only intrinsic variable)

This example shows the different variables that impact population regulation and growth and how they can be different types; not only density independent variables affect the thrips population