Populations can be described by Distribution Numbers Age structure Density

Populations cannot grow indefinitely (r) because there are limited resources and habitats (k)

ClumpingUniform dispersionRandom dispersion

i.e., Plethodon sp. salamanders are found clumped under fallen logs in the forest

the night lizard Xantusia sp. is found clumped within fallen Joshua trees in the Mojave desert

Plants often clump because their seeds fall close to the parent plant or because their seeds only germinate in certain environments. Impatiens capensis seeds are heavy and usually fall close to the parent plant-this species grows in dense stands.

Species may clump for safety, or social reasons. Ground nesting bees Halictus sp. prefer to nest in the presence of other bees, forming aggregations of solitary nests

This generally happens because of interactions between individuals in the population.

Competition: Creosote bushes in the Mojave desert are uniformly distributed because competition for water among the root systems of different plants prohibits the establishment of individuals that are too close to others.

Territoriality: The desert lizard Uta sp. maintains somewhat regular distribution via fighting and territorial behavior

Human Intervention: I.e., the spacing of crops.

This pattern occurs in the absence of strong attraction or repulsion among individuals. It is uncommon.

The trees of some forest species are randomly distributed within areas of suitable habitat. For example, fig trees in the amazon rain

forest. This random distribution might be due to seed dispersal by bats.

Simply the number of individuals in the population at any given time. Sometimes called abundance.

This is the relative number of individuals at different ages.

The number of individuals in the population per unit area or unit volume. For many organisms, it is the density of

a population rather than its actual numbers, that exerts a real effect on the organism.

There are 10,400 mice living in a 1000m x 1000m field. What is the density of this population?

The area of the field is 1,000,000 square meters (m2).

The density of mice is therefore 10,400 mice/1,000,000m2=.0104/m2.

Birth (Natality)Death (Mortality) ImmigrationEmigrationPopulation change= (B+I) – (D+E)

This is probably the best, simple, model of population growth…it predicts the rate of growth, or decay, of any population where the per capita rates of growth and death are constant over time. In exponential growth models, births deaths,

emigration and immigration take place continuously

This is a good approximation for the growth of most biological populations

i.e., human populations grow exponentially when resources are plentiful

Density independent

Results from sudden crash in population size

Outstrip resource limit

catastrophe

N(t)=N0ertwhere r is the exponential growth

parameterN0 is the starting population t is the time elapsed r=0 if the population is constant, r>0

if population is increasing, r<0 if the population is decreasing.

The human population of the earth is growing at approximately 1.8%per year.

The population at the start of 2001 was approximately 6 billion.

If nothing were to slow the rate of population growth, what would the population be in the year 2101?

N(t)=N0ert

r= .018 t=100 yearsN0=6 billionN(100)=N0ert

N(100)=6x109ert =6x109*e1.8

N(100)= 6x109*6.04 = 36.3 billion

Populations tend to grow to the maximum extent possible given environmental conditions

Biotic potential Dependent on innate biological

principlesIntrinsic rate of increase (r)=

rate of population growth if unlimited resources available

Biotic potential cannot be sustained

Environmental resistanceNegative feedback▪Snowshoe hare and lynx

Populations grow until one or several limiting resources become rare enough to inhibit reproduction so that the population no longer grows.

The limiting resource can be light, water, nesting sites, prey, nutrients or other factors.

Eventually, every population reaches its carrying capacity, this is the maximum number of individuals a given environment can sustain.

Density dependent

Recycling and renewal of resources

Establishes equilibrium around carrying capacity

A: lag phaseB: acceleration

phaseC: exponential

growthD: deceleration

phaseE: equilibriumG: dynamic

fluctuations

Adaptive traitsTechnological advances

Density- dependent factors Intraspecific competition Predation Disease

Density-independent factors Natural disasters Pollution Habitat destruction (deforestation)

Examples: For Neodiprion sawflies, winter surviorship is greatly affected by the weather, which is density-independent.

During the summer, however, parasitic wasps impose very high density-dependent mortality.

Pacific mussels, Mytillus sp., are largely limited by density-dependent competition for space on rocky outcrops. Occasionally, density -independent disturbance by floating logs decimates populations.

Stable Rainforest species

Irruptive Insects

Cyclic Lemmings▪ http://www.youtube.com/watch?v=pDqlZjpSJCc

Wolf-moose interactions Irregular

Due to catastropies

Delayed density dependence

Sexual vs. Asexual

r-selected Reproduce early and

put most of their energy into reproduction

Many small offspring High growth rate

Population size fluctuates wildly around carrying capacity

Low ability to compete Most offspring die

before reaching reproductive age

K-selected Fewer, larger

offspring High parental care Most offspring survive

to reach reproductive age

Lower growth rate High ability to

compete Population size fairly

stable around carrying capacity

Positive: enhance population growth

Negative: reduce population growth

Can be positive or negativeLichensCoral and zooxanthelae algae

Wood termites and protozoaHumans and gut flora

Can be positive or negativeMigrationTerritory behaviorSocieties and hierarchiesMating and courtshipColors, patterns, physical characteristics

PredatorsDiseasesPheromones

Type I: large animals immune to predation, live to old age

Type II: mostly prey, predation is constant throughout lifespan

Type III: large numbers of young because most will be eaten, only few adults survive