Population Biology

Definition A population is a group of organisms of the same species, interbreeding or closely related through interbreeding and evolving as a unit.

Review Community: a group of interacting plants and animals forming an identifiable group

Review Biosphere: the entire part of the earth where organisms are found

Review Habitat: the place where an organism naturally lives or grows

Review Niche: position or function of an organism in its community – its occupation

Population Size Calculations Simplest calculations involve changes in populations over a time period in a defined space. gr= N/ t

Ex) What is the growth rate? Initial Population gulls move in (immigration) 20 0 gulls move out (emigration) 0 32 chicks hatch (natality) chicks die-10 2 adults die -2 Time1 year Total240 Annual Increase of +40 birds per year

Density Formula: D = N/A or N/V Density equals number of organisms divided by area (or volume or space) Eg. 200 bison in a 100 acre pasture is a density of 2 bison per acre

Rate of Change Often uses density R = D / t Rate of density change equals change in density over change in time.

Example In 1996 there were 10 Grizzly Bears in a ha forest. In 2005 there are only 8. What is the rate of density change? R = D / t R = – / 2005 – 1996 R= bears/ha/y

Per Capita Growth Rate Or cgr The amount that a population changes per individual over a set period of time Cgr = N / N Per capita growth rate = change in number divided by initial population size

CGR A lynx population was 19 per sq. km in In 1993 it was 3 per sq. km. What was the cgr of this population from 1991 to 1993? Cgr = N/ N = -16 / 19 = per lynx

Distributions of Populations Can be clumped – more individuals together than apart Often involves cooperation among group members (eg. Herd, pack)

Distribution of Populations Can be random – not seen often in nature Organisms have no effect on each other

Distribution of Populations Can be uniform: evenly distributed Usually due to competition between individuals Territories, etc.

Open Populations Are those where organism can enter or leave Often have S-shaped curves

Closed Populations True closed populations are rare On islands, isolated communities

Population Growth Graphs typically have numbers on vertical axis and time on horizontal axis

Logistic Growth S shaped curves are typical of stable populations.

Eg. Wild Horses on reserve land in AB

Exponential Growth J shaped curve (initially) occurs with short-lived populations that rapidly deplete their environment

Eg. Flies on a carcass

Overshoots Result when k is greatly exceeded and the environment deteriorates

Carrying Capacity Is the number of organisms a habitat can sustain over the long term “k” Influenced by Biotic Potential and Environmental Resistance

Biotic Potential maximum number of offspring produced capacity of offspring to survive to reproduce number of times per year an organism reproduces age at which offspring are reproductively mature

Generally speaking, smaller, simpler organisms have a higher biotic potential than larger organisms.

Environmental Resistance limiting factors on a population Availability of resources (food, water, space, etc.) Competition for resources with other organisms Intra-specific – within a species Inter-specific – between a species Predators Disease Climate change

Environmental Resistance Puts brakes on biotic potential (B) – maximum reproductive rate

Environmental Resistance B K

Limiting Factors on Populations Law of the Minimum: if any one of many needed nutrients/limiting factors is reduced below the required levels, the population growth rate declines

Limiting Factors Can be density independent – those that will affect a population regardless of its size Eg. Cold winter

Limiting Factors Can be density dependant – those that increase when the population size increases Eg. Disease Predation Food Supply

Gause’s Law Competitive exclusion No two species can remain in competition for a limited resource

Other Graphs Survivorship curves

Other Graphs Age distribution pyramids

Population Histograms are graphs showing the composition by age and gender of a population at a specific time. Population histograms have the following characteristic shapes:

R and K Population Strategies This is a continuum Most populations fall between these two extremes

K Selection k-selected species is one that typically has: Stable environmental conditions Slow growing individuals Low reproduction rate (B) Parental care of offspring

R- Selection An R-selected species is one that typically has: Unpredictable environment Small individuals with short life spans Reproduce at a high rate Little or no parental care

Life History Patterns Some organisms undergo regular patterns of growth and decline known as population cycles Small rodents, rabbits, lemmings often cycle every 1 – 4 years

Population Cycles Can be due to fluctuations in food supply, predation, or both

Predator - Prey Cycles

Chaos Theory Used by population biologists to study the general trends in populations Small uncertainties in short-term prediction of individual events may be magnified to such an extent that complex systems become quite unpredictable

Chaos The ‘butterfly effect” This is the sensitivity of a system to the initial conditions Change any starting parameter slightly and the resulting changes magnify until the result is very different from the initial prediction

Technologies Include: Radio collars

Technologies Sampling methods Quadrats – counting organisms in defined areas

Transects – counting organisms that touch line

Technologies Mark/recapture studies

Symbiosis – a relationship between two individuals of different species. Parasitism – one species lives in or on another where it obtains food and resources. The host is usually harmed by the relationship. Ex. Tapeworm Symbiotic Relationships

Many parasites are r-selected Eg. Tapeworm

Commensalism – one species lives on or near another, but while one species benefits, the other is unaffected.

Mutualism – two species that live in close association with one another, where both benefit from the relationship. Ex. honeybee and flower Ex. shark and remora

Ecological Succession Ecological succession – is the gradual and orderly change of a community as it is either developed from bare land or replaced by another community.

Succession Primary Succession – the gradual colonization of an area that has not supported an ecosystem before. (from bare rock) PrimarySuccession

Secondary Succession – the colonization of an area that once supported an ecosystem that was destroyed by fire, flood, etc. Exploring Time Gallery Display

Pioneer Community – is the first species to appear during succession. Climax Community – is the final stable community that results at the end of succession.

Generalizations about Succession: Species composition changes more quickly at earlier stages. Total number of species increases dramatically at early stages, levels off at intermediate phases and declines at the climax stage. Food webs develop in complexity as succession progresses. Total biomass increases during succession and levels off at the climax stage.