Population Biology Definition A population is a group

Population Biology

Definition �A population is a group of organisms of the same species, interbreedingor closely relation 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 involved changes in populations over a time period in a defined space. �gr = N/ t �gr � = growth rate N = change in number t = change in time

Ex) What is the growth rate? Initial Population 20 gulls move in (immigration) 200 20 0 gulls move out (emigration) 0 32 chicks hatch (natality) 10 chicks die, 2 adult die (mortality) Time Total Annual Increase of +40 birds per year 32 -10 + -2 = -12 1 year 240

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 �R = �Rate uses density D/ t of density change equals change in density over change in time

Example �In 1996 there were 100 Grizzly Bears in a 1 000 ha forest. In 2005 there are only 80. What is the rate of density change? �R �R = D/ t = (0. 08 – 0. 1) / (2005 – 1996) �R = -0. 02 bears/ha/year

Per Capita Growth Rate �Or “cgr” �Is the change in number of individuals in a population relative to the original number of individuals. �Cgr �Per = N/N capita growth rate = change in number divided by initial population size

CGR �A lynx population was 19 per 10 000 sq. km in 1991. In 1993 it was 3 per 10 000 sq. km. What was the cgr of this population from 1991 – 1993? �Cgr = �-16/ 19 �-0. 84 N/N

Distributions of Populations �Can be clumped – more individuals together than apart ◦ Often involves cooperation among group members (ex. Pack, herd, flock)

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 organisms can enter or leave �Often have S-shaped curves (on a graph showing growth)

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

Population Growth Graphs �Typically have numbers on vertical (y) axis and time on the horizontal (x) axis.

Logistic Growth �S-shaped curves are typical of stable populations

�Example: The wild horses of Alberta

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

�Flies on a carcass

Overshot �Results when “k” (carrying capacity) is greatly exceeded and the environment deteriorates.

Carrying Capacity (k) 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. VS

Environmental Resistance �Limiting factors on a population ◦ Availability of resources (food, water, space, etc. ) ◦ Competition for resources with other organisms ◦ INTRAspecific – within a species (example: two beavers competing for space in a river to build a damn ◦ INTERspecific – between a species (example: a beaver and a moose competing for a food source)

Environmental Resistance �Predators �Disease/parasites �Climate change

Environmental Resistance �Puts the brakes on biotic potential (B) – maximum reproductive rate �*concept check* �Mutualism vs Commensalism vs Parasitism

Environmental Resistance

Environmental Resistance

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. �Ex an extremely cold winter.

Limiting Factors �Can be density dependent– those that increase when the population size increases ◦ Ex. Disease, predation, food supply

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

Other Graphs �Survivorship curves

Other Graphs �Age distribution pyramids

Population Histogram �Graphs showing the composition by age and gender of a population at a specific time.

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

K Selection �A 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

Technologies �Transects touch line -counting organisms that

Technologies �Mark/recapture studies

Symbiotic Relationships �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

�Many parasites are r-selected

�Commensalism - one species lives on or near another, but while one species benefits, the other is unaffected. �Very few examples of this

�Mutualism - two species that live in close association with one another, where both benefit from the relationship. �Ex: Honeybee and flowers

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

Succession �_________Succession – the gradual colonization of an area that has not supported an ecosystem before (from bare rock, a newly formed volcanic island, etc. )

�__________Succession the colonization of an area that once supported an ecosystem that was destroyed by fire, flood, etc. –

�_________Community – is the first species to appear during succession �___________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 numbers 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