C4.1 Populations and Communities

population

group of individual organisms of the same species living in a given area

random sample

every member of a population has an equal chance of being selected

quadrats

square sample areas

• quadrat sampling - repeatedly placing quadrat frame at random positions in a habitat and recording the number of organisms present at a time for reliable estimate of a sessile (fixed position) population

capture-mark-release-recapture method to estimate population size for motile organisms

• capture as many individuals as possible in area occupied by animal population

• mark captured individual safely

• release all marked individuals, resettle into habitat

• assume: no migration in/out of the population, no deaths or birth, marked individuals mix back with the population and have the same chance of being recaptured as unmarked individuals, marks remain visible, markings to not affect survival rate

• recapture as many individuals as possible, count number of marked and unmarked

• calculate estimated population using Lincoln index

estimating population using Lincoln index

M*N/R

• M as initial number of individuals caught and marked

• N as total number of individuals recaptured

• R as total number of individuals recaptured with marks

carrying capacity

maximum size of population that environment can support due to limited resources (e.g. water, light, soil, nitrogen, breeding space, food, dissolved oxygen)

density-independent factors

same effect regardless of population size (e.g. flooding, forest fires)

density-dependent factors

increased effect as population grows larger, basis for negative feedback mechanisms - reduce larger populations, allow smaller populations to increase

• competition or limited resources

• predation

• infections, disease, parasitism, prey infestation

sigmoid curve, population growth curves

graph makes J → S shape

• exponential phase - population increases more and more rapidly

• transitional phase - population growth slows as limiting factors have increasing effects

• plateau phase - population remains close to carrying capacity

community

group of populations living together in an ecosystem

intraspecific relationships

relationship existing between individuals of the same species

• competition - individuals share ecological niche, likely require the same resources, natural selection of traits that allow individuals to compete more effectively

  • examples*

• cooperation - mainly in social animals, all individuals benefit - e.g. communal roosting

  • examples*

herbivory

primary consumers feed on producers which may or may not be killed

• bison grazing grasses

• more examples*

predation

predator kills and eats prey

• anteaters eating ants or termites

• more examples*

interspecific competition

two or more species competing for the same resources

• ivy climbing and competing for light with oak trees

• more examples*

mutualism

two species live in a close, mutually beneficial association

parasitism

parasite species live inside or on the outer surface of the host species and obtains food from them; host harmed, parasite benefits

• ticks live on skin of deer and suck blood

• more examples*

pathogenicity

pathogen lives inside host, causing disease in host

• potato blight fungus Phytophthora infecting potato plants

• myxomatosis virus infecting rabbits

Fabacae and Rhizobium as an example of mutualism

• root nodules in Fabacae can only use nitrogen from ammonium (NH₄⁺) or nitrate (NO₃^-), develops mutualistic relationship with nitrogen-fixing bacteria Rhizobium for abundant supply of ammonium

• bacteria lives in root nodules, protected from consumers

• maintenance of low O2 conditions suitable to Rhizobium

• sugars made by photosynthesis provide energy for Rhizobium

• Rhizobium absorbs N2 and fixes it to produce ammonium, which is supplied to plant, preventing nitrogen deficiency

mycorrhizae in Orchidaceae as an example of mutualism

• orchids depend on mycorrhizal relationship with fungi in soil, two-way exchange of materials once the orchid starts to photosynthesize

• orchid supplies carbon compounds made by photosynthesis to fungus

• fungus absorbs nitrogen (mostly supplied in the form of ammonium or amino acids) and phosphorus from soil, supplies to orchid

• fungus supplies fixed carbon in organic compounds obtained from soil by digesting dead organic matter

• fungus supplies orchid with water absorbed from soil

zooxanthellae in hard corals as an example of mutualism

• coral’s calcium carbonate skeleton provides safe and protected environment for zooxanthellae

• coral grows close to surface of the sea, allowing photosynthetic algae to have reliable light source

• corals supply zooxanthellae with carbon dioxide produced by cell respiration

• zooxanthellae supply carbon compounds and oxygen produced by photosynthesis

endemic species

species naturally occurring in an area

invasive species

alien species which increases in number and spreads rapidly

competitive exclusion principle

two species cannot occupy the same ecological niche indefinitely, invasive species successfully competing causes endemic species to occupy a smaller realized niche

• examples*

tests for interspecific competition

• competitive exclusion may discourage two species from growing together

• H₀ (null hypothesis) - two species are distributed independently

• H₁ (alternative hypothesis) - two species are associated

• chi-squared test

• stronger evidence for competition through field manipulation and laboratory experiments

chi-squared test

used for testing independence.association, goodness of fit

• use of contingency table

• expected frequency = row total * column total / grand total

• rejecting null hypothesis is not proof of competition/alternative hypothesis