C4.1 - Populations & Communities

Population

A group of individuals of the same species living in a specific area at the same time, sharing resources and interacting with one another.

Estimation of population size

• Random sampling: every member of a population has an equal chance of being selected for the sample

  • sampling errors are always possible: difference between the true and estimated value

    • to reduce sampling error —> bigger quadrants

• methods depend on if sessile or motile

  • Sessile: random quadrat sampling

    • Mark boundary + generate random numbers to determine coordinates + estimate how much of total area covered by quadrants + estimate population size

    • Standard deviation indicates the degree of variability

  • Motile: capture-mark-release-recapture + the Lincoln index

    • capture - mark - release - recapture - calculate Lincoln indexx

    • m = number of individuals captured + marked first time

    • N = total number of individuals captured in 2nd sample

    • R = number of recaptured marked individuals

    • M x N/R

    • Assumptions made: no migration, no deaths, marked + unmarked have same chance of being recaptured, no births, marks remain visible (carving shell), marks do not affect survival (maybe it causes them to be eaten by predators)

Population growth curves

Graphical representations of how populations change over time, often including phases such as lag, exponential, transitional + plateau/carrying capacity phases - Sigmoid-curve

EXAMPLE: Eurasian collared dove

• Lag

  • An adaptation period

  • Doves are adjusting to their new conditions

  • The length of the lag phase can vary considerably - based on how different the conditions are from the conditions that the bacteria came from

• Exponential: happens when density dependent factors are not effective/movement into a new niche where resources are abundant

  • reproduction

  • little to inhibit growth (like resources or space is not a limiting factor)

  • Positive feedback (more doves = more doves)

• Transitional

  • Growth slows

  • Not exponential

  • Resources are limiting factor

• Plateau

  • Equal births and deaths

  • Max population number the ecosystem can support

Modelling of the sigmoid population growth curve

• Using duckweed / yeast

• Start with small number of organisms + abundant resources - exponential growth stage

• As time goes on can estimate populations + at what population size is carrying capacity reached

• Can investigate carrying capacity or variables that affect growth

Negative feedback control of population size by density-dependent factors

Negative feedback control: populations might rise and fall periodically but are relatively stable over time

• factors bringing populations down when too high and up when too low

Density-Dependent: have an increasing effect when the population is bigger

• Disease/parasites

• Predation

• Competition

Density-Independent: same effect no matter the population size

• Flooding

• Drought

• Forest fire

Example:

• if way over carrying capacity with more breeding + fewer deaths - density dependent factors play a big role in population size

  • Density dependent factors cause more deaths and fewer births

• if way below carrying capacity with more deaths + fewer births - density dependent factors don’t play a big role

  • Density dependent factors cause more birth + fewer deaths

Competition vs cooperation in intraspecific relationships

Relationships that exist between individuals of same species

• Competition: occupy the same ecological niche

  • In plants = Competition for light: Wild garlic = crowded in woodland so not all get light, Competition for pollinators in flowering plants: honey bee is dusted with pollen as feeds on nectar in dandelion, Competition for soil nutrients

  • In animals = competition for food, territory, mates

  • Leads to natural selection because some individuals will have traits that help them outcompete others

• Cooperation: mutually beneficial relationship

  • Parental care in animals: one female eider duck take care of 40+ offspring not only own offspring

  • Defense against predation: California sea lions circle ‘bait ball’ of Mackerel - tightly packed + move fast

  • Communal roosting in animals: Emperor penguins huddling to conserve body heat

Community

Group of populations living/interacting together in an area

Herbivory as a category of interspecific relationship within communities

Primary consumers feeding on producers

• may or may not kill the producer

• E.g.

  • Sheep grazing grasses

  • Aphids eating sap from phloem

Predation as a category of interspecific relationship within communities

One consumer species (predator) killing + eating another consumer species (prey)

• E.g.

  • hawk eating a mouse

  • Dolphin eating a fish

  • Ladybug eating an aphid

• Predator-Prey relationships as an example of density-dependent control of animal populations

  • Cyclical process - more prey = more predators food = more predators = eat more prey = less prey = less predator food = less predators = less predation = more prey

Mutualism as a category of interspecific relationship within communities

Two species living in a close association, both benefiting from the association

• usually from different kingdoms because they bring very different things to the relationship

E.g.

• Photosynthesising zooxanthellae (algae) living in polyp cells of hard corals + exchanging materials with corals

  • Coral produces co2 for algae + algae produces o2 + carbohydrates for coral

  • Coral provides protected environment close to the surface where algae can absorb light

• Mycorrhizal fungi growing into roots of orchids + exchanging nutrients

  • Mycorrhizal fungi penetrate seed + provide mineral ions (nitrogen, phosphorus) + water

  • Orchids is dependant on mutualistic relationships because is tiny seed with little food storage

  • When orchid photosynthesis = provide mycorrhizae with carbohydrates

• Rhizobium bacteria living in root nodules of Fabaceae family (peas) + exchanging materials with the plant

  • Rhizobium bacteria can ‘fix’ nitrogen from air + convert to ammonium/nitrate ions and supply that to plant

  • Fabaceae plant supply bacteria with carbohydrate + protected place to live

  • Nitrogen availability = limiting factor to growth - cannot obtain from air only from soil

Parasitism as a category of interspecific relationship within communities

One species (parasite) living inside/on surface of host + obtaining food from the host - host is harmed but not killed, parasite benefits

E.g.

• Ticks on skin of deer feeding by sucking blood

• Roundworm living in gut of raccoons + absorbing foods digested by racoon

Pathogenicity as a category of interspecific relationship within communities

One species (pathogen) living inside other species (host) + causing disease in host

E.g.

• Potato blight fungus infecting potato plants

• Tuberculosis bacterium infecting badgers

• HIV in humans

Interspecific relationship vs Infraspecific relationship

Interspecific: relationships between different species

Infraspecific: relationships between individuals of the same species

Interspecific competition as a category of interspecific relationship within communities

Two species competing for a limited resource (plants = water, light, nutrients / animals =water, food, territory, oxygen)

• E.g. Barnacles competing for space + food on rocky shores/trees

Tests for interspecific competition

• Testing for interspecific competition:

  • Chi-squared test for species association

    • Field observation: random quadrat sampling to collect quantitative data over time - works very well for sessile organism

      • How many times species A occurs by itself and how many times species B occurs by itself

    • Null hypothesis: two species are distributed independently

    • Alternative hypothesis: two species are associated

    • Rejection of null hypothesis does not necessarily mean they are competing

      • Further investigation would be needed:

        • Field manipulation: removing one of competing species from area + observing consequences

        • Laboratory experiments: using controlled conditions to explore interactions between species + consequences when isolated

Use of the chi-squared test for association between two species

Expected: row total x column total/ grand total

To calculate X²: (O-E)²/ E

• then add together all values which gives u X²

P-value is always: 0.05

Degrees of freedom: (# rows - 1) x (# columns - 1) - will always be one for BIOLOGY

Look at critical value table

• where degrees of freedom = 1 and p value is 0.05 the critical value is 3.84

• If X² > critical value you reject null

Resource competition between endemic + invasive species

• Endemic species: occur naturally in an area

• Alien species (introduced outside their range by human activity) become invasive species if successful enough

• Invasive (only invasive if spread rapidly) species: alien to that environment spread rapidly due to lack of density-dependent factors like natural predators + introduced artificially (deliberately or on accident)

  • Endemic + invasive species competing for one+ same resources means endemic realised niche is reduced + species may go extinct by outcompeting

• E.g. Eurasian red squirrel (endemic) + grey squirrel (invasive)

  • 30 grey squirrels introduced from U.S as an ornamental species

  • Now: red squirrel is only found in North England, Wales, and Scotland

Antibiotics in interspecific competition

Most metabolic pathways are common to most organisms but targeting a unique metabolic pathway can control that population

Secreted by microorganisms to kill other competing microorganism

• E.g. Penicillium fungi - secrete penicillin (antibiotic) which kills saprotrophic gram-positive bacteria in soil

Allelopathy in interspecific competition

Most metabolic pathways are common to most organisms but targeting a unique metabolic pathway can control that population

Some plants release secondary metabolites in soil which negatively impact growth + create a competitive disadvantage for neighboring plants

• E.g. Eastern black walnut tree which releases chemical into soil —> inhibits root growth + photosynthesis in competitors

Top down control of populations

Something from higher in the food chain affects a lower level (predation)

• E.g. Wolves in Yellowstone National Park (more wolves = less elk = more plants)

• Predator population goes up = less herbivores = more producers = less nutrients

Bottom up control of populations

Something from a lower level in the food chain affects a higher level (nutrient sources)

• Most common limitations on producers = mineral nutrients in soil, light, water

• Not enough nutrients in soil = not many producer = not many herbivores = not many predators

Meres

Small lakes

• Shallow meres: sunlight penetrate to bed, water plants photosynthesise to top, invertebrates shelter among water plants + feed on algae (keep water clear)

• Deep meres: sunlight can’t penetrate far (bc dense population of algae), fish easily find invertebrates, invertebrates feed less on algae, dense growth of algae

Carrying capacity

Maximum population size that an organism can support