C4.1: Populations and Communities

Population

Group of organisms of the same species living in a given area

can interbreed, reproductive isolation like geographical barriers is used to distinguish one population from another

Population Size Estimation

Population size is difficult to estimate by counting as organisms may use camouflage or be spread over large areas

requires estimation based on evidence

Random Sampling

Should allow every member of population to have an equal chance of being selected

to avoid bias, random number generators should be used and multiple samples should be collected instead of one

may still result in sampling error

Sampling Error

The difference between estimate of population and actual size

Quadrat Sampling

A rectangle of known dimensions used to record the number of organisms present

uses a quadrat frame (sample areas) at random positions

Suitable for SESSILE organisms that do not move (plants)

Quadrat Sampling Procedure (4)

1. Use measuring tape to mark a base line along habitat's edge

2. Generate 2 random numbers, first determines distance along one axis, second determines distance across another, at a RIGHT angle, all distances must be EQUALLY likely

3. Place a quadrat determined by the two random numbers

If done correctly with enough replications, will provide reliable estimate of population size

Capture-Mark-Release-Recapture Method (5)

Suitable for motile organisms

1. Capture as many organisms as possible in an area by netting, trapping, or searching

2. Mark each organism w/out making them more visible to predators

3. Release back into habitats and allow them to reintegrate

4. Recapture as many organisms as possible and count how many are marked/unmarked

5. Calculate estimated population size w/ Lincoln Index

Lincoln Index

Population Size = (M*N)/R

M = number of organisms caught and marked initially

N = total number of organisms recaptured

R = number of recaptured organisms with marks

Assumptions in Lincoln Index (4)

1. No migration into/out of population

2. No births/deaths

3. Marked organisms have equal chance of being recaptured due to successful reentry into community

4. Marks are visible and do not increase risk of predation

Carrying Capacity

Maximum population size that environment can support based on available resources, environment can vary in abundance of resources, but all resources are limited

Competition for Limited Resources

If population becomes too large, competition will occur

organisms who do not obtain enough resources may die, decreasing population size

Plant Limiting Resources

H2O, light, nitrogen in soil

Animal Limiting Resources

H2O, space, food, dissolved O2

Over time, population sizes can... (3)

Increase long term due to unoccupied niche

or

Decrease long term due to negative impacts like habitat construction

or

Remain stable via negative feedback control

Factors that Influence Population Size

Density-Independent and Density-Dependent Factors

Density-Independent Factors

Have same effect regardless of size (forest fires), cause fluctuations in population size

Density-Dependent Factors

Have greater effect as population size increases (disease), pushes population size towards carrying capacity via negative feedback

Density Dependent Factors Examples (3)

Competition, Predation, Pathogens or Pests

Competition Density-Dependent Factor

Limited resources like water, food (animals), and light (plants)

Predation Density-Dependent Factor

More intense as population increases as that of prey

Pathogens or Pests

More easily spread as population density increases

Exponential Growth

Populations increase at accelerating rates in absence of limiting factors, involves positive feedback where breeding increases population and larger population breeds more

however density-dependent factors prevent this via negative feedback

Exponential Growth Example

Species find an unoccupied ecological niche in a new area

ex. Eurasian collared doves moved from Asia to Europe and thrived w/ abundant food from bird feeding and agriculture

Limits to Exponential Growth

Growth eventually stops due to carrying capacity, resources are limited and other density-dependent factors may have an impact

Population graph changes from J to S shape over time

Sigmoid Curve

Shows population stabilization as it nears carrying capacity

Boom and Bust Cycles

Not all populations reach a plateau after growth, instead greatly exceeds carrying capacity then drops below carrying capacity

Reaches peak then crashes due to toxin accumulation or environment stress

Modeling Sigmoid Population Growth

Ecosystem growth can be modeled using duckweed

-introduce small number of organisms with abundant resources

-measure population growth via data logging or counting

Duckweed

Stemless photosynthetic water plants found in ponds/lakes

-small floating fronds w/ single root

-reproduces asexually through new fronds that separate

-Grow in beakers/cups

Intraspecific Relationship

Relationship between individuals of same species, usually within same population

Competition

Results because of limited resources, all organisms are harmed to some degree, some are more successful than others which increases their chances of survival and reproduction (natural selection)

Competition Examples (3)

1. For pollinators - dandelions use bright colors/nectar to attract honey bees, then dust them with pollen

2. For food - Bohemian waxwings compete for berries and migrate south when they run out

3. For breeding - guillemots compete for ledges of sea cliffs

Cooperation

Advantageous over competition since all organisms involved benefit

the degree of cooperation varies with social animals like termites benefitting the most

Cooperation Examples (3)

1. Roosting - fantails and emperor penguins conserve body heat by doing it together

2. Defense against predation - fish form tightly packed, fast-moving "bait balls" to make it harder for predators

3. Parental care - one or more eider ducks care for offspring of multiple parents

Community

A group of populations living together and interacting in an area including plants, animals, fungi, and bacteria

hundreds or thousands of species may exist within an ecosystem

Types of Interspecific Relationships (6)

Herbivory, Predation, Interspecific Competition, Mutualism, Parasitism, Pathogenicity

Herbivory

Primary consumers feed on producers; producer may or may not be harmed

ex. bison grazing on grass, aphids feeding on phloem

Predation

One consumer (predator) kills and eats another consumer (prey)

ex. anteaters feeding on ants/termites, starfish eating oysters

Interspecific Competition

Two or more species use same resources; amount taken by one reduces availability for other

ex. ivy climbing on oak trees to compete for light, barnacles competing for space/food on rocky shores

Mutualism

Two species both benefit from relationship

ex. mycorrhizal fungi grow into roots of orchids and exchange nutrients, zooxanthellae in coral cells photosynthesize and exchange materials

Parasitism

One species (parasite) lives inside or on surface of another (host) to obtain food from and cause them harm

ex. ticks live on skin of deer and feed on their blood, roundworms live in gut of racoons and absorb digested food

Pathogenicity

One species (pathogen) lives inside another (host) and causes disease

ex. potato blight fungus infects potato plants, tuberculosis bacterium infects badgers

Mutualism Examples (3)

Root nodules in Fabaceae and Rhizobium

Mycorrhizae in Orchidaceae

Zooxanthellae in hard corals

Root nodules in Fabaceae and Rhizobium

Plants need nitrogen, but cannot use atmospheric nitrogen (N2) and requires a fixed form like ammonium (NH4+) or nitrate (NO3-)

Plant Provides: root nodules (protected environment), low O2 conditions, sugars via photosynthesis shared w/ bacteria

Rhizobium Provides: fixation of nitrogen (N2) into ammonium

Mycorrhizae in Orchidaceae

Orchid seeds do not have food reserves, fungal hyphae penetrate root of seedling and provides nutrients for growth

Orchid Provides: Carbon compounds via photosynthesis (later in life)

Fungus (mycorrhizae) Provides: absorption of nitrogen, phosphorous, and water from soil, organic nitrogen from digested dead organic matter

Zooxanthellae in Hard Corals

Corals secrete calcium carbonate (CaCO3) to form skeleton in which polyps live, most contain zooxanthellae, a photosynthetic algae

Coral Provides: Save/protected environment (skeleton), growth near surface for abundant light for photosynthesis, CO2 from respiration, used by algae

Zooxanthellae Provides: Carbon compounds from photosynthesis, O2 from photosynthesis, used by coral

Endemic Species

Species that occur naturally in an area, regulated by density-dependent factors

Alien Species

Species introduced by humans, deliberately or accidentally

unregulated since density-dependent factors like predators or pests are absent in new habitat

Competitive Exclusion Principle (For Alien Species)

alien species outcompete endemic species for resources, considered "invasive"

endemic species then occupy a smaller realized niche, decreasing their population or becoming extinct

Argentine Ant

Introduced in SoCal from S. America, forms large, cooperative colonies that can monopolize resources like food and habitat from native species (harvester ants)

Chi-Square

A statistical technique used to test for associations (statistical significance) between species in a habitat during quadrat sampling

Process:

1. Hypothesis (null vs alternative)

2. Table of Contingency and Frequencies (observed and expected)

3. Chi-Square Formula

4. Calculate degrees of freedom (always 1 for this sort problem)

5. Find p-value that matched Chi-square (want p<0.05)

Predator-Prey Relationships

In many communities, predator and prey populations remain stable due to similar birth/death rates

However, in some, they undergo cyclical oscillations

Predator-Prey Cyclical Oscillations

1. Increase prey, increase predator food availability, increase # predators

2. Increase # predators, increase predation, decrease # prey

3. Decrease # prey, decrease predator food availability, decrease # predators

4. Decrease # predators, decrease predation, increase # prey

Impact of Weather Conditions

Warmer spring/summer causes more plant growth, which provides more food for herbivores, increase in herbivores changes predator-prey dynamics

Red Fox and Mountain Hare

Observed in Sweden

Outbreak of sarcoptic mange (skin disease) in foxes causes a sharp decline in fox #s in 1980s, but outbreak ends in 1990s which resumes normal fox-hare population cycles

Control of Populations

Interaction btwn trophic levels in food chains serve as population control

either direct or indirect interactions

Direct Interactions

Predators feed on prey or herbivores feed on producers

Indirect Interactions

Predators influence producers that herbivores eat (ex. by fertilizing soil w/ feces)

Direction of Interactions

either top-down or bottom-up, although both are possible, one is more likely to be dominant in community

Top-Down Control

Control from higher trophic levels to lower ones, ex. increase predator leads to decrease in prey

Bottom-Up Control

Control from lower trophic levels to higher ones, ex. decrease soil nutrients leads to decrease in producers

Primary Metabolites

products of metabolism essential to survival

Secondary Metabolites

organic compounds that are not directly involved in the normal growth, development, or reproduction of an organism, can be toxins that deter competitors

Antibiotic Production by Penicillium

Penicillium secretes penicillin to prevent competition for digested good

Penicillium

Genus of fungi found in soil and on decaying food, it is a saprotrophs that digests carbon compounds externally and uses hypae to absorb digested products

Antibiotics

toxins secreted by microorganisms to kill/inhibit other microorganisms

Penicillin

An antibiotic secreted by penicillin that inhibits cross-linking of peptidoglycan in cell walls of bacteria, causing them to burst/die, only released when food supplies are scarce

Allelopathic Agents

toxins secreted into soil by plants to kill/inhibit neighboring plants

Tree of Heaven

Invasive tree species in N. America which uses allelopathy, releases ailanthone from root/stem bark