BIOL 413 midterm 1

Scales of ecology

Individual→ population→ community→ ecosystem→ biosphere

Population

Individuals of the same species lining in a defined area

Studies at population level:

• Emphasis on variation

  • Number

  • Density

  • Composition

Geographic range

The extent of land or water within which a population lives

Abundance

The total number of individuals

Density

The number of individuals per unit are

Composition

The “makeup” in terms of age, sex or genetics

Community

Collection of all populations living together in a defined area

• boundaries are not always rigid and may cover small or large areas

• Include many types of interactions

  • Predation

  • Competition

  • Herbivory

Studies at community level:

• diversity

  • Richness (Total)

  • Evenness (distribution)

• Species interactions

Abuses Paramecium experiment

Tested theory on protozoan pop. growing in small bottles, found that species grown separately achieved stable densities but when pairs of species were grown together in a simple environment one species always won out and the other species became extinct

competitive exclusion principle

Two species cannot coexist on one limited resource

Niche

Range of conditions that a species can tolerate

Fundamental niche

Parts of the environment that a species could occupy in the absence of interactions with other species - abiotic conditions

• range of temperatures

• Humidity

• Salinity

Realized niche

the range of biotic and abiotic conditions under which a species can persist (biotic: competition, predation)

Small scale variation

Creates geographic ranges that are composed of small patches of suitable habitat

Reciprocal transplant experiment

When planted outside their natural elevations, the two species grew poorly and experienced lower survival

Limiting similarity

Minimal niche difference between two competing species that would allow coexistence

d/w ~ 1

Four factors to explain species diversity

1. Process that determine “success” of species

2. Changes in relative abundance due to chance events

3. Movements of spp in/ out of communities

4. Generation of new species

Latitudinal diversity gradient

As one gets closer to the equator more species are seen

• diversity as species richness - large charismatic organism

Measuring diversity

• defining a community boundary is arbitrary (quadrant, transect)

• Richness strongly correlates (+) with sample size

• Richness scales non-linearly with sample size/effort - also arbitrary

• Species richness only one aspect of diversity:

  • genetic diversity

  • Functional diversity

  • Phylogenetic diversity

Species accumulation curve

A biologist walking in a straight line through a community, counting the number of species they encounter

Initially every new individual encountered is likely to be different species, but at some point species will be encountered that have already been seen and the line becomes horizontal for a while, then a new species is encountered and the line goes up by one species again

• if curve is up “increasing deceleration function”

• If curve is down “decreasing deceleration function”

Highly uneven distribution curves with lots of rare species increase slowly

Alpha diversity

The number of species found at a local scale

Beta diversity

Measure of difference in species composition or species turnover between two or more habitats or local sites within a region

Gamma diversity

A measure of species richness in a region

B= a/y

Factors that change alpha & beta diversity

1. Unevenness ( lower a + increased B)

2. Dispersion (clumped vs random) - lower a + increased b

   • similar affect s unevenness

   • Faster increase with random

3. Higher regional (gamma diversity) - decreased a + increased B

4. Smaller local plot area - decreased a + increased b

   • like sampling fewer individuals

5. Lower density of individuals - increase a + decrease B

   • fewer individuals per plot

Species-area relationship (SAR)

S = cA^z

S=number of species

A=area

C&z=fitted constant

• Large areas contain more species than smaller areas

• Larger areas contain greater variety of habitat types

• Different species have different habitat affinities

• Larger areas = more species

  • larger areas support larger populations (lower chance of extinction)

SARs two drivers

• immigration:

  • rate declines with # of resident species (0 when source and sink have the same species)

• Extinction:

  • rate increases with number of resident species

  • Just more species to go extinct

  • Number of individuals/species decreases as total residence increases (smaller populations)

• Even in areas of uniform habitat, larger areas= more species

  • larger areas support larger population (lower chance of extinction)

  • Intersection =equilibrium point (immigration-extinction equilibrium richness

Are most species common or rare?

• most communities:

  • few common species

  • Many rare species

• Many potential causes

  • periodic disturbances (fire, salt marshes)

  • Sampling & transient (migrating) species

    • imperfect

• Competitive exclusion

  • few dominants outcompete

• Freq. dep predation

  • common vs rare

• Genetic variation

  • small = pop low genetic var (vulnerable to disturbance/disease

Productivity and species richness

• broad scales: species richness increases with productivity

  • productivity = conversion of resources to biomass

  • Regional (large scale) positive (sometimes decelerating)

    • high productivity = high richness

• Smaller scale: varies patterns - positive, negative “hump shape”, “U shape”

• productivity peaks at intermediate species richness

  • richness limited by abiotic stress in unproductive environments and a species interaction in productive ones

  • Nutrient limitation + light limitation

  • High/low productivity environments rare

  • Varies from low to high annually average intermediate