Lec 28 (Disturbance)

Question 1

Succession?

Answer 1

a gradual change in communities
in an area following a disturbance or the
creation of new substrate

Question 2

Disturbance?

Answer 2

any relatively discrete event that disrupts an
ecosystem, community, or population structure
and changes resources, substrate availability,
or the environment

Question 3

The type of succession that occurs depends on

how extreme the disturbance was…

Answer 3

Primary succession= When disturbance creates/exposes new substrate (gradual change)

Secondary succession= Disturbance removes most organisms (but leaves/doesn’t destroy the soil)

Question 4

Succession can?

Answer 4

cycle indefinitely, or end with stability (until the next disturbance)

Question 5

Climax community?

Disclimax community?

Answer 5

Climax community: a community that occurs
late in succession and whose population
remains stable until disrupted by disturbance

Disclimax community: a community whose
species composition is maintained through
frequent disturbances

Question 6

What is The Intermediate Disturbance Hypothesis?

Answer 6

The theory that “intermediate” levels of disturbance allow for the greatest species diversity.

Question 7

Levels of disturbance are defined by?

Answer 7

– Frequency of the disturbance

– Intensity of the disturbance

Question 8

Intermediate levels of disturbance promote higher
diversity.

True or False

Answer 8

True

Question 9

Intermediate levels of disturbance promote higher

diversity….but why?

Answer 9

At Intermediate Disturbance:

– A moderate amount of change

– Sufficient time between disturbances for species
to colonize, but not enough time to allow
competitive exclusion.

– So, community contain organisms that are good
competitors AND organisms that are good
colonizers

Question 10

What about low disturbance?

Answer 10

• Little change

• Contain mostly organisms that are good competitors
– Also called ‘climax
species’
– K-selected species

Question 11

What about high disturbance?

Answer 11

• Lots of change

• Contain mostly organisms that are good colonizers
– Also called pioneer
species
– r-selected species

Question 12

Evidence Supporting the Intermediate

Disturbance Hypothesis: Grasslands…

Answer 12

Grasslands can experience a lot of disturbance (Grazing, Fire and Drought)

Burrowing by prairie dogs can lead to patchiness,
creating patches of grass, grass with a few forbs, and forbs with a little grass.

Question 13

How much evidence is there for the

intermediate disturbance hypothesis?

Answer 13

Some ecologists have NOT found support when looking at different ecosystems

Question 14

Some ecologists have NOT found support when looking at different ecosystems… Why?

Answer 14

Ecosystems are complex: effect of disturbance depends on the biology of the organisms in the system and the details of the disturbance.

…. So not all communities will change equally due to
disturbance.

Question 15

Why would communities NOT change when

disturbed?

Answer 15

Community Stability

Stability: the ability of a community to withstand
or recover from a disturbance

Question 16

What Drives Stability?

Answer 16

Resistance and Resilience

Question 17

What is Resistance and Resilience?

Answer 17

Resistance: The capacity of a community or ecosystem
to maintain structure and/or function in the
face of potential disturbance.

Resilience: The capacity to recover structure and/or function after disturbance; a highly resilient community or ecosystem may be completely disrupted by disturbance, but quickly return to its former state.

Question 18

Evidence for Stability… Park Grass Experiment

Answer 18

• Long term experiment (started in 1856!)

* Originally intended on testing the effects of fertilizer on hay yield

Question 19

-What were the results of the park grass experiment?

Answer 19

• Without fertilizer, grasses dominated
• With fertilizer, other species like legumes increased in frequency
• Whether a community is stable depends on how we view it!

-Scale is very important when measuring any
ecological phenomenon

Question 20

Increased _________leads to

greater stability.

Answer 20

diversity (both genetic diversity within

species and a high diversity of species)

Question 21

Insurance hypothesis?

Answer 21

theory that increased diversity increases community stability due to an increased probability of there being some species present in the community able to cope
with any particular disturbance.

Question 22

-What temperature results in the most diversity?

Answer 22

Average temperatures

Question 23

Biodiversity, stability and the insurance

hypothesis on coral reefs…

Answer 23

• Algae competes with coral for space
• Herbivory reduces competition
• Many types of herbivores on reefs
• High herbivore diversity and evenness supports coral community stability!