How do Communities Change?

Question 1

Why would a species establish or not?

Answer 1

• dispersal limitation
• niche requirements
• competition
• exploitation
• associated mutualists

Question 2

Chronosequence

Answer 2

• series of communities or ecosystems representing a range of ages or times since disturbance
• farther distance from glacier represents going forward in time

Question 3

Reiners et al. 1971 (experiment and results)

Answer 3

• glacier bay glacial extent since 1794
• took advantage of existing knowledge on when ice retreated
• n = 7 sites
• after a steep upward trend, the increase in species richness slowed down towards an asymptote

Question 4

Example of secondary succession (in a boreal forest)

Answer 4

• fires are a natural disturbance in the boreal forest and can differ in severity and impact
• sites that recently experienced fires were dominated by aspen
• some species, like pin cherry and willow were only found shortly after fire
• older stands were dominated by white cedar
• aspen declined
• balsam fir and white spruce with constant abundance

Question 5

Why do we see a change in species composition?

Answer 5

one explanation is that species have different adaptations (life-history traits) that allow them to colonize, compete, and persist

Question 6

Mechanism of succession in a boreal forest

Answer 6

adaptation to fire
- many spruce require fire to release seeds from cones
- creates high light conditions for aspen seedlings
- earn hardwood species like aspen actually inhibit forest fires (don’t burn very easily)
- spruce burn better
-

Question 7

Longleaf pine Savannah

Answer 7

• another example of adaptions to fire
• maintained indefinitely as endpoint of succession as long as fire continues (fire climax)
• depend of summer fires to kill fire-intolerant hardwood trees
• adapted to tolerate (even promote) fires
• in absence of fore, hardwoods replace pine savanna

Question 8

Succession in Freshwater Lakes

Answer 8

experience two time scales
- first type of succession occurs over a relatively short time scales, across seasons are few years, in response to disturbance events (like freezing, flooding)
- second type occurs over geological time scales

Question 9

What are some ecosystem properties that can change during the course of succession?

Answer 9

• biomass
• primary production
• respiration
• nutrient retention

Question 10

Ecosystem changes at Glacier bay

Answer 10

• increase in soil depth during succession
• proportion of organic and litter horizon increases with succession
• changes drives by biomass accumulation

Question 11

What are the Three Models of Succession?

Answer 11

1. facilitation model
2. tolerance model
3. inhibition model

Question 12

Facilitation model?

Answer 12

early species modify the environments in ways that hinder themselves and benefit later species

Question 13

Tolerance Model

Answer 13

• early species modify the environment but its does not positively or negatively affect later species
• later species dominate due to slow growth and environmental tolerance (life history characteristics)

Question 14

Inhibition Model

Answer 14

• early species modify conditions in ways that hinder later species
• disturbance or stress needed for later species to become abundance
• basically opposite of facilitation