BIO 262 Succession + Island biogeography

Succession

Process by which communities change over time from one type (generally refers to vegetation type) to another, eventually coming to a 'steady state' climax community

Primary succession

Occurs in areas with bare rock

I. Glacial retreat

II. After volcanic eruption

Secondary succession

Occurs in areas where disturbance has removed vegetation without destroying the soil

I. Abandoned farm fields

II. After regular, low-intensity fires

Climax community

Community whose populations and species composition remain stable until disrupted by disturbance

Variation in successional stages in Scotland (Miles)

I. Heavy grazing --> climax stage is grassland

II. Moderate grazing --> climax stage is moor & bracken fern

III. No grazing --> climax stage is forest

Factors changing during terrestrial succession

I. Species diversity

II. Biomass

III. Productivity (NPP)

Factors changing during terrestrial succession

I. Species diversity

a. Glacier Bay (Reiners, Worley, Lawrence 1971)

i. Species diversity increased over time, plateaus in later-successional stages

ii. Time to maximum species diversity varies by plant type

Factors changing during terrestrial succession

I. Species diversity

c. Sycamore Creek in AZ (Fisher et al 1982)

i. Species richness plateaus quickly, then declines in later-successional stages

ii. Similar pattern for several different types of organisms

Factors changing during terrestrial succession

II. Biomass

a. Biomass accumulation model (Bormann and Likens 1981) for recovery of ecosystems following disturbance:

i. Reorganization phase: Forest loses biomass and nutrients, despite accumulation of living biomass (10-20 years).

ii. Aggradation phase: Ecosystem accumulates biomass and eventually reaches peak biomass (100+ years).

iii. Transition phase: Biomass declines somewhat from the peak.

iv. Steady-state phase: Biomass fluctuates around the mean

Factors changing during terrestrial succession

III. Productivity (NPP)

Increases but levels off and often eventually decreases somewhat

Mechanisms of succession

A. Facilitation

B. Tolerance

C. Inhibition

Mechanisms of succession

A. Facilitation

I. Many species attempt to colonize open space: however, only suitable "pioneer" species can actually succeed.

II. Pioneer species modify the environment in such a way that it becomes less suitable for themselves and more suitable for later-successional species

Mechanisms of succession

B. Tolerance

I. All species attempt to and are capable of colonizing open space

II. Early-colonizing species do not facilitate colonization by later-arriving species

III. Succession proceeds because later-successional species are longer-lived and more tolerant of environment conditions

Mechanisms of succession

C. Inhibition

I. All species attempt to and are capable of colonizing open space

II. Early-colonizing species modify the environment in such a way as to make it less suitable for later-arriving species

III. Later-successional species can only invade an area when the early-arriving species die or are removed

Why study islands?

A. Conservation Importance

B. Metapopulation framework - islands as a 'population of populations'

Why study islands?

A. Conservation importance

I. 'Real' islands are surrounded by water, BUT...

II. Islands can also be defined any 'patch' in a surrounding 'matrix'

a. Pond surrounded by land

b. River systems

c. Mountain vegetation surrounded by desert

d. Fragmented forest surrounded by development

III. Defined broadly, islands are ubiquitous

Why study islands?

B. Metapopulation framework - islands as a 'population of populations'

I. Levin's model - islands are all the same size

II. Island-mainland model: one larger 'mainland', surrounded by smaller 'islands'. Mainland is source of colonists, not the islands themselves. Also called' source-sink' model (mainland is the source, where births>deaths, islands are the sink, where deaths> births)

MacArthur-Wilson species equilibrium model of island biogeography

A. This model was first presented as a graph of gross extinction and immigration rates against the number of species present on the island.

I. Rate of immigration of new species (those not yet on the island) decreases with increasing number of species already present. It reaches zero when all species in the source area (there are P of them) are on the island

II. Rate of extinction of species increases as the number of species increases (the more species there are, the more to go extinct). Assumptions imply:

III. An equilibrium between immigration and extinction will eventually occur, at which time the immigration and extinction rates will equal the same value, called the turnover rate at equilibrium

MacArthur-Wilson species equilibrium model of island biogeography

B.

B. S^ is the number of species on the island at equilibrium. The y-intercept of the intersection is the turnover rate at equilibrium.

MacArthur-Wilson species equilibrium model of island biogeography

C. Two additional assumptions allow some more predictions:

I. Near islands have immigration rates higher than far, for the same number of species present.

II. Small islands have extinction rates higher than large, for the same number of species present

MacArthur-Wilson species equilibrium model of island biogeography

D. These imply:

I. Near islands of the same size as far have more species.

II. Large islands of the same distance as small have more species.

Tests of the MacArthur-Wilson (M-W) model of island biogeography

A. Krakatoa - volcano erupted in 1883, buried island in 30-60 m of ash

B. Diamond (1969): Balance of immigration and extinction in bird populations on CA Channel Islands shows predicted patterns for near versus far islands.

C. Simberloff and Wilson (1969): Defaunated (exterminated all organisms on) islands using fumigation, then watched re-colonization of arthropods

Other natural evidence

A. When exploring species-area relationships, how do you disentangle the impact of habitat diversity from the impact of area? Since habitat diversity increases as area increases, it can be hard to tell which factor is responsible for increasing species number. Answering this question, requires manipulating island area while keeping habitat diversity constant. Simberloff 1976: Starting with identical mangrove islands, kept some as controls and removed area from others. How did islands of reduced area differ in terms of species diversity?

I. In areas with identical habitat, larger islands had more species

II. Positive correlation between species richness and island area per se

Other natural evidence

B. Bahamas - Schoener & Spiller

I. Simulated hurricanes on small islands by removing vegetation...that failed, but then Hurricane Lili came through in 1996 and did the job for them.

II. On remote islands, spiders regained equilibrium species density in ≈ 1 year.

III. On remote islands, lizards never recolonized - why?

a. Spiders are good colonizers, so reach islands quickly.

b. Lizards are poor colonizers, so may not make it out to remote islands before hurricanes come and eliminate them

IV. So, M-W works for spiders, but not for lizards (due to high hurricane frequency and poor lizard dispersal ability)