Why does succession take so long?

• Different plant species have different ecological requirements. A beech or live-oak needs shade as a seedling.

• They also need soil moisture which means the soil must have a high organic content.

• So succession is also the development of soil and colonization by soil organisms.

3 models of successionConnell and Slatyer (1979)

Facilitation

• Initially thought that all succession was due to facilitation.

Example of facilitation: shade provided by pines allows seedlings of broad-leaved trees to survive.

Or

Growth of a nitrogen-fixing plant on sandy (nutrient poor) soils such as alder enriches the soil sufficiently for other species to colonize.

Tolerance

• All species could live at all stages of the succession, but differing dispersal abilities/adaptations ensures earliest stages occupied by pioneer-type species.

• As succession proceeds fewer and fewer of the early successional species can tolerate the new conditions and so the system matures toward D.

• Dispersal distance is therefore also a big factor to be considered.

Inhibition

• Species mutually inhibit one another through competition. System can only change when an individual dies an is replaced.

• What will influence that replacement?

Soil maturity and succession

• Soil accumulates organic matter as succession proceeds.

• Increased ability to hold moisture.

• Pioneer species are shaded out.

The balance

• Now understood that facilitation, tolerance, and inhibition all combine to produce succession.

Clements vs Gleason

• We now know that species respond individualistically to change.

• Communities are not superorganisms and will not always return to a predictable equilibrium.

Colonization of new areasPrimary Succession: Colonization of new areas

Colonization of a new area

• Follows succession from pioneers to competitors…..but all have to disperse there.

• Distance from source is important…can larvae survive long enough to be transported there? Can seeds be blown there? Can mammals swim there? Can birds fly there?

Two ways to study succession

• Follow one location from disturbance to maturity, ex. Krakatau, Mt St Helens.

• Select similar habitats at different times since similar disturbance, ex. Glacier Bay, building riverbank

• Macroalgal succession over 30 months on experimental concrete blocks.

UlvaSea lettuce

Intertidal succession

• Steadily retreating glacier since 1850s.

• New land surface revealed…primary succession.

• Oldest succession where ice first retreated.

1850

1912

Primary succession in Glacier Bay, Alaska

Glacier Bay Succession

• Retreating glaciers expose new land surface of till.

• Rate of retreat ca. 65 km in 200 years

• Succession follows broadly predictable path

Nutrient changes at

Glacier Bay• The initial soil is nutrient

poor.• Alder is an N-fixer, spruce

and hemlock are not.• “forest floor” reflects N in

leaf and wood litter.• Why is there a peak in

forest floor N at the transition to spruce-hemlock.

• Why does soil N decline in the spruce-hemlock zone?

AT least that has been the accepted story..BUT!

• Fastie (1995, Ecology) shows that alder may actually slow the succession through competition.

• The succession to Sitka Spruce was much faster in the sites deglaciated in the 1780s-1840s than the later sites.

• And hemlock has not begun to grow at any site that initiated after 1840.

Simulation showing nitrogen inputs during 2ndry succession

Importance of alder (ALRU) as a nitrogen fixer and Ceanothus (CEVE), early in succession.

Nitrogen sources

Rotmoos Glacier, Italian alps 1895

Rotmoos Glacier, Italian alps 1999

Rotmoos Glacier, Italian alps

• 1895 glacial tongue evident in valley

• 1999 2km of retreat evident

• Following is work by Kauffman, Ecology (2001)

First 50 yrs

• Sparse vegetation means little local productivity.

• Surprisingly, insects are primarily predators relying on allochthonous (derived from elsewhere) sources of prey

5 yrs: Harvestman- a glacial specialist Predator.

10 yrs: 4 spp. Of ground beetle, occupying separate niches.

20 yrs: assorted spidersAbundant.

30 yr Centipedes, under rocks50 yrs: herbivorous beetles as vegetation density increases.

50-150 yrs first

appearances

• As vegetation increases in density a more normal insect spectrum is represented with food chains supported by herbivores. Detrital cycle also evident. System now autochthonous (productivity is local).

70 yr: millipedes are important decomposers.

100 yrs: 1 sp. Of ant occupies sunniest locations

140 yrs: Densest Vegetation supportsgrasshoppers

Effect of succession on adjacent waters

• Shading of margins• As succession increases soil organic content will

also increase dissolved organic carbon (DOC), e.g. humic acids and tannins.

• Leaching into waterways these chemicals color the water and reduce transparency.

Dissolved Organic Carbon, e.g humic acids influences aquatic

foodchainsMacroZooplanktonSpp.#

2

3-4

5

DOC

Low

Moderate

High

Mt St. Helens example of interference

• Lupins are N fixers and were colonists after the Mt St. Helens eruption.

• Lupin expansion rapid at first but slowed after a few years.

Fagan & Bishop(American Naturalist 2000)

• At expansion edge Lupin expansion limited by herbivory.

• In center of lupin range herbivory limited by predation.

Interactive effects of

herbivory and predation

This river is building a spit

• As the spit grows the youngest vegetation will be on the tip.

• Often a clear succession, both in terms of age and species composition, is evident.

Succession in streams &rivers

• Note the change in size and species composition beside the channel

Can we make predictions about the colonization process?

• Here is an island.

• Most plants are going to arrive either by wind, sea, or bird/bat.

Krakatau case study

• Krakatau is an island group in Indonesia.

• A volcanic eruption sterilized the islands in 1883.

Krakatau: The best example of primary succession

• Aug 27th 1883 Volcanic explosion sterilized islands

• 2000 times the power of the bomb dropped on Hiroshima.

• Generated tsunami that killed 36,000 people• 100 m thickness of new ash coated the

islands…new land surface. A natural laboratory.

• Colonization of plants and animals documented since 1884.

Krakatau is west of Java

Immediately after the eruption• 1884: no plant life found, some blue-green algae growing

on ash.

• 1896: there were some coastal shrubs, scattered grasses and shrubs in the interior.

• 1908: Interior a “parkland” of grasses and clumps of trees.

The succession continues

• 1928-1932: forests close over the grassland.• 1979-1992: forests changing in species composition.

Earliest trees now 60-80 years old.

Increase in species diversity• Plant species

continue to colonize the islands.

• New plants provide new opportunities for animals.

• Animals cannot colonize until foodplant is present.

Pattern evident in dispersal

mechanism of arrivals

• At first wind and sea dispersed species.

• Later trees dominated by bird and bat dispersed species.

• Wind dispersal still brings orchids and ferns.

Large pigeons and bats regularly move between islands and

mainland

Development of structure

• As succession proceeds the physical structure of the vegetation becomes more complex, offering more niches. Trunk-cavities, vines, larger limbs.

• How would this affect recruitment?

Why structure matters

• The more layers in the canopy the higher the animal diversity.Offer different feeding opportunities.

• Trunk-cavities provide nest sites for birds and insects.

• Vines provide food, cover and nest sites.• Dead wood for decomposers • Larger limbs better attachment sites for

epiphytes.

Biotic and abiotic influences

• Nitrogen likely to be limiting nutrient early in succession….why?

Source of recovery will be different in

primary and secondary succession

How habitat quality

influences succession

• Walker and Chapin’s model considers the importance of major ecological factors in succession in terms of severe and favorable landscapes.

Walker and Chapin Cont.

• Note how differently facilitation and competition influence succession.