Transcript Slide 1

Disturbance & Ecological Succession
Hurricane Katrina
Aug. 29, 2005
Image from http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/katrina_goe_2005241_lrg.jpg
Disturbance & Ecological Succession
Succession – directional change in community composition at a site
(as opposed to simple fluctuations), initiated by natural or
anthropogenic disturbance, or the creation of a new site
Some biologists restrict the definition to directional
replacement of species after disturbance
Disturbance – a discrete event that damages or kills residents
on a site; either catastrophic or non-catastrophic
(Platt & Connell 2003)
Photo of W. J. Platt at Camp Whispering Pines, LA from K. Harms; photo of J. H. Connell from UCSB
Disturbance & Ecological Succession
Catastrophic disturbance – a disturbance that kills all residents of
all species on a site; i.e., creates a “blank slate” (Platt & Connell 2003)
Mt. St. Helens, Washington, U.S.A.
May 18, 1980
Photo of Mt. St. Helens from Wikipedia
Disturbance & Ecological Succession
Non-catastrophic disturbance – a disturbance that falls short of
wiping out all organisms from a site; i.e., leaves “residual organisms”
or “biological legacies” (Platt & Connell 2003)
Yellowstone Nat’l. Park, U.S.A.
just after 1988 fires
Luquillo Experimental Forest, Puerto Rico
just after 1989 Hurricane Hugo
Photo of Yellowstone in 1988 from Wikipedia;
Photo of Luquillo Forest, Puerto Rico in 1989 from http://pr.water.usgs.gov/public/webb/hurricane_hugo.html
Disturbance & Ecological Succession
Primary Succession – succession that occurs after the creation of
a “blank slate,” either through catastrophic disturbance or
de novo creation of a new site
Mt. St. Helens, Washington, U.S.A.
May 18, 1980
Anak Krakatau, Indonesia
appeared above water ~ 1930
Photo of Mt. St. Helens in 1980 from Wikipedia;
Photo of Anak Krakatau from http://amazingindonesia.net/2008/06/mount-krakatoa-the-wrath-of-earth
Disturbance & Ecological Succession
Secondary Succession – succession that occurs after
non-catastrophic disturbance (including “old fields”)
Yellowstone Nat’l. Park, U.S.A.
just after 1988 fires
Luquillo Experimental Forest, Puerto Rico
just after 1989 Hurricane Hugo
Photo of Yellowstone in 1988 from Wikipedia;
Photo of Luquillo Forest, Puero Rico in 1989 from http://pr.water.usgs.gov/public/webb/hurricane_hugo.html
Disturbance & Ecological Succession
Henry David Thoreau (1859) is often credited with coining
“succession” as applied to directional changes in plant communities
Thoreau made many remarkable observations at a time when many
still believed in such phenomena as spontaneous generation
“Though I do not believe that a plant will spring up
where no seed has been, I have great faith in a seed.
Convince me that you have a seed there,
and I am prepared to expect wonders.”
Photo of Thoreau from Wikipedia
Disturbance & Ecological Succession
A brief history of observations and ideas…
H. Cowles (1899) – stressed the dynamic nature of
“plant societies” (“phytosociology”)
Examined species composition of Lake
Michigan sand dunes & concluded that
the dunes were older further inland, i.e.,
formed a “chronosequence” from which
temporal change could be inferred
(space-for-time substitution)
Believed that succession tended toward a
stable equilibrium that was never (or at
least rarely) reached
Photo of Cowles from http://oz.plymouth.edu/~lts/ecology/ecohistory/cowles.html;
photo of Lake Michigan sand dune from http://ebeltz.net/folio/cfol-5.html
Disturbance & Ecological Succession
A brief history of observations and ideas…
F. Clements (1916, 1928) – radical, “superorganism”
view of communities; species interact to promote a
directed pattern of community development through
“seral” stages, ending in a “climax” community
H. Gleason (1926, 1939) – “individualistic view of
succession” in which “every species is a law unto
itself”
Our modern population-biology view derives primarily
from Gleason’s conceptual model, even though
Clementsian ideas of deterministic progression through
seral to climax stages dominated ecological theory well
into the 20th century (see Connell & Slatyer 1977)
Photos from http://oz.plymouth.edu/~lts/ecology/ecohistory/history.html
Disturbance & Ecological Succession
A brief history of observations and ideas…
F. Egler (1954) – made distinctions between primary succession
(“relay floristics,” in which initially there is no vegetation) vs.
secondary succession (following non-catastrophic disturbance of
existing vegetation)
Egler thought secondary successional patterns were driven by
propagules present when the disturbance occurs (“initial floristic
composition hypothesis”)
In addition, he thought that changes in species abundances reflected
differences in longevity of species
Disturbance & Ecological Succession
A brief history of observations and ideas…
Four classic papers demonstrate the maturation of thought concerning
the nature of trade-offs & colonization history within Gleason’s
“individualistic” framework
Horn & MacArthur (1972) – mathematical models of competition
among fugitive species in a harlequin environment
Drury & Nisbet (1973) – verbal models of succession driven by
differences in dispersal & competitive ability, growth & survival
Platt (1975) – empirical demonstration of mechanisms of coexistence
of fugitive species on badger-mound disturbances
Bormann & Likens (1979) – introduced the “shifting-mosaic steadystate” concept; within-patch non-equilibrial dynamics average to an
equilibrium pattern at the scale of many such patches taken together
Disturbance & Ecological Succession
A brief history of observations and ideas…
Connell & Slatyer (1977) – Reacted against an emphasis on life-history
strategies & competition alone; recognized a variety of species
interactions that could impact succession
Three models of succession:
1. Facilitation – Early species enhance the establishment of later
species (if it occurs, it is perhaps most likely in primary succession)
2. Tolerance – Early species have no effect on later species
3. Inhibition – Early species actively inhibit later species
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
One of the world’s most rapid and extensive glacial retreats in modern
times (so far); eliminated ~2500 km2 of ice in ~200 yr, exposing large
expanses of nutrient-poor boulder till to biotic colonization
Photo of Glacier Bay National Park, Alaska from Wikipedia
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
Classical view of Glacier Bay succession based on 50 yr of research,
which employed the simple chronosequence assumption:
- Mosses
- Mountain Avens (Dryas); shallow-rooted herbs
- Willows (Salix); first prostrate, then shrubby species
- Alder (Alnus crispus); after 50 yr forms thickets to 10 m
- Sitka Spruce (Picea sitchensis); invade alder thickets
- Hemlock (Tsuga heterophylla); establish last
Succession is driven by N-fixation (Dryas & Alnus)
Alnus acidifies the soil, allowing Picea invasion
Accumulation of soil carbon through succession improves soil texture
and water retention, ultimately allowing invasion by Tsuga
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
Fastie (1995) – Reconstructed patterns of stand development at
several sites within the chronosequence; intensively analyzed tree-rings
Figure from Fastie (1995)
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
Fastie (1995) – Identified 3 alternative pathways of compositional
change (not a single chronosequence of events):
1. Sites deglaciated prior to 1840 were colonized early by
Picea & Tsuga
2. Sites deglaciated since 1840 were the only sites colonized
early by N-fixing Alnus
3. Sites deglaciated since 1900 were the only sites dominated
relatively early by black cottonwood (Populus trichocarpa)
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
Oldest sites:
Intermediate sites:
Youngest sites:
Dryas  Picea & Tsuga
Dryas  Alnus  Picea
Dryas  Alnus  Populus  Picea
What accounts for these among-site differences in composition?
Differences are unrelated to soil parent material
Strong effect of seed source: Refugial Picea stands are
concentrated at the mouth of the bay; distance from the
nearest seed source explains 58% of among-site
variance in early Picea recruitment
Younger sites received more of their seed rain from new
communities colonizing exposed surfaces than from refugial
populations
Disturbance & Ecological Succession
Primary succession along the Glacier Bay chronosequence
What about facilitation?
Succession of Alnus to Picea was considered a textbook example of
facilitation in the mid- to-late 20th century
The real pattern is more complex!
Alnus was absent on older sites, so Picea does not require it for
establishment
Alnus may either inhibit or facilitate seedling establishment of Picea
Chapin et al. (1994) – Found net positive effects of Alnus on
Picea on glacial moraines, but net negative effects on
floodplains
Disturbance & Ecological Succession
Facilitation along cobble beaches of New England
Bruno (2000) – Determined mechanisms by which Spartina alterniflora
is a facilitator of relatively large impact on the community (i.e., a
“foundation species” - Drayton [1972]; “keystone modifier” - Bond
[1993]; “ecosystem or keystone engineer” - Jones et al. [1994])
Observations:
Spartina occurs along the
shore; cobble-beach plants
occur behind Spartina
Cobble-beach community is
absent along breaks in the
Spartina phalanx
Photo by J. Bruno
Disturbance & Ecological Succession
Facilitation along cobble beaches of New England
Bruno (2000)
Question:
At which life stage(s) is colonization cobble-beach plants limited to sites
behind Spartina?
Experiment:
Addition experiments to determine limiting life stages (seed supply,
seed germination, seedling emergence, seedling establishment & adult
survival) for cobble-beach plants
Results:
Only seedling emergence & establishment were adversely affected
by the absence of Spartina
Disturbance & Ecological Succession
Facilitation along cobble beaches of New England
Bruno (2000)
Question:
By what mechanism(s) does Spartina facilitate seedling emergence &
establishment of cobble-beach plants?
Experiment:
Conducted manipulations of water velocity, substrate stability, herbivory
& soil quality in sites lacking Spartina
Results:
Substrate stability increased seedling emergence & establishment,
whereas manipulations of the other factors had limited influence
Disturbance & Ecological Succession
Facilitation along cobble beaches of New England
Bruno (2000)
Conclusions:
Spartina alterniflora acts as a foundation species, keystone modifier &
ecosystem engineer) by stabilizing the substrate, enabling seedlings of
cobble-beach plants to emerge & survive
Photo by J. Bruno
Disturbance & Ecological Succession
Primary succession on Krakatau & Anak Krakatau
Explosion of Krakatau (1883)
The loudest explosion ever
heard by humans
Created tsunamis that killed
30,000 people on larger
islands & mainland
Anak Krakatau
The island was effectively “sterilized”
Anak Krakatau (“Child of Krakatau”) appeared out of the ocean in
~1930 & has been growing ever since
First analyses of colonizing vegetation were by Doctors van Leeuwen
(~1930s); more recent expeditions by Robert J. Whittaker
Photo of Anak Krakatau from http://amazingindonesia.net/2008/06/mount-krakatoa-the-wrath-of-earth
Disturbance & Ecological Succession
Primary succession on Krakatau & Anak Krakatau
Whittaker (1994) – Examined dispersal characteristics of plant arrivals
Nearest mainland site is Sumatra (~ 50 km away);
Nearest island is ~ 21 km away
First arrivals (within 4 yr of eruption) were either wind or water
dispersed
Early zoochorous plants were dominated by figs; 17 of 24 fig species
on the island arrived in the first 30 yr and are now canopy dominants,
which suggests that bats have been very important dispersal vectors
or mobile links (Old World bats have gut-retention times up to 12 hr)
Disturbance & Ecological Succession
Primary succession on Krakatau & Anak Krakatau
Whittaker (1994) – There are now 124 zoochorous species on Anak
Krakatau
Doves and pigeons (> 4 hr gut retention time) have been important
dispersers subsequent to colonization of the island by figs (an indirect
mechanism of facilitation by bats operating through figs?)
Many large-seeded species are absent relative to Sumatra & the
mainland flora
Disturbance & Ecological Succession
Primary succession on Krakatau & Anak Krakatau
Anak Krakatau
Image taken June 11, 2005 from Ikonos satellite
Image from http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/krakatau.IKO2005_06_11_lrg.jpg
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
May 18, 1980 – the north face of the
previously symmetrical mountain
collapsed in a rock-debris avalanche
that essentially wiped clean 60 km2
of forest
Fagan & Bishop (2000) – Examined
the influence of herbivores on the rate
of spread of lupines (Lupinus lepidus),
the site’s main “colonizing” species
Mt. St. Helens, Washington, U.S.A.
May 18, 1980
Photo of Mt. St. Helens from Wikipedia
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
Lupines are efficient N-fixers
& trap detritus; they are often
facilitators in ecological
succession
Lupines colonized from
remnant populations
elsewhere on the
volcano to form patches
Spread rapidly initially
and then slowed
Why?
Figure from Fagan & Bishop (2000)
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
Fagan & Bishop (2000) – Ruled out various alternative explanations
for slowed population growth rates & focused on the effect of insect
herbivores, whose colonization lagged behind the lupines by 10 yr
Experimental test:
Established plots at the center of lupine patches (core) and at the edge
of expanding patches (edge)
Sprayed half of the plots with pyrethroid insecticide
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
Much higher incidence of
damaging insects at
patch edges
Higher leaf damage at
patch edges
Figure from Fagan & Bishop (2000)
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
Lower seed production
at patch edges
Edge
Site
Core
Site
Why was there more herbivore activity at the edge?
Densities of predators (e.g., spiders) & parasitoids (e.g., a tachinid fly)
were 4x higher at the core vs. edge
Predators may be more abundant in the core where plant density &
productivity are higher
Figure from Fagan & Bishop (2000)
Disturbance & Ecological Succession
Primary succession on the flanks of Mount St. Helens
Fagan and Bishop (2000) – Diffusion model showed how reduced seed
production at the edge affects rates of lupine spread (assuming no longdistance, jump-dispersal events)
Figure from Fagan & Bishop (2000)
Disturbance & Ecological Succession
Modeling secondary succession – Horn (1975)
Developed simple Markov models of successional replacement
of temperate-zone tree species
Forest consists of cells, each occupied by a single tree
Probability of replacing an individual tree with a new individual of
a given species is calculated from a transition matrix
Example of transition matrix for four species
(GB=grey birch; BG=black gum; RM=red maple; BE=beech)
GB
BG
RM
BE
GB
0.05
0.01
0
0
BG
0.36
0.57
0.14
0.01
RM
0.50
0.25
0.55
0.03
BE
0.09
0.17
0.31
0.96
Initial composition vector: (100, 0, 0, 0)
After 1 time step:
(5, 36, 50, 9)
Iterate this process & plot the
changes in relative abundance…
Disturbance & Ecological Succession
Modeling secondary succession – Horn (1975)
GB
BE
RM
BG
Figure from Horn (1975)
Disturbance & Ecological Succession
Modeling secondary succession – Horn (1975)
One approach for estimating transition probabilities: proportional to the
fraction of each species as saplings beneath adults, e.g., if 5% of
saplings beneath GB are GB, then P(GB|GB)=0.05
If the same transition matrix is used throughout, then a stable composition
(the dominant Eigenvector) will result (here dominated by BE)
However, the Markov approach is phenomenological, so…
Why do recruitment probabilities vary, i.e., what biological traits lead to
different colonization rates & relative abundances?
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
The most recent generation of forest simulation models; precursors
include FORET (Shugart & West 1977)
Spatially explicit, mechanistic simulation model developed to predict
dynamics of succession for 9 species of northeastern U.S.A. hardwoods
Early occupation by Red Oak (Quercus rubra) & Black Cherry (Prunus
serotina) followed by late dominance by Beech (Fagus grandifolia) &
Hemlock (Tsuga canadensis), with Yellow Birch (Betula alleghaniensis)
present in gaps
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
Basics of SORTIE:
Spatially explicit model predicting the fate of every individual tree
throughout its life
Individual performance is affected by resource availability at each tree’s
location (original SORTIE only included competition for light)
Species-specific functions predict each individual’s growth, mortality,
fecundity & dispersal; estimated from data collected in the field
Four sub-models determine the fate of each individual throughout its life
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
(1) Resource (light) submodel: Calculates light available to an
individual based on its neighborhood; the process is analogous to taking
a fisheye photo above each plant
Calculates a projected cylindrical crown for each individual based on data
relating crown diameter & depth to stem diameter
Computes whole-season photosynthetically active radiation (PAR) for
each plant based on the location & identity of neighbors
Figure from Pacala et al. 1996
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
(2) Growth sub-model: Species-specific equations predict radial growth
from diameter & light availability
Figure from Pacala et al. 1996
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
(3) Mortality sub-model: Species-specific equations predict probability
of death from an individual’s growth rate over the past 5 yr
Figure from Pacala et al. 1996
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
(4) Recruitment sub-model: Species-specific equations predict the
number & spatial locations of seedlings based on the sizes of adult trees
Figure from Pacala et al. 1996
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
Community-level output: From randomly seeded initial composition
Hemlock & Beech clearly dominated after 500 yr
Figure from Pacala et al. 1996
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
The mechanistic approach taken in this model allows one to ask:
Which key traits define species performance?
How sensitive are model predictions to parameter values (and therefore
sampling errors in parameter estimation)?
How would hypothetical species with different parameter values perform
in this community? What would constitute a “superspecies” (i.e., one of
J. Silvertown’s ecological / evolutionary “demons”)?
How many species could potentially coexist,
e.g., > 50 spp. for > 10,000 yr?
How would changing abiotic / biotic conditions affect forest trajectories?
Disturbance & Ecological Succession
Modeling secondary succession – Pacala et al. (1996; SORTIE)
See Doug Deutschman’s on-line visualization of SORTIE!
Link to SORTIE visualization
Baseline without
disturbance
Figures from Deutschman et al. 1997
Heavy
disturbance
Large, circular
clear-cut
Disturbance & Ecological Succession
Succession may involve changes beyond species composition…
Community and Ecosystem Properties:
Diversity – often increases throughout succession
Standing-crop biomass – often increases throughout succession
Elemental cycling & other biogeochemical processes –
e.g., the Hubbard Brook experiments in New Hampshire,
and Peter Vitousek’s work in Hawaii
Susceptibility to disturbance – may be a function of time since last
disturbance, e.g., fire and the accumulation of fuel loads
Anthropogenic Disturbance & Ecological Succession
If “all species have evolved in the presence of disturbance, and thus are
in a sense matched to the recurrence pattern of the perturbation”, why
are anthropogenic disturbances often so damaging? (Paine et al. 1998)
Anthropogenic disturbances often differ from the natural
disturbance regime in timing, frequency, or intensity
Paine et al. (1998) also argued that: “more serious ecological
consequences result from compounded perturbations within the
normative recovery time of the community in question”
Anthropogenic Disturbance & Ecological Succession
A marine example: Corals in the Caribbean
Hughes (1994, Science)
One-two punch of overfishing (“selective disturbance”) & “natural”
mass mortality of dominant urchins (Diadema) has created a
“phase-shift” from coral-dominated to macroalgae-dominated reefs
Caribbean coral reefs may never recover!
Photo of macroalgae-dominated reef from http://news.mongabay.com/2008/0108-hance_coral.html
Anthropogenic Disturbance & Ecological Succession
A terrestrial example: Dipterocarps in southeast Asia
Curran et al. (1999, Science)
One-two punch of logging & increased
frequency of El Niño events (due to
anthropogenically induced climate change?)
resulted in elimination of recruitment by
dipterocarps in forests of Borneo!
May result in a large-scale “phase-shift” away
from dipterocarp domination of the forests
[dipterocarps are the principal food of giant
squirrels, bearded pigs, several species of
parakeet & myriad specialist insects, etc.]
Photo of dipterocarp forest from http://biology.ucsd.edu/news/article_012706.html