Nov 7 2013
Mount St. Helens
Mount. St Helens is located in Washington State, as part of the geologically active Cascade
Massive volcanic eruption in 1980: This caused an explosive eruption and the largest avalanche
in recorded history. New habitats were created devoid of living organisms
Effects of the eruption varied depending on distance from the volcano
and habitat type.
The pumice plain was a large gently sloping moonscape of a
place below the volcano that was pelted in hot sterilizing
pumice/volcanic rock (lacked life and organic matter of all
forms) The blowdown zone was an area in which the majority
of the landscape consisted of blown down trees, covered with
rock, gravel and mud tens of meters deep in some places. The
scorch zone was an area where fires killed all the trees in the
forests but left them standing.
A surprising number of species survived. Some were still dormant
under winter snows. Others were in burrows, or under ice-covered lakes, or were plants with
In many cases, these species were thrust into novel physical environments without time to
adapt. Some thrived, others fared poorly. However, their adaptability and unpredictability was
surprising. Unlikely alliances were made that hastened succession.
Gophers survived in their tunnels. Grassy meadows, their preferred habitat, expanded after the
eruption. Their burrowing activities facilitated plant succession by bringing organic soil, seeds,
and fungal spores to the surface.
Newly-formed and isolated ponds were colonized by amphibians much faster than was thought
possible. Frogs and salamanders were using tunnels created by northern pocket gophers to
make their way from one pond to another across the arid landscape.
Multiple mechanisms were responsible for primary succession:
Facilitation by dwarf lupines (first plant to arrive) on the Pumice Plain. They trap seeds
and detritus, and increased the nitrogen content of the soil through their symbiotic
association with nitrogen-fixing bacteria.
Lupines were inhibited by insect herbivores, which controlled the pace of succession.
Tolerance was observed in some primary successional habitats. Douglas fir and
herbaceous species lived together in some habitats. Introduction
Communities are always changing, some more than others.
For example, in comparison of a desert (large stoic cacti) with a rocky intertidal zone, it
appears that deserts rarely change while the species in a rocky intertidal zone
continuously come and go. However, deserts are just changing on a much slower pace.
Human actions are becoming one of the strongest forces behind community change, and we
have an imperfect understanding of the consequences of those actions.
Agents of change act on communities across all temporal and spatial scales. Agents of change
may be subtle and catastrophic, natural and human caused. Agents of change can be abiotic or
biotic. Abiotic agents of change can be classified into disturbances and stresses. Disturbances
are abiotic events that physically injures or kills some individuals and creates opportunities for
other individuals to grow and reproduce. A stress is an abiotic event that reduces the growth or
reproduction of some individuals creating opportunities for other individuals. Biotic agents of
change include positive/negative species interactions, as well as the actions of ecosystem
engineers and keystone species. In addition, abiotic and biotic factors often interact to produce
changes in communities.
Consider a coral reef
community in the
Indian Ocean. If you
could view it over the
last few decades, you
would observe slow
and subtle changes, as
well as catastrophic
ones. Today, the result
of these agents of
change are a coral reef
that has many fewer
coral species than it
did a few decades ago
Species interactions such as competition, predation and disease can lead to the subtle and gradual
displacement of species over time.
More catastrophic changes may include the massive deaths of corals in the last decade
due to natural and human causes. Catastrophic disturbances such as a tsunami may
cause massive injury and death in coral species, leading to the replacement of some
species with other species or no replacement at all. Changes in abiotic conditions, such
as sea level (rising sea level decreases amount of light reaching corals; more tolerant
species may replace), ocean acidification (dissolves coral skeletons hindering their
growth) and water temperature can cause physiological stress, coral bleaching (loss of
symbiotic algae as a result of high water temperatures from climate warming) and
Succession is the directional change in species
composition over time as a result of abiotic
(physical and chemical) and biotic agents of
change. Mechanistically, succession involves
the colonization and extinction of species in a
community due to agents of change.
Studies of succession often focus on
vegetative change, but the roles of animals,
fungi, bacteria, and other microbes are
Agents of change vary in frequency and
intensity (amount of damage and death it
causes). How often and at what magnitude agents of change act determines the temp of
The image to the right shows the spectrum of disturbance. How much biomass is removed
(intensity) and how often it is removed (frequency) can influence the amount of disturbance
(red circles) that occurs and the type of succession that is possible afterwards. Communities
cannot form where extremely intense and frequent disturbances occur.
For example, Mt St Helens produced an extremely intense disturbance (both because of
massive physical force and area covered) but had low frequency (such eruptive episodes
are rare) At the opposite end of the spectrum are weak and frequent disturbances that
have more subtle effects.
Succession progresses through various stages that include a climax which is thought to be a
stable end point that experiences little change until a particularly intense disturbance sends the
community back into an earlier stage.
Two types of succession that differ in their initial stages:
Primary succession involves the colonization of habitats devoid of life (e.g., volcanic
rock). This occurs when the effect of a disturbance is catastrophic leading to destruction of all life or because they are newly created habitats (volcanic rock) A pioneer stage is
formed in which recolonization occurs.
Primary succession can be very slow because the initial conditions pioneers
must face are very inhospitable.(lacks most basis resources such as soil,
nutrients and water)
The first colonizers (pioneer or early successional species) tend to be stress-
tolerant, and transform the habitat in ways that benefit their growth and that of
Secondary succession involves re-establishment of a community in which some, but not
all, organisms have been destroyed.
Secondary succession occurs after fires, storms, logging, herbivory etc. The
legacy of the preexisting species and their interactions with colonizing species
play larger roles than in primary succession.
The blowdown zone at St. Helens was an area in which several species managed
to survive and secondary succession took place
Early History of Ecology is a Study of Succession
One of the pioneers of modern ecology was Henry Cowles (1899), who studied succession of
vegetation on sand dunes along Lake Michigan. In this location, the sand dunes are continuously
growing as new sand is deposited at the shoreline. (new sand blown onshore from sand deposits
exposed during droughts)
He assumed that plant assemblages farthest from the lake’s edge were the oldest (late
successional stages); the ones nearest the lake were the youngest (early successional
stages), representing a time series of successional stages.
The first stages were dominated by a hardy ecosystem engineer, American beach grass,
which traps sand and creates hills, providing refuge for plants less tolerant of burial and
scouring experienced on the beachfront.
Different plant assemblages seen in different positions on a dune represent different
successional stages. Cowles could predict how communities would change over time
without actually waiting for the pattern to unfold, which would have taken decades to
centuries (space for time substitution). Frederick Clements believed plant communities are like “superorganisms,” groups of species
working together toward some deterministic end. Thus, succession is similar to the
development of an organism with a beginning, middle and end.
Each community has its own predictable life history and, if left undisturbed, ultimately
reached a stable end point. This is called the “climax community,” which is composed of
dominant species that persist over many years and provide stability that can be
Henry Gleason thought communities are the random product of fluctuating environmental
conditions acting on individual species. Each community was the product of a particular place
and time, and was thus unique in its own right. Gleason felt that the model proposed by
Clements ignored the responses of individual species to prevailing conditions.
Communities are not the predictable and repeatable result of coordinated interactions
among species; each community is unique.
Connell and Slatyer (1977) reviewed the literature on succession and proposed three models:
1. Facilitation model, inspired by Clements. Early species modify the environment in ways
that benefit later species but hinder their own continued dominance. Initial species are
stress tolerant and able to modify the environment. The sequence of species
facilitations leads to a climax community (species no longer facilitate other species and
are displaced only by disturbances).
2. Tolerance model also assumes the earliest species modify the environment, but in
neutral ways that neither benefit nor inhibit later species. Sequences of succession are
thus entirely dependent on life-history characteristics such as the specific amount of
energy a species allocates to growth (allow them to tolerate environmental/biological
stresses) The climax community is composed of the most “tolerant” species that can co-
exist with other species in a more densely populated area. Eventually, dominant species
replace or reduce pioneer species abundance through competition. 3. Inhibition model assumes early species modify conditions in negative ways that hinder
later successional species. For example, pioneer species might modify the environment
through rapid growth and make the area increasingly shady (essentially increasing
competition for light). The environment is thus less hospitable to other potential
colonizing species. The only possibility for new growth/colonization arises when a
disturbance leads to dominating species being destroyed, damaged, or removed. This
frees up resources and allows for the invasion of other species that were not previously
Mechanisms of Succession
Mechanisms driving succession
rarely conform to any one of the
above models, but instead are
dependent on the community and
context in which experiments are
Glacier Bay, Alaska is one of
the best studied examples
of primary succession.
Melting glaciers have led to
a sequence of community
change that reflects
succession over many
centuries (over more than
200 years, glacier melting
has exposed bare rock to
colonization and succession).
Permanent plots were established
that have allowed researchers to
observe the pattern of commun