Ecology Lecture No. 16: Change In Communities
Thursday November 1 , 2012
Case Study – Effects Of Volcanic Eruption:
-When Mt. Saint Helens erupted in 1980, a surprising number of species survived. Gophers survived in
their tunnels as their preferred habitat (grassy meadows) 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 very quickly as they were exploiting
tunnels created by northern pocket gophers to make their way from one pond to another.
-Multiple mechanisms responsible for primary succession included: Facilitation (trapping seeds and
detritus and having nitrogen fixing bacteria) by dwarf lupines on the Pumice Plain, inhibition of lupines
by insect herbivores (which controlled the pace of succession) and tolerance among Douglas fir and
herbaceous species living together in habitats.
-Communities are always changing, some more than others. 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. Various examples of agents of change include both abiotic (waves and currents, wind, water
supply, chemical composition, temperature, and volcanic activity) and biotic (negative interactions)
phenomena. Agents of change vary in frequency and intensity.
-Succession is the directional change in species composition over time as a result of abiotic and biotic
agents of change. The two types of succession are primary succession (which involves the colonization of
habitats devoid of life like volcanic rock) and secondary succession (which involves the reestablishment
of a community in which some, but not all, organisms have been destroyed).
-Primary succession can be very slow as the initial conditions are very inhospitable. 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 other species. Secondary succession occurs after events such as fires,
storms and logging. The legacy of the pre-existing species and their interactions with colonizing species
play larger roles than in primary succession.
Hypotheses For Changing Communities:
-Cowles (1899) assumed that plant assemblages farthest from the lake’s edge were the oldest and the
ones nearest the lake were the youngest, 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. In this way, Cowles could
predict how communities would change over time without actually waiting for the pattern to unfold.
-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.
Each community reaches a stable end point called the “climax community,” which is composed of
dominant species that persist over many years and provide stability that can be maintained indefinitely.
-Henry Gleason thought communities are the random product of fluctuating environmental conditions
acting on individual species. Communities are not the predictable and repeatable result of coordinated
interactions among species (each community is unique).
Three Models Of Succession:
-There were three models of succession developed after numerous ecological field experiments: The
facilitation model (early species modify the environment in ways that benefit later species; the sequence
of species facilitations leads to a climax community), the tolerance model (earliest species modify the
environment, but in neutral ways that neither benefit nor inhibit later species ) and the inhibition model
(early species modify conditions in negative ways that hinder later successional species).
Mechanisms Of Succession:
-Glacier Bay, Alaska is one of the best studied examples of primary succession and demonstrates
mechanisms for all 3 models of succession. Early stages showed aspects of facilitation as plants modified
the habitat in positive ways for other plants and animals. Later, species such as alders had negative
effects (inhibition) on later successional species. In the spruce stage, where dominance was an artifact of
slow growth and long life, succession was driven by life history characteristics, a signature of the
-Many experimental studies show that succession is driven by many mechanisms. N