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Lecture 16

Biology 2483A Lecture 16: Changes in Communities

Course Code
BIOL 2483A
Hugh Henry

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Effects of the eruption varied depending on distance from the volcano and habitat type.
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 underground parts.
Their burrowing activities facilitated plant succession by brining organic soil, seeds, and
fungal spores to the surface.
Gophers survived in their tunnels. Grassy meadows, their preferred habitat, expanded after
the eruption.
Newly-formed and isolated pons were colonized by amphibians much faster than was though
Frogs and salamanders were using tunnels created by northern pocket gophers to make their
way from one pond to another.
Mount St. Helens
Lupines were inhibited by insect herbivores, which controlled the pace of succession.
Tolerance: Douglas fir and herbaceous species lived together in some habitats.
Mechanism for Primary Succession:
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 al temporal and spatial scales.
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 (bleaching), as well as
catastrophic ones (tsunamis).
Example: Corals in India
Succession is the directional change in species composition over time as a result of abiotic and
biotic agents of change.
Lecture 16: Changes in Communities
Thursday, November 26, 2015
10:09 AM
Lecture Slides Page 1

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biotic agents of change.
Studies of succession often focus on vegetative change, but the roles of animals, fungi,
bacteria, and other microbes are equally important.
Circles represent the amount of community change. You can have intense changes (like
volcanic eruptions) that happen less frequently.
You can get areas that are low intensity at low frequency
Note: you can't get thing s that high intensity and high frequency because nothing would
be able to live there.
Agents of change vary in frequency and intensity
Can be very slow and initial conditions are very inhospitable.
The first colonizers (pioneer or early successional species) ten to be stress-tolerant, and
transform the habitat in ways that benefit their growth and that of other species.
Primary Succession:involves the colonization of habitats devoid of life (volcanic rock)
Occurs after fires, storms, logging, etc.
The legacy of the preexisting species and their interactions with colonizing species play
larger roles than in primary succession.
Secondary Succession: involves reestablishment of a community in which some, but not all,
organisms have been destroyed.
Assumed the plant assemblages farthest from the lake's edge were the oldest, the ones
nearest the lake were the youngest, representing a time series of successional stages.
Example: Succession of Sand Dunes
Types of Succession:
Lecture Slides Page 2

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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
Coweles 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 "super organisms" 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
Henery Gleason thought communities are the random product of fluctuating environmental
conditions acting on individual species.
Communities are not the predictable repeatable result of coordinated interactions among
species, each community is unique.
Even though you see climax events, Henry is more right.
Note: you don’t have species working together to an end point. Instead, you have random
products of changes in the environment working on the species.
Three Models of Succession
Facilitation model - inspired by Clements. Early species modify the environment in ways that
benefit later species. The sequence of species facilitations leads to a climax community.
Tolerance model - also assumes the earliest species modify the environment, but in neutral
ways that neither benefit nor inhibit later species.
Inhibition model - assumes early species modify conditions in negative ways that hinder later
successional species.
Mechanisms of Succession
You end up with sites that are newly exposed with glaciers
Example: Glacier Bay, Alaska is one of the best studied succession.
Melting glaciers have led to a sequence of communities that reflect succession over many centuries.
Lecture Slides Page 3
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