Lecture 15 – Community change
Agents of Change
Agents of change act on communities across multiple temporal and spatial scales.
Consider a coral reef community in the Indian Ocean. If you could look back at it over the last few
decades, you would observe slow and subtle changes, as well as catastrophic ones.
o Figure 16.3 – Both natural and human caused.
Species interactions, such as competition, predation and disease, can cause the
gradual replacement of species over time.
Changes in abiotic conditions such as sea level and water temperature can cause
physiological stress, coral bleaching, and eventually mortality
Catastrophic disturbances, such as tsunamis, can cause massive injury and death
in coral reefs.
o Table 16.1 – Communities and the species contained within them, change in response to a
number of abiotic and biotic factors. (page 346)
In the 1980s. an unusual alga was found in the Mediterranean Sea
It was native of warm Caribbean waters (1820 degrees). It had never been found in colder waters
(1213) nor in such densities. French marine biologists calculated its rate of spread at 1 hectare in
o Figure 15.2 – Caulerpa was unintentially released by the Oceanographic Museum of
Monaco in 1984 and by 2000 Caulerpa had spread as far as Tunisia.
A coldresistant strain of Caulerpa had been sent to them from a zoo in Germany, to use as a
backdrop for tropical fish aquaria. The museum released Caulerpa in the process of cleaning tanks,
thinking it would die in the cold Mediterranean.
o This dramatically changed the way native species interacted, and thus the structure and
function of native communities.
o Seagrass meadows dominated by Posidonia oceanica were overgrown by Caulerpa. The
seagrass meadows support a multitude of species.
Caulerpa acts as an ecosystem engineer, accumulating sediments around its roots more readily
than Posidonia, which changes the invertebrate community.
There is also a significant drop in the numbers and sizes of fish after Caulerpa invades, suggesting
the habitat is no longer suitable.
Natural Experiment of Mountainous Proportions
Mt. St. Helens – May 18, 1980 – Devastation created new habitats devoid of any living organisms.
Figure 16.2 – Organisms on Mount St. Helens were scorched, pounded by pumice, covered in
mud, and blown down by eruptions. The eruption had different effects on the geology of the
mountain at different locations, creating many new habitats.
Agents of Change
Succession is the change in species composition in communities over time. It is the result of both
biotic (physical and chemical) and abiotic factors.
o Abiotic factors in the form of climate, soils, nutrients, and water, vary over daily,
seasonal, decadal, and even 100,000 – year time scales.
Abiotic agents of change can be put into two categories:
o Disturbance – an event that injuries or kills some individuals and creates opportunities for
Result of an earthquake o Stress – an abiotic factor reduces the growth or reproduction of individuals (ex:
Frost on the tree, frost may not kill the tree but it will
effect the fruit production.
Abiotic and biotic factors often interact to produce community change.
o An ecosystem engineer causes changes in abiotic conditions
that can cause species replacement.
A beaver creating a wetland leads to species
o Figure 16.4 Spectrum of Disturbance – How much biomass is
removed and how often it is removed can influence the amount
of disturbance that occurs and the type of succession that is
possible afterward. Communities cannot form where extremely intense and frequent
disturbances occur (ex: active volcano’s)
Basics of Succession:
Theoretically, succession progresses through various stages that include a climax stage – a stable
end point that experiences little changes.
There is some argument about whether succession can ever lead to a
stable end point.
Figure 16.5 a simple model of succession involves
transitions between stages driven by species
replacements over time. Theoretically, these
changes ultimately result in a climax stage that
experiences little change.
When the effect of a disturbance is catastrophic,
destroying a life a pioneer stage is formed through the process of
primary succession. Secondary succession occurs after a
disturbance alters a community greatly but does not destroy all life.
Two types of succession differ in their initial stage:
o Primary succession involves the colonization of habitats devoid of like (volcanic rock) –
doesn’t happen too often
o Secondary succession involves reestablishment of a community in which some, but not
all, organisms have been destroyed.
Modern ecology got its start by people interested in the succession of plant species.
o Henry Cowles, studied the successional sequence of vegetation on sand dunes along Lake
o He assumed that plant assemblages farthest from the lake’s edge were the oldest
successional stage; the ones nearest the lake were the youngest.
o He could see successional stages arranged spatially. This allowed him to predict how a
community would change over time without actually waiting for the pattern to unfold,
which would have taken decades to centuries.
o This is called “space for time substitution” and is used frequently today
It assumes that time is the main factor causing communities to change and that
unique conditions in particular locations are inconsequential.
Two other early ecologist
o Frederick Clements believed plant communities were like “superorganisms” groups of
species working together toward some deterministic end.
Felt that each community had a predictable life history and, if left undisturbed,
ultimately reached a stable end point called the climax community. Climax
community was composed of species that dominated and persisted over many years and provided the type of stability that could potentially be maintained
o Henry Gleason thought that communities were the random product of fluctuating