Nov 14, 2013
Communities vary in the numbers and kinds of species they contain.
Species diversity differs among communities due to variation in regional species pools, abiotic
conditions, and species interactions.
Species diversity at the local scale: We must ask two important questions.
What are the factors that control species diversity within communities?
What is the effect of species diversity on community function?
Communities vary drastically in their species richness and composition. These species must all
come together. We can explain this by
considering the factors that control species
Distribution and abundance of species in
communities depends on:
1. Regional species pools and
dispersal ability (species
supply/what can get there).
Species that can disperse to the
community pass through the first
2. Abiotic conditions. Species that
can tolerate the abiotic
conditions in the community pass
through the second filter.
3. Species interactions. Species restricted by and/or dependent on particular
species interactions in the community pass through the third filter.
These factors act as “filters,” which exclude species from (or include species in) particular
communities. Species are lost at each "filter" so local communities only contain a fraction of the
species in the regional pool.
Regional Species Pools/Dispersal Ability
The regional species pool provides an upper limit on the number and types of species that can
be present in a community. This is because the regional pool is what supplies the communities with species. Regions of high species richness tend to have communities of high species
The importance of dispersal can be seen in cases of non-native species invasions.
Humans have greatly expanded regional species pools by serving as vectors of dispersal
(methods of transport for non native species to new communities.
Aquatic species travel around the world in ballast water carried by ships.
Seawater is pumped in and out of ballast tanks which serve to balance/stabilize
cargo ships (organisms from the sea are taken up with it and released near
ports). Ships are now larger and faster, so trans-ocean trips take less time—
species are more likely to survive (thus non native species introduction via
ballast water has increased).
Examples that have been introduced by ballast water are the zebra mussel
(introduced to great lakes by ballast water. These became destructive invaders
to the inland waterways of the U.S) and the release of the comb jelly
Mnemiopsis Leidyi into the Black Sea.
A species may be able to get to a community but be unable to tolerate the abiotic conditions.
Example: A lake might not support organisms that require fast-flowing water. In
addition, lakes are good places for fish, aquatic plants, plankton etc. but not terrestrial
Differences among abiotic environments are obvious constraints that determine where
particular species can and cannot occur.
Humans transport many more species that can actually survive in the new location to which
they are carried. For example, many species that are dispersed in ballast water can’t survive in a
new habitat because of temperature, salinity, amount of light etc. As a result, many of these non
native species die before they can become a threat. However, we can’t rely on physiological
constraints to exclude invaders, as in the case of Caulerpa in the Mediterranean Sea. With
multiple introductions, these invader species may adapt to the point where they are able to
survive and reproduce.
Evidence shows that global climate warming is leading to alterations in the abiotic conditions of
communities. It may facilitate the invasions of species that would be unable to survive in cooler
Coexistence with other species is also required for community membership
If a species depends on another for its growth, reproduction, and survival, those other species
must be present.
A species may be excluded from a community by competition, predation, parasitism, disease. For example, a lake may be a good habitat for many fish species but, but limited
resources may prevent all of these species from living together in harmony. In this case,
certain species may dominate leading to the extinction of weaker species.
Some non-native species do not become part of the new community. This is attributed to
interactions with native species.
Biotic resistance occurs when interactions with the native species exclude or slow the
population growth of the non native species/invader.
Example: Native herbivores can reduce the spread of non-native plants.
Mortality of non native plants due to native species is quite high.
Not a lot is known about biotic resistance. This is because ecologists tend to focus on
whether species spread ONCE they have been incorporated into a community, rather
than looking at whether a species BECOMES a member of a community. This is also
partly because failed introductions of non-native species tend to go undetected
However, many species are able to coexist in a single habitat. Numerous factors attribute to
Resource partitioning: Competing species
coexist by using resources in different ways. It
reduces competition and increases species
richness. This has been shown with the Lotka
Volterra competition model.
In a simple model, each species’ resource use
falls on a spectrum of available resources. The
resource spectrum represents different
nutrients, prey sizes, habitat types. In the
image to the right, each curve represents the
resource use of a different species in the community. Species resource use lies
somewhere along the spectrum, and we have overlapping of resource use by some of
The more overlap of resource use, the more competition between species. An extreme
of this would be complete overlap and competitive exclusion. The less overlap, the more
specialized species have become (increased resource partitioning), and the less strongly
Species richness may increase as a
result of resource partitioning. In the
narrow spectrum, species show a high
degree of specialization/partitioning
(little overlap) in their resource use.
More species can be packed into a
community. This lower overlap may be
due to the evolution of specialization or
character displacement. In the broad spectrum, there are more kinds of resources
available to support more species. MacArthur (1958) studied resource partitioning in a community of warblers in
New England forests. He wanted to see how they coexist in the face of similar
resource needs. He helped understand how the principle "species that compete
can coexist by using resources in different ways" can be applied to entire
communities, where multiple interactions are occurring at once. He recorded
feeding habits, nesting locations, and breeding territories. When he mapped the
locations of warbler activity he found that the birds were using different parts of
the habitat in different ways.
It was found that Warbler species partition resources by feeding in different
parts of the trees. The shading in the below image indicates where on the tree
each species of Warbler chooses to feed. It was also noticed that the nesting
heights and locations of breeding territories varied as well. Although using the
same habitat and resources, the Warbler community was able to partition the
resources so that they could all coexist.
To explain how diatom species coexist in nature, Tilman proposed the resource
ratio hypothesis: Species coexist by using resources in different proportions.
Two diatom species (Cyclotella and Asterionella) were grown in media with
different SiO 2PO r4tios.
Tilman found that Cyclotella dominated only when the ratio was low,
Asterionella dominated when the ratio was high. Coexistence occurred only
when SiO and PO were limiting to both species.
2 4 The above study works best outside of a laboratory setting when the natural
resources vary naturally within an area. Robertson et al. (1988) mapped soil
moisture and nitrogen concentration in an abandoned field and found variation
over small spatial scales. If the two maps are overlapped, smaller patches
corresponding to different proportions of these two resources emerge. This
suggests that resource partitioning could occur in plants. Processes that Promote Coexistence
Processes such as disturbance, stress, predation, and positive interactions can mediate resource
availability, thus promoting species
coexistence and species diversity
Examples have shown that when the
dominant competitor is unable to reach its
own carrying capacity, competitive
exclusion can’t occur, and coexistence will
be maintained. This was shown in the case
of sea palms and mussels that compete for
space in the Rocky Intertidal Zone. Mussels
are the dominant competitors and Sea
Palms can only coexist when the mussels
are frequently disturbed by waves. As long
as the dominant competitor is unable to
reach its own carrying capacity because of
reductions in its abundance due to
disturbance, stress, or predation, competitive exclusion cannot occur and coexistence will be
Charles Darwin showed that disturbance is a mechanism for the maintenance of species
diversity (experiment in which he left a meadow unmowed except for one small patch.
With mowing, dominant competitors were removed and weak competitors could thrive
leading to increased richness). This supported the argument that nature applies limits to
the tendency of species to increase in abundance and outcompete other species.
G. E. Hutchinson revived the idea in his paper “The Paradox of the Plankton” (1961). He
provided one of the first mechanistic descriptions of how coexistence could be
maintained under fluctuating environmental conditions. He focused on phytoplankton
in a lake. These communities have very high diversity (30–40 species), all using the same
limited resources, in a homogeneous environment (even distribution of resources). His
explanation for coexistence was that conditions in the lake changed seasonally, which
kept any one species from outcompeting the others. As long as conditions changed
before the competitively superior species reached carrying capacity (thereby eliminating
other species), coexistence would be possible.
Positive interactions may also have an effect on coexistence. For example, species that might
normally be unable to tolerate stressful conditions can maintain viable populations under
stressful conditions because of the facilitative effects of other species.
Intermediate Disturbance Hypothesis
This hypothesis explains species diversity under variable conditions. It was proposed to explain
how gradients in disturbance affect species diversity in communities. The frequency and intensity of disturbance experienced by a particular community could have
dramatic effects on the species diversity.
The intermediate disturbance hypothesis:
Species diversity should be greatest at
intermediate disturbance, and lowest at high
and low disturbance. At low disturbance,
competition determines diversity (dominant
competitors can outcompete weak
competitors) . At high disturbance, many
species can not survive (high mortality rates).
Intermediate disturbance allows for a nice
balance between disruption of competition and mortality.
There have been many tests of this hypothesis.
Sousa studied communities on intertidal boulders in southern California that were
overturned by waves. Small boulders were overturned frequently (disturbance), large
boulders were overturned
less often. Intermediate
boulders were turned over
at intermediate frequencies.
After 2 years, it was
determined that most of the
small boulders had only one
species (early successional
species with high
dominance). The large
boulders had two species
(for the most part) which
were late successional
species (most recent
because early colonizers
were killed off by high disturbance levels). The intermediate boulders had 4-7 species
(mix of early, mid and late successional species)
Several people have elaborated on the intermediat