Lecture 18: Species Diversity in Communities (Chapter 18)
Recall from chapter 11
• The competitive exclusion principle: two same species that use a limiting resource in the same way cannot
• Joseph Grinnell first used the term “niche” in 1917, as: “no two species of bird or mammal will be found
to occupy precisely the same niche.”
Paradox of the Plankton
• Many plankton; few resources
• Proposed solutions to the “paradox of plankton”: heterogeneity, fluctuations/disturbances and
18.1 Community Membership
• Species diversity/richness differs among communities due to variation in regional pools, abiotic
conditions, and species interactions.
• How do collections of species end up coming together and forming communities with different species
compositions and richnesses? One way to answer is to consider the factors that control species membership in
• Distributions and abundances of organisms within communities are dependent on three interaction faces:
regional species pools and dispersal ability (species supply), abiotic conditions, and species interactions.
These can be though of as “filters” that act to exclude species from (or include them in) particular
• Figure 18.4
Figure 18.4: Community Membership: A series Filter
• Species end up in a local community by passing through a
series of ”filters” that determine community membership.
Species are lost at each filter, so local communities contain
only a faction of the species in the regional pool.
• In practice, all the filters work at the same time, rather than in
series as the figure suggests.
• (A) From the regional species pool, species that can disperse
to the community pass through the first filter.
• (B) Species that can tolerate the abiotic conditions in the
community pass through the second filter.
• (C) species restricted by and/or dependent on particular
species interactions in the community pass through the third
Species Supply is the “first cut to community membership
• Regional species pool provides an absolute upper limit on the numbers and types of species that can be
present within communities. We saw that regions of high species richness tend to have communities of high
o This relationship is due to the role of the regional species, and more specifically, of dispersal in
“supply” species to communities.
• The importance of dispersal can be seen in cases of nonnative species invasions. Nowhere is the controlling
effect of dispersal on community membership more evident than in the invasion of communities by non
• Humans have greatly expanded the regional species pools of communities by serving as vectors of dispersal.
o For example, we know that many aquatic species travel to distant parts of the world that they could
not otherwise reach in the ballast water carried by ships. Most of the time the water (with organisms it contains like bacteria to planktonic larvae) is taken up and released to close ports,
where some of the organisms have the chance to colonize near shore communities.
o As many as 5000 freshwater and marine species are transported in the ballast water of oceangoing
vessels each day.
Figure 18.5A: large and fast oceangoing ships are carrying marine species to all parts of
the world in their ballast water.
o In 1993, Carlton and Geller listed 46 known examples of ballastmediated invasions in the previous
20 years. One species, the zebra mussel, arrived in North America in the late 1980s in the ballast
water discharged into the great lakes. It has had communitychanging effects on inland waterways
and native bivalves.
Figure 18.5B: the zebra mussel, a destructive invader of the inland waterways of the US,
was carried there from Europe in ballast water.
Abiotic conditions play a strong role in limiting community membership
• A species may be able to get to a community, but may fail to become a member of the community because it
is physiologically unable to tolerate the abiotic conditions of the environment.
• Some abiotic constraints are obvious (e.g. fish on land, or more subtle; a lake might not support organisms
that require fastflowing water and therefore require fast streams).
• These differences among abiotic environments are obvious constraints that largely determine where particular
species can and cannot occur within a region.
• For example, in the discussion about ballast water, it was clear that humans transport many more species than
can actually survive in the new locations that they’re carried too. Some species may not have the temperature,
salinity, or light regimens they need to survive or grow.
Who interacts with whom makes all the difference in community membership
• The final cut to community membership is coexistence with other species.
• For species that depend on other species for growth, reproduction, or survival, those species must be present
if it is to gain membership in a community.
• Some species may be excluded from a community by competition, predation, parasitism, or disease.
• For example, we might assume that lakes are suitable habitats for many fish species, but could those species
all live together in one lake, given that resources are limiting?
o A simple view suggests that the best competitors or predators should dominate the lake, thus
excluding weaker competitors and resulting in a lowdiversity ecosystem.
• Most communities are full of species that are actively interacting and coexisting.
• The failure of nonnative species to become incorporated into communities has been attributed to interactions
with native species that exclude or slow the population growth of the nonnative species a phenomenon that
ecologists called biotic resistance.
o Many studies have shown that native herbivores have the ability to reduce the spread of nonnative
plants in substantial ways. In the study it was found that mortality of nonnative plants due to native
herbivores can be quite high. While native herbivores can kill nonnative plants, it is still unknown
how important native species are in completely excluding nonnative species from a community.
• There are three schools of thought what controls community diversity:
1. Equilibrium theory: ecological and evolutionary compromises lead to resource partitioning.
2. Nonequilibrium theory: fluctuating conditions keep dominant species from monopolizing
3. Neutral theory: species do not differ and diversity patterns are a product of dispersal, speciation
and demographic stochasticity. .
18.2 Resource Partitioning
• The competitive exclusion principle: two species that use a limiting resource in the same way cannot
coexist. • The paradox of the plankton: high species diversity despite extreme resource limitation.
• Resource partitioning: competing species are more likely to coexist when they use resources in a different
• Resource partitioning among species in a community reduces competition and increases species richness.
Resource partitioning allows more species to coexist along a resource spectrum
• A simple model of resource partitioning envisions each type of resource available in a community as varying
along a “resource spectrum.” (See fig. 18.7.
• The resource spectrum could represent, for example, different nutrients, prey sizes, or habitat types.
• Figure 18.7: species coexistence within communities may depend on how the species divide resources. (A)
The principle resource partitioning along a resource spectrum. (B,C) Two characteristic of communities that
can result in a higher species richness.
(A) Each curve represents the resource
use of a different species in the
(B) Species in this community show a
high degree of specialization (little
overlap) in their resource use.
(c) In this community, the resource
spectrum is broad, making more kinds of
resource available to support more
• You can assume that each species’ resource use falls somewhere along the spectrum and overlaps with the
resource use of other species.
o The more overlap, the more competition between species, with the extreme being complete overlap
and competitive exclusion.
o The less overlap, the more partitioning of resources has occurred, and the less strongly species will
complete with one another. (18.7a)
• Species richness could be high in some communities because species show a high degree of partitioning
along the resource spectrum (18.7b).
o More species can be “packed” into a community if the overlap in resource use among species is
low, leading to less competition and ultimately higher species richness.
o The lower overlap could be due to the evolution of specialization or character displacement, which
reduces competition overtime.
• Species richness could be high in some communities because resource spectrum is broad (18.7C). A broader
resource spectrum would make a diversity of resources available to be used by a wider variety of species,
resulting in higher species richness.
Specialization is the main mechanism of coexistence (Equilibrium Theories)
• Robert Macarthur looked at resource partitioning in whole communities.
• Macarthur studied warblers (small and brightly colored birds
that cooccur in the forests of northern North America) • Yellowrumped warbler: one warbler species fed from the
• Figure 18.8: Resource Partitioning by Warblers: Robert middle parts of trees to forest floor
studied the habitat and food choices of five species of
warblers in New England forests. He found that warblers • Blackthroated green warbler/baybreasted warbler: Two
partition resources by feeding in different parts of the same species fed in the middle parts of trees, both inside and
toward the outside of tree canopy.
• Cape May warbler/blackburnian warbler: two other species
fed on the outer tops of trees.
• Macarthur also found that nesting heights of these 5 birds
varied, as did their use of breeding territories. Taken
together, this supported his hypothesis that the warblers,
although using the same habitat and food resources, were
able to coexist by petitioning those resources in slightly trees. The shaded areas in each tree diagram represent the parts of trees where each warbler species fed most
• Macarthur, along with John Macarthur, extended these ideas about resource portioning in a study of the
relationship between bird species diversity (calculated using the Shannon index) and foliage height diversity
(a measure of the number of vegetation layers in a community that serves as an indication of habitat
complexity, also calculated using the Shannon Index). They found a positive relationship between the two in
13 tropical and temperate bird habitats from Panama to Maine.
o Figure 18.9: Bird species Diversity is higher in more complex habitats: Macarthur an