Lecture 17 – Species Diversity in Communities (chapter 18)
The competitive exclusion principle: Two species that use a limiting resource in the
same way cannot coexist.
Joseph Grinnell first used the term ‘niche’ in 1917, as : “no two species of birds or
mammals will be found to occupy precisely the same niche”
Why do we see in natural communities that there is this overwhelming case that there is
extremely high species diversity despite the fact that there is extreme resource limitation.
One example of this: Paradox of the Plankton – GE Hutchinson
- He observed that there were 30-40 phytoplankton species in the freshwater lake
system. All of these species competed for the same set of limited resources (eg:
carbon dioxide). Even though they were competing for these resources, there was
still a very high amount of species diversity.
- How can we explain this?
Heterogeneity: in terms of species pool size, the abiotic conditions in which we may have
species, and also species interactions that control species diversity.
Species richness differs among communities due to variation in regional species
pools, abiotic conditions, and species interactions.
Glacier National Park in USA: made up of very distinct communities such as meadows,
lakes, forests, etc. Each of these communities is highly distinct and they have their own
species compositions, species richness, etc. Some species can move between communities
but they still remain relatively distinct.
So how do these species get to the communities that they end up in? We need to look at
community membership at the local scale. Figure 18.4 Community Membership: A Series of Filters
There are 3 interacting filters that work to include and exclude species from local
communities. There is no one single process that is responsible for the distribution and
abundance of species rather the interaction of these factors that both include and exclude
species from local communities.
1. The regional species pool provides an upper limit on the number and types of
species that can be present in a community. The regional species pool acts as a
supplier to the local communities through species dispersal and immigration.
Regions of high species richness have communities of high species richness.
The importance of dispersal can be seen in cases of non-native species invasions.
Humans have recently expanded the regional species pool of communities by acting as
vectors of dispersal by bring species previously limited in their dispersal capabilities into
new communities. Eg: Zebra mussel – it is a huge threat to our local marine ecosystems. They were brought
over by the ballast water carried by ships. It is a destructive invader of the inland
waterways of North America and was brought there from Europe. Wrecked havoc on the
ecological systems and human infrastructure.
Figure: Regions of high species richness will also have communities of high
species richness because of the role of dispersal within these communities.
2. Abiotic Factors: species may be able to reach a community but be
physiologically unable to tolerate the biotic or abiotic conditions of the
Some abiotic constraints are obvious (e.g., fish on land, or more subtle; a lake might not
support organisms that require fast-flowing water). Eg: when the ballast water of ships from Asia come to North America then the non native
species may find themselves in waters that may not have the right temperature or salinity
in order to survive and grow.
3. Species Interactions: The final cut requires coexistence with other species.
For species that depend on other species for growth, reproduction, or
survival, those other species must be present.
Species may be excluded from a community by competition, predation, parasitism, or
Eg: Biotic resistance – the native species of a community have a biotic resistance to the non
native species of a community.
- When a non- native species enters a community, the native species may choose to
exclude or slow down the growth of the non-native species. For example, the non
native species Garlic Mustard, there is an issue in Ontario, may have to face
herbivores that it may not have to face in its native range and also other issues such
as competition and disease.
How are species able to coexist?
There are three schools of thought what controls community diversity:
Equilibrium theory—ecological and evolutionary compromises lead to resource
partitioning in order to reduce competition.
Nonequilibrium theory—fluctuating conditions keep dominant species from
monopolizing resources. (eg: disturbance, predation, etc.) Neutral theory—species do not differ and diversity patterns are a product of dispersal,
speciation and demographic stochasticity. The individual species do not matter, what is
important is the dispersal, etc of the species.
The competitive exclusion principle: Two species that use a limiting resource
in the same way cannot coexist. (We don’t always end up seeing this in nature)
Resource partitioning among the species in a community reduces competition and
increases species richness.
Resource partitioning—competing species are more likely to coexist when they use
resources in different ways.
On the x axis is resource partitioning which can represent different nutrients an the habitat
type that a species requires. The species are each of the curves so in total there are 5
species in this community. On the y axis is the amount of variability of available resources
(not the amount). There is overlap between the curves which means that there is more
competition between the species. When there is total overlap, we expect to see competition
exclusion so that one species would drive the other species to extinction. Areas of less
overlap between the curves show less partitioning of resources so the species compete less
intensely with one another. There are a number of ways in which resource partitioning may result in species richness.
In B species richness can be high because they show a degree of partitioning along the
different spectra. Especially when there are more species being packed into a community,
we notice that there is very little overlap between species which can be a result of
specialization or character displacement which both reduce competition over time. In C,
species richness can be high because there is a broader resource spectrum. There is a
greater diversity of resources available to be used by a wider variety of species. Equilibrium Theories
Figure 18.8 Resource Partitioning by Warblers
MacArthur (1958) looked at resource partitioning in whole communities. He recorder the
feeding habitats, nesting locations, and breeding territories of 5 species of Warblers to find
out how they coexist with one another in the face of resource need. He found that the
Warblers used different parts of the habitat or tree canopy in different ways by using
different heights and different depths of the tree canopy. The species may overlap but
mostly they use different parts of the canopy..
This supported his hypothesis that the Warblers are able to coexist despite the fact that
they have to use the same habitat for those food resources.
In further studies, MacArthur and MacArthur (1961) looked at bird communities in 13
There was a positive relationship between bird species diversity and foliage height
diversity (number of vegetation layers, a measure of habitat complexity, a proxy for how
broad the spectrum is in which they are in). He was asking if there is a greater number
of resource diversity and if there is a correlation between the number of species in the
system. Figure 18.9 Bird Species Diversity is Higher in More Complex Habitats.
He found that there is a positive relationship between foliage height diversity and birds
species diversity. However, interestingly, bird species diversity was not related to plant
diversity other than height diversity. This suggests that trees species identity was less
important that the structural complexity of the habitat. MacArthur found a strong
correlation between broader resources and high species richness.
Tillman’s experiment of two diatoms that competed for silica. When he varied the
different ratios of silica to phosphorus depending on whether it was high or low, what he
observed was that in every scenario, one species would drive the other species to
extinction (complete exclusion). However, in nature, both species are able to coexist. Why
can’t we relicate this in experimental settings?
To explain how diatom species coexist in nature, Tilman proposed the resource ratio
hypothesis—species coexist by using resources in different proportions.