Title: The Nature of Communities
1The Nature of Communities
2The Nature of Communities
- What Are Communities?
- Community Structure
3Introduction
- Two-way species interactions considered
- competition
- predation (and herbivory)
- parasitism
- commensalism
- mutualism
- In reality
- all species experience multiple interactions
- These interactions shape the communities in
which they live.
4What Are Communities?
Communities are groups of interacting species
that occur together at the same place and time.
- Interactions among multiple species produce an
entity that supercedes the sum of its parts. - In practice, biological or physical guidelines
used to define a community. - 1. physically defined communities all species
- in a sand dune
- in Clear Creek
- in Clear Creek under the Lowell Blvd. overpass
- in Vogel Canyon of southeastern Colorado
5Figure 15.3 A Defining Communities
6- 2. Biologically defined communities all species
associated with - a kelp forest
- a maple forest
- a coral reef.
- Dominant species the basis for the community
delineation - kelp
- Sugar Maple
- coral
7Figure 15.3 B Defining Communities
?
8- Taxonomic identification of species problematic.
- Therefore, counting all species difficult to
impossible. - Therefore, ecologists usually consider a subset
of species when they define and study
communities. - 1. Taxonomic affinity
- Aspidoscelis tesselata, A. sexlineata, and their
arthropod prey (Orders). - 2. Guildsgroups of species that use the same
resources. - 3. Functional groupspecies that function in
similar ways, but do not necessarily use the same
resources.
9Figure 15.4 Subsets of Species in Communities
10- 4. Food webs
- Ecological organization of species based on their
trophic or energetic interactions. - Functional (not taxonomic) classification.
- Trophic levels defined by groups of species that
have similar ways of obtaining energy - Producers
- Primary consumers
- Secondary consumers
- Tertiary consumers
- Food webs provide little information about the
strength of interactions or their importance in
the community.
11- Some species span two trophic levels, and some
species change feeding status as they mature. - Some species are omnivores, deriving energy from
more than one trophic level.
chuckwalla
12Figure 15.5 Four-Level Food and Interaction Webs
Interaction webs more accurately describe both
the trophic (vertical) and non-trophic
(horizontal) interactions in a traditional food
web.
13Concept Species richness and species diversity
are important descriptors of community structure.
- Community structure is the set of characteristics
that shape communities. - Species richnessthe number of species in a
community. - Species evennessrelative abundance of the
various species. - Species diversity combines species richness and
species evenness.
14- Example Two communities with four species each
(species richness is equal). - In community A, proportion of yellow species is
0.85 proportion of each of the other three
species is 0.05. - In community B, each species has the same
proportional abundance 0.25. This community has
higher diversity.
15- There are several quantitative species diversity
indices. The one most commonly used is the
Shannon index
pi proportion of individuals in the ith
species s number of species in the community
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18- Oh! Oh! Species diversity is often used to mean
the number of species in a community. - Biodiversity describes the diversity of
important ecological entities. - These entities can span multiple spatial scales
alleles - species
- communities
- Implicit in biodiversity is the
interconnectedness of all components of diversity.
19Figure 15.7 Biodiversity Considers Multiple
Spatial Scales
20- Species diversity indices
- Used to compare different communities.
- Rank abundance curves used to provide information
on commonness or rarity of species. - The proportional abundance of each species (pi)
is plotted relative to the others in rank order
(most abundant, next most abundant, etc.).
21- Relative abundances can suggest the types of
species interactions that might occur. - Example In Community A, the dominant species
might have a strong negative effect on the three
rare species. - Experiments that add or remove species are used
to explore such hypotheses.
22- An example.
- Two soil bacteria communities in pastures in
Scotland. - One pasture had been fertilized regularly.
- Bacteria species can be identified quickly using
DNA sequencing of 16S ribosomal DNA. The bacteria
can then be grouped using phylogenetic analysis. - Species diversity and rank abundance curves were
determined. - McCaig et al. (1999) found 22 phylogenetic groups
of bacteria.
23Figure 15.9 Bacterial Diversity in Pastures in
Scotland
Both pastures had very similar community
structure. A few species were abundant most
species were rare.
24Community Structure
- Species accumulation curvesspecies richness is
plotted as a function of the total number of
individuals that have been counted with each
sample. - These curves can help determine when most or all
of the species in a community have been observed.
25Figure 15.10 When Are All the Species Sampled?
26Community Structure
- As additional samples are taken, additional
species will tend to be sampled. - At some point, the curve will reach a threshold
at which no new species are added despite
additional sampling.
27Community Structure
- Hughes et al. (2001) compared species
accumulation curves for 5 different communities - Temperate forest in Michigan.
- Tropical bird community in Costa Rica.
- Tropical moth community in Costa Rica.
- Bacterial community from a human mouth.
- Bacterial community from tropical soils.
The 5 communities varied significantly in the
amount of sampling effort necessary to determine
their species richness.
28- Species compositionthe identity of species
present in the community. - Two communities could have identical species
diversity values, but have completely different
species. - The identity of species is critical to
understanding community structure.