Transcript Concept 16.3 - practical ecology

16
The Nature of Communities
Chapter 16 The Nature of Communities
CONCEPT 16.1 Communities are groups of
interacting species that occur together at the
same place and time.
CONCEPT 16.2 Species diversity and species
composition are important descriptors of
community structure.
CONCEPT 16.3 Communities can be
characterized by complex networks of direct
and indirect interactions that vary in strength
and direction.
Introduction
Although so far we have considered
species interactions in two-way
relationships, in reality, species
experience multiple interactions that
shape the communities in which they
live.
THIS is the dominant direction of
community ecology.
CONCEPT 16.1
Communities are groups of interacting
species that occur together at the same
place and time.
Concept 16.1
What Are Communities?
In practical terms, ecologists usually
define communities based on physical
or biological characteristics.
A physically defined community might
encompass all the species in a sand
dune, a mountain stream, or a desert.
Concept 16.1
What Are Communities?
A biologically defined community might
include all the species associated with a
kelp forest, a freshwater bog, or a coral
reef.
This approach emphasizes the
importance of an abundant species,
such as kelp.
Figure 16.3 Defining Communities
Concept 16.1
What Are Communities?
Ecologists often define a community
arbitrarily, based on the questions they
are posing.
Example: A study of marine invertebrates
in seaweed might restrict the community
to that interaction, and not include
mussel-eating birds, etc.
Concept 16.1
What Are Communities?
Counting all the species in a community
is essentially impossible, especially if
small or unknown species are
considered.
Ecologists usually consider a subset of
species when they define and study
communities.
Concept 16.1
What Are Communities?
Subsets of species can be defined by:
• Taxonomy (e.g., all bird species in a
community)
• Guild—group of species that use the
same resources
• Functional group—species that
function in similar ways
Figure 16.4 Subsets of Species in Communities
Concept 16.1
What Are Communities?
Food webs organize species based on
trophic or energetic interactions.
Trophic levels:
• Primary producers—autotrophs
• Primary consumers—herbivores
• Secondary consumers—carnivores
• Tertiary consumers—carnivores
Figure 16.5 Food Webs and Interaction Webs (Part 1)
Concept 16.1
What Are Communities?
Food webs tell little about the strength of
interactions or their importance in the
community.
Some species span two trophic levels,
and some change feeding status as they
mature.
Some species are omnivores, feeding on
more than one trophic level.
Concept 16.1
What Are Communities?
Food webs do not include nontrophic
interactions (horizontal interactions,
such as competition).
Interaction webs more accurately
describe both trophic (vertical) and nontrophic (horizontal) interactions.
Figure 16.5 Food Webs and Interaction Webs
CONCEPT 16.2
Species diversity and species composition
are important descriptors of community
structure.
Concept 16.2
Community Structure
Community structure is the set of
characteristics that shape communities.
Community structure is descriptive in
nature, but provides the basis for
generating hypotheses and experiments
to understand how communities work.
Concept 16.2
Community Structure
Species diversity combines species
richness and species evenness.
• Species richness—the number of
species in a community.
• Species evenness—relative
abundances compared with one
another.
Concept 16.2
Community Structure
Compare 2 communities with 4 species
each (species richness equal).
Community A: One species constitutes
85% of the individuals, the other species
each constitute 5%.
Community B: Abundance is equally
divided, each species constitutes 25%.
This community has higher diversity.
Figure 16.6 Species Richness and Species Evenness
Concept 16.2
Community Structure
The most commonly used species
diversity index is the Shannon index:
s
H   pi ln pi 
i 1
pi = proportion of individuals in the ith
species
s = number of species in the community
Concept 16.2
Community Structure
Species diversity (and biodiversity) are
often used more broadly to mean the
number of species in a community.
Biodiversity describes diversity at
multiple spatial scales, from genes to
species to communities.
Implicit is the interconnectedness of all
the components.
Figure 16.7 Biodiversity Considers Multiple Spatial Scales
Concept 16.2
Community Structure
Graphical representations of species
diversity show commonness or rarity.
Rank abundance curves plot the
proportional abundance of each species
(pi) relative to the others in rank order.
Figure 16.8 Are Species Common or Rare?
Concept 16.2
Community Structure
Relative abundances can suggest what
species interactions might be occurring.
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 these relationships.
Concept 16.2
Community Structure
Species diversity and rank abundance
curves were determined for soil bacteria
communities in two pastures.
One pasture had been fertilized regularly.
Bacteria species can be identified using
DNA sequencing of 16S ribosomal DNA
and grouped using phylogenetic
analysis.
Concept 16.2
Community Structure
20 phylogenetic groups of bacteria were
found in the pastures.
Both had similar community structure. A
few species were abundant; most
species were rare.
Figure 16.9 Bacterial Diversity in Pastures in Scotland
Concept 16.2
Community Structure
Species accumulation curves: Species
richness plotted as a function of total
number of individuals counted.
These curves can help determine when
most or all of the species in a
community have been observed.
Figure 16.10 When Are All the Species Sampled?
Concept 16.2
Community Structure
In reality, the threshold where no new
species are counted never occurs
because new species are constantly being
found.
Hughes et al. (2001) compared species
accumulation curves for five different
communities.
Figure 16.11 Communities Differ in Their Species Accumulation Curves
Concept 16.2
Community Structure
The communities varied greatly in the
amount of sampling effort needed to
determine species richness.
Temperate forest and tropical bird
species were adequately represented
before half the individuals were counted.
For tropical soil bacteria, more effort was
needed to sample this extremely diverse
community.
Concept 16.2
Community Structure
Species composition: Identity of species
in a community.
Two communities could have identical
species diversity values, but completely
different species.
The identity of species is critical to
understanding community structure.
CONCEPT 16.3
Communities can be characterized by
complex networks of direct and indirect
interactions that vary in strength and
direction.
Concept 16.3
Interactions of Multiple Species
In a community, multiple species
interactions generate a multitude of
connections.
Direct interactions: Between two
species (e.g., competition, predation,
facilitation).
Indirect interactions: The relationship
between two species is mediated by a
third (or more) species.
Figure 16.12 Direct and Indirect Species Interactions
Concept 16.3
Interactions of Multiple Species
Indirect effects are often discovered by
accident when species are
experimentally removed to study the
strength of direct interactions.
Concept 16.3
Interactions of Multiple Species
Trophic cascade:
A carnivore eats an herbivore (a direct
negative effect on the herbivore).
The decrease in herbivore abundance
has a positive effect on a primary
producer.
Concept 16.3
Interactions of Multiple Species
In kelp forests, sea otters feed on sea
urchins, which feed on the kelp.
Sea otters have a positive indirect effect
on kelp.
Kelp, in turn, can positively affect
abundance of other seaweeds, which
serve as habitat and food for marine
invertebrates and fishes.
Figure 16.13 Indirect Effects in Interaction Webs (Part 1)
Concept 16.3
Interactions of Multiple Species
Trophic facilitation:
A consumer is indirectly facilitated by a
positive interaction between its prey and
another species.
Example: Interactions between salt marsh
plants affect aphids.
Figure 16.13 Indirect Effects in Interaction Webs
Concept 16.3
Interactions of Multiple Species
A sedge (Juncus) and a shrub (Iva) have
a commensalistic relationship.
Juncus shades the soil surface,
decreasing evaporation and salt buildup.
Juncus also has aerenchyma, tissue that
allows oxygen to move to the roots.
Some oxygen moves into the soil where
other plants can use it.
Concept 16.3
Interactions of Multiple Species
Experimental removal of Juncus
decreased the growth rate of Iva, but
removing Iva had no effect on Juncus.
Population growth rates of aphids on Iva
were significantly higher when Juncus
was present.
Juncus has an indirect positive effect on
the aphids.
Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 1)
Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 2)
Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 3)
Concept 16.3
Interactions of Multiple Species
Juncus has both positive and negative
effects on Iva (it improves soil conditions,
but also facilitates the aphids).
But since Iva cannot survive indefinitely
without Juncus, the positive effects
outweigh the negative.
Concept 16.3
Interactions of Multiple Species
Competitive networks: Interactions
among multiple species in which every
species has a negative effect on every
other species.
In a network, as opposed to a linear
hierarchical system, no one species
dominates the interaction, allowing for
coexistence.
Figure 16.15 Competitive Networks versus Competitive Hierarchies
Concept 16.3
Interactions of Multiple Species
Interaction strength: Magnitude of the
effect of one species on the abundance
of another species.
It is measured by removing one species
(the interactor species) from the
community and observing the effect on
the other species (the target species).
Concept 16.3
Interactions of Multiple Species
If removal of the interactor species results
in a large decrease of the target
species, the interactor has a strong
positive effect on the target species.
If removal of the interactor causes an
increase of the target species, the
interactor has a strong negative effect
on the target species.
Concept 16.3
Interactions of Multiple Species
Per capita interaction strength =
C 
 
E

ln
I
C = Number of target individuals with interactor
present
E = Number of target individuals with interactor
absent
I = Number of interactor individuals
Concept 16.3
Interactions of Multiple Species
Interaction strength may depend on
environmental factors.
Menge et al. (1996) measured interaction
strength of sea star (Pisaster) predation
on mussels (Mytilus) in wave-exposed
versus wave-protected areas.
Interaction strength was greater in
protected areas. Pisaster was a less
efficient predator in crashing waves.
Ecological Toolkit 16.1, Figure A How Much Does Predation by Sea Stars Matter? It Depends
Concept 16.3
Interactions of Multiple Species
Dominant species or foundation
species: Have large effects on other
species, and thus species diversity, by
virtue of their considerable abundance
or biomass.
Figure 16.17 Dominant versus Keystone Species
Concept 16.3
Interactions of Multiple Species
Some dominant species are ecosystem
engineers—they create, modify, or
maintain physical habitat for themselves
and other species.
Example: Trees can have a large physical
influence on the structure of the forest
community, in addition to providing food
and habitat for many other species.
Figure 16.18 Trees Are Dominant Species and Ecosystem Engineers
Concept 16.3
Interactions of Multiple Species
Keystone species have strong effects
because of their role in a community.
Their effect is large in proportion to their
biomass or abundance.
They usually influence community
structure indirectly, via trophic means,
as in the case of sea otters.
Concept 16.3
Interactions of Multiple Species
Some keystone species are also
ecosystem engineers.
Example: Beavers—a few individuals can
have a large impact by building dams.
Dams can transform a swiftly flowing
stream into a marsh with wetland plants.
Concept 16.3
Interactions of Multiple Species
The view of species interactions as
context-dependent, or changeable under
different environmental conditions, is
relatively new to ecology.
Some keystone species play important
roles in their communities in one
context, but not in others.