16 The Nature of Communities. Chapter 16 The Nature of Communities CONCEPT 16.1 Communities are...

16The 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


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


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


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


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


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


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


Food webs do not include nontrophic interactions (horizontal interactions, such as competition).

Interaction webs more accurately describe both trophic (vertical) and non-trophic (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

iii ppH

1

ln

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


Indirect effects are often discovered by accident when species are experimentally removed to study the strength of direct interactions.

Concept 16.3


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


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


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


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


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)

Concept 16.3


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


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


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


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


Per capita interaction strength =

IEC

ln

C = Number of target individuals with interactor present

E = Number of target individuals with interactor absent

I = Number of interactor individuals

Concept 16.3


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


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


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


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


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


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.

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