Chapter 16- The Nature of Communities

Communities

Groups of interacting species that occur together at the same place and time

Practical definition of communities used by ecologists

• Ecologists usually define communities based on physical or biological characteristics

• Ecologists often define a community arbitrarily based on the questions they are posing

• Ex: A study of marine invertebrates in seaweed might restrict the community to that interaction, and not include mussel-eating birds, etc.

Subsets of species can be defined by 3 features

1. Taxonomic Affinity

2. Guild

3. Functional Group

Taxonomic Affinity

• Groups of species classified together because of evolutionary lineage

• E.g. all bird species in a community

Guild

• Group of species that use the same resource, even though they might be taxonomically distant

• Ex: some birds, bees, and bats feed on flower pollen, thus forming a guild of pollen-eating animals

Functional Group

• Species that function in similar ways but may or may not use similar resources

• For example, nitrogen-fixing plants (legumes) can be placed in the same functional group

Ecologists often characterize communities based on transfer of energy via…

Trophic Interactions (or energetic interactions). The network of interactions is called a food web

Food web

a representation of the trophic or energetic connections among species within a community

What can food web be organized by?

Trophic levels

Trophic Levels

• A measure of how an organisms obtains energy within a food web

• Groups of species that have similar ways of interacting and obtaining energy.

Primary Producers

Autotrophs

Primary Consumers

Herbivores

Secondary Consumers

Carnivores

Tertiary Consumers

Carnivores

What is the variation in food web complexity based upom

The community structure, and (from an ecologist’s perspective) on the question being asked

What do food webs tell us little about?

• Strength of interactions or their importance in the community

  • Ex: some species span the trophic levels, and some change feeding status as they mature

  • Some species are omnivores, feeding on more than one trophic level

  • Idealized foods webs often do not included important elements such as symbionts (including mutualists and parasites) and detritivores

  • Does not include noon-trophic interactions such as competition and some positive interactions

Detritus

Organic Matter

Detritivores (mostly fungi and bacteria)

Consume detritus through a process called decomposition

Horizontal interactions

• Non-trophic interactions

• Ex Competition and some positive interactions

Interaction Web

• More accurately describes both the trophic (Vertical) and Non-trophic (horizontal) interactions among the species in a traditional food web

2 descriptor components of community structure

1. Species Diversity

2. Species Composition

Community structure

• The set of characteristics that shape a community

• provides the necessary quantitative basis for generating hypotheses and experiments directed at understanding how communities work.

Species diversity

a measure that combines the number of species (species richness) and their abundances compared with those of the other species (species evenness) within the community

Species richness

the number of species in a community

Species evenness

which tells us about the commonness or rarity of species, requires knowing the abundance of each species relative to those of the other species within the community, a harder value to obtain

The contributions of species richness and species evenness to species diversity can be illustrated using a hypothetical example

• Let’s imagine two meadow communities, each containing four species of butterflies. Both communities have the same butterfly species richness, but their species evenness differs. In community A, one species constitutes 85% of the individuals in the community, while the other four species constitute only 5% of the individuals in the community; thus, species evenness is low.

• In community B, the number of individuals are evenly divided among the four species (25% each), so species evenness is high. In this case, even though each community has the same species richness (four species), community B has the higher species diversity because it has higher species evenness

Shannon Index

Commonly used species diversity index

Shannon Index Formula

H= -∑pᵢ ln(pᵢ)

H= Shannon Index Value

pᵢ= = the proportion of individuals found in the ith species

ln = the natural logarithm s = the number of species in the community

Claude Shannon

• Mathematician / engineer / computer scientists / unicyclist / juggler

• His Master’s thesis laid the foundation for digital computing

• He conceived and laid the foundations for information theory, which has been essential for the electronic communications networks that now lace the earth

Components of Shannon Index

• The lowest possible value of H is zero. The higher a community’s H value, the greater its species diversity.

• These calculations show that community A has the lower Shannon index value (H), confirming mathematically that this community has lower species diversity than community B.

• Given that both communities have the same species richness, the difference in species diversity is driven by the lower species evenness in community A.

Biodiversity

• Describes diversity at multiple spatial scales, from genes to species to communities.

• Implicit in the term is the interconnectedness of genes, individuals, populations, species, and even community-level components of diversity.

Population’s viability

• its chance of persistence

• genetic diversity among individuals within a population influences that population’s viability, which in turns affects species' diversity in a community

The number of community types in an area is critical to…

diversity at larger regional and latitudinal scales.

1 thing Species diversity indices allows ecologists to compare

• Different communities

Graphical Representations of Species Diversities

• Give us a more explicit view of the commonness or rarity of the species in communities

• Rank Abundance Curve

Rank abundance curves

Plot the proportional abundance of each species (p ) relative to the others in rank order, from most abundant to least abundant

Outcomes of Figure 16.6

• If we use rank abundance curves to compare our two butterfly communities from Figure 16.6, we can see i that community A has one abundant species (i.e., the blue butterfly) and three rare species (i.e., the yellow, pink, and orange butterfly species), whereas in community B, all the species have the same abundance

• These two patterns could suggest the types of species interactions that might occur in these two communities. For example, the dominance of the blue butterfly in community A might indicate that it has a strong effect on one or more of the other species in the community. In community B, where all the species have the same abundance, their interactions might be fairly equivalent, with no one species dramatically affecting the others.

20 phylogenetic groups of bacteria were found in the fertilized / unfertilized pastures in Scotland

• 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

Species composition

• Identity of species in a community

• obvious but important characteristic that is not revealed in species diversity indices

• For example, two communities might have the same species diversity value but have completely different members

• In the case of the bacterial communities in Scottish pastures that we considered in, although diversity indices for two communities were nearly identical, their composition differed. Five taxonomic groups of bacteria out of the 20 the researchers found were present in one or the other pasture, but not in both.

Whaat does this graph show?

• Community A (1.1) and Community B (1.09), Community D (1.04): These values are very close to 1. This proximity indicates that each of these communities is highly dominated by one species, with the other species present in very low abundances. Essentially, even though there might be several species recorded, the contribution of most of them to the overall community is minimal.

• Community C (1.61): A higher value like 1.61 suggests that the species in this community are more evenly distributed. In other words, rather than one species overwhelmingly dominating, several species contribute more significantly to the community’s overall makeup. This higher number can be viewed as indicating a modestly more diverse—and balanced—system compared to the others

Species accumulation Curve

• 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.

Hughes et al. (2001) compared species accumulation curves for five different communities.

• Hughes and colleagues found that communities of five different types varied greatly in the sampling effort that would be needed to estimate their species richness.

• A comparison of species accumulation curves not only provides valuable insight into the differences in species richness among communities, but also demonstrates the influence of the spatial scale at which sampling is carried out. For example, if we were to sample the richness of bacteria in tropical soils at the same scale at which we sampled Costa Rican moths, the bacterial richness would be immense in comparison. But such comparisons do suggest that our ability to sample all the bacteria in the human mouth is roughly equivalent to our ability to sample all the moth species in a few hundred square kilometers of tropical forest. The work of Hughes et al. also reminds us how little we know about the community structural characteristics of rarely sampled assemblages, such as microbial communities

Communities are characterized by complex networks of…

Direct and Indirect interactions that vary in strength and direction

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

Positive interactions on graph

• Positive (+). The sign refers to slope of this relationship

• This can be interpreted as: As species A increases, species B increases

• Or, as species A decreases, species B decreases

Negative interactions on graph

• Negative ( - )

• As speciesA increases, species B decreases

• Or, as species A decreases, species B in increases

Drawing of Direct Interaction

Drawing of Indirect Interaction

Trophic Cascade

• Changes in abundance at one trophic level can affect abundance at other trophic levels.

Trophic Facilitation

• A consumer is indirectly facilitated by a positive interaction between its prey and another species.

Indirect Effects in Interaction Webs

• Pay attention to the + or – symbols in the relationships

• The overall effect of an indirect interaction can be determined by multiplying the signs of all the direct interactions together:

  • + • + = +

  • + • - = -

  • - • - = +

Results of Trophic Facilitation in a New England Salt Marsh

In summary, the graph demonstrates that in a New England salt marsh, Juncus exerts a strong trophic facilitation effect—its presence transforms the environment to allow aphid populations to multiply dramatically, while in its absence, conditions restrict aphid growth. This finding not only highlights the importance of specific plant species in shaping community dynamics but also offers insights into how modifications to a habitat could have cascading effects through the food web.

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.

Competitive Hierarchy

• one species dominates the interaction

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).

Per Capita Interaction Strength Formula

(ln(C/E)) / I

• LN = natural log (log base e)

• C = Number of target individuals with interactor present

• E = Number of target individuals with interactor absent

• I = Number of interactor individuals

If C>E

Positive Interaction

If C<E

Negative Interaction

What did Menge et al. (1996) show?

• interaction strength can vary based on environmental factors

Dominant or foundation species:

have large effects on other species and species diversity, due to their considerable abundance or biomass.

Keystone species

• have strong effects because of their role in a community.

• effect is large in proportion to their biomass or abundance.

• usually influence community structure indirectly, via trophic means, as in the case of sea otters.

Ecosystem Engineers

create, modify, or maintain physical habitat for themselves and other species

Redundant species

• having the same function as other species within a larger functional group.

• They contribute to overall diversity but their presence or absence has little significance for the ultimate regulation of the community

Context- Dependent Species Interactions

• changeable under different environmental conditions

Example: Some keystone species play important roles in their communities in one context, but not in others.

• sea stars in protected vs. unprotected habitats