Lecture 14: The Nature of Communities

Question 1

What are communities?

Answer 1

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

Question 2

Physically defined community

Answer 2

Might encompass all the species in a sand dune, mountain stream, or a desert

Question 3

Biologically defined community

Answer 3

• Might include all species associated with a kelp forest, freshwater bog, or a coral reef
• This approach emphasizes importance of an abundant species, such as trees

Question 4

Subsets of species can be defined as __

Answer 4

• Taxonomic affinity
• Guild
• Functional group

Question 5

Taxonomic affinity

Answer 5

all bird species in a community

Question 6

Guild

Answer 6

group of species that use the same resources

Question 7

Functional group

Answer 7

species that function in similar ways, but do not necessarily use the same resources

Question 8

Food Webs

Answer 8

• organize species based on trophic or energetic interactions
• tell little about the strength of interactions or their importance in the community
• don’t include non trophic interactions

Question 9

Trophic levels

Answer 9

First level: Primary producers (autotrophs) – plants and algae

Second level: Primary consumers – herbivores

Third level: Secondary consumers – carnivores

Fourth level:Tertiary consumers – carnivores

Question 10

Omnivores

Answer 10

• feeding on more than one trophic level

Question 11

Interaction webs

Answer 11

more accurately describe both the trophic (vertical) and non-trophic (horizontal) interactions than a traditional food web

Question 12

Community structure

Answer 12

The set of characteristics that shape communities
- Species richness
- Species Evenness
- Species diversity

Question 13

Species richness

Answer 13

number of species in a community

Question 14

Species evenness

Answer 14

relative abundances compared with one another

Question 15

Species diversity

Answer 15

combines species richness and species evenness

Question 16

Shannon Index

Answer 16

• equation in notes
• pi = proportion of individuals in the species
• s = number of species in the community

Question 17

Biodiversity

Answer 17

• describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of all the components

Question 18

Genetic Diversity

Answer 18

affects the viability of populations, which in turn affects species diversity in a community

Question 19

Rank abundance curves

Answer 19

• plot the proportional abundance of each species (pi ) relative to the others in rank order
• Relative abundances can suggest what species interactions might be occurring

Question 20

Species composition

Answer 20

• identity of species in a community
• Two communities could have identical species diversity values, but have completely different species.

Question 21

Species accumulation curves

Answer 21

• species richness is plotted as a function of the total number of individuals that have been counted
• These curves can help determine when most or all of the species in a community have been observed.

Question 22

Spatial scale

Answer 22

• really important
• If we sampled bacteria in tropical soils at the same scale as Costa Rican moths, the bacterial diversity would be immense in comparison.
• The study highlights how little we know about community structure of rarely studied assemblages, such as microbial communities.

Question 23

Direct Interactions

Answer 23

occur between two species competition, predation, and facilitation
- A –> B

Question 24

Indirect interactions

Answer 24

• Occur when the relationship b/w two species is mediated by a third (or more) species
• Ex. Pollination depends on bees; the bee population is influenced by mice that prey on bees’ nests; mice are eaten by cats. An increase in the cat population can impact the flowers

Question 25

A trophic cascade

Answer 25

-Strong linear relationship of direct effects between organisms that eat one another
- ex. 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

Question 26

Trophic facilitation

Answer 26

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

Question 27

Competitive networks

Answer 27

• Competitive interactions among multiple species in which every species negatively interacts with every other species
• Strong direct competition is buffered, competitive interactions weaker, and no one species dominates
• allow coexistence of competitors and thus maintain species richness

Question 28

Interaction strength

Answer 28

• Magnitude of the effect of one species on the abundance of another species.
• measured by removing one species (the interactor species) from the community and observing the effect on the other species (the target species)
• may depend on environmental factors

Question 29

Dominant species/ Foundation species

Answer 29

• have large effects on other species
• they create, modify, or maintain physical habitat for themselves and other species
  Ex. 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

Question 30

Keystone species

Answer 30

• have a strong effects because of their roles in the community
• Their effect is large in proportion to their biomass or abundance.
• They usually influence community structure indirectly, via trophic means

Question 31

Some keystone species are ecosystem engineers

Answer 31

Ex. 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.
- At the landscape level, beavers can create a mosaic of wetlands within a larger forest community, which increases regional biodiversity.