Lecture 17: Species Diversity in Communities

Question 1

Distribution and abundance of species in communities depends on:

Answer 1

• Regional species pools and dispersal ability
• Abiotic conditions
• Species interactions

Question 2

Regional species pools and dispersal ability

Answer 2

• The regional species pool provides an upper limit on the number and types of species that can be present in a community
• The importance of dispersal can be seen in cases of non-native species invasions

Question 3

Biotic resistance

Answer 3

• occurs when interactions with the native species exclude the invader
• Ex. Native herbivores can reduce the spread of non-native plants.

Question 4

Resource partitioning

Answer 4

• Competing species coexist by using resources in different ways. It reduces competition and increases species richness
• X-axis: resource spectrum (size of prey, seeds, etc.)
• The more overlap of resource use, the more competition between species. The less overlap, the more specialized species have become, and the less strongly they compete

Question 5

Resource ratio hypothesis

Answer 5

Species coexist by using resources in different proportions

Question 6

Processes that promote coexistence

Answer 6

• disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity
• When the dominant competitor is unable to reach its own carrying capacity, competitive exclusion can’t occur, and coexistence will be maintained

Question 7

The Paradox of the Plankton

Answer 7

• Lake phytoplankton communities have very high diversity (30–40 species), all using the same limited resources, in a homogeneous environment
• As long as conditions changed before competitively superior species reached carrying capacity, coexistence would be possible

Question 8

The intermediate disturbance hypothesis

Answer 8

Species diversity should be greatest at intermediate disturbance. At low disturbance, competition determines diversity. At high disturbance, many species can not survive

Question 9

Potential role of positive interactions

Answer 9

• High tidal – less likely for tide to come in
• Low tidal - most likely for tidal action
  Experiment:
• In the low zone physiological stress (frequent inundation) was the main controlling factor
• In the middle intertidal zone, the reed Juncus facilitated other plant species. Without Juncus, most species died.
• They concluded that positive interactions were critically important in maintaining species diversity, especially at the intermediate stress levels.

Question 10

Lottery models

Answer 10

• emphasize the role of chance in maintaining species diversity
• all species have equal chances of obtaining resources that were made available by disturbances, and this allows coexistence
• Species must have similar interaction strengths and growth rates, and be able to respond quickly to disturbances that free up resources
• most relevant in very diverse communities where many species overlap in their resource requirements.

Question 11

The diversity-stability theory

Answer 11

• A long-standing idea in ecology is that species richness is positively related to community stability—the tendency of a community to remain the same in structure and function.

Question 12

Three hypotheses proposed to explain the positive relationship between species diversity and community function and their 2 variables

Answer 12

• Complementary hypothesis
• Redundancy hypothesis
• Idiosyncratic hypothesis
  1) Degree of overlap in ecological function of species.
  2)Variation in strength of the ecological functions of species

Question 13

Complementary hypothesis

Answer 13

• As species richness increases, there will be a linear increase in community function.
• Each species added has an equal effect.

Question 14

Redundancy hypothesis

Answer 14

• The functional contribution of additional species reaches a threshold.
• As more species are added, there is overlap in their function, or redundancy among species
• Start to see more redundancy, overlap in function
• More realistic

Question 15

Idiosyncratic hypothesis

Answer 15

• Dominant species have a much larger effect on community function than other species
• Key members of community driving ecological function
• Very important whether they are included or not

Question 16

Can species diversity suppress human diseases?

Answer 16

• Reservoir of human diseases typically come from animals
• Number of individuals that carried hantavirus increased in plots; infections increased
• Lower diversity, rather than interacting with those without the virus, interact with ones that carry it