Lecture 17

Question 1

Distribution and Abundance of Species in Communities Depends on:

Answer 1

1. Regional species pools and dispersal ability
2. Abiotic conditions
3. Species interactions
   - These factors act as “filters” which exclude species from (or include species in) particular communities

Question 2

Regional Species Pools and Dispersal Ability

Answer 2

• Regional species pool provides an upper limit on the number and types of species present in a community
• Importance of dispersal can be seen in cases of non-native species invasions
• Humans have greatly expanded regional species pools by serving as vectors of dispersal
  Ex; aquatic species travel around the world in ballast water carried by ships

Question 3

Abiotic Conditions

Answer 3

• A species may be able to get to a community but be unable to tolerate the abiotic conditions
  Ex. a lake might not support organisms that require fast-flowing water
• Many species that are dispersed in ballast water can’t survive in a new habitat because of temperature, salinity, etc
• We can’t rely on physiological constraints to exclude invaders

Question 4

Species Interactions

Answer 4

• Coexistence with other species is also required for community membership
• Other species may be required for growth, reproduction, or survival
• Species may be excluded from a community by competition, predation, parasitism or disease

Question 5

Biotic Resistance

Answer 5

• occurs when interactions with the native species exclude the invader
  Ex. native herbivores can reduce the spread of non-native plants
• Not much is known about biotic resistance
  because failed introductions of non-native species tend to go undetected

Question 6

Resource Partitioning

Answer 6

• competing species coexist by using resources in different ways. It reduces competition and increases species richness
• In a simple model, each species’ resource use falls on a spectrum of available resource
• more overlap: more competition between species
• less overlap: more specialized species have become and the less strongly they compete

Question 7

MacArthur

Answer 7

• 1958
• Studied resource partitioning in a community of warblers in New England forests
• Recorded feeding habits, nesting locations, and breeding territories

Question 8

When MacArthur mapped the locations of warbler activity

Answer 8

• he found that the birds were using different parts of the habitat in different ways

Question 9

Diatom Species Coexist in Nature

Answer 9

• To explain this Tilman proposed the resource ratio hypothesis

Question 10

Resource Ratio Hypothesis

Answer 10

• pecies coexist by using resources in different proportions

Question 11

Tilman’s Experiment

Answer 11

• Two diatom species were grown in media with different SiO2 : PO4 ratios
• Cyclotella dominated only when the ratio was low (asterionella cant dominate because there are not enough cilia)
• Asterionella dominated when the ratio was high
• Coexistence occurred only when SiO2 and PO4 were limiting to both species

Question 12

Proccesses that Promote Coexistence

Answer 12

• 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 13

Intermediate Disturbance Hypothesis

Answer 13

• Species diversity should be greeted at intermediate disturbance

Question 14

At Low Disturbance

Answer 14

• competition determines the diversity

Question 15

At High Disturbance

Answer 15

• many species cannot survive

Question 16

Sousa

Answer 16

• Studied communities on intertidal boulders in southern California that were overturned by waves
• Small boulders were overturned frequently (disturbance)
• Large boulders were overturned less often
• The high and low disturbance is relative
• High diversity: well-balanced

Question 17

Potential Role of Positive Interactions

Answer 17

• New England salt marsh, 3 intertidal zones were identified with different species composition
• Middle intertidal zone had the greatest species richness
• Low zone physiology stress (frequent inundation) was the main controlling factor
• Concluded that positive interactions were critically important in maintaining species diversity, especially at intermediate stress levels

Question 18

Menege-Sutherland Model

Answer 18

• Separates the effects of predation from those of disturbance and stress
• The above theories assume an underlying competitive hierarchy

Question 19

Lottery Models

Answer 19

• Emphasize the role of chance in maintaining species diversity
• All species have equal chances of obtaining resources that were made available by disturbances (Which allows coexistence)

Question 20

Species Must Have

Answer 20

• similar interaction strengths
• similar growth rates
• the ability to respond quickly to disturbances that free up resources

Question 21

Sale (1977)

Answer 21

• Looked at patterns of occupation of new sites by three fish species, and found it to be random
• The lottery model may be most relevant in very diverse communities but decreases in communities which species have large disparities in interaction strength

Question 22

One Important Component of this System

Answer 22

• Fishes produce many mobile juveniles that can saturate a reef and quickly take advantage of open space

Question 23

The Consequences of Diversity

Answer 23

• The central idea in ecology is: that species diversity can control community functions, such as plant productivity, soil fertility, water quality, etc

Question 24

Many Community Functions also Provide Valuable Services to Humans

Answer 24

• Food and fuel production
• Water purification
• O2 and CO2 exchange
• Protection from catastrophic events, such as floods

Question 25

Diversity-Stability Theory

Answer 25

• Long-standing idea that species richness is positively related to community stability
• Tendency of a community to remain the same in structure and function

Question 26

Three Hypotheses Have Been Proposed To ___

Answer 26

• explain the positive relationship between species diversity and community function

Question 27

2 Variables in all the Hypotheses

Answer 27

• Degree of overlap in ecological function of species
• Variation in the strength of the ecological functions of species

Question 28

Complementarity Hypothesis

Answer 28

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

Question 29

Redundancy Hypothesis

Answer 29

• the functional contribution of additional species reaches a threshold
• As more species are added, there is overlap in their function or redundancy among species

Question 30

Idiosyncratic Hypothesis

Answer 30

• Dominant species have a much larger effect on community function than other species