Chapter 18: Factors Influencing the Structure of Communities

Question 1

The fundamental niche is a ____ on community structure?

Answer 1

• constraint

Question 2

The set of adaptations a species has defines its what?

Answer 2

fundamental niche

Question 3

Along with an environmental gradient, fundamental niches indicate what?

Answer 3

• only a subset of species can survive, grow and reproduce in that range of environmental conditions
  ie a set number of resources for a set number of species

Question 4

What does the null model assume?

Answer 4

the presence and abundance of species in a community is the result of independent responses to the physical enviornment. These species can live here because the resources they need occur there - no consideration of other species that could be living in that enviornment with them …ie no interaction consideration

Question 5

Species interactions can modify ___, influence ___ and ___.

Answer 5

• the fundamental niche
• abundance
• distribution

Question 6

Often species interactions can be diffuse, meaning what?

Answer 6

that they involve a number of species.
***because of this experiments examining 1 or 2 species may underestimate species interactions on the function and structure of the community.

Question 7

Apply the concept of diffuse and different species interactions to the food web for boreal forests of northwestern Canada example!

Answer 7

• snowshoe hare populations have ebb and flow in abundance over time
• variety of predators that prey on them
• not just lynx that controls snowshoe hares
• straight lines = predation
• arrows upon each other =cannibalism
• combined predation of great horned owl, lynx and coyote controlling snowshoe hare populations trending over time
• a single predator can have very limited effect on their prey species but combined prey effect really shows the diffuse nature of interaction in the community
• White spruce= no direct link between lynx and white spruce BUT lynx can have a controlling influence on white spruce bc they will eat the thing that also preys on white spruce (indirect interaction)

Question 8

What is an indirect interaction?

Answer 8

• a species affects another indirectly through its interaction with a third species that has a direct interaction with the second
  ex: lynx can have positive indirect effect on white spruce

Question 9

Keystone predation is an example of what kind of interaction?

Answer 9

• indirect

Question 10

Role of predation in communities?

Answer 10

• role that predation has in shaping the structure of communities is the influence of all competitive interactions (diffuse, direct and indirect)

Question 11

Explain the Keystone predation example with the food web of pacific northwest intertidal community ?

Answer 11

• thought that the removal of piaster (starfish) would increase its prey species
• HOWEVER, muscles and barnacles are the only two that become really abundant and everything else declines, this is because barnacles and muscles are much more competitive;therefore, piaster keeps levels of its prey down to an extent that they all can thrive in the area.

Question 12

Explain keystone predation.

Answer 12

• indirect interaction

- predator enhances inferior competitor by reducing abundance of superior competitor

Question 13

How can we understand which interactions are important for community structure?

Answer 13

• one way ecologists simplify the task of understanding species relationships is to group the species
  eg functional groups or more broadly trophic levels

Question 14

Are all species equally important for community structure?

Answer 14

• normally in a lot of ecosystems after the removal of some species the community can bounce back
• there are a couple of species really important in the system so if you remove one of them it can have devastating effects on the community BUT if you remove some of the less important ones the community can adapt and be alright over time
• Basically we do not know what a lot of species do to help the ecosystems function the way they are suppose to

Question 15

Structure of food webs can suggest some questions about the processes that control community structure such as what two control types?

Answer 15

• Bottom-up control: are populations limited by the trophic level below ( abundance of their food)
• Top-down control: or limited by the trophic level above them (i.e. predation)

Question 16

Biological structure of community reflects what?

Answer 16

• both species’ response to environmental conditions and their interactions

Question 17

Do the set of species and interactions vary among locations due to environmental conditions?

Answer 17

YEP
- community diversity is structured by what is present in the environment and the set of interactions with the species they have in that environment

Question 18

Explain the salt marsh donation example!

Answer 18

• complex relationship with competition along environmental gradient
• involves resource source and non-resource variables
• trade off between below ground competition (nutrients getting from soil) and ability to tolerate stresses (oxygen availability and salt tolerance) in the environment controls what happens in the enviornment
• stress gradient does not correspond to the resource gradient
• allows characteristics of strs tolerance to enhance resource availability
• if you can take the salt stress and get access to the resources there you can out compete other species
• patterns of temp and moisture (climate) are the major determinant of the patterns in both terrestrial and aquatic environment we see
• they form the basis for a lot of the vegetative classifications we will see
• when classifying different ecosystems that is very important

Question 19

Factors affecting diversity:

- environmental heterogeneity can result in ??

Answer 19

• an area being able to support more species

Question 20

Factors affecting diversity:
- environmental heterogeneity:
L> example of environmental heterogeneity would be soil moisture. Explain this example!

Answer 20

• differences in soil moisture in what could be described as one ecosystem/plot
  -distribution and abundance of species in this plot will change depending on their soil moisture tolerance
• ## everything is not equal in an environment

Question 21

Factors affecting diversity:
- environmental heterogeneity:
L> example of environmental heterogeneity in a forest?

Answer 21

• heterogeneity in a light environment on the forest floor via a tree falling allowing sunlight to come in to a area that was shaded. This gives new heterogeneity into the landscape changing the subset of species that can live there. We end up with an increase in species diversity bc we get plants that cannot live in the shade coming in and colonizing because they now have this resource. As they grow and compete the species diversity will god own bc of adaptation differences between them….in another 100-200 years a tree will grow up closing off the canopy and it will be back to the diversity it once was.

Question 22

Explain the relationship between bird species diversity and foliage hight diversity example!

Answer 22

-measured bird species diversity and structural heterogeneity of vegetation in 13 different communities, the have a variety of structures grass lands to forests etc
- quantified this variability via evaluating relationship between bird species diversity and foliage height diversity. They found that as foliage height diversity increased so did bird species diversity. As you increase diversity of a habitat you can also increase the species using those habitats - may end up with same biomass in the environment
-

Question 23

Does increasing species diversity always equal an increase in the individuals of that environment?

Answer 23

no

Question 24

Why would it be important to increase diversity and not individuals?

Answer 24

• community persistence
• if a disease came in that wipes out a whole species..if only that one species was present the community would be gone. High diversity leads to a more stable environment bc you can lose species and still maintain a lot of that ecosystem’s function.

Question 25

Resource availability and diversity in plant communities?

Answer 25

• diversity is often lower when nutrient availability is higher

Question 26

Resource availability and diversity:

- Tree species richness vs soil fertility in Costa Rica example?

Answer 26

• negative relationship between nutrient availability and plant diversity in communities
• ## doesn’t mean we will get higher diversity. It can increase how well and fast they grow but just bc there is more doesn’t = more species

Question 27

Reduced diversity with higher nutrients seems counterintuitive. Why might this happen?
L> think with respect to plants as an example

Answer 27

• the actions of different species within the community
• experiments have been done which reveals a shift in importance below ground and above ground competition
• when nutrient availability is low, plant growth rate, size and density is low which means all competition is happening below ground. In this situation the diversity is maintained - not so great at competing so they are all equally managing. Competitive displacement is quite low.
• As nutrient availability increases, plant growth rate, size and density increases which means competition is happening above ground. Competition for light and space resources is now more important. If good at growing quickly you can out compete species and kill them off for access to resources thereby decreasing diversity. Competitive displacement s now present.