Chapter 17: Community Structure Flashcards
Ecological Hierarchy?
- Individual
- Population: group of individuals of the same species that occupy a given area
- Community; assemblage of species that occupy a given area, interacting directly or indirectly
The biological structure of a community is?
- the mix of species (number and relative abundance)
- some communities have a few more common species while others have a wide variety of species
Species Diversity:
- Species richness?
- number of species
Species Diversity:
- species evenness?
- how individuals are apportioned among the species
Species Diversity:
- relative abundance of species?
of individuals of species A/ total # of individuals of alls species
Species Diversity:
- community with some individuals of many species is ____ than one with few species.
ex?
- more diverse
ex: 100 individuals and 10 species
Maximum diversity: 10 of each species
Lowest diversity: 91 of one species and 1 of each of the other species
**lowest diversity: you have a 91% chance of identifying it
If diversity is very high you have a better chance of ___ environmental changes, impacts of pollution, disease etc
- standing /tolerating
- lots of species interactions
- if low diversity = more likely to be affected
Indices of Diversity:
- many diversity indices used to quantify and compare diversity. They tend to differ in?
- their weighting of evenness and species richness
Indices of Diversity:
- which index did we use for the forest lab?
- Shannon- seiner index
Indices of Diversity:
- aside from Shannon-weiner index, there are other indices that are also based on ??
proportional abundance of species ex Simpson’s
Simpson’s index?
- number of times we would have to take pairs of individuals at random to find a sprit of the same species. Inverse of Simpson’s dominance index
Simpson’s index:
- equation?
D= summation(niIN)^2
D= dominance (opposite of diversity is dominance. If you have high diversity you have low dominance)
- summation= summation of all species
- ni= number of individual species i
- IN= total number of individuals of all species
Simpson’s index:
- As D increases??
- as D increases(greater chance of finding pairs of same species), diversity decreases
Simpson’s index:
- Simpson’s reciprocal index??
= 1/D ranges from 1 to species richness
*less sensitive than other indices to speceis richness s
Shannon-Weiner Index?
- measures degree of uncertainty. If diversity is high, certainty of picking a particular species at random is low
Shannon-Weiner Index:
- first step in calculation?
- calculate relative abundance of each species:
- pi= ni/N
Shannon-Weiner Index
- after calculating relative abundance of each species what is then computed?
- Shannon index (H)
H= -summation(pi)(lnPi)
- summation: summation of all species
- ln= log base e
- pi= proportion of individual species i
- how diverse are they in terms of the proportion of each individual species…picking two at random what is the chance they are the same
- if each species was even the huber would be higher therefore greater uncertainty
Why is quantifying biological diversity within a community a necessary step in assessing the impact of human activities?
- better way of comparing sites vs descriptive means
- helps us examine our impact in areas
- repeatable
Dominance?
- single or few species that predominate in a community
Dominance’s relationship with diversity?
dominance is converse of diversity
Dominance is usually measured by?
- abundance but may also be biomass
* In forest lab: dominance is measured as basal area: total basal area of tree species/area sampled.
Keystone species?
- dominance is only one measure of effect go a species on community
- they function in unique manner. Effect disproportionate to abundance
- a dominant species has an effect on its ecosystem that is proportionate to its abundance (t
- community importance ( the change in a community or ecosystem trait per unit change in abundance of a species)
- **keystone species have an impact much greater than their biomass
graph: total impact of a species (y) and proportional biomass (x)
Ecologists often focus on what when examining structure of communities?
- feeding relationships
Two models of feeding relationship?
- Food chain: descriptive diagram eg grass->grasshopper-> sparrow -> hawk
* *very simplified.. - Food web: made up of numbers food chains, highly interwoven with linkages representing a wide variety of species interactions
* mor realistic
* *predation and herbivory are the two most important interactions
Trophic levels?
- are feeding groups
1. autotrophs (primary producers like plants)
2. heterotrophs ( secondary producers: herbivores, carnivores and omnivores)
aka cannot fix carbon..consume things that can
Trophic levels:
- guilds?
- sometimes divided into groups of species that use a common resource in a similar way ex: nectar-feeding bird
Trophic levels:
- Functional type?
- a broader grouping based on function in community;
feeding start food category
shredders dead leaves
colloectors fine organic
filter feeders particles in water column
miners buried particles
Physical structure of communities?
- abiotic factors: depth and water flow in river ex
- biotic factors such as the spatial arrangements of organisms ex vegetation n forest
Vertical Stratification of Communities?
- most communities have a distinctive vertical structure
- On land determined largely by lang life form
- In lake, layers determined by light penetration
- more complex habits have more species
Zonation?
pattern of spatial variation in community structure
- common to all environments
- generally occurs along with physical gradients
- ex salt marsh
- demonstrates abiotic attributes of an environment really set the stage.
Explain changes in species composition of forest stands along topographic gradient in Virginia via at the bottom of ahill
changes in species composition in trees as you go down the hill top to the water
physical gradient? amount of water
lower area probably have a lot of flooding
top you probably get really dry
soil moisture in summer and flooding during other times of the year
what do the trees have to be able to do down there? tolerate flooding…..you get specialists that can do well down there…as you move up the hill its not that these trees cannot tolerate the dryer condition its that they cannot out compete the other species in more fertile environments
Boundaries between communities?
- difficult to define if transition gradually
- communities often distinguished based on species assemblages that are characteristic of different physical environments
- indices of community can be used to compare communities e.g. Jaccard’s coefficient
Comparison of diversity indices does not tell you if there are ?
- the same species in two communities. Can use a similarity index?
Jaccard’s coefficient?
- 1= total species overlap
- 0 = no overlap
- a= number of species in community
- B= number in community 2
Formula:
Cj= j/(a+b-j)
- just using species diversity tells you nothing about the composition
- *does not use this form according the instagram
Why do both similarity and diversity indices? What do they tell you that are different?
-because diversity indexes only tell us relative diversity, species richness, evenness and abundance but similarity tells us the composition and how similar or dissimilar they are
diversity does not tell you just what species are there
most people present both in papers
If you use more than one diversity index, can you directly compare your numbers for two different diversity indices?
if you were to use simpsons and shannon it would be meaningless
-you couldn’t …you would essentially be comparing to different summaries of the data which wouldn’t make sense
point of these is to compare between sites
