Ecology Unit 4.docx - Ecology Unit 4 Chapter 18: Community...

Ecology Unit 4
Chapter 18: Community Structure
 Community structure
o Community diversity includes both the number and relative
abundance of species
o Resources, habitat, keystone species and disturbances can
play roles in organizing communities
o Communities are organized into food webs
 Terrestrial community organization changes with altitude , which
impacts:
o Temperature
o Wind
o Soil moisture
 Aquatic community organization changes with depth
o Species distributions may be limited by abiotic interactions
i.e. the tides
 Upper intertidal zones
o Also may be limited by biotic interactions like competition
and predation
 Lower zones
 An ecotone is the distinct boundary between communities
o Plant biology
o Can be caused by abrupt changes in conditions like soil
composition

 Serpentine (rocky) soil prevents tree growth
o Some species prefer the
ecotone to the adjacent
habitats, but why?
o
 Most likely due to
competitive exclusion at
one time
 Forced into less
than ideal territory
due to competition
between species
and adapt to the
new environment
o Some species are adapted to
all 3 types of soils
 Hawkweed, Fescue, and
Snakeroot trees
 Due to many reasons but often related to the
evolutionary history of that species
 Species within communities may be interdependent or
independent
o Tree species
appeared and
disappeared
throughout the
study area
according to the
moisture gradient
but without reference to each other
2

 The trees are independent of one another
 Can subsist without each other just fine
 Stability of community is not dependent on each
species abundance
o Interdependence can be measured as survival and
reproduction rates
 Extreme environments (high elevations), neighbor
removal hurts survival and reproductive efforts of the
focal species
 Low elevation, neighbor removal increases
reproduction of focal species
 Plant is released from competition
 Rank-abundance curves can be used to tease apart species
evenness and relative abundance
o Width of each curve indicates the species’ richness (how
many of that species you actually have)
o Species abundance : how many of each type?
o Species evenness : are all species in the community equally
abundant?
o Log-normal distribution : most species have neither very few
or very many individuals within the community
 Normal bell curve
 While species richness may be identical, species evenness may still
vary between communities
3

o Theoretical comparison:
 Community A has 5
species, 1 is much
more abundant
 Community B has 5
species, all equally
abundant
 Therefore,
community B has
greater species evenness
o Lizards in Brazil:
 Eucalyptus plantation is more
even
 Primary forest is more
species rich
 Calculating diversity
o Simpson’s diversity index (D)
 A simple mathematical measure that characterizes
species diversity in a community
 The proportion of species, I, relative to the total
number of species,
p
i
, is calculated and squared
