Notes 21: Community Ecology
• Introduction
o Definitions of Community
§ Broad
• A group of populations at the same place and time
• Most ecologists define a community as an assemblage of
populations interacting in the same place at the same time
• Often identify the community by the dominant species
o Like oak-hickory community
§ Narrow
• Functionally or taxonomically
• Might refer to small-mammal community of southeastern US
o Predatory mice like grasshopper mouse and seed-eating
mice
• Might refer to guild
o Group of species that uses same resources same way
§ Functional subgroup of populations in an area that
use a common set of resources in similar ways
o Might think of a guild of medieval craftsmen that all do
similar things
o Ex: large carnivores on the plains of Africa
§ Complex
• Communities connected by migration or energy flow
• Energetic relationships between communities separated in space
• Species might migrate between them, linking communities into a
complex
• Dynamics of a population must be understood in both
communities
• Songbirds in eastern US declining for last 50 years, even though
forested habitat has been increasing
o Migrate to tropical forests of central and South America,
which have been shrinking
o Constrained by loss of habitat in wintering grounds, so
reproductive rate in summer breeding grounds is not
enough to offset these losses
• Think of terrestrial and aquatic communities as separate, but not
really
o Trees transfer energy to stream through dropped leaves
o Energy that feeds aquatic communities
o Fish affect reproduction of surrounding plants
o Ponds without fish have lots of dragonfly larvae, which
develop into dragonflies that eat pollinators and reduce
plant reproduction
, o Fish eat dragonfly larvae, so this keeps pollinator densities
and plant reproduction rates high
o Development of the Community Concept
§ Clements – Community as “superorganism”
• Prairie has certain species that live together and forests had
different species that lived together
• Sand dunes on great lakes had different assemblages at discrete
distances from shore
• Interpreted correlated distributions as casual
o Need one another must live together so collectively act as
one large system
o Like a big organism
o Discrete transitions between communities
• Over time, community develops like an organism
o Into climax community, or mature state
o Not replaced by another type of community
§ Gleason – the individualistic concept of community
• Communities are assemblages of species that happen to have
overlapping ranges
• Eastern deciduous forest
• Some species are found together but not always found together
• Species have overlapping distributions, but these are largely
independent
o Dependent on the responses of each species to its
environment
§ Whittaker – Gradient Analysis
• Species respond independently to environmental gradients
o Steep gradients create abrupt transitions between
community types
• Appreciated different types of communities
• Some composed of a specific set of species (closed)
• Some had a large degree of overlap in species composition (open)
• Abruptness with which communities would change over space
depends on abruptness of environmental change
o When environment changed abruptly, communities
changed abruptly and appear as closed
o When environment changed gradually, communities
changed gradually
• Ecotone: where environments change abruptly and discrete
communities meet
, •
• A transition in community type at serpentine boundary/soil
o Serpentine soil have chromium, nickel, magnesium
o Usually an abrupt change in soil concentrations
o Abrupt change in community type
o Hyperaccumulation is extra absorption of toxic metals
• More gradual environmental change means less abrupt
community transitions
o Although each species is most abundant under certain
moisture and elevation conditions in the Smokies, co-
occur over a wide range of conditions
o Only relative abundances may vary
o Red oak most abundant in drier, higher elevation
§ But extends into many other forest types
o Beech, white oak, and red oak occur together in many
areas
§ Beech in most wet, white oak drier, red oak driest
and highest elevation
§ Pickett and White – Patch-Dynamic Theory
• Environment not only determinant of community type
o Variation in community type may not be just a function of
changes in environmental conditions
o May be function of changes in disturbance regime, time
since last disturbance, and successional stage of
community
, • Consider history of site
• Even in a large expanse of habitat, there will be a series of patches
of different successional ages
o Representing communities that are recovering from a
disturbance at some time in the past
• Community is a dynamic mosaic of successional patches of
different ages
o Variation in community type do not need to indicate
differences in environmental conditions, but differences in
the frequency of disturbance
o Or time since last disturbance
• Difference in pine and oak communities may not be due to
moisture
o Could be due to time since last fire
o Key Descriptors
§ Species Richness
• Number of species in the community
§ Species Diversity
• References relative abundance of species in community
• If all species equally abundant, community is very diverse
• If some species are very abundant and others very rare,
experience a less diverse community
• Measure diversity
o Affected by changes in richness and relative
abundance/evenness
!
o Simpson’s reciprocal index of diversity: ∑($ )"
!
§ pi is proportion of total community represented by
Ith species
§ Membership
• Most fundamental descriptor
• Not quantitative, so overlooked
• Which species are present
o Not number of species or relative abundance
• Two communities could have same richness and diversity but
contain completely different species
§ Trophic Relationships
• Who eats whom
o Food webs define trophic relationships between
species/taxa
• Functional context
• First define nodes/interactors
o Typically species
• Introduction
o Definitions of Community
§ Broad
• A group of populations at the same place and time
• Most ecologists define a community as an assemblage of
populations interacting in the same place at the same time
• Often identify the community by the dominant species
o Like oak-hickory community
§ Narrow
• Functionally or taxonomically
• Might refer to small-mammal community of southeastern US
o Predatory mice like grasshopper mouse and seed-eating
mice
• Might refer to guild
o Group of species that uses same resources same way
§ Functional subgroup of populations in an area that
use a common set of resources in similar ways
o Might think of a guild of medieval craftsmen that all do
similar things
o Ex: large carnivores on the plains of Africa
§ Complex
• Communities connected by migration or energy flow
• Energetic relationships between communities separated in space
• Species might migrate between them, linking communities into a
complex
• Dynamics of a population must be understood in both
communities
• Songbirds in eastern US declining for last 50 years, even though
forested habitat has been increasing
o Migrate to tropical forests of central and South America,
which have been shrinking
o Constrained by loss of habitat in wintering grounds, so
reproductive rate in summer breeding grounds is not
enough to offset these losses
• Think of terrestrial and aquatic communities as separate, but not
really
o Trees transfer energy to stream through dropped leaves
o Energy that feeds aquatic communities
o Fish affect reproduction of surrounding plants
o Ponds without fish have lots of dragonfly larvae, which
develop into dragonflies that eat pollinators and reduce
plant reproduction
, o Fish eat dragonfly larvae, so this keeps pollinator densities
and plant reproduction rates high
o Development of the Community Concept
§ Clements – Community as “superorganism”
• Prairie has certain species that live together and forests had
different species that lived together
• Sand dunes on great lakes had different assemblages at discrete
distances from shore
• Interpreted correlated distributions as casual
o Need one another must live together so collectively act as
one large system
o Like a big organism
o Discrete transitions between communities
• Over time, community develops like an organism
o Into climax community, or mature state
o Not replaced by another type of community
§ Gleason – the individualistic concept of community
• Communities are assemblages of species that happen to have
overlapping ranges
• Eastern deciduous forest
• Some species are found together but not always found together
• Species have overlapping distributions, but these are largely
independent
o Dependent on the responses of each species to its
environment
§ Whittaker – Gradient Analysis
• Species respond independently to environmental gradients
o Steep gradients create abrupt transitions between
community types
• Appreciated different types of communities
• Some composed of a specific set of species (closed)
• Some had a large degree of overlap in species composition (open)
• Abruptness with which communities would change over space
depends on abruptness of environmental change
o When environment changed abruptly, communities
changed abruptly and appear as closed
o When environment changed gradually, communities
changed gradually
• Ecotone: where environments change abruptly and discrete
communities meet
, •
• A transition in community type at serpentine boundary/soil
o Serpentine soil have chromium, nickel, magnesium
o Usually an abrupt change in soil concentrations
o Abrupt change in community type
o Hyperaccumulation is extra absorption of toxic metals
• More gradual environmental change means less abrupt
community transitions
o Although each species is most abundant under certain
moisture and elevation conditions in the Smokies, co-
occur over a wide range of conditions
o Only relative abundances may vary
o Red oak most abundant in drier, higher elevation
§ But extends into many other forest types
o Beech, white oak, and red oak occur together in many
areas
§ Beech in most wet, white oak drier, red oak driest
and highest elevation
§ Pickett and White – Patch-Dynamic Theory
• Environment not only determinant of community type
o Variation in community type may not be just a function of
changes in environmental conditions
o May be function of changes in disturbance regime, time
since last disturbance, and successional stage of
community
, • Consider history of site
• Even in a large expanse of habitat, there will be a series of patches
of different successional ages
o Representing communities that are recovering from a
disturbance at some time in the past
• Community is a dynamic mosaic of successional patches of
different ages
o Variation in community type do not need to indicate
differences in environmental conditions, but differences in
the frequency of disturbance
o Or time since last disturbance
• Difference in pine and oak communities may not be due to
moisture
o Could be due to time since last fire
o Key Descriptors
§ Species Richness
• Number of species in the community
§ Species Diversity
• References relative abundance of species in community
• If all species equally abundant, community is very diverse
• If some species are very abundant and others very rare,
experience a less diverse community
• Measure diversity
o Affected by changes in richness and relative
abundance/evenness
!
o Simpson’s reciprocal index of diversity: ∑($ )"
!
§ pi is proportion of total community represented by
Ith species
§ Membership
• Most fundamental descriptor
• Not quantitative, so overlooked
• Which species are present
o Not number of species or relative abundance
• Two communities could have same richness and diversity but
contain completely different species
§ Trophic Relationships
• Who eats whom
o Food webs define trophic relationships between
species/taxa
• Functional context
• First define nodes/interactors
o Typically species
