Nature of the Community – Chapter 20

Community

• Any assemblage of populations of living organisms in a prescribed area or habitat– Can be of any size: user defined– Community of animals on a rotting log– Community of plants in the beech-maple

deciduous forest

Rivet Model of Communities• Species in a community are like rivets in the

wings of airplanes• You can remove a few rivets, but how many are

too many?• Obligate association vs. obligate exclusion

– ‘tight’ communities

Redundancy Model of Communities

• Most species have little to do with each other– ‘loose’ communities

• If one predator disappears, another takes its place

Community Characteristics• Biodiversity – what species of animals and plants

live there?– What controls biodiversity

• Growth form and structure – major vegetation categories– Trees, shrubs, herbs, mosses– Broadleaf vs. needles– Stratification (vertical layering) of the community

• Relative Abundance – are species equally abundant– Eveness

• Trophic Structure – Who eats whom?– Determines flow of energy and materials– Determines biological organization

Community Characteristics

• Temporal – can be studied in a changing community or one that is in equilibrium– A stable community is referred to as a climax

community– A changing community is considered to be going

through succession

• Spatial – studied across environmental gradients– How does the community change along a moisture or

temperature gradient

Community Boundaries

• If communities are functional units, then there should be sharp boundaries between communities

• However, most plant communities have a complex continuum of populations

• Gradient analysis can be used to study the continuous variation of vegetation in relation to environmental variables

Fundamental Unit

Individualistic Continuum

Resource-partitioned continuum

Resource-partitioned continuum with several strata

Distributional Relations• If the separate stands that make

up a community are similar, then all or many of the species in the community must have similar geographic distributions

Tension Zones

• Areas between floristic provinces– Coincide with the distributional limits of many

species

Index of Similarity

• If two communities are similar, then they should have many of the same speciesX = number of species in community 1

Y = number of species in community 2

Z = number of species occurring in both

Index of similarity = 2z

X + Y

(2)(17)26+27 = 0.64

Similarity of Crustacean Zooplankton of the Great Lakes

Index of Similarity

Lakes Superior and Michigan 0.81

Lakes Michigan and Huron 0.93

Lakes Erie and Ontario 0.90

Indicator Species

• Can be used to define a community without having to catalog every species present

• They can be a ‘signpost’ for the community or they can be a ‘health’ indicator

• Fish indicator species can be used to estimate water quality– If fish are present that can not tolerate poor water

quality, the water quality must be good– Presence of fish that can withstand poor water quality

does not mean that he water quality is poor

Criteria for Indicator Species

• Should be well known and easily identifiable

• The biology and natural history of the organism must be well known so that we have a good understanding of its tolerances and requirements– Should normally be a permanent resident of that

community

• Should be easily surveyed so non-professionals can be involved in surveys

• Should be specialized to one community or habitat, or to the set of conditions it is supposed to indicate– Specialists are better than generalists

• Should be closely associated with a group of other species it is supposed to indicate

Types of Indicator Species

• Umbrella species – indicator species with large area requirements (grizzly bear)– Take care of land requirements for this species, many

other smaller ones will benefit

• Flagship species – charismatic species that serve as conservation symbols and rallying points for the protection of areas (Bengal tiger)

• Keystone species – pivotal species in a community that maintain the structure of the community (starfish; chp 23)

Tiger beetles as indicator species for disturbance