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Community structure

I. Community structure - communities are characterized not only by the organisms that live there, but by the physical characteristics of the area (which are tied to the organisms by their tolerances, etc.)

A. Physical structure - of a community includes:

1. Abiotic factors, slope, depth of water, soil type, etc.

2. Biotic factors - spatial configurations of organisms

B. Vertical structure (within a community) - on land and in aquatic environments, this is determine by the vertical gradient of light
Fig. 20.3 (on land) canopy, under-story, shrubs, herbaceous layer, organic layer of soil
In water, the types of algae and sea grasses present will vary with depth: brown (can be exposed), greens & sea grasses (must remain submerged), red (can live with less light).

C. Horizontal structure (within a community) - gaps or other temporary irregularities caused by the death of canopy trees, changes in plant type due to soil structure, moisture availability, environmental tolerances of plant species, etc.

II. Zonation - spatial variation between communities - changes in the physical and biological structure of communities as you move across the landscape and abiotic factors vary - tied to environmental tolerances of the species

A. Transition zones (boundaries) - an abrupt change in physical and biological structure, areas of high animal diversity (many habitats and niches interact here), can very depending on whether or not the two communities intermingle.
- defining boundaries between communities can be difficult and can very depending on whether or not the two communities intermingle (we will deal with this in more detail later when we discuss two different concepts of communities, Chapter 22).

1. Permanence of transition zones
   a. inherent - long-term boundaries, stable
   b. induced - temporary boundaries induced by man or periodic disturbances

2. Types of transition zones
   c. Edge - (Fig 21.3a) - where two vegetative communities meet
   d. Ecotone - (Fig. 21.3b,c,d) - where two vegetative communities meet and intergrade (still a rather abrupt change in communities is noticeable).

B. Community concepts (Fig 7.24 in Stiling) - based on/defined by presence/absence of transition zones

1. Closed community - species belonging to a community are closely associated with one another, suggesting co-evolution of species as reason for similar environmental tolerances. See an edge between these communities.

2. Open community - each species is distributed independently based on its own requirements, co-occurrance and the resulting communities are the result of similar tolerances and competitive & consumer-resource interactions). May see overlap of species with bordering communities. See ecotones between these communities.

3. Gradient/continuum between these two concepts is often the reality.

III. Landscape fragmentation

A. Edges - all habitats have edges, areas where the structure of the habitat is changing, mixing with another habitat: good living conditions for some species, poor for others.

B. Interior - an area of uninterrupted habitat within the edges: required by some species to survive.

C. Metapopulations - a number of subpopulations with an area, interacting among fragmented habitat patches.
Population - all of the members of a given species in a given area.
Subpopulations - dividing up population in a larger area by level of interaction

D. Fragmentation of habitats - habitat areas may be fragmented (by natural causes or by man) into smaller patches. Decreases the interior and increases the edge space - may cause a shift in the types of species occupying the patches (edge species increase, interior species decrease)

1. Decreases the interior and increases the edge space - may cause a shift in the types of species occupying the patches (edge species increase, interior species decrease)

2. Islands - an isolated piece of a given habitat type

3. Corridors - habitat pathways connecting habitat patches, may become severed, interrupting the interaction of the sub-populations and potentially stranding (and dooming to extinction) some sub-populations.
   - e.g. think of amphibians or FW fish, rather than bird, who can move about quite easily.
   - corridors can be reduced to fragmented 'stepping stones', which some species may be able to travel, but others may not.
   - creation of stepping stones or corridors between habitat patches is one way in which diversity in fragmented communities can be increased, by increasing interaction among the sub-populations in otherwise isolated habitat fragments.

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