Community Structure and Species Abundance

• A community is an association of interacting species inhabiting a defined space.
• Community structure includes species number, relative abundance, and diversity.
• A guild is a group of organisms making their living similarly, such as seed-eating desert animals.
• Life-form is a plant's structural combination and growth dynamics, similar to a guild.

16.1 Species Abundance

• There are consistent patterns in species' relative abundance across ecosystems.
• Most species are moderately abundant, with very few being extremely abundant or rare.
• Preston studied relative abundance and developed the concept of commonness and rarity distribution.

The Lognormal Distribution

• Preston graphed species abundance in collections as frequency distributions, where each interval doubles the preceding one on a log2 scale.
• Lognormal distributions result in a bell-shaped curve.
• Sample size greatly affects lognormal distributions; significant effort is needed to capture rare species.

16.2 Species Diversity

• Species diversity comprises two factors: richness and evenness
• Species richness is the number of species in a community.
• Species evenness is the relative abundance of species.

Quantitative Index of Species Diversity

• The Shannon Wiener Index is used: H' = -Σ pi loge pi
• H' is the diversity index value.
• pi is the proportion of the ith species.
• Loge is the natural logarithm of pi.
• s is the number of species in the community.

Rank-Abundance Curves

• Rank-abundance curves can portray relative abundance and diversity by plotting species' relative abundance against their abundance rank.
• Greater species evenness is indicated by a lower slope, and less evenness by a steeper slope.

16.3 Environmental Complexity

• Species diversity generally increases with environmental complexity/heterogeneity.
• MacArthur found warbler diversity increased as vegetation stature (foliage height) increased.
• Many studies show a positive relationship between environmental complexity and species diversity.

Niches and Heterogeneity

• Hutchinson described the "paradox of the plankton"; phytoplankton communities in simple environments compete for the same nutrients, but many species coexist without competitive exclusion.
• Similar paradoxes exist for terrestrial plants.
• Environmental complexity may account for a significant portion of diversity.

Niches of Algae and Terrestrial Plants

• Algal niches seem to be defined by nutrient requirements.
• Tilman found that coexistence of freshwater diatoms depended on the ratio of silicate and phosphate.
• Diatoms vary in phosphorus uptake rate
• The ratio of silicate to phosphate varies across lakes, affecting diatom dominance.
• Variation in limiting nutrients' availability contributes to environmental complexity.

Complexity in Plant Environments

• Lebo et al. studied spatial variation in nutrients and particulate concentrations in Pyramid Lake where there is not a uniform solution.
• Robertson et al. studied nitrogen and moisture variation in an abandoned field exhibiting considerable patchiness.
• Spatial variability affects plant community structure.

Soil and Topographic Heterogeneity in Tropical Forests

• Jordan studied vegetation and soils in the Amazon forest.
• Forest diversity is organized in the following ways:
• A large number of species within tropical forest communities
• A large number of plant communities in a given area, each with a distinctive species composition.
• Variation in soil influences the number of communities.

Nutrient Enrichment

• A negative relationship has been observed between nutrient availability and algal and plant species diversity.
• As nutrients increase, the diversity of plants and algae declines because increased nutrient availability reduces limiting nutrients.
• Light becomes a single limiting resource; the species most effective at competing for light becomes dominant.

Nitrogen Enrichment and Fungal Diversity

• Areas of high atmospheric nitrogen deposition have fungal diversity declines, possibly from shifts from above- to below-ground growth.
• Studies in Alaska found ectomycorrhizal fungal taxa declines along a soil nitrogen gradient and corresponded to a soil pH decline.
• Environmental complexity can account for a portion of plant species diversity.

16.4 Disturbance and Diversity

• Intermediate disturbance levels promote higher diversity.
• Equilibrium is when environmental conditions remain relatively stable and are maintained by opposing forces.
• Most environments are subject to disturbance.

The Nature and Sources of Disturbance

• Disturbance is difficult to define and varies with organisms and environments, dependent on spatial and temporal scale
• Abiotic forces, biotic factors, and human activities are potential sources of disturbance.

Intermediate Disturbance Hypothesis

• Connell proposed that disturbance significantly influences community diversity and that high diversity is a result of constantly changing conditions. • High and low disturbance reduces diversity.
• Frequent, intense disturbance and infrequent, low-intensity disturbance both reduce diversity.
• Intermediate disturbance levels allow species to colonize, but competitive exclusion does not occur.

Disturbance and Diversity in the Intertidal Zone

• Sousa studied disturbance effects on algae and invertebrates growing on intertidal zone boulders.
• Predicted that disturbance level depends on boulder size.
• Boulders supporting the greatest species diversity were subject to intermediate disturbance levels.

Disturbance in Temperate Grasslands

• Whicker and Detling designated prairie dogs as a source of disturbance on N. A. prairies because they build extensive burrow systems.
• Prairie dogs remove vegetation, opening areas to colonization.
• Pest control programs reduced prairie dog populations by 98%, and in general eliminate dynamic influence of plant communities.

Applications: Disturbance by Humans

• Effects of human-caused disturbance are everywhere.
• Examples include deforestation and pollution.
• High disturbance levels lead to species diversity declines.
• Thousands of species are threatened with extinction.

Urban Diversity

• 80% of people in developed countries live in cities.
• Urban biota is diverse; urbanization can reduce bird species richness and evenness, but substantial diversity levels also exist in some landscapes.
• Bird species richness peaks with intermediate land use intensity.

The content covers community structure, species abundance, and diversity within ecosystems. It explores how species interact and are distributed. Key concepts include guilds, life-forms, and the lognormal distribution of species abundance, which reveals patterns of commonness and rarity.