Species Abundance and Diversity (Ch 16)

From Populations to Communities

  • Definitions:

    • Population: A group of individuals of a single species inhabiting a specific area.

    • Community: Defined by Moles & Sher as "An association of interacting species inhabiting some defined area".

    • Also defined as an association of interacting populations.

Species Abundance & Diversity: Big Questions

  • Key inquiries include:

    • “Why are there so many species?”

    • Why do groups of competing species coexist?

    • Why are some communities more diverse than others?

Species Abundance & Diversity: Chapter 16

  • Key Topics:

    • 16.1: How do we measure species diversity?

    • 16.2: How do we quantify species diversity?

    • 16.3 & 16.4: What environmental factors might lead to high species diversity?

From Populations to Communities

  • Further definition of Community: A group of interacting populations, addressing questions about species diversity within competitor species.

  • Guild: Defined by Moles & Sher as “A group of organisms that all make their living in a similar way”.

    • A subset of a community associating and partially competing for shared resources.

  • Life Form (Growth Form): Groups of plants with similar structures and growth dynamics (e.g., trees, vines, grasses, forbs).

  • Trophic Level: Group of organisms sharing the same feeding relationship to primary producers or competing for food sources.

Guilds Addressed in Lab

  • Examples include:

    • Algae

    • Stream diatoms

    • Terrestrial plants: Flowering plants (Forbs)

    • Invertebrates: Earthworms (soil inverts), Pollinator Insects

Defining Diversity: Richness and Evenness

  • Species Richness: The total number of species in a guild or community.

  • Species Evenness: Assessment of the variation in the relative abundance of species.

  • Figure 16.5: Depicts species richness and evenness.

Variation in Species Richness and Evenness

  • Commonly, species abundance is distributed lognormally.

    • Most communities exhibit:

    • A few rare species

    • A few abundant species

    • Many moderately abundant species

  • Moles & Sher Figure 16.3: Illustrates community composition variability

  • Sample size effects:

    • Abundant species are easier to detect, while rare species may be difficult.

Community Composition

  • Species Rarefaction Curves: Indicate sampling quality.

    • After sampling, once new species are no longer added, confidence in diversity assessment increases.

Rank Abundance Curves

  • Definition: A graphical representation showing species proportion and rank.

    • Y Axis: Proportional Abundance

    • X Axis: Species Rank

    • Rank 1 represents the most abundant species, while the lowest rank shows the least abundant species.

    • A gradual negative slope indicates a relatively even community; a steep slope indicates an uneven community.

  • Case Comparison:

    • Community A is dominated by one species, showing lower species diversity.

    • Community B has equal proportions of species leading to higher species diversity.

Shannon-Wiener Diversity Index

  • A metric summarizing community richness and evenness.

  • Refer to Moles & Sher Figure Table 16.1 & Figure 16.6 for visualization.

Coexistence of Species

  • Questions addressed in Moles & Sher Sections 16.3 and 16.4

    • Notion: Higher diversity in complex environments

    • Factors include:

    • Spatial niche partitioning

    • Different limiting resources

    • Intermediate disturbances

Microclimates and Niche

  • Microclimate influences species niches, particularly regarding transpiration rates.

    • Example of two species:

    • E. farinosa: Low transpiration, cool due to reflective leaves, grows in shallow soils.

    • E. frutescens: Higher transpiration rate due to availability of deeper soil water.

Coexistence Factors

  • Variation in nutrient levels or other resources that can lead to microclimate differences.

  • Liebig’s Law of the Minimum: Plant growth is limited by the most scarce resource available.

    • This principle posits that competitors can coexist when they each exploit different limiting resources optimally.

SimUText: Competition Overview

  • To achieve stable coexistence:

    • If $K1 = K2$ and $ ext{if } orall ext{ i,j: } ext{ } rac{α{12}}{1} < 1 ext{ and } rac{α{21}}{1} < 1$

  • Nutrient enrichment may decrease environmental complexity, potentially favoring competitive dominance.

  • Intermediate Disturbance Hypothesis:

    • Favors species richness at intermediate levels of disturbance.

Life History Strategies Overview

  • Tolerance tradeoffs (Grime 1979):

    • Factors considered:

    • Intensity of Stress (water limitation)

    • Intensity of Disturbance (grazing/fire)

    • Classifications:

    • Competitive

    • Ruderal

    • Stress Tolerant

Anthropogenic Influences on Diversity

  • Discussion about the relationship between biodiversity and anthropogenic disturbances in grasslands.

  • Mention of the findings surrounding bird richness at different land use intensities.

    • Observations of species loss correlating with land use intensification.

Niche vs Neutral Theory

  • Niche Theory: Suggests unique niches for species allowing coexistence through diverse resource utilization.

  • Neutral Theory: Considers that diversity can be maintained through stochastic events, with no fixed niche requirements.

Species Divergence Drivers

  • Factors include speciation not limited to niche partitioning; randomness plays a significant role.