Species Diversity in Communities

Community Membership

  • Central Questions:
      - How do collections of species come together to form communities?
      - What factors determine species inclusion or exclusion in communities?

Filters Determining Community Membership

  • Factors include:
      1. Species Supply:
         - Regional species pools
         - Species’ dispersal abilities
      2. Environmental Conditions (Abiotic Filter):
         - Species’ environmental tolerances
      3. Species Interactions (Biotic Filter):
         - Positive and negative interactions affecting coexistence.

Resource Partitioning

  • Defined as niche partitioning:
      - Species diverge in their use of limiting resources to reduce competition, thus allowing coexistence.
      - Example given: Species evolving to utilize resources in distinct but complementary ways.

Impact of Resource Spectrum on Diversity

  • Higher species richness when:
      - High resource partitioning leads to more species being “packed” into the community.
      - A broad resource spectrum can support a greater array of species.

Robert MacArthur’s Study of Warblers

  • Studied 5 species of warblers in Maine and Vermont.
      - Documented feeding habits, nesting sites, and territories.
      - Resulted in findings that different species used varying parts of tree canopies, demonstrating resource partitioning.

Foliage Height Diversity and Bird Species Diversity

  • Empirical data on how foliage height diversity correlates with bird species diversity:
      - Graphical representation shows positive correlation, with higher foliage height supporting more bird species.

Resource Mediation and Species Diversity

  • Disturbance and stress can affect resource availability, influencing species diversity.

  • Discussion Prompt: How does disturbance impact diversity?

  • Example discussed of predator interactions and disturbance influence.

Evidence from Paine’s Experiment

  • Analyzed the impact of removing sea stars on local species diversity.
      - Observed shifts in species abundance after predator removal.

  • Importance of predation as a disturbance source impacting community composition.

Intermediate Disturbance Hypothesis

  • Concept that species diversity is maximized at intermediate levels of disturbance.
      - Evidence presented from Sousa’s study on rocky intertidal ecosystems:
        - Infrequently disturbed areas had low diversity (primarily late successional species).
        - Intermediately disturbed areas had the highest diversity (mix of early, mid, and late successional species).
        - Frequently disturbed areas showed reduced diversity with dominance from early colonists.

Herbivory as a Disturbance Factor

  • Jane Lubchenco’s research on herbivorous snails impacting algal species richness in the rocky intertidal as a non-physical disturbance source.

Consequences of Diversity

  • Community Diversity Relationships:
      - Positive correlation between community diversity (species richness, etc.) and community functions, foundational to ecosystem services.

  • Community functions affect:
      - Primary production, resistance to disturbances, etc.

  • Societal relevance due to concerns over species loss and its implications for ecosystem services.

Tilman’s Experimental Studies on Drought Resistance

  • Conducted on abandoned agricultural land to explore plant species richness effects on community function.

  • Notable findings regarding species richness and drought resistance:
      1. Curvilinear relationship observed: greater richness (up to ~10-12 species) increased resistance, beyond which redundancy occurred.
      2. Follow-up experiments confirmed these findings, showing more efficient nitrogen use with increased richness.

Disagreements Among Ecologists on Diversity-Function Relationships

  • Alternative hypotheses:
      1. Complementary Hypothesis: Each species contributes uniquely and equally to community function; predicts linear increases in function with richness.
      2. Redundancy Hypothesis: Some species have overlapping effects; once a threshold of richness is reached, additions do not increase function.
      3. Idiosyncratic Hypothesis: Different species have varying individual effects; dominant species exert more substantial influence on function.

Visual Representation of Hypotheses

  • Select graphs illustrating relationships based on the three hypotheses discussed regarding species richness and community function.

Zoonotic Diseases

  • Definition and Relevance: Diseases transmitted from animals to humans.

  • Connections drawn between biodiversity loss and increased emergence of zoonotic diseases due to habitat fragmentation, deforestation, and urbanization.

  • Case Study: Hantavirus linked to increased infection prevalence as small mammal community diversity declines:
      - Observed infection rates rising from 2% to 14% in field studies as diversity decreased.

Mechanisms by which Diversity Reduces Disease Transmission

  • Hypotheses Discussed:
      1. Loss of competing/predatory species contributes to increased host population densities and enhanced disease spread.
      2. More diverse communities facilitate less transmission likelihood among infected hosts and non-host species.
      3. Species composition influences susceptibility to zoonotic diseases, necessitating protection of biodiverse communities to suppress disease transmission.

Conclusion

  • The interconnectedness of species diversity, community function, and human health underscores the importance of preserving ecological diversity to support ecosystem resilience and mitigate disease risks.

Questions and Further Discussion Prompts

  • Reflection on the concepts of community membership, resource partitioning, and diversity’s implications for ecology and health.