Lecture 22 - Community Ecology 14 - Biodiversity Drivers 01 11/10/2025
Introduction
The session's topic is drivers of biodiversity.
The class aims to discuss explanations for variations in biodiversity across space and time.
Focus areas include:- Competition
Predation
Introduction of two important theories:
Theory of Island Biogeography
Latitude and Gradient of Biodiversity
Recap of Previous Lessons
Importance of examining competition and predation as drivers of biodiversity.
"Drivers" explained as factors that impact the variation in biodiversity and areas with higher species diversity.
Grading Updates
Grades available on Brightspace and Gradescope.
An assignment for regrade is posted on Brightspace, with specific conditions outlined.
Due date for the assignment is next Monday.
A related paper must be read and questions submitted by next Monday.
Reminder of an upcoming exam in a week and a half, with an offer for questions regarding any assignments.
Patterns of Biodiversity
Overview of previously provided graphs regarding biodiversity.
Graph analysis indicating the relationship between the number of species and different metrics related to biodiversity.
These graphs often visually represent:
Species-Area Curves: Typically showing a positive, non-linear relationship (often a power-law, S=cAzS=cAz, where SS is species number, AA is area, and cc and zz are constants) indicating that larger areas generally host more species.
Species-Distance Relationships: Illustrating an inverse correlation where species richness tends to decrease as the distance from a source pool increases.
Gradients with Environmental Factors: Depicting how species diversity changes along gradients such as depth, altitude, and latitude.
Succession Stages
Days after colonization noted as part of the study of species patterns.
Analysis of succession stages to explain biodiversity recovery.
Graphs tracking succession often show an initial rapid increase in species richness following a disturbance or colonization event, followed by a plateau or even a slight decline as competitive exclusion or other limiting factors begin to take effect.
Area and Distance
Discussion concerning the relationship between species number and area size or distance.
Exploration into how depth, altitude, and latitude correlate with species diversity over time.
For instance, plots of species richness versus area commonly follow a logarithmic or power-law curve, mathematically expressed as S=cAzS=cAz. Similarly, the relationship with distance often shows a decay curve, demonstrating fewer species or lower immigration rates at greater distances.
Competition as a Driver of Biodiversity
Competition allows coexistence when resources can be partitioned effectively.
Example: Darwin's Finches illustrating character displacement where variation in seed size leads to diverse feeding strategies.
Figures here typically show frequency distributions of beak sizes for different finch species. In areas where two competing species coexist (sympatry), their beak size distributions are often more distinct (divergent) compared to areas where they live alone (allopatry), demonstrating the morphological shift due to competition for resources.
Discussion on limiting resources: Definition emphasizes resources critical for species survival. The number of available limiting resources is correlated with species richness.
Interspecific Competition
Intraspecific competition only allows diversity if multiple resource dimensions are exploited.
Spatial spread of available resources is as critical as dietary resources.
Example: Different types of anemones colonized by various fish species, emphasizing how body size and habitat impact resource utilization.
Predation as a Driver of Biodiversity
Overview of how predation affects community structure by controlling competitor populations.
Discussed examples include
Starfish controlling strong competitors leading to increased biodiversity.
Parasites affecting plant health and species dominance.
If the preferred prey is dominant, intermediate levels of predation can enhance biodiversity; conversely, without competitive imbalances, predation may not support biodiversity.
Theory of Island Biogeography
Detailed explanation of the theory published in 1967 by Robert MacArthur and Edward O. Wilson.
Discussed two observed patterns:
Larger area supports more species.
Proximity to source areas influences species richness: closer areas yield greater species diversity.
Equilibrium model introduced as the balance between immigration of new species and extinction of existing species.
Explanatory Mechanism
Mechanisms governing these patterns discussed in terms of immigration and extinction rates.
Relationship between area size and extinction rate: Larger areas have reduced extinction rates due to larger populations and increased survival probabilities.
Area and immigration dynamics illustrate how larger areas support more species through effective habitat presence.
The equilibrium model is often visualized with a graph where the X-axis represents the number of species on an island and the Y-axis represents the rate of immigration or extinction. The immigration rate curve typically slopes downwards (fewer new species arrive as most niches are filled), while the extinction rate curve slopes upwards (more species lead to more competition and thus higher extinction risk). The intersection of these two curves indicates the equilibrium number of species for that island. Variations in island size and distance from the mainland shift these curves (e.g., larger islands have lower extinction rates, closer islands have higher immigration rates), leading to different equilibrium points.
Further Discussion on Distance
Examined how distance impacts immigration rates; shorter distances equate to higher immigration rates, thereby increasing species richness.
Emphasized constant dynamics of species movement, colonization, and extinction over ecological time.
Important Questions Raised
Why does immigration rate decrease as species numbers increase?
As species occupy niches, the number of new immigrants decreases, impacting potential for new species colonization.
Conclusion of Island Biogeography Considerations
Importance of equilibrium dynamics between immigration and extinction modeled as a tool to explain local biodiversity.
Exploration of area and distance as critical factors in community ecology with potential indirect influences.
Latitude-Gradient Biodiversity
Shifts focus to global patterns of biodiversity with emphasis on areas closer to the equator having higher species richness.
Noted standard patterns across different taxa: e.g., bivalves, butterflies, mammals, and trees demonstrate this pattern.
Graphs illustrating this typically plot species richness (Y-axis) against latitude (X-axis, often from 0exto0exto at the equator to 90exto90exto at the poles in both hemispheres). These figures consistently show a peak in species diversity around the equator and a marked decline as latitude increases towards higher latitudes.
Proposed explanations include:
Energy availability (sunlight for photosynthesis)
Temperature effects on metabolic rates and ecological interactions.
Higher temperatures enhance species survival and reactions, driving biodiversity evolution.
Energy and Temperature Relations
Two primary components driving biodiversity gradients are identified: energy and temperature.
Energy increases productivity, correlating positively with biodiversity.
The role of temperature is less direct but facilitates higher metabolic rates and adaptive responses over time.
Emphasized that increased productivity alone doesn't guarantee biodiversity; a rich variety of resources is also necessary.
Empirical Evidence Overview
Summarized the importance of studies documenting these relationships, examining species richness, temperature, and productivity across geographic scales.
Correlations within ecological studies varying based on spatial scale, emphasizing the need for larger observational frameworks.