Neutral Theory and Niche Theory: Species Abundance and Ecological Models

Introduction

Focus on higher scale ecological processes.
System biology framework applies both to molecular and ecological scales.
Topics: neutral theory vs niche theory in ecology.
Relationship between molecular interactions and higher level ecological patterns.

Study on why some species are more common or rare.
Example of species count on Barro Colorado Island (BCI) in Panama:
- 21,457 trees counted.
- 225 distinct species identified.
- Data collection: canopy trees (> 10 cm DBH).

The main contribution: derivation of closed-form equation for species abundance pattern.
Mathematical model vs simulation for understanding species distribution.
Insights from neutral theory in contrast to niche theory.

Analysis of species abundance patterns using log scales.
Histogram plot interpretation:
- Most common number of individuals per species is 1.
- Abundance has a long tail; rare species are relatively common.
- Misinterpretation risk with transformed scales.
Example data from BCI:
- 9 species with 18 individuals, etc.
Large proportion of species with low individual counts; few species with high counts.
Rare species’ prevalence as a recurring ecological pattern.

Assumes species differences in terms of resource usage.
Broken stick model: Resource axis divided randomly; species abundance proportional to resource segment length.
- Fails to produce log-normal distribution due to lack of long gaps/short segments.
Hierarchy Models: Sequential breaking of resources, leading to log-normal distribution.
- Analogy to crushing stone; multiple levels of resource division.
- Can explain observed species distributions effectively.

Assumes all individuals and species are demographically identical (birth, death rates, etc.).
Core Concept: Metacommunity (large) vs an island (smaller, isolated).
- Migration rate dictates species migration to islands.
Dynamics: Moran-like process for population changes on the island.
- Draw individuals randomly; decide their fate probabilistically from the metacommunity or current island pool.
- Species extinction vs recolonization balance.
Comparison to niche models:
- Neutral model more parsimonious but not accounting for actual species differences.
- Predicts less informative patterns but can sufficiently explain many observed patterns.

Predicted distribution for neutral theory’s species abundance.
Comparison to Poisson distribution in mRNA models:
- Proportional birth/death rates vs constant transcription rates in molecular contexts.
Fisher log series used to describe species abundance in metacommunities.
- Yields faster fall-off than empirical island distributions due to migration effects.
Real-world justifications and assumptions: how closely these models describe specific ecologies (e.g., BCI’s location and history).

Challenges in fitting models to observed data:
- Both niche and neutral models can fit data well but with different parameterizations.
- Discussion around the number of parameters, model parsimony, and practical interpretability.
Niche models explain species-area relationships better.
Neutral models predict transient species dynamics effectively.

Highlight the importance of both model types in explaining biodiversity.
Reflect on practical implications of theoretical models in ecology.
Acknowledgment of the lecture’s educational value and encouragement to continually explore systems biology.