Paine 1996 (Food Web Complexity and Species Diversity)

An observational, comparative, and manipulative study examining the ecological relationships between food webs and species diversity in Echinoderms and their prey

Background Research, Rationale, Objectives

Why study? Species diversity is a fundamental aspect of ecological stability, influencing the resilience and functioning of ecosystems. Understanding the mechanisms that regulate biodiversity is critical for ecological theory used in future studies and conservation efforts. Predation’s precise role in maintaining species diversity remains underexplored. Rocky intertidal ecosystems provide an ideal setting to investigate these processes due to their well-defined trophic interactions and accessibility for field studies. This study examines how food web complexity, particularly the role of predation, influences local species diversity.

Predation’s Role on Diversity and Resource Monopolization

• Competitive dominance by a single species can lead to resource monopolization, reducing biodiversity.

• Predators can disrupt this monopolization, allowing multiple species to share key resources BY PREVENTING competitive exclusion, which increases habitat availability

• Prior research suggests that higher predation intensity correlates with greater species diversity, especially in marine ecosystems.

  • Pisaster ochraceus (a keystone predator) is known to regulate barnacle and mussel populations

Role of Resource Monopolization in Diversity Loss

• Physical space is a limiting resource in many ecosystems, especially in intertidal zones where organisms compete for attachment surfaces.

• Barnacles and mussels can dominate space, preventing colonization by other species, leading to simpler, less diverse communities.

Hypothesis

Local species diversity is directly influenced by the efficiency of predators in preventing resource monopolization. When effective predators are present, species diversity remains high. When predators are removed or absent, dominant species monopolize resources, leading to lower diversity.

High predator abundance = high species diversity

Low predator abundance = species monopolize = lower diversity

Objectives

1. Examine how predation regulates species diversity in rocky intertidal food webs.

2. Determine if predator removal leads to competitive exclusion and reduced biodiversity.

   “How does the removal of top predators alter community structure?“

3. Compare food web complexity across different geographic regions and assess its impact on local diversity

Methodology

Study Approach

1. Construct food webs to analyze species interactions

2. Compare diversity across different geographic regions to assess the role of predation

3. Conduct a predator removal experiment to evaluate how predation influences community structure

Study Sites

Research conducted in three rocky intertidal zones along a latitudinal gradient:

• Mukkaw Bay, Washington (49°N, temperate zone)

  • Relatively simple food web

  • Dominant predator: Pisaster ochraceus (a keystone starfish).

  • Main prey: Mussels (Mytilus californianus), barnacles (Balanus glandula), chitons, and limpets.

  • Predator Removal Experiment (Conducted at Mukkaw Bay, Washington)

    • Site: A section of shoreline 8m × 2m

    • All Pisaster ochraceus individuals were manually removed.

    • A control area maintained with an undisturbed population of Pisaster ochraceus.

    • Species composition and diversity were monitored at both sites over time.

    • Changes in barnacle and mussel populations were measured to assess the impact of predator removal.

• Northern Gulf of California, Mexico (31°N, subtropical zone)

  • Complex food web with additional predators and prey.

  • Dominant predator: Heliaster kubiniji (a starfish).

  • Additional predatory gastropods: Muricanthus nigritus, Hexaplex spp.

  • Main prey: Herbivorous gastropods, bivalves, barnacles.

• Golfo de Nicoya, Costa Rica (10°N, tropical zone)

  • Simpler food web with no major top predator.

  • Main predators: Small gastropods (Thais biserialis and Acanthina brevidentata).

  • Main prey: Barnacles and mussels

Food Web Analysis

• Predator-prey interactions were observed and recorded.

• Prey species were identified and classified by trophic levels.

• Food web diagrams were constructed to visualize energy flow and determine energy contribution by converting prey biomass into caloric values.

Data Collection and Analysis

• Species richness and diversity were estimated by counting species in each food web.

• Predator diet composition was analyzed through direct observations.

• Comparisons were made across regions to assess how predator presence/absence influenced diversity.

• Changes in species abundance over time were recorded to measure the effects of predator removal.

Results

1. Temperate Region: Mukkaw Bay, Washington (49°N)

   • The top predator (Pisaster ochraceus) preyed upon dominant space-occupying species (like barnacles and mussels).

   • Relatively simple food web, with low trophic complexity but high species diversity

   • Predator-to-prey ratio

     • 0.18 (proportion of carnivores in the community).

   • Effects of Predator Removal (Mukkaw Bay Experiment)

     • Before removal:

       • The intertidal zone had 15 species, including barnacles, mussels, chitons, limpets, anemones, algae, and sponges.

       • Pisaster ochraceus fed primarily on barnacles and mussels

     • After predator removal:

       • Species richness declined to 8 species.

       • Barnacle (Balanus glandula) populations increased, occupying 60-80% of available space.

       • Mussels (Mytilus californianus) outcompeted other species, further reducing diversity.

       • Chitons, limpets, anemones, and benthic algae disappeared due to a lack of available space.

       • Ecosystem became trophically simpler, with fewer species and fewer functional groups.

2. Subtropical Region: Northern Gulf of California, Mexico (31°N)

   • The top predator (Heliaster kubiniji) fed on barnacles and other prey, preventing competitive exclusion.

   • A more complex food web was observed, with multiple predatory species (Heliaster kubiniji, Muricanthus nigritus, Hexaplex spp.).

   • Predator-to-prey ratio

     • 0.24, indicating a higher proportion of predators compared to Mukkaw Bay.

3. Tropical Region: Golfo de Nicoya, Costa Rica (10°N)

   • No strong top predator was present.

   • The food web was simpler, dominated by barnacles and mussels, which monopolized space

   • Lower species diversity was observed compared to the other sites.

   • The absence of a dominant predator allowed competitive exclusion to occur.

Key Findings

1. Predation prevents competitive exclusion

   • In the presence of a top predator (Pisaster ochraceus or Heliaster kubiniji), no single species was able to monopolize resources, allowing for a diverse community.

   • In sites where a strong predator was absent or removed, barnacles and mussels dominated the habitat, reducing species diversity.

2. Higher trophic complexity is associated with greater species diversity

   • The subtropical food web (Northern Gulf of California), which contained multiple predator species, supported higher species diversity than the simpler temperate and tropical food webs.

     • Predator-to-prey ratio was highest in the subtropical region

3. Predator removal leads to ecosystem simplification

   • When Pisaster ochraceus was removed from Mukkaw Bay, species richness dropped from 15 to 8 species, and the system became dominated by mussels and barnacles.

     • Algae, chitons, limpets, and anemones disappeared due to space monopolization by dominant species.

4. Tropical food webs without strong predators have lower diversity

   • The Costa Rica intertidal site lacked a major top predator, leading to lower species diversity compared to other sites.

   • Without a dominant predator, competitive exclusion occurred, reducing species richness.

Big Picture:

• Local species diversity is directly related to the presence and efficiency of predators in an ecosystem.

  • Where predators are absent or removed, ecosystems become simplified and dominated by a few competitive species.

• Predators prevent competitive exclusion by dominant species, maintaining higher species richness.

• Ecosystems with strong predation pressure support more complex food webs and greater biodiversity.

Discussion

Alternative Factors Influencing Diversity

1. Climate Stability vs. Diversity:

   • Tropical environments are often assumed to support greater species richness due to climatic stability.

   • However, Costa Rica (tropical site) had the lowest diversity, showing that predation, not just climate, determines local diversity.

2. Environmental Heterogeneity:

   • Some studies suggest that more complex physical environments promote diversity by providing more niches.

   • However, topographical differences between study sites did not strongly correlate with diversity patterns, further emphasizing the role of predation.

3. Primary Productivity and Energy Flow:

   • Higher primary productivity should support more trophic levels.

   • However, nutrient availability alone cannot explain species diversity—predation determines how resources are distributed among species.

Implications for Ecological Theory

1. Predation is a primary force shaping ecological communities

   • Supports Hutchinson’s (1959) idea that biological interactions influence diversity

   • Challenged the current assumptions that climate alone dictates biodiversity patterns

2. Keystone predators regulate ecosystem complexity.

   • The concept of keystone species (later developed by Paine himself in 1969) emerges from this study.

   • Removing a single predator drastically alters community structure.

3. Higher predator-to-prey ratios correlate with greater diversity.

   • Diverse systems (e.g., Northern Gulf of California) contain more predators, leading to higher species richness.

     • Tropical regions with weak predation (Costa Rica) show lower diversity despite stable environmental conditions.

Paper Analysis

Strengths

• Introduction of Keystone Predation: The study provided the first empirical evidence that a single predator (Pisaster ochraceus) can regulate entire ecosystems, laying the foundation for the keystone species concept (Paine, 1969)

  • Challenging Ecological Assumptions: The findings contradicted succession theory (Margalef, 1958), showing that ecosystems tend toward simplicity, not complexity, when predators are removed.

• Food Web Complexity & Predator-to-Prey Ratio: The study introduced the predator-to-prey ratio as a measure of food web complexity, showing that ecosystems with more predators support higher species richness.

• Multi-faceted Scientific Approach: Paine conducted a predator removal experiment, demonstrating a direct link between predation and species diversity. He also included comparisons and observational data

• Comparative Analysis Across Ecosystems: By examining temperate, subtropical, and tropical intertidal zones, Paine highlighted how predation intensity correlates with biodiversity, challenging the assumption that tropical regions are always more diverse.

Limitations

❌ Lack of Quantitative Diversity Metrics: Paine used species richness (number of species) instead of formal indices (e.g., Shannon-Weaver Index), which overlooks species evenness and abundance.

❌ Limited Geographic Scope: Only three locations were studied, which may not fully capture global patterns.

❌ Environmental Factors Not Fully Explored: Climate, habitat heterogeneity, and productivity were mentioned but not directly tested as drivers of diversity.

❌ No Analysis of Predator Behavior: The study assumes predators act uniformly but does not explore feeding preferences or interaction strengths within the food web.

Future Research Directions

📊 Quantitative Food Web Analysis – Using network models, energy flow metrics, and biodiversity indices for deeper insights into trophic complexity.

🐺 Expanding Keystone Species Research – Investigating keystone predators in terrestrial and marine systems (e.g., wolves, sharks, large herbivores).

🌍 Climate Change & Predation – Studying how climate variability affects predator-prey dynamics and biodiversity.

🦀 Predator Functional Diversity – Exploring how different predator feeding strategies influence species richness and community structure.