Herbivores as drivers of negative density dependence in tropical forest saplings

By: Forrister et al (2019)

Summary of the paper

The article by Forrister et al. (2019) explores how herbivores influence plant diversity in tropical forests by driving negative density dependence (NDD) in young tree saplings. NDD is a process where a plant’s growth and survival decrease as its local population density increases, often because of factors like resource competition or shared threats (e.g., herbivores and pathogens). This effect helps maintain high species diversity by preventing any one species from becoming too dominant.

The study focuses on Inga tree species in Panama, where researchers examined two primary mechanisms of NDD: resource competition and herbivore pressure. The team tested whether neighboring trees with similar traits related to resource use and defense were more susceptible to NDD. They found that saplings surrounded by others with similar chemical defenses against herbivores grew slower and had lower survival rates, likely because they shared herbivores. In contrast, similarity in resource acquisition traits did not significantly affect sapling performance.

This work supports the idea that herbivore pressure, rather than resource competition, primarily drives NDD among Ingasaplings. This herbivore-driven NDD reduces growth and survival rates, especially when saplings are close to others with similar chemical defenses, reinforcing species diversity in tropical forests.

Outline of the paper

I. Introduction

  • Context: Tropical forests have high biodiversity with numerous species coexisting.

  • Focus: Negative density dependence (NDD) helps maintain species diversity by decreasing growth and survival in denser populations, preventing species dominance.

  • Mechanisms: Two proposed drivers of NDD:

    1. Resource Competition: Plants compete for limited resources.

    2. Herbivore Pressure: Plants share herbivores and pathogens, which reduce growth and survival in high-density areas.

  • Objective: Determine if herbivore pressure or resource competition drives NDD in saplings, focusing on Inga tree species in Panama.

II. Study Design & Methodology

  • Study Site: Barro Colorado Island (BCI), Panama.

  • Focal Species: Nine species of Inga trees, which show high diversity and coexistence in small areas.

  • Data Source: Long-term forest dynamics plot data (tree locations, species, sizes).

A. Experimental Design

  • Trait Selection:

    • Resource Acquisition Traits: Wood density, leaf morphology, tree height, and leaf elemental composition.

    • Defense Traits: Traits affecting herbivore preference, such as leaf chemistry, timing of new leaf production, and trichome density.

  • Herbivore Identification: DNA barcoding to identify and catalog herbivores feeding on Inga saplings.

  • Statistical Analysis: Generalized linear models (GLMs) used to assess relationships between neighborhood trait similarities, herbivore communities, and focal tree growth/survival.

B. Hypotheses

  • If resource competition drives NDD: Growth and survival should decrease in areas where neighboring plants have similar resource acquisition traits.

  • If herbivore pressure drives NDD: Growth and survival should decrease in areas where neighboring plants have similar chemical defenses and share herbivore communities.

III. Results

A. Impact of Resource Acquisition Traits

  • Finding: No significant effect of resource acquisition trait similarity on growth or survival of Inga saplings.

B. Impact of Herbivore Pressure

  • Chemical Similarity:

    • Saplings growing near neighbors with similar chemical defenses had lower growth rates and survival.

    • Chemical similarity negatively affected survival more than growth.

  • Herbivore Community:

    • Saplings with similar neighboring species shared more herbivores.

    • Increased herbivore sharing correlated with decreased growth and survival.

    • Herbivore community similarity was overdispersed, meaning neighboring Inga species hosted distinct herbivore communities, likely to reduce shared herbivore loads.

C. Light Availability Control

  • Findings:

    • Tree growth increased with the presence of gap-specialist trees (light availability).

    • However, survival decreased in high-density areas of gap specialists, suggesting a trade-off between growth benefits and density-related stress.

IV. Conclusions

  • Primary Driver of NDD: Herbivore pressure, not resource competition, is the main mechanism driving NDD in Inga saplings.

  • Implications for Forest Diversity:

    • Herbivore-driven NDD promotes species diversity by decreasing growth and survival of chemically similar neighbors, limiting the success of high-density conspecific groups.

    • This supports the idea that natural enemies, particularly herbivores, play a critical role in maintaining tropical forest diversity.

V. Broader Implications

  • Ecological Significance: Understanding NDD mechanisms in tropical forests could inform conservation efforts by highlighting the importance of herbivore-plant interactions in maintaining biodiversity.

  • Future Research: Further studies on other plant and herbivore interactions across different forest sites would provide additional insight into the role of natural enemies in forest diversity.