Detailed Study Notes on Plant Growth Evolution Post Herbivore Removal

Abstract

  • Objectives (Purpose of Study):

    • Quantified differences in individual plant growth rate, tolerance to herbivory, competitive ability, and secondary metabolites.

Experimental Design (Purpose of Study & Methodology)

  • Purpose: Investigate the evolutionary changes in plant traits due to the presence or absence of rabbit herbivores.

  • Questions addressed:

    1. Does herbivore removal stimulate directional evolutionary changes in growth, defenses, and tolerance?

    2. Do evolved traits affect interspecific ecological interactions between plant competitors?

  • Expected results (Hypothesis):

    • Defensive traits to decrease post-herbivore removal due to presumed costs; growth rates to vary inversely with herbivore presence.

    • Competitive ability expected to increase with herbivore exclusion.

Methods (Methodology)Study System and Exclosures

  • Conducted at Silwood Park, England (51.409°N, 0.642°W).

  • Introduced 22 rabbit exclosures from 1984-2009, allowing for varying herbivore exposure (4 months to 26 years).

  • Collected seeds from Rumex acetosa for comparative analysis; defined populations based on exclosure origin while accounting for limited gene flow due to vegetative reproduction.

Greenhouse Experiment

  • Seeds germinated in a common environment to control for maternal effects.

  • 24 plants grown from each seed source; measured growth responses under varying conditions.

Trait Assessments

  • Tolerance and Competitive Ability:

    • Damage manipulated to simulate grazing; competition tested against Oenothera biennis.

    • Tolerance calculated as:
      Tolerance=(damaged biomass−undamaged biomass)undamaged biomass\text{Tolerance} = \frac{(\text{damaged biomass} - \text{undamaged biomass})}{\text{undamaged biomass}}.

  • Chemical Defenses:

    • Analyzed concentrations of oxalates and tannins in plant tissues to assess defensive compound levels.

    • Oxalate Measurement: Processed dried tissue in heated water to extract soluble oxalates and measured using absorbance.

    • Tannin Measurement: Utilized radial diffusion assays to quantify soluble protein-binding tannins.

Statistical Analysis

  • Regression analyses assessed the relationship between traits and exclosure age.

  • Evolutionary change rates computed using Haldanes and Darwins metrics for standardization over time.

Results (Main Findings)

Growth Rate

  • Result: Significant decline in growth rate correlated with exclosure age: 30% reduction in growth rate over 26 years, consistently across various damage and competition treatments indicating a clear evolution in plant growth response.

  • Result: Highest observed evolutionary rate at 0.080.08 Haldanes.

Chemical Defenses and Tolerance

  • Result: No significant evolutionary changes found in chemical defenses or tolerance despite variations in growth rate.

  • Result: Negative evolutionary rate observed in both tannin and oxalate concentrations, underscoring static chemical response in mutual influence of herbivore presence, though not statistically significant.

Competitive Ability

  • Result: Competitive ability among Rumex acetosa showed no directional change attesting to negligible impact of evolutionary changes on interspecific interactions with competitors.