Herbivore vs. nutrient control of marine primary producers: context-dependent effects

Area of Science:

• Marine ecology
• Ecosystem dynamics
• Food web interactions

Background:

• Human activities like overharvesting consumers and nutrient loading disrupt ecosystem forces.
• Understanding top-down (consumer) and bottom-up (nutrient) forces is crucial for ecosystem structure.

Purpose of the Study:

• To quantify the effects of herbivory and nutrient enrichment on primary producers.
• To determine the relative and synergistic roles of these forces across different habitats and producer types.

Main Methods:

• Factorial meta-analysis of 54 field experiments.
• Orthogonal manipulation of herbivore pressure and nutrient loading.
• Focus on benthic marine habitats and primary producers.

Main Results:

• Both herbivory and nutrient enrichment significantly affected primary producer abundance.
• Nutrient enrichment effects were greater in the absence of herbivores.
• Herbivores had stronger effects than nutrients in tropical macroalgae and seagrasses.
• Effects varied by latitude, producer type, ecosystem productivity, and algal functional groups.

Conclusions:

• Human alterations to food webs and nutrient availability significantly impact primary producers.
• The loss of herbivores can exacerbate eutrophication effects, particularly in tropical reef systems.
• Ecosystem productivity and producer type mediate the relative importance of top-down and bottom-up forces.