In coral reef ecosystems, herbivorous fishes are critical in maintaining reef health by controlling algal growth, cycling nutrients, and supporting overall biodiversity. While herbivorous fish species share broad similarities, they fulfill different ecological functions based on their diet and feeding strategies. Generalist species have broad diets, allowing them to consume various foods, which makes them more resilient to environmental changes and human impacts, such as pollution and habitat degradation. In contrast, specialist fish rely on specific food types or feeding strategies and often perform unique ecological tasks, such as removing certain algae or sediment, contributing to the reef’s health.
Functional homogenization occurs when generalist species become more abundant, while specialists decline or disappear. This shift reduces ecosystem resilience and functionality, as the loss of specialists weakens the ecosystem’s ability to respond to further stress, potentially worsening coral reef degradation.
This study investigated the relationship between functional homogenization in herbivorous fish assemblages and human impacts across over 3,000 Pacific coral reef sites. Researchers used an index of fish diet specialization, a functional trait matrix, fish abundance data, and the NCEAS Human Impact Score, which represents cumulative human stressors beyond fishing alone. This score encapsulates influences from urban development, pollution, and habitat alteration, creating a holistic measure of human impacts.
Findings showed distinct herbivore assemblage patterns across islands, with regional differences occasionally masking the direct impacts of human activity. However, when these differences were accounted for, key trends emerged: in areas with greater human influence, such as Oʻahu, Kauaʻi, Maui, and Guam, generalist species like Acanthurus nigrofuscus were more dominant, while overall herbivore diversity was lower. This reduction in diversity, a sign of biotic homogenization, was associated with the loss of dietary specialists, which are more abundant on healthier, less degraded reefs (e.g., islands within the Papahānaumokuākea Marine National Monument.)
Human impacts also influence herbivore specialization indirectly through habitat complexity and benthic cover. Significant correlations between these factors and functional homogenization suggest that habitat structure is a strong predictor of herbivore specialization. Although direct links between human impact scores and specialization were subtler, the overall trend supports that human influences lead to functional homogenization across islands.
This study highlights that diverse human activities—not just fishing—affect herbivore diversity and specialization. These findings underscore the need to address multiple human stressors to sustain reef resilience and biodiversity, with important implications for reef management under escalating environmental pressures.
Implications for managers
- Focus on conserving herbivore diversity, emphasizing the protection of specialized species to maintain functional diversity within the ecosystem.
- Continuously monitor and manage human stressors, including pollution, overfishing, and habitat degradation. Understanding the specific impacts of these stressors on herbivorous fish communities will enable targeted interventions to mitigate their effects.
- Even in areas with low human impact, consider site characteristics (e.g., wave action, reef area, current and historical land use) that may influence reef fish diversity.
- Use comprehensive measures to assess the resilience of reef fishes to human impacts, incorporating both ecological and genetic diversity.
Author: Nalley, E.M., A. Heenan, R.J. Toonen and M.J. Donahue
Year: 2024
Ecological Indicators 2024; 162:111622. doi: 10.1016/j.ecolind.2024.111622
