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Baselines and degradation of coral reefs in the Northern Line Islands - PubMed

  • ️Tue Jan 01 2008

Baselines and degradation of coral reefs in the Northern Line Islands

Stuart A Sandin et al. PLoS One. 2008.

Abstract

Effective conservation requires rigorous baselines of pristine conditions to assess the impacts of human activities and to evaluate the efficacy of management. Most coral reefs are moderately to severely degraded by local human activities such as fishing and pollution as well as global change, hence it is difficult to separate local from global effects. To this end, we surveyed coral reefs on uninhabited atolls in the northern Line Islands to provide a baseline of reef community structure, and on increasingly populated atolls to document changes associated with human activities. We found that top predators and reef-building organisms dominated unpopulated Kingman and Palmyra, while small planktivorous fishes and fleshy algae dominated the populated atolls of Tabuaeran and Kiritimati. Sharks and other top predators overwhelmed the fish assemblages on Kingman and Palmyra so that the biomass pyramid was inverted (top-heavy). In contrast, the biomass pyramid at Tabuaeran and Kiritimati exhibited the typical bottom-heavy pattern. Reefs without people exhibited less coral disease and greater coral recruitment relative to more inhabited reefs. Thus, protection from overfishing and pollution appears to increase the resilience of reef ecosystems to the effects of global warming.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Location of study sites at Kingman, Palmyra, Tabuaeran and Kiritimati atolls in the Line Islands.

Sites were located at semi-exposed fore reef habitats between 10 and 12 m in depth, and approximately one km apart from each other. Reef fishes were surveyed at all sites (n = 25 sites per atoll), and benthic communities at a subset of sites (n = 10–12 sites per atoll).

Figure 2
Figure 2. General aspect of fore reef habitats (left column) and representative 0.5-m2 photos of the bottom (right column) at Kingman (A–B), Palmyra (C–D), Tabuaeran (E–F), and Kiritimati (G–H), showing the degradation from a reef dominated by top predators and corals (Kingman) to a reef dominated by small planktivorous fishes and algae.

Photo credits: A by Zafer Kizilkaya, B–H by Jennifer Smith.

Figure 3
Figure 3. Fish biomass (A) and abundance (B), and cover of major benthic functional groups (C) across the northern Line Islands.

Note that human impacts increase from left to right in this and subsequent figures. CCA = crustose coralline algae.

Figure 4
Figure 4. Size frequency distribution of corals (A), and coral disease prevalence and recruit density (B) across the northern Line Islands.

Because recession in adult corals can often lead to fragmentation and the development of many small colonies that may have all been part of one larger colony, recruits are defined here as colonies less than 5 cm in diameter from coral taxa demonstrating essentially unidirectional growth (i.e., Acropora, Pocillopora and Fungiids), and thus represent individuals known to be young relative to larger individuals. Note that there were no colonies larger than 160 cm at Kiritimati.

Figure 5
Figure 5. Principal component analysis (PCA) of major fish and benthic groups from all sites with photoquadrat data (n = 10 per atoll, except Kingman with n = 9).

Atolls cluster in sequence along the first principal component axis (PC1), shifting from left to right from Kingman to Kiritimati. The major loadings on PC1 include biomass of top predatory fish, coral and CCA cover (to left, i.e., decreasing disturbance) and turf algal cover (to right, i.e., increasing disturbance). Note that herbivore biomass, the putative intermediate in models of simple trophic cascades, is orthogonal to the dominant axis correlated with apex predators.

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