Reef fish responses to climate change
Understanding how fish communities in the Lakshadweep cope with change
Tropical reefs have an uncertain future, and every coral mass mortality event further tests the resilience of these systems. While coral communities in many reefs in the Lakshadweep have shown a remarkable ability to withstand and recover from these events, how are its fish populations faring?
Fish play key roles in reef resilience
The role of fish communities in mediating reef resilience has long being recognized. Top predators are often ecosystem keystones and their removal can lead to a range of flow-on consequences as the complex network of interactions they maintain unravel. Their loss has been associated with declines of coral and the inability of reefs to recover from occasional disturbances. Additionally, herbivore fish communities are also critical, playing much more direct roles in controlling algal growth in the aftermath of major bleaching events. We have been tracking how fish communities respond to catastrophic coral die-offs in the Lakshadweep since 1998. After the mass bleaching event of 1998, the worst affected reefs saw a major loss of fish species including top predators and coral feeding specialists. In contrast, herbivore fish quickly dominated the community helping maintain reefs relatively free of algae and facilitating a rapid recovery of coral in many reefs. Critically, the reefs of the Lakshadweep, were, until recently not heavily fished, a significant component of the native resilience of this archipelago.
Top predators need stable regimes
We have been tracking benthic trends in the Lakshadweep since 1998 and a clear geographical pattern of recovery is emerging, linked to local hydrodynamics. While some reefs show very rapid declines in the wake of bleaching events, they have the ability to recover their structure very rapidly. The benthic structure at more protected and deeper locations in contrast does not change much through time. After the 2010 bleaching event, we conducted an archipelago-wide survey to explore how benthic predators like groupers responded to this mortality. Our studies are showing that grouper numbers and biomass were highest only on the most stable reefs across the island group. At more structurally dynamic reefs, groupers could apparently not exploit the increasing benthic structural complexity that is so vital to them. This pattern is particularly pronounced for larger, longer-lived species of grouper. What this means is that the ability of coral to recover quickly on reefs may not necessarily augur a return of ecological function. Keystone predators like groupers, that are long lived and may be highly territorial, require stable, unchanging reefs to thrive. These stable locations become particularly important as these reef systems are subject to increasingly frequent coral mass mortality events.
Our studies are now exploring the mechanisms that drive grouper distributions across these stability regimes.
- Journal Article2017Coping with catastrophe: foraging plasticity enables a benthic predator to survive in rapidly degrading coral reefsAnimal Behaviour, Vol 131: 13-22. http://dx.doi.org/10.1016/j.anbehav.2017.07.010Download
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Human-induced rapid environmental change (HIREC) disproportionately affects species with specialist traits and long generation times. By circumventing prolonged evolutionary processes, behavioural plasticity is critical in allowing species to cope with rapid environmental changes within their lifetimes. Coral reefs have faced multiple mass mortality events of corals related to climate change in the last two decades. The consequent loss of structural complexity adversely impacts long-lived, structure-dependent fish predators. We attempted to determine how well a guild of groupers (Pisces: Epinephelidae) copes with rapid structural change in the lightly fished Lakshadweep Archipelago, Indian Ocean. Of the 15 species, territorial and site-attached groupers declined exponentially with decreasing structural complexity, while widely ranging species showed no change. However, one site-attached species, the peacock grouper, Cephalopholis argus, maintained high densities across the structural gradient. We explored the mechanisms this species employs to cope with declining habitat structure. Our observations indicate that both a potential release from specialist competitors and plasticity in foraging behaviour (foraging territory size, diet and foraging mode) appeared to favour the peacock grouper's survival in sites of high and low structure. While specialist competitors dropped out of the assemblage, the foraging territory size of peacock groupers increased exponentially with structural decline, but remained constant and compact (50 m2) above a threshold of structural complexity (corresponding to a canopy height of 60 cm). Interestingly, despite significant differences in prey density in sites of high and low structure, gut content and stable isotope analyses indicated that peacock groupers maintained a specialized dietary niche. In-water behavioural observations suggested that diet specialization was maintained by switching foraging modes from a structure-dependent ‘ambush’ to a structure-independent ‘widely foraging’ mode. The remarkable foraging plasticity of species such as the peacock grouper will become increasingly critical in separating winners from losers and may help preserve specialist ecosystem functions as habitats collapse as a result of climate change.
- Dataset2016Long-lived groupers require structurally stable reefs in the face of repeated climate change disturbances.Karkarey R, Kelkar N, Lobo AS, Alcoverro T, Arthur R (2014) Data from: Long-lived groupers require structurally stable reefs in the face of repeated climate change disturbances. Dryad Digital Repository. http://dx.doi.org/10.5061/dryad.d7j02
- Popular Article2016Living with change: local responses to global impactsCurrent Conservation, issue 10.2 http://www.currentconservation.org/?q=issue/10.2
- Journal Article2016Homeward bound: fish larvae use dispersal corridors when settling on coral reefsNatural History Notes: Frontiers in Ecology and the EnvironmentDownload
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- Journal Article2014Long-lived benthic predators require structurally stable reefs in the face of repeated climate-change disturbancesCoral Reefs. 33: 289-302
Benthic recovery from climate-related disturbances does not always warrant a commensurate functional recovery for reef-associated fish communities. Here, we examine the distribution of benthic groupers (family Serranidae) in coral reef communities from the Lakshadweep archipelago (Arabian Sea) in response to structural complexity and long-term habitat stability. These coral reefs that have been subject to two major El Nin ̃o Southern Oscillation-related coral bleaching events in the last decades (1998 and 2010). First, we employ a long-term (12-yr) benthic- monitoring dataset to track habitat structural stability at twelve reef sites in the archipelago. Structural stability of reefs was strongly driven by exposure to monsoon storms and depth, which made deeper and more sheltered reefs on the eastern aspect more stable than the more exposed (western) and shallower reefs. We surveyed groupers (species richness, abundance, biomass) in 60 sites across the entire archipelago, representing both exposures and depths. Sites were selected along a gradient of structural complexity from very low to high. Grouper biomass appeared to vary with habitat stability with significant differences between depth and exposure; sheltered deep reefs had a higher grouper biomass than either sheltered shallow or exposed (deep and shallow) reefs. Species richness and abundance showed similar (though not significant) trends. More interestingly, average grouper biomass increased exponentially with structural complexity, but only at the sheltered deep (high stability) sites, despite the availability of recovered structure at exposed deep and shallow sites (lower-stability sites). This trend was especially pronounced for long-lived groupers (life span [10 yrs). These results suggest that long-lived groupers may prefer temporally stable reefs, independent of the local availability of habitat structure. In reefs subject to repeated disturbances, the presence of structurally stable reefs may be critical as refuges for functionally important, long-lived species like groupers.