Turf Wars

Understanding turtle-fisher conflicts in Lakshadweep seagrass meadows

Fishers in the seagrass meadows of the Lakshadweep associate a decline in fish catch with increases in green turtles over the last two decades. Our work has been exploring the ecological and economic drivers of this conflict in these lagoons 

Troubled Waters

Seagrass meadows form important habitats for lagoon fishers in the Lakshadweep Islands for regular small-scale, subsistence-level fishing. Fishers have, however, been reporting declines in fish catches from seagrasses over the last decade or so, and many fishers attribute this decline to an increase in green turtles numbers. Although easy to discount this as just another disaffected fisher tale, fishers in the Lakshadweep propose a series of very believable mechanisms linking turtles to fish decline which are much more difficult to dismiss offhand. Apart from turtles breaking nets and driving away fish by swimming, fishers we interviewed were adamant that the herbivorous turtle overgrazed seagrass, driving a decline in adult fish that used the meadow as well as reducing recruitment by many lagoon fish. In 2005, conflicts between local lagoon fishers of the Agatti Island and green turtles escalated, resulting in clandestine killing of turtles by angry fisher folk. 

Dead 20turtle

Conflict between turtles and fishers often results in turtles being killed

Problems of plenty

Over the past few years we have been trying to validate the complex ecological pathways hypothesized by local fishers, through both observational and experimental research on turtle herbivory itself. We first estimated the magnitude of turtle herbivory in the Lakshadweep Islands, and established that the current levels of herbivory do represent a significant overgrazing, and that the herbivory rates and turtle densities in the Lakshadweep meadows are indeed among the highest in the world. We used seagrass clipping experiments to show that turtles can cause population and biomass declines in seagrasses and explore the mechanisms by which turtle herbivory causes  species shifts in seagrass meadows from long-lived, hardy species to short-lived, sediment-tolerant seagrasses. In this process of changing meadow succession, turtles are also seriously affecting the capacity for seagrass recovery. 

Many paths to conflict

We have also been tracking fisher perceptions about the turtle problem on these islands, and set out to validate these hypotheses through studies on both turtle herbivory affecting seagrass structural complexity, and most importantly, the response of fish populations to this change across a gradient of differentially grazed meadows. Our results provide considerable support for the fact that the fishers, for the most part, appear to have gotten it right in ‘blaming’ the turtles for their declining catch. We also show, through a basic economic evaluation of the costs incurred by fishers due to turtle conflict, that indirect and unobvious pathways of conflict might be far more serious than direct mechanisms such as breakage of nets, which, despite having smaller costs, are generally the focal point of conflict mitigation measures.  

People

Funding

Publications

  • Journal Article
    2014
    Seagrasses in the age of sea turtle conservation and shark overfishing
    Heithaus, M.R., Teresa Alcoverro, Rohan Arthur, Burkholder, D.A, Coates, K.A, Christianen, M.J.A., Nachiket Kelkar, Manuel, S.A, Wirsing, A.J., Kenworthy, W.J., Fourqurean, J.W.
    Frontiers in Marine Science 1:28. doi: 10.3389/fmars. 2014.00028.
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    PDF, 1.95 MB

    Efforts to conserve globally declining herbivorous green sea turtles have resulted in promising growth of some populations. These trends could significantly impact critical ecosystem services provided by seagrass meadows on which turtles feed. Expanding turtle populations could improve seagrass ecosystem health by removing seagrass biomass and preventing of the formation of sediment anoxia. However, overfishing of large sharks, the primary green turtle predators, could facilitate turtle populations growing beyond historical sizes and trigger detrimental ecosystem impacts mirroring those on land when top predators were extirpated. Experimental data from multiple ocean basins suggest that increasing turtle populations can negatively impact seagrasses, including triggering virtual ecosystem collapse. Impacts of large turtle populations on seagrasses are reduced in the presence of intact shark populations. Healthy populations of sharks and turtles, therefore, are likely vital to restoring or maintaining seagrass ecosystem structure, function, and their value in supporting fisheries and as a carbon sink.

  • Journal Article
    2013
    Greener pastures? High-density feeding aggregations of green turtles precipitate species shifts in seagrass meadows
    Journal of Ecology. 101: 1158-1168

    1. Historical declines of marine megaherbivores have led to a view of seagrass communities structured largely by abiotic disturbance and plant competition. There is, however, growing recognition of the significance of top-down control through herbivory, on seagrass ecosystem processes, raising the question of how meadows functioned under historically high populations of megaherbivores. 2. We assess the impacts of such intense herbivory on seagrass meadow composition in the Lakshadweep islands (India), where high-density feeding aggregations of green turtles have persisted for over a decade. We use a series of complementary approaches: (i) natural herbivory exclosures (ii) published data on seagrass composition before and after turtles established (at one atoll: Agatti) and (iii) present species composition along a turtle herbivory gradient over multiple atolls.  

    ...to read more, download the full paper

  • Journal Article
    2013
    Green turtle herbivory dominates the fate of seagrass primary production in the Lakshadweep islands (Indian Ocean)
    Marine Ecology Progress Series. 485:235-243

    Historical global declines of megaherbivores from marine ecosystems have hitherto contributed to an understanding of seagrass meadow production dominated by detrital path- ways — a paradigm increasingly being questioned by recent re-evaluations of the importance of herbivory. Recoveries in green turtle populations at some locations provide an ideal opportunity to examine effects of high megaherbivore densities on the fate of seagrass production. We conducted direct field measurements of aboveground herbivory and shoot elongation rates in 9 seagrass meadows across 3 atolls in the Lakshadweep Archipelago (India) representing a gradient of green turtle densities. Across all meadows, green turtles consumed an average of 60% of the total leaf growth. As expected, herbivory rates were positively related to turtle density and ranged from being almost absent in meadows with few turtles, to potentially overgrazed meadows (ca. 170% of leaf growth) where turtles were abundant. Turtle herbivory also substantially reduced shoot elongation rates. Simulated grazing through clipping experiments confirmed this trend: growth rates rapidly declined to almost half in clipped plots relative to control plots. At green turtle den- sities similar to historical estimates, herbivory not only dominated the fate of seagrass primary pro- duction but also drastically reduced production rates in grazed meadows. Intensive turtle grazing and associated movement could also modify rates of detrital cycling, leaf export and local carbon burial, with important consequences for the entire seascape.

  • Journal Article
    2013
    Complex ecological pathways underlie perceptions of conflict between green turtles and fishers in the Lakshadweep Islands.
    Biological Conservation 167: 25-34

    Managing human–wildlife conflict is often complicated by apparent mismatches between community perceptions and measures of directly incurred losses. Fishers in Agatti Island (Lakshadweep, India) associate recent increases in green turtle (Chelonia mydas) populations with declining fish catches, resulting in targeted killing of turtles. We compared fisher perceptions in Agatti with a very similar atoll, Kadmat, with much lower turtle densities. Nearly 90% of Agatti fishers interviewed blamed turtles for declining catch compared with 20% in Kadmat and proposed two mechanisms for this decline: direct interference (e.g., turtles damaged gear) which we define as first order conflict, and indirect mechanisms (second order conflict): turtles overgrazed seagrasses, thereby reducing fish catch. We evaluated the magnitude of gear loss with interviews and tested proposed indirect mechanisms with a turtle density gradient, before–after comparisons (taking advantage of an increase in turtles in Kadmat and concurrent decrease in Agatti) and a natural herbivore exclosure. These complementary approaches supported fisher-pro- posed second-order mechanisms: at high densities, turtles heavily grazed seagrasses, significantly reduced canopy heights, lowered fish recruit abundance, food fish biomass and catch. Estimates of losses incurred in Agatti show that first-order conflict cost fishers USD 0.6 fisher-1 year􏰄1, while second-order pathways accounted for USD 887 fisher-1 year-1. Our results show that local perceptions are fueled by often-complex mechanisms that, though not always straightforward to measure, are very important in generating conflict. Reconciling the human–wildlife interface requires an adequate accounting of direct and indirect mechanisms to more completely reflect true losses communities bear for living with wildlife.

  • Journal Article
    2010
    Implications of conserving an ecosystem modifier: Increasing green turtle (Chelonia mydas) densities substantially alters seagrass meadows
    Aparna Lal, Rohan Arthur, Nuria Marbà, Adrian Lill, Teresa Alcoverro
    Biological Conservation 143: 2730-2738

    Ecosystem modifiers have the ability to significantly alter the ecosystem they inhabit sometimes with serious consequences for their own populations. We evaluated the ability of green turtles (Chelonia mydas) to modify seagrass ecosystems by their foraging activity. This study was conducted in a seagrass-dominated lagoon in the Lakshadweep Islands, Indian Ocean, where a stable high-density congregation of green turtles is present. We determined a gradient of turtle density in the lagoon and measured the intensity of turtle herbivory across the gradient. We then measured the impact of increasing grazing on seagrass structural parameters, growth and flowering along this gradient. Our results indicate that turtles substantially change seagrass meadow structure (canopy height, shoot length, width and density), reduce flowering and can potentially even cause changes in the species composition of the meadow. We discuss the implications of these results for seagrass ecosystem function, green turtle movement and human attitudes. When conserving ecosystem modifiers like the green turtle, any management strategy needs to include a detailed knowledge of the roles these species play in the ecosystems they inhabit.

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