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Seagrass systems under nutrient loads, hydrodynamics & green turtle grazing. Do green turtles rule the seagrass world?
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Despite their high economic and ecological value, seagrass meadows are experiencing chronic declining at a global scale (Waycott et al. 2009), predominantly due to eutrophication, high turbidity due to land use change, but also due to other factors such as mechanical disturbance (Short et al. 1996, Orth et al. 2006) and shifts in top-down controls of seagrass consumers due to overfishing of top-predators (Moksnes et al. 2008). In a more pristine past large grazers such as green turtles and dugongs were reported to be very abundant in many seagrass meadows (Jackson et al. 2001, Jackson et al. 1997). Nowadays most meadows grow in the absence of megaherbivores, which had coevolved with seagrasses over the last 50 million years (Domning 2001, Les et al. 1997, Bowen et al. 1993). Although there are many studies focusing on separate effects of eutrophication, grazing or hydrodynamics on seagrass, relatively little is known on the interactive effects. This raises the general question whether and how the large-scale loss of megaherbivores affects the resilience of seagrass systems with respect to the impact of other environmental stressors. To address this issue, the present thesis focused on how green turtles affect seagrass dynamics directly and indirectly, via interaction between grazing effects, eutrophication and hydrodynamics. By understanding the drivers and interactions responsible for the functioning and degradation of seagrass systems we were also able to develop indicator tools and to provide insight for the management of seagrass ecosystems and, at a larger scale, marine protected areas (MPAs).
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... The observed hyper-abundance of turtles in this study is likely to be the result of three interacting processes. First, a chronic decline of seagrass habitat has occurred in non-reserve areas around this MPA, probably because of high sedimentation rates, turbidity, eutrophication and mechanical disturbance [25]. The decline in seagrass habitat is a global phenomenon [26] and can be expected to cause increased turtle abundance in remaining habitats. ...
... The detectability was high as the water was clear and shallow (less than 3 m), and turtles were counted only on days with calm weather conditions. Turtles were observed to forage almost exclusively on seagrass biomass [25] and grazed year round at our field site at constant densities, as shown by extra monthly assessments of densities throughout 2009. Green turtles were captured on the seagrass meadow using the rodeo method [29] in December 2009 (n ¼ 116) and December 2011 (n ¼ 141). ...
Marine protected areas (MPAs) are key tools for combatting the global overexploitation of endangered species. The prevailing paradigm is that MPAs are beneficial in helping to restore ecosystems to more 'natural' conditions. However, MPAs may have unintended negative effects when increasing densities of protected species exert destructive effects on their habitat. Here, we report on severe seagrass degradation in a decade-old MPA where hyper-abundant green turtles adopted a previously undescribed below-ground foraging strategy. By digging for and consuming rhizomes and roots, turtles create abundant bare gaps, thereby enhancing erosion and reducing seagrass regrowth. A fully parametrized model reveals that the ecosystem is approaching a tipping point, where consumption overwhelms regrowth, which could potentially lead to complete collapse of the seagrass habitat. Seagrass recovery will not ensue unless turtle density is reduced to nearly zero, eliminating the MPA's value as a turtle reserve. Our results reveal an unrecognized, yet imminent threat to MPAs, as sea turtle densities are increasing at major nesting sites and the decline of seagrass habitat forces turtles to concentrate on the remaining meadows inside reserves. This emphasizes the need for policy and management approaches that consider the interactions of protected species with their habitat.
... Manfaat dan fungsi ekologis lamun mempunyai peranan penting sebagai penghasil produk primer dalam rantai makanan. Lamun juga merupakan makanan untuk biota laut hewan seperti Dogong dan Penyu (Christianen, 2013). ...
... The interactive effects between megaherbivores and invasive seagrass may impact seagrass species coexistence and species competition. Green turtles are described to have a foraging preference for seagrass species with the highest palatability and nutrient content (Bjorndal, 1997) which are characteristics attributed to fastgrowing species (such as H. wrightii) over slower growing species (such as T. testudinum; Christianen, 2013). The invasive H. stipulacea seems to be an exception to this rule. ...
Our knowledge of the functional role of large herbivores is rapidly expanding, and the impact of grazing on species coexistence and nonnative species expansion has been studied across ecosystems. However, experimental data on large grazer impacts on plant invasion in aquatic ecosystems are lacking.
Since its introduction in 2002, the seagrass species Halophila stipulacea has rapidly expanded across the Eastern Caribbean, forming dense meadows in green turtle (Chelonia mydas)—foraging areas. We investigate the changes in seagrass species coexistence and the impacts of leaf grazing by green turtles on nonnative seagrass expansion in Lac Bay (Bonaire, Caribbean Netherlands).
Green turtle grazing behaviour changed after the introduction of nonnative seagrass to Lac Bay in 2010. Field observations, together with time‐lapse satellite images over the last four decades, showed initiation of new grazing patches (65 ha, an increase of 72%). The sharp border between grazed and ungrazed seagrass patches moved in the direction of shallower areas with native seagrass species that had previously (1970–2010) been ungrazed. Green turtles deployed with Fastloc‐GPS transmitters confirmed high site fidelity to these newly cropped patches. In addition, cafeteria experiments indicated selective grazing by green turtles on native species. These native seagrass species had significantly higher nutritional values compared to the nonnative species. In parallel, exclosure experiments showed that nonnative seagrass expanded more rapidly in grazed canopies compared to ungrazed canopies. Finally, in 6 years from 2011 to 2017, H. stipulacea underwent a significant expansion, invading 20–49 fixed monitoring locations in Lac Bay, increasing from 6% to 20% in total occurrence. During the same period, native seagrass Thalassia testudinum occurrence decreased by 33%.
Synthesis. Our results provide first‐time evidence of large‐scale replacement of native seagrasses by rapidly colonizing Halophila stipulacea in the Caribbean and add a mechanistic explanation for this invasiveness. We conclude that green turtle leaf grazing may modify the rate and spatial extent of this invasive species' expansion, due to grazing preferences, and increased space for settlement. This work shows how large herbivores play an important but unrecognized role in species coexistence and plant invasions of aquatic ecosystems.
... Manfaat langsung lamun bagi kehidupan manusia secara signifi kan hampir tidak ada namun fungsi ekologis lamun mempunyai peranan yang sangat besar sebagai penghasil produk primer dalam rantai makanan. Lamun merupakan makanan untuk Dogong dan Penyu ( Christianen 2013). Dugong dan penyu menyukai Halodule uninervis sebagai menu utama ( de Iongh et al 2007). ...
Buku ini memuat diversity dan sebaran biota laut di Indonesia, mulai dari Protozoa, Plankton, Porifera, Coelenterata, Polychaeta, Krustasea, Moluska, Ekhinodermata dan Ikan. Selain itu, buku ini juga menekankan pada biota ekonomis penting yang ada.
... Some preference feeding tests were carried out by Al-Ajzoon (1993) in the Gulf of Oman with young adults of the green turtle, which were offered clumps of macroalgae (preference: Sargassopsis > Sargassum > Ulua) and seagrasses (preference: Halophila > Syringodium > Halodule) (Al-Ajzoon 1993). Sea turtles are described to have a foraging preference for seagrass species with the highest palatability and nutrient content (thesis Christianen 2013, Bjorndal 1997 which are characteristics attributed to fast growing species, such as H. stipulacea. Generally green turtles favor fast-growing species (such as H. stipulacea) over slower-growing species (such as T. testudinum) (thesis Christianen 2013). ...
In this report we examined the proximate response of fish assemblages, queen conch, and sea turtles on H. stipulacea meadows in Lac Bay, Bonaire, Caribbean Netherlands. Here we primarily focused on the differences between the invasive species H. stipulacea and the principal species of native sea grass in Lac Bay, namely turtle grass Thalassia testudinum http://library.wur.nl/WebQuery/wurpubs/456875
... The carbonate substrate had a median grain size of 591630 mm (d 50 , mean 6 SE, Malvern Laser Particle Sizer) and did not differ significantly between stations. The canopy was of low structural complexity as a result of intensive grazing by green sea turtles (Chelonia mydas, 20.6 individuals 62.2 ha 21 , [38]). The hair-like leaves were short (,5 cm), narrow (,1 mm) and thin (,0.2 mm). ...
One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.
Tropical deforestation continues to be a major driver of biodiversity loss and greenhouse gas emissions. Remote sensing technology is increasingly used to assess changes in forest cover, species distributions and carbon stocks. However, satellite and airborne sensors can be prohibitively costly and inaccessible for researchers in developing countries. Here, we describe the development and use of an inexpensive (<$2,000) unmanned aerial vehicle for surveying and mapping forests and biodiversity (referred to as ‘Conservation Drone’ hereafter). Our prototype drone is able to fly pre-programmed missions autonomously for a total flight time of ~25 minutes and over a distance of ~15 km. Non-technical operators can program each mission by defining waypoints along a flight path using an open-source software. This drone can record videos at up to 1080 pixel resolution (high definition), and acquire aerial photographs of <10 cm pixel resolution. Aerial photographs can be stitched together to produce real-time geo-referenced land use/cover maps of surveyed areas. We evaluate the performance of this prototype Conservation Drone based on a series of test flights in Aras Napal, Sumatra, Indonesia. We discuss the further development of Conservation Drone 2.0, which will have a bigger payload and longer range. Initial tests suggest a flight time of ~50 minutes and a range of ~25 km. Finally, we highlight the potential of this system for environmental and conservation applications, which include near real-time mapping of local land cover, monitoring of illegal forest activities, and surveying of large animal species.
An understanding of the mineral nutrition of plants is of fundamental importance in both basic and applied plant sciences. The Second Edition of this book retains the aim of the first in presenting the principles of mineral nutrition in the light of current advances. This volume retains the structure of the first edition, being divided into two parts: Nutritional Physiology and Soil-Plant Relationships. In Part I, more emphasis has been placed on root-shoot interactions, stress physiology, water relations, and functions of micronutrients. In view of the worldwide increasing interest in plant-soil interactions, Part II has been considerably altered and extended, particularly on the effects of external and interal factors on root growth and chapter 15 on the root-soil interface. The second edition will be invaluable to both advanced students and researchers.
The aim of this research is to gain an understanding of the structure of the Indo-Pacific green turtle meta-population in general and the green turtle population in Aru in particular and obtain insight into the effects of large-scale commercial exploitation of this species. This study intends to present a backbone against which conservation strategies can be drafted and management decisions can be made. Chapter two presents a detailed description of the green turtle nesting population of Aru and provides the essential demographic data needed for population persistence and metapopulation studies. The data for this chapter were obtained during four consecutive census periods from 1997 to 2000, as well as from an in-situ
experiment on the influence of nesting substrate type on hatching and emergence success. Genetic analyses using mtDNA variation in combination with mark-recapture data is used in chapter three to assess the geographic range occupied by individual breeding populations and the distribution of such populations through Australasia. Assessments of mtDNA haplotype variation among 27 green turtle rookeries is used to try to reveal the level at which adult female disperse among nesting sites in the region. Genetic tests of population
differentiation of these rookeries have been used to characterise distinct breeding populations or groups of populations (referred to as breeding stocks). In chapter four the same genetic tests were used to assess migratory connectivity between feeding grounds and the breeding stocks. Special attention is given to the migratory pattern of the Aru stock that migrates away from the Aru feeding grounds to test the hypothesis that this stock is for the most part sedentary in that it breeds and forages in the same region, and therefore is likely to be more vulnerable under the pressure of the commercial harvest. In chapter five an age-based
stochastic population model is used to assess the impact of the observed harvest on persistence of the Aru stock and investigate the effect of several management approaches aimed to reduce the probability of extirpation. A more detailed discussion of various forms of green turtle exploitation in Aru and the conservation status of the Aru population in the context of the Aru Tenggara marine reserve is presented in chapter six. The major findings of this study are presented in the synopsis in the summary chapter.
Herbivores can directly increase nitrogen mobility by increasing the quality of organic matter entering the decomposition cycle, but they also may decrease nitrogen mobility by decreasing the biomass of high-nitrogen species in the plane community. We assessed effects of voles (Microtus) on nitrogen dynamics using exclosures in two riparian meadows (Crystal Bench and Blacktail Deer Creek) in Yellowstone National Park (USA). At both sites, the quantity of plant litter was decreased by herbivory following a vole population peak in 1992. At Crystal Bench, removal of voles caused a decrease in the nitrogen concentration and an increase in the C:N ratio of plant litter over the four years of the study. The higher quality litter produced in the presence of voles at Crystal resulted in a larger pool of potentially mineralizable nitrogen in soil from control plots relative to soils from plots that had not been accessible to voles. At Crystal, vole removal did not cause a change in plant community composition. However, at Blacktail, after several years of vole exclusion, legumes became more common in exclosures than in control plots that were accessible to voles. Selective herbivory on high-nitrogen legumes kept the litter quality outside exclosures low, whereas higher legume biomass caused a decrease in C:N ratio of plant litter inside exclosures. The removal of voles at Blacktail caused a 15% increase in the fraction of the soil nitrogen that was rapidly mineralizable. Our results show that voles increased nitrogen mobility, especially during and after population peaks. However, that increase was offset by decreases in nitrogen mineralization over longer periods when voles caused a decrease in high-quality plant litter produced by preferred forage plants, especially legumes: Thus, both the mechanisms by which voles affected nitrogen dynamics and the net effects of voles varied over time and space. The balance of direct and indirect effects may provide a general mechanistic explanation of whether herbivores increase or decrease the rate of nitrogen cycling.
