Meta-Analysis of Reciprocal Linkages between Temperate Seagrasses and Waterfowl with Implications for Conservation (original) (raw)
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Estuaries and Coasts, 1998
We evaluated the distribution of waterfowl in relation to a seagrass (Ruppia maritima) patch, to infauna, and on its relationship with substrate characteristics. An experiment performed in the Mar Chiquita coastal lagoon (37'46'S, 57"27'W, Argentina), was used to evaluate the effect of berbivory on widgeon grass, R. maritima. Control plots of equal size, located between bird exclosures, were exposed to berbivory. The experiment ran for 31 d, starting on December 1, 1994. Censuses showed that black-necked swan (a = 50 birds ha-', SD = 37, n = 10) and coots (X = 42 bids ha-', SD = 28, n = 10) were the most important (always present) of the waterfowl seen feeding on R. maritima. The results of the experiment showed greater leaf lengths, lower belowground (rhizomes and roots) biomass, greater aboveground (leaves and shoots) biomass, and greater abundance of the polycbaete Hetermna.stus sin&s in exclosure plots. There were no exclosure effects on total biomass (belowground plus aboveground), reproductive parts (fruits and pre-and postpollination flowers), or abundance of most benthic infauna. Topographic surveys showed that sediment surface was higher within the R muriElma patches, but there were no significant differences in gramdometric composition. Surveys also showed that bid distribution was strongly associated with the presence of R. maritima.
Role of Larger Herbivores in Seagrass Communities
Estuaries, 1984
The nutritional ecology of macroherbivores in seagrass meadows and the roles of grazing by urchins, fishes and green turtles in tropical systems and waterfowl in temperate systems are discussed in this review. Only a few species of animals graze on living seagrasses, and apparently only a small portion of the energy and nutrients in seagrasses is usually channeled through these herbivores. The general paucity of direct seagrass grazers may be a function of several factors in the composition of seagrasses, including availability of nitrogen compounds, presence of relatively high amounts of structural cell walls, and presence of toxic or inhibitory substances. The macroherbivores, however, can have a profound effect on the seagrass plants, on other grazers and fauna associated with the meadow, and on chemical and decompositional processes occurring within the meadow. Grazing can alter the nutrient content and digestibility of the plant, as well as its productivity. Removal of leaf material can influence interrelations among permanent and transient faunal residents. Grazing also interrupts the detritus cycle. Possible consequences of this disruption, either through acceleration or through decreased source input, and the enhancement of intersystem coupling by increased export and offsite fecal production, are discussed. The extent and magnitude of these effects and their ecological significance in the overall functioning of seagrass meadows only can be speculated, and probably are not uniform or of similar importance in both tropical and temperate seagrass systems. However, areas grazed by large herbivores provide natural experiments in which to test hypotheses on many functional relations in seagrass meadows.
Seagrass herbivory:evidence for the continued grazing of marine grasses
Marine Ecology Progress Series, 1999
Unlike the majority of marine plants, seagrasses are believed to experience little damage from the feeding activities of marine herbivores. Based on our previous work, plus a review of the literature, we suggest that this paradigm significantly underestimates the importance of seagrass herbivory in nearshore environments. In this review, we provide evidence from over 100 publications, showing that grazing on seagrasses is widespread in the world's oceans. Overwhelmingly, reports of grazing on seagrasses are based on observations, laboratory measurements, and bioenergetic calculations. To date, few field experiments have been conducted to evaluate the importance of seagrass grazing in the nearshore environment. Of these, even fewer have considered the possibility that herbivores may stimulate rates of primary production of the role of belowground nutrient reserves in determining the impacts of grazers on seagrasses. We contend that the currently accepted view that herbivory plays a minor role in the energetics of seagrass habitats needs to be reexamined by measuring seagrass responses to grazer induced tissue losses in controlled field manipulations. Only then will we be able to determine the in~portance of the seagrass-grazing pathway in marine food webs.
Driving factors of biogeographical variation in seagrass herbivory
Science of The Total Environment
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Feeding preferences of herbivores in a relatively pristine subtropical seagrass ecosystem
Marine and Freshwater Research, 2012
Understanding forage choice of herbivores is important for predicting the potential impacts of changes in their abundance. Such studies, however, are rare in ecosystems with intact populations of both megagrazers (sirenians, sea turtles) and fish grazers. We used feeding assays and nutrient analyses of seagrasses to determine whether forage choice of grazers in Shark Bay, Australia, are influenced by the quality of seagrasses. We found significant interspecific variation in removal rates of seagrasses across three habitats (shallow seagrass bank interior, shallow seagrass bank edge, deep), but we did not detect variation in gazing intensity among habitats. In general, grazers were more likely to consume fast-growing species with lower carbon : nitrogen (C : N) and carbon : phosphorus (C : P) ratios, than the slower-growing species that are dominant in the bay. Grazer choices were not, however, correlated with nutrient content within the tropical seagrasses. Slow-growing temperate se...
Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea
Helgoland Marine Research, 2000
During autumn migration (September to December), brent geese (Branta b. bernicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in the nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment eight times within this 3-month period. In addition, brent geese excavated pits 3–10 cm deep by trampling in order to feed on below-ground phytomass. About 12% of the seagrass beds became pitted to an average depth of 4.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m–2 of above-ground and 28 g of below-ground phytomass. This corresponds to 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were excluded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The underlying process seems to be a self-inhibition of dense overwintering seagrass by mud accretion. Assuming our experimental results can be scaled up to the entire seagrass bed, we hypothesize that in the sheltered upper intertidal zone, seasonal erosion caused by herbivorous geese and ducks is necessary for the persistence of Z. noltii.
The Role of Herbivory in Structuring Tropical Seagrass Ecosystem Service Delivery
Frontiers in plant science, 2018
Seagrass meadows support key ecosystem services, via provision of food directly for herbivores, and indirectly to their predators. The importance of herbivores in seagrass meadows has been well-documented, but the links between food webs and ecosystem services in seagrass meadows have not previously been made explicit. Herbivores interact with ecosystem services - including carbon sequestration, cultural values, and coastal protection. Interactions can be positive or negative and depend on a range of factors including the herbivore identity and the grazing type and intensity. There can be unintended consequences from management actions based on a poor understanding of trade-offs that occur with complex seagrass-herbivore interactions. Tropical seagrass meadows support a diversity of grazers spanning the meso-, macro-, and megaherbivore scales. We present a conceptual model to describe how multiple ecosystem services are influenced by herbivore pressure in tropical seagrass meadows. ...
Estuarine, Coastal and Shelf Science, 2015
Herbivory on Mediterranean seagrass species is generally low compared to consumption of some other temperate and tropical species of seagrasses. In this study we: (1) investigate the feeding preference of the two dominant Mediterranean seagrass herbivores, the sea urchin Paracentrotus lividus and the fish Sarpa salpa, on Posidonia oceanica and Cymodocea nodosa and (2) elucidate the role of epiphytes in herbivore choices. We assessed consumption rates by tethering seagrass shoots, and preferences by food choice experiments with the following paired combinations: 1) Epiphytized leaves of both C. nodosa vs. P. oceanica (CE vs PE); 2) Non-epiphytized leaves of C. nodosa vs. P. oceanica (CNE vs. PNE); 3) Epiphytized vs non-epiphytized leaves of C. nodosa (CE vs. CNE) and 4) Epiphytized vs non-epiphytized leaves of P. oceanica (PE vs PNE). We found that preference for C. nodosa was weak for S. salpa, but strong for P. lividus, the species responsible for most consumption at our study. Overall both herbivores showed preference for epiphytized leaves. The higher nutritional quality of C. nodosa leaves and epiphytes together with the high coverage and diversity of the epiphyte community found on its leaves help explain the higher levels of herbivory recorded on epiphyted leaves of C. nodosa. Other factors such as seagrass accessibility, herbivore mobility and size, and behavioral responses to predation risks, may also affect the intensity of seagrass herbivory, and studies addressing the interactions with these factors are needed to improve our understanding of the nature, extent and implications of herbivory in coastal ecosystems.
Marine Ecology Progress Series, 2002
The widespread occurrence and persistence of modern day seagrass habitats has led many to hypothesize that grazing on seagrasses is minimal. On a global scale this may well be true as the numbers of large vertebrate herbivores (e.g. sea turtles, manatees and dugongs) and waterfowl, grazers that can greatly alter seagrass density, have been dramatically reduced in coastal ecosystems. Nonetheless, numerous observations indicate that smaller herbivores (e.g. the bucktooth parrotfish Sparisoma radians), grazers that may not be able to reduce seagrass density substantially, still commonly feed on seagrasses in the subtropical and tropical western Atlantic Ocean. These observations led us to quantify the role that seagrass herbivory plays in modern-day seagrass food webs. In the first phase of this study, digitally scanned seagrass leaves were clipped to ropes and placed at 3 sites in Hawk Channel, in the northern Florida Keys (USA). Areal loss from the tethered leaves provided a daily estimate of seagrass grazing rates. These losses, coupled with local estimates of net aboveground primary production, allowed us to determine the proportion of seagrass production consumed at our study sites during 4 separate seasons. We found that seagrass grazing varied greatly both spatially and seasonally at our sites but, on average, grazers consumed virtually all of the aboveground production at 2 of the 3 sites. When experiments were repeated in the summer of a second year at 6 sites, seagrass grazing again varied greatly among sites, but at 3 of the sites most of the daily production of seagrass shoots was consumed by small herbivorous fishes. These results suggest that while it is undoubtedly true that modern day grazing by mammals, turtles and waterfowl on seagrass is reduced, small vertebrate grazers consume substantial amounts of seagrass production in the northern Florida Keys.