Sesarmid crabs dominate Indo West-Pacific mangroves, and consume large amounts of mangrove litter. This is surprising, since mangrove leaves have high tannin contents and C/N ratios that far exceed ...17, normally taken as the maximum for sustainable animal nutrition. This paradox has led to the hitherto untested hypothesis that crabs let leaves age in burrows before consumption, thereby reducing tannin content and C/N ratio. We excavated burrows of Neosarmatium meinerti within high-shore Avicennia marina mangroves, and investigated whether burrow leaves had C, N or C/N values significantly different from those of senescent leaves. Leaves were found in <45% of burrows, mostly only as small fragments, and N concentrations and C/N ratios of burrow leaves never varied significantly from senescent leaves. The leaf-ageing hypothesis was therefore not supported. In the field N. meinerti and Sesarma guttatum fed on sediment in 76% and 66-69% of observations, respectively, and on leaves in <10% of observations. Sediments from two A. marina mangroves had a mean C/N ratio of 19.6. Our results, and the literature, show that mangrove leaves are unlikely to fulfil the N requirements of crabs, whether or not leaf ageing takes place. Sediment detritus could be a richer source of N, as shown by lower C/N ratios and regular ingestion by crabs. By fragmenting leaves crabs may be elevating the nutritional quality of the substrate detritus.
The Common Redshank Tringa totanus breeding population on British saltmarshes has declined by over 50% since 1985, with declines linked to changes in grazing management. Conservation initiatives have ...encouraged low‐intensity grazing of less than one cattle per hectare, but Redshank have continued to decline, even in regions where light grazing was predominant. This study quantified effects of grazing intensity on Redshank nest survival over six lightly grazed saltmarshes with livestock densities between 0 and 0.82 cattle per hectare, in the Ribble Estuary, northwest England. We assessed whether grazing resulted in nest mortality directly through cattle trampling and/or indirectly through grazer modification of habitat that accelerates predation risks. Cattle density was recorded both during the Redshank breeding season and for 1 year prior to the study, to account for both short‐term trampling effects and the longer term effects on vegetation. Results showed that risk of nest loss to trampling increased from 16% at 0.15 cattle per hectare to 98% at 0.82 cattle per hectare in the breeding season. The risk of a nest being predated increased from 28% with no grazing to 95% at 0.55 cattle per hectare based on all year grazing data. These results suggest that even light conservation grazing at less than one cattle per hectare can reduce Redshank nest survival rates to near zero. It may therefore be appropriate to reduce saltmarsh grazing intensities, or change the timing of saltmarsh grazing to reduce the number of livestock present during the Redshank breeding season.
Grassland carbon capturing and storage (CCS) is thought to benefit from regulation of grazing. The impact is likely to depend on livestock density. Yet, few studies have tested this principle or ...evaluated the consistency of grazer-carbon relationships across multiple sites. We sampled four intertidal zones across 22 salt marshes along a 650 km stretch of coast in the UK to examine the impact of livestock density on globally important saltmarsh “blue carbon” stocks. Although there were marked impacts of grazing pressure on above ground vegetation composition, structure and biomass, there was no detectable relationship between grazing intensity and soil organic carbon, irrespective of tidal zone in the marsh or soil depth-layer analyzed. A substantial spatial variation in soil carbon was instead explained by contextual environmental variables. There was evidence that compensatory responses by vegetation, such as increased root growth, countered carbon loss from grazing impacts. Our work suggests that grazing effects on carbon stocks are minimal on broader scales in comparison with the influence of environmental context. The benefits of grazing management to carbon stores are likely to be highly context dependent.
Conservation grazing for breeding birds needs to balance the positive effects on vegetation structure and negative effects of nest trampling. In the UK, populations of Common redshank Tringa totanus ...breeding on saltmarshes declined by >50% between 1985 and 2011. These declines have been linked to changes in grazing management. The highest breeding densities of redshank on saltmarshes are found in lightly grazed areas. Conservation initiatives have encouraged low‐intensity grazing at <1 cattle/ha, but even these levels of grazing can result in high levels of nest trampling. If livestock distribution is not spatially or temporally homogenous but concentrated where and when redshank breed, rates of nest trampling may be much higher than expected based on livestock density alone. By GPS tracking cattle on saltmarshes and monitoring trampling of dummy nests, this study quantified (i) the spatial and temporal distribution of cattle in relation to the distribution of redshank nesting habitats and (ii) trampling rates of dummy nests. The distribution of livestock was highly variable depending on both time in the season and the saltmarsh under study, with cattle using between 3% and 42% of the saltmarsh extent and spending most their time on higher elevation habitat within 500 m of the sea wall, but moving further onto the saltmarsh as the season progressed. Breeding redshank also nest on these higher elevation zones, and this breeding coincides with the early period of grazing. Probability of nest trampling was correlated to livestock density and was up to six times higher in the areas where redshank breed. This overlap in both space and time of the habitat use of cattle and redshank means that the trampling probability of a nest can be much higher than would be expected based on standard measures of cattle density. Synthesis and applications: Because saltmarsh grazing is required to maintain a favorable vegetation structure for redshank breeding, grazing management should aim to keep livestock away from redshank nesting habitat between mid‐April and mid‐July when nests are active, through delaying the onset of grazing or introducing a rotational grazing system.
In the UK, populations of Common Redshank Tringa totanus breeding on saltmarshes have declined by >50% since 1985, and declines have been linked to changes in grazing management. By GPS tracking cattle on saltmarshes and monitoring trampling of dummy nests, this study quantified (i) the spatial and temporal distribution of cattle in relation to the distribution of redshank nesting habitats and (ii) trampling rates of dummy nests. The distribution of livestock was highly variable depending on both time in the season and the saltmarsh under study, with cattle using between 3% and 42% of the saltmarsh extent and spending most their time on higher elevation habitat within 500m of the sea wall, where redshank breed. This overlap in both space and time of the habitat use of cattle and redshank means that the trampling probability of a nest can be much higher than would be expected based on standard measures of cattle density.
1. Habitat structure, including vegetation structural complexity, largely determines invertebrate assemblages in semi‐natural grasslands. The importance of structural complexity to the saltmarsh ...invertebrate community, where the interplay between vegetation characteristics and tidal inundation is key, is less well known.
2. It was hypothesised that canopy complexity would be a more important predictor of spider and beetle assemblages than simple vegetation attributes (e.g. height, community type) and environmental variables (e.g. elevation) alone, measured in two saltmarsh regions, south‐east (Essex) and north‐west (Morecambe Bay)
U.K. C
anopy complexity (number of non‐vegetated ‘gaps’ in canopy ≥ 1 mm wide) was assessed using side‐on photography. Over 1500 spiders and beetles were sampled via suction sampling, winter and summer combined.
3. In summer, saltmarshes with abundant spider and beetle populations were characterised by high scores for canopy complexity often associated with tussocky grass or shrub cover. Simple vegetation attributes (plant cover, height) accounted for 26% of variation in spider abundance and 14% in spider diversity, rising to 46% and 41%, respectively, with the addition of canopy complexity score. Overwintering spider assemblages were associated with elevation and vegetation biomass. Summer beetle abundance, in particular the predatory and zoophagous group, and diversity were best explained by elevation and plant species richness.
4. Summer canopy complexity was identified as a positive habitat feature for saltmarsh spider communities (ground‐running hunters and sheet weavers) with significant ‘added value’ over more commonly measured attributes of vegetation structure.
Recent studies in salt marshes have demonstrated the role of plant roots in sediment stabilisation, and hence the importance of marshes in providing coastal protection. However, the relative role of ...root traits and environmental factors in controlling sediment stability, and how intraspecific variability of root traits vary within and among marshes, remain poorly understood. In this study, we investigated which root trait(s) drive sediment stability (resistance to lateral erosion) in two marsh species with an important role in coastal protection (Spartina anglica and Atriplex portulacoides) and how the environment affects the expression of these traits. We sampled three marshes along salinity gradients in each of two estuaries in Wales (UK), establishing replicate plots in the respective dominant zones of each species. In all plots we sampled abiotic variables (sand, redox potential, pH, salinity) and root traits (root density, specific root density, root volume, root length density); in a subset of these plots (three per species in each marsh) we extracted soil-plant cores and assessed their erosion resistance in a flume. Sediment stability was enhanced by increases in root density and reductions in sand content. Abiotic variables affected root density in different ways depending on species: in S. anglica, redox was the only significant factor, with a positive, linear effect on root density; in A. portulacoides, redox had a non-linear (U-shaped) effect on root density, while sand had a negative effect. Collectively, these results show that (i) intraspecific variability in root density can influence sediment stability in salt marshes, and (ii) sediment properties not only influence sediment stability directly, but also indirectly via root density. These results shed light on spatial variability in the stability of salt marshes to lateral erosion and suggest that root density should be incorporated into coastal vegetation monitoring programs as an easy-to-measure root trait that links the environment to sediment stability and hence to the function and services provided by marshes.
Biological rhythms with lunar components are common in nature. In the sea, the moon's gravitational pull on earth is the principal cause of the tides, which normally reach maximum amplitudes every ...new and full moon. Many populations synchronize spawning to this time. Some choose either the new or the full moon, implying that moonlight is important; but one lunar phase usually has higher tides than the other, and many species select the phase with the higher tide to improve the offshore transport of their progeny. However, tidal dominance by one lunar phase is not constant; it switches between new and full moon every seven months. We tested the influence of this 14-month "syzygy inequality cycle" (SIC) on lunar synchrony by sampling 11 populations of intertidal crabs at two locations in East Africa for 21 months. Eight populations synchronized larval release with the SIC. Tidal cues were more important than moonlight in entraining the reproductive rhythm, although two populations synchronized spawning to the new moon. SIC synchrony increased with population shore level, because only the higher lunar tide permitted top-shore spawning. Top-shore species therefore have a restricted lunar choice. SIC synchrony could be common, given that it occurs in most marine environments.
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•Saltmarsh carbon stocks can vary by 52 times depending on how upscaling is done.•IPCC’s recommended carbon sampling procedure can inflate marsh stock calculations.•Regression ...upscaling produce detailed maps of carbon distribution in marshes.•Simple SOC content × area calculations are comparable to machine learning algorithms.•Uncertainty maps make it easy to assess marsh-scale carbon stock accuracy.
Calculating the amount of soil organic carbon (SOC) stored in coastal environments, including salt marshes, is needed to determine their role in mitigating the Climate Crisis. Several techniques exist to calculate the SOC content of a unit of land from the upscaling of soil cores. However, no comprehensive assessment has been made on the performance of commonly used SOC upscaling techniques until now. We measured the SOC content of soil cores gathered from two Scottish salt marshes. Two SOC values were used for upscaling; SOC content for a 1 m standardised depth (as recommended by the IPCC), and SOC content of the modern marsh deposit (identified in the stratigraphy as a transition from organic-rich (marsh) to mineral-rich (intertidal flat) soil. Twenty-two upscaling techniques were used (SOC content × area, interpolative, and regression-based extrapolative calculations). Leave-one-out cross-validation procedures and prediction interval widths were used to assess the accuracy of each technique. Digital Terrain Models and Normalized Difference Vegetation Indices were used as covariate surfaces in the regression models. We found that marsh-scale SOC stocks varied by as much as fifty-two times depending on which sampling depth and upscaling technique was used. The largest differences emerged when comparing SOC stocks upscaled from 1 m deep and modern marsh deposits. Using the IPCC recommended 1 m sampling depth inflated the SOC stocks of salt marshes, as intertidal flat environments were included in the calculation. Ensemble regression models from the weighted average of seven machine learning algorithm outputs produced the highest upscaling accuracies across marshes and sampling depths. Simple SOC content × area calculations produced marsh-scale SOC stocks that were comparable to stock values produced by more advanced ensemble regression models. However, regression models produced detailed maps of SOC distribution across a marsh, and the associated uncertainty in the SOC values. Our findings are broadly applicable for other environments where large-scale SOC stock assessments and uncertainty are needed.
Anthropogenic impacts on biodiversity across a wide range of ecosystems are well documented; however the responses of ecosystems to reduced diversity are still poorly understood. We investigated the ...effects of species richness, species identity and environmental variables on aboveground biomass increment using replanted mangroves at Gazi Bay, Kenya. We planted 32 plots (36 m²) with 8 treatments: all possible combinations of the treesAvicennia marina,Bruguiera gymnorrhiza, andCeriops tagaland an unplanted control. Trees were planted in July and August 2004 and monitored annually until 2009. Growth was slow in the first 2 yr of the study, but by 2007 there was a significant treatment effect on aboveground biomass.A. marinashowed strong competitive traits, with the best growth overall and enhanced growth of individual trees when planted in mixed species plots. The highest biomass was recorded in 3-species mixes; partitioning the net effects of species mixing showed a strong species selection effect, but there was also a complementarity effect in some of the three species plots. Biomass was positively correlated with presence ofA. marinaand negatively correlated with sediment salinity. We conclude that there is variation in the stages of plant development at which species richness effects manifest themselves; in addition the effects of environmental variables have a bearing on the nature and direction of the relationship between species richness and ecosystem function.
•Livestock grazing in saltmarshes reduces bee abundance and diversity.•Reductions are driven by decreased cover of two key food plants.•Increased plant diversity with grazing does not compensate for ...negative effects.•Negative effects are stronger at higher grazing intensity.•Ungrazed saltmarshes rank highly for bees compared to terrestrial habitats.
Global declines in pollinator populations and associated services make it imperative to identify and sensitively manage valuable habitats. Coastal habitats such as saltmarshes can support extensive flowering meadows, but their importance for pollinators, and how this varies with land-use intensity, is poorly understood. We hypothesised that saltmarshes provide important bee foraging habitat, and that livestock grazing either suppresses or enhances its value by reducing the abundance - or increasing the diversity - of flowering plants. To test these hypotheses, we surveyed 11 saltmarshes in Wales (UK) under varying grazing management (long-term ungrazed, extensively grazed, intensively grazed) over three summers and investigated causal pathways linking grazing intensity with bee abundance and diversity using a series of linear mixed models. We also compared observed bee abundances to 11 common terrestrial habitats using national survey data.
Grazing reduced bee abundance and richness via reductions in the flower cover of the two key food plants: sea aster Tripolium pannonicum and sea lavender Limonium spp. Grazing also increased flowering plant richness, but the positive effects of flower richness did not compensate for the negative effects of reduced flower cover on bees. Bee abundances were approximately halved in extensively grazed marshes (relative to ungrazed) and halved again in intensively grazed marshes. Saltmarsh flowers were primarily visited by honeybees Apis mellifera and bumblebees Bombus spp. in mid and late summer. Compared to other broad habitat types in Wales, ungrazed saltmarshes ranked highly for honeybees and bumblebees in July-August, but were relatively unimportant for solitary bees. Intensively grazed saltmarshes were amongst the least valuable habitats for all bee types.
Under appropriate grazing management, saltmarshes provide a valuable and previously overlooked foraging habitat for bees. The strong effects of livestock grazing identified here are likely to extend geographically given that both livestock grazing and key grazing-sensitive plants are widespread in European saltmarshes. We recommend that long-term ungrazed saltmarshes are protected from grazing, and that grazing is maintained at extensive levels on grazed marshes. In this way, saltmarshes can provide forage for wild and managed bee populations and support ecosystem services.
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