Shorebirds Affect Ecosystem Functioning on an Intertidal Mudflat

Abstract

Ecosystem functioning and services have provided a rationale for conservation over the past decades. Intertidal muddy sediments, and the microphytobenthic biofilms that inhabit them, perform crucial ecosystem functions including erosion protection, nutrient cycling and carbon sequestration. It has been suggested that predation on sediment macrofauna by shorebirds may impact biofilms, and shorebirds are known to consume biofilm, potentially causing significant top-down effects on mudflat ecosystem functioning. We carried out an exclusion experiment on the Colne Estuary, Essex, to examine whether shorebird presence significantly affects sediment erodibility measured with a Cohesive Strength Meter (CSM) and microphytobenthos biomass measured using PAM fluorescence (Fo) and chlorophyll a content. We also tested for treatment effects on sediment-water nutrient fluxes [nitrate, nitrite, ammonia, phosphate and dissolved organic carbon (DOC)] during periods of both dark and light incubation. Excluding shorebirds caused statistically significant changes in regulating and provisioning ecosystem functions, including mudflat erodibility and nutrient fluxes. The presence of shorebirds lowered the sediment critical erosion threshold τcr, reduced nitrate fluxes into the sediment under illumination, lowered nitrate efflux, and reduced phosphate uptake, compared to sediments where birds were excluded. There were no significant differences in macrofauna community composition within the sediment between treatments after 45 days of bird exclusion, suggesting a direct link between shorebird presence or absence and the significant differences in biofilm-related variables. This study introduces previously unknown effects of shorebird presence on ecosystem functions within this system and highlights an area of shorebird science that could aid joint conservation and human provisioning action.

A tale of two key species in a subtropical mudflat: four-fold density increases produce minimal ecological response in macrofauna

“…Coastal ecosystems are under intense pressure from anthropogenic activities (Williams et al 1990;MEA 2005;IPCC 2014;McPhee 2017;Newton et al 2020), and an improved understanding of their dynamics is required to anticipate how marine ecosystems, and the services they deliver, might change in the future. In situ experimental density manipulations of organism densities have shown how communities and the associated ecosystem functions respond to change (Wilson 1991;Bertness et al 2014;Ling et al 2015;Aguilera et al 2018;Booty et al 2020). Therefore, we hypothesised that the macrofaunal assemblage structure, their collective biological traits (and, by extension, mudflat functioning), and the abiotic environment would respond to increased densities of the crab Macrophthalmus setosus and the mud whelk Pyrazus ebeninus.…”

“…P. ebeninus (Edwards and Welsh 1982;DeWitt and Levinton 1985) and large vertebrates that disturb surface sediments (e.g. shorebirds; Booty et al 2020) or burrowers can affect surface deposit feeders by decreasing microphytobenthic production (Botto and Iribarne 1999; Webb and Eyre 2004;Lomovasky et al 2006;. For example, the complete removal of Macrophthalmus japonicus from temperate mudflats of the Tama Estuary, Japan, during autumn increased mean macrofaunal density by 53% in the subsequent summer compared with controls.…”

“…Experimental studies in laboratory mesocosms (Biles et al 2003;Waldbusser et al 2004;Braeckman et al 2010;Karlson et al 2016) and in the field (O'Connor and Crowe 2005;Clare et al 2016) have examined how the biological traits of macrofauna influence functioning. Manipulative in situ field experiments can provide more realistic insights because they integrate the species interactions and abiotic processes within natural systems (Paine 1966;Dayton et al 1974;Menge 1976;Wilson 1991;Raffaelli and Moller 1999;Booty et al 2020;Hawkins et al 2020). However, rigorous design is required to minimise artefacts of the experimental set-up, such as those owing to the presence of artificial structures, which may bias the outcomes (Hulberg and Oliver 1980;Woodin 1981;Hall et al 1990;Underwood 1990;Peterson and Black 1994;Benedetti-Cecchi and Cinelli 1997;Como et al 2006;O'Malley and Hunt 2020).…”

Combining Multispectral and Radar Imagery with Machine Learning Techniques to Map Intertidal Habitats for Migratory Shorebirds

“…To classify the different intertidal habitat types, we combined individual bands and sets of band indexes derived from Sentinel-1 and Sentinel-2 imagery with a DEM, produced in this study from a time series of Sentinel-2 scenes representing different tidal stages, to use as potential predictors. Our workflow consisted of (1) an extensive collection of quantitative field reference data, including grain size analysis of sediment samples; (2) the defining of target shorebird intertidal habitats through a set of quantitative empirical thresholds; (3) the collecting and computing of a set of predictors from Sentinel-1 and Sentinel-2 imagery, including a Sentinel-2-derived DEM; (4) the employment of a modified recursive feature elimination algorithm to identify a minimal set of predictors; (5) the training of a random forest predictor with the selected predictors; (6) the validation of the quality of the final model against a fully independent dataset; (7) the production of a classification map for the entire study area; and (8) the employment of a post-classification filter to produce the final map. In addition, we assessed the benefits (in terms of classification accuracy) of combining data from different sensors by comparing the final model and the final map with models and maps produced by alternative models without Sentinel-1 and DEM layers.…”

“…They are estimated to cover at least 127,921 km 2 [1] and include a variety of important habitat types, such as rocky reefs, shell beds, seagrass and macroalgae mats, and soft exposed sediments [2]. Intertidal flats support high biodiversity levels and complex benthic food webs [3], including high-level consumers such as migratory shorebirds, which play an important role in structuring processes in intertidal food webs [4,5]. These birds depend entirely on intertidal areas for foraging during their non-breeding periods [6], where they congregate at low tide to feed on benthic invertebrate prey [7].…”