“…) and have also provided evidence that shorebirds substantially deplete prey stocks during winter (Ahmedou Salem et al . ). Such evidence suggests that strong interspecific competition might drive resource partitioning at Banc d'Arguin.…”
1. Food webs and trophic dynamics of coastal systems have been the focus of intense research throughout the world, as they prove to be critical in understanding ecosystem processes and functions. However, very few studies have undertaken a quantitative comparison of entire food webs from a key consumer perspective across a broad geographical area, limiting relevant comparisons among systems with distinct biotic and abiotic components. 2. We investigate the structure and functioning of food webs in four tidal ecosystems of international importance for migratory shorebirds along the East Atlantic Flyway: Tejo estuary in Portugal, Sidi Moussa in Morocco, Banc d'Arguin in Mauritania and Bijag os archipelago in Guinea-Bissau. Basal food sources, shorebirds and their prey (benthic invertebrates) were sampled in all areas, and Bayesian stable isotope mixing models and community-wide metrics were used in a comparative analysis among areas. 3. Significant differences among study areas were found in the structure of food webs, as well as in the relative importance of basal resource pools supporting each food web. Overall, the food web of Banc d'Arguin was characterized by lower trophic diversity and higher functional redundancy than the other sites. This result might be explained by the low number of trophic pathways of organic matter transfer in this seagrass-dominated system which, as a fossil estuary, lacks inputs from both freshwater and nutrient-rich offshore oceanic waters. 4. Structure of shorebird communities was consistent with the main organizational patterns found for each food web, highlighting the less diverse character of the community of Banc d'Arguin. At Banc d'Arguin and Bijag os archipelago, which displayed the smallest and largest isotopic niche widths in bird assemblage, respectively, mean niche overlap among species was low, suggesting high interspecific partitioning in resource use. Tropical systems typically offer comparatively lower harvestable prey biomass for shorebirds and might thus strengthen interspecific competition, leading to low niche overlap among species. 5. Our study reveals relevant differences in the structure of food webs and shorebird communities in coastal areas along an avian flyway. While differences in trophic redundancy of food webs point to distinct levels of ecosystem resilience, contrasts in the organization of shorebird communities highlight the plasticity in the foraging behaviour of species inhabiting areas with distinct environmental conditions.
“…) and have also provided evidence that shorebirds substantially deplete prey stocks during winter (Ahmedou Salem et al . ). Such evidence suggests that strong interspecific competition might drive resource partitioning at Banc d'Arguin.…”
1. Food webs and trophic dynamics of coastal systems have been the focus of intense research throughout the world, as they prove to be critical in understanding ecosystem processes and functions. However, very few studies have undertaken a quantitative comparison of entire food webs from a key consumer perspective across a broad geographical area, limiting relevant comparisons among systems with distinct biotic and abiotic components. 2. We investigate the structure and functioning of food webs in four tidal ecosystems of international importance for migratory shorebirds along the East Atlantic Flyway: Tejo estuary in Portugal, Sidi Moussa in Morocco, Banc d'Arguin in Mauritania and Bijag os archipelago in Guinea-Bissau. Basal food sources, shorebirds and their prey (benthic invertebrates) were sampled in all areas, and Bayesian stable isotope mixing models and community-wide metrics were used in a comparative analysis among areas. 3. Significant differences among study areas were found in the structure of food webs, as well as in the relative importance of basal resource pools supporting each food web. Overall, the food web of Banc d'Arguin was characterized by lower trophic diversity and higher functional redundancy than the other sites. This result might be explained by the low number of trophic pathways of organic matter transfer in this seagrass-dominated system which, as a fossil estuary, lacks inputs from both freshwater and nutrient-rich offshore oceanic waters. 4. Structure of shorebird communities was consistent with the main organizational patterns found for each food web, highlighting the less diverse character of the community of Banc d'Arguin. At Banc d'Arguin and Bijag os archipelago, which displayed the smallest and largest isotopic niche widths in bird assemblage, respectively, mean niche overlap among species was low, suggesting high interspecific partitioning in resource use. Tropical systems typically offer comparatively lower harvestable prey biomass for shorebirds and might thus strengthen interspecific competition, leading to low niche overlap among species. 5. Our study reveals relevant differences in the structure of food webs and shorebird communities in coastal areas along an avian flyway. While differences in trophic redundancy of food webs point to distinct levels of ecosystem resilience, contrasts in the organization of shorebird communities highlight the plasticity in the foraging behaviour of species inhabiting areas with distinct environmental conditions.
“…Furthermore, habitat quality will vary within and between years. If conditions in West Africa deteriorate throughout most wintering periods, for example through depletion of food stocks (Ahmedou Salem, Geest, Piersma, Saoud, & Gils, ), individuals staging at these sites during southward migration (Figure ) may initially encounter profitable conditions and decide to stay, but get ‘trapped’ when conditions get worse.…”
Section: Discussionmentioning
confidence: 99%
“…In these tropical areas, the relatively low harvestable prey biomass (Aharon-Rotman et al, 2016;Catry et al, 2016;Piersma et al, 1993) would result in low fuelling rates and low body masses at northward departure in a closely related High Arctic sandpiper, the Red Knot Calidris canutus (Piersma et al, 2005). Indeed, food availability during migratory fuelling has been shown to positively influence timing of migration and survival of Red Knots and Bar-tailed Godwits Limosa lapponica (e.g., Atkinson et al, 2007;Rakhimberdiev et al, 2018) and explained why less proficiently foraging juvenile shorebirds may skip northward migration (Hockey, Turpie, & Velásquez, 1998 & Gils, 2013), individuals staging at these sites during southward migration ( Figure 1) may initially encounter profitable conditions and decide to stay, but get 'trapped' when conditions get worse.…”
Evolutionary theories of seasonal migration generally assume that the costs of longer migrations are balanced by benefits at the non‐breeding destinations.
We tested, and rejected, the null hypothesis of equal survival and timing of spring migration for High Arctic breeding sanderling Calidris alba using six and eight winter destinations between 55°N and 25°S, respectively.
Annual apparent survival was considerably lower for adult birds wintering in tropical West Africa (Mauritania: 0.74 and Ghana: 0.75) than in three European sites (0.84, 0.84 and 0.87) and in subtropical Namibia (0.85). Moreover, compared with adults, second calendar‐year sanderlings in the tropics, but not in Europe, often refrained from migrating north during the first possible breeding season. During northward migration, tropical‐wintering sanderlings occurred at their final staging site in Iceland 5–15 days later than birds wintering further north or south. Namibia‐wintering sanderlings tracked with solar geolocators only staged in West Africa during southward migration.
The low annual survival, the later age of first northward migration and the later passage through Iceland during northward migration of tropical‐wintering sanderlings, in addition to the skipping of this area during northward but not southward migration by Namibia‐wintering sanderlings, all suggest they face issues during the late non‐breeding season in West Africa.
Migrating sanderlings defy long distances but may end up in winter areas with poor fitness prospects. We suggest that ecological conditions in tropical West Africa make the fuelling prior to northward departure problematic.
“…This explains the negative correlation between the relative amount of Loripes in the diet and the available density of Dosinia (figure 5 a ): although the absolute rate at which Loripes was eaten was likely to be constant each year (equal to toxin constraint q ), the absolute rate at which Dosinia was eaten increased with the available Dosinia density as long as birds were not digestively constrained (this would occur at a Dosinia density of 0.6–0.7 g AFDM flesh m −2 ). A recent study showing year-round changes in Dosinia and Loripes densities also suggests that the relative contribution of Loripes to the diet of knots increased as Dosinia stocks became depleted throughout winter [39]. Figure 5.( a ) How the amount of Loripes in the diet (relative to Dosinia ) relates to the available density of Dosinia , both theoretically (TDRM) and empirically.…”
Section: Discussionmentioning
confidence: 98%
“…Knots face a trade-off between feeding on the superabundant but toxic Loripes [37] and a much less abundant but non-toxic prey, Dosinia isocardia (hereafter Dosinia ); numerically, Loripes and Dosinia together make up 75 per cent of all molluscs that are ingestible by knots [38,39] and dominate the diet of knots [40]. Knots face an additional trade-off: Loripes has a very thin shell, whereas Dosinia has a thicker armature.…”
Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d'Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.
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