Meta-Analysis of Reciprocal Linkages between Temperate Seagrasses and Waterfowl with Implications for Conservation

Abstract

Multi-trophic conservation and management strategies may be necessary if reciprocal linkages between primary producers and their consumers are strong. While herbivory on aquatic plants is well-studied, direct top-down control of seagrass populations has received comparatively little attention, particularly in temperate regions. Herein, we used qualitative and meta-analytic approaches to assess the scope and consequences of avian (primarily waterfowl) herbivory on temperate seagrasses of the genus Zostera. Meta-analyses revealed widespread evidence of spatio-temporal correlations between Zostera and waterfowl abundances as well as strong top-down effects of grazing on Zostera. We also documented the identity and diversity of avian species reported to consume Zostera and qualitatively assessed their potential to exert top-down control. Our results demonstrate that Zostera and their avian herbivores are ecologically linked and we suggest that bird herbivory may influence the spatial structure, composition, and functioning of the seagrass ecosystem. Therefore, the consequences of avian herbivory should be considered in the management of seagrass populations. Of particular concern are instances of seagrass overgrazing by waterfowl which result in long-term reductions in seagrass biomass or coverage, with subsequent impacts on local populations of waterfowl and other seagrass-affiliated species. While our results showed that bird density and type may affect the magnitude of the top-down effects of avian herbivory, empirical research on the strength, context-dependency, and indirect effects of waterfowl–Zostera interactions remains limited. For example, increased efforts that explicitly measure the effects of different functional groups of birds on seagrass abundance and/or document how climate change-driven shifts in waterfowl migratory patterns impact seagrass phenology and population structure will advance research programs for both ecologists and managers concerned with the joint conservation of both seagrasses and their avian herbivores.

High Grazing Pressure of Geese Threatens Conservation and Restoration of Reed Belts

“…Recently, the role of herbivores in limiting the abundance of aquatic plants has been highlighted and quantified (Bakker et al, 2016a , b ; Kollars et al, 2017 ; Wood et al, 2017 ). Aquatic herbivores remove on average 40–48% of vascular plant biomass in freshwater and marine ecosystems, which is typically 5–10 times greater than reported for terrestrial ecosystems (Bakker et al, 2016a ).…”

“…In marine ecosystems, the disappearance of large predators and the creation of marine protected reserves has locally resulted in high grazing pressure on seagrass beds by green sea turtles ( Chelonia mydas L.) (Christianen et al, 2014 ). Similarly, in temperate regions, overgrazing by waterfowl may threaten the existence of seagrass beds (Kollars et al, 2017 ). Therefore, the conservation dilemma that we describe for the protection of reed beds likely exists for the conservation of multiple types of aquatic plants.…”

Latitudinal variation in plant defence against herbivory in a marine foundation species does not follow a linear pattern: The importance of resource availability

“…We tested these predictions by performing a simulated herbivory experiment with the seagrass Zostera marina. Zostera marina is a dominant foundation species in estuaries and coastal areas across the Northern Hemisphere and is consumed by a wide variety of grazers including birds and small invertebrates (e.g., amphipods, isopods), which can strongly influence eelgrass abundance and distribution (Kollars et al, 2017;Reynolds et al, 2012;Tomas et al, 2011).…”

Spatial patterns and predictor variables vary among different types of primary producers and consumers in eelgrass (Zostera marina) beds

“…The amount of production by epiphytic and benthic algae sometimes exceeds the production by seagrass [ 8 ]. Consumers in seagrass beds are also diverse and consist of small invertebrates such as gastropods, amphipods, shrimps, and annelids, and some vertebrates such as rabbitfish, green sea turtles, manatees and waterfowl [ 12 , 13 ]. The invertebrates can be categorized as epifauna or infauna depending on where they inhabit the seagrass beds, and they utilize the food sources and habitats provided by the primary producers, as most of the invertebrates are grazers and detritivores [ 14 ].…”

“…There have been many studies that have examined the variations in primary producers and consumers in seagrass beds [ 15 , 16 ]. Investigations have been conducted on plant-plant interactions, such as the competition for light and nutrients between seagrass and algae [ 17 ], and plant-animal interactions, such as the control of plant abundance by herbivores [ 12 , 18 ]. For epifaunal consumers such as invertebrate herbivores, the bottom-up effects from plants on animals have been investigated, and these investigations have focused on the relationships among the abundances of associated animals, microalgae and seagrass (e.g., [ 19 – 21 ]).…”