Growth and flowering responses of eelgrass to simulated grazing and fecal addition by Brant Geese

Abstract

Seagrasses provide a wide range of ecosystem services and are prioritized in conservation planning. Management of this dynamic community requires describing seagrass responses to repeated grazing by dependent herbivores. Using two experimental randomized block designs in Humboldt Bay, California, we tested responses of eelgrass (Zostera marina) to separate and combined effects of simulated Brant Goose (Branta bernicla) grazing (clipping to a uniform height) and fecal pellet addition, and then to medium (MED) and severe (SEV) intensities of simulated Brant flock visitation. In Experiment 1, clipping with fecal addition (FC) stimulated more clonal addition of shoots than controls or clipping and fecal addition alone (standardized β = 0.26 relative to control). The FC treatment also had the largest positive, delayed effect (β = 0.135) on density of flowering shoots. In Experiment 2, the MED intensity treatment had positive effects on shoot density (β = 0.149), rates of leaf extension (β = 0.16), and leaf productivity (β = 0.20). These MED treatment responses resulted in the highest measures of above-ground (β = 0.099) and below-ground (β = 0.32) biomass. Whole shoot (β MED = 0.33, β SEV = −0.36) and landscape (β MED = 0.34, β SEV = −0.23) levels of productivity for the MED treatment reflected the parabolic shape predicted by the compensatory regrowth hypothesis, likely because light and nutrient resources were available to the eelgrass and meristems that were not damaged. Meristem injury in the SEV treatment likely explained why productivity was low, but the high SEV resource levels allowed these plants to recover to control levels in approximately eight to 10 weeks after the last treatment applications. The MED level of shoot and landscape productivity would optimize low intertidal accessibility for Brant to the most nutritiously valuable leaves. Brant grazing and detrital pathways of carbon flow are likely linked with Brant-induced productivity contributing to the detrital pathway when Brant are present and after the birds leave the estuary. The effect of Brant grazing on eelgrass sexual reproduction may increase long-term resilience of estuarine ecosystems.

Disturbance decreases genotypic diversity by reducing colonization: Implications for disturbance–diversity feedbacks

“…This result also contrasts with previous studies suggesting that physical disturbance increases eelgrass genotypic diversity by facilitating seedling recruitment (Foster et al, 2021;Reusch, 2006;Zipperle et al, 2010). The reductions in flowering effort we observed in clipped treatments (see also Shaughnessy et al [2021]) might partially explain reductions in colonization if the colonizing genets are new seedlings, especially given that contributions to the seed bank come from both local and distant sources (Furman et al, 2015;Harwell & Orth, 2002;Ruckelshaus, 1998;Zipperle et al, 2010). However, the magnitude of local seed production is likely not the only process influencing the number of genets gained in our experiment.…”

“…Within a localized area in the meadow, the colonization of new genets may arise from seedling recruitment (reviewed in Kendrick et al, 2012), clonal encroachment from genets outside the patch, the reestablishment of vegetative ramets dislodged from other locations (present study), and potentially somatic mutations (Yu et al, 2020). Previous studies predicted that disturbance, such as grazing by waterfowl, could alter Zostera genetic diversity via effects on sexual reproduction (Kollars et al, 2017; Shaughnessy et al, 2021), but the magnitude of this influence depends on the disturbance intensity. For example, clipping vegetative shoots to a height of 45 cm in the late winter increased the production of flowering shoots the following spring (Shaughnessy et al, 2021), but severe disturbances that dislodge rhizomes, such as grazing by Canada geese ( Branta canadensis ), can drive perennial populations to extinction in the absence of adequate seedling recruitment (Rivers & Short, 2007).…”

“…Previous studies predicted that disturbance, such as grazing by waterfowl, could alter Zostera genetic diversity via effects on sexual reproduction (Kollars et al, 2017; Shaughnessy et al, 2021), but the magnitude of this influence depends on the disturbance intensity. For example, clipping vegetative shoots to a height of 45 cm in the late winter increased the production of flowering shoots the following spring (Shaughnessy et al, 2021), but severe disturbances that dislodge rhizomes, such as grazing by Canada geese ( Branta canadensis ), can drive perennial populations to extinction in the absence of adequate seedling recruitment (Rivers & Short, 2007).…”

Remote sensing in seagrass ecology: coupled dynamics between migratory herbivorous birds and intertidal meadows observed by satellite during four decades

“…Another type of information concerns the seagrassherbivores relationship at habitat/population level over larger time scales. This interaction is much more complex to investigate because it depends on grazing magnitude and frequency, on seagrass physiological responses with long-term effects (e.g., compensatory growth mechanisms, investment in sexual reproduction), and on indirect mechanisms associated to herbivory (e.g., soil fertilization with faeces, seed propagation, relief of competition by other vegetal species) (Shaughnessy et al, 2021;Valentine and Duffy, 2006). Interestingly, most herbivorous birds feeding on seagrass are migratory birds.…”

“…compensatory growth mechanisms, investment in sexual reproduction), and indirect mechanisms associated with herbivory (e.g. soil fertilization with faeces, seed propagation, relief of competition by other vegetal species) (Shaughnessy et al, 2021;Valentine & Duffy, 2006). Interestingly, most herbivorous birds feeding on seagrass are migratory birds.…”