The Seagrass Effect Turned Upside Down Changes the Prospective of Sea Urchin Survival and Landscape Implications

Abstract

Habitat structure plays an important mediating role in predator-prey interactions. However the effects are strongly dependent on regional predator pools, which can drive predation risk in habitats with very similar structure in opposite directions. In the Mediterranean Sea predation on juvenile sea urchins is commonly known to be regulated by seagrass structure. In this study we test whether the possibility for juvenile Paracentrotus lividus to be predated changes in relation to the fragmentation of the seagrass Posidonia oceanica (four habitat classes: continuous, low-fragmentation, high-fragmentation and rocks), and to the spatial arrangement of such habitat classes at a landscape scale. Sea urchin predation risk was measured in a 20-day field experiment on tethered individuals placed in three square areas 35×35 m2 in size. Variability of both landscape and habitat structural attributes was assessed at the sampling grain 5×5 m2. Predation risk changed among landscapes, as it was lower where more ‘rocks’, and thus less seagrass, were present. The higher risk was found in the ‘continuous’ P. oceanica rather than in the low-fragmentation, high-fragmentation and rock habitats (p-values = 0.0149, 0.00008, and 0.0001, respectively). Therefore, the expectation that juvenile P. lividus survival would have been higher in the ‘continuous’ seagrass habitat, which would have served as shelter from high fish predation pressure, was not met. Predation risk changed across habitats due to different success between attack types: benthic attacks (mostly from whelks) were overall much more effective than those due to fish activity, the former type being associated with the ‘continuous’ seagrass habitat. Fish predation on juvenile sea urchins on rocks and ‘high-fragmentation’ habitat was less likely than benthic predation in the ‘continuous’ seagrass, with the low seagrass patch complexity increasing benthic activity. Future research should be aimed at investigating, derived from the complex indirect interactions among species, how top-down control in marine reserves can modify seagrass habitat effects.

Spatially explicit modelling of kelp-grazer interactions: a seascape approach for effective habitat enhancement using artificial reefs

“…Both between-species and species-seascape interactions can be considered as an emergent property of the heterogeneity due to the spatial configuration of the landscape (Didham, 2010). For example, meadow proximity influences the trajectory of free-ranging bison moving in a patchy forest landscape (Dancose, Fortin, & Xulin, 2011), corridors of vegetation showing low-resistance to movement ease the spread of mountain pine beetles (Powell et al, 2018), while the spatial arrangement of seagrass affected the predation on sea urchins (Farina et al, 2016). Our models included variables describing the spatial configuration of both the seascape and the artificial blocks (Table 2), however the limited spatial extent of our study or the abundance of cells containing boulders (Fig.…”

“…Despite an overall lack of seascape maps of proper resolution and the inability to track animals compared to their terrestrial counterpart, marine ecologists are increasingly considering the spatially explicit seascape configuration to better resolve the mechanisms underlying the ecological processes. For example, predation risk for Mediterranean sea urchin Paracentrotus lividus and predator identity emerged as a function of the arrangement of seagrass patches (Farina et al, 2016). Aerial imagery in clear tropical water systems allowed to show that grazing pressure on macroalgae decreases with the distance from the nearest coral reef providing shelter to fish (Gil, Zill, & Ponciano, 2017; Madin, Madin, & Booth, 2011).…”

Peer Review #1 of "The challenge of managing the commercial harvesting of the sea urchin Paracentrotus lividus: advanced approaches are required (v0.1)"

“…Farina et al (2009Farina et al ( , 2014 found high proportions of middle-sized sea urchins in patchy meadows in accordance with the efficient shelter that Posidonia oceanica leaves provide from the visual mechanism of predatory fish. However, the three-dimensional structure of large seagrass meadows can become a 'death trap' in the presence of high densities of bottom predators (Farina et al, 2014(Farina et al, , 2016Schmidt & Kuijper, 2015). For example, Sector 5, in the Gulf of Oristano, is PeerJ reviewing PDF | (2020:03:47281:2:0:NEW 3 Sep 2020)…”

“…), which is known as effective predators of sea urchins (e.g. Farina et al, 2016) and whose proliferation is probably favoured by the bio-deposits that accumulate beneath the nearby mussel farms (Inglis & Gust, 2003).…”

Generation and maintenance of predation hotspots of a functionally important herbivore in a patchy habitat mosaic

“…These species move relatively freely across the seascape, and are critical agents for sea urchin population control (Guidetti et al, 2004;Hereu, Zabala, Linares, & Sala, 2005). While other bottom-associated species also prey on urchins, their effect on urchin populations is relatively minor (Sala & Zabala, 1996; but see Farina et al, 2016).…”