Distribution and abundance of the acmaeid limpet, Patelloida latistrigata, and its interaction with barnacles

Nutrient conditions determine the strength of herbivore‐mediated stabilizing feedbacks in barrens

Abiotic environmental conditions can significantly influence the way species interact. In particular, plant–herbivore interactions can be substantially dependent on temperature and nutrients. The overall product of these relationships is critical for the fate and stability of vegetated ecosystems like marine forests. The last few decades have seen a rapid spread of barrens on temperate rocky reefs mainly as a result of overgrazing. The ecological feedbacks that characterize the barren state involve a different set of interactions than those occurring in vegetated habitats. Reversing these trends requires a proper understanding of the novel feedbacks and the conditions under which they operate. Here, we explored the role of a secondary herbivore in reinforcing the stability of barrens formed by sea urchin overgrazing under different nutrient conditions. Combining comparative and experimental studies in two Mediterranean regions characterized by contrasting nutrient conditions, we assessed: (i) if the creation of barren areas enhances limpet abundance, (ii) the size‐specific grazing impact by limpets, and (iii) the ability of limpets alone to maintain barrens. Our results show that urchin overgrazing enhanced limpet abundance. The effects of limpet grazing varied with nutrient conditions, being up to five times more intense under oligotrophic conditions. Limpets were able to maintain barrens in the absence of sea urchins only under low‐nutrient conditions, enhancing the stability of the depauperate state. Overall, our study suggests a greater vulnerability of subtidal forests in oligotrophic regions of the Mediterranean and demonstrates the importance of environment conditions in regulating feedbacks mediated by plant–herbivore interactions.

Ecological and environmental context shape the differential effects of a facilitator in its native and invaded ranges

Invasive species often exhibit disproportionately strong negative effects in their introduced range compared to their native range, and much research has been devoted to understanding the role of shared evolutionary history, or lack thereof, in driving these differences. Less studied is whether introduced species, particularly those that are important as facilitators in their native range, have persistent positive effects in their invaded range despite a lack of a shared evolutionary history with the invaded community. Here, we manipulated the density of a habitat-forming facilitator, the high intertidal acorn barnacle Balanus glandula, factorially with herbivore density in its native range (Bluestone Point, British Columbia, Canada) and invaded range (Punta Ameghino, Chubut Province, Argentina) to determine how this facilitator differentially affects associated species at these two locations. Given that high intertidal species at Punta Ameghino (PA) are evolutionarily naïve to barnacles, we predicted that the positive effects of B. glandula at PA would be absent or weak compared to those at Bluestone Point (BP). However, we found that B. glandula had an equally positive effect on herbivore biomass at PA compared to BP, possibly because the moisture-retaining properties of barnacle bed habitats are particularly important in seasonally dry Patagonia. Barnacle presence indirectly decreased ephemeral algal cover at BP by increasing grazer pressure, but barnacles instead facilitated ephemeral algae at PA. In contrast, B. glandula increased perennial algal cover at BP, but generally decreased perennial algal cover at PA, likely due to differences in dominant algal morphology. Though our experiment was limited to one location on each continent, our results suggest that shared evolutionary history may not be a prerequisite for strong facilitation to occur, but rather that the nature and strength of novel species interactions are determined by the traits of associated species and the environment in which they occur.

Bioerosive and bioprotective role of barnacles on rocky shores

Bioerosion and bioprotection (bio-remodeling) is the action exerted by biota colonizing rocky shores. It represents an important component among processes responsible for shaping coastal landforms, and a clear evidence of interaction between the biosphere and the solid earth. Barnacles extensively colonize the midlittoral belt of rocky shores in the Mediterranean Basin. Previous research, mostly based on laboratory evidence, suggests that barnacles are bioprotectors, in that they protect the rock surface from different types of physical and chemical weathering. In this paper, we present the results of a field experiment carried out at different spatial scales at two study areas along the moderately energetic and microtidal coast of NW Italy. Barnacles were removed from the sandstone bedrock in replicated plots (manipulated plots) arranged according to a hierarchical spatial design. After four months rock hardness was tested on each plot with both Schmidt hammer and Equotip Piccolo devices, as well as on a corresponding number of control plots. Data were processed by means of a multifactorial analysis of variance (ANOVA). In control plots, rock hardness tested with Schmidt hammer exceeded that measured in previously manipulated plots. Testing with Equotip yielded the opposite results. This experimental evidence confirmed that barnacles play a bio-protective role in the midlittoral at sub-surficial level, while adding the key aspect that this effect is generalizable to spatial scales ranging from a few centimeters up to tens of kilometers. In addition, our results showed, for the first time, that at surface level they can simultaneously act as bioeroders, likely causing corrosion of the rock surface by fostering dissolution of the sandstone carbonate matrix.

Trying to fit in: are patterns of orientation of a keystone grazer set by behavioural responses to ecosystem engineers or wave action?

The distribution of animals varies at different temporal and spatial scales. At the smallest scale, distribution may be orientated in regard to particular environmental variables or habitat features. For animals on the rocky intertidal, the processes which set and maintain patterns of distribution and abundance in wave-exposed areas are well studied, with explanatory models focused on wave action and, more recently, the role of biogenic habitats. In contrast, patterns of orientation by intertidal animals have received less attention, although having ecological and fitness consequences. Here, we report tests of competing models to explain the observation that limpets on steeply sloped surfaces orientate downwards. A greater proportion of downwards-facing limpets was found in sheltered sites and areas without barnacles and this pattern was consistent across many shores and sampling occasions. Additionally, the frequency at which limpets were dislodged after a storm was independent of orientation. To test whether orientation is a behavioural response to habitat-forming barnacles, barnacles were removed and/or killed from patches of substrata and the change in proportion of downwards-facing limpets measured. The proportion increased with barnacle removal and this behaviour was a response to the structure of the barnacles, not a biotic effect associated with the living organism. Our study suggests that biogenic habitat not wave action sets patterns of limpet orientation and barnacle shells, regardless of whether the barnacle is alive or not, limit the ability of limpets to adopt a downward orientation.

Urban structures provide new insights into interactions among grazers and habitat

A major challenge in ecology is to understand and predict consequences of environmental changes to biological assemblages. Urbanization and associated alteration and destruction of habitat cause profound changes in local biodiversity and the ecology of and interactions among organisms. This study tested hypotheses about interactions among intertidal species on urbanized shores to examine predictions from their known ecology. On natural shores, grazing limpets, Cellana tramoserica, outcompete the smaller limpets, Patelloida latistrigata. The latter shelter among barnacles. On seawalls in Sydney Harbour (Australia), P. latistrigata were found among and on oysters. C. tramoserica were found directly on walls where there were no oysters. To explain these observations, several models were proposed and tested by manipulative experiments in the field: (1) oysters provide habitat for P. latistrigata; (2) negative interactions with C. tramoserica cause P. latistrigata to move on and among oysters; and (3) in areas with oysters, space available for grazing is insufficient for C. tramoserica to survive well. The results showed that C. tramoserica had a negative impact on the survival of P. latistrigata, although not causing the latter to move onto oysters. The oysters directly and indirectly affected P. latistrigata. First, oysters provided habitat and increased survival of P. latistrigata. Second, oysters influenced the interaction between C. tramoserica and P. latistrigata. This "interaction modification" diminished the negative impact of C. tramoserica. The material used to construct a wall did not directly influence the distribution of the limpets. Maintaining oysters on seawalls is, however, important because of their direct and indirect effects on P. latistrigata. It was possible to predict some processes on urbanized shores from known ecology. Other processes could not be predicted, making it necessary to do experiments to understand how built structures influence biological assemblages. This research contributes to understanding how to conserve biodiversity in urban areas.

Quantification of radular marks as a method for estimating grazing of intertidal gastropods on rocky shores

Wax discs have been used previously on intertidal rocky shores to record the grazing activity of gastropods. This study has evaluated this methodology for recording grazing of four common intertidal microalgal grazers on intertidal shores in New South Wales, Australia. In the laboratory, the four species examined-the patellid limpet, Cellana tramoserica (Sowerby), the trochid, Austrocochlea porcata (A. Adams), the neritid, Nerita atramentosa Reeve and the littorinid, Bembicium nanum (Lamarck)-made distinctive marks in the wax. These allowed identification of each species or combinations of species grazing over the different discs. Field experiments showed that the intensity of grazing, as indicated by the mean number of scratches per disc, was positively related to the number of gastropods in the surrounding area during low tide for C. tramoserica. The number of scratches per disc in any area was correlated with the percentage of discs scratched. The relationship for C. tramoserica was found at two scales-in sites (approximately 3x3 m) and also in plots (50x50 cm) within sites. Therefore, densities that were measured when these limpets were inactive during low tide provided good estimates of grazing activity during high tide. This is largely because these limpets do not move far between where they rest and where they feed. The amount of microalgal food in the vicinity was not correlated with density, nor with grazing intensity. No relationship between density and grazing intensity was found for N. atramentosa, although experiments were only done in the field at one spatial scale (in sites, 3x3 m). Results obtained in the laboratory and in the field show that wax discs are useful to distinguish grazing by different species of gastropods on Australian rocky shores and allow tests of hypotheses about grazing activity at different spatial scales.

Grazing patterns in Siphonaria gigas (Mollusca, Pulmonata) on the rocky Pacific coast of Panama

The pulmonate limpet Siphonaria gigas is the most abundant molluscan grazer in the mid zone on rocky, wave-exposed shores of the Pacific coast of Panama. Erect macroalgae and sessile invertebrates are rare; crustose algae cover ∼90% of the rock. The relative abundance of a common blue-green algal crust (Schizothrix calcicola?) is negatively correlated with Siphonaria's abundance. Large-scale removals of the limpet cause rapid increases in percent cover of Schizothrix and concomitant decreases in other crusts, but no changes in the abundance of erect algae or sessile invertebrates. Removing Siphonaria also (1) increases recruitment of crustose algae and barnacles onto new rock and plexiglass substrata, and (2) decreases the abundance of a calcified form of Schizothrix.Harsh conditions during daytime low tides and foraging by fishes at high tide control the microdistribution of most of this region's mobile and sessile benthic organisms. Wave action and substratum heterogeneity modify these constraints: Siphonaria is rare or absent in sheltered areas, especially on homogeneous surfaces, and is most abundant at wave-exposed sites. However, at extremely wave-beaten sites, Siphonaria and other benthic consumers are rare and ineffective. Crustose algae are reduced in abundance and space is dominated by erect macroalgae and/or barnacles. These normally rare species can outcompete crusts only when thermal or desiccation stress and the effects of benthic consumers and fishes are drastically reduced.