Trophic compensation stabilizes marine primary producers exposed to artificial light at night

Artificial light at night (ALAN) is a widespread phenomenon along coastal areas. Despite in - creasing evidence of pervasive effects of ALAN on patterns of species distribution and abundance, the potential of this emerging threat to alter ecological processes in marine ecosystems has remained largely unexplored. Here, we show how exposure to white LED lighting, comparable to that experienced along local urbanized coasts, significantly enhanced the impact of grazing gastropods on epilithic microphytobenthos (MPB). ALAN increased both the photo-synthetic biomass of MPB and the grazing pressure of gastropods, such that consumers compensated for the positive effect of night lighting on primary producers. Our results indicate that trophic interactions can provide a stabilizing compensatory mechanism against ALAN effects in natural food webs

Spatial correlation reverses the compound effect of multiple stressors on rocky shore biofilm

Understanding how multifactorial fluctuating environments affect species and communities remains one of the major challenges in ecology. The spatial configuration of the environment is known to generate complex patterns of correlation among multiple stressors. However, to what extent the spatial correlation between simultaneously fluctuating variables affects ecological assemblages in real-world conditions remains poorly understood. Here, we use field experiments and simulations to assess the influence of spatial correlation of two relevant climate variables - warming and sediment deposition following heavy precipitation - on the biomass and photosynthetic activity of rocky intertidal biofilm. First, we used a response-surface design experiment to establish the relation between biofilm, warming, and sediment deposition in the field. Second, we used the response surface to generate predictions of biofilm performance under different scenarios of warming and sediment correlation. Finally, we tested the predicted outcomes by manipulating the degree of correlation between the two climate variables in a second field experiment. Simulations stemming from the experimentally derived response surface showed how the degree and direction (positive or negative) of spatial correlation between warming and sediment deposition ultimately determined the nonlinear response of biofilm biomass (but not photosynthetic activity) to fluctuating levels of the two climate variables. Experimental results corroborated these predictions, probing the buffering effect of negative spatial correlation against extreme levels of warming and sediment deposition. Together, these results indicate that consideration of nonlinear response functions and local-scale patterns of correlation between climate drivers can improve our understanding and ability to predict ecological responses to multiple processes in heterogeneous environments

Ecological feedback mechanisms and variable disturbance regimes: the uncertain future of Mediterranean macroalgal forests

Loss of algal canopies can result in a shift towards a turf-dominated state, where variability in species life-history traits can determine new mechanisms of feedback, and influence the degraded system under variable regimes of disturbance. By focusing on rockpools dominated by Cystoseira brachycarpa, we tested the hypothesis that the alga Dictyopteris polypodioides could take advantage of extreme regimes of disturbance related to storms, and outcompete other turfs through a distinctive combination of life traits. Replacement of the canopy was initially driven by a mix of taxon-specific life-traits and resulting assemblages were susceptible to intense events of disturbance. Subsequently, D. polypodioides dominated removal quadrats, favored by density-dependent abilities to intercept more light and reach larger size than the rest of turf. These new positive feedbacks may contribute to maintain the modified state of the system and influence its ability to withstand extreme abiotic conditions

Hybrid datasets: integrating observations with experiments in the era of macroecology and big-data

Understanding how increasing human domination of the biosphere affects life on earth is a critical research challenge. This task is facilitated by the increasing availability of open-source data repositories, which allow ecologists to address scientific questions at unprecedented spatial and temporal scales. Large datasets are mostly observational, so they may have limited ability to uncover causal relations among variables. Experiments are better suited at attributing causation, but they are often limited in scope. We propose hybrid datasets, resulting from the integration of observational with experimental data, as an approach to leverage the scope and ability to attribute causality in ecological studies. We show how the analysis of hybrid datasets with emerging techniques in time series analysis (Convergent Cross Mapping) and macroecology (Joint Species Distribution Models) can generate novel insights into causal effects of abiotic and biotic processes that would be difficult to achieve otherwise. We illustrate these principles with two case-studies in marine ecosystems and discuss the potential to generalize across environments, species and ecological processes. If used wisely, the analysis of hybrid datasets may become the standard approach for research goals that seek causal explanations for large-scale ecological phenomena. This article is protected by copyright. All rights reserved

Temporal clustering of extreme climate events drives a regime shift in rocky intertidal biofilms

Research on regime shifts has focused primarily on how changes in the intensity and duration of press disturbances precipitate natural systems into undesirable, alternative states. By contrast, the role of recurrent pulse perturbations, such as extreme climatic events, has been largely neglected, hindering our understanding of how historical processes regulate the onset of a regime shift. We performed field manipulations to evaluate whether combinations of extreme events of temperature and sediment deposition that differed in their degree of temporal clustering generated alternative states in rocky intertidal epilithic microphytobenthos (biofilms) on rocky shores. The likelihood of biofilms to shift from a vegetated to a bare state depended on the degree of temporal clustering of events, with biofilm biomass showing both states under a regime of non-clustered (60 d apart) perturbations while collapsing in the clustered (15 d apart) scenario. Our results indicate that time since the last perturbation can be an important predictor of collapse in systems exhibiting alternative states and that consideration of historical effects in studies of regime shifts may largely improve our understanding of ecosystem dynamics under climate change

Experimental evidence of spatial signatures of approaching regime shifts in macroalgal canopies

Developing early warning signals to predict regime shifts in ecosystems is a central issue in current ecological research. While there are many studies addressing temporal early warning indicators, research into spatial indicators is far behind, with field experiments even more rare. Here, we tested the performance of spatial early warning signals in an intertidal macroalgal system, where removal of algal canopies pushed the system toward a tipping point (corresponding to approximately 75% of canopy loss), marking the transition between a canopy- to a turf-dominated state. We performed a two-year experiment where spatial early warning indicators were assessed in transects where the canopy was differentially removed (from 0 to 100%). Unlike Moran correlation coefficient at lag-1, spatial variance, skewness, and spatial spectra at low frequency increased along the gradient of canopy degradation and dropped, or did not show any further increase beyond the transition point from a canopy- to a turf-dominated state (100% canopy removal). Our study provides direct evidence of the suitability of spatial early warning signals to anticipate regime shifts in natural ecosystems, emphasizing the importance of field experiments as a powerful tool to establish causal relationships between environmental stressors and early warning indicators

Connell and Slatyer's models of succession in the biodiversity era

Understanding how species interactions drive succession is a key issue in ecology. In this study we show the utility of combining the concepts and methodologies developed within the biodiversity–ecosystem functioning research program with J. H. Connell and R. O. Slatyer’s classic framework to understand succession in assemblages where multiple interactions between early and late colonists may include both inhibitory and facilitative effects. We assessed the net effect of multiple species interactions on successional changes by manipulating the richness, composition, and abundance of early colonists in a low-shore assemblage of algae and invertebrates of the northwestern Mediterranean. Results revealed how concomitant changes in species richness and abundance can strongly alter the net effect of inhibitory vs. facilitative interactions on succession. Increasing richness of early colonists inhibited succession, but only under high levels of initial abundance, probably reflecting the formation of a highly intricate matrix that prevented further colonization. In contrast, increasing initial abundance of early colonists tended to facilitate succession under low richness. Thus, changes in abundance of early colonists mediated the effects of richness on succession

Subtle differences in growth rate drive contrasting responses of ephemeral primary producers to recurrent disturbances

Although the importance of time after disturbance is well established in the ecological literature, studies examining how differences in growth rate affect species recovery and persistence in relation to the interval between recurrent perturbations are rare. We examined the response of two ephemeral primary producers inhabiting high-shore rock pools, epilithic microphytobenthos (EMPB), and green filamentous algae, to disturbance regimes varying for the time interval between consecutive events. Informed from an empirically parametrized growth model's outcomes, we tested the hypothesis that EMPB would be able to recover from more frequent disturbance compared with filamentous algae in a field experiment involving three physical disturbance patterns differing for the clustering degree: high, moderate, and low (20, 40, and 80 days between disturbances). We predicted that: high clustering would prevent the recovery of both taxa; moderate clustering would prevent the recovery of the slower growing taxon only (filamentous algae); both taxa would recover under low clustering. Results showed that EMPB persisted independently of the clustering degree, whereas filamentous algae did not withstand any disturbance regime. Dramatically different effects of disturbance on organisms with subtle differences in their growth rate indicate that even stronger responses may be expected from taxa with more markedly contrasting life histories

Effects of grazer diversity on marine microphytobenthic biofilm: a ‘tug of war’ between complementarity and competition

Species loss is one of the most striking problems related to human-driven environmental changes. Nevertheless, biodiversity and ecosystem functioning experiments have mainly focused on primary producers, paying less attention to the consequences of changing diversity at higher trophic levels. We performed a field experiment using cage enclosures to test the effects of species richness, identity and density of gastropod grazers on the photosynthetic efficiency and biomass of intertidal biofilm on an exposed rocky shore in the northwest Mediterranean. The diversity and composition of intertidal grazers affected the photosynthetic efficiency of biofilm with only negligible effects on biomass. Individual species showed strong identity effects. In assemblages of 2 or more species, positive or negative complementarity effects occurred. The magnitude of the ecosystem response is expected to depend on the particular species assemblage and its density, which will determine whether niche partitioning or competition is the prevailing process. Grazer preference in specific components of biofilm, characterized by different photosynthetic efficiency and competitive abilities, might explain concomitant changes in photosynthetic efficiency and comparable levels in biomass among treatments. The effects of grazers declined following the natural trend of decreasing biomass of biofilm during the study period, highlighting the importance of considering temporal variability in the effects of biodiversity on ecosystem functioning. This work emphasizes the key role of species identity to predict effects on their resources and ecosystem functioning.This work was partially supported by the University of Pisa