A food web including parasites for kelp forests of the Santa Barbara Channel, California

Understanding diversity-synchrony-stability relationships in multitrophic communities

Understanding how species loss impacts ecosystem stability is critical given contemporary declines in global biodiversity. Despite decades of research on biodiversity-stability relationships, most studies are performed within a trophic level, overlooking the multitrophic complexity structuring natural communities. Here, in a global analysis of diversity-synchrony-stability (DSS) studies (n = 420), we found that 74% were monotrophic and biased towards terrestrial plant communities, with 91% describing stabilizing effects of asynchrony. Multitrophic studies (26%) were representative of all biomes and showed that synchrony had mixed effects on stability. To explore potential mechanisms, we applied a multitrophic framework adapted from DSS theory to investigate DSS relationships in algae-herbivore assemblages across five long-term tropical and temperate marine system datasets. Both algal and herbivore species diversity reduced within-group synchrony in both systems but had different interactive effects on species synchrony between systems. Herbivore synchrony was positively and negatively influenced by algal diversity in tropical versus temperate systems, respectively, and algal synchrony was positively influenced by herbivore diversity in temperate systems. While herbivore synchrony reduced multitrophic stability in both systems, algal synchrony only reduced stability in tropical systems. These results highlight the complexity of DSS relationships at the multitrophic level and emphasize why more multitrophic assessments are needed to better understand how biodiversity influences community stability in nature.

Effects of protection and temperature variation on temporal stability in a marine reserve network

Understanding the drivers of ecosystem stability has been a key focus of modern ecology as the impacts of the Anthropocene become more prevalent and extreme. Marine protected areas (MPAs) are tools used globally to promote biodiversity and mediate anthropogenic impacts. However, assessing the stability of natural ecosystems and responses to management actions is inherently challenging due to the complex dynamics of communities with many interdependent taxa. Using a 12-year time series of subtidal community structure in an MPA network in the Channel Islands (United States), we estimated species interaction strength (competition and predation), prey species synchrony, and temporal stability in trophic networks, as well as temporal variation in sea surface temperature to explore the causal drivers of temporal stability at community and metacommunity scales. At the community scale, only trophic networks in MPAs at Santa Rosa Island showed greater temporal stability than reference sites, likely driven by reduced prey synchrony. Across islands, competition was sometimes greater and predation always greater in MPAs compared with reference sites. Increases in interaction strength resulted in lower temporal stability of trophic networks. Although MPAs reduced prey synchrony at the metacommunity scale, reductions were insufficient to stabilize trophic networks. In contrast, temporal variation in sea surface temperature had strong positive direct effects on stability at the regional scale and indirect effects at the local scale through reductions in species interaction strength. Although MPAs can be effective management strategies for protecting certain species or locations, our findings for this MPA network suggest that temperature variation has a stronger influence on metacommunity temporal stability by mediating species interactions and promoting a mosaic of spatiotemporal variation in community structure of trophic networks. By capturing the full spectrum of environmental variation in network planning, MPAs will have the greatest capacity to promote ecosystem stability in response to climate change.

Consumption of trematode parasite infectious stages: from conceptual synthesis to future research agenda

Given their sheer cumulative biomass and ubiquitous presence, parasites are increasingly recognized as essential components of most food webs. Beyond their influence as consumers of host tissue, many parasites also have free-living infectious stages that may be ingested by non-host organisms, with implications for energy and nutrient transfer, as well as for pathogen transmission and infectious disease dynamics. This has been particularly well-documented for the cercaria free-living stage of digenean trematode parasites within the Phylum Platyhelminthes. Here, we aim to synthesize the current state of knowledge regarding cercariae consumption by examining: (a) approaches for studying cercariae consumption; (b) the range of consumers and trematode prey documented thus far; (c) factors influencing the likelihood of cercariae consumption; (d) consequences of cercariae consumption for individual predators (e.g. their viability as a food source); and (e) implications of cercariae consumption for entire communities and ecosystems (e.g. transmission, nutrient cycling and influences on other prey). We detected 121 unique consumer-by-cercaria combinations that spanned 60 species of consumer and 35 trematode species. Meaningful reductions in transmission were seen for 31 of 36 combinations that considered this; however, separate studies with the same cercaria and consumer sometimes showed different results. Along with addressing knowledge gaps and suggesting future research directions, we highlight how the conceptual and empirical approaches discussed here for consumption of cercariae are relevant for the infectious stages of other parasites and pathogens, illustrating the use of cercariae as a model system to help advance our knowledge regarding the general importance of parasite consumption.

The Blob marine heatwave transforms California kelp forest ecosystems

Ocean warming has both direct physiological and indirect ecological consequences for marine organisms. Sessile animals may be particularly vulnerable to anomalous warming given constraints in food acquisition and reproduction imposed by sessility. In temperate reef ecosystems, sessile suspension feeding invertebrates provide food for an array of mobile species and act as a critical trophic link between the plankton and the benthos. Using 14 years of seasonal benthic community data across five coastal reefs, we evaluated how communities of sessile invertebrates in southern California kelp forests responded to the "Blob", a period of anomalously high temperatures and low phytoplankton production. We show that this event had prolonged consequences for kelp forest ecosystems. Changes to community structure, including species invasions, have persisted six years post-Blob, suggesting that a climate-driven shift in California kelp forests is underway.

Habitat associations and impacts on a juvenile fish host by a temperate gnathiid isopod

The distribution and abundance of organisms is typically shaped by multiple biotic and abiotic processes. Micropredators are parasite-like organisms that are smaller than their hosts and/or prey and feed on multiple hosts during a given life stage. Unlike typical parasites, however, they spend much or most of their time free-living, associating only temporarily with hosts. In the ocean, micropredators can impact multiple fish species, and in particular can have significant lethal and sub-lethal effects on newly settled fish. Although gnathiid isopods are abundant and primary micropredators in coral reef ecosystems, their impacts are relatively unexplored within sub-tidal temperate rocky reefs. We investigated the distribution of juvenile gnathiid isopods along sub-tidal temperate rocky reefs and tested trap methodology. We also quantified both the sub-lethal and lethal impacts of feeding-stage juvenile gnathiid isopods on juvenile, post-settlement reef fish, Heterostichus rostratus (giant kelpfish). We were most interested in determining the relationship between gnathiid infestation level and fish swimming performance, in particular swimming metrics relevant to predator avoidance maneuvers. We found that Gnathia tridens was present in rocky reefs rather than embayments along the Southern California coastline and that within rocky reefs, gnathiids occurred in the highest densities in lighted traps. Surprisingly, we observed almost no influence of fish size or gnathiid sub-lethal infestation level on ambient or burst swimming performance metrics. However, burst duration was reduced by gnathiid infestation, which is important in predator avoidance. There were significant differences in survivorship among small fish compared to large fish as a result of gnathiid infestation. Larger fish survived higher numbers of gnathiids than smaller fish, indicating that parasite-induced mortality is greater for smaller fish. Investigations of the effects of micropredators on subsequent predator-mediated mortality, including the susceptibility of fishes and their individual responses to micropredators, can further contribute to our understanding of processes affecting recruitment in resident reef fish populations. Further research, especially within temperate sub-tidal ecosystems, is needed to understand and highlight the overlooked importance of micropredation in shaping fish populations within a reefscape.

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Gnathiid isopods are more abundant in sub-tidal rocky reef than in bay habitats off southern California.

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Among the trap designs tested, lighted traps were most effective.

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Wave height and lunar period also impacted capture rates.

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For H. rostratus, gnathiids alter fish swimming performance at varying levels of infestation.

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Mortality rates from gnathiid infestation were inversely related to the size of fish host.