The consequences of mass mortality events for the structure and dynamics of biological communities

Fjord oceanographic dynamics provide refuge for critically endangered Pycnopodia helianthoides

Disease outbreaks as a driver of wildlife mass mortality events have increased in magnitude and frequency since the 1940s. Remnant populations, composed of individuals that survived mass mortality events, could provide insight into disease dynamics and species recovery. The sea star wasting disease (SSWD) epidemic led to the rapid >90% decline of the sunflower star Pycnopodia helianthoides. We surveyed the biomass density of P. helianthoides on the central British Columbia coast before, during and after the arrival of SSWD by conducting expert diver surveys in shallow subtidal habitats from 2013 to 2023. We found a rapid decline in biomass density following the onset of SSWD in 2015. Despite consistent recruitment post-outbreak to sites associated with outer islands, we found repeated loss of large adult individuals over multiple years. Within nearby fjord habitats, we found remnant populations composed of large adult P. helianthoides. The interaction of temperature and salinity with the biomass density of P. helianthoides varied by location, with high biomass density associated with higher temperatures in the outer islands and with lower temperatures and higher salinity in the fjords. These patterns suggest that fjords provide refuge from consequences of SSWD and protecting these populations could be imperative for the species.

Catastrophic fish mass mortality events in Moroccan freshwater ecosystems: alarming trends and impacts on biodiversity

Fish mass mortality events (FMMEs) represent an escalating ecological crisis, significantly threatening aquatic biodiversity, particularly in North African freshwater ecosystems. Addressing a critical knowledge gap in this region, our study presents the first comprehensive assessment of FMMEs in Moroccan aquatic ecosystems, including freshwater systems and estuaries, based on meticulous monitoring from January 2020 to December 2022. During this three-year period, we documented 18 FMMEs across 16 distinct ecosystems, with a notable increase in frequency observed during the summer and autumn months. Estuaries emerged as critical hotspots for these events, exhibiting the highest frequency of FMMEs and highlighting their vulnerability to climatic and anthropogenic pressures. Our findings indicate a staggering loss of at least 7.8 million fish, with Atherina boyeri, accounted and identified as the most affected species by FMMEs. The families Cyprinidae and Mugilidae experienced the most substantial impacts, including significant biomass losses in Chelon saliens, Chelon labrosus, and Cyprinus carpio. Additionally, endemic species such as Luciobarbus maghrebensis and Luciobarbus rabatensis also faced considerable declines. These events underscore severe ecological disruptions and provide novel insights into species distribution and interactions, including the first recorded presence of Oreochromis niloticus in previously undocumented regions. This research underscores the urgent need for targeted conservation strategies and proactive interventions to mitigate the ecological and socioeconomic ramifications of FMMEs. By addressing these critical issues, we can better protect Moroccan freshwater ecosystems that are at risk of further biodiversity loss.

From carrion to soil: microbial recycling of animal carcasses

Decomposer microbial communities are gatekeepers in the redistribution of carbon and nutrients from dead animals (carrion) to terrestrial ecosystems. The flush of decomposition products from a carcass creates a hot spot of microbial activity in the soil below, and the animal's microbiome is released into the environment, mixing with soil communities. Changes in soil physicochemistry, especially reduced oxygen, temporarily constrain microbial nutrient cycling, and influence the timing of these processes and the fate of carrion resources. Carcass-related factors, such as mass, tissue composition, or even microbiome composition may also influence the functional assembly and succession of decomposer communities. Understanding these local scale microbially mediated processes is important for predicting consequences of carrion decomposition beyond the hot spot and hot moment.

Beyond the kill: The allometry of predation behaviours among large carnivores

The costs of foraging can be high while also carrying significant risks, especially for consumers feeding at the top of the food chain. To mitigate these risks, many predators supplement active hunting with scavenging and kleptoparasitic behaviours, in some cases specializing in these alternative modes of predation. The factors that drive differential utilization of these tactics from species to species are not well understood. Here, we use an energetics approach to investigate the survival advantages of hunting, scavenging and kleptoparasitism as a function of predator, prey and potential competitor body sizes for terrestrial mammalian carnivores. The results of our framework reveal that predator tactics become more diverse closer to starvation, while the deployment of scavenging and kleptoparasitism is strongly constrained by the ratio of predator to prey body size. Our model accurately predicts a behavioural transition away from hunting towards alternative modes of predation with increasing prey size for predators spanning an order of magnitude in body size, closely matching observational data across a range of species. We then show that this behavioural boundary follows an allometric power-law scaling relationship where the predator size scales with an exponent nearing 3/4 with prey size, meaning that this behavioural switch occurs at relatively larger threshold prey body size for larger carnivores. We suggest that our approach may provide a holistic framework for guiding future observational efforts exploring the diverse array of predator foraging behaviours.

Predator mass mortality events restructure food webs through trophic decoupling

Predators have a key role in structuring ecosystems1-4. However, predator loss is accelerating globally4-6, and predator mass-mortality events7 (MMEs)-rapid large-scale die-offs-are now emblematic of the Anthropocene epoch6. Owing to their rare and unpredictable nature7, we lack an understanding of how MMEs immediately impact ecosystems. Past predator-removal studies2,3 may be insufficient to understand the ecological consequences of MMEs because, in nature, dead predators decompose in situ and generate a resource pulse8, which could alter ensuing ecosystem dynamics by temporarily enhancing productivity. Here we experimentally induce MMEs in tritrophic, freshwater lake food webs and report ecological dynamics that are distinct from predator losses2,3 or resource pulses9 alone, but that can be predicted from theory8. MMEs led to the proliferation of diverse consumer and producer communities resulting from weakened top-down predator control1-3 and stronger bottom-up effects through predator decomposition8. In contrast to predator removals alone, enhanced primary production after MMEs dampened the consumer community response. As a consequence, MMEs generated biomass dynamics that were most similar to those of undisturbed systems, indicating that they may be cryptic disturbances in nature. These biomass dynamics led to trophic decoupling, whereby the indirect beneficial effects of predators on primary producers are lost and later materialize as direct bottom-up effects that stimulate primary production amid intensified herbivory. These results reveal ecological signatures of MMEs and demonstrate the feasibility of forecasting novel ecological dynamics arising with intensifying global change.

Species' attributes predict the relative magnitude of ecological and genetic recovery following mass mortality

Theoretically, species' characteristics should allow estimation of dispersal potential and, in turn, explain levels of population genetic differentiation. However, a mismatch between traits and genetic patterns is often reported for marine species, and interpreted as evidence that life-history traits do not influence dispersal. Here, we couple ecological and genomic methods to test the hypothesis that species with attributes favouring greater dispersal potential-e.g., longer pelagic duration, higher fecundity and larger population size-have greater realized dispersal overall. We used a natural experiment created by a large-scale and multispecies mortality event which created a "clean slate" on which to study recruitment dynamics, thus simplifying a usually complex problem. We surveyed four species of differing dispersal potential to quantify the abundance and distribution of recruits and to genetically assign these recruits to probable parental sources. Species with higher dispersal potential recolonized a broader extent of the impacted range, did so more quickly and recovered more genetic diversity than species with lower dispersal potential. Moreover, populations of taxa with higher dispersal potential exhibited more immigration (71%-92% of recruits) than taxa with lower dispersal potential (17%-44% of recruits). By linking ecological with genomic perspectives, we demonstrate that a suite of interacting life-history and demographic attributes do influence species' realized dispersal and genetic neighbourhoods. To better understand species' resilience and recovery in this time of global change, integrative eco-evolutionary approaches are needed to more rigorously evaluate the effect of dispersal-linked attributes on realized dispersal and population genetic differentiation.

Acute resource pulses from periodical cicadas propagate to belowground food webs but do not affect tree performance

Acute resource pulses can have dramatic legacies for organismal growth, but the legacy effects of resource pulses on broader aspects of community structure and ecosystem processes are less understood. Mass emergence of periodical cicadas (Magicicada spp.) provides an excellent opportunity to shed light on the influence of resource pulses on community and ecosystem dynamics: the adults emerge every 13 or 17 years in vast numbers over much of eastern North America, with a smaller but still significant number becoming incorporated into forest food webs. To study the potential effects of such arthropod resource pulse on primary production and belowground food webs, we added adult cicada bodies to the soil surface surrounding sycamore trees and assessed soil carbon and nitrogen concentrations, plant-available nutrients, abundance and community composition of soil fauna occupying various trophic levels, decomposition rate of plant litter after 50 and 100 days, and tree performance for 4 years. Contrary to previous studies, we did not find significant cicada effects on tree performance despite observing higher plant-available nutrient levels on cicada addition plots. Cicada addition did change the community composition of soil nematodes and increased the abundance of bacterial- and fungal-feeding nematodes, while plant feeders, omnivores, and predators were not influenced. Altogether, acute resource pulses from decomposing cicadas propagated belowground to soil microbial-feeding invertebrates and stimulated nutrient mineralization in the soil, but these effects did not transfer up to affect tree performance. We conclude that, despite their influence on soil food web and processes they carry out, even massive resource pulses from arthropods do not necessarily translate to NPP, supporting the view that ephemeral nutrient pulses can be attenuated relatively quickly despite being relatively large in magnitude.