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The spatial organization of populations determines their pathogen dynamics. This is particularly important for communally roosting species, whose aggregations are often driven by the spatial structure of their environment.We develop a spatially explicit model for virus transmission within roosts of Australian tree‐dwelling bats (Pteropus spp.), parameterized to reflect Hendra virus. The spatial structure of roosts mirrors three study sites, and viral transmission between groups of bats in trees was modelled as a function of distance between roost trees. Using three levels of tree density to reflect anthropogenic changes in bat habitats, we investigate the potential effects of recent ecological shifts in Australia on the dynamics of zoonotic viruses in reservoir hosts.We show that simulated infection dynamics in spatially structured roosts differ from that of mean‐field models for equivalently sized populations, highlighting the importance of spatial structure in disease models of gregarious taxa. Under contrasting scenarios of flying‐fox roosting structures, sparse stand structures (with fewer trees but more bats per tree) generate higher probabilities of successful outbreaks, larger and faster epidemics, and shorter virus extinction times, compared to intermediate and dense stand structures with more trees but fewer bats per tree. These observations are consistent with the greater force of infection generated by structured populations with less numerous but larger infected groups, and may flag an increased risk of pathogen spillover from these increasingly abundant roost types.Outputs from our models contribute insights into the spread of viruses in structured animal populations, like communally roosting species, as well as specific insights into Hendra virus infection dynamics and spillover risk in a situation of changing host ecology. These insights will be relevant for modelling other zoonotic viruses in wildlife reservoir hosts in response to habitat modification and changing populations, including coronaviruses like SARS‐CoV‐2., The authors give compelling evidence that spatial structure and bat aggregation may be a missing piece in understanding infection dynamics and spillover risk from flying‐fox roosts.

From April 2003 to April 2004 monthly sea surveys were conducted around 2 fish aggregating devices (FADs) moored at 2000 and 2500 m depth in Martinique (Lesser Antilles). The use of a dual frequency splitbeam echosounder combined with an underwater camera and fishing methods allowed assessment of average space and time distribution of pelagic fish aggregated beneath the FADs, as well as identification of their overall size and species composition. At daytime, 4 fish aggregations were identified at each FAD, representing 4 distinct types: i) an aggregation of small juvenile tuna (mean fork length, FL: 30 cm) observed very close to the surface in 25% of daytime periods; ii) a small surface aggregation dominated by carangids, Caranx crysos, present in 65% of daytime periods; iii) a large sub-surface aggregation observed during all daytime periods: this aggregation appeared to be primarily comprised of 58 cm FL blackfin tuna (Thunnus atlanticus), mixed with yellowfin (Thunnus albacares) and skipjack (Katsuwonus pelamis) tunas of the same size; and iv) sub-surface scattered large predators (mainly blue marlin, Makaira nigricans) present in 10% of daytime periods. A smaller sub-surface aggregation comprised of medium tuna mixed with "extranatants" (fishes which remain within 10 to 50 m of a FAD) was observed in 75% of night-time periods, whereas unidentified scattered fishes were detected from 70 to 400 m depth. The low daytime vulnerability of medium sub-surface tunas to applied line techniques leads us to assume that their feeding motivation was low during daytime. These tunas could preferentially feed on mesopelagic organisms during night-time and transition periods around Martinican moored FADs. Local fishermen mainly targeted the large scattered predators using very small tunas as living bait. The sub-surface tuna aggregation hence appeared to be currently unexploited by local fisheries, though it represented the large majority of the pelagic biomass around the moored FADs., Des campagnes scientifiques mensuelles ont été menées autour de 2 dispositifs de concentration de poissons ancrés à 2000-2500 m de profondeur, en Martinique d'avril 2003 à avril 2004. L'utilisation combinée d'un échosondeur monofaisceau multifréquence, d'une caméra vidéo sous-marine et de techniques de pêche conventionnelles a permis de déterminer la distribution spatio-temporelle moyenne des poissons agrégés autour des DCP ancrés ainsi que la composition générale des agrégations. Les techniques de vidéo sous-marine ont joué un rôle majeur dans l'identification des espèces et l'observation de leur comportement. De jour; quatre types d'agrégations ont été définis : i) une agrégation de thonidés juvéniles (longueur à la fourche, LF, moyenne : 30 cm) observée très près de la surface durant 25 % des périodes diurnes échantillonnées ii) une petite agrégation composée essentiellement de Caranx crysos présente en surface lors de 65 % des phases diurnes iii) une grande agrégation de thons observée en sub-surface lors de toutes les phases diurnes; cette agrégation était constituée essentiellement de thons noirs (Thunnus atlanticus) de 58 cm LF, associés de façon saisonnière à des albacores (Thunnus albacares) et des listaos (Katsuwonus pelamis) de taille similaire iv) des grands prédateurs assez dispersés (essentiellement le marlin bleu, Makaira nigricans), observés lors de 10 % des phases diurnes. Une agrégation de sub-surface, aux dimensions plus réduites et composée de thons et "d'extranatants" (poissons restant à distance de 10 à 50 m du DCP) a été observée lors de 75 % des périodes nocturnes; ainsi que des organismes non-identifiés dispersés entre 70 et 400 m de profondeur. La faible vulnérabilité diurne des thons de sub-surface aux techniques de ligne utilisées indique qu'ils ne se nourrissent pas activement durant la journée. Ces poissons pourraient se nourrir préférentiellement d'organismes mésopélagiques durant les phases de transition et nocturnes autour des DCP ancrés. La pêcherie artisanale martiniquaise n'exploite qu'une faible part des ressources agrégées autour des DCP ancrés. Les principales espèces-cibles sont les grands prédateurs capturés de jour en utilisant des thonidés juvéniles comme appât vivant. L'agrégation de thons de sub-surface représente la grande majorité de la biomasse autour des DCP mais elle est quasiment inexploitée par les pêcheries locales.

Fish lice (Argulus spp.) are obligate ectoparasites, which contrary to most aquatic parasites, retain the ability to swim freely throughout the whole of their life. In fish farms, they can quickly increase in numbers and without effective control cause argulosis, which results in the reduced growth and survival of their fish hosts. The morphology of Argulus spp, including their sensory organs, is suitable for both parasitism and free-swimming. By spending a considerable amount of time away from their host, these parasites risk being excessively dispersed, which could endanger mating success. Here we present a review of recent studies on the behaviour of Argulus spp, especially the aggregative behaviour that mitigates the dilution of the parasite population. Aggregation of parasites, which is especially important during the period of reproduction, occurs on different scales and involves both the aggregation of the host and the aggregation of the parasites on the host. The main behavioural adaptations of Argulus spp, including searches for hosts and mates, host manipulation and host choice, are all focused on the fish. As these ectoparasites repeatedly change hosts and inflict skin damage, they can act as vectors for fish pathogens. The development of environmentally friendly measures for the control and prevention of argulosis needs to take into account the behaviour of the parasites.

Fish lice (Argulus spp.) are obligate ectoparasites, which contrary to most aquatic parasites, retain the ability to swim freely throughout the whole of their life. In fish farms, they can quickly increase in numbers and without effective control cause argulosis, which results in the reduced growth and survival of their fish hosts. The morphology of Argulus spp, including their sensory organs, is suitable for both parasitism and free-swimming. By spending a considerable amount of time away from their host, these parasites risk being excessively dispersed, which could endanger mating success. Here we present a review of recent studies on the behaviour of Argulus spp, especially the aggregative behaviour that mitigates the dilution of the parasite population. Aggregation of parasites, which is especially important during the period of reproduction, occurs on different scales and involves both the aggregation of the host and the aggregation of the parasites on the host. The main behavioural adaptations of Argulus spp, including searches for hosts and mates, host manipulation and host choice, are all focused on the fish. As these ectoparasites repeatedly change hosts and inflict skin damage, they can act as vectors for fish pathogens. The development of environmentally friendly measures for the control and prevention of argulosis needs to take into account the behaviour of the parasites. peerReviewed

International audience; Sea cruises were conducted for 57 days over 16 months to characterise pelagic fish aggregations around two moored fish aggregating devices (FADs) in Martinique (Lesser Antilles). Echosounder surveys run in a star pattern were used in conjunction with obliquely beamed sonar observations. An echo-integration-by-shoal algorithm was implemented to isolate pelagic fish shoals from sound scattering layers and to compute mean morphometric, positional and density parameters. Tree regressions were used to select and classify pelagic fish target strengths (TS), with reference to their spatial and temporal characteristics. The main type of pelagic fish aggregation was a large sub-surface aggregation. It was observed during all daytime periods within a radius of 400 m. of the FAD. A smaller type of aggregation was observed closer to the surface and to the FAD in 65% of daytime periods. Large scattered fish were observed in 16% of daytime periods. At night, a medium-sized aggregation was detected in the sub-surface in 75% of night-time periods. The sizes of the fish inside the aggregations (determined from TS values) were lower in the small near-surface aggregation than in the large sub-surface aggregation. Mean packing densities of sub-surface medium fish and near-surface small fish aggregations (determined from TS and shoal acoustic density) were respectively 0.2 and 1.3 fish per m(3). The acoustic methodology and results are discussed with reference to the characteristics and performance of the echosounder and to the spatial structure of pelagic fish aggregations around moored FADs in Martinique.

In recent years it has become apparent that when studying plant-herbivore interactions, it is important to also consider the multitrophic context, i.e. to what extent generalist and specialist enemies act as bodyguards for the plant. I have performed a field experiment with 25 genets, varying in plant defense levels, of meadowsweet (Filipendula ulmaria) and its herbivore, the chrysomelid beetle Galerucella tenella. Each genet was treated with four different density manipulations of G. tenella (5, 15, 30, and 60 larvae per genet). The experiment was set up on an archipelago island with natural levels of a specialist hymenopteran parasitoid Asecodes parviclava and generalist natural enemies. I set up the following hypotheses: 1) larval survival will be higher on less defended plants than on more defended plants, 2) the proportion of parasitoids will be higher on more defended plants, and 3) larval survival will increase with increasing larval densities. The most important findings of the experiment were an overall very low total survival of larvae (G. tenella pupae and A. parviclava mummies), a generally high parasitism by A. parviclava on more defended plants, and that two groups of generalist predators (hunting and web-forming spiders) were more frequent on less defended plants. My study suggests that herbivore survival may not be higher on less defended plants under situations with abundant occurrence of generalist predators, as they are able to trace high-quality foraging sites.

The spatial organization of populations determines their pathogen dynamics. This is particularly important for communally roosting species, whose aggregations are often driven by the spatial structure of their environment. We develop a spatially explicit model for virus transmission within roosts of Australian tree-dwelling bats (Pteropus spp.), parameterized to reflect Hendra virus. The spatial structure of roosts mirrors three study sites, and viral transmission between groups of bats in trees was modelled as a function of distance between roost trees. Using three levels of tree density to reflect anthropogenic changes in bat habitats, we investigate the potential effects of recent ecological shifts in Australia on the dynamics of zoonotic viruses in reservoir hosts. We show that simulated infection dynamics in spatially structured roosts differ from that of mean-field models for equivalently sized populations, highlighting the importance of spatial structure in disease models of gregarious taxa. Under contrasting scenarios of flying-fox roosting structures, sparse stand structures (with fewer trees but more bats per tree) generate higher probabilities of successful outbreaks, larger and faster epidemics, and shorter virus extinction times, compared to intermediate and dense stand structures with more trees but fewer bats per tree. These observations are consistent with the greater force of infection generated by structured populations with less numerous but larger infected groups, and may flag an increased risk of pathogen spillover from these increasingly abundant roost types. Outputs from our models contribute insights into the spread of viruses in structured animal populations, like communally roosting species, as well as specific insights into Hendra virus infection dynamics and spillover risk in a situation of changing host ecology. These insights will be relevant for modelling other zoonotic viruses in wildlife reservoir hosts in response to habitat modification and changing populations, including coronaviruses like SARS-CoV-2.