Gregory F. Albery

Emerging infectious diseases are a hallmark of the Anthropocene ^1,2 . Human-driven ecosystem changes and human–wildlife–livestock contact can drive the emergence of zoonotic and vector-borne diseases ^3–5 , but how much these processes shape landscapes of outbreak risk is poorly understood, beyond a few well-studied systems ^6–8 . Here we consolidate 58,319 outbreak event records for 32 diseases and systematically test how 16 hypothesized social and environmental drivers impact outbreak geographies, while accounting for detection and reporting biases. We show that outbreak risks are typically highest in mosaic landscapes where people and livestock live alongside forests and fragmented ecosystems. These combined factors, along with long-term declines in precipitation, share strong impacts across several vector-borne diseases (for example, dengue, Lyme disease and zoonotic arboviruses). By contrast, directly transmitted zoonoses (for example, Ebola and mpox) share few common drivers, and the impacts of most other anthropogenic pressures (for example, deforestation, climate warming and agricultural intensification) vary widely between diseases. Most consistently, the observed geography of outbreaks is shaped by healthcare access: reporting declines by a median of 32% (range across diseases: 1.2–96.7%) for each additional hour’s travel time from a health facility. Our findings underscore that infectious disease spillover and emergence are multi-causal, and that no one-size-fits-all strategy can prevent epidemics and pandemics. Ecosystem-based public health interventions ⁹ should always follow system-specific evidence, and be paired with greater investment in health systems and One Health pathogen surveillance.

Although strong evidence links social connectedness to improved fitness, there is pronounced among-individual variation in social behaviour. This variation may be maintained if different types of social relationships (i.e. having many versus strongly connected partners) have environment-dependent fitness benefits, but this has rarely been tested. We applied network analysis to 37 years of social association data in female Soay sheep to test the relative importance of relationship quantity (mean group size, network degree and participation coefficient) and quality (mean strength and social differentiation) for overwinter survival. We find that both relationship quantity and quality predict survival the following winter. Crucially, the relative importance of relationship quantity and quality depends on weather conditions: harsh conditions favour quantity (specifically large mean group sizes), likely owing to thermoregulatory benefits, while benign conditions favour quality (specifically mean association strength), possibly because conspecific familiarity reduces conflict. Our second measure of relationship quality—social differentiation—had a positive association with survival that was not environment-dependent, illustrating the value of strong, stable relationships in fluid societies. These findings advance our understanding of social benefits in fission–fusion systems and show that changing environmental conditions may be an important mechanism maintaining variation in social behaviour in natural populations.

Resource availability is crucial in shaping exposure and resistance to parasites. Although effects are expected to vary across parasite species, most of our knowledge comes from single-host, single-parasite studies. In a wild wood mouse population (Apodemus sylvaticus), we supplemented dietary resources over 3 years and quantified effects on a diverse parasite community alongside host behaviour and physiology. Resource supplementation had diverse impacts on 12 parasite taxa including positive (3/12), negative (6/12), and null (3/12) results. Supplementation also influenced host biology, with supplemented mice moving less, experiencing higher conspecific densities, improved conditions, and increased reproduction. These findings experimentally demonstrate that resource availability can multifacetedly affect disease dynamics. The complexity likely arises due to combined effects on exposure and susceptibility processes. Our study advances understanding of disease drivers at the human-wild interface and underscores the importance of assessing whole parasite communities to predict responses to changing resource landscapes in the wild.

Infectious diseases present a growing risk to wildlife biodiversity. Predicting outbreak dynamics and developing effective management tools require knowledge on the mechanisms of infection transmission. Specifically, understanding the relative contributions of direct host-to-host transmission versus indirect environmental transmission is critical. Here, we leverage a large dataset from long-term monitoring of a free-living population of eastern water dragons (Intellagama lesueurii) experiencing an outbreak of the fungal pathogen Nannizziopsis barbatae to investigate this pathogen’s transmission. Using different host behaviours to model direct and indirect transmission of infection in this population, we demonstrate N. barbatae’s ability to infect hosts through indirect transmission. Our findings shed light on the danger N. barbatae poses to Australian reptiles but also provide insights into how outbreaks should be managed. Additionally, we provide a methodology to elucidate the transmission mechanisms of driving the spread of emerging fungal pathogens.

Alien mammals introduced beyond their native distribution ranges can bring novel pathogens into the colonised communities and alter pathogen transmission dynamics. Past studies identified immunological and ecological drivers of cross-species viral transmission, but this knowledge has rarely been applied to alien species, leading to an underestimation of their role in disease emergence. Here we predict the viral sharing network resulting from the establishment of 67 alien mammals introduced globally in the last 50 years, using a trait- and phylogeny-informed viral sharing model. We show that the introduction of alien mammals can result, on average, in six novel viral sharing events per introduction (95% CI = 5.03–6.98), potentially reshaping the viral sharing networks of local communities. Phylogenetic relatedness emerged as the strongest predictor of viral sharing between alien and native species, with additional contributions from trait-based, dietary, and habitat similarities. Predicted viral sharing was concentrated in the Global North, reflecting potential geographic biases in both introduction records and viral surveillance. Our approach provides a quantitative tool to estimate viral hazards driven by established alien species that can be used to support risk assessment frameworks and international policy on biological invasions.

Inbreeding depression is the reduction in fitness of inbred individuals relative to their more outbred counterparts. Parasitism also reduces fitness and is a route by which inbreeding depression may operate, yet the complete pathway from inbreeding to parasitism to fitness has almost never been documented in the wild. We investigated parasite-mediated inbreeding depression in a wild population of a large mammal (red deer, Cervus elaphus ), using high-quality individual-level data on fitness in juveniles and adult females, longitudinal infection data for three gastrointestinal helminth parasites, and genomic inbreeding coefficients. We found evidence for parasite-mediated inbreeding depression via strongyle nematodes in juvenile survival, independent of direct adverse effects of inbreeding on survival and indirect effects of inbreeding on survival via birth weight. Inbreeding also reduced fitness in reproductive adults by reducing overwinter survival. Our study reveals three independent pathways by which inbreeding depresses fitness and highlights the rarely-studied route of parasitism.

Theory predicts that high population density leads to more strongly connected spatial and social networks, but how local density drives individuals’ positions within their networks is unclear. This gap reduces our ability to understand and predict density-dependent processes. Here we show that density drives greater network connectedness at the scale of individuals within wild animal populations. Across 36 datasets of spatial and social behaviour in >58,000 individual animals, spanning 30 species of fish, reptiles, birds, mammals and insects, 80% of systems exhibit strong positive relationships between local density and network centrality. However, >80% of relationships are nonlinear and 75% are shallower at higher values, indicating saturating trends that probably emerge as a result of demographic and behavioural processes that counteract density’s effects. These are stronger and less saturating in spatial compared with social networks, as individuals become disproportionately spatially connected rather than socially connected at higher densities. Consequently, ecological processes that depend on spatial connections are probably more density dependent than those involving social interactions. These findings suggest fundamental scaling rules governing animal social dynamics, which could help to predict network structures in novel systems.

Exposure to environmentally transmitted parasites should increase with population density due to accumulation of infective parasites in space. However, resource competition also increases with density, lowering immunity and increasing susceptibility, offering an alternative pathway for density-dependent infection. To test the relationships between these two processes and parasitism, we examined associations between host density, resource availability, immunity, and counts of 3 common helminth parasites using a long-term study of red deer. We found evidence that immunity increased with resource availability while parasite counts declined with immunity. We also found that greater density correlated with reduced resource availability, and while density was positively associated with both strongyle and tissue worm burdens, resource availability was independently and negatively associated with the same burdens. Our results support separate roles of density-dependent exposure and susceptibility in driving infection, providing evidence that resource competition is an important driver of infection, exacerbating effects of density-dependent increases in exposure.

An individual’s social connections have strong effects on fitness. Despite this, there is pronounced among-individual variation in social behaviour. This variation may be maintained if different types of social connections have environment-dependent fitness benefits, but this has rarely been tested. We applied network analysis to 37 years of social association data in Soay sheep to test this hypothesis. Our results show that both relationship quantity (having many association partners) and relationship quality (having strong associations) are linked with survival. Crucially, the relative importance of relationship quantity and quality depends on overwinter weather, with harsh conditions favouring quantity over quality, likely due to thermoregulatory benefits. Relationship quality, meanwhile, was most beneficial under benign weather conditions, possibly due to enhanced familiarity with conspecifics. These findings advance our understanding of social benefits in fission-fusion societies and show that changing environmental conditions may be an important mechanism maintaining variation in social behaviour in natural populations.

Sociality is hypothesised to buffer populations against environmental stochasticity by stabilising key vital rates, yet this has rarely been tested across diverse taxa. We compiled demographic time series from 87 populations for 66 animal species spanning 12 taxonomic classes to test whether sociality promotes demographic buffering and whether this relationship depends on environmental context. Using stochastic elasticities and phylogenetically comparative analyses, we find broad support for the social buffering hypothesis: more social species exhibit lower sensitivity of population growth to vital rate variance (lower stochastic elasticity to vital rate variances, | T _σ |), particularly in juvenile survival and reproduction. Contrary to expectations, juvenile vital rates, but not adult survival, showed the strongest buffering patterns in social taxa, suggesting life-stage-specific social mechanisms. Furthermore, we show that social species invest more in maximising the average performance of adult survival and reproduction (higher stochastic elasticity to vital rate means, T _μ ), consistent with adaptive strategies favouring canalisation of fitness-related traits. This buffering benefit is especially relevant in highly unpredictable environments: in climates with erratic precipitation, social species have greater demographic advantages, exhibiting even lower T _σ values. Together, these findings provide support for the social buffering hypothesis, especially in highly stochastic environments. Social traits confer some demographic stability under moderate climatic conditions, but may become more relevant under extreme stochasticity—an insight particularly important for understanding population resilience in a rapidly changing world.