Beyond the triad: why ecological theory must anchor One Health

Abstract

One Health has been highly effective at emphasizing the interconnectedness of human, animal, and environmental health. However, One Health must more explicitly incorporate ecological theory to move beyond documenting pathogen presence or co-location toward understanding the mechanisms that lead to infection and transmission. Here, we argue that ecological theory provides the missing mechanistic layer between pathogen detection and prevention, and we identify four practical areas where this matters: landscape connectivity and contact networks, biodiversity and microbiomes, ecological interpretation of environmental sampling, and long-term ecological capacity. Together, these areas can help identify which human–animal–environment connections matter most for disease dynamics and where prevention efforts can most effectively interrupt transmission. Therefore, ecological theory can move One Health from documenting connections toward identifying mechanisms, directing prevention, supporting wildlife conservation, and promoting human well-being.

1. Ecology is the missing middle between detection and prevention

One Health is effective at emphasizing that human, animal, and environmental health are connected, but it is often less explicit about the ecological mechanisms that turn pathogen presence into transmission potential. Ecological theory is the body of formal, and often quantitative, conceptual or predictive frameworks that help explain how biological organisms interact with each other and with their abiotic environment, and how those interactions generate patterns and processes observed in nature [1]. These frameworks offer a powerful scaffold for One Health, which seeks to safeguard human, animal, and ecosystem health from zoonotic and environmentally acquired diseases in an integrated approach. Our argument is not simply that ecology should be included in One Health, but that ecological theory is the missing middle connecting pathogen detection to actionable prevention by linking detections to sources, transmission routes, hosts, environmental contexts, and interventions.

Zoonotic risk is determined not only by the co-occurrence of humans, animals, and environments, but also by the ecological processes that connect them. Disease dynamics are shaped by intra- and interspecific interactions and by environmental conditions, which vary across space and time. Environmental and ecological disturbances can create new species interactions and contact zones among humans, livestock, wildlife, and invertebrate vectors [2]. An inherent property of disease systems is that they often behave nonlinearly, which means that small ecological changes may have little effect until a threshold is crossed, after which transmission rapidly increases or decreases. Examples include vector population thresholds, host-density thresholds, pathogen dose thresholds, or landscape-connectivity thresholds, which all operate at different scales. Rather than documenting events after they occur, One Health should ask which ecological principles apply to a particular disease system and which interventions interrupt transmission. Although evolutionary and developmental biology are important for One Health [3], we do not discuss these concepts here.

Spillover events likely occur far more often than they are detected and are not restricted to rare pandemic events. They occur routinely through endemic and enzootic zoonoses such as brucellosis, Q fever, Rift Valley fever, and tularemia, where repeated animal, vector, or environmental exposures generate recurrent human infections that may remain localized, underdiagnosed, or unable to sustain onward transmission. An ecologically integrated One Health approach should not be built only around dramatic disease events like Ebola, Nipah, or highly pathogenic H5N1 avian influenza. It is also needed for the everyday zoonoses to reduce transmission through important human-animal-environmental interfaces.

One Health needs to move beyond co-location of disease events to understanding entire disease systems so that transmission pathways can be inferred and broken. Concepts and principles such as niche theory, community assembly, trophic interactions, and source-sink dynamics can illuminate how ecological systems and processes reduce or enhance spillover and spillback. Here, we discuss four ways in which ecological theory can better connect and inform the practitioners of One Health to go beyond the triad of human, animal and environmental health. First, metacommunity dynamics, landscape connectivity, and contact networks can distinguish how physical habitat configuration, species-specific movement, and transmission networks shape zoonotic exposure. Second, understanding mechanisms that structure biodiversity can help identify when host communities dilute or amplify infection risk. Third, environmental sampling of infectious pathogens that is not based on the ecological dynamics of a system may be greatly limited at best, and misleading at worst. Lastly, long-term ecological research and global ecological capacity are needed to capture temporal and spatial variability, rare spillover events, and nonlinear dynamics, while ensuring that local knowledge helps translate ecological theory into One Health practice.

2. Identifying transmission connections across landscapes

Landscape connectivity is not a single concept, but a family of related concepts that can be used to understand how pathogens, hosts, and vectors move across space. Structural connectivity describes the physical arrangement of the landscape matrix (e.g., habitat patches, corridors, barriers, and resistance surfaces) [4], whereas functional connectivity describes realized movement, which depends on species behavior, habitat perception, dispersal capacity, and resource use [4]. Contact networks represent actual or potential transmission contacts among individuals, species, farms, herds, or human communities [5]. These approaches can be coupled when spatial connectivity is used to estimate contact probabilities, such as when landscape resistance is used to parameterize likely contacts among different hosts, including people [6], [7]. Similarly, source–sink dynamics should distinguish demographic sources and sinks from those of pathogens. In a disease context, pathogen sources are populations or habitats that maintain or amplify infection, whereas sinks receive repeated introductions but fail to sustain transmission [8]. This distinction links landscape ecology to reservoir competence and helps identify whether management should preserve connectivity, interrupt specific contact pathways, or reduce exposure at high-risk interfaces.

The implications of increased connectivity are context dependent. Connectivity may increase exposure when connectivity creates repeated contact among different hosts. It can also increase transmission when it facilitates movement of infected individuals, highly competent reservoir hosts, and vectors into susceptible populations. However, connectivity can reduce transmission when it supports stable habitat use, maintains predator or competitor communities that suppress competent reservoirs, or allows animals to meet resource needs without moving into peri-urban areas or other high-contact settings. This means that One Health practitioners should not treat ecological corridors, habitat restoration, and land-use planning as uniformly beneficial or risky. Instead, they should evaluate the conservation benefits as well as the disease-transmission consequences. As ecological corridors expand globally, their implications for disease transmission remain unclear and require careful evaluation.

Hendra virus in Australia illustrates how connectivity can generate or reduce zoonotic exposure depending on which connections are strengthened. Flying foxes (Pteropus spp.) are the reservoir hosts, and bat presence alone does not explain spillover risk. Long-term data suggests that land-use change, climate variability, and loss of winter-flowering plants have resulted in bats moving into agricultural and urban areas, where contact with horses become more likely [9]. Periodic food shortages from loss of winter flowering habitat preceded Hendra virus spillover events, while pulses of winter flowering in remnant forests prevent spillover. In this system, habitat loss disrupted ecological connectivity to food resources and increased functional connectivity between bats and areas where horses are kept. Therefore, ecological countermeasures to reduce the risk of spillover events include protecting and restoring winter-flowering habitat and managing horse access to bat foraging and roosting areas. Hendra virus shows why One Health must distinguish between connectivity that supports conservation from connectivity that increases transmission opportunities.

3. Host and microbial communities shape transmission

Biodiversity can either dilute or amplify pathogen transmission depending on which hosts, vectors, and ecological interactions dominate a system. The dilution effect occurs when greater species diversity reduces transmission, often because vectors or pathogens encounter more “dead-end” or low-competence hosts, lowering the probability that infection reaches highly competent reservoirs or susceptible livestock and humans [10]. In contrast, amplification occurs when diversity adds competent reservoir hosts, increases vector abundance, or creates more contact opportunities among susceptible hosts. The One Health implication is that diversity itself is not automatically protective or harmful; its effect depends on community composition, population demographics, competence, behavior, density, and land-use. A strong ecological approach would ask not whether a system is diverse, but which species are present, how they interact, and whether those interactions reduce or intensify transmission. Landscape connectivity further determines whether these community effects translate into transmission by shaping how competent reservoir hosts, vectors, domestic animals, and people encounter one another across space.

Microbial ecology is also essential to a more ecologically grounded One Health framework because pathogens enter complex microbial communities that can either resist or facilitate colonization [11]. The diversity, composition, and functional capacity of host microbiomes can shape pathogen establishment through competition for nutrients and attachment sites, production of inhibitory metabolites, and immune modulation. Microbiome structure can also influence transmission by affecting pathogen shedding intensity and duration [12]. For One Health, this means treating microbiomes as dynamic ecological communities that mediate host susceptibility and infectiousness, while providing practical intervention targets. Additionally, host immunocompetence can be shaped by environmental drivers and life-history trade-offs, which in turn can influence microbiome structure, pathogen susceptibility, and shedding. Together, host community ecology and microbial ecology help identify which species maintain pathogens, which hosts amplify or interrupt transmission, and which individual hosts are most likely to shed pathogens at levels relevant to environmental contamination and onward spread.

4. Distinguishing signal from risk in environmental sampling

One Health surveillance should move beyond interpreting positive soil, water, wastewater, or wildlife samples as isolated detections, and instead make ecological context central to evaluating plausible exposure and transmission pathways. Wastewater and environmental surveillance are beginning to take a more integrated approach, but One Health applications still too often treat positive soil, water, or wildlife detections as isolated findings rather than ecological signals. A detected environmental signal is only meaningful if it can be connected to a possible transmission pathway, such as contaminated waterlines, fomites, wildlife incursions, dust or aerosol movement, runoff, or grazing patterns. These are routes that could realistically carry an organism from the environment into livestock or humans. Without ecological and management context, it is difficult to know whether a positive sample represents an active threat or a dead-end detection. Therefore, a One Health approach should connect detection to transmission relevance and intervention design by pairing detections with comprehensive metadata and ecological context to assess transmission potential.

5. Capturing disease dynamics through long-term research and global ecological capacity

Long-term research is essential for documenting ecological dynamics over many years and decades. Disease systems often change year to year with infections becoming more or less severe during certain conditions. For example, arbovirus outbreaks are generally worse in certain years and mild in others. These differences are partly shaped by temperature, drought, precipitation, mosquito ecology, avian communities, and patterns of human activity, but the strength and direction of these effects can vary regionally [13]. Another example is Ebola virus spillover to humans in which outbreaks are sporadic, spatially clustered, and difficult to predict from short observation windows. Modeling studies increasingly show that Ebola spillover is shaped by interacting ecological, epidemiological, climatic, and socioeconomic factors [14], meaning that short-term sampling campaigns are often insufficient. Long-term datasets are necessary to identify lagged effects, thresholds, nonlinear dynamics, and recurring environmental conditions that precede elevated transmission.

Strengthening global ecological capacity would allow One Health programs to move from reactive outbreak response toward local prevention and adaptive management. Capacity building should include sustained support for field ecology, wildlife and vector monitoring, environmental data collection, ecological modeling, and training programs that integrate ecologists with veterinarians, physicians, microbiologists, public health practitioners, and local communities. Ecologists should be included in One Health planning to provide the ecological context that is essential for connecting pathogen detection to actionable prevention. Building this capacity globally is especially important because many high-risk interfaces occur in regions where ecological data are sparse, monitoring programs are intermittent, and local expertise is under-resourced. Ultimately, One Health implementation will be stronger if long-term data and ecological expertise are included in surveillance design, outbreak investigation, epidemiological modeling, and mitigation planning.

6. Conclusion

We argue for a more ecologically integrated One Health approach to explain how pathogen infection and transmission change across space and time and where prevention efforts can most effectively break transmission. We describe four ways ecological theory can be better integrated into One Health. Together, these priorities can move One Health from documenting connections among humans, animals, and environments toward a more mechanistic understanding of disease systems. By doing so, an ecologically grounded One Health approach can help preserve wildlife, limit pathogen propagation, and promote human well-being, which is the essence of One Health.

Declaration of generative AI and AI-assisted technologies

During the preparation of this work, the authors used ChatGPT, GPT-5.5 Thinking (OpenAI) for minor copyediting and language refinement. After using this tool/service, the authors reviewed and edited the content as needed and take full responsibility for the content of the published article.

CRediT authorship contribution statement

Andrew W. Bartlow: Writing – original draft, Conceptualization. Jeanne M. Fair: Writing – review & editing, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.