The Impact of Disease on Biodiversity and Ecosystem Health

Disease is often overlooked as a driver of biodiversity loss – overshadowed by habitat destruction, climate change, and overexploitation. Yet pathogens have the power to devastate wildlife populations with frightening speed, sometimes pushing already-vulnerable species to the brink of extinction. As human activities continue to reshape natural landscapes, the relationship between disease and biodiversity is becoming increasingly urgent. Understanding how pathogens affect ecosystems – and what we can do about it – is central to any serious conversation about conservation today.

The role of pathogens in biodiversity decline 🔗

Pathogens – including viruses, bacteria, fungi, and parasites – are a natural part of every ecosystem. In balanced systems, disease acts as a regulator, maintaining population sizes and preventing any one species from dominating. The trouble begins when new pathogens are introduced into ecosystems where species have had no prior exposure and therefore no evolved immunity.

Research published in Animal Conservation confirms that while infectious disease alone rarely drives species to global extinction, diseases can greatly impact local species populations by causing temporary or permanent declines in abundance, and pathogens can interact with other driving factors such as habitat loss, climate change, overexploitation, invasive species, and environmental pollution to contribute to local and global extinctions.

One of the most striking fungal examples is Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis. Bd has been implicated in the extinction of the golden toad in Costa Rica, as well as the sharp-snouted day frog and two species of gastric-brooding frogs from Australia. These were not species already on the edge – Bd alone was sufficient to eliminate entire populations.

A key concept here is the dilution effect. In areas of naturally high biodiversity, there may be a larger potential source pool for pathogens, but biodiversity loss frequently increases disease transmission. Preserving intact ecosystems and their endemic biodiversity should generally reduce the prevalence of infectious diseases. In other words, a species-rich habitat can act as a buffer, diluting the chance that any single pathogen finds enough susceptible hosts to cause an outbreak. When biodiversity erodes, that buffer disappears.

Impact on endangered species 🔗

Rare and endangered species face a disproportionately high risk from disease outbreaks. Their already-small population sizes mean that even moderate mortality rates can be catastrophic. Small, isolated populations also tend to have reduced genetic diversity, which limits their ability to mount effective immune responses against new pathogens.

Research using the IUCN Red List found that pathogens appear to increase in importance as species move towards extinction, though this varies with host taxonomy. Disease is rarely the first threat a species faces, but it frequently becomes a decisive final blow when populations are already weakened by other pressures.

Ebola and great apes 🔗

The impact of Ebola virus on gorilla populations is one of the most documented and disturbing examples of disease-driven population collapse in endangered wildlife. Ebola outbreaks have caused severe declines in chimpanzee and gorilla populations in Central Africa, with mortality rates estimated at around 90-95% in affected populations. During the Lokoué outbreak in Odzala-Kokoua National Park in 2004, individuals living in social groups were more heavily affected than solitary males, highlighting the cost of sociality in terms of disease risk.

In Lossi sanctuary alone, it was estimated that the Ebola virus killed 5,000 wild gorillas. The severe population decline, as a result of illegal hunting, disease and habitat loss, contributed to the 2007 shift of the conservation status of western gorillas from “endangered” to “critically endangered.”

The situation for mountain gorillas is equally alarming. Ebola virus mortality rates of up to 98% have reduced the global gorilla population by approximately one-third, and with just over 1,000 mountain gorillas remaining in the world, a single outbreak could decimate the population.

Amphibians: the most disease-threatened vertebrate group 🔗

Among assessed taxa, 32% of amphibians are currently threatened with extinction – the highest proportion of any major vertebrate group. Fungal pathogens like Bd have been central to this crisis, spreading across continents and infecting species that had never encountered such pathogens before. Because many amphibians have limited mobility and small geographic ranges, a localized disease outbreak can eliminate an entire species with no possibility of recolonization.

Human activities that promote disease spread 🔗

Disease dynamics in wildlife do not exist in isolation from human behavior. The growing frequency and geographic reach of wildlife disease outbreaks is closely linked to the ways humans alter natural environments.

Habitat fragmentation 🔗

When forests and other natural habitats are fragmented by roads, agriculture, and urban development, wildlife populations are divided into smaller, isolated groups. Habitat fragmentation can disrupt connectivity and gene flow between host populations, leading to declines in disease resistance. Land-use change can also introduce human-associated invasive species such as rats, cats, and pigs, altering the ecological community in ways that affect the pathogen community. These introduced species frequently act as bridge hosts, moving pathogens between wildlife and human communities in ways that would not occur in intact ecosystems.

Invasive species as vectors 🔗

Invasive species do not just compete with native wildlife for food and space – they can introduce entirely new pathogens into ecosystems where local species have no immunity. Diseases are often transmitted between wild and domestic species, as well as from invasive species into resident populations. The emergence of new infectious diseases frequently results from a change in ecology of host or pathogen, and when these relationships are disrupted, ecological effects may extend to many other parts of the ecosystem.

Climate change and shifting disease ranges 🔗

The intensifying emergence of infectious pathogens can be attributed to climate change, biodiversity loss, habitat degradation, and an increasing rate of wildlife-human contacts. As temperatures rise and precipitation patterns shift, disease vectors such as mosquitoes and ticks are expanding into previously inhospitable regions. Climate change is forcing many species into new geographic distributions, altering the animal communities in certain regions. As a result, some species will become extinct while others will expand, setting up new host-parasite and host-host interactions. This reorganization of species communities creates novel opportunities for pathogen spillover and outbreak.

Wildlife trade and human encroachment 🔗

The legal and illegal wildlife trade, as well as the expansion of human settlements into natural areas, dramatically increases contact between humans, domestic animals, and wildlife. Zoonotic diseases are more likely to jump to human populations when human activities – including changes in land use, farming of wild animals, and the illegal wildlife trade – bring people and domestic animals into close proximity with wild animals. This proximity doesn’t just threaten human health; it also exposes wildlife to human and livestock pathogens against which they have little natural defense.

Preventing disease spread to conserve biodiversity 🔗

Given how deeply disease is entangled with other conservation threats, preventing its spread requires a broad, coordinated strategy – one that treats ecosystem health, wildlife health, and human health as interconnected.

Habitat protection and restoration 🔗

The most fundamental preventive measure is maintaining intact, connected habitats. Disease prevention is far more effective and less costly than disease control. Conservation practices that achieve intact and diverse ecosystems, as well as connectivity between wildlife habitats and populations, help provide a buffer to increasing stressors on wildlife health. Protected areas limit deforestation, reduce human encroachment, and preserve the species diversity that naturally suppresses pathogen transmission through the dilution effect.

Conservation efforts such as habitat protection and restoration can mitigate pathogen spillover by reducing human-wildlife and livestock-wildlife interactions, enhancing dilution effects, and lowering reservoir host density. Deforestation prevention and ecosystem restoration are therefore not just conservation tools – they are also disease prevention strategies.

Wildlife disease surveillance and monitoring 🔗

Early detection is critical. The IUCN recognizes wildlife disease surveillance as a key pillar of biodiversity conservation, noting that surveillance programmes should provide a baseline understanding of wildlife health, detect immediate or potential threats including emerging diseases, and support species conservation assessments and the development of action plans.

According to the U.S. Fish & Wildlife Service, once a new wildlife disease has been introduced to a population, eradication may not be possible. This underscores why disease contingency plans, wildlife disease surveillance, and investigation of mortality events are vital components of disease management – to catch outbreaks before they spiral out of control.

Disease monitoring hotspots should be focused in areas with habitat loss such as through deforestation, with special emphasis on the edges between habitat remnants and areas where domestic species move into natural habitats, as well as where hydrological, agricultural, or aquaculture development projects are taking place.

Biosecurity and translocation protocols 🔗

Moving animals between locations – whether for conservation reintroductions or captive breeding programs – carries significant disease risks. The Zoological Society of London (ZSL) has developed frameworks for disease risk analysis that are now adopted as IUCN guidelines, recognizing that veterinary work is critical to the success of translocation projects because of the high potential for spreading disease, and can make the difference between success and failure by tackling disease threats before they become a problem.

Reducing human-wildlife contact 🔗

Preventive measures to decrease the spread of disease include reducing activities that unnaturally congregate animals into small geographic areas, restricting wildlife rehabilitation and release practices, curtailing wildlife translocations, and reducing interactions between humans, domestic animals, and wildlife. Alongside these practical steps, protecting bat roosting areas and food sources is also increasingly recognized as important – when bats lose their natural habitats and winter food sources, their populations splinter and migrate to agricultural and urban areas where, partly due to inadequate food sources, bats become stressed and shed more virus.

The One Health approach 🔗

Effectively addressing disease as a threat to biodiversity requires recognizing that human health, animal health, and ecosystem health are inseparable. A collaborative effort of multiple disciplines in a One Health context is crucial if the health of human beings, livestock, wildlife, and the environment is to be improved. This means veterinarians, ecologists, public health officials, and policymakers working together – sharing data, coordinating responses, and designing conservation strategies that account for disease dynamics from the outset.

Disease is not an inevitable price of nature’s complexity. It becomes a crisis when ecosystems are degraded, species are isolated, and human activities create new pathways for pathogens to spread. Protecting biodiversity and preventing disease outbreaks are, in this sense, the same goal – pursued through the same means.

What do you think? Given that habitat destruction both drives biodiversity loss and amplifies wildlife disease outbreaks, should disease prevention be formally integrated into national conservation planning frameworks – and if so, who should be responsible for funding and enforcement? As climate change continues to push species into new regions, creating novel host-pathogen encounters, how should conservationists prioritize which ecosystems and species to monitor first?

References

1. https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/j.1469-1795.2008.00228.x
2. https://iucn.org/resources/issues-brief/wildlife-disease-surveillance
3. https://www.fws.gov/testimony/examining-impacts-disease-wildlife-conservation-and-management
4. https://www.zsl.org/what-we-do/projects/wildlife-disease-risk-analysis-and-health-surveillance