Biodiversity loss: The hidden catalyst behind rising infectious diseases, says study

By Vivek Saini

In an age where pandemics threaten global health, understanding the root causes of disease emergence is more critical than ever. Surprisingly, not the abundance of wildlife but rather the loss of biodiversity plays a pivotal role in the rise of infectious diseases. The lush rainforests, vibrant coral reefs, and expansive grasslands of our planet, often seen as bastions of life, are now recognized as unlikely yet significant contributors to the spread of zoonotic diseases—those that jump from animals to humans. A landmark study published in Nature has unveiled a startling truth: biodiversity loss stands as a major environmental factor driving these outbreaks. As habitats are destroyed and ecosystems disrupted, the intricate balance that once kept pathogens in check is lost, leading to increased human exposure to new and dangerous diseases.

How Habitat Loss Breeds Infectious Diseases

The lush rainforests, teeming coral reefs, and vast grasslands we often associate with biodiversity might seem like breeding grounds for disease. However, the reality is far more nuanced. A recent study published in Nature analyzing nearly 1,000 studies revealed a surprising truth: biodiversity loss, not a surplus of life, is the most significant environmental driver of infectious diseases emerging and jumping from animals to humans (zoonotic diseases).

Dr. Petch Manopawitr, Conservation Scientist and Technical Advisor on Biodiversity Conservation, Protected Area Management and Sustainability while interacting with CFC India, offers valuable insights, clarifying that “although the study does mention that deforestation and forest fragmentation are comparatively unimportant (as drivers of disease), this doesn’t necessarily mean that all types of habitat loss have equal effects.” Wetlands, for example, are critical ecosystems potentially impacted by degradation. He emphasizes that “when wetlands are degraded or destroyed, it can lead to changes in the distribution and abundance of these species, potentially bringing them into closer contact with human populations or livestock. This increased contact could facilitate the transmission of zoonotic diseases.”

Dr. Manopawitr emphasizes that “The health impacts of biodiversity loss and associated disease emergence are likely to be felt most acutely by vulnerable populations, such as those living in poverty, in rural or remote areas, or in regions with limited access to healthcare services. These populations may be more directly dependent on natural resources and ecosystem services and may have fewer resources available to cope with the health and economic consequences of disease outbreaks. To prevent another catastrophic pandemic like COVID-19, we must take decisive action to maintain ecosystem health, reduce greenhouse gas emissions, control invasive species, and halt biodiversity loss at all costs” 

This seemingly counterintuitive finding highlights the delicate balance that healthy ecosystems maintain. Biodiversity acts as a natural buffer against disease outbreaks, and its decline disrupts this equilibrium in several ways:

• Increased Host-Pathogen Encounters: Imagine a diverse ecosystem as a bustling social gathering. Different animal species, each potential hosts for various pathogens (viruses, bacteria, parasites), coexist in a complex web of interactions. However, habitat loss fragments and destroys these ecosystems, forcing remaining wildlife into closer proximity. This creates a scenario akin to a crowded party where everyone interacts with everyone else. The increased frequency of contact between various animal species significantly increases the chances of pathogens “spilling over” from their usual hosts to new ones, potentially including humans who encroach on these shrinking habitats. Studies like the one published in Ecology Letters demonstrate this link, showing how habitat fragmentation in the Amazon rainforest led to a higher prevalence of zoonotic diseases in primates.

• Loss of Reservoir Dilution: Think of a healthy forest ecosystem with various mammal species. Some of these mammals might be natural carriers of specific pathogens. Still, their populations are kept in check by the overall health of the ecosystem and the presence of competing species. However, when habitat loss shrinks this ecosystem and eliminates some species, the remaining carrier population can experience a boom. This creates a more extensive “reservoir” of the pathogen, making it more likely to spill over to other species, including humans. A study published in Emerging Infectious Diseases supports this concept, highlighting how deforestation in Malaysia led to increased human exposure to the Nipah virus, carried by fruit bats whose populations boomed due to habitat changes.

• Disruption of Predator-Prey Dynamics: Predators play a crucial role in maintaining healthy ecosystems by keeping prey populations in check. However, habitat loss often disproportionately affects predator populations, leading to an abundance of prey species that might also act as carriers for specific pathogens. As described in a study published in the Journal of Applied Ecology, this scenario can create ideal conditions for pathogens to thrive within these unchecked prey populations and potentially spill over to other species.

Beyond the Pandemic: Why Protecting Nature is Crucial for Public Health

The recent COVID-19 pandemic was a stark reminder of human health’s vulnerability to emerging infectious diseases. However, the link between environmental changes and disease outbreaks goes far beyond the current crisis. A growing body of research, including the abovementioned study in Nature, highlights a critical truth: protecting biodiversity is not just about preserving beautiful ecosystems. It’s a cornerstone of safeguarding public health.

Dr. Manopawitr, a specialist in this field, sheds light on the complexities of how biodiversity loss impacts disease risk across regions and ecosystems. He highlights that “studies have shown that the diversity of many taxa, including pathogens and their hosts, tends to increase towards the equator. This means that tropical regions, which harbor a greater diversity of species, may be more susceptible to the effects of biodiversity loss on disease dynamics. It’s very important to be aware that tropical regions definitely have more to lose when it comes to biodiversity loss and disease outbreaks.” The impact can also vary depending on the ecosystem. For example, “in forest ecosystems, the loss of predators or other key species can lead to changes in the abundance and behavior of reservoir hosts, potentially increasing the risk of zoonotic disease transmission. In aquatic ecosystems, the loss of biodiversity can alter the balance between pathogens and their hosts, leading to increased disease outbreaks in fish or amphibian populations.”

Additionally, Dr. Manopawitr emphasizes the role of host specificity: “the degree of host specificity exhibited by pathogens can also influence the impact of biodiversity loss on disease dynamics. In regions where pathogens are highly host-specific and endemic, the loss of biodiversity may have a more direct impact on disease risk, as the removal of key host species can disrupt pathogen transmission cycles.” Finally, he points out the potential dangers of human expansion: “In regions where human populations are rapidly expanding into previously undisturbed areas, the effects of biodiversity loss on disease risk may be amplified by increased contact between humans, wildlife, and potential disease reservoirs.” Understanding these factors is crucial for developing targeted strategies to mitigate disease risks in different contexts.

Here’s why a healthy natural world is essential for preventing future pandemics:

• Reduced Disease Transmission Risk: As discussed earlier, biodiversity is a buffer against disease outbreaks. Diverse ecosystems with many species create complex interactions that keep pathogens in check. This reduces the likelihood of any single pathogen becoming dominant and spreading to humans. A study published in Science Advances exemplifies this concept, demonstrating how increased plant diversity in agricultural landscapes can lower the transmission risk of mosquito-borne diseases.

• Natural Disease Reservoirs: Many wild animals, particularly bats and rodents, harbor viruses and bacteria that can potentially jump to humans. However, predators and competing species keep these “natural reservoirs” in check in a healthy ecosystem. When habitat loss disrupts these ecosystems, it can lead to population explosions in these carrier species, increasing the risk of spillover events. A study published in PLOS ONE reinforces this concern, highlighting a link between deforestation in Southeast Asia and the emergence of the Hendra virus in fruit bats.

• Early Warning Systems: A diverse and healthy natural world offers valuable clues about potential disease threats. Scientists can monitor wildlife populations and track the prevalence of pathogens in these reservoirs to identify possible outbreaks before they spill over to humans. This “early warning” system nature provides is crucial for developing preventive measures and mitigating the impact of emerging diseases. A study published in Philosophical Transactions of the Royal Society emphasizes the importance of wildlife disease surveillance for preventing future pandemics.

By protecting biodiversity, we safeguard the natural world and invest in a healthier future for ourselves. Understanding this critical connection is essential for informing policies and promoting practices that ensure the well-being of humans and the planet.

How Environmental Changes Fuel Disease Emergence

The link between biodiversity loss and infectious diseases, although crucial, is just one piece of the puzzle. A broader understanding of how environmental changes create a breeding ground for emerging diseases is essential. Here’s how various environmental factors are contributing to this complex issue:

• Climate Change: Rising global temperatures are altering weather patterns and expanding the geographic range of disease-carrying vectors like mosquitoes and ticks. This can introduce previously unseen pathogens into new regions, making populations more vulnerable. A study published in PLOS Medicine explores this connection, demonstrating how climate change is expected to increase the global suitability of malaria transmission zones by up to 5%.

• Intensification of Agriculture: Modern agricultural practices, characterized by monoculture farming and heavy reliance on pesticides, contribute to environmental degradation and biodiversity loss. This can disrupt natural predator-prey relationships, leading to population explosions in potential disease reservoirs like rodents. A study published in Ecology Letters investigates this link, highlighting how agricultural intensification can increase the risk of spillover events from wildlife reservoirs to humans.

• Wildlife Trade and Consumption: The global trade and consumption of wild animals, particularly bushmeat in some regions, creates opportunities for pathogens to jump from wildlife to humans. During these interactions, people come into close contact with various viruses and bacteria that can spill over and cause outbreaks. A review published in Clinical Microbiology Reviews explores this risk factor, highlighting the role of the wildlife trade in the emergence and spread of infectious diseases.

Understanding these complex interactions between environmental changes is essential for developing effective strategies to prevent future disease outbreaks. By addressing the root causes, such as climate change and unsustainable practices, we can create a healthier planet for humans and wildlife.

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