A new study titled A meta-analysis on global change drivers and the risk of infectious disease and published in the journal, Nature on May 08, 2024 reveals that four human-induced global changes–biodiversity loss, introduction of non-native species, climate change, and chemical pollution are the primary drivers of disease spread among not only humans but also plants and animals.
The study was done by 20 researchers from the US-based universities, including the University of Notre Dame, Yale University, Oregon State University, and the University of Connecticut.
Anthropogenic or human-induced change is simply defined as alterations in the environment that result from human action or presence. This might be intentional, such as in the case of landscape modification, the introduction of new species, or clearing land for agriculture. These human activities can lead to unrecognised or poorly understood side effects such as infectious diseases.
Previous studies have also highlighted the connection between disease spread and environmental changes, but their approach was more targeted. For instance, a study conducted in 2023 discovered that warming temperatures might be contributing to a wider spread of malaria in Africa.
Objective of the Study
The main objective of the study is to ascertain the degree to which drivers of global change are linked to the risk of infectious diseases and if these relationships vary on ecological settings, such as host or parasite taxon or human vs non-human diseases.
Some Major Findings of the Study
Although the spread of diseases can be influenced by a variety of factors, researchers have identified five key global change drivers that influence disease spread: (i) biodiversity loss, (ii) climate change, (iii) chemical pollution, (iv) non-native species, and (v) habitat loss. After compiling information from around a thousand earlier researches, the researchers examined how these drivers affected the incidence and severity of infectious diseases in humans, animals, and plants on all continents except Antarctica.
The study reveals that biodiversity loss, which refers to the loss of plant or animal species at a local or worldwide scale, has the most significant impact. Biodiversity-rich areas are believed to mitigate the spread of infectious diseases through a phenomenon known as the ‘dilution effect’. The hypothesis states that parasites and pathogens that cause disease have evolved to spread and survive more readily when they infect common host species rather than rarer host species because there are more hosts for them to infect. As humans destroy biodiversity, rarer species tend to disappear first. Consequently, the abundant species and the diseases that they carry may increase, raising the overall risk of infectious disease outbreaks.
The US Lyme disease is a prime example of how human-induced biodiversity loss leads to a higher risk of disease. The white-footed mice are the most abundant and the most competent host; whereas, larger mammals tend to be rarer and less competent. As we lose larger mammals due to human-induced biodiversity loss, we are left with proportionally more white-footed mice and therefore, a higher risk of disease.
According to the World Wide Fund for Nature’s Living Planet Report 2022, the human activities have led to a significant decline in plant and animal species populations, with an average decline of 69 per cent of wildlife population in the past 50 years. Additionally, as per a report by the Royal Society, between 1990 and 2020, around 420 million hectares of forest (mainly tropical forest) were lost, and a further 10 million hectares are being lost each year.
Other global change factors can also substantially amplify the disease spread in several ways. When non-native species are introduced into an environment, they bring new pathogens and parasites, which can lead to the spread of new diseases. For example, the Asian tiger mosquito arrived in Europe from Asia and brought along diseases such as dengue and chikungunya. Climate change can change the migratory patterns of species, causing them to relocate to new areas, interact with local species, and exchange pathogens.
Habitat loss is the startling discovery of the study. Habitat loss occurs when a habitat is no longer able to support a certain species or group of species and, appears to reduce the spread of disease. The reason might be the rapid urbanisation, which limits habitat for wild hosts and parasites while ushering in better sanitation and health facilities than rural areas.
Conclusion
Overall, the study reveals that biodiversity loss, chemical pollution, climate change, and introduced species are associated with increases in disease-related end points or harm, whereas urbanisation is associated with decreases in disease end points. Deforestation, forest fragmentation, and natural biodiversity gradients are either comparably insignificant or idiosyncratic as drivers of disease.
The study has certain limitations, such as the previous meta-analyses often focus on a single global change driver, ignoring the fact that organisms may be experiencing multiple of these drivers simultaneously in the real world. For instance, climate change and chemical pollution can cause habitat loss and change, which in turn can cause biodiversity loss and facilitate species introductions. Therefore, future research must determine if the combined effect of these factors increases, decreases, or multiplies the risk of infectious disease outbreaks.
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