The next epidemic caused by an emerging pathogen is already foreseeable – it’s time to take the ecological crisis seriously. The Ilkka Hanski Nature Network is a local initiative which contributes to the sustainability of global ecosystems.
The pandemic caused by COVID-19 is not the first, nor the last, disease outbreak where we can trace the origins back to human resource use and the resulting environmental change.
The COVID-19 disease is caused by a zoonotic virus, which are pathogens that are transmitted between animals and humans. The trend of emerging diseases is increasing, and indeed, this is not the first emerging zoonotic virus to infect humans. Currently approximately 260 viruses are known to have successfully made this transition. We have long battled with AIDS, Avian flu, SARS, MERS, Ebola, Zika, Dengue and Chikungunya as well as multiple other diseases caused by zoonotic viruses that have received less media attention.
These viruses are just the tip of the iceberg. It has been estimated that the known 5 400 bird and mammal species are host to some 1.67 M viruses still unknown to science. Of these, 631,000-827,0000 may be considered zoonotic with potential to transmit to humans.
Viruses are ubiquitous across plant and animal kingdoms, even fungi and bacteria host high diversity of viruses. Only recently scientists have had the sequencing-based tools required to efficiently unravel this enormous, often unseen, diversity of viruses. Only a fraction of this diversity is considered to consist of highly pathogenic viruses.
We are still far from understanding the ecological role of virus diversity found in nature. Categorizing the ecological outcome of host-virus interactions is not straightforward, as has become apparent also during the COVID-19 pandemic. The same virus may be lethal to some hosts, while asymptomatic on others.
A harmless virus can become highly virulent following a shift in the environment or a jump to a new host. As we are modifying our environment, the contact rates between humans and wildlife – often through livestock – have rapidly increased, offering opportunities for host shifts. Currently humans, their pets and livestock comprise approximately 96 percent of all mammal biomass on Earth, while wild mammal populations make of up the remaining four percent.
This ratio has been rapidly changing as natural habitats are declining following agricultural intensification, urbanization, and forestry. As a consequence, the population sizes in natural habitats have declined on average by some 60% in recent decades, and the current species extinction rates are estimated to be some hundreds to a thousand times higher than their natural background extinction rate.
Contrary to what one might expect, the threat of zoonotic pathogens to humans increases with declining wildlife populations. Ecological disease research has repeatedly demonstrated that the loss of native biodiversity increases the infection risk of individuals in the local host community. The mechanisms behind this phenomenon are currently under intensive investigation. However, some of the underlying causes are well understood.
Population declines and species loss in disturbed areas favor species that are characterized by a high reproductive rates and short life cycles. Such populations are typically characterized by high densities, and can be highly conductive to disease transmission. Infectious diseases that are transmitted among hosts benefit from such homogenization of their host communities.
Natural food webs are disturbed as species decline and go extinct. This creates pressure for changes in resource use at higher trophic levels for species such as pathogens. The need to survive and reproduce is a powerful driver of both ecological and evolutionary change. RNA viruses such as SARS-CoV-2 are often characterized by a high mutation rates that enable them to rapidly adapt to changing conditions.
In species rich communities interactions such as competition, predation and parasitism among species are effective mechanisms regulating population growth. As species are lost, these natural mechanisms are disturbed, and population sizes of individual species can rapidly increase.
These ecological rules also govern natural pathogen populations. Viruses are expected to be strongly regulated by competition for limited host resources with other species. These regulatory mechanisms are liable to breakdown following changes in host community diversity.
When these ecological mechanisms beak, the risks of disease emergence increases. When faced with a large, immunologically naïve host populations like humans, the stage is set for a pandemic.
Anna-Liisa Laine
Professor of Ecology, University of Zürich
Visiting Professor, University of Helsinki
Director of the Research Centre for Ecological Change