By: Dana Sackett, PhD
Given the unprecedented times we are in and that most with an interest in fish biology also have an interest in ecology and nature, I decided to take some liberties in this week’s The Fisheries Blog article. So please bear with me while I diverge from our typical fisheries-centered topics and speak more generally about ecology and biodiversity and how it may be able to help us in the future. I do throw in a fish example, so keep an eye out for it.
High levels of biodiversity (measured as the number of different ecosystems, species, and even genes in a population) provide essential services to life on our planet such as sequestering atmospheric carbon, recycling nutrients, and many others (for more on this see our article “The Value of a Species”). One, maybe unexpected, benefit of having high biodiversity (a large variety of species in a habitat for example) is the reduction of disease transmission to people. The majority of human infectious diseases have animal origins, most of which are from mammals. These types of diseases even have their own name, ‘zoonotic infectious diseases’ or those that affect people but initially come from animal sources. The current infectious disease caused by a coronavirus wreaking havoc around the world is an example of a zoonotic infection and one that originated from a mammal.
Numerous studies have found that zoonotic infections in people increase with the loss of natural habitat and biodiversity. This link has been found so often by scientists that many have been able to predict the areas and likelihood of zoonotic diseases emerging and affecting humans (called a ‘disease hotspot’; a quick google search can show just how many different predictive maps exist). While more biodiverse areas can harbor more potential viruses (because there are more species) this does not seem to translate to more diseases in humans. It may seem counter-intuitive that more species in an area would result in it being less likely for those species to transmit a disease to people but it makes more sense when the reasons for this correlation are explained. I attempt to do this below.
First, when people encroach on natural habitats for farming, harvesting resources, or to establish settlements they often destroy the natural habitat that is already there causing a decline in the number of individuals, species, and ecosystems (loss of biodiversity). When people intrude into these new areas they are exposed to a vast diversity of animals and microorganisms, many that people may have never encountered before. Also, the more people encroaching into these new habitats the more likely one will encounter an animal that may be carrying a microorganisms that can cause a potential zoonotic disease (for example rodents, bats, primates, and carnivores are the most common carriers of diseases that have the potential to affect people).
If an encounter between an animal and person in this new area does result in disease transmission to the person (this is called spillover: when a virus jumps to a new host species), it only becomes problematic at the population level if there are a lot of encounters between the infected animals and people or if the disease is infectious from person to person and there are a large number of people nearby for the virus to spread to. Thus, areas where natural habitat is being disturbed near large populations of people create conditions for a new disease to emerge and spread. Indeed, almost half of the zoonotic diseases that have emerged in humans since 1940 resulted from changes in land use or hunting wildlife because these activities increase contact between humans and animals. For instance, a virus called Nipah jumped from wild fruit bats to domestic pigs in Malaysia where farmland encroached on natural habitat. The high densities of pigs in those local farms facilitated pig-to-pig transmission and allowed the virus to spillover from pigs to humans. In humans, this virus had a devastating 40% to 90% mortality rate.
The loss of species in a community has also been more directly linked to increased disease in humans. To better understand how this can occur I will use a simplified hypothetical fish example. Let us imagine that there is a pond that can support 50 fish, which are made-up of 10 different fish species. Two of those 10 species have the potential to harbor a virus that can infect a person (it is important to note that fish viruses generally do not harm humans but for this example we will pretend they can). Under normal conditions this variety of fish species would help to reduce the total number of virus-carrying fish in the pond and reduce their encounter rate with other fish that can carry the disease (see below figure on the left). However, if the pond is disturbed, causing 6 species (that cannot be infected by the virus) to die-out, the remaining 4 species (two of which can spread the disease) would increase in abundance, filling the empty habitats in the pond. This would cause more encounters between fish that can carry and spread the disease.
In addition, a person that ventured into that hypothetical disturbed pond would now have a much higher likelihood of encountering a fish that has the disease. While this is an extremely simplified hypothetical situation that ignores many other factors that can contribute to disease spread (such as crowding of a species preferred habitat, behavior, stress from disturbance, higher levels of intraspecific competition, and possible trophic cascades), it has happened in numerous ecosystems. West Nile virus, a mosquito-transmitted virus that is carried by passerine birds is one example. Studies found that communities with low bird diversity tended to be dominated by species that could spread the virus. This led to a high occurrence of infection in mosquitoes and people. Communities with high bird diversity, on the other hand, had many birds that were not hosts to the mosquito that carried the virus, which reduced the spread of the disease.
But what if the species that harbors and spreads the disease is the one that is lost? If that is the case, instead of amplifying the spread of the disease, that loss would reduce disease risk. However, consistently this does not seem to be the case. Habitually, the species most likely to be lost from an ecosystem as diversity declines are the ones that reduce the spread of disease. For example, in eastern North America the white-footed mouse is simultaneously the most abundant host species for Lyme disease and the most resilient to habitat loss and disturbance. In contrast, Virginia opossums are poor hosts for Lyme disease because they kill the vast majority of ticks that attempt to feed on them. They are also one of the first species to disappear when forests are degraded and biodiversity declines. The presence of opossums and other “dead-end” host species essentially dilutes the presence of the infected white-footed mice reducing the chance of transmission to people (this phenomenon is called the “dilution effect”). The frequency with which this dilution effect is seen in nature seems to suggest that the traits that make an animal resilient to a loss in biodiversity may also make them susceptible to disease.
Predicting where and when disease outbreaks will occur remains one of the biggest scientific challenges of our time. But as stated by Han and other scientists in 2016: “… the frequency with which new infectious diseases are emerging underscores the necessity of shifting from a reactionary to a pre-emptive approach to mitigating infectious disease.” This statement is so much more poignant today than it was when stated four years ago. My hope is that we can come out of this pandemic with the mindset that taking preventative steps to avoid disease outbreaks, and prepare for those outbreaks that we know are likely to occur, are essential to our future. To this point, models have suggested that linking conservation efforts with smaller-scale land-use approaches will help lessen the occurrence and impact of emerging infectious diseases in people.
Thank you for bearing with me on my tangential topic this week. We will be back to your more fisheries-based topics next week!
References and additional reading material:
Allen T, Murray KA, Zambrana-Torrelio C, Morse SS, Rondinini C, Di Marco M, Breit N, Olival KJ, Daszak P. 2017. Global hotspots and correlates of emerging zoonotic diseases. Nature Communications 8: 1124. doi:10.1038/s41467-017-00923-8
French RK, Holmes EC. 2020. An ecosystems perspective on virus evolution and emergence. Trends in Microbiology, 28: 165-175. https://doi.org/10.1016/j.tim.2019.10.010
Han BA, Kramer AM, Drake JM. 2016. Global patterns of zoonotic disease in mammals. Trends Parasitol 32: 565–577. doi:10.1016/j.pt.2016.04.007
Keesing F, Belden LK, Daszak P, Dobson A, Harvell CD, Holt RD, Hudson P, Jolles A, Jones KE, Mitchell CE, Myers SS, Bogich T, Ostfeld RS. 2010. Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468: 647–652. doi:10.1038/nature09575
Nabia G, Siddiqueb R, Alid A, Khan S. 2020. Preventing bat-born viral outbreaks in future using ecological interventions. Environmental Research 185: 109460 doi.org/10.1016/j.envres.2020.109460
Ostfeld RS. 2009. Biodiversity loss and the rise of zoonotic pathogens. Clinical Microbiology and Infection. 15: 40-43. doi.org/10.1111/j.1469-0691.2008.02691.x
Patil RR, Kumar CS, Bagvandas M. 2017. Biodiversity loss: public health risk of disease spread and epidemics. Annals of Tropical Medicine and Public Health 10: 1432-1438.
Pongsiri MJ, Roman J, Ezenwa VO, Goldberg TL, Koren HS, Newbold SC, Stfeld RS, Pattanayak SK, Salkeld DJ. 2009. Biodiversity loss affects global disease ecology. Bioscience 59: 945-954. doi:10.1525/bio.2009.59.11.6
Rulli MC, Santini M, Hayman DT, D’Odorico P. 2017. The nexus between forest fragmentation in Africa and Ebola virus disease outbreaks. Science Reports 7: 41613. doi:10.1038/srep41613
Granter, SR, Ostfeld RS, Milner Jr DA. 2016. Where the Wild Things Aren’t: Loss of biodiversity, emerging infectious diseases, and implications for diagnosticians. Amercian Journal of Clinical Pathology 146:644-646. doi: 10.1093/AJCP/AQW197
Wang L-F, Walker PJ, Poon LLM. 2011. Mass extinctions, biodiversity and mitochondrial function: are bats ‘special’ as reservoirs for emerging viruses? Current Opinion in Virology 1:649–657. doi:10.1016/j.coviro.2011.10.013
Wilkinson, DA, Marshall JC, French NP, Hayman DTS. 2018. Habitat fragmentation, biodiversity loss and the risk of novel infectious disease emergence. J. R. Soc. Interface 15: 20180403. doi.org/10.1098/rsif.2018.0403
Young HS, Parker IM, Gilbert GS, Guerra AS, Numm CL. 2017. Introduced species, disease ecology, and biodiversity–disease relationships. Trends in Ecology & Evolution. 32: 41-54. doi.org/10.1016/j.tree.2016.09.008