Biodiversity, globalization and shifting disease ecologies

Urban areas intrude on once-wild places. On the edge of the Everglades, in Florida.

As I was researching a story for the Observer, I had to go poking through the December 2009 issue of BioScience, and I stumbled across an interesting article reviewing biodiversity declines and global disease ecology. The authors assert that multiple factors working synergistically are leaving humans more at risk of contracting infectious diseases — some new and some classical but re-emerging. This is the gist of their paper:

“We propose that habitat destruction and biodiversity loss associated with biotic homogenization can increase the incidence and distribution of infectious diseases affecting humans.” {1}

This idea is a chronic, low-grade theme in media stories in recent time: Swine flu, ebola virus, nipah virus, West Nile virus, the re-emergence of cholera, and the forces of climate change altering the disease dynamics of insect-borne diseases like malaria, to name a few.

The authors write that the coupling of both globalization — and its ability to shuttle rapidly people and organisms across the globe — and ecological disruption (in the form of degraded or altered habitat) are linked to emerging and re-emerging infectious diseases. While many people have hashed out whether we are or are not in middle of a sixth mass extinction event (most Big Thinkers believe we are), the authors say that missing from this conversation is the link between biodiversity declines and emerging and re-emerging pathogens. They then layout several case studies describing just this. If ever there were a reason people should care about species extinctions, the idea that this trend is linked to increasing relative risk of human disease should be The One Reason they pay attention. (Personally, I happen to love and support and care for all Life on earth, and I don’t need a reason like this to get me to care about species extinctions… but I know that some people will never care one iota unless it directly affects them personally.)

Not only have high-speed transportation networks across the globe helped pathogens and invasive species to spread more quickly, but deforestation and habitat fragmentation have led to biodiversity losses that alter the structural diversity and habitat complexity that help some ecosystems be more resilient to disease or invasive species. Biodiversity losses can also lead to altered species compositions or homogeneity where disease could be spread more quickly. Take sudden oak death, for example, it has more than 100 species that act as vectors for its spread but only one or two species are the real drivers for the infectious cycle. One of these species, California bay laurel, quickly colonizes areas where fires wiped out older mature forests that were diverse, thereby speeding the infectious process across the landscape and changing the structural diversity of the new habitat.

Unless you have been living in a cave, you’ve probably noticed a huge push to combat malaria globally in the past several years. But have you thought about how “habitat alteration, fragmentation, and deforestation can increase the risk of malaria transmission through effects on mosquito survival, density, and distribution”? I hadn’t… Deforestation coupled with a warming climate can create microclimates where mosquitoes digsst their blood meals more efficiently, leading them to feed and lay eggs more frequently — perhaps altering the disease dynamics and abundance of the malarial parasite in the environment. Deforestation has already been documented to boost the populations of Anopheles darlingi in the Amazon, the most efficient vector of malaria in this region, and to be linked with higher rates of humans being bitten. Food for thought next time you think you’ve no need to care about Amazonian deforestation.

And if you don’t have enough reasons to care about vertebrate diversity, here’s another one: it may protect you from hantaviruses. The authors review a case study in Panama where it was found that areas with human activities that cause a decline in vertebrate diversity, combined with dry spells followed by rain, are more prone to hantavirus outbreaks. Apparently having a high diversity of rodents around decreases the density of rodents that act as the resevoir host, thereby reducing the relative risk that a human will breath in the aerosolized rodent urine or feces that transmit the virus and cause pulmonary problems and even death. (Not a good way to go, is my guess.)

As development creeps into what was formerly wild places, people are placed in closer proximity to wild species. And if they have livestock, it may not even take direct contact for transmission of bacteria, viruses or parasites to occur. By focusing on human diseases as an outcome of biodiversity loss and globalization, the authors create a strong frame with which to communicate the importance of biodiversity conservation.

The intrinsic value of biodiversity does not register on everyone’s barometer of values. So next time you find yourself talking to someone who does not care about biodiversity loss, thinks climate change is overblown or thinks its fine to dump concrete all over the planet, use the ideas in this post to frame a message that in the end, these things will just circle back to bite humanity in the rump. And the threat to each of us may be as close as an airplane ride away.

NOTES:

1. I originally wrote this post in September 2009; but in switching over to using ResearchBlogging.org I needed to re-post it for their system to recognize it. Thanks for your patience.

{1} Pongsiri, M., Roman, J., Ezenwa, V., Goldberg, T., Koren, H., Newbold, S., Ostfeld, R., Pattanayak, S., & Salkeld, D. (2009). Biodiversity Loss Affects Global Disease Ecology BioScience, 59 (11), 945-954 DOI: 10.1525/bio.2009.59.11.6

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