Bardian SPRING 2011

A female Anopheles gambiae mosquito
seen at 125x magnification. The
Anopheles gambiae is predominant in
Africa and is a disease vector for the
Plasmodium protozoa that cause
malaria.
©David Scharf/Getty Images
When we first made these discoveries about Lyme disease, we thought it was a coincidence that
the host that remained when diversity declined was also the best host for the pathogen and the vector.
But in the past few years, we have seen study after study that shows the same pattern for a number of
other disease systems. We suspect that some underlying biological reason explains why species that
thrive in low-diversity habitats are good hosts for pathogens and vectors, but we do not yet know what
it is. One hypothesis posits that pathogens evolve to be transmitted most efficiently by the host species
they encounter most frequently; abundant species also tend to be ecologically resilient so that they
persist when diversity is lost. That would explain the pattern we so frequently see between diversity
declines and increased disease transmission. Another hypothesis suggests that species that are
“weedy”—short-lived and fast reproducing—tend to invest less energy in defending their bodies from
certain kinds of attacks by pathogens. In other words, they may invest less in certain aspects of their
immune defenses. A number of species have been shown to conform to this pattern. Pathogens may
be able to adapt to these species by circumventing the immune defenses they do have, and these hosts
then become good at transmitting the pathogen. Weedy species also tend to thrive in low-diversity
habitats, so again, this would also explain the widespread correlation between reduced diversity and
increased disease transmission. A final possibility is that both of these hypotheses could be correct:
both pathways work together to reinforce the pattern we see.
Whatever the underlying reason, the connection between diversity and disease is sufficiently clear
and widespread that it lends extra importance to efforts to preserve biological diversity around the
world. We know how to conserve diversity in theory—we need to keep natural areas as large as possible
because larger areas of habitat have higher diversity. We also need to reinforce efforts to preserve diversity
in the face of real-world challenges, such as economic development that appears to be at odds
with preservation of natural habitats. The protection of human health is a powerful incentive for us
to seek and adopt the appropriate strategies.
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