The biodiversity cure

MILLBROOK — Richard Ostfeld shared a decade of scientific research on the ecology of human health at The Cary Institute of Ecosystem Studies in Millbrook, at a free public lecture Friday evening, March 5. His explanation of the proven link between the decline of biodiversity and the spread of emerging infectious diseases immediately commanded attention in the packed auditorium.

Ostfeld began his lecture with unpleasant, undeniable facts. An unprecedented global extinction of species, larger than that which occurred during the time of the dinosaurs, has already occurred in the modern era.  In fact, 2,000 species of Pacific birds have disappeared forever, and it is estimated that every 20 minutes another species becomes extinct.  At the same time, zoonotic diseases, like Lyme and West Nile virus, have multiplied, especially in the temperate zones of the Northeast in the United States and in Western Europe.  

Ostfeld’s hypothesis, called the dilution effect, posits that loss of vertebrate diversity increases the spread of zoonotic diseases to humans. He explained that zoonotic diseases, like Lyme, involve a pathogen, a human, a vertebrate reservoir species, a vector species and potentially other species that interact with these organisms. As calculated by the CDC (Centers for Disease Control and Prevention), reported Lyme disease cases between 1982 and 2008 grew from zero to more than 30,000. The actual number of cases is estimated at 300,000, primarily in the Northeast, the Upper Midwest and the West Coast.  

To understand the transmission of  the Lyme disease bacteria Borrelia burgdorferi, which cause Lyme disease, you must begin with the two-year life cycle of Ixodes scapularis, more commonly known as the deer tick.  The almost invisible larvae are hatched without the Lyme infection, but when they feed on the blood of an infected host the larvae grow into infected nymph ticks that can transmit the disease to humans. The larvae come alive in the spring, feed and drop off their hosts, and then emerge as infected nymphs in the middle of the summer, growing into adult ticks by late fall.

Ostfeld has been counting larvae and nymphs and ticks for the last 15 years and discovered that the number of larvae in a season does not predict the number of infected nymphs because of the variability of the responses by potential hosts. Every type of host, from opossum to mouse, responds differently to larvae.  The controlled scientific research to measure these responses in a lab is labor and time intensive.

But the conclusion drawn from recording the responses of six host species (white-footed mice, squirrels, chipmunks, opossums, veeries and cat birds) is dramatic. For example, white-footed mice are much more likely to become infected with tick nymphs and less likely to kill them. For every 50 nymphs a mouse kills, it is calculated that a squirrel kills 843 and an opossum kills over 5,686.

The dilution effect occurs when natural habitats are degraded, usually by human land development. The mice remain and other species disappear. As non-mice species become less present, tick meals on infected mice increase and the likelihood of transmission to humans grows. If opossums disappear from an environment the number of infected nymphs rise 30 percent. But if mice were to be removed, Lyme-infected nymphs would decrease by 75 percent. High host diversity reduces the density of the nymphs simply because there is a lower percentage of mice that are the prime “reservoir†species for ticks.

Ostfeld’s dilution effect research proves that the presence of more species of hosts in a habitat results in fewer mice to become easily infected by ticks.   The next question was, could the dilution effect be used to predict where there will be more infected nymphs? Ostfeld’s research examined 14 forest fragments and determined that forests smaller than 5 acres had four to five times the number of Lyme infected mice as parcels 6 to 16  acres in size. This is a result of  their lack of biodiversity and large mouse populations on smaller pieces of land.

The Cary Institute’s research also analyzed West Nile virus, which spread across the continent from New York to California in less than four years. The disease was carried by common birds — blue jays, grackles, crows, house finches, robins and sparrows — which are present everywhere.  Using national bird count numbers, the Cary  team predicted that counties with high bird diversity would have a lower incidence of West Nile disease. Research confirmed that counties with low bird diversity had 10 times the number of West Nile cases than those places with an abundance of bird species.

Ostfeld then applied the research to Hantavirus pulmonary syndrome, which is spread by the deer mouse, schistosomiasis, which sickens over 200 million people in sub-Saharan Africa and Southeast Asia, and even rust fungus on grasses and grains such as wheat and rice.  The dilution effect hypothesis holds true that reduced diversity results in increased infections.

Regardless of the disease, the “reservoir†species that act as carriers share common traits.   Ostfeld described them as “fast.† They are species like mice, “Who proliferate dramatically, have a low immune investment,†and are very adaptable in a degraded environment. They thrive where other species cannot, which is why you are five times more likely to find a Lyme disease-bearing tick in your wooded backyard than in a large state forest. Diverse animal populations that help minimize exposure to diseases are threatened by habitat loss, climate change and the incursion of invasive species.  

In closing, Ostfeld thanked Dutchess County for its support of Cary Institute research, and reminded everyone that 2010 is the “International Year of Biodiversity.†There was not enough time to answer all the questions. Do fungus sprays work? What should you do with a tick infected athletic field?  Is citronella better than Deet?   Persistent questioners mobbed  Ostfeld at the podium. He did say there was no correlation between the number of deer and the number of infected larvae.

For information on upcoming lectures in April and the Cary Institute Summer Ecology Camp, go to the Cary Web site at caryinstitute.org.