Thursday, April 18, 2013

Biodiversity and disease risk

Lets think of all the mammals that currently inhabit Pound Ridge, NY: chipmunk, grey squirrel, mice, vole, groundhogs, moles, opossum, cottontail rabbit, raccoon, red fox, gray fox, bobcat, mink, white tailed deer, bats, coyote and bear.  Now think of the additional mammals that were here before the eastern forests were cut down: there were mountain lion, wolf, porcupines, beaver, woodland elk and weasels of all sizes.  When you stop to list them all, this diversity seems impressive.  It should be obvious this wildlife diversity varies greatly from place to place and depends on the presence of suitable habitat.

Consider our Lower Hudson/ Southern New England/ NY Metro region and all of the landscape pieces that make it up.  We have cities, state parks, county parks, small lot suburbs, large lot suburbs, farms of all sizes, pasture, orchards, wooded wetlands, lakes, ponds, rives, stream, town centers, tree farms, and (thanks to groups like PRLC) small nature preserves.  As you can see, there are a lot of different places for our mammals to live and each of these landscape pieces support a unique diversity of mammals.  
For example, the diversity of mammals in Van Cortland Park  is expected to be lower than that in Ward Pound Ridge Reservation.  Certain mammals need large tracts of land on which to roam (bobcat, for example), while other animals do fine in a heavily fragmented suburban setting (such as the white footed mouse).

Here is the $64,000 question: does a place's diversity affect how that place functions?  For years Dr. Richard S. Ostfeld from the Cary Institute of Ecosystem Studies has been asking this question.  Specifically, Dr. Ostfeld studies the ecological components that contribute to Lyme disease risk across the tri-state region.  His findings are shocking!  In a nutshell, here is what he has concluded:

First, a bit of background:

Back legged ticks (Ixodes scapularis) carry a spirochete (Borrelia burgdorfii) which, when entered into humans through a bite, causes Lyme disease.  These ticks are not born with the spirochete, they must pick it up from a vertebrate (commonly a bird or mammal) during a blood meal.  The animal that initially gives the spirochete to the tick is called a reservoir.  Just stop here for a second.  This is an important piece to understand.  Without the reservoirs, ticks in Pound Ridge wouldn't carry the spirochete that causes Lyme disease.  It is the reservoirs that keep infecting new generations of ticks with the infectious spirochete.    

Now on to Dr. Ostfeld's research.  He has found that:
1) Not all potential reservoirs are equally good at giving the spirochete to ticks.  Specifically, ticks that feed on white footed mice are very likely to obtain the spirochete during their blood meal, whereas ticks on opossum are much less likely to obtain the spirochete. (*The reason behind this is not totally understood but it likely has to do with the reservoir's immune system*).  This means that we would be better off with less mice and more opossums.  There's more.

This cartoon shows the differences between two disease reservoirs

2) Reservoirs vary in the amount of ticks that they carry.  Specifically, the average mouse carries roughly 50 ticks whereas the average squirrel carries roughly 150.

3) Reservoirs vary in the amount of ticks that they remove from their body.  Specifically, mice remove far fewer ticks from their body than opossums.

A white footed mouse with an ear full of ticks

4) A tick's chances of surviving the winter (an important component of their life cycle and therefore, disease transmission) depends on where it obtained its pre-winter blood meal.  Specifically, mice-fed ticks survive the winter better than opossum and squirrel-fed ticks.

From these findings we begin to realize that it matters very much which animal an uninfected tick feeds on.  Now on to the question of diversity.  If it matters which animals an uninfected tick feeds on and we know that different places have different groups of animals and levels of diversity, can we then begin to predict which places are more likely to have infected ticks (and sadly, infected humans)?  This is exactly what Dr. Ostfeld has done.

As diversity declines, disease risk goes up.   

Think about it this way: as a forested landscape gets fragmented by development the first animals to be lost are those that are long-ranging - animals like the bobcat and bear that just love to roam.  As we continue to whittle away at the landscape animals drop out in a predictable pattern based on their habitat requirements such as area, cover, and food source.  Some animals do very poorly in the modified environments that are now common throughout the region while other animals do very well.  You don't see a lot of porcupines around your house but you do see a lot of chipmunks, right?   What Dr. Ostfeld has discovered is that the species that thrive in our fragmented and human dominated environments (specifically, the white footed mouse) are the most efficient disease reservoirs.  His research shows that as animals are taken out of an ecosystem (aka, as diversity decreases), the risk of Lyme disease goes up.  He also shows that as predators are removed from an ecosystem the number of mice (very efficient disease reservoirs) goes up.

For these reasons, managing for vertebrate biodiversity should be a priority.  Predators like the red fox and bobcat are important in keeping down mice populations and critters like raccoon and opossum are helpful in diluting the reservoir effect or Lyme disease.  It makes me happy to know that the compost at the Armstrong House is visited by raccoons and opossum!

More resources:
This book is a fantastic read on the subject.
Click here to watch Dr. Ostfeld cover a lot of this material in a lecture.

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