Rediscovering Biology: Molecular to Global Perspectives
Biodiversity Expert Interview Transcript: Rick Ostfeld, PhD
Rick Ostfeld, PhD
Animal ecologist at the Institute of Ecosystem Studies in Millbrook, New York. His research focuses on the interactions among organisms that influence the risk of human exposure to vector-borne diseases and the dynamics of terrestrial communities. His recent research has focused on the causes of the spread of Lyme disease in New England.
Rick Ostfeld, PhD, is an animal ecologist at the Institute of Ecosystem Studies in Millbrook, New York. His research focuses on the interactions among organisms that influence the risk of human exposure to vector-borne diseases and the dynamics of terrestrial communities. His recent research has focused on the causes of the spread of Lyme disease in New England.
What is meant by the term ‘ecosystem services’?
Ecosystem services are things that natural ecological systems, ecosystems, provide for human beings, and they include things like filtering water, purifying water, purifying the air. Ecosystem services include agricultural ecosystems that provide food for us. They provide oxygen because trees are producers of oxygen, and they are members of ecosystems. So those are many of the things that life depends on are produced by ecosystems and those are what are considered services.
And these services are provided for all life forms?
That’s true, although oftentimes when people think of ecosystem services they are specifically referring to those that are supplied to humans, although I certainly agree that ecosystems provide services to all the components that occur in those ecological systems.
What is meant by ‘habitat destruction’?
A habitat is defined as sort of the address where an organism lives. And so if you can describe all those different features of that address, then you are describing the habitat of an organism.
So, for instance, a salamander might like moist forest, and it might like to live in places where there are a lot of fallen logs, so its habitat would be intact, maybe old mature forest that has a lot of fallen, woody debris, as it’s called, lying on the ground.
So any activity that destroys or removes that kind of habitat is considered “habitat destruction.” You’re actually taking away the home that that particular organism needs in order to make its living.
What is the role of that loss in terms of lost biodiversity?
Well, probably the principal cause of the loss of biological diversity or “biodiversity” is the destruction of habitat. There are other causes of the loss of biodiversity of certain species. There are some species that are specifically targeted for exploitation, such as over-fishing or over-hunting of particular animals, and they can be lost as a component of biodiversity that way.
But by and large, the way species are lost and populations and genetic strains which are all components of biodiversity, the way they’re lost from natural systems is through destroying the habitat, taking away the places that they need in order to make their living in the world.
What is a zoonotic disease?
A zoonotic disease is a disease in which the pathogen, the virus, bacterium, fungus, protozoan-there are many different kinds of pathogens, of disease-causing organisms-in the case of zoonotic diseases, the origin of that pathogen is in some animal population in the wild.
So there are diseases like influenza or chicken pox-things that we pass back and forth between one another-and those are not zoonotic diseases. But if we acquire the pathogen that causes the disease from some animal in nature, then that’s considered a zoonotic disease.
And what is a ‘disease vector’?
A disease vector is an organism that is actually responsible for transmitting the disease from one victim to another. So probably the most familiar disease vectors are mosquitoes. West Nile virus is a disease in which the mosquito at one point in its life bites a bird, and if that bird is infected with the virus, the mosquito will acquire the virus, carry it around as it flies around, and then if it bites a human being later on in its life then it can transmit that disease. That mosquito is a vector. It carried the disease agent from one host, the bird, to another host, the person.
So does such a vector suffer the symptoms of the disease?
It’s an interesting question whether vectors actually suffer symptoms of the disease, or are affected by these pathogens. In many cases, they do have the bacteria or viruses in the stomach or gut tissues, which is oftentimes where these pathogens exist in the mosquito or tick vector.
It sometimes does cause some loss of viability, so some modest impact on the behavior or lifespan of the vector. Oftentimes it’s extremely subtle, and the vectors do just fine even with a bunch of viruses or bacteria living inside them.
What is a spirochete?
A spirochete is a classification of bacteria that, as the name suggests, typically is found in a spiral type shape. Sometimes they look like a curly French fry in profile. And spirochete bacteria are responsible for diseases like syphilis, and Lyme disease, as well as other types of diseases like Relapsing Fever and a bunch of exotic ones.
So Borrelia burgdorferi, which causes Lyme disease, is a spirochete?
Borrelia burgdorferi is in fact a spirochete bacterium, yes.
Earlier, we saw Vicki doing some work in the field. Can you explain a bit about how that work relates to your research?
Vicki and the crew that she was leading are actually trying to understand the population size of different vertebrate animals, mammals and birds in particular, here in southeastern New York. Those are the important animals and vertebrates for our purposes.
We need to understand the population size of those animals, their reproductive rate, how many ticks are on an average member of each species in the forest. And also what the diversity is of different kinds of mammals and birds in the forest, because of our suspected impact that biodiversity has on Lyme disease.
So they’re actually censusing the animals, and of course to census things like white-footed mice, chipmunks, shrews, and the like, you can’t just send them a form and hope to get a good return rate. You have to actually go out into the field, capture them in live traps, so they’re captured alive and provided with food overnight. These are nocturnal animals so they’re active at night. And then they’ve given a numbered metal ear tag that actually gives them an identity.
And so we know for each individual, where it was caught, how often it was caught, when it goes into reproductive condition, when it has babies, and when it disappears, which approximates when we think it dies, and how many ticks it has on its body throughout its lifetime. So we actually are gathering as accurate estimates as we can of who exists out in the forest that can possibly supply blood meals to the tick parasite population.
What are the specific measuring techniques?
Well, here on site we’re actually not doing a species area curve. We’re relying on other scientists who are working in highly fragmented areas. We’ve just actually gotten word that we will be funded over the next four years to do research in fragmented landscapes, areas where a lot of the forest has been destroyed, and what remains occurs as small fragments in the northeastern U.S. But other scientists who are interested in similar things have looked at the loss of species from forest fragments as they get smaller and smaller, and we’re quite interested in which species do get lost as formerly continuous forest gets chopped up into little bits.
We think that the order with which species get lost from these forests actually plays a key role in the ecology of Lyme disease risk, because each species of host plays a unique role in feeding ticks or infecting ticks and supporting the tick population, such that the loss of certain species is much more important than the loss of other species.
Can you just briefly summarized how Lyme disease arose in the human population?
Well, Lyme disease is actually probably an ancient disease, although it was undiscovered until about the mid-1970s’. In the mid-70s, there was a cluster of cases of childhood arthritis in Lyme, Connecticut. It was very mysterious to the medical professionals.
It was discovered that at least for some of these kids, there was an association between their case of arthritis and tick bites, although ticks in that part of the country weren’t thought to be very common occurrence. Then further searching identified a spirochete bacterium that was causing these disease symptoms in the children, and it was identified that the bacterium came from these tick bites.
So there’s a lot of biomedical detective work that helped to discover this new, or previously un-described species of spirochete bacterium, Borrelia burgdorferi, in a tick that was thought to be a brand new species of tick as well, which later was found to be just a northern population of a well known and widespread tick.
And since then, Lyme disease cases have been identified in 49 of 50 states–that’s grown tremendously as a public health threat since the mid-1970s.
Has our understanding of Lyme disease improved?
The amount of information that was collected about where Lyme disease comes from and how it gets to us is really quite phenomenal. It’s taken a lot of basic biological research.
What is the present rate of infection?
Well, there are about 15,000 to 20,000 cases of Lyme disease in the United States that are reported to the Centers for Disease Control and Prevention (CDC) every year. That’s thought to be a gross underestimate of the true number of Lyme disease cases for a variety of reasons. The criteria for reporting are very stringent, and so oftentimes if you don’t meet every particular what’s called “surveillance case definition,” then it’s not of an official case of Lyme disease.
There are probably something on the order of ten times that, so a hundred and 50 to 200,000 cases in the U.S. every year, and that’s clustered in certain parts of the country. So for instance, the northeastern and mid-Atlantic regions from essentially Massachusetts to Maryland have the highest number of cases of Lyme. Probably a great deal of that number fall into that part of the country, and then the upper Midwest-Minnesota and Wisconsin-have a fair number as well. California is about third in terms of the frequency of Lyme cases in the U.S.
What has been the role of habitat destruction in Lyme disease?
Habitat destruction is something that hasn’t really been thought of until very recently as affecting Lyme disease risk very much, so this is work that’s very much in progress as we speak. And it’s a fairly complicated story.
One of the very important things to note about the Lyme disease epidemic is the role that’s played by a ubiquitous creature in forests of most of the North American continent and that’s called the white-footed mouse. It’s a very, very common, very widespread mouse species, but most people have no idea it exists because it’s so secretive and small and nocturnal. You never detect that they’re out there, but the forest is often crawling with mice.
Mice play a very key role, because they are what we call the natural reservoir for the Lyme spirochete. And what that means is that the ticks pick up a Lyme disease infection by biting a mouse. That’s where they get the infection. If they don’t bite a mouse, or one of a very small number of other hosts, then they’re not going to become infected, and then they never become dangerous to people.
So the more mice there are, the greater the opportunity for these ticks to bite mice, the greater the opportunity for them to get infected, the greater the risk to people of being exposed to an infected tick, and that’s a risk measure for Lyme disease.
What we’ve been finding is that the greater the diversity of other hosts for these ticks, that is non-mouse hosts for ticks, the more of the ticks bite something that isn’t unlikely to infect them with Lyme disease bacteria, so you may have a reasonably high tick population, but a very small fraction of them will be infected and the risk of human exposure will be lower.
So what that means is that we need to know how these various different host species respond to habitat destruction, and it turns out that the short answer to that question is the white-footed mice love fragmented, disturbed, degraded habitats. They do very well in very small patches of forest in a suburban setting or an industrial or agricultural setting. Why they do so well in those kinds of settings we’re not entirely sure, but it could have something to do with the fact that many of the species that compete with them for food, or actually prey upon them and use them for food, disappear as forests are fragmented into smaller and smaller patches.
So you get a sort of a two-pronged effect on Lyme disease risk when you lose species from smaller and smaller, more degraded forest. One is you lose these alternative hosts that are unlikely to infect the ticks. And the other is you lose the species that are most likely to regulate the population size of white-footed mice and keep them in check.
The result is that white-footed mice go through the roof in very small fragments of forest, and the tick population as a result also goes through the roof, and a very high percentage of them become infected and dangerous to people.
And what we’re now working on in my lab is understanding exactly what the mechanisms are for this effect, and how widespread is this impact of habitat fragmentation on Lyme disease risk.
What can humans do to keep a cap on Lyme disease?
Essentially, if you’re in a Lyme disease endemic area–an area where Lyme disease is a big risk and a chronic risk–it’s there, and there’s very little you can do to actually eradicate it. But there are ways to reduce the probability of encountering an infected tick. And what our data suggest is that the best way to do this in some of these suburban industrial areas mixed with forest types of landscapes is to keep forest patches as large as possible.
Our studies actually looked at the effect of forest fragmentation from patches that are about 15 to 20 acres in size down to ones that are less than one acre in size, so that’s the range in which we were looking at the impact on Lyme disease risk. And over that range there was a dramatic impact where the Lyme disease risk, the number of infected ticks, I think increased by a factor of four or five as you got down below about two or three acres in size.
So that’s an important message for housing developers or planners who are trying to decide how to put up houses and how to interpose them with forest patches. Essentially the message is the bigger the forest patch, the better, in terms of protecting health.
Is there a role for acorns?
It turns out that acorn production in any oak-dominated forest, anywhere in the world in fact is a very interesting phenomenon. What tends to happen is that one year out of every three or four, all the oak trees let loose with a huge bumper crop of acorns in synchrony, so you might be walking through the forest floor and it’s almost as though you’re skating on ball bearings. There are so many acorns in any patch of forest, but the next year there might be no acorns at all or only a handful. So this is a real boom or bust cycle.
It looks like the evolutionary reason why the trees are doing this is to avoid having all of their seeds, which is what acorns are, consumed in any given year. So if you produce a whole lot, the term for this is “predator satiation,” which is a fancy way of saying you’re producing so many seeds that the things that eat your seeds can’t possibly eat them all. They’re satiated. And so you have a few acorns survive. They germinate. You have the next generation of oak trees.
One of the consequences of this is that the white-footed mice populations go through their own boom and bust cycles. White-footed mice and a lot of other wildlife love to eat acorns. They’re very nutritious. They have a long shelf life. They can be stored all winter long, and the mice do that. In fact, they store them underground or in tree holes by the dozens or even hundreds, and any year in which acorn production is very high, the mice survive winter very well. They have a good food supply. They don’t have to go running around where they’re vulnerable to owls and foxes. They can sit there and hunker down in their burrow and eat acorns.
They also tend to reproduce early in the season, so that the summer following an acorn year is the time when you really see a huge population of white-footed mice.
And what our research has done is to link acorn production to the risk of exposure to Lyme disease after a time delay, by monitoring acorns, mice, and both ticks and the percent of ticks that are infected with Lyme disease spirochetes every year over the past nine or ten now, and we see a very strong association between acorn production, mice the following year, and infected ticks two years later. So we’re using acorns now as a leading indicator or Lyme disease risk.
So what message does your research have to offer in terms of larger biodiversity models?
There are a number of messages from our research that have to do with biological diversity. You know we were talking earlier about ecosystem services. It’s really never been recognized before that an ecosystem service can be protection of human health against these zoonotic diseases like Lyme disease, and we’re very interested in the extent to which biodiversity actually provides us a protective health service in the real world, and the evidence is starting to become I think very strong that, in fact, it does do this–in the case of Lyme disease but also other vector-borne zoonotic diseases, and there are many, many of them.
But another interesting thing that our research illustrates, I think, is the importance of understanding the complexity of interactions among species in these ecosystems. If we can predict risk of exposure to a disease like Lyme it then we can avoid it much more easily. So the acorns to mice to ticks, these are ecological connections-the type of thing that professional ecologists love to study and love to try to figure out.
Usually for the sake of understanding nature better–and that’s in fact one of my main motivations to do this–these connections become very important in predicting public health risk as well. What this kind of research does is it links ecological field research, natural history understanding, and interconnections of species to public health in a fairly unusual way.
What is the ‘dilution effect’?
Well, the dilution effect is a term we use to describe the impact of high diversity among mammals and birds and lizards in reducing the risk of human exposure to Lyme disease.
The way that diversity can be seen as a diluting impact is that the greater the number of non-mouse hosts for ticks that you have in any given environment, the greater your ability to dilute the impact of white-footed mice, this main reservoir for Lyme bacteria.
So what we’re finding is that the more raccoons and skunks, the possums, squirrels, shrews, oven birds, lizards-the list goes on and on-the more alternative non-mouse hosts you have in these communities, the greater the dilution impact. You’re diluting the effect of the mice, weakening it, and reducing the number of infected ticks that can go biting people and making them sick.
How does the ‘rescue effect’ fit in?
The rescue effect is an interesting situation, in which it’s possible for non-mouse hosts to actually play a strong role in maintaining an infected tick population when mouse populations decrease for whatever reason. So there are some situations in which there is a low mouse population-possibly due to an acorn failure, for instance-and some other host steps in and ends up feeding and infecting a fair number of ticks.
And so we think that this is the situation with shrews, which is another very small mammal that runs around on the forest floor. They’re not nearly as good as white-footed mice are at infecting ticks, but they’re moderately good. They infect about 40% of the ticks that feed on them, whereas mice infect more than 90%. But that 40% for shrews is still a lot more than most hosts out there in the woods.
So if you have a very low mouse population, shrews might actually be rescuing the ticks and their spirochete populations, and maintaining at least a moderate Lyme disease risk for people. The disease is rescued, in a sense.
Do those other variables, like the shrew population, complicate things?
The rescue effect definitely adds complexity to the system, but it’s very illustrative, in that we can’t simply assume that all the species out there in the forest are interchangeable, that they’re all performing the same functions. And the rescue effect teaches us that there are certain species that, even though they’re not playing as strong a role as mice are, they are particularly important, and they’re important in different ways. So there are other hosts that we call “dilution hosts,” that are particularly spectacularly good at reducing Lyme disease risk.
And some of the things that we’re finding are particularly good dilution hosts are grey squirrels, red squirrels, and we think possibly flying squirrels as well. And what makes you a good dilution host is if you feed a large number of ticks, yet you infect very few of them with the spirochete. And this is what we’re finding, in grey squirrels in particular, and some of these other squirrels as well. So each species plays a unique role in the number of ticks it feeds, and in how likely it is to infect them, and what interaction it has with white-footed mice.
So the complexity is something that’s just unavoidable. It is an integral part of these ecological systems, and if you want to understand an outcome like Lyme disease, there’s no way around it: you have to understand the roles that each species plays.
What are some Lyme disease myths?
There are a lot of poor understandings of ticks. I’ve read various recommendations about how to avoid ticks that include tying up your hair if you have long hair, and I’ve never understood what that’s supposed to do for you.
Ticks actually seek a host by hanging around either on the forest floor or on vegetation just a couple of centimeters, a few inches above the forest floor. And they’re very immobile. They don’t crawl very well, and they’re incredibly tiny, so they just don’t cover very much distance.
And so tying up your hair, unless your hair is down to your ankles, it’s really not going to get in their way. It’s not going to attract ticks to you. There are people who think that ticks can hop or fly, and that’s not correct, or that you should wear a bandana to keep ticks from jumping out of trees onto you, and that’s also not accurate.
It’s easy to get very paranoid about ticks, particularly in many places where you just can’t come in out of the woods without worrying that you have a few ticks on you, and they are so tiny that they’re very difficult to detect. So you get a little bit of folklore about the behavior of ticks.
There’s a lot of disagreement about early symptoms of Lyme disease, and late symptoms as well. There are places where folks who wake up with vertigo or a little bit of memory loss or they feel a little lethargic some Monday morning when they wake up, in some parts of the country, the first thing they’ll think is, “Oh, I’ve got Lyme disease,” and they’ll seek some medical attention and be very frustrated when the doctor doesn’t find any sign of Lyme disease at all.
So there’s a lot of hysteria about Lyme disease, but there’s also very well-warranted concern. So I don’t mean to downplay people who are concerned about tick bites or about the risk of Lyme disease. And in fact, the disease has so many different symptoms, and they’re so generalized that sometimes they’re right when they have memory loss or lethargy or get some chills, sometimes they do have Lyme disease.
What’s the name of the tick that carries the disease?
The tick that transmits Lyme disease used to be called the “deer tick.” That’s when it was thought to be a brand new tick to science. It had never been described before. A lot of people still call it the “deer tick,” although that’s technically not correct anymore. It’s the “black-legged tick.” Deer tick rolls off the tongue a little more easily.
The reason it was called that is because the adult stage in the tick’s lifecycle is a specialist on white-tailed deer. That’s the one stage in the lifecycle of the tick that crawls up off the forest floor to a couple feet up off the floor onto vegetation in order to seek a host, and they sit there and they wave their little arms around waiting for something warm and furry to brush by. And when you’re up two feet off the forest floor in many parts of the country the most likely thing to brush by is gonna be a white-tailed deer.
So the adult ticks do feed on deer. That’s where they mate. That’s where the females engorge with blood and expand from a little sesame seed-sized creature into the size of a small jellybean and that’s all deer blood that’s causing that growth. They mate well on the deer, and the females then drop off after feeding for a week or so, and then lay their eggs in the leaf litter.
And so we’ve been using that information-the fact that the majority of adult ticks feed on deer-to try to predict where the next generation’s crop of newly hatched larval ticks is going to be. And in fact we’re linking the size of the newly hatched larval tick population to the activity by white-tailed deer during the past fall. That’s another of the acorn connections, since deer love to eat acorns as well. The bigger the acorn crop, the more deer are clustered into acorn-producing areas of the forest, and we find that’s where there are huge outbreaks of larval ticks the next summer.
That’s of limited use in terms of Lyme disease risk, because the larvae don’t transmit the disease. They hatch out of eggs free of Lyme bacteria. They have to get it somewhere before they can become dangerous to us. That’s where the mice come into play. They tend to get the disease by feeding off a mouse, and then when they bite us later we can get it too.
What are some of the ways people measure biological diversity, and what are the difficulties?
Biological diversity is defined in actually a lot of different ways. The most common way that ecologists define it is simply by counting the number of species that you have in an area, although that’s not the only way to measure biological diversity, or biodiversity.
But even if we take that simple of a measure, just the number of different species that occur in an area, it’s very difficult to measure it accurately, and also typically you’re limiting your universe to a subset of all the species that can occur.
If we were really interested in the true biological diversity measured by the number of species that occurred in any forest fragment, we would have to be worried about hundreds, maybe thousands, of species of bacteria in the forest, fungi that decompose the leaf litter, all the little plants and mosses and ferns and trees, all the bugs–the insects are usually the biggest component of biodiversity.
So in our case, we’re lucky, because we’re really interested mostly in the species that can potentially act as hosts for ticks, and that means we only have a few dozen that we need to worry about. But still, measuring the presence or absence, much less the abundance, of those few dozen species is quite a challenge.
Some of them are easily counted, like the birds that are singing. You can go out and just listen carefully, and you know whether you have wood thrushes in your patch of woods. Others are secretive and nocturnal, and unless you set hundreds of live traps, you’ll never know whether you have shrews or flying squirrels or White-footed mice. Other things, like foxes or coyotes, bobcats, you need to set up another method to examine whether they occur there or not. Things like remotely triggered cameras. You draw them in to a ‘bait’. They pass through an infrared beam-CLAPS-it takes a picture of them, and you know they’re there.
But they’re all so different. All these different species are different. The methods of detecting them are very different and they vary in their sensitivity, too, so you might miss things without knowing it, which could give you a bias, and what we’re trying to do in science is avoid those kinds of biases.
So even in a boiled down form, biodiversity is difficult and complex to study?
Yes, even boiling down biological diversity into its simplest form is still a complex challenge to be able to measure. And as has been pointed out by certain scientists like E.O. Wilson, we have no idea. We haven’t even described a large fraction of the species that exist within certain groups of organisms, like the beetles, for instance. Probably only a small fraction of the actual number of beetle species that exist on Earth have ever been described by scientists. So we have a long way to go before we’re going to be able to do a decent job at describing biodiversity. We don’t have that much time, though, to get it right.
And why not?
Because many of these species are disappearing, either in local areas or globally. They’re gone once and for all before they’re ever catalogued by science, before the roles that they play in ecological systems are ever determined, before their potential benefits to humans or other organisms are ever understood. So the loss of biological diversity is an enormous problem from a variety of different angles, and we can’t even measure what we’re losing when we’re losing species.What do you see as a worst-case scenario?
I think that the sixth mass extinction is going on. There seems little doubt of that, even if we were to slow down the rate of extinction, it would still be much, much higher than the background rate that we see without human influences like we’re having today.
It is very much an open question as to what extent that’s going to impact life on Earth. Paul Ehrlich has developed an analogy of what is called the “rivet effect,” that essentially our ecosystems are like an aircraft that’s held together by a bunch of rivets, and you can lose a few of those rivets and still have the integrity of the aircraft, and you’re not going to crash and burn. But once you lose too many of them, then the aircraft is no longer held together very well, and you’re a goner.
We don’t really know the extent to which that’s a good analogy for natural ecological systems, but a worst case scenario is that, in fact, not only is it a good analogy, but that we’ve already lost too many rivets, and there are so many time delays in ecological systems that we don’t know it yet, but we’re going to see the demise of a lot of these ecosystem services that we talked about, that ecological systems, as they lose species, will no longer be able to function in a in a healthy way. They’ll no longer be able to absorb carbon dioxide, that greenhouse gas, out of the atmosphere, and produce oxygen for us, filter water and air, provide for us foods through crops the like.
So we don’t really know, because we have such a poor idea of the roles that various different species play-and they play multiple roles. So, the white-footed mouse, for instance, we’ve been talking about as a villain when it comes to Lyme disease. It’s also a villain when it comes to a lot of other diseases: Hanta viruses, Leptospirosis, and Trypanosomiasis. I mean they’re a little breeding ground for nasty bugs, but they’re also a hero when it comes to eating the pupae stage in the gypsy moth lifecycle. So the gypsy moth is a devastating forest pest that’s been introduced to this country from Europe, and the white-footed mice are pretty much all we’ve got right now that keep gypsy moths in check.
That’s typical of species. They play multiple roles, so when we lose them it’s very hard to predict what the net effect will be. The white-footed mice, if we lose them then owls and foxes, coyotes, and weasels all have less to eat and they’re likely to do something differently, whether that’s decline, go extinct themselves, move somewhere else, we don’t really know.
What does the term ‘biocomplexity’ mean?
Biocomplexity is a fairly new term, and I think it’s still being defined by ecologists and other scientists. Biodiversity typically refers to entities like species or genes, or there can be a diversity of different habitat types within a landscape.
Biocomplexity can also refer to different interactions among species, and the kinds of chains of reactions that I’ve been referring to earlier, that biocomplexity can include other things besides just those physical entities. But it’s still being defined. It’s a little early to know whether the concept is going to have staying power.
What do you see as a best-case scenario?
The best-case scenario is actually kind of an unfortunate one, because we-I mean humans-have already caused the extinction of many, many species, either directly or indirectly. The best-case scenario is we stop that, and we’ve already seen the last human-caused extinction of an organism.
We still don’t know whether it’s already too late for certain ecosystem functions or for certain other species that interact with ones that we’ve already driven extinct.
So the best-case scenario is a rather sobering one, in which it doesn’t get any worse, but we really haven’t measured how bad it is. The hope is that we can either dramatically slow or stop the rate of human-caused extinctions, at least until we can collect the information about what roles they play, what utility they might have, and that’s not even considering the ethical argument that we have no right to directly or indirectly force species into permanent extinction.
Is there any hope for a better balance?
I’m optimistic. The surveys that have been performed, at least for U.S. citizens, show that biodiversity–it’s interesting–it’s not understood very well, but it’s valued nevertheless, which actually reflects scientific understanding as well. We don’t really understand biodiversity as scientists, which reflects the public not understanding what the term really means.
But people do quite strongly value the preservation of life forms on Earth, and that bodes well for the future. Obviously, that has to be translated into policy, and we’re not doing a terribly good job of translating those societal values into policy.
One of the things that needs to be done is to once and for all debunk the notion that economic development and biodiversity preservation are somehow incompatible. They’re not incompatible.
Another thing that needs to happen is that the science needs to progress and to be supported by society. There’s a lot of very good science that’s being done right now, on which policy can be based.
One of the main efforts is the identification of what are called “biodiversity hot spots.” Are there particular places on the planet that are crucial because they house or protect a lot of the biodiversity of particular groups? That’s often an economic argument. Are you able to generate enough money to actually protect for very long periods of time–centuries, millennia–certain biodiversity hot spots on the planet?
We’ve done a pretty good job of identifying where they are, but now we have to have the political will to actually protect them.
Let’s shift back to your research of dispersal dynamics of animal populations.
Well, we’re interested in dispersal dynamics, and dispersal simply means one-way movements by animals from one home to another home. And that becomes important in landscapes such as this one, which is southeastern New York, with a lot of forest patches that are interspersed with residential areas, agricultural areas, industrial areas, and the like.
You can see almost a patchwork if you look at an aerial photo of this area where you have different forest types and open fields and such, and we’re interested in whether movements by animals, particularly in light of the Lyme disease issue, can influence Lyme disease risk, numbers of infected ticks from one of these patch types to another patch type.
What techniques are involved in gathering this data?
So we’re interested in animals making one-way movements, so they’re born in one particular place, and they end up moving from that particular “home range” to some other part of the world. And it’s very difficult to do that, especially with tiny little animals. It’s very hard to put, for instance, a radio transmitter on them and figure out where they move from one place to the next.
So we really need to take a series of snapshots in time to find out about where the animals are. One way we do that is by setting up hundreds and hundreds of live traps, little metal boxes that catch these animals alive, and we let them go after a night in the trap, but we know every individual in the mouse population, chipmunk population, everyone–by number, not by name–in the forest And so we know when they were first caught in one spot, and if they appear suddenly in another spot that they’ve dispersed, they’ve moved.
We also mark babies, again with little metal ear tags, in so-called “nest boxes,” which look like bird houses or bird boxes, but they’re used by mice in the forest. So we can open them up, take out the mother mice and their babies, individually mark them, and then we know where the mice show up once they get to maturing age and once they’re independent of their mothers.
And what procedures do you use to get tick samples from the field?
We need to worry about getting accurate estimates for how many ticks there are in any given place and any given time. And what we do is a very low-tech but pretty effective method called “drag sampling,” and that means we simply drag a piece of white corduroy cloth along pre-marked “transects,” so these are lines through the forest, and every ten meters along the transect is marked, so we know how far we’ve dragged the cloth. We pick up the cloth every 20 meters along the transect and actually hang it up on a branch so that we can get a good look at it, and pluck off the ticks one by one. Sometimes that means only one or two ticks; other times–I actually hold the record for the largest number of ticks in a single 20-meter drag and that was 1,200 ticks so we really hit a little mini hot spot of ticks that were looking for hosts.
And that’s a good way of sampling them, because what the ticks are doing is sitting there, waiting for something to brush by, and we’re giving them something to brush by. It happens to be a piece of cloth, but it does well represent our risk of brushing by a tick and having them grab hold.
And what further steps do you use to study the ticks?
Well, the different life stages of tick get treated differently. The newly hatched larval ticks, we know they’re not infected with Lyme disease bacteria, so we actually catalog those. We put them in ethanol in a little glass vial, and keep them in the lab in case we get the appearance of some unusual species of tick in our population sample.
But the other two stages, the nymph and the adult, we bring back alive to the laboratory, and then later grind them up in a small plastic tube and assess them for their infection status. Are they infected or not with Lyme disease bacteria, and that’s done using some laboratory reagents, a fluorescent molecule that allows the bacteria to actually light up under the microscope.
What major questions still drive you?
One of the questions I’m very interested in is whether other vector-borne diseases, other tick-borne diseases and mosquito-borne diseases-things that are transmitted by biting flies, and there are dozens of these types of diseases–whether they are also subject to the dilution effect. To what extent does biodiversity impact those diseases? Is it similar to the way that it does Lyme disease? And that’s something that we really know very little about so far. I suspect that the dilution effect operates very broadly among various different vector-borne diseases, but we don’t really know that yet.
Another big question for me is about our results that we recently got here in Duchess County, New York, where small fragments-less than about an acre or two-were the riskiest places for Lyme disease. Is that a widespread phenomenon? Does that occur throughout the northeastern, mid-Atlantic U.S.? Does it occur in the upper Midwest, and along the West Coast where Lyme disease is a problem? So it’s actually asking about the generality of this fragmentation impact on human health.
What do you most want the audience to understand?
Well, for Biology teachers, one of the messages that I think is very important is that biodiversity and biocomplexity does matter, that it matters beyond simply an aesthetic or a moral point of view, that we need to protect our biological heritage. Now this is something that I believe, but it’s a belief. It’s a point of view, not a scientific statement.
But in this case, the protection of human health in at least these specific situations is an ecosystem function I think everyone can relate to-students in particular-and so it emphasizes the fact that understanding how nature works is something that can be very important to protecting our health and that of our families, beyond just pursuing knowledge for the sake of knowledge.
What are the ways in which humans as one species are causing extinction of others?
This often happens with introduced species, exotic species. There’s one instance of an animal called the brown tree snake, which is a tropical snake that’s actually fairly benign, a well-behaved little snake. It’s not venomous. It’s not big. It doesn’t strangle you. But it’s been introduced into Pacific Islands, where it’s an extremely effective predator of birds, and these birds have never in their evolutionary history experienced this brown tree snake before, so they have no defense mechanism whatsoever, and there are species of bird that have actually been driven to extinction by this exotic snake species.
So is this simply a natural process?
Throughout the fossil record, we have these five major waves of extinction from natural causes, like meteorite impacts and the like, or Ice Ages that occurred naturally, but this wave is human-caused. I don’t know of any examples of a non-human influenced case of a species driving another species to extinction in nature.
And actually, when you think about that for a moment, that’s not surprising. These the species that occur on Earth on average have existed for about a million years. That’s the average life expectancy for a species-sometimes somewhat longer than that.
And in order to survive for a million years, they have to be able to get along with their natural enemies-things that make them sick, things that eat them, things that compete with them or parasitize them. And so there are many different mechanisms that species have for getting along with things that even act as their enemies in order to avoid being driven to extinction.
One of the situations that causes species to go extinct is that they experience a brand new predator, a brand new parasite or pathogen. In those situations, they can go extinct sometimes very rapidly.
What have the big surprises been to you in recent studies of biodiversity?
Well, I guess one of the things that has been most surprising to me is the degree to which these species are actually interconnected. Probably the single most surprising thing to me in the research I’ve done in the past ten years is how strongly white-footed mice act as what I consider a “hub” species. They really interact very strongly with a bunch of other species. So I’ve mentioned the ticks that feed on them, the bacteria that live in their bodies. I’ve mentioned the gypsy moth pupae that they prey on. They also attack the eggs and sometimes even the nestlings of ground nesting birds, and we’re finding that they have a strong impact on survival of certain birds, some of which are of conservation concerns, so there’s part of the biodiversity angle as well.
As the mice go through their ups and downs, so too do certain hawk species, we’re finding-things like Sharpshin hawks and Coopers hawks-are actually fluctuating in synchrony with the mice, but with a one-year time lag, which is what you would expect from their lifecycle.
The mice are amazingly good at consuming tree seeds on the forest floor, so if there are enough mice running around in any given year, they can cause complete failure in a tree seed crop and cause lack of regeneration of the trees in that particular year.