Human Footprint

If you were to make a map of the total influence that humans have had on planet Earth, you’d have what some scientists call the “human footprint.”

Eric Sanderson is with the Wildlife Conservation Society — headquartered at the Bronx Zoo in New York. He was part of a collaborative project to measure the human footprint.

He used data sets representing several major kinds of human influence on our world — human population density, human transformation of the land, human land use, access from roads and waterways, and electrical power and infrastructure. He found that we humans have made our mark — one or more of those factors influences 83% of the land surface on Earth.

Eric Sanderson: When you see on TV, you see wildebeasts, you see these remarkable displays of fish in the ocean, or big movements of animals in South America, that we need to realize that human beings have responsibilities for those creatures. If we want to have wildlife and wildplaces in the world, have a rich and beautiful environment for ourselves, then we need to moderate the negative impacts of our influence, while enhancing the positive things we can do. And there’s a lot of things we can do.

A couple of years ago, a group of scientists started the human footprint project as a way to think about what wild places mean for us on this planet. Using a variety of data sets, including the latest in satellite sensing, they learned that 83% of the Earth’s surface has been influenced in one way or another by humans. Dr. Eric Sanderson was part of the human footprint project.

Eric Sanderson: I hope that the 21st century sees a reawakening, a rethinking of our relationship to the environment, and that human beings choose to commit ourselves as societies, as individuals, to having respect for the integrity of life on Earth, and to try and modify our lifestyles and our ways of living. Not so that we can’t make our living, but to modifiy the ways we make our living so that we can live with wildlife and wild places, so we can live with the full range of nature.


Sanderson: The Wildlife Conservation Society has a mission, tryin to save wildlife and wildplaces around the world. And a couple of years ago, we wanted to start a project that to try and think about what wildplaces mean for us. So we teamed up with a group of geographers and political scientists at Columbia University and the center for International Earth Science Information Network. We searched through the scientific literature to try and find geographic data sets, or maps in the computers, that represent factors of human influence on the environment. And, fortunately, because of changes in 1990, increased availability from remote sensing information from satelites distributed by organizations like NASA and other government agencies, the movement to make government more effecient in this country led to various government agencies releasing more data in the public domain. So we were able to find datasets representing four major kinds of human influence on the environment — human population density, human transformation of the land, human land use, access from roads and waterways, and electrical power and infrastructure. So basically we took those four maps, the maps of human population density over the land surface, the map of roads and waterways, the maps of human landuse — urbanization and agricultural land use and settlements, the map of lights you can see from a satelite at night, we added those maps up together in our computer, and the result is a map that we call the human influence index which shows, at a gradient, the level of human influence on ecosystems all over the world. And we measured that influence in different ecosystems in the wild, around the world, in different regions, different ecosystems in different regions, and we map what we call the human footprint, which gives you a map of the gradient of human influence, from the most influenced, to the least influenced places, and all the biomes, and all the large ecosystems in all the regions of the world. So that’s what the human footprint is. So it’s based all on those kind of data I was telling you about — roads, land use, access, electrical power infrasturcture, all from 1990s data sets all of the most current, globally available data sets we could find. And we added those together, and you get the human footprint. And when you analyze the human footprint, one or more of those factors influences 83% of the land surface. And if you comapare it to geographic estimates of where we can grow rice or wheat or maize, from the food and agricultural organization, the FAO, we influence 98% of those areas. So 98% of the places where you can grow our main staple crops are influenced by human beings. The actual levels of that influence vary from place to place. When we say influence, what we really mean are places where you might find human footprints on the land surface, whether that’s literally a footprint of someone who left a road and walked into a forest to go hunting or to go fishing, whether that’s the human footprint that comes when people cut down forests or develop grasslands, or whether that’s actually urbanization — whether that’s pavement or concrete and all the things that we associate with our cities — with a gradient of human influence across the land surface.

A biome is a large, regional ecosystem somewhere on the land surface that is defined by climate and physical factors like the soil type, and that have a characteristic flora and fauna, a characteristic set of plants and animals. So there’s various ways of mapping the biomes. You’ve probably seen them in fourth grade geography classes. A grassland is a biome, a tropical savannah is a biome, a decidous forest like where I live in the Northeast is a biome. So we looked at biomes around the world, we seperated biomes in different regions so that we just compared human influence values within a biome. We would take, for example in the deciduous forests of the Northeast, we would find the area that was least influenced, and call that a zero. And we find the area that was most influenced, and call that a hundred. And we’d stretch all the other values, that’s a scientific process called normalization. So we normalize by all the biomes. This way, human influence in the Northeast deciduous forests is really different than human influence in the tropical forests of Africa, for example. So whe’re not directly comparing the human influence in North American Deciduous forests with tropical forests in Africa, because they have diferent biota, they have different plants and animals that live there, and they have a different social-cultural context. That means there’s different conservation alternatives, whether you’re talking about tropical forests in Africa, or deciduos forests in North America.

People have been looking at human influence on the environment — in the paper we quote George Marsh, who in 1843 asked, How do human activities threaten natural processes? And so that’s a central question to environmental science and ecology. What’s different about this study is that we took all these as-up-to-date-as-possible data sets, some of them which have only become available in the last few years, and added them together. So there’s been studies about human population density, and how that relates to environmental change. There’s been studies about land use, and how that relates to changes in the environment, changes in habitat and so forth. There’s been studies in how road access leads to increased levels of hunting, modification of water flows and soil processes. We’re really taking those factors together and adding them to a single index to try and estimate a proxy for human influence on the land surface. And the reason we did that was because it’s really important for conservation. The kind of options that you’re going to have for conservation are going to vary a lot, depending on the levels of human influence. In places where there’s less human influence, where there’s less human infrastructure, where there’s fewer roads and fewer settlements, then we expect that more often we’ll find the intact, full range of nature — all the plants and animals that live in a place — and we expect that we’ll find the processes of nature in its fullest range. And that gives you a lot of options for conservation. You can set up protected areas, like national parks and reserves. You can work with the people that live there and try to moderate the way they live with the environment so that they can make their living while still allowing nature to survive. As you move up the level of human influence to more impacted areas, the kind of options you have are less. You can imagine in a sort of area of moderate influence like in a lot of rural areas in the United States, some species are there and they’re doing fine. Others species aren’t there anymore. We’ve lost a lot of our large mammals across the United States, particularly the canivoirs and large ungulates that have a hard time living with human beings. Also, more sensitive species tend to be lost out of systems that have moderate human influence. And so there we’re looking at more mixed strategies of conservation within the context of human use. Strategies that are about restoration of species population, or possibly preservation of certain of populations that still exist. There’s implementation of certain populations. And then there’s places like cities, like New York City where I live, where there’s really a heavy level of human influence. But there also are some aspects of nature that still survive here. You can go to parks, and those parks have lots of invasive species in them, but they still have trees and New York has actually been really successful in bringing back migrating bird populations. There are still 400 bird species that are regularly seen in the New York City region. The cleaning up of the harbor waters in the Hudson River after the Clean Water Act.. has really cleaned up the water and allowed fish populations to come booming back. You know, 10 or 15 years ago striped bass weren’t found in New York Harbors in anywhere near the kinds of numbers that they’re found today. So there’s a lot of things that human beings can do. But, from a conservation planning perspective, one of the first things we have to realize is that different levels of human influence impact the kinds of actions and the kinds of opportuninties that we’re going to have for conservation. Sometimes I say, that when you look at the human footprint map, it’s really dramatic. We map heavier level of human influence in colors that are sort of black, purple, and red, and then you can see the gradient through yellows, light-greens, to darker greens indicating areas of less human influence. And when you first look at the map, you’re really taken, particularly the map of the United States, how heavy human inflluence is in the United States. That’s not to say that there aren’t aspects of wildness in all parts of the United States. But compared to big areas like the Amazon or the Boreal forests of Canada, the United States is pretty heavily influenced.

But what the map means to me is that we can do something about it. We can realize that human beings are really calling the shots. And therefore we should look at the green areas and think of those as places where there’s great opportunities for conservation still, and we should look at the red areas and the purple areas as places where we need to work really hard to have nature with us, near the places we live. So that it touches our daily lives. And so I think that’ll make us appreciate not just the places, not just the parks where we live around, the nearby forests and streams, but also to start to care about places that are farther away from us, whether those are also the United States out West, or in Alaska, or whether those are overseas, in Asia, or Africa, or Latin America, where there still are remarkable opportunities for conservation.

So the project started about 2 years ago. And we got a little bit of funding frome Comumbia University to start us off. And we got some software support from a GIS software maker, ESRI. A little bit of money from a foundation here in New York called the Prospect Hill Foundation. And that spurred on this collaborration between us and these political scientists and geographers at Columbia University. My expertise is really in biological conservation. I’m a landscape ecologist by training. So working with them really helped out. So we worked for about 2 years. We produced this paper that just appeared in the journal Bioscience this October. and this led to a lot of press interest in the story.

And this is just the beggining of the work, it’s not the end. Now we’re working on what the specific policy and conservation implications of the work are. We’re doing studies to look at how large the species with wide ranges, like the tigers and lions bears, african wild dogs have changed with respect to level of human influence over the last hundred years. We’re doing studies that look at the level of human influence within other countries, and how that gradient changes in particular places. And we’re trying to get together funds to do a bunch of outreach activities, enhance our web site, produce a poster. WCS has a large educational department that write high school curriculum for lower grades, and they’re interested in incorporating the human footprint in their teaching curriculum. So we’re trying to get the message out.

We’re trying to get the message of the human footprint out to the public, so that people realize that yes, human beings have a lot of responsibilities for nature. It goes along with our rights as living things to have responsibilities for the other living things that live around us. And that there’s a lot we can do to make a more balanced relationship between human beings and the rest of nature.

So what we did was, we tried to pick factors that we knew were important for human influence based on scientific studies, and that had a global influence. We were restricted to picking only ones that we could find with the kinds of geographic data sets that we could use on our computers with. And so that’s why we didn’t include things like global warming, which of course has a world-wide influence. And we didn’t include pollution, because there’s no map per say that shows the levels of pollution around the world. But we could find data on what we think are the primary drivers of human influence which are: human population density, land transformation, access and electrical power infrastructure.

In terms of human population density, peopl from the time of Malthus have been concerned that as you have more people, and you multiply people by their consumption rate, that says something about their human influence. That, in some sense, is their ecological footprint. Many of your listeners have probably head about that human population has been shown to directly affect the local environment. For example, there’s a study in Ghana that looked at human population density in areas around national parks, and within 50 KM of national parks, and if you did just a simple comparison of the size of the park and the numbe of people living within 50 KM of the park, and the levels of extinction within that park over a thirty year period, you find that 98% of the extinctions can be explained statistically based on the human population density and the size of the park. There’s been similar studies of national parks in the Western United States and for reserves in Africa. So we know that there’s an important relationship between the human population density just by itself and the survival of other species around us.

In terms of land transformation, scientists know that there’s many different consequences when we take a piece of land and we convert it from some sort of natural land cover into human use. Whether that’s for a settlement or agriculture or a road or for a railway or for a city. Simply taking a piece of land and converting it to a land use that’s more dedicated to exclusive human use means that it’s not as good a habitat for the species that used to be there. Also, the pattern of landuse leads to fragmentation of habitat. If you have a forest, and you fragment 1/3 of it on one side and 1/3 of it on the other side, then you kind of isolate the middle part. And some species then, through habitat fragmentation, can’t live there anymore. It’s too small, it’s not connected with the rest of the habitat, they can’t move around the way they used to, therefore they can’t survive. And different human land uses have diferent levels of effect. In terms of agriculture, you have, in the United States, leads to the pollutants that are out of the soil. The levels of nitrogen in the soil, the levels of phosphorus, those run off, and then drain into streams, and then into lakes, into the ocean and so forth. Agriculture is also associated with the loss of topsoil. And that’s a really significant problem. It takes a long time for soil to build up. And if you lose them in a short period of time, it’s going to be difficult to restore systems like that.

In terms of access, there’s been a lot of studies that show that areas near roads are more affected by roads than places that are farther away. And roads have direct effects, they modify the soil because you have to dig up the soil and build it into the roadway. The materials that are used to build roads can leach into the soil. The traffic going back and forth along the road kills animals, you have roadkills. We cite a study that estimated that over million vertebrates are killed everyday on roads. And we know that people use roads as access points. People drive their vehicles along those roads. They get out of their vehicles, and then they move into the countryside. Sometimes that’s fine. There’s certain uses that are compatible, you know people going hiking and camping, in general has a lesser influence. People also hunt from roads. Any hunter will tell you that the more roads there are, the more hunters you’re likely to see in the forest. And that hunting is controlled in some countries, like in the U.S. and Europe. In other countries, there’s a lot less control. And so WCS has recently finished some major studies that show that when a logging road is built into an area, the major effect can be not from the logging itself, which might be just taking out one tree per hectare, but essentially the hunters that come in behind that road and kill all the wildlife, you can map it. There’s like a 10 KM buffer around the road, on either side of the road where all the animals are taken out. And those animals are then exported out to cities where they’re eaten.

And finally, power infrastructure. Power infrastructure is a very dramatic factor in developed parts of the world. If you look at the map of lights at night, in the United States and Europe, it pretty closely maps onto the map of population density. In other parts of the world, in Latin America or Africa, it doesn’t map very closely to human population density. Because those countries don’t have the same kind of technological development. But what’s significant about it, in terms of land transformation, is that electrical power is also associated with the capacity to convert fossil fuels into work to modify the environment. And if you think about it, one guy with a bull dozer has 300 horsepowers of strength to modify the environment. That’s more influence, more power, for that one person than one of the Egyptian pharoahs 5000 years ago could have brought to the single problem, just because you can’t get 300 horses to pull on one thing, or 2000 slaves to push on something. So one guy with a bulldozer can make a lot more progress than 1000 guys with axes could have 500 years ago. And that means that the speed and the extant with which with we’re able to transform the environment with developed power infrastructures is much higher. And then of course there’s all the effects that cars have, in terms of affecting the environment, in terms of habitat destruction associated with roads and the factors of increased access to wild areas and in terms of our resource base, the kinds of resources we need to acquire from the environment in order to build the roads and build the cars and build all the planes that we need to support the kind of lifestyle that we have in the United States right now.
In terms of taking our human footprint elsewhere, there’s not much left on the land surface to take it elsewhere. So what we hope people take away from this is the message that the human footprint is a global driver of conservation crises on the planet. When you see on TV, you see wildebeasts, you see these remarkable displays of fish in the ocean, or big movements of animal in South America, that we need to realize that human beings have responsibilities for those creatures. You just can’t get around it anymore. There’s no really big wild places, and most of the biomes that are left don’t have some level of human influence. And to me, that implies that we need to have some level of responsibility for them. If we want to have wildlife and wildplaces in the world, have a rich and beautiful environment for ourselves, that we need to moderate the negative impacts of our influence, while enhancing the positive things we do. And there’s a lot of things we can do. In some cases that’s going to mean actully limiting our influence. Not building that next road, or not taking out that segment of land and converting it to agriculture. In some places that means conserving what we call the Last of the Wild, those few last places that are relatively less influenced by human beings in all the biomes around the world. But I think another part of the solution is that we need to become better stewards of the environment, across the gradient of human influence, and realize the things that we can do and the things that we can’t do at different points across the gradient of human influence, whether that’s preservation, or restoration, or management. There’s a lot of things that scientists, conservation biologists, restoration ecologists have figured out that we can do to help wildplaces and wildthings to survive around us. But the most important thng I’d like to say to your listenters is, for human beings as individuals, and for our institutions and our govenments, to make the choice to moderate our influence in return for a healthier relationship with the natural world. I kind of think that we’re out of balance with the natural world right now, and I think that’s causing a lot of our problems, directly and indirectly. I hope that the 21st century sees a reawaking, a rethinking of relationship to the environment, and that human beings chose to committ ourselves as societies, as individuals, to having resepect for the integrity of life on earth, and to try and modify our lifestyles and our ways of living. Not so that we can’t make our living, but to modifiy the ways we make our living so that we can live with wildlife and wildplaces, so we can live with the full range of human nature. I think that’s really what it’s all about, what life’s all about. And I think that if you come from it from a spritual perspective, or a religious perspective, or a moral perspective, or a scientific perspective, you come to the same place — that we need to be responsible for the way that we interact with nature and use our creativity and ingenuity for modifying the environment to try and limit the kinds of negative influences that we have on wildlife and enhance the positive things we can do for wildlife. and we can do it. We can do it. Particularly us in the first world, those of us that live in developed countries. We have a lot wealth, we have a lot of education, we have a lot of smarts, basically, and I’m an American. And I think as an American that I want to have a better relationship with my American natural resources and my American wildlife and my American wildplaces. And we can do that. We can do that. We can start today. And I hope we do start today.

You know that when the map first came out on the plotter, we were shocked. It’s our business to know what human influence on the environment is. But when the map came out, and we got the numbers, we realized that 83% of the land surface was influenced by human beings, 98% of the places where we can grow food. We were shocked, and I was pretty depressed actually for a couple of weeks afterwards. Because even though you read these numbers, Peter Vetusic and his colleagues wrote a study a number of years ago that said that 48% of the primary productivity of the green material on the Earth’s surface is used by human beings in one way or the other. Large amounts of our freshwater resources or marine resources, you hear everyday another typically depressing story about how human beings are drawing down our natural capital, our natural resources, and yet — 83% — that’s pretty dramatic. And so then we started talking to each other. We talked to our colleagues at Columbia, I talked to the colleagues I have here at the Wildlife Conservation Society, I talked to my colleagues in Latin America, Asia, and Africa, all over the world about this to sort of get what their take was, what they understood, and what did it mean for their cultures. What we came to realize, the general message is that human beings have a choice about their relationship with the environment. And right now, we’re choosing, typically, in very selfish ways too take down the environment, draw down our natural resources. Organizations like the Wildlife Conservation Society are tying to encourage societies to make the opposite decision, and trying to show them ways that we can live with our natural resources in a more intelligent and compassionate way.

I’d just like to encourage them to think out of the box a little bit. I think sometimes that people hear environmental scientists talk about decline in species, or the decline in our air, water, or soil. And I think that most people want to do something about it. And I think the things we do often help, whether that’s recycling, or encouraging our kids not to kill wildlife, not to smash bugs in the garden. I think that’s all great. But I think it’s not enough, frankly. I think we need to tell our political leaders and tell our international conservation organizations, and tell our local leaders, tell our mayors, our town halls nature near our places. We want to have nature away from where we are. We want to have nature close to us. So I hope your listeners will think about doing that.

The true story is, we have a lot of influence. That influence varies from place to place. And we can do something about it. And I hope that’s the story that you’re able to carry to your listeners.

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