U.S. Department of the Interior 

U.S. Geological Survey 

USGS FS-127-03

December 2003

Background

Humans have dramatically 

altered wildlands in the western 

United States over the past 100 

years by using these lands and 

the resources they provide. 

Anthropogenic changes to the 

landscape, such as urban expansion and development of rural 

areas, influence the number 

and kinds of plants and wildlife that remain. In addition, 

western ecosystems are also 

affected by roads, powerlines, 

and other networks and land 

uses necessary to maintain 

human populations. 

The cumulative impacts 

of human presence and actions 

on a landscape are called the 

“human footprint.” These 

impacts may affect plants and 

wildlife by increasing the number 

of synanthropic (species that 

benefit from human activities) bird 

and mammal predators and facilitating their movements through the 

landscape or by creating unsuitable 

habitats. These actions can impact 

plants and wildlife to such an extent 

that the persistence of populations 

or entire species is questionable. 

For example, greater sage-grouse 

(Centrocercus urophasianus) once 

were widespread throughout the Great 

Basin, but now are a focus of conservation concern because populations have 

declined for the past three decades across 

most of their range. At the USGS Forest 

and Rangeland Ecosystem Science Center, 

we are developing spatial models to 

better understand potential influences of 

the human footprint on shrubland ecosystems and associated wildlife in the western 

United States.

The Human Footprint in the West: 

A Large-scale Analysis 

of Human Impacts

Development of the Human Footprint Map 

We are developing the map of the human footprint 

for the western United States from an analysis of the 

landscape structure and anthropogenic features. The map 

focuses on shrubland ecosystems and combines models 

of habitat use by synanthropic predators (“top-down” 

effects, see inside right) and the risk of invasive plant 

presence (“bottom-up” effects, see insight right) to estimate the total influence of human activities. All datasets 

used in the analysis are archived on the SAGEMAP 

website (http://sagemap.wr.usgs.gov), maintained by the 

USGS at the Forest and Rangeland Ecosystem Science 

Center’s Snake River Field Station.

We are modeling human activities that benefit synanthropic predators in order to understand the top-down 

interaction between predators and shrubland wildlife. 

For example, power lines are used by common ravens 

(Corvus corax) and raptors for nesting and for hunting 

perches. Human impacts in rural areas, including agriculture, landfills, and recreational sites, often provide 

abundant and new food sources which potentially increase the numbers of common ravens, American crows 

(Corvus brachyrhynchos), black-billed magpies (Pica 

hudsonia), brown-headed cowbirds (Molothrus ater), and 

red foxes (Vulpes vulpes). Linear 

features such as railroads, primary and secondary roads, and 

irrigation channels enhance 

the movements of predators 

into previously unused regions 

(Fig. 1, inside). black-billed magpie
Figure 2

Anthropogenic factors that act as dis pers al 

agents for invasive plants. The fi nal model 

will depict exotic plant invasion risk across 

landscapes in the western United States.

The spread of invasive plant species often is facilitated along 

networks of primary and secondary roads, railroads, and power 

lines. Disturbance associated with creating and maintaining 

these linear features also enhances the establishment of invasive 

species. To evaluate these bottom-up effects, we model the risk 

of exotic plant species invasion along linear corridors (Fig. 2). 

We also model anthropogenic factors that interrupt connectivity and increase fragmentation of wildlands, such as roads, 

railroads, and agricultural lands (Fig.3).

Figure 3

Anthropogenic factors that infl uence wildland connectivity and vertebrate 

dispersal. Our fi nal model will be based on math e mat i cal functions that 

in cor po rate inter-patch distance, patch size, and density.

Interstates and Highways

Other Roads

Figure 1

Anthropogenic factors that 

infl uence avian predator 

numbers and habitat use. To 

model predators’ habitat use, 

we will employ mathematical 

functions that refl ect daily 

movement patterns of 

American crows and 

common ravens.

Above and left – invasive 

plants trailing along roads. 

Human Disturbance 

(Agricultural land, human 

settlement, railroads, powerlines, and roads) 

Rest Areas

Campgrounds

Landfills

Interstates and Highways

Railroads

Powerlines

Population > 1 individual/ha

Linear Density > 3.5 km/km2

Agriculture

•

•

•

–

–

–

Far left —

Amercian crows

Left —

common raven

Photo courtesy of 

Roarke Donnelly

Left – Urban sprawl in 

Boise, Idaho.

Below – Irrigation 

allows cultivation of 

hay in the high desert 

of Central Oregon.
Top-down Versus Bottom-up Effects

An important question in ecology is how food web interactions vary through time and space. Population ecologists 

and conservation biologists study population dynamics of 

species by calculating the magnitude and direction of speciesspecific population growth rates. This measure of population 

fluctuation is derived from demographic rates such as reproductive, death, and dispersal (the movement of an individual 

from one to another population), which are affected by factors such as climate, stochastic events (i.e., rare but extreme 

weather patterns such as severe flooding), predators, parasites, 

and social structure. Merging the influences of each factor and 

the complex interactions among them helps population ecologists form hypotheses regarding “top-down” versus “bottomup” population processes. Top-down (consumer) processes 

occur when, for example, predators keep prey populations at 

levels below the population size that would be observed in 

the absence of predators. On the other hand, if factors such as 

food and/or habitat availability are the main influences driving 

population fluctuations, a population is regulated by bottomup (resource) processes. Populations also can be regulated 

by both processes, either through a seasonal shift from one 

process to the other, or when both processes act in concert. 

Is a population of a species regulated via top-down or 

bottom-up processes in an ecosystem of interest? The answer 

may depend on the ecosystem (e.g., marine versus terrestrial), 

but it is generally assumed that bottom-up processes are most 

important in explaining vertebrate population fluctuations. 

Yet, top-down processes may override bottom-up processes in 

cases where ecosystems are exposed to atypical disturbance, 

such as anthropogenic habitat alterations. These anthropogenic 

factors may affect population size in three ways. They may act 

top-down by altering habitat or food sources to a degree that 

allows predator populations to increase and consume more of 

the species of interest. They may act bottom-up by increasing 

habitat loss and exploitation to a threshold above which the 

habitat becomes unsuitable for a given species. Or they may 

act by altering both top-down and bottom-up processes. Understanding how anthropogenic factors influence the population regulatory processes helps ecologists predict how humans 

are affecting landscapes and environmental changes. 

Suggested Readings

Bolger, D. T. 2001. Urban birds: population, community, and 

landscape approaches. Pages 155-177 in Avian Ecology 

and Conservation in an Urbanizing World (J. M. Marzluff, 

R. Bowman and R. Donnelly, eds.). Kluwer Academic 

Publishers, Norwell, MA.

Gratton, C. and R. F. Denno. 2003. Seasonal shift from bottom-up to top-down impact in phytophagous insect populations. Oecologia 134: 487-495.

Sinclair, A. R. E. and J. R. Krebs. 2002. Complex numerical 

responses to top-down and bottom-up processes in vertebrate populations. Philosophical Transactions of the Royal 

Society of London, Series B 357: 1221-1231.

Worm, B. and R. Myers. 2003. Meta-analysis of cod-shrimp 

interactions reveals top-down control of oceanic food webs. 

Ecology 84: 162-173.

Habitat dominated by native 

plants provides essential factors 

for survival, such as food and 

shelter.

The synergistic effects of short-cycle and highintensity fi res along with the resultant invasion of 

the exotic cheat grass, Bromus tectorum, will render 

habitat to be unsuitable for the greater sage-grouse.

Native forb species 

such as largefl ower 

hawksbeard, Crepis 

occidentalis (above), 

and sagebrush , 

Artemisia sp. (left), are 

some food items in the 

diet of the greater 

sage-grouse.

Anthropogenic changes to the 

landscape can favor the presence 

of predatory species such as the 

red fox (below).

This chart depicts 

the top-down and 

bottom-up processes, 

and the interactions 

among them that 

potentially affect prey 

populations. We used 

the greater sagegrouse to illustrate 

how anthropogenic 

factors can infl uence 

both top-down and 

bottom-up processes.

Above, below and right:

Agriculture encroachment 

on wildlands.
• Developing models to predict the spread of invasive 

plants accompanying the human footprint 

Linear features of the footprint provide corridors along which 

invasive species can spread. Further analysis of the footprint 

may permit better models that lead to control or slowing 

invasive species. This is particularly important for shrublands 

in which the synergistic effects of fire and invasive plants 

convert ecosystems dominated by native plants to systems 

dominated by exotic plants.

The Human Footprint: 

Management and Conservation Implications

The human footprint aids managers in planning, implementing land-use actions, and developing strategies to 

conserve habitats and wildlife. Modeling the human footprint 

across large landscapes also allows researchers to generate 

hypotheses about ecosystem change and to conduct studies 

in regions differing in potential impact. Because funding for 

restoration and conservation projects is limited, and because 

there is little room for errors in the management of species

 of concern, land managers are able to maximize restoration 

and conservation efforts in areas minimally influenced by 

the human footprint. As such, the human footprint model is 

an important first step toward understanding the synergistic 

effects acting on shrublands in the western United States.

Credits:

Photos – USGS researchers 

and the U.S. Fish and Wildlife 

National Image Library, 

except as noted. 

Graphic Design – Erik Ackerson

Contacts:

USGS Forest and Rangeland Ecosystem Science Center, 

Snake River Field Station, 970 Lusk Street, Boise, ID 83706

Matthias Leu

(208-426-2598; email: mleu@usgs.gov)

Steve Hanser

(208-426-2892; email: shanser@usgs.gov)

Steven T. Knick

(208-426-5208; email: steve_knick@usgs.gov)

greater sage-grouse

The Human Footprint: Next Steps

Our maps of the human footprint represent 

preliminary models of the effects of land uses and 

developments over the western United States. We 

are planning to test the predictions and associated 

implications to better understand human influence on 

shrubland ecosystems in the western United States. 

Specifically, our next steps include:

• Testing how predators respond to the 

human footprint

Our predictions from the model of synanthropic bird 

predation risk were substantiated with independent 

data from the USGS Breeding Bird Survey; however, 

we need to further test the model with additional surveys of avian as well as mammalian predators.

• In ves ti gat ing how species of con ser va tion 

concern respond to the human footprint 

Greater sage-grouse, Gunnison sage-grouse (Centrocercus minimus), and pygmy rabbits (Brachylagus 

idahoensis) are declining in numbers. We need to 

better understand how the human footprint influences 

the distribution and abundance of multiple species 

of concern in shrubland ecosystems. An important 

component of our analysis is to determine which 

anthropogenic factors or combination of features may 

act as barriers to vertebrate movements or dispersal.

• Determining how the pop u la tion dynamics of 

wildlife are influenced by the human footprint 

Changes in habitat and predation risk associated with 

the human footprint may have significant impacts on 

the ability to maintain wildlife populations. Therefore, we need to assess the relationship between 

dynamics of wildlife populations and individual and 

cumulative features of the footprint.

• Understanding the regional effects of the 

human footprint on migratory bird pop u la tions

Because many bird species that breed in northwestern 

shrublands winter in the southwestern United States, 

we can determine the relative impact of the human 

footprint on different periods of the annual cycle.

pgymy rabbit

Photo courtesy of 

Mike Gillingham