Snow/Wolf/Elk Interactions in Yellowstone National Park

Craig Anderson

Institute of Arctic and Alpine Research Department of Geography University of Colorado-Boulder Boulder, Colorado

Overview• Introduction

• Background– History– Winter Mortality– Index of Winter Severity

• Current Research

• My Research

• Summary

Introduction• Reintroduction of grey wolves (Canis

lupus) into Yellowstone National Park 1995

• Provided new opportunities to study several aspects of wolf predation

– Effects of winter severity on predation patterns

– Specifically elk (Cervus elaphus)

Introduction

• Previous studies focused on long-extant wolf-prey systems

• Reintroduction allowed for studies focusing on mutual development of learned wolf and elk behavior simultaneously

Possible Questions1. How would the winter severity affect wolf

predation on a previously wolf-free elk herd?

2. Would introduced wolves tend to take older, malnourished or otherwise vulnerable elk?

3. What is the kill rate and amount of prey consumed?

Possible Questions4. Would reintroduced wolves find

predation so easy that they would eat small amounts from each kill?

5. Finally, and perhaps most importantly, how would these relationships be affected by winter severity?

Other questions ???

Background

• Northern Rocky Mountain wolves native to Yellowstone in 1872

• Predator control prominent in late 1800’s and early 1900’s

• Between 1914-1926 at least 136 wolves killed

Background• Yellowstone wolves believed to be extinct by

1940

• Wolves persistent in lower 48 states only in Minnesota and on Isle Royal in Michigan

• Early 1980’s, wolves began to reestablish near Glacier National Park in northern Montana

• By 1996, estimated 75 wolves inhabited Montana

Background• Wolves are listed as “endangered” throughout

its historic range in lower 48 except Minnesota where it is threatened

• NPS policy calls for restoring native species under certain circumstances:

a) Sufficient habitat existsb) Management can prevent threats to outside interestsc) Restored species resembles extirpated speciesd) Extirpation resulted from human activities

Background

• October 1991 EIS begun on restoring wolves to Yellowstone and central Idaho

• Several years of study and near record number of public comments

• Finally, two wolf packs released in 1995 and 1996

Background• USFS and

Canadian wildlife biologists captured the wolves in Canada

• The release was successful and wolves seen preying on elk shortly thereafter

Winter Mortality

• The primary prey for wolves in YNP is ungulates, specifically elk

• Until reintroduction, mortality of most ungulates in winter due to malnutrition

Winter Mortality

• At reintroduction, the YNP elk herd was near or at ecological carrying capacity

• Herd contained high numbers of old and vulnerable individuals

• Herd #’s estimated between 14,000-20,000

Winter Severity• Widely accepted that

severe winter conditions are the primary cause of elk mortality in YNP

• Heavy snowpack, cold temperatures, and inadequate forage contribute to significant mortality

Winter Severity• Large ungulates can travel and forage in deep

snow, but it also limits their mobility and increases overall energy expenditures

• Predation, locomotion, browsing strategies, thermodynamic regulation and herd distribution all affected by winter snowpacks

• Snowpack properties can play a significant role: density, hardness, depth, stratigraphy, SWE

Winter Severity• However, lacking a

substantive definition of “winter severity”

• Yet to be adequately quantified

• “Its like porn, you can’t define it, but you know it when you see it”

Current Research• Index of Winter Severity (Farnes et al., 1999) Snowpack Distribution Across Yellowstone

National Park http://nrin.nbii.gov/climate/ynp_report.pdf

– IWS intended to estimate animals’ response to climatic and vegetative conditions on the winter range

– Obtained by combining snow water equivalent (SWE),

cumulative minimum temperatures below the effective critical temperature and forage availability on the winter range

Index of Winter Severity (IWS)

• Snow Water Equivalent (SWE)– Amount of water produced by melting snowpack– Snow depth x (snow density/water density)– Obtained from SNOTEL, snow courses and CLIM

stations– As SWE increases:

• Foraging area decreases• Energy expended in locomotion and foraging

increases• Competition for forage increase

Index of Winter Severity (IWS)• Effective Critical Temperatures (ECT)

– When air temps are below the threshold ECT, elk must increase basal metabolic rate to maintain body temperature

– Must utilize energy from fat reserves to maintain adequate body temps

– Cumulative losses thru the winter

– Calculated by subtracting ECT from minimum temperature and accumulated over the winter

Index of Winter Severity (IWS)

• Forage production

– Production of summer forage critical to winter survival

– Amount produced usually a function of available spring moisture and temperature

– Uses Keetch-Byram Drought Index (KBDI) to determine amount of annual forage produced

Index of Winter Severity (IWS)

• Combination of these three variables produces an IWS for elk

• Values range from -4 to +4

• Variables weighted as:– SWE = 45%– ECT = 35%– KBDI = 20%

Current Research• Winter Severity and Wolf Predation on a Formerly Wolf-

Free Elk Herd (Mech et al., 2001)http://www.npwrc.usgs.gov/resource/2002/severity/severity.htm

• Studied wolf predation on elk in YNP in 1997-98 during severe and mild winter seasons

• Examined carcasses for age, sex, estimated amount eaten

• Collected bone marrow samples and mandibles

• Concluded that the effect of wolves on elk herd numbers is related to the magnitude of winter severity

Current Research

• Others:

– Density dependence modeling (Lora Ballinger 1999)

– Bioenergetics/Spatial modeling (Dr. Monica Turner – University of Wisconsin at Madison

– Several others

My Research• Complexity Across Boundaries –

Coupled Human and Natural Systems in the Yellowstone Northern Elk Winter Range

– NSF funded under Biocomplexity program

– Develop a Multi-Agent Simulation Model (MAS) that explores NEWR vulnerability to environmental stressors

– Understanding ecological systems where humans are significant factors

– http://gis.iowa.uiowa.edu/private/yellowstone/

Biocomplexity Websitehttp://gis.iowa.uiowa.edu/private/yellowstone

Objectives• Investigate snow/elk interactions while

focusing on snow component by characterizing spatial and temporal distribution of snowpack properties

• Use a combination field data, model evaluation and model development to make contributions in snow hydrology and ecology

• To provide snowpack properties as input into the elk bioenergetics model

Questions• Which snow parameters are significant with respect to elk

bioenergetics?– Depth, density ,SWE, hardness– Snowfall and snowmelt magnitude and timing

• Which elk energetic expenditures are significant components?– Forage, locomotion, winter survival

• What is winter severity?

• Model Resolution?– Spatial and temporal

• What level of complexity is needed for most robust model?– Parameter selection and estimation

• Deterministic and Stochastic modeling approaches

Site Description•Spans ~80,000 hectares

•Lands managed by

•NPS, USFS, MFWP, private ownership, others

•Contentious land management issues

•Characterized by elevation, precipitation and temperature gradients

•Inverse relationship b/t precip and forage

Evaluation of NREL Yellowstone Snow Model

• Snow Model for Yellowstone National Park developed by Francis Singer, Gary Wockner, Mike Coughenour of the Natural Resource Ecology Lab (NREL) and Phil Farnes of Snowcap Hydrology in 2001

• Purpose: To produce prediction maps of snow water equivalence (SWE) in Yellowstone National Park (YNP)

• Snowpack influences the feeding habitat, locomotion, migration timing, vulnerability to predation and overall energy expenditures of elk

Field Work to DateField Methods:

• Snow Probe – snow depth measurements at 50-m grid spacing– GPS point location

• Snow Pits– Depth, density, temperature, hardness, grain size,

grain shape and stratigraphy – GPS point location

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Yellowstone National Park

NEWR

Crystal Creek Drainage

Field Work to Date

Field Work to Date

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Crystal Creek Snow Survey

• April 1-4, 2003

• 300 GPS depths

• 6 reference snow pits

Crystal Creek Snow Survey

• January 15-17, 2003

• 1200 GPS depths

• 21 reference snow pits

Field Work to Date

Sample snow pit profile from Crystal Creek April ‘03

crusts

Additional Field Data (YERC)

• Lamar Valley - 31 additional snowpits dug early and late season ’03

• Blacktail Plateau – 30 additional snowpits dug early and late season ‘03

• Evaluation– NREL model evaluation and validation– Running model for days with corresponding field data– 24 days/119 snow pits– Measured vs. Modeled

Measured vs. Modeled SWE Winter '03: Crystal Creek, Lamar Valley, Blacktail Plateau

y = 0.2298x + 35.397R2 = 0.1244

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Measured SWE (mm)

Mod

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Current Status

SNTHERM Overview

• Process driven, one-dimensional energy and mass balance model

• Developed by Rachel Jordan at CRREL in 1991

• Models changes in snowpack properties through time

• Model driven by first principles not empirical relationships

SNTHERM Overview

• Explicitly distributes energy, mass and momentum on a layer-by-layer basis throughout the snowpack

• 14 constant parameters– Elevation, slope, aspect, surface roughness, etc.

• 20 state variables– Snow density, temperature, depth, grain size, SWE, etc.

• Driving Variables– Air temp, relative humidity, wind speed, incoming/outgoing

shortwave radiation and incoming longwave radiation

Geostatistical Overview

• Understanding spatial distribution of snowpack properties challenging problem in snow hydrology

• Past and current spatially distributed snow models differ in the process representation they involve

• Use geostatistical techniques to spatially distribute measured snowpack properties

Summary

• Reintroduction of wolves in YNP contentious issue

• Provides the opportunity to study the effects of wolves on a previously wolf-free elk herd

• What are the effects of winter severity on elk/wolf/snow relationships?

Summary

• What is winter severity and what role does it play in elk-wolf interactions?

• Current research efforts– Index of Winter Severity (Farnes et al)

• intended to estimate animals’ response to climatic and vegetative conditions on the winter range

• SWE, ECT and KBDI

– Winter Severity and Wolf Predation on a Formerly Wolf-Free Elk Herd (Mech et al., 2001)

Summary

• Mech et al– Concluded that the effect of wolves on elk herd

numbers is related to the magnitude of winter severity

• Complexity Across Boundaries – Coupled Human and Natural Systems in the Yellowstone Northern Elk Winter Range– Develop a Multi-Agent Simulation Model

(MAS) that explores NEWR vulnerability to environmental stressors

Summary• Biocomplexity continued

– Understanding ecological systems where humans are significant factors

– Investigate snow/elk interactions while focusing on snow component by characterizing spatial and temporal distribution of snowpack properties

– Use a combination field data, model evaluation and model development to make contributions in snow hydrology and ecology