1983 ernie p. wiggers - tdl

CHARACTERIZATION OF ADJACENT DESERT

MULE AND WHITE-TAILED DEER

HABITATS IN WEST TEXAS

by

ERNIE P. WIGGERS, B.S., M.S.

A DISSERTATION

IN

AGRICULTURE

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

DOCTOR OF PHILOSOPHY

Approved

Chairman of the Committee

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(L4^ //, •X L .wZ .—

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u ccepted

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ean of the G ^ d u a t e School

May, 1983

'I)

ACKNOWLEDGEMENTS

In any study which encompasses as much t e r r i t o r y as t h i s study

did i t i s necessary to acknowledge numerous agencies and individuals

for the i r con t r ibu t ions . F i r s t , I want to thank Dr. Samuel L.

Beasom for select ing me as the research a s s i s t an t for t h i s project

and for h i s advice, support, and ed i to r i a l comments on the

manuscripts we coauthored. I want to thank my advisory committee,

Drs. J. Knox Jones, Henry A. Wright, Fred C. Bryant, and John R.

Garidino, for the i r ins t ruc t ions and comments which I believed

improved me as a profess ional . I am indebted to Scott Bebber, Larry

Howe, Austin Templer, and Jim Woods for the i r unfailing work in the

f ie ld ass i s t ing me in the col lec t ion of the data presented in t h i s

d i s s e r t a t i o n . I thank the individuals of the Texas Parks and

Wildlife Department, Soil Conservation Service, National Park

Service, and Bureau of Land Management who assis ted me in locat ing

study areas and making i n i t i a l landowner contac t s . I am appreci t ive

for the cooperation of the more than 30 west Texas ranchers who

graciously allowed me to invade the i r lands and co l l ec t the

necessary vegetat ive da t a . To the graduate students within the

Department of Range and Wildlife Management, I am grateful for the i r

fr iendship and ass i s t ance , espec ia l ly Luke Celentano and Dave Wester

for the i r help with the computer management and s t a t i s t i c a l

computation of the da t a .

i i

Funding for this study was provided by the USDA Forest Service

Rocky Mountain Forest and Range Experiment Station through the Great

Plains Wildlife Research Laboratory, Dr. Fred Stromer, Project

Leader. Research f a c i l i t i e s were provided by the Department of

Range and Wildlife Management, Dr. Henry A. Wright, Chairman.

Finally, I thank my wife, Hope, for her inspiration,

encouragement, support, and love throughout the work on my doctorate

degree.

I l l

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

ABSTRACT v

LIST OF TABLES vii

LIST OF FIGURES viii

CHAPTERS

I. INTRODUCTION 1

II. THE STUDY AREAS 3

III. METHODS 10

IV. ANALYSIS 15

V. RESULTS 16

Population Estimates 16

Habitat Characterization: The Trans-Pecos Region . . . . .16

Habitat Characterization: The Panhandle Region 26

VI. DISCUSSION 30

LITERATURE CITED 36

Abstract

Fourteen vegetat ive parameters and a Land Surface Ruggedness

Index (LSRI) were quantified on 306,000 ha of rangeland that

d i f f e r e n t i a l l y supported high and low dens i t i e s of mule deer

(Odocoileus hemionus crooki) and white- tai led deer (0 . virginianus

texanus) in west Texas to evaluate species-specif ic habi ta t

parameters. The only s ign i f i can t ly di f ferent (P<0.05) vegetat ive

component between the habi ta t s of the 2 species was percent woody

cover. On the high density white-tai led deer hab i t a t s , woody cover

averaged 63/E, and on the high density mule deer habi ta t s i t averaged

43%. Increases in white- tai led deer dens i t i e s were pos i t ive ly

correlated with increases in percent woody cover. The re la t ionsh ip

between woody cover and responses in desert mule deer numbers was

not s igni f icant (P>0.05), but deser t mule deer nunbers tended to

decrease as percent woody cover increased. Mule deer recent ly

disappeared from 2 areas where woody cover exceeded 75?.

The measurement of percent woody cover cor rec t ly c lass i f i ed 83

and 65% of the study areas into high and low density c l a s s e s ,

respec t ive ly , for white- ta i led deer and deser t mule deer . The

threshold value for discriminating between hab i ta t s of high or low

deer densi ty was approximately 53 and 50% woody cover for white-

t a i l ed deer and mule deer , r espec t ive ly .

V

Average LSRI values where white- ta i led deer were sighted during

ae r i a l surveys were l e s s than where deser t mule deer were s ighted.

The LSRI for mule deer ranged from 2 to 232 and for white- ta i led

deer from 0 to 136. In areas exhibi t ing a wide variance in

topographic ruggedness white- tai led deer appeared to be r e s t r i c t ed

to locat ions of l e sse r ruggedness, but mule deer were no t .

Coexistence of deser t mule deer and white- tai led deer in west

Texas i s possible because of the i r divergent habi ta t select ion for

percent woody cover and topography. However, because woody cover

can change temporally, the pos s ib i l i t y ex i s t s for competitive

exclusion to occur as deer dens i t i e s shif t in response to changes in

woody cover. (k)mpetitive exclusion apparently has occurred in west

Texas as whi te- ta i led deer have successfully supplanted mule deer in

localized a reas . Monitoring of changes in woody cover should be

used as an a p r io r i method for identifying areas where the potent ia l

for competitive exclusion e x i s t s . The use of habi ta t manipulative

prac t ices tha t can rebalance the r a t i o of preferred habi ta t s for

each deer species seems essen t ia l to ensure the continued presence

of both species in t h i s portion of the i r range.

V I

LIST OF TABLES

Page

1. Habitat component comparisons within deer density classes

and between deer species in the Trans-Pecos region,

Texas 17

2. Multiple regression models for desert mule deer and white-

tailed deer densities in the Trans-Pecos region, Texas. . . . 20

3. Correlation coefficients of multple regression model

variables determined for desert mule deer and white-tailed

deer densities in the Trans-Pecos region, Texas 22

4. Habitat component comparisons between desert mule deer and

white-tailed deer habitats in the Panhandle region of

Texas and southeastern New Mexico 27

5. Deer species-specific habitat component comparisons between

high deer density habitats in the Trans-Pecos region and

deer habitats in the Panhandle region and southeastern

New Mexico 28

Vll

LIST OF FIGURES

Page

1. Location of study areas for desert mule deer and white-

tailed deer habitat characterization in west Texas and

southeastern New Mexico 5

2. Desert mule deer and white-tailed deer densities in

relation to total percent aerial woody cover in

the Trans-Pecos region, Texas 25

Vlll

CHAPTER I

INTRODUCTION

The Trans-Pecos and Panhandle regions of west Texas accommodate

populations of deser t mule deer and white-tai led deer. Within

recent years landowners, sportsmen, and b io logis t s have expressed

concern about an apparent population decline in mule deer and an

increase in white- ta i led deer in areas t r ad i t i ona l l y considered

deser t mule deer range (Harwell and Gore 1981). Although

unsubstant iated, t h i s s i tua t ion may r e su l t from brush infes ta t ion

(Humphrey 1958, Johnston 1963, USDA 1964, Hasting and Turner 1965)

tha t now favors the white- tai led deer (Harwell and Gore 1981).

A survey of landowners in the Pecos River area indicated that

brush, as measured by changes in mesquite (Prosopis sp.) and

juniper (Juniperus s p . ) , has increased in r e l a t i ve abundance since

1900, and that white- ta i led deer have been disproport ionately

favored by t h i s increase (Wiggers 1982). Similar pat terns of white-

ta i led deer encroachment have been observed in the westcentral

United States and southern Canada (Kramer 1972). Kramer suggests

tha t the increasing dens i t i e s of woody vegetation along water

channels may have influenced a shif t in species r a t i o favoring

whi te- ta i led deer in these areas by providing habi ta t corr idors for

range expansion.

Presently l i t t l e i s known regarding the habitat requirements for

either deer species in west Texas, especial ly on sympatric ranges.

The lack of such information res tr i c t s the formulation of managonent

guidelines for possible habitat improvement for either species.

Given continued white-tailed deer encroachment, th i s knowledge i s

essential for addressing the concerns for the future status of the

desert mule deer. The objective of th i s study was to determine i f

selected habitat components di f ferent ia l ly influenced population

leve l s of desert mule deer and white-tailed deer.

CHAPTER I I

The Study Areas

Texas Parks and Wild l i fe Department (TPWD) deer d i s t r i b u t i o n maps

i n d i c a t e t h a t sympatric popula t ions of mule deer and w h i t e - t a i l e d

deer occur in the Panhandle and Trans-Pecos geographic reg ions in

west Texas (Harwell and Gore 1981, Russ 1981). Within these

r e g i o n s , s e l e c t i o n of s p e c i f i c study a r ea s which were known to

support or p rev ious ly t o have supported popula t ions of both deer

spec i e s was aided by TPWD and USDA Soil Conservation Service (SCS)

pe r sonne l . The a reas se lec ted included a l l or p a r t s of 30 p r i v a t e

r anches , 1 s t a t e and 1 f e d e r a l l y administered park in west Texas.

In a d d i t i o n , t he Mescalero Sands Area, administered by the Bureau of

Land Management (BLM), in southeas te rn New Mexico was included as a

s tudy area with the Panhandle region ( F i g . 1) . These a reas

represented 306,000 ha of sympatric or a t l e a s t adjacent d e s e r t mule

deer and w h i t e - t a i l e d deer h a b i t a t s .

In the Panhandle r e g i o n , TPWD maps i n d i c a t e t h a t deer a re

a s soc ia t ed p r imar i l y with the the Rolling P la ins Physiographic Area

(Harwell and Gore 1981, and Russ 1981). This area has a tempera te ,

sub t rop i ca l c l i m a t e , cha rac t e r i zed by long summers and dry win te r s

(Richardson e t a l . 1974). The Rolling P la ins occupies a zone

between the a r i d , d e s e r t r eg ions to the southwest and the more humid

NM

Study Areas

m Edwards Plateau

Rolling Plains

^ Mescalero Sands

Trans-Pecos

r eg ions t o the e a s t . P r e c i p i t a t i o n i s extremely v a r i a b l e , both

seasona l ly and annua l ly , but a long term annual average i s about 52

cm (Cor re l l and Johnston 1970). The major i ty occurs as r a i n during

spr ing and summer thundershowers . Snow f a l l s occas iona l ly but

u s u a l l y remains on the ground for only a few days . Extremes in

seasonal tempera tures a re e v i d e n t . From November to March s t rong ,

f a s t moving cold f ron t s can r e s u l t in pronounced and rapid drops in

d a i l y t empe ra tu r e s . The average minimum temperature for January

through March, the 3 co ldes t months, i s - 1 . 7 C. Summers are hot

with an average maximum temperature during June through August of 34

C. The l eng th .of the f ros t f ree period i s 200-220 days (Koos e t a l .

1966, Richardson e t a l . 1974).

The area i s cha rac t e r i zed by r o l l i n g topography, except in

l oca l i zed a reas where stream channel erosion has produced prominent

escarpments and canyons. So i l s are p r imar i ly deep to very shallow

c l a y , s i l t , and sandy loams under la in by a l ayer of ca l i che and

sandstone (Jacquot e t a l . 1965, Wright and Bailey 1982). Twenty

t h r e e range s i t e s were evident on the study a r e a s . Of t h e s e , 10

comprised 88% of the study a r e a s . These were, in descending order

by a r e a : deep sand, rough b r e a k s , mixedland s lopes , mixedlands, deep

h a r d l a n d s , sandy loams, sandy bot tomlands , very shal low, shallow

r e d l a n d s , and hardland s l o p e s .

The Rolling Plains are contained within the mixed grassland

vegetative type of the Southern Great Plains. Dominant woody

species include mesquite and juniper on most s o i l s , with sand

shinnery oak (Quercus havardii) and sand sagebrush (Artemisia

f i l i f o l i a ) on sandy s o i l s (Correll and Johnston 1970). Some

important subdcminant shrubs include lotebush (Zizyphus

obtusifolia) , ephedra (Ephedra sp.) , and aromatic sumac (Rhus

trilobata) (Wright and Bailey 1982). Prairie grasses include l i t t l e

bluestem (Schizachyriim scoparium) , big bluestem (Andropogon

gerardii) , sideoats grama (Bouteloua curtipendula) , and hairy grama

(B. hirsuta); with buffalo grass (Buchloe dactyloides) . tobosagrass

(Hilaria mutica) , and three awn (Aristida sp.) grasses increasing

under heavy grazing pressure (Correll and Johnston 1970).

The Mescalero Sands are located just below the western edge of

the Llano Estacado escarpment in southeastern New Mexico. The most

prominent physiological features of th i s area are 3 large, mobile

sand dunes (Smith 1971). The climate i s semi-arid with an average

rainfal l of 38 cm and a frost-free period of approximately 200 days

(Tuan et a l . 1969). The 2 major range s i t e s in this area are

sandyland and duneland.

The Trans-Pecos region includes the Trans-Pecos and the western

edge of the Edwards Plateau Land Resource Areas (Rives 1980). This

region has a warm semi-arid climate (Turner and Fox 1974, Rives

8

1980). Annual p rec ip i t a t ion i s about 30 cm, although t h i s var ies

grea t ly between years (Correll and Johnston 1970). Most of t h i s

p rec ip i t a t ion r e s u l t s from spring and summer thundershowers. Snow

accumulations are infrequent and do not represent a s ignif icant

source of moisture. The average minimum winter temperature for the

3 coldest months, January through March, i s 1.1 C. Average maximum

summer temperature i s 35 C for June through August. The length of

the frost free period i s 240-265 days (Tunner and Fox 1974, Rives

1980).

Topography cons i s t s of broad, level plateaus and ro l l ing to steep

h i l l s and canyon wal ls . Soils are shallow to deep and gravelly on

the limestone outcrops, gravelly and*loamy on the upland s i t e s , and

clayey and loamy on the flood p la ins . Concentrations of gypsum and

lime occur in many s o i l s of t h i s region (Carter and Cory 1930).

Several range s i t e s are evident in the Trans-Pecos region, but only

4 s i t e s were present on the study a reas . These were, in descending

order by a rea : steep rocky, gravel ly , deep s o i l , and bottomland.

The Trans-Pecos region i s on the eastern edge of the deser t

shrub-grass vegetat ive type. Major woody species include mesquite,

creosote bush (Larrea t r i d e n t a t a ) . tarbush (Flouren sia cernua) . and

fourwing saltbush (Atriplex canescens) (Rives 1980, Wright and

Bailey 1982). The more xeric s i t e s support arid-land plants such as

lechegui l la (Agave l e c h e g u i l l a ) , oco t i l l o (Fouquieria splendens),

and several species of yucca (Yucca sp.) (Correll and Johnston

1970). Prevalent grasses include black grama (Bouteloua eriopoda) .

alkal i sacaton (Sporobolus airoides) . tobosagrass, and burrograss

(Scleropogon brevifolius) (Bunting 1978, Rives 1980).

Ranching i s the major agricultural act ivity conducted on the

study areas in both regions. In the Panhandle region, ranching

operations u t i l i ze almost exclusively c a t t l e , whereas in the Trans-

Pecos region c a t t l e , sheep, goats, and combinations thereof are

u t i l i z e d . Since the cat t l e industry predominates in the Rolling

Plains, there i s l i t t l e effort between landowners to control

predators. However, where sheep and goats are managed in the Trans-

Pecos region extensive control efforts by landowners, primarily for

coyotes (Canis latrans) , are evident.

CHAPIER I I I

Methods

A l a t e September-early October he l i cop t e r survey was used to

determine s p e c i e s - s p e c i f i c deer d e n s i t i e s for 2 parks and 11 ranches

in the Panhandle region in 1980 and 17 ranches in the Trans-Pecos

region in 1981. Deer populat ion d e n s i t i e s for the Mescalero Sands

area were obtained from a e r i a l surveys conducted in 1979 and 1980 by

the BLM (ELM unpublished d a t a ) . All a e r i a l surveys were conducted

only during the 3 hours a f t e r sunr i se and 3 hours before s u n s e t .

During the a e r i a l survey the number of deer sighted was recorded ,

and the l o c a t i o n of each s igh t ing was marked on accompanying

topographic maps for l a t e r a n a l y s e s .

In the Panhandle r eg ion , deer popula t ions tended to be sparse and

l oca l i z ed so t h a t f l i g h t - l i n e s were concentrated in a reas where the

i n v e s t i g a t o r s f e l t the chance of s igh t ing deer was g r e a t e s t . This

procedure more e f f i c i e n t l y u t i l i z e d a v a i l a b l e f l i g h t t ime . Because

deer s i g h t i n g s were l o c a l i z e d , a study area was r e s t r i c t e d to the

area within a 2.15 km rad ius c i r c l e inscr ibed around a e r i a l

s i g h t i n g s of d e e r . The 2.15 km rad ius encompassed an area, t h a t

exceeded the average home range s ize of sedentary deer in t h i s

reg ion (Koerth 1981). S igh t ings l e s s than 2.15 km apar t were

considered to be within the same study a r e a , and t h i s combined area

10

11

c o n s t i t u t e d one study a r e a . Areas which did not over lap were

considered to be separa te study a r e a s . This procedure r e s u l t e d in

21 s tudy a r e a s in the Panhandle r e g i o n . In the Trans-Pecos region

the a e r i a l t r a n s e c t l i n e s t r aversed each e n t i r e ranch because deer

in t h i s reg ion were more numerous and more uniformly d i s t r i b u t e d .

Here each ranch was considered as a separa te study a r e a .

Vegetat ion was sampled during February through April and May

through September to determine seasonal extremes. Sampling was

r e s t r i c t e d to the Panhandle study a reas during the winter and summer

seasons of 1980 and on the Trans-Pecos study a reas in the winter and

summer of 1981. To minimize v a r i a t i o n between sample p l o t s , each

area was s t r a t i f i e d and sampled according to SCS range s i t e

c l a s s i f i c a t i o n s . Sample p lo t c e n t e r s were equa l ly spaced at 75 m

d i s t a n c e s along a randomly se lec ted t r a n s e c t through each range

s i t e . The same p l o t cen te r was used for a l l sampling procedures .

An average of 154 and 91 p l o t c e n t e r s were located and sampled per

s tudy area in the Panhandle and Trans-Pecos r e g i o n s , r e s p e c t i v e l y .

Standing biomass was sampled by c l i pp ing a l l herbaceous p lan t

ma te r i a l wi thin 0.25 m^quadrats a t each p l o t c e n t e r . All p l an t s

were sor ted in to g r a s s or forb groups and weighed to the nea res t

whole gram. Any herbaceous ma te r i a l l e s s than 1 gr in biomass was

recorded as 0.5 g r . Representa t ive samples of a l l p lan t ma te r i a l

were oven-dried (40 C) and percent mois ture content de termined.

12

Standing biomass was adjusted for moisture content and expressed as

(kg/ha) oven-dried biomass.

Percent of horizontal screening cover was measured at each plot

center for 5 height intervals on a 2.5 m x 0.15 m density board

(Nudds 1977). The proportion of area screened from a kneeling

observer in each 0.5 m interval at 15 m and 30 m distances was

estimated to 1 of 8 percentage c lasses ; 0, 1-5, 6-25, 26-50, 51-75,

76-95, 96-99, and 100 %. The mid-point value of each percentage

c lass was recorded as the percent of screening cover. This

measurement was repeated for 2 randomly selected directions from the

sample plot center. The average percent screening cover for the

0,0-1,5 m intervals was used to estimate a mean distance where total

deer screening at deer height occurred (Tanner et a l . 1978). An

identical procedure was used to determine the mean distance to total

screening above deer height using the percent screening cover for

the 1.6-2.5 m intervals . If no screening cover was measured, then

distance to total screening was recorded as 1,500 m.

Woody plant density was determined by counting stems of a l l woody

plants and cacti in 0,01 ha circular quadrats around each plot

center. Density calculations were made for each species, for al l

woody plants, and for tree species which could support a canopy

cover above deer height. In addition, the canopy diameter of

randomly selected specimens of each species on each range s i te and

13

area was r eco rded . Diameter measurements were used to c a l c u l a t e an

average canopy area assoc ia ted with each woody s p e c i e s . Average

canopy area and stem dens i t y were used to determine percent a e r i a l

cover of each woody spec i e s on a range s i t e using the following

e q u a t i o n : % a e r i a l woody cover = canopy area (m ) * dens i ty

( s t ems /ha) /10 ,000 mVha *100, The sum of the percent a e r i a l cover

for a l l woody spec i e s equal led the t o t a l percent a e r i a l woody cover

for a range s i t e , and the sum for a l l the t r e e species equalled the

percent a e r i a l t r e e cover . Canopy diameter was included as a

v e g e t a t i v e measurement only for the Trans-Pecos study a r e a s .

Topographic ruggedness was determined by c a l c u l a t i n g a Land

Surface Ruggedness Index (LSRI) a t each map loca t i on of deer

s i g h t i n g s recorded during the he l i cop t e r survey. LSRI va lues were

used to a s s e s s land ruggedness as a h a b i t a t f ea tu re for each deer

s p e c i e s . The LSRI was determined using a do t -g r id composed of 96

uniformly spaced do t s on a c i r c l e of t r a n s p a r e n t p l a s t i c sheet ing

which represented a 40 ha area on topographic maps. The cen te r of

t h i s gr id was ove r l a id to match the point marked as a deer s igh t ing

on the topographic map. The number of dot-contour l i n e

i n t e r s e c t i o n s for each s igh t ing was counted and recorded as the LSRI

v a l u e . Because twice and four t imes the number of contour l i n e s a re

requi red to d e p i c t the r e l i e f for a given area using 3.04 versus

6.10 and 12.20 m contour i n t e r v a l s , r e s p e c t i v e l y , a l l LSRI va lues

were s tandardized to t h a t expected for the 3.04 m contour i n t e r v a l s .

14

This was accomplished by multiplying the LSRI value by a correction

factor equal to the contour l ine interval of the map divided by 3.04 m.

CHAPTER IV

Analysis

Mean vegetat ive measurements from sample p lots within range s i t e s

were determined for each study area. Mean values were 'weighted' in

proportion to the area occupied by each range s i t e in that study

area. The data for the Mescalero Sands study area was included with

the data for the study areas in the Panhandle region during

analyses . Because of ex i s t ing differences between habitat and

envirorment, data for the Panhandle and Trans-Pecos regions were

analyzed independently. Population d e n s i t i e s were used as

indicators of habitat preference, assuming high d e n s i t i e s were «

ref lect ive of high habitat preference. The low deer densit ies

determined for the Panhandle region precluded their separation into

low and high density c lasses for habitat contrast evaluations.

Instead, a l l observations were assigned a low density

c las s i f i ca t ion . Deer densi t ies in the Trans-Pecos region were 2 2

grouped into low (<8.0 deer/0.4 km ) and high O8.0 deer/0.4 km )

density c lasses for analyses, A IXincan's multiple range test was

used to determine i f significant differences existed between

parameters in desert mule deer and white-tailed deer habitats within

deer density c la s se s , A discriminant analysis was used to determine

i f selected habitat parameters could accurately c lass i fy habitats

into low or high deer density c la s se s ,

15

CHAPTER V

Resul ts

Population Estimates

Densi ty e s t i m a t e s in the Trans-Pecos region ranged from 2 - 2

0 . 0 - 2 9 . 0 / 0 , 4 km (X=5,5) for d e s e r t mule deer and 0 .3 -27 .0 /0 .4 km

(X=10.3) for w h i t e - t a i l e d d e e r . Although mule deer were not

observed on 2 a r e a s , landowner records i nd i ca t e t h a t t h i s spec ies

had occupied these a reas within the past 40 years (W. A. Wroe,

Personal Communication), Population es t imates for the Panhandle

region were lower and narrower in range , 0 .6 -9 .1 /0 .4 km^ (Y=3.0) and 2 —

0 . 6 - 8 . 9 / 0 . 4 km (X=3.3) for mule deer and wh i t e - t a i l ed dee r , r e s p e c t i v e l y .

Habi ta t Clharacter iza t ion: The Trans-Pecos Region

In the Trans-Pecos r eg ion , percent woody cover was the only

v e g e t a t i v e parameter which was s i g n i f i c a n t l y d i f f e r e n t (P<0.05)

between h a b i t a t s occupied by mule deer and wh i t e - t a i l ed deer (Table

1 ) . Aerial woody cover averaged 63% on a reas support ing high whi te-

t a i l e d deer d e n s i t i e s . This value was s i g n i f i c a n t l y g rea te r (P<0.05)

than the 43% a e r i a l woody cover determined on a reas support ing high

d e n s i t i e s of mule deer (Table 1) . Conversely, woody cover averaged

43% on the low d e n s i t y w h i t e - t a i l e d deer h a b i t a t s and 56% on the low

d e n s i t y d e s e r t mule deer h a b i t a t s . A s i g n i f i c a n t d i f fe rence

16

17

Table 1. Habitat component comparisons within deer density classes and

between deer species in the Trans-Pecos region, Texas. Measurements

followed by a different lower case letter are significantly different

(P< 0.05).

Habitat component

Forage biomass. summer (kg/ha)

Grass biomass. summer (kg/ha)

Forb biomass. Slimmer (kg/ha)

Forage biomass, winter (kg/ha)

Grass biomass. winter (kg/ha)

Forb biomass, winter (kg/ha)

Total woody density (stems/ha)

Tree density (stems/ha)

Total woody cover (%)

Tree cover (%)

n <a a•nr•« tn rlosur(

High Mule deer

507 a

315 a

192 a

365 a

196 a

169 a

3,937 a

649 a

43 a

20 a

a 194 a

deer

N

1 5

5

5

5

5

5

5

5

5

5

5

density White-tailed deer

860 a

524 a

336 a

690 a

256 a

434 a

3,044 a

811 a

63 b

35 a

151 a

N

8

8

8

8

8

8

8

8

8

8

8

Low Mule deer

820 a

507 a

313 a

690 a

308 a

382 a

3,321 a

732 a

56 a

31 a

152 a

deer

N

12

12

12

12

12

12

12 3

12

12

12

12

density White-ta deer

611 a

386 a

225 a

508 a

291 a

217 a

,909 a

616 a

43 a

21 a

176 a

ilec N

9

9

9

9

9

9

9

9

9

9

9

0.0-0.5 m strata, summer (m)

Distance to closure 943 a 1.6-2.5 m strata, summer (m)

Distance to closure 377 a 0.0-1.5 m strata, winter (m)

Distance to closure 917 a 1.6-2.5 m strata, winter (m)

893 a 8 904 a 12 936 a 9

5 387 a 8 317 a 12 288 a 9

781 a 8 863 a 12 967 a 9

18

Table 1.—Continued

High deer density Low deer density Mule White-tailed Mule White-tailed

Habitat component deer N deer N deer N deer N

LSRI 30 a 615^ 29 a 779 40 a 122 31 b 242

1 2 Number of study areas; Number of deer sighting locations,

19

(P<0,05) was determined between the average LSRI values for mule

deer versus white- ta i led deer sightings on low deer densi ty areas

(Table 1) . Overall LSRI measurements of mule deer sightings were

greater and had a greater range (2-232) than those of white-tai led

deer (0-136).

Within deer species , t o t a l herbaceous biomass and grass biomass

in the summer were s igni f icant ly lower (P<0.05) on the high density

versus the low densi ty mule deer hab i t a t s . Canopy cover for a l l

woody plants and for t r ee s only were s igni f icant ly greater (P<0.05)

on high versus low densi ty white- ta i led deer hab i t a t s . The

remaining habi ta t parameter measurements characterized high density

whi te- ta i led deer habi ta t s as having greater amounts of both

screening cover and herbaceous plant material than did low density

whi te- ta i led deer h a b i t a t s . Conversely, screening cover and

herbaceous biomass was greater on low mule deer density habi ta t s

than on high mule deer densi ty h a b i t a t s .

Total percent ae r i a l woody cover and percent aer ia l cover of the

t r ee species accounted for 63% of the variat ion in white-tai led deer

dens i t i e s in a stepwise mult iple regression model (Table 2) , Total

ae r i a l woody cover accounted for 48% of the va r ia t ion , but the

increase in the regression coeff ic ient when percent t ree ae r ia l

cover was included was s ign i f i can t . Two dif ferent var iables

resul ted vrtien mule deer densi ty was used as the dependent variable

20

Table 2. Multiple regression models for desert mule deer and white-

tailed deer densities in the Trans-Pecos region, Texas.

Dependent variable = Mule deer

density

Intercept

B value

3.961

P F R P F

0.55 0.0036

Grass biomass, summer (kg/ha) -0.030 0.0016

Distance to closure 1.6-2.5 m strata, summer (m)

0.017 0.0042

Dependent variable = White-tailed

deer density

0.63 0.0009

Intercept

Total woody cover

Tree canopy cover

-10.900

56.832

-32.102

0.0004

0.0291

21

(Table 2 ) . The measurements of summer grass biomass and mean

dis tance to closure at deer height accounted for 55% of the

var ia t ion in mule deer d e n s i t i e s .

Correlations between regression model variables and species-

specif ic deer dens i t i e s are given in Table 3. Total percent aer ia l

woody cover was the most highly correlated parameter. Increases in

percent ae r i a l woody cover were s igni f icant ly (P<0.05) correlated

with increases in white-tai led deer d e n s i t i e s . Mule deer dens i t i es

were negatively associated with percent aer ia l woody cover, although

t h i s was not a s igni f icant (P>0.05) re la t ionsh ip . No habitat

parameters were s ignif icanty correlated with mule deer dens i t i e s .

The strongest r e la t ionsh ip with mule deer dens i t ies was indicated

for standing grass biomass during the summer, and t h i s was an

inverse r e l a t i onsh ip . In addit ion, mule deer dens i t i es were

inversely correlated with increases in white-tailed deer dens i t i e s .

Percent ae r i a l woody cover was inversely related to to t a l woody

density (r=-0,47, P<0,05) but strongly correlated with the dominant

brush species , mesquite (r=0.80, P<0.001). Although not

s ign i f i can t , the percent mesquite aer ia l cover was posi t ively

re la ted with increases in white-tai led deer numbers and inversely

related with increases in mule deer numbers.

Because aer ia l woody cover was the only vegetative parameter

measured that was s ign i f ican t ly d i f ferent between deer species, i t

22

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23

was further investigated to determine i f i t could be used to

discriminate between deer densi ty c l a s se s . The measurement of t o t a l

percent ae r i a l woody cover was successful in correc t ly classifying

65 and 83% of the observations for mule deer and white-tailed deer,

r e spec t ive ly , into deer densi ty c lasses (Fig, 2 ) . Of the 6 wrong

c l a s s i f i c a t i o n s for mule deer where the discriminate c lass i f i ca t ion

did not agree with those according to observed deer d e n s i t i e s , 5

occurred when low densi ty observations were placed in the high

densi ty c l a s s . The apparent s ingle threshold value for

discriminating between deer density c lasses occurred at about 50%

t o t a l woody cover. Habitats supporting a percent aer ia l woody cover

l e s s than 50% were c lass i f ied into high mule deer density c l a s ses .

Of the 3 areas supporting 75% woody cover or more, mule deer have

disappeared on 2 a reas , and thei r density was l ess than 4/0.4 km^ on

the other a rea .

The threshold value used to discriminate between white-tai led

deer densi ty c lasses occurred at 53% to ta l aer ia l woody cover.

Areas where percent ae r ia l woody cover was greater than th i s were

c lass i f ied as high white- tai led deer densi ty h a b i t a t s . Two of the 3

misc lass i f i ca t ions for white-tai led deer resulted when high deer

densi ty observations were c lass i f ied to the low densi ty c l a s s . All 2

whi te- ta i led deer densi ty est imates that exceeded 20 deer per 0.4 km

occurred when t o t a l ae r ia l woody cover exceeded 65% and they were

co r rec t ly c l a s s i f i e d .

25

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26

Habi ta t C h a r a c t e r i z a t i o n : The Panhandle Reg ion

In the Panhandle r e g i o n , no s i g n i f i c a n t (P>0.05) d i f f e rences were

determined between the vege t a t i ve components of mule deer and whi te-

t a i l e d deer h a b i t a t s (Table 4 ) . A s i g n i f i c a n t (P<0.05) d i f fe rence

between deer spec ies was shown only for the LSRI value associa ted

with each s igh t ing (Table 4 ) . Not only was the mean LSRI

measurement a s soc ia ted with mule deer l o c a t i o n s s i g n i f i c a n t l y higher

than those for w h i t e - t a i l e d dee r , but the LSRI range was much

broader (10-176 and 0-90, r e s p e c t i v e l y ) and i t s maximum value was

almost twice as l a r g e as t h a t for wh i t e - t a i l ed dee r .

No s i g n i f i c a n t (P>0.05) c o r r e l a t i o n s or regress ion models could

be determined between s p e c i e s - s p e c i f i c deer d e n s i t i e s and h a b i t a t

measurements in the Panhandle r eg ion . The s t ronges t r e l a t i o n s h i p

was ind ica ted between LSRI measurements and mule deer d e n s i t i e s

( r = 0 . 6 l , P<0.10) ,

Evaluat ions of deer h a b i t a t s in the Panhandle were attempted by

comparing h a b i t a t measurements from t h i s region to measurements

determined on the high d e n s i t y deer h a b i t a t s in the Trans-Pecos

region (Table 5 ) . Comparisons reveal dramatic d i f f e rences which

might be i n d i c a t i v e of h a b i t a t q u a l i t y between the 2 r e g i o n s . Most

no t ab l e were the s i g n i f i c a n t (P<0.05) or otherwise s u b s t a n t i a l

d i f f e r e n c e s in seasonal forb product ion . During the winter season.

27

Table 4. Habitat component comparisons between desert mule deer and

white-tailed deer habitats in the Panhandle region of Texas and south

eastern New Mexico. Measurements followed by a different lower case

letter are significantly different.

Habitat component Mule deer N White-tailed deer N

Forage biomass, summer 579 a 9 538 a 12 (kg/ha)

Grass biomass, summer 436 a 9 456 a 12 (kg/ha)

Forb biomass, summer 143 a 9 83 a 12 (kg/ha)

Forage biomass, winter 270 a 9 378 a 12 (kg/ha)

2 Grass biomass, winter 208 a 9 327 b* 12 (kg/ha)


Total woody density 16,806 a 9 21,975 a 12 (stmes/ha)


Distance to closure, 0.0-1.5 m strata, summer (m)

579 a

436 a

143 a

270 a

208 a

62 a

16,806 a

617 a

283 a

9I

9

9

9

9

9

9

9

9

LSRI

50 a 12

996 a 12

177 a 12

779 a 9 818 a 12

376 a 9 204 a 12


Distance to closure, 0.0-1.5 m strata, winter (m)

Distance to closure, 1,016 a 9 1,028 a 12 1.6-2.5 m strata, winter (m)

81 a 191^ 19 b** 12

^Number of study areas; * = P<0.10, ** = P<0.01; ^Number of deer

sightings.

28

Table 5. Deer species-specific habitat component comparisons between

high deer density habitats in the Trans-Pecos region and deer habitats

in the Panhandle region and southeastern New Mexico. Measurements

followed by a different lower case letter are significantly different.

Mule deer White-tailed deer

Habitat component Trans-Pecos Panhandle Trans-Pecos Panhandle

Forage biomass, summer (kg/ha)

Grass biomass, summer (kg/ha)

Forb biomass summer (kg/ha)

Forage biomass winter (kg/ha)

Grass biomass, winter (kg/ha)


Total woody density (stems/ha)






LSRI

507 a

315 a

192 a

365 a

196 a

169 a

3,937 a

649 a

194 a

579 a

436 a

143 a

270 a

208 a

62 b

16,806 a

617 a

283 a

943 a

377 a

917 a

30 a

779 a

376 a

1,016 a

81 b***

860 a

524 a

336 a

690 a

256 a

434 a

3,044 a

811 a

151 a

893 a

387 a

781 a

29 a

538 a

456 a

83 b***'

378 a

327 a

50 b*

21,975 a

996 a

177 a

818 a

204 a

1,028 a

19 b**

* = P<0.10; ** = P<0.05; *** = P<0.01.

29

mean forb production on the high density white-tailed deer habitats

in the Trans-Pecos region were more than 800% greater than for

white-tailed deer habitats in the Panhandle region. Although not as

dramatic, similar dispari t ies occurred when mule deer habitats were

compared. In addition, LSRI measurements were significantly

different (P<0,05) for both deer species between regions.

CHAPTER VI

Discussion

Mean measurements of 63% woody cover on areas supporting high

w h i t e - t a i l e d deer d e n s i t i e s and 43% on areas supporting low

d e n s i t i e s agree s t rong ly with the 60% and 43% brushy cover reported

for high and low d e n s i t y w h i t e - t a i l e d deer h a b i t a t s in south Texas

( S t e u t e r and Wright 1980) . Approximately 53% woody cover appears

essen t i a l before high dens i t i e s of white-tailed deer can be expected

on rangeland in southwest Texas. However, as indicated by the

missed c l a s s i f i c a t i ons in the discriminate analys is , percent aer ia l

woody cover alone does not ensure high deer d e n s i t i e s . Steuter and •

Wright (1980) reported that brushy cover accounted for 67% of the

va r ia t ion in white- tai led deer dens i t i e s in south Texas. This value

approximated the percent determined for the white-tailed deer in the

Trans-Pecos region. Other inves t iga tors have reported the

requirements for suff ic ient brushy cover by white-tailed deer on

rangeland in Texas (Horejsi 1973, McMahan and Ingl is 1974, Ingl is e t

a l . 1978)

The r e l a t ionsh ip between brushy cover and desert mule deer

populations i s l e s s apparent because of the wide var ia t ions in the

response of mule deer dens i t i e s to that habitat parameter. But, the

30

31

data strongly suggest that desert mule deer can maintain higher

population nunbers in areas where there i s substantially lower

amounts of aerial woody cover than can white-tailed deer. Short

(1977) concluded that the mule deer has benefited from brush

infestation in i t s range since many of the forage species i t uses

are those that have invaded and proliferated on the semidesert

grasslands. This benefit apparently ex is t s only i f l e s s than 50%

woody cover i s maintained on the rangeland. If woody cover

development continued with brush infestation, then a shift in

habitat characteriztics favoring white-tailed deer would be

expected.

The wide variations in mule deer densit ies in response to

different l eve l s of woody cover indicates that this deer has l e s s

specific requirements for woody cover than white-tailed deer.

However, the presence of high densit ies of white-tailed deer on all

areas with high amounts of woody cover precluded a definit ive

determination since i t could not be distinguished v*iether habitat

preferences alone or the presence of white-tailed deer restricted

mule deer occupation. Kramer (1971) observed interactions between

Rocky Mountain mule deer (0, h, hemionus) and white-tailed deer (fi,

V, dacotensis) on overlapping range and concluded that negative

aggressive behavior was not prevalent between the 2 deer species.

If similar interactions are assimed for sympatric populations in

west Texas, then the conclusion that habitat selection alone

32

partition the 2 deer species on their range i s strengthened.

The recent disappearance of mule deer on 2 areas currently

supporting high white-tailed deer densit ies and a high percent woody

cover indicates that white-tailed deer can replace desert mule deer

in marginal mule deer habitat (MacArthur 1958). Similarly, i t has

been shown that mule deer can replace white-tailed deer given the

proper habitat changes (Anthony and anith 1977).

As a vegetative component, woody cover can be influenced by a

variety of environmental and man-made actions which can alter i t s

structure. During response to these actions, alterations in the

percent aerial woody cover may result in habitat selection shifting

between deer species so that a temporary opportunity for

interspecif ic competition and competitive exclusion exists (Anthony

and anith 1977). A survey by Wiggers (1982) suggests that this type

of exclusion has occurred in the Pecos River area of Texas, They

reported that brush infestation has continued throughout this

century resulting in a shift in habitat characteristics which now

favors the white-tailed deer. As a resu l t , white-tailed deer have

expanded into new locations in this area at a rate twice that of

mule deer. Further, the extirpation rate of mule deer on individual

ranches was 700% greater than that for white-tailed deer. Thus, i t

appears that competitive exclusion by white-tailed deer has occurred

and may s t i l l be occurring in this area as a result of habitat

33

preference sh i f t s from increases in aerial woody cover. The

opposite trend in habitats and population dynamics between desert

mule deer and Coues white-tailed deer (fi, v , couesi) was reported by

Anthony and Smith (1977) in southern Arizona. They attributed this

occurrence to an upward shift in altitude of the desert shrub

vegetational zone favored as habitat by the desert mule deer and a

concurrent retreat upward in altitude of the oak woodland,

chaparral, coniferous forest zone favored as habitat by the white-

tai led deer. The desert shrub type was characterized as being

overgrazed and supporting lesser abundance of grasses and herbaceous

plant material than the vegetation zones favored by white-tailed

deer. Thus, the study by Anthony and Smith (1977) collaborates my «

findings that desert mule deer habitats differ from white-tailed

deer habitats by the amount and structure of the existing

vegetation.

The s ignif icantly greater LSRI values for locations where mule

deer were sighted compared to where white-tailed deer were sighted

indicated that mule deer can occupy areas of greater topographic

ruggedness. Other investigators have proposed land ruggedness as a

possible habitat partitioning factor (Kramer 1972, Krausman 1978).

However, their observations were based on primarily subjective

evaluations of topography. Thus, additional investigations where

land surface ruggedness i s quantitatively analyzed throughout the

range of deer habitat overlap are needed to determine i t s full

34

importance and the specific conditions under which i t becomes a

factor.

The lower population estimates of both deer species in the

Panhandle region probably ref lects a generally lower habitat quality

for deer. Because forbs comprise a major proportion of the diet of

each species (Chamrad and Box 1968, Anderson et a l . 1965, Short

1977. Sowell 1981), the lower habitat quality in the Panhandle

region may be a reflection of the low forb production. Lower forb

production may also account for the low nutritional plane determined

for mule deer in this region (Sowell 1981). A similar nutritional

plane would be expected for white-tailed deer because of the diet

s imilarity of the 2 deer species (Krausman 1971, Kramer 1972,

Anthony and Smith 1977).

The contributions of other factors in restricting deer population

growth in th is region, such as coyote predation or the

interrelationship between predation and habitat quality, i s not

known but warrants further investigations. Studies have shown

coyotes to be a major mortality factor of deer fawns in Texas (Cook

et a l . 1971, Beasom 1974). Given already low deer dens i t ies , a

skewed predator-prey ratio in favor of the coyote might surppress

deer populations in this region (Mech 1970, Connolly 1978).

35

Kramer (1972) concluded that since large scale coexistence of

mule deer and white-tailed deer occurs, then each species must

occupy divergent niches to ensure minimum competitive overlap. The

determination of 2 habitat selection differences between mule and

white-tailed deer on overlapping range in west Texas supports

Kramer's conclusions of divergent niches between these 2 wildlife

species . Coexistence of mule and white-tailed deer in west Texas i s

possible because of differences in each deer's habitat selection for

percent aerial woody cover and topographic ruggedness. Therefore,

dual management of these 2 deer species i s possible in areas of

overlap in west Texas. Since percent aerial woody cover may change

temporally due to a variety of factors, monitoring of th i s habitat

component seems essential for proper deer management. Chronic

monitoring should be used to determine the rate at which preferred

habitat of each deer i s shrinking or expanding, and to identify

areas where the opportunity for competitive exclusion e x i s t s .

Further, management actions designed to reverse trends of continued

woody cover development may become necessary to ensure that

preferred desert mule deer habitat remains, and that this species

remains a prominent big game animal on this portion of i t s range.

LITERATURE CITED

Anderson, A, E . , W. A. Snyder, and G. W, Brown, 1965. Stomach

^ con ten t a n a l y s i s r e l a t e d to condi t ion in mule dee r , Guadalupe

Mountains, New Mexico. Journal of Wildl i fe Management.

29:352-366.

Anthony , R. G., and N, S, Smith. 1977. Ecological r e l a t i o n s h i p s

between mule deer and w h i t e - t a i l e d deer in southeas tern Arizona,

Ecological Monographs, 47:255-271,

Beasom, S. L, 1974. Rela t ionships between predator removal and

w h i t e - t a i l e d deer net p r o d u c t i v i t y . Journal of Wildl i fe

Management. 38:854-859-

Bunting, S. C. 1978. The vege ta t ion of the Guadalupe Mountains.

D i s s e r t a t i o n . Texas Tech Univers i ty , Lubbock, Texas, USA.

C a r t e r , W. T. , and V. L. Cory. 1930. So i l s of the Trans-Pecos,

Texas and some of t h e i r vege t a t i ve r e l a t i o n s h i p s . Transact ions

of the Texas Academy of Science . 15:19-37.

Chamard, A. D. and T. W. Box. 1968. Food h a b i t s of wh i t e - t a i l ed

deer in south Texas. Journal of Range Management. 21:158-164.

Connolly, G. E. 1978. Predators and predator c o n t r o l . Pages

369-394 in J . L. Schmidt and D. L. G i l b e r t , e d i t o r s . Big Game

of North America, Ecology and Management. Stackpole Books,

Har r i sbu rg , Pennsylvania , USA.

36

37

Cook, R, S , , M, White, D, 0, T ra ine r , and W. C, Glazener. 1971.

Mor t a l i t y of young w h i t e - t a i l e d deer fawns in south Texas.

Journal of Wild l i fe Management. 35:47-56.

C o r r e l l , D, S . , and M. C, Johnston. 1970. Manual of the vascular

p l a n t s of Texas, The Texas Research Foundation, Renner, Texas,

USA,

Harwell , W, F , , and H, G. Gore, 198I. Whi te - ta i led deer

popula t ion t r e n d s . Job Performance Report. Federal Aid Project

No. W-IO9-R-4. Job No. 1, Texas Parks and Wildl i fe Department,

Aus t in , Texas, USA,

Has t ing , J , R,, and R, M. Turner . 1965. The changing m i l e .

Univers i ty of Arizona P re s s , Tucson, Arizona, USA.

H o r e j s i , ' R. G. 1973. Influence of brushlands on wh i t e - t a i l ed deer

d i e t s in North-Central Texas. Thes i s . Texas Tech Univers i ty ,

Lubbock, Texas, USA.

Humphrey, R.R. 1958. The d e s e r t g r a s s l a n d . A h i s t o r y of

v e g e t a t i o n a l change and an a n a l y s i s of c a u s e s . Botanical

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