THE POSSIBLE INFLUENCE OF THE MOUNTAIN LION
ON MULE DEER POPULATIONS
Maurice Hornocker
Unit Leader
Idaho Cooperative W i l d l i f e Research Unit
College of F o r e s t r y , W i l d l i f e and Range Sciences
U n i v e r s i t y of Idaho
Moscow, Idaho 83843
Any d i s c u s s i o n of t h e i n f l u e n c e o r e f f e c t o f
m u n t a i n l i o n p r e d a t i o n on prey populations becomes,
of n e c e s s i t y , a d i s c u s s i o n of predator-prey r e l a t i o n s h i p s . The reason is obvious - t h e r e a r e p r a c t i c a l l y
no d a t a a v a i l a b l e on lion-prey i n t e r a c t i o n s . Lions
a r e known t o k i l l d e e r wherever t h e two s p e c i e s
occur t o g e t h e r b u t t h i s merely e s t a b l i s h e s t h e f a c t .
The e f f e c t o f t h i s k i l l i n g on d e e r numbers - t h e '
r e a l l y meaningful and important a s p e c t t o c o n s i d e r h a s s c a r c e l y been looked a t i n an o b j e c t i v e way.
Therefore, today we a r e a b i t l i m i t e d . We c a n ' t
c r i t i c a l l y review t h e l i t e r a t u r e because t h e r e i s n ' t
any. We w i l l d i s c u s s what is known of l i o n s and
t h e i r e f f e c t on prey from r e s e a r c h thus f a r r e p o r t e d .
We w i l l t a l k about t h e p o s s i b l e i n f l u e n c e and e f f e c t ,
o r l a c k of e i t h e r , of l i o n s on mule d e e r p o p u l a t i o n s
and t h e f a c t o r s t h a t a r e important i n t h e s e r e l a t i o n s h i p s . We'll a l s o consider some n e g a t i v e evidence
concerning t h e e f f e c t of l i o n p r e d a t i o n , t h e r o l e
c o n t r o l might p l a y , and needed r e s e a r c h .
2.
d e n s i t y of t h e p r e d a t o r p o p u l a t i o n
3.
c h a r a c t e r i s t i c s of t h e prey
4. d e n s i t y and q u a l i t y of a l t e r n a t e foods
available f o r t h e predator
5.
c h a r a c t e r i s t i c s of t h e p r e d a t o r
Each o f t h e s e v a r i a b l e s may e x e r t c o n s i d e r a b l e
i n f l u e n c e and t h e e f f e c t of any one may depend upon
changes i n a n o t h e r . Considered s e p a r a t e l y each
t e l l s l i t t l e b u t when considered i n combination,
sound i n t e r p r e t a t i o n s o f t h e r e l a t i o n s h i p s can b e
made.
F u r t h e r , i t is important t o know t h e h i s t o r y
of i n t e r a c t i n g p o p u l a t i o n s . Did t h e s p e c i e s evolve
t o g e t h e r ? Is t h e r e l a t i o n s h i p an o l d one o r i s i t
of r e c e n t o r i g i n ? I s t h e ecosystem s t a b l e o r
changing? Knowing t h i s , we can make some g e n e r a l
statements :
I d o n ' t i n t e n d t o launch i n t o a g e n e r a l i z e d
d i s c u s s i o n of p r e d a t i o n - I d o n ' t b e l i e v e t h a t ' s
necessary h e r e . I t w i l l , however, be h e l p f u l t o outl i n e a few of t h e t h i n g s b a s i c t o predator-prey
r e l a t i o n s h i p s . None o f t h e s e a r e my o r i g i n a l
thoughts - they a r e t h o s e of numerous a u t h o r s s e t
down over a long p e r i o d of time.
1. The l i m i t i n g e f f e c t s o f p r e d a t i o n tend t o
b e reduced and t h e r e g u l a t o r y ( o r s t e a d y s t a t e )
e f f e c t s i n c r e a s e d where t h e i n t e r a c t i n g p o p u l a t i o n s
have a comnnn e v o l u t i o n a r y h i s t o r y and they occur
i n a r e l a t i v e l y s t a b l e ecosystem.
F i r s t , what a r e t h e p o s s i b l e e f f e c t s of l i o n
p r e d a t i o n ( o r any p r e d a t i o n f o r t h a t m a t t e r ) on prey
populations?
2. V i o l e n t predator-prey i n t e r a c t i o n s happen
f r e q u e n t l y when t h e i n t e r a c t i o n is o f r e c e n t o r i g i n
o r when t h e r e h a s been r e c e n t l a r g e s c a l e d i s t u r bance of t h e ecosystem by c l i m a t i c change, n a t u r a l
h o l o c a u s t , o r by man.
1. The p r e d a t o r can be s t r o n g l y l i m i t i n g t o
t h e p o i n t of reducing t h e prey t o e x t i n c t i o n o r n e a r
extinction.
2 . The p r e d a t o r can b e r e g u l a t o r y i n t h a t i t
h e l p s keep prey p o p u l a t i o n s w i t h i n t h e c a r r y i n g
c a p a c i t y of t h e i r r e s o u r c e s , o r another way, it cont r i b u t e s t o a s t e a d y s t a t e i n t h e d e n s i t y of t h e prey.
3. The p r e d a t o r may be n e i t h e r l i m i t i n g n o r
r e g u l a t i n g ; i n o t h e r words, t h e p r e d a t o r is
i n s i g n i f i c a n t i n t h e population dynamics o f t h e prey.
Which s i t u a t i o n e x i s t s between t h e p r e d a t o r and
i t s prey depends on d i f f e r e n t f a c t o r s . Leopold (1933)
c l a s s i f i e d t h e s e i n t o 5 groups:
1. d e n s i t y of t h e prey p o p u l a t i o n
So much f o r t h e p o s s i b l e e f f e c t s of p r e d a t i o n .
What a r e i t s i n f l u e n c e s ?
1. P r e d a t i o n can dampen and p r o t r a c t v i o l e n t
f l u c t u a t i o n s i n numbers of prey animals. Damping
can a c t t o reduce range damage and s e r i o u s overs t o c k i n g may b e a v e r t e d u n t i l such time a s o t h e r
l i m i t i n g f o r c e s , such a s d i s e a s e , may come i n t o
operation.
2. Predation is a l s o a strong s e l e c t i v e force
a c t i n g t o remove prey i n d i v i d u a l s p o s s e s s i n g l e s s
d e s i r a b l e adaptive characters.
3. P r e d a t o r s may a l s o a c t t o d i s p e r s e prey
animals and thus cause them t o be more evenly d i s t r i b u t e d on c r i t i c a l range.
The Possible Influence of the Mountain Lion
These a r e t h e t h e o r e t i c a l p o s s i b l e e f f e c t s and
i n f l u e n c e s of mountain l i o n s on mule d e e r p o p u l a t i o n s .
What i s the real-world evidence f o r any of t h e s e
r e l a t i o n s h i p s ? This i s , o r could be, q u i t e important
a t a time when we a r e experiencing a region-wide
d e c l i n e i n mule deer p o p u l a t i o n s .
Unfortunately we don't have much information.
Published information l a r g e l y , a g a i n , d e a l s w i t h t h e
f a c t , n o t t h e e f f e c t . There a r e s e v e r a l r e s e a r c h
p r o j e c t s c u r r e n t l y under way, b u t most of them a r e
i n t h e i r e a r l y s t a g e s . The work by my colleagues and
I i s t h e only r e c e n t published d a t a on t h e i n t e r a c t i o n of l i o n s and mule deer and t h e e f f e c t s of t h a t
l i o n p r e d a t i o n . I n 10 y e a r s of s t u d y we found t h a t
l i o n s were n o t l i m i t i n g p o p u l a t i o n s of mule d e e r i n
a w i l d e r n e s s environment i n c e n t r a l Idaho. T h e i r
p r e d a t i o n had l i t t l e e f f e c t on u l t i m a t e numbers of
mule deer during t h i s p e r i o d . The deer p o p u l a t i o n
i n c r e a s e d s t e a d i l y during t h e f i r s t f i v e y e a r s ,
s t a b i l i z e d and remained s t a b l e f o r t h e remainder of
t h e p r o j e c t . A t t h e same time t h e mountain l i o n
population remained s t a b l e because of a f a i r l y r i ~ i d
I n t h i s s i t u a t i o n under
system of t e r r i t o r i a l i t y .
t h e s e e c o l o g i c a l c o n d i t i o n s , l i o n p r e d a t i o n was considered beneficial.
Harley Shaw ( p e r s . comm.) of t h e Arizona Game
and F i s h Department has worked w i t h a l i o n population
f o r s e v e r a l y e a r s i n Arizona. He has a s s e s s e d t h e
l i o n k i l l of domestic s t o c k and has recorded k i l l s of
d e e r . Shaw concludes t h a t l i o n p r e d a t i o n i s an
important f a c t o r i n determining mule d e e r numbers on
h i s study a r e a . He b e l i e v e s t h a t deer numbers could
be i n c r e a s e d on h i s a r e a by removing l i o n s .
These d i f f e r i n g r e s u l t s p o i n t up t h e f a c t t h a t
d i f f e r i n g e c o l o g i c a l u n i t s have t h e i r own p r o p e r t i e s
and each must be looked a t i n d i v i d u a l l y i f v a l i d
i n t e r p r e t a t i o n s a r e t o be made. This i s p a r t i c u l a r l y
important w i t h t h e mountain l i o n , a reasonably
adaptable predator.
We a r e , of course, s p e c i f i c a l l y concerned about
the mule deer d e c l i n e . I t i s v a l i d t o q u e s t i o n what
r o l e l i o n s p l a y i n t h i s d e c l i n e . Lacking o b j e c t i v e
d a t a , i t may prove u s e f u l t o examine p a s t h i s t o r i e s
of i n t e r a c t i n g p o p u l a t i o n s .
I t i s g e n e r a l l y accepted t h a t mule d e e r populat i o n s peaked i n western North America during t h e
period 1940-1970.
During t h i s time, mountain l i o n s
were regarded a s vermin i n a l l western s t a t e s , and
unregulated k i l l i n g was encouraged. We could p o i n t
t o t h e c o n t r o l of l i o n s a s t h e f a c t o r r e s p o n s i b l e f o r
high deer numbers, b u t t h e r e i s n e g a t i v e evidence t o
r e f u t e t h i s . F i r s t , the d e c l i n e began b e f o r e unregul a t e d l i o n k i l l i n g ceased. Deer numbers r o s e
s i m i l a r l y during t h i s period i n a r e a s where t h e r e was
no l i o n c o n t r o l a t a l l - i n s e v e r a l of our l a r g e
n a t i o n a l parks and some of t h e l a r g e r r e l a t i v e l y
i n a c c e s s i b l e wilderness a r e a s . S i m i l a r n e g a t i v e
evidence i s a v a i l a b l e today - t h e d e c l i n e of mule
deer appears uniform throughout t h e West i n a r e a s
where few o r no l i o n s occur a s w e l l a s a r e a s where
they a r e b e l i e v e d t o be numerous. F u r t h e r , whitet a i l deer populations a r e not d e c l i n i n g . On t h e
c o n t r a r y , they appear t o be f l o u r i s h i n g , a t l e a s t i n
t h e Northwest, both where l i o n s occur and where they
do n o t .
The E'ossib
d
Influence of the Mountain Lion
So we r e a l l y d o n ' t know a g r e a t d e a l about t h e
e f f e c t and i n f l u e n c e of mountain l i o n s on mule d e e r
throughout t h e i r range. We do know t h a t once prey
population s i z e i s lowered then any d e p r e s s i n g
f a c t o r may have a p r o p o r t i o n a t e l y g r e a t e r e f f e c t on
t h a t prey p o p u l a t i o n , a c t i n g t o depress i t even
f u r t h e r . I t appears s e l f - e v i d e n t t o many people
t h a t removal of any d e p r e s s i n g i n f l u e n c e w i l l r e s u l t
i n g r e a t e r numbers of a prey s p e c i e s . P r e d a t o r s a r e
the most obvious of t h e d e p r e s s i n g f a c t o r s and
n a t u r a l l y a t t r a c t t h e most a t t e n t i o n . I t is h e r e
t h a t p u b l i c and p o l i t i c a l p r e s s u r e comes f o r c o n t r o l .
We know t h a t p r e d a t o r c o n t r o l i n some c a s e s w i l l
o b t a i n t h e d e s i r e d r e s u l t s . We a l s o know t h a t t h e s e
r e s u l t s o f t e n a r e s h o r t - l i v e d and do n o t s o l v e t h e
long-range problem.
F u r t h e r , t h e c o s t s a r e high,
both i n an economic and e c o l o g i c s e n s e a s pointed
o u t by a u t h o r s from E r r i n g t o n ' s time on. To t h e s e
we can now add c u l t u r a l c o s t s - p u b l i c r e l a t i o n s
problems. At t h e same time, some c o n t r o l programs
w i l l always be i n i t i a t e d on a b a s i s of p o l i t i c a l o r
emotional expediency only, w i t h no b i o l o g i c a l o r
ecological considerations a t a l l .
To r e c t i f y t h i s s i t u a t i o n we need t h e r i g h t
kind of r e s e a r c h . This r e s e a r c h needs t o address
i t s e l f , simultaneously and c o n c u r r e n t l y , t o t h e
p r e d a t o r p o p u l a t i o n , t h e prey p o p u l a t i o n , and an
assessment of t h e prey h a b i t a t . Further, t h e e f f e c t s ,
and n o t merely t h e f a c t , of t h e p r e d a t i o n on t h e
prey p o p u l a t i o n should be t h e foremost o b j e c t i v e .
Too o f t e n i n t h e p a s t , t h i s kind of r e s e a r c h h a s
g o t t e n side-tracked on p r e d a t o r biology only, on
t h e r e v e a l i n g f a c t s o f p r e d a t i o n (gee whiz, t h e s e
critters
s u r e knocking o f f t h e d e e r ! ) , on
i r r e l e v a n t f a c t s of a l l k i n d s . These t h i n g s a r e
important, b u t t h e r e a l i s s u e , t h e e f f e c t s of predat i o n , a r e never r e a l l y l e a r n e d . At t h e same time
we need t o s t u d y populations n o t s u b j e c t e d t o
predation
what happens t o d e e r population l e v e l s
i n t h e absence of p r e d a t i o n ? This o f t e n i s longterm r e s e a r c h and s o f a r f e d e r a l and s t a t e a g e n c i e s
h a v e a ' t been e a g e r t o provide funding. But s h o r t term e f f o r t s won't h e l p .
are
-
I f I may, I ' d l i k e t o quote from some t h i n g s I
s a i d about p r e d a t o r s and p r e d a t o r c o n t r o l i n 1 9 7 2 .
I b e l i e v e t h e s e comments s t i l l a r e v a l i d , and a r e
a p p r o p r i a t e f o r a d i s c u s s i o n of our understanding of
mountain lion-mule deer i n t e r a c t i o n s :
"We need t o know more of t h e biology
and ecology of p r e d a t o r y s p e c i e s . We need
t o know more about s e l f - r e g u l a t i n g
mechanism and how they can be used to
advantage. We need t o s t u d y and compare,
simultaneously, e x p l o i t e d and unexploited
p o p u l a t i o n s . We need t o recognize t h e
g e n e t i c d i f f e r e n c e s between populations
of p r e d a t o r s and prey t h a t have evolved
t o g e t h e r and those t h a t have n o t . We
need o b j e c t i v e assessments of t h e socioeconomics of p r e d a t o r management. We
need r e s e a r c h on a l t e r n a t i v e s t o d i r e c t
k i l l i n g , o r reduction control.
I
b e l i e v e t h e a p p l i c a t i o n of s p e c i e s b i o l o g y ,
a s suggested by Knowlton (1972), i s t h e
key t o f u t u r e p r e d a t o r management. But
f i r s t , we must know s p e c i e s biology."
...
Summing up:
1. We d o n ' t have much o b j e c t i v e d a t a concerning
lion-mule deer r e l a t i o n s h i p s .
2. Lion p r e d a t i o n , l i k e any o t h e r p r e d a t i o n ,
under c e r t a i n c o n d i t i o n s a c t t o l i m i t prey popul a t i o n s . When a prey population is lowered d r a s t i c a l l y by whatever f a c t o r then any d e p r e s s i n g f a c t o r
g a i n s more importance. I n t h i s s i t u a t i o n , p r e d a t o r
c o n t r o l may h e l p , b u t u s u a l l y d o e s n ' t s o l v e t h e
problem.
3. Lion p r e d a t i o n , l i k e any o t h e r p r e d a t i o n ,
normally is i n e f f e c t i v e i n d r a s t i c a l l y reducing
numbers of prey s p e c i e s when t h a t prey s p e c i e s has
suitable habitat.
4. Predator c o n t r o l i s a v a l i d w i l d l i f e management t o o l , b u t i f s u i t a b l e h a b i t a t i s n o t a v a i l a b l e
f o r a prey s p e c i e s , then no amount of p r e d a t o r
c o n t r o l w i l l b r i n g about f l o u r i s h i n g p o p u l a t i o n s of
t h a t prey s p e c i e s .
Mountain l i o n s and mule d e e r evolved t o g e t h e r i n
t h e West. Both have s u r v i v e d , o f t e n f l o u r i s h e d .
T h e i r r e l a t i o n s h i p i s a s i m p l i f i e d s t r a i g h t l i n e one:
H a b i t a t j m u l e d e e r 3 l i o n s . Mule d e e r depend on
h a b i t a t , l i o n s depend on mule d e e r . I f we improve
h a b i t a t , we can i n c r e a s e t h e numbers o f d e e r i n t h e
l o n g run; i f we d e c r e a s e l i o n s o n l y , w i t h no h a b i t a t
change, t h e r e s u l t s won't be t h e same.
LITERATURE CITED
Hornocker, M. G. 1972. P r e d a t o r ecology and management - what now? J. Wildl. Manage. 36:401-404.
Knowlton, F. F. 1972. Preliminary i n t e r p r e t a t i o n s
of coyote p o p u l a t i o n mechanics w i t h some management i m p l i c a t i o n s . J. Wildl. Manage.
36: 369-382.
Leopold, A. 1933. Game management. Charles
S c r i b n e r s Sons, New York. 481pp.
109
The Possible Inftuence of the Mountain Lion