what it is in June; different when the animal is 
hungry from what it is when satiated. The 
niche is also mutable, subject to evolution, as 
when the insect population evolves resistence 
to the hydrocarbon or the blackbird habituates 
to the scarecrow. 
Biotic Requirements 
Usually the more enlightened ecological 
approach is to deal with the positive features 
in the environment, the animals' require- 
ments--to put an embargo on vulnerable req- 
uisites in short supply. 
One of the pressing conflicts between verte- 
brate numbers and man's economic, aesthetic, 
and other vital interests is the recent popula- 
tion explosion of herring gulls (Larus ar- 
gentatus) in the Northeast. Gulls plunder crops, 
pollute domestic water supplies, rob feeds, 
collide with airplanes, compete with more 
desirable species, and otherwise offend our 
tidy lives. At the turn of the century the 
herring gull was rare as a breeding birdin the 
United States. Today they are counted by the 
tens of thousands and have extended their 
breeding range south to the Carolinas. The 
growth of this population coincides with the 
growth of open-faced metropolitan dumps, 
sewerage outlets, fish piers, and other sources 
of what we might call "gurry"' along the sea- 
board. Whether or not the correlation is causal, 
no one can deny that herring gulls today derive 
a large percentage of their food from these 
festering sources of nourishment. Biotic con- 
trol of this scavenger could be achieved, with- 
out doubt, simply by doing away with the gurry-- 
but doing away with it is not simple, for this 
involves training that most intractable of all 
animals, man. 
Any of the countless species that we now 
esteem, whose habitats have been ruined by 
man, could have become pests if their biotic 
requirements had been more fully met rather 
than less fully met. The overabundant elk 
(Cervus canadensis) of Yellowstone National 
Park is a case in point. 
Environmental control at the site of the 
problem is usually the most satisfactory an- 
swer to vertebrate pest problems, Continental 
population reduction is seldom if ever the 
reasonable solution. The next best thing to 
removal of local food supply is the removal 

of the vegetative cover immediately around the 
problem site. Proper grazing practices tend 
to eliminate mice, ground squirrels, and 
rabbits in agricultural areas where they con- 
stitute a pest. 
The most extraordinary example in my ex- 
perience of lack of biotic control was seen on 
our inspection of the Logan International Air- 
port in Boston, Mass., after the tragic collision 
of an aircraft with a flock of birds in 1960. 
Logan Airport was virtually a wildlife sanc- 
tuary with many small blocks of varied habitat, 
including ponds, lush borders, phragmites 
roosts, brush copses, tidal flats, and of course 
a garbage dump. The Boston Port Authority is 
doing much to change all this. 
There are instances where vertebrate pests 
can not be controlled by removing the biotic 
requirements, for some species have been put 
into direct competition with our own, but always 
new, biotic requirements. Where our own re- 
quirements can not be protected by positive 
physical or cultural means, the population of 
pests must be reduced by negative means, 
Biotic Intolerances 
Natural predation helps keep populations in 
check, but there is a necessary population 
threshold (seldom low enough to suit our ob- 
jectives), below which the predator suffers 
more than the prey. In very limited situations 
animal predators may be released with some 
effectiveness, as inthe following two examples, 
but their use can hardly be generally rec- 
ommended. At several Canadian and European 
airfields trained falcons have recently been 
employed to reduce the numbers of birds that 
present a potential hazard to aircraft opera- 
tion. Raccoons (Procyon lotor) were released 
on islands off Massachusetts in 1964 by Dr. 
John A. Kadlec to discourage nesting of gulls. 
Warm-water fishpond management, where 
the operator has the advantage of working with 
a closed system, is largely founded on control 
by predation (Swingle and Smith, 1941). Both 
forage and carnivorous species of fish mustbe 
stocked together in the correct proportions to 
assure both types reaching legal catching size. 
Too many forage fish result in their stunted 
growth. With few forage fish carnivorous fish 
extirpate them and then begin eating their own 
offspring. 
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