Download Niche Expansion and Contraction in a Variable Environment1

AMER. ZOOL., 26:71-79 (1986)
Niche Expansion and Contraction in a Variable Environment1
H. RONALD PULLIAM
Department of Zoology, University of Georgia,
Athens, Georgia 30602
SYNOPSIS. When food is abundant, sparrows are found in great variety of habitats and
appear to specialize on particularly profitable types of seeds. However, during periods of
food scarcity, each sparrow species occupies a very narrow range of habitats but consumes
a great variety of seeds within each occupied habitat. As a result, average dietary overlap
may underestimate the potential for competition for food within habitats during periods
of food shortage whereas average habitat overlap overestimates the potential for competition since each species occupies a different habitat during periods of food shortage.
occur in approximately the same relative
abundance when food is plentiful and when
it is scarce. Quite to the contrary, Schoener
(1974, 1982) argues that when food is
abundant, certain food types are "superabundant" and these food types are particularly attractive to all consumer species
by virtue of their abundance. According to
Schoener's view, when food becomes scarce
diets diverge because each species is forced
to specialize on the particular prey types
that it consumes most efficiently.
In this paper, I examine various aspects
of niche expansion and contraction for several species of sparrows in a variable environment, the arid grasslands of southeastern Arizona. During the non-breeding
season these sparrows eat seeds and seed
production can vary by orders of magnitude between years. First, I discuss habitat
utilization of sparrows during years of high
and low seed production. I then discuss the
relationship between changes in habitat
utilization and shifts in geographic range.
Finally, I examine the diet of one species,
Chipping Sparrow, in detail and discuss the
potential for seed size partitioning within
habitats during years of seed scarcity.
INTRODUCTION
Many ecologists have argued that high
overlap in resource utilization invariably
leads to competition and competitive
exclusion of inferior species. Resource
levels, however, fluctuate greatly, and many
species may be resource limited only during infrequent periods of resource shortage (Wiens, 1977). If two species overlap
greatly when resources are abundant but
diverge in resource utilization during
infrequent periods when resources are
scarce, any measure of average resource
overlap will greatly overestimate the
potential for competition. On the other
hand, if resource utilization converges
between species during periods of shortage, average overlap may underestimate
the potential for competition.
A minor controversy has arisen concerning whether dietary overlap between closely
related species increases or decreases during periods of food shortage. Most of the
theory and empirical results of optimal foraging studies suggest that animals are most
specialized when food is most abundant
(Pyke et al., 1977; Krebs et al, 1983).
According to this view, when food becomes
scarce animals are less selective and this
results in greater dietary overlap between
species. In other words, food scarcity leads
to dietary convergence because all species
become generalists. An implicit assumption of this argument is that food types
HABITAT UTILIZATION
The abundance of wintering sparrows in
grassland and woodland habitats at The
Research Ranch in southeastern Arizona
is strongly influenced by seed production
(Pulliam and Parker, 1979). The seeds consumed by sparrows are mostly produced in
the fall and the magnitude of fall seed pro1
From the Symposium on Mechanistic Approaches to duction varies enormously depending on
the Study ofNatural Communities presented at the Annual
Meeting of the American Society of Zoologists, 2 7 - summer rainfall. Table 1 shows the fall seed
production of all grasses and those forbs
30 December 1983, at Philadelphia, Pennsylvania.
71
72
H. RONALD PULLIAM
TABLE 1. Seed production and sparrow densities (mean ±sumes roughly 1 kg of seeds each winter
SE) in woodland habitats at The Research Ranch.'
(mid-September through March). In years
of low seed production (< 10 to 20 kg ha"1),
sparrows consume most of the seeds proSparrow diensity
Grasses Forbs'
(ind ha
duced; however, in many years seed production is far greater and sparrows have
1972-73
little impact on overall seed density. Based
Oak woodland
10.0
0.0
7.0 + 1.4
Riparian woodland
88.9
on the relationship between rainfall and
6.1 13.0 + 3.5
seed production, Pulliam and Parker (1979)
1973-74
estimated that sparrows are locally food
Oak woodland
1.2
0.9 + 0.4
tr*
limited at the ranch, in the sense that the
Riparian woodland
2.0
1.6 + 1.1
1.6
wintering sparrow population is large
1974-75
enough
to consume over 75% of the seeds
5.4 + 1.4
Oak woodland
23.6
0.3
produced, roughly one in every five years.
Riparian woodland
12.2
18.8 + 5.5
82.4
In years of low seed production, fewer
1975-76
sparrow
species were found in each habitat
Oak woodland
3.1
tr
2.2 + 0.7
at The Research Ranch than in years of
1981-82
high seed production (Table 2). Two vegtr
Oak woodland
43.8
11.8 + 5.0
etatively
distinct grassland habitats and two
a
Seed production was not measured in the riparian
distinct woodland habitats were censused
woodland in 1975-76 and no censuses were con- each year from mid-September through
ducted in riparian woodland in 1981-82.
b
Includes Amarantkus spp., Mollugo verticillata, and mid-April for four years. A sparrow species
was considered resident in a particular habPotulaca spp.
c
Production < 0.05 kg/ha.
itat only if one or more individuals were
seen in that habitat on more than half of
the censuses conducted. As shown in Table
known to be eaten by sparrows (see Pul- 2 there were four winter resident species
liam, 1980). Based on the calculations of in the riparian woodland during the winter
Pulliam and Brand (1975) and Pulliam and of 1974-75 when seed production was 82
kg ha~' but none in 1973-74 when seed
Parker (1979), a typical 20 g sparrow conSeed production
(kg ha-)
TABLE 2. The resident sparrow species in four habitats and grass seed production (kg ha ')for those years."
Grassland #1
1972-73
Seed production
Resident sparrows
23.0
Ammodramus
Grassland #2
Oak woodland
Riparian woodland
7.2
10.0
88.9
none
Spizella
Spizella
Pipilo
r
1973-73
Seed production
Resident sparrows
1973-74
Seed production
Resident sparrows
0.2
—
1.2
Spizella
2.0
none
none
none
19.1
Ammodramus
Pooecetes
6.9
23.6
82.4
Pooecetes
Spizella
Pipilo
Spizella
Pipilo
Zonotrichia
Pooecetes
1975-76
—
3.1
6.0
Seed production
7.6
Spizella
Resident sparrows
Ammodramus
Ammodramus
none
* The sparrow species included are Grasshopper Sparrow (Ammodramus savannarum), Brown Towhee (Pipilo
fuscus), Vesper Sparrow (Pooecetesgramineus), Chipping Sparrow (Spizella passerina) and White-crowned Sparrow
(Zonotrichia leucophrys).
73
NICHE EXPANSION AND CONTRACTION
production was only 2 kg ha~'. Comparing
all 14 site-year combinations for which both
seed production and sparrow densities were
measured, the rank correlation between
seed production and number of resident
species per habitat is statistically significant
(P < 0.05, Kendall's tau).
Typically, each sparrow species was
found to be resident in a lesser variety of
habitats when seed production was lower.
For example, Chipping Sparrows were resident in two habitats during each of the
two years of high seed production (1972—
73 and 1974-75) but resident in only one
habitat during each of two years of low
seed production (1973-74 and 1975-76).
Similarly, Vesper Sparrows were absent
from the study site in all years except 197475, the year of the greatest seed production when they were resident in three different habitats. Seed production in arid
grasslands is closely correlated with late
summer rainfall (Pulliam and Parker, 1979)
and Figure 1 shows the overall relationship
between the number of habitats occupied
by each species and total summer (JuneOctober) rainfall.
In years of high seed production, relatively more sparrows stay in the northern
portion of their wintering range (Pulliam
and Parker, 1979). My study sites at The
Research Ranch in southern Arizona are
near the northern edge of the wintering
range of a large population of Chipping
Sparrows. The wintering range of the
Western Chipping Sparrow (Spizella passerina arizonae) consists of two disjunct
regions, one on the far west coast, extending from central California to northern
Baja California and the second beginning
in central Arizona and extending south into
the Sierra Madre mountains in Mexico. The
entire wintering population that inhabits
the Sierra Madre Occidental of northwestern Mexico probably migrates through
southern Arizona or southwestern New
Mexico.
Grass seed production in the oak woodland site was only 1 kg ha~' during the fall
of 1973 as compared to 24 kg ha" 1 in the
same site during the fall of 1974. Chipping
Sparrow density in the winter of 1973-74
was only 0.8 individual ha~' as compared
O Chipping Sparrow
I3
A Brown Townee
•
Grasshopper Sparrow
X Vespers Sparrow
200
225
-LXA250
275
Summer precipitation
300
L_
325
FIG. 1. The range of habitats occupied by sparrow
species expands during years of high rainfall and high
seed production.
to 5.1 in 1974-75. During February 1974,
Parker and I censused sparrows along a 15
km route in Nonoava, Chihuahua (27°28'N)
which is in the southern portion of the winter range of 5. passerina arizonae and along
nearly 34 km near Mesa Tres Rios, Sonora
(29°52'N) in the central portion of the wintering range. We relocated and recensused
the same routes in February of 1975.
Because the habitats are quite different in
the different portions of the wintering
range, I do not compare sparrow densities
between sites but only at the same sites,
comparing sparrow populations in years of
high and low seed production.
The migratory habit and winter distribution of Chipping Sparrows respond to
variation in seed production in the northern portion of the wintering range. As
shown in Figure 2, in contrast to the
increase in the density of Chipping Sparrows in the northern portion of the wintering range, when seed production was
high, the relative density of Chipping Sparrows in the southern portion of the wintering range was sharply reduced. I compared the number of 0.5 km segments of
the census routes that had more sparrows
in 1974 to the number that had more in
1975 and found the difference between
years at Nonoava statistically significant.
There was, however, no statistically difference in the relative Chipping Sparrow densities for the two years at Mesa Tres Rios
in the central portion of the wintering
range. The overall pattern indicates a shift
74
H. RONALD PULLIAM
10
>,
<7i
c
8
CD
The Reseach
Ranch, Arizona
|f 6
(31° 2 8 ' ;
I? 4
High seed
production
(1974-75)
search of a suitable habitat when food is
scarce. In years of high seed production,
sparrows occupy a greater variety of habitats and tolerate greater population densities in each habitat in the northern portions of the wintering range. In years of
low seed production individuals are apparently more selective and subsequently travel
farther south in search of an appropriate
habitat.
DIETARY EXPANSION
Sparrows wintering at The Research
125
Ranch consume seeds produced in the late
summer and early fall and the abundance
100
of these seeds declines during the winter
Mesa Tres
(Pulliam and Brand, 1975). As discussed
75
(32)
Rios, Sonora
below, Chipping Sparrows consume a
(29°52')
50
greater variety of seed species towards the
end of winter after they have depleted the
25
supply of preferred seed species. Almost
(35)
0
all of the seeds fall from the plants onto
Feb. 1975
the ground by late fall and the sparrows at
I 300
the ranch forage almost exclusively on the
w
ground, only rarely taking seeds from seed
250
c
heads still on the plants.
Nonoava,
I 200
In mid-January and again in early March
Chihuahua
o
(27° 28')
of 1975,1 caught Chipping Sparrows in an
150
oak woodland at the ranch and gave each
an
oral emetic to induce regurgitation of
100
the crop contents (Pulliam, 1980). On the
(29)
day following each emetic sample, soil sam50
o
(I)
ples were collected for subsequent analysis
Feb. 1975
0
Feb. 1974
of seed densities in the soil. Table 3 lists
FIG. 2. The geographic wintering range of Chipping the common seed species identified in the
Sparrows expands during years of low seed produc- diet samples. The list accounts for approxtion. Seed production in the fall of 1974-75 at The imately 95% of the seeds consumed in both
Research Ranch was more than ten times greater than
it was in the fall of 1973-74. More Chipping Sparrows January and March. For the January diet
over-wintered at the ranch and fewer showed up in sample, the weighted average seed size was
the southern portions of the wintering range when 0.2 mg and the usable energy content was
seed production was high at the ranch.
approximately 0.8 cal per seed. Because an
active Chipping Sparrow in mid-winter uses
upwards of 15 kcal day"1, each individual
southward of the sparrow population when bird requires about 20,000 seeds per day.
seed production is low in the northern end Based on 10 hr of available daylight in midwinter, a Chipping Sparrow must locate
of the wintering distribution.
and
consume one seed every 1.5 to 2.0 secThe southward shift in sparrow populations during years of low seed production onds just to meet its daily energy requireappears also to occur for species other than ments.
Chipping Sparrow (Pulliam and Parker,
Chipping Sparrows may meet their daily
1979). The shift is apparently the result of requirements either by consuming very
individuals having to travel farther in large numbers of small, very abundant
75
NICHE EXPANSION AND CONTRACTION
TABLE 3. Characteristics of seeds eaten by Chipping Sparrows.
Weight (mg)
Husking time (sec)
I. Unarmoured seeds
Amaranlhus sp.
Eragrostis lehmanniana
Mollugo verticillata
Portulaca oleracea
P. retusa
0.05
0.11
0.06
0.12
0.19
0.2
0.6
0.6
0.9
0.6
3.6
19.3
37.6
1.5
28.6
II. Armoured seeds
Aristida divaricata
A. hamulosa
Bouleloua chrondrosiodes
B. curtipedula
B. gracilis
Hackelochloa granularis
Leptochloa dubia
Panicum obtusum
Schkuhria wislezeni
Trichachne californica
0.62
0.54
1.00
0.63
0.39
0.63
0.52
1.00
0.81
0.49
3.2
2.8
1.1
3.9
0.9
2.3
0.1
1.3
-—
1.4
1.3
—
1.2
4.5
4.7
2.3
Density (x ± SE)
±
±
±
±
±
2.5
4.7
15.0
0.4
9.2
± 0.4
± 2.5
± 0.6
± 0.6
± 0.1
± 0.6
0.0
0.9 ± 0.3
1.7 ± 0.7
0.1 ± 0.1
Variance/mean
48.8
32.7
168.0
2.5
83.5
4.0
45.1
11.4
3.8
0.9
7.7
—
3.0
8.6
1.0
* Unarmoured seeds are small and have very short husking times. Armoured seeds are larger, have much
heavier husks and require more time to husk. The seed density data is for the January sample and seed density
is given in seeds per 100 cm8.
seeds or by searching for less abundant, but
more profitable, larger seeds. Table 3
divides the seeds eaten by Chipping Sparrows into two categories. The so-called
"armoured" seeds are all relatively large
(0.4 to 1.0 mg) and all have heavy husks
and often have conspicuous hairs, awns or
other projections from the husks.
Armoured seeds are always individually
husked and each seed requires 1 to 5 seconds husking time. The "unarmoured"
seeds are all very small (0.1 to 0.2 mg) and
most are very abundant. Handling times
for unarmoured seeds are very small (0.2
to 0.9 sec) and some species are frequently
swallowed unhusked (see Pulliam, 1980).
As shown in Table 3, most seed species
have a clumped dispersion pattern (variance to mean ratio greater than 1.0). This
is particularly true of unarmoured seeds
that tend to be found in dense clumps at
the base of the plants that produced them.
Furthermore, different species of unarmoured species tend to co-inhabit disturbed areas with the result that their seeds
are positively associated in the soil samples.
For example, Mollugo verticillata and Portulaca retusa, the two most common unarmoured seeds, were common in the same
soil samples (r = 0.87, n = 28, P < 0.01).
The armoured seeds are more widely dispersed, probably because they are borne
on relatively tall stalks and more prone to
wind dispersal.
Small, unarmoured seeds account for
93% by number and 68% by weight of all
seeds found in Chipping Sparrow diets in
January. As shown in Figure 3, by March
Chipping Sparrows had shifted towards
eating more large, armoured seeds. In the
March sample, the unarmoured seeds still
accounted for 89% of the seeds in the diet
by number but only 39% of the seeds by
weight. The data presented in Figure 3
suggest that by March, some individuals
were specializing on armoured seeds. As
shown in Figure 4, there were strong positive correlations between pairs of
armoured seeds in the sparrow diets. Interestingly, none of these seeds species were
significantly correlated in the soil samples.
These data indicate that, whereas some
individual sparrows were avoiding the
armoured seeds altogether, those that ate
any armoured seeds tended to eat a lot of
several different species.
The shift away from specializing on
clumps of unarmoured seeds and towards
eating more armoured seeds was accompanied by an increase in the variety of seeds
76
H. RONALD PULLIAM
IO
JANUARY
."2 0
0
10
20
30
40
50
60
70
80
90
100
10
JANUARY SAMPLE
MARCH
(n = l6)
0
10
20 30 40 50 60 70 80
Percent unormored seeds in diet
90
100
FIG. 3. Chipping Sparrows in Oak woodland habitat
ate mostly unarmoured seeds; however, after the
unarmoured seeds were depleted, they consumed more
armoured seeds.
consumed. Table 4 lists seven seed species
that were eaten in March that were not
found in the January diet. At least two of
these species, Hackelochloa granularis and
Schkuhria wislezeni were very abundant in
the soil samples on both dates and are
unlikely to have been missed due to sampling error alone (see Pulliam, 1980).
This dietary expansion coincides with a
decline in the abundance of seeds in the
soil. The density of all seeds known to be
eaten by sparrows in the January soil samples was 105.2 seeds 100cm~2. The density
of these same seeds in the March soil samples was only 69.4 seeds 100 cm~2, a decline
of 34%. During this period the density of
unarmoured seeds declined by 40% as
compared to a decline of only 5% in
armoured seeds.
Unarmoured seeds of the more common
species disappeared more rapidly from
areas with dense concentration of these
seeds than from areas were the same seed
species occurred in lesser concentrations.
This pattern of seed disappearance presumably resulted from patch choice by
sparrows. Figure 5 shows the distribution
of the number of seeds per soil sample for
the two most common seed species, Por-
MARCH SAMPLE
FIG. 4. Sparrows that ate a lot of any one species of
armoured seeds tended to eat a lot of other species
as well. The numbers are correlation coefficients
between the abundance of one species of seed and
the abundance of a second species in the sparrow diet
samples. None of these seed species are significantly
correlated in the soil samples.
tulaca retusa and Mollugo verticillata. In both
cases, the soil samples with the most seeds
of the given species contained a far smaller
proportion of the total seeds of that type
in March than in January. Since the seeds
of the unarmoured seed species tended to
be positively associated in the soil samples,
I also looked at the frequency distribution
of unarmoured seeds of all types combined
in the soil samples. In January, 6 of 28 soil
samples contained more than 150 unarmoured seeds each and 60% of the unarmoured seeds occurred in samples with
more than 150 unarmoured seeds each. In
March only 1 of 28 samples contained more
than 150 unarmoured seeds and only 19%
of the unarmoured seeds occurred in samples with more than 150 seeds each.
NICHE EXPANSION AND CONTRACTION
77
TABLE 4. Seed species eaten by Chipping Sparrows in
March but not found in the January diet samples.
Bouteloua gracilis
Croton sp.
cf. Cyperus sp.
Hackelochloa granularis
Paspalum sp.
Schkuhria wislezeni
Talinum sp.
These data indicate that Chipping Sparrows consumed a greater variety of seed
species as the density of seed declined. One
way of assessing the magnitude of such a
dietary expansion is to compare the energetic value of the seeds being consumed to
the rate of food intake required to maintain a positive energy balance. Seed value
is measured as the usable energy content
(cal) or mass (mg) of the seed divided by
the time (sec) required to husk the seed
(handling time). An "energetically profitable" seed is one whose seed value (cal sec"1
or mg sec"1) exceeds the rate of food intake
required to meet the animals' 24 hr energy
requirements. If all profitable seeds
encountered are consumed, the diet is said
to be "fully expanded."
Pulliam (1980, 1985) calculates that during a typical 10 hr mid-winter day, a Chipping Sparrow requires approximately 0.15
mg sec"1 seed intake while foraging in
order to meet its 24 hr energy requirements. In the March dietary sample, Chipping Sparrows were consuming 14 of the
15 seed species with known seed value
exceeding 0.15 mg sec"1 and none of the
4 seed species with seed value of less than
0.15 mg sec"1. This suggests that by the
end of the season the diets of Chipping
Sparrows were fully expanded in the sense
of eating all energetically profitable seeds.
The dietary expansion observed for
Chipping Sparrows indicates that sparrow
diets are flexible and respond in a predictable fashion to variation in food supply.
In a year of moderately low seed production (~ 25 kg• ha"•), Chipping Sparrow diets
were fully expanded by the end of the season. Though there is no data on Chipping
Sparrow diets in a year of very low seed
production, it is reasonable to assume that
I
5
10
15
20
Sample number (rank ordered)
25
FIG. 5. The distribution of the number of seeds per
sample of the two most common unarmoured seed
species show that they tended to occur in dense clumps.
Notice that the tails of the two curves are very similar,
but that fewer soil samples in March contain large
numbers of seeds. This pattern suggests that sparrows
concentrate their foraging effort on the larger clumps
of seeds.
dietary expansion would occur even more
rapidly and the sparrows would consume
all energetically profitable seeds encountered for most of the winter months.
DISCUSSION
According to optimal foraging theory,
animals should specialize on the most profitable prey types when these are abundant
but accept less profitable prey when more
profitable ones are scarce. If, during
periods of scarcity, all food types are
roughly equally reduced in abundance,
each consumer species should eat a greater
variety of prey, resulting in greater dietary
78
H. RONALD PULLIAM
overlap between consumer species. However, as pointed out by Schoener (1974,
1982), if during periods of abundant food,
some prey types become "superabundant"
relative to others, several consumer species
may all specialize on the same superabundant prey, leading to potentially greater
overlap during periods of plenty.
The dietary preferences of sparrows in
nature appear to be influenced by both the
energetic values of seeds and by seed abundance and spatial distribution. In laboratory preference experiments, Chipping
Sparrows prefer seeds weighing about 0.5
mg to seeds of all other sizes (Pulliam,
unpublished data). However, seeds of this
size are not particularly well represented
in Chipping Sparrow diets in nature,
whereas much smaller seeds (0.1 to 0.2 mg)
are over-represented in natural diets.
Though these small unarmoured seeds
have relatively low energetic value individually, Chipping Sparrows can apparently
achieve high rates of intake by concentrating on dense patches of them.
The abundance of small, unarmoured
seeds varies widely from year to year. Table
1 gives an indication of the magnitude of
this variation, since most of the small, unarmoured seeds are forbs and most of the
larger, armoured seeds are grasses. Though
forb seeds are usually most abundant in
years of high seed production, this is not
always the case. Most of the common forbs
flower earlier than do the most common
grasses. If the summer rains start early,
forb seed production is very high but late
summer rains can result in low forb seed
production and relatively high grass seed
production. Very early rains followed by a
relatively dry late summer and early fall
can result in superabundant forb seed production and relatively low grass seed production.
Dietary overlap between Chipping Sparrows and other sparrow species is likely to
be particularly low during years of superabundant forb seed production. This is
because most forb seeds are too small to
be profitably consumed by larger sparrow
species. For example, Pulliam (1985) calculates that under typical mid-winter conditions, White-crowned Sparrows (23 g)
cannot profitably consume seeds smaller
than 0.25 mg. Chipping Sparrows (13 g)
on the other hand can profitably consume
seeds as small as 0.1 mg and, as already
discussed, appear to specialize on these
small seeds when they are abundant.
During years of abundant seed production two or more sparrow species occupy
oak woodland habitat at The Research
Ranch, but as shown in Table 2, during
years of low seed production, only Chipping Sparrows are resident in oak woodlands. Chipping Sparrows are particularly
well adapted to oak woodlands because they
can consume virtually the entire range of
seed sizes produced there. Chipping Sparrows can profitably consume most seeds in
the size range of 0.1 to 1.0 mg (Pulliam,
1985) and only about 5% of the grass and
forb seeds produced in oak woodland weigh
more than 1.0 mg. Thus, Chipping Sparrows can profitably consume most of the
seeds available to larger sparrow species,
in addition to being able to consume seeds
too small to be profitably eaten by the larger
sparrow species.
I postulate that during periods of food
scarcity, each sparrow species specializes
on a few habitats where it can outcompete
all other sparrow species. The suitability
of a particular habitat depends, in part, on
the sizes of seeds produced there, as discussed for Chipping Sparrows in oak woodland. Habitats, no doubt, differ in other
ways that make them more or less suitable
for a particular sparrow species. Chipping
Sparrows, for example, require habitats
where foraging sites are in close proximity
to trees and shrubs to which they retreat
when disturbed by predators. Grasshopper
Sparrows, on the other hand, are most
common in open grasslands with widely
scattered trees and shrubs. Pulliam and
Mills (1977) argue that differences in crypticity and social behavior adapt different
sparrow species to habitats with very different vegetative characteristics.
If each sparrow species is best adapted
to a few habitats where it can outcompete
all other sparrow species, why do they
occupy other habitats during periods of
food abundance and only specialize on
favored habitats during periods of food
NICHE EXPANSION AND CONTRACTION
shortage. As previously discussed, The
Research Ranch is near the northern end
of the wintering distribution of several
sparrow species. When seed production is
high, more individuals can be supported in
prime habitats in the northern portion of
the wintering distribution. If seeds are sufficiently abundant, feeding rates will be
more than adequate to meet energetic and
nutritional requirements of sparrows in a
variety of otherwise suboptimal habitats.
In this case the benefit to be derived from
finding a slightly better wintering habitat
farther south might not compensate for
the cost of migration. The result would be
a greater concentration of individuals in a
greater variety of habitats in the northern
end of the wintering distribution. During
periods of low seed production, Chipping
Sparrows consume virtually all of the seeds
produced in occupied habitats at The
Research Ranch. Greater densities of overwintering sparrows in the northern end of
the wintering distribution would undoubtedly result in over-exploitation of the available food and high mortality of sparrows.
In this case the cost of migration is bound
to be more than compensated for by an
increase in survival probability. As pointed
out by Pulliam and Parker (1979), rainfall,
and presumably seed production, is more
reliable and the density of potential competitors is lower in the southern portion of
the wintering range of Chipping Sparrows.
Sparrow population in arid grasslands
appear to be food limited only during
occasional years of unusually low seed production (Pulliam and Parker, 1979). As discussed in this paper, each sparrow species
appears to occupy a narrow range of habitats and to consume a wide variety of seeds
within each habitat during periods of food
scarcity. Since sparrows appear to specialize on particularly profitable seeds when
food is abundant, an estimate of average
dietary overlap would underestimate the
potential for competition for food within
habitats. On the other hand, an estimate
of average habitat overlap would overesti-
79
mate the potential for competition, since
habitat overlap appears to be lowest when
food is scarce.
For many species of animals, resource
utilization is highly flexible and responds
adaptively to changes in resource abundance and distribution. For all such species,
average niche overlap may be a poor index
of competitive interactions. Resource utilization must be studied under a variety of
conditions with special attention paid to
the utilization of limiting resources during
periods of resource scarcity.
ACKNOWLEDGMENTS
Portions of the research reported here
were supported by DEB-8206936 and previous grants to the author from the
National Science Foundation. I thank Craig
Benkman, Mary Price, Tom Schoener,
Gary Grossman, John Glasser and Graham
Pyke for useful comments on an earlier
version of this paper. Finally, I express my
appreciation to the sparrows and woodlands at The Research Ranch.
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