AMER. ZOOL..19:331-343 (1979).
Temperature as an Ecological Resource
JOHN J. MAGNUSON, LARRY B. CROWDER, AND PATRICIA A. MEDVICK
Laboratory of Limnology and Department of Zoology, University of Wisconsin-Madison,
Madison, Wisconsin 53706
SYNOPSIS. Ectothermic vertebrates respond to the temperature of their habitat in a manner
that is remarkably similar to their response to more traditional ecological resources such as
food. We review the response to temperature primarily from literature on fishes in terms
of ecological concepts related to niche theory and competition. The width of the fundamental thermal niche is about 4°C when measured by a mean plus and minus one standard
deviation of the distribution of temperature occupied in a laboratory gradient. Fish of
temperate freshwater appear to fall into three thermal guilds along the temperature resource axis —cold, cool, and warm water fishes. Realized thermal niches are similar in
central tendency to fundamental niches, but niche width appears to be more narrow for
the realized niche in limited sample data. The success of interference competition for space
with preferred temperature is tied to social dominance in a manner analogous to food
competition. Thermal niche shifts in the face of interspecific competition for preferred
temperature appear supported by one laboratory study. Exploitation competition in respect to temperature seems nebulous. If animals successfully compete for their thermal
niche, growth and perhaps other measures of fitness are maximized. Cost/benefit models
for thermal resources and food resources lead to similar predictions about resource use.
We suggest that viewing temperature and other niche axes in the way ecologists have
viewed food resources would be useful.
THERMAL NICHE
INTRODUCTION
Our purpose is to view temperature by
analogy as an ecological resource instead
of as a factor influencing the behavior and
physiology of the animal per se. Temperature is a characteristic of an animal's
habitat, an axis of its multidimensional
niche. We will argue that animals compete
for and partition thermal resources, that
the success of this process contributes directly to fitness of the animal, and that the
thermal niche of an organism can be
treated as quantitatively as can a consumable resource such as food.
The ideas presented developed during research
projects supported by Madison Gas and Electric
(Proposal 12813), University of Wisconsin Sea Grant
Program (Project 144-L947), and N.S.F. (Grant OCE
77-08531) to J.J.M. Dr. Medvick's current address is:
Department of Biology, Montana State University,
Bozeman, Montana 59717. Publication costs for this
paper were supported by N.S.F. Grant PCM 7805691 to William W. Reynolds, the organizer of the
Symposium.
Measuring the thermal niche
Considerable work in physiological, behavioral, and applied ecology has provided
a number of options to us in measuring the
thermal niche of fishes, reptiles, and amphibians. The thermal niche might be
defined by lethal limits, physiological or
metabolic optima, by behavioral performance optima, or by behavioral preferences.
We choose to represent the thermal niche
by behavioral preferences as it makes it
easier to conceptualize more active competition for the resource, and because the
physiological and performance measures
as functions of temperature, seem more
analogous to measures of fitness that result
from successful competition. Lethal temperatures set an ultimate bound on a thermal niche but are so extreme that they
usually have little to do with the fine tuning
of an animal's resource utilization.
Measurements of niche breadth are then
obtained from the behavioral thermoregulation of the animal and can be char331
332
MAGNUSON ET AL.
acterized as behavioral or statistical end
points from laboratory determined temperature preferences. These measures
should, of course, be made without the
complicating aspects of inter- or intraspecific competition or variation in other
physical resources of the habitat. Possible
measures include the range of temperature occupied in a thermal gradient, some
statistically determined portion of that
range, or the temperatures bracketed by
the upper and lower avoidance temperatures. These measures were reviewed by
Reynolds and Casterlin (1976) for centrarchid fishes, by Magnuson and Beitinger
(1979) for centrarchid fishes and desert
reptiles, and by several authors in this sym-
|
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TEMPERATURE CO
FIG. 1. Comparison of measures of niche breadth
for temperature using laboratory data on behavioral
thermoregulation for green sunfish (Beitinger et al.,
19756) and the desert iguana (DeWitt, 1967; Vaughn
et al., 1974). Histograms are the percent of time animals spend at each temperature in laboratory temperature gradients.
posium. They are diagrammed in Figure 1
for a green sunfish (Lepomis cyanellus Rafinesque) and a desert iguana (Dipsosaurus
dorsalis Baird and Girardi).
We propose that the median plus or
minus 33 percent of the laboratory determined thermal distribution will be the
most useful measure of the thermal niche.
The use of the range is probably a poor
choice because it is too dependent on sample size and is greatly influenced by rare
excursions into extreme temperatures. A
second approach would be to use the preferred range of temperatures as defined by
Neill and Magnuson (1974) and Beitinger
etal. (19756) as the temperatures bracketed
by the upper and lower turnaround temperatures. This has the advantage of using
the animal to set the limits more directly
and it also includes about 70 percent of the
data in a temperature distribution histogram which is close to the 66 percent included in plus and minus one standard deviation. A difficulty with this measure is
that it is easier to obtain from certain types
of thermal gradients — namely the temporal gradients used by Neill et al. (1972)
and Reynolds and Casterlin (1976) or devices similar in principle in the terrestrial
reptiles literature (see Magnuson and
Beitinger, 1979). Most laboratory data
collected in vertical or horizontal spatial
gradients of temperature are easier to
analyze with statistical end points rather
than behavioral end points because the
latter require continuous monitoring of
the fishes temperature. In the temporal
gradients for fishes, water temperatures
are monitored continuously as the fish activates, by its behavior, alternate periods of
slow heating and cooling (Neill et al., 1972).
A niche breadth of plus and minus one
standard deviation from the mean has
been used for other resources (May and
MacArthur, 1972; Werner, 1977) and
would have the advantage of being consistent with those measures. However, as reviewed by DeWitt and Friedman in this
symposium, the thermal distribution of
animals in laboratory gradients are often
skewed. Thus, a more appropriate statistical approach would be to use the median
and quartiles or to be consistent with
TEMPERATURE AS AN ECOLOGICAL RESOURCE
333
earlier niche breadth measurements the avoidance temperatures of Beitinger et al.
median plus or minus the percentage en- (1975b) (-70% of observations) and plus
closed in one standard deviation on a nor- and minus one standard deviation of the
mal distribution, i.e., ± 33% of the points. temperature distribution (—66% of obserSince we use a variety of data sources in vations) yielded a mean of 3.76°C (95% CI
this paper, we will be using several diffei
[3.29 - 4.23], n = 18). Plus and minus one
ent measures that include 66 to 70 percent standard deviation for 15 species (summer
of the distribution.
preference) from Reutter and Herdendorf
The breadth of the thermal niche is (1974) averaged 4.33°C (95% CI [2.93 similar for fishes that prefer different 5.73], n = 17). Reutter and Herdendorf"s
temperatures (Fig. 2). When measured by (1974) data are much more variable, but
the mean plus and minus one standard de- nonetheless, these mean niche breadths
viation, it is 3.8°C for the green sunfish, are not significantly different. Taken to3.6°C for yellow perch (Perca flavescens gether, available data on the thermal niche
(Mitchill)) and including the extreme breadths of 24 species average'd 4.04°C
points at 27 and 28°C, 5.4°C for rainbow (95% CI [3.35 - 4.72], n = 35). Thus,
trout (Salmo gairdneri Richardson). The to- fishes with markedly different thermal
tal range occupied is again similar—11°C niches in terms of central tendency have
for green sunfish, 10°C for yellow perch, somewhat similar niche breadths.
and excluding the extreme points, 11°C for
Quantitative measures of the thermal
rainbow trout. An analysis of this niche niche and its breadth for species are prebreadth for nine species from Coutant requisites for ecological analysis of inter(1977), based on upper and lower and intraspecific species interactions in respect to the temperature resources. Such
measures also make it possible to consider
questions such as partitioning of thermal
resources, thermal guilds, and intra- and
interspecific competition in respect to
30
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TEMPERATURE (°C)
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FIG. 2. Niche breadths for temperature for a warm,
cool, and cold water fish determined in laboratory
temperature gradients. Histograms are the percent1 is
age of time spent at each temperature. | •
the mean plus a n d minus o n e standard deviation.
Data sources a r e : green sunfish in a temporal g r a dient (Beitinger et al., 19756), yellow perch in vertical
gradient (McCauley a n d Read, 1973), a n d rainbow
t r o u t in a vertical g r a d i e n t (McCauley a n d P o n d ,
1971).
Freshwater fishery scientists have historically referred to fishes as warm water
fishes—the sunfish and the like; cool water
fishes—yellow perch and the like; and cold
water fishes — rainbow trout and the like.
More recently, Hokanson (1977) began to
more formally classify fishes by their thermal requirements. He called the three
groups temperate eurytherms, temperate
mesotherms, and temperate stenotherms.
The adjective, temperate, was to acknowledge that each group in temperate climates
is exposed to water below 4 C during the
winter months, regardless of the summer
temperature needs. His classification was
based on fitness characteristics such as
temperature required for gonadal growth,
for spawning to occur, for physiological
optima, and on the basis of lethal limits.
While he grouped the fish into three
categories, he acknowledged that the clus-
MAGNUSON ET AL.
334
ters could be real or only arbitrary divisions along a continuum.
To examine the classification we have
plotted the distribution of fishes along the
thermal resource gradient separately for
laboratory and field studies (Fig. 3). Only
summer estimates from freshwater species
of North America are included. If more
than one estimate is available for the same
species, they are averaged for adults or for
young, but young and adults if identifiable
are plotted as two points. Each is plotted as
a single point without any indication of
either the niche breadth or the range of
temperatures occupied in the field. There
are many possible sources of bias in the
presentation such as differences in methods, acclimation histories, and criteria
The evidence is clear that species are not
distributed uniformly along the temperature gradient based on laboratory estimates of temperature preference. Cold
10
15 20
25
30
TEMPERATURE CO
35
40
FIG. 3 Frequency distributions of temperature
preference (laboratory) and temperature occupation
(field) by freshwater fishes found in North America at
temperate latitudes. An average temperature for each
species was obtained and plotted separately for young
and adults. Winter, spring, and fall data were deleted
when identified in Coutant (1977). Numbers in histogram represent families of fishes: 1 = Centrarchidae; 2 = Percidae; 3 = Esocidae; 4 = Ictaluridae; 5
= Salmonidae; 6 = Cypnnidae; 7 = Percichthyidae; 8
= Cottidae; 9 = Catostomidae; and 10 = other
families. Brackets diagram thermal niches of a representative fish in the cold, cool, and warm water
guilds as 4°C and 10°C niche breadths. Data sources
are mostly from Coutant (1977) but also from Otioet
al. (1976), McCauley et al. (1977), Brandt (1978),
Reynolds and Casterlin (1978A,<r), Reynolds et al.
(\978d,t), Richards and Ibara (1978).
water species are clearly separated from
cool and warm water species. Considerable
overlap occurs between cool and warm
water species but the skewing of the distribution of the species between 20 and
35°C suggests an interpretation of a cool
water and a warm water group with some
overlap near 25 and 26°C. We will refer to
the three groups as thermal guilds. Guild is
used in the sense of Root (1967) and is interpreted here with respect to temperature
to mean fish which use the same temperature resources in similar ways.
Fishes in several common families, indicated in Figure 3 by numbers are, with
rare exception, in single guilds. On the
basis of temperature preference, centrarchids (1), ictalurids (4), and percichthyids
(7) are in the warm water guild, percids (2)
and esocids (3) are in the cool water guild,
and salmonids (5) are in the cold water
guild. In contrast cyprinids (6) appear to
be more diverse with some species apparently in each of the three guilds. The temperatures that are centered in each guild,
11.0-14.9°C, 21.0-24.9°C, and 27.0-30.9°C,
are not uncommon in lakes and streams of
the U.S.A. and Canada during summer. In
Lake Mendota, Wisconsin, for example,
hypolimnetic temperatures in July and
August are in the 11 to 13°C range, the
epilimnion in the 21 to 24°C range. Littoral
zone and pond temperatures are often as
warm as 25 to 28°C. The warm, cool, and
cold water fishes would then be expected
to partition the thermal resources of
stratified lakes such as Lake Mendota.
Since fish communities in a single lake
often include members from each of the
three guilds, and since a wide range of
temperatures can occur in a single lake,
partitioning of thermal resources would be
expected to reduce interactions among
species (MacLean and Magnuson, 1977).
Ontogenetic changes in the temperature
preferred by some fishes leads to a partitioning of thermal resources between size
or age classes of a single species. The best
example is perhaps that of the alewife
(Alesa psudoharengw (Wilson)) in the Laurentian Great Lakes (Brandt, 1978). In vertical laboratory gradients young-of-theyear alewife preferred 25°C while adults
335
TEMPERATURE AS AN ECOLOGICAL RESOURCE
preferred 16°C (Otto etal., 1976). A difference this large, 9°C, results in little or no
overlap in the thermal niche of young and
adults and would be expected to almost
completely separate the two groups in a
thermally stratified lake. In Lake Michigan
the adult alewife also occupied cooler temperatures than the young and were spatially segregated from their young during
summer (Brandt, 1978).
Field temperature distributions would
not necessarily be expected to correspond
too closely to laboratory determined thermal niches. As with a resource such as
food, the preferred resources may not be
in the environment, other resource needs
may alter their use of the thermal resources, or perhaps species interactions
exclude a species from thermal resources.
Regardless (Fig. 3) there are modes in the
field distribution in the temperature range
of the cold, the cool, and the warm water
fishes. Other modes appear as well. Catostomids are broadly distributed in temperature, but little data are available from the
laboratory. While some families tend to occur in specific thermal zones, many seem to
be somewhat more widely distributed than
observed from the laboratory preferences.
COMPETITION FOR THERMAL RESOURCES
Fundamental and realized niches
Fundamental niches (Hutchinson, 1957)
of coexisting species along a thermal gradient may be inferred from laboratory
preference data. Overlap of fundamental
niches on a single axis is related to the po-
tential for competition in that region of
niche space. Of course, viewing one dimension of niche space alone may yield a
misleading picture of the ecological interaction. Often, two or more niche axes
must be considered (Schoener, 1974). We
have examined the overlap along temperature and food niche axes of three centrarchids, the bluegill (Lepomis macrochirus
Rafinesque), green sunfish, and largemouth bass (Micropterus salmoides Lacepede) at a body mass of 4 g (Fig. 4). Large
overlaps of 30 to 85 percent occur between
species along single axes of the fundamental niche (Table 1). Green sunfish is intermediate to largemouth bass and bluegill on
the food axis but on the temperature axis,
largemouth bass is intermediate. On both
axes, singly and together, the overlaps between largemouth bass and green sunfish
are greatest. Werner (1977) argued, based
on the theory of limiting similarity (May
and MacArthur, 1972; but see Turelli,
1978), that the three species could not
stably coexist on the food size axis. Analogous arguments could be made that the
fishes are also too similar to coexist this
tightly packed along temperature axis.
Consideration of both temperature and
food size axes indicates that the largemouth bass X green sunfish interaction is
potentially the most important in determining the realized distribution of these
fishes in niche space.
Field ecologists rarely deal with the fundamental (noninteractive) niche of a
species, but with the realized (interactive)
niche. Midpoints of field temperatures occupied by fishes are plotted against their
TABLE 1. Overlap in the fundamental niche of three centrarchids in respect to temperature and log food size, measured
as a percentage of each axis occupied by both species. *
Niche axes
percent overlap
Species interactions
Bluegill x Largemouth bass
Bluegill x Green sunfish
Largemouth bass x Green sunfish
* See Figure 4 for data sources.
Temperature
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Temperature x
Log food size log food size
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336
MAGNUSON ET AL.
Interference and exploitation
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FIG. 4. Food and thermal niche of three centrarchids at a mass of 4 g. The food niche is the mean log
prey weight ± 1 S.D. as interpolated from Werner
(1977). The thermal niche is median ± the upper and
lower turnaround temperatures for green sunfish
(Beitinger el al., 1975*) and largemouth bass (Neill
and Magnuson, 1974) and the mean at a 14:10
photoperiod for bluegill (Magnuson and Beitinger,
1979).
laboratory preference in Figure 5. Twenty-five percent of field temperatures fall
within a 4°C temperature range or plus
and minus 2°C around the laboratory
preference. Sixty-eight percent of the field
temperatures fall within a 10°C range.
Only thirty-two percent of the field data
exceed temperatures the fish would not
choose in the laboratory. Realized thermal
niches seem to be more dispersed than
fundamental niches and there may be a
slight bias toward occupying lower temperatures in the field than preferred in the
laboratory.
In addition, realized (field) thermal
niches of coexisting species in Lake Monona, Wisconsin, tend to be narrower
than fundamental (laboratory) niches (Fig.
6). We had expected a higher variance in
temperature occupied in the field than in
the laboratory, but it appears that thermal
niches are more constricted in the field.
This may be a result of species interactions
(niche compression is not unusual for other
niche axes), but this hypothesis clearly requires a more detailed examination.
Ecologists usually do not consider competition for thermal resources as it is
difficult to envision the consumptive use of
temperature. If temperature is considered
at all, it is likely to be as a property of some
space or time for which competition is
more easily envisioned. However, just as all
physical and chemical variables are spatially and temporally distributed, all
ecologically interesting space or time has
physical and chemical properties. The
same relation holds for food and space;
food has an obvious spatial distribution
and certain habitats may be characterized
by their food distribution. This has led to
some confusion in food-space niche studies
in which it is often difficult to resolve competition for space (e.g., habitat partitioning) and competition for food (e.g., food
size partitioning) related to spatial habitat
patches.
When we consider the analogy between
the partitioning of thermal resources
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TEMPERATURE PREFERENCE
(LABORATORY)
FIG. 5. Relation between central tendencies of temperature occupied in the field (realized niche) and
those preferred in the laboratory (fundamental
niche) for freshwater fishes of temperate North
America in summer. The 45° line is bounded by ±2°C
and ± 5CC boundaries of the thermal niche. Each
point represents a species (adult and young separately). Numbers and data sources are identified in
the legend for Figure 3.
337
TEMPERATURE AS AN ECOLOGICAL RESOURCE
(temperature-space) and food resource
partitioning in terms of competition, we
find some interesting parallels. Interference competition for thermal resources
within a species appears to be analogous to
that of food resources. Socially dominant
individuals often exploit the preferred
range of the resource and exclude subordinate conspecifics. For example, in the
laboratory small bluegills shift out of their
fundamental thermal niche when a larger,
dominant individual is present on the side
of an aquarium with preferred temperatures (Fig. 7) (Beitinger and Magnuson,
1975a). Several authors, concerned with
measuring temperature preferences, have
noted that social interactions {e.g., aggression) can bias their estimates (Pearson,
1952; Barans and Tubb, 1973; Beitinger
and Magnuson, 1975a). Peek (1965)
showed that the largest smallmouth bass
(Micropterus dolomieui Lacepede) in a
spatial-temperature gradient occupied the
preferred temperature. Smaller individuals occupied progressively more marginal
temperatures. Medvick, Magnuson and
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FIG. 7. Difference in the temperatures occupied by
a small bluegill with and without a larger bluegill on
the side of the aquarium with the preferred temperature of 31°C. The other side of the aquarium was 4°C
below or about 31°C. Data are from Beitinger and
Magnuson (1975a) for fish in aquaria with two sections.
Sharr (unpublished data) found that
dominant bluegill almost always established their territory on the side of an
aquarium that had the preferred temperature. The relation of size to behavioral
dominance and use of the thermal re4.0
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TEMPERATURE PREFERENCE (°C)
per aquarium. But when population den(Fundamental Niche Breadth)
sity was high dominant fish were unable to
FIG. 6. Width of realized thermal niche and funda- successfully defend their preferred temmental thermal niche for young warm water fishes
from the littoral zone of Lake Monona, Wisconsin, perature territory. Defense of preferred
during daylight in August. The width of the realized temperature space further deteriorated
niche is ± 1 S.D. of the mean body temperature calcu- when the temperature gradient in the
lated from Figure 12 in Xeill and Magnuson (1974). aquarium was large, i.e., thermal stakes
The fundamental niches are the differences between
were high for subordinates. The response
the upper and lower turnaround temperatures averof
territorial animals to decreasing per
aged forday and night in Neill and Magnuson (1974).
338
MAGNUSON ET AL.
capita food and increasing population density is similar (Magnuson, 1962; Simon,
1975).
Temperature like food influences the
intensity of aggression. In juvenile golden
medaka (Oryzias latipes (Temminck and
Schlegel)) aggression was most intense
when food was limited and least intense
when food was present in excess (Magnuson, 1962). In the yellow bullhead (Ictalurus natalis (Lesueur)) aggression was least
intense at temperatures near preferred
and more intense at cooler and warmer
temperatures. The preferred temperature
of yellow bullheads is abut 28.4°C
(Reynolds and Casterlin, 1978c) and aggression is least between 28 and 30°C
(McLarney etal., 1974). One interpretation
of these observations is that when optimum temperatures are obtained, interference competition is reduced. The analogy to food is that when excess food is obtained, interference competition is reduced.
Interspecific interactions along a temperature gradient also mimic interactions
on other niche axes. In the laboratory, the
Comanche Springs pupfish (Cyprinodon elegans (Baird and Girard)) shifts to cooler
water in a horizontal temperature gradient
when in the presence of the Pecos gambusia {Gambusia nobilis (Baird and Girard))
(Fig. 8). This evidence for thermal segregation in the presence of another species is
striking and may be analogous to a niche
shift as observed by Werner and Hall
(1976) for centrarchids competing for
food resources. The modes (Fig. 8) of the
temperature distribution for each species
also are higher with both species present.
This observation suggests an interactive
compression of their thermal niches, but
the data are too preliminary to confirm
this.
Species may also partition thermal resources temporally. Reynolds and Casterlin (1978a) have shown, based on laboratory data, that largemouth and small-
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and Comanche Springs pupfish in a horizontal gradient when tested alone and the temperatures oc-
cupied by each in the same gradient when tested together. Data are from the afternoon testing by Gelbach t-t al. (1978).
TEMPERATURE AS AN ECOLOGICAL RESOURCE
mouth bass exhibit complementarity of
temperature preference on a diel basis. It
is not known whether these fish show diel
thermal segregation or complementarity in
the field, but this pattern would be consistent with temporal partitioning of other
resources.
Exploitation competition for temperature is perhaps the weakest part of the
analogy between thermal resources and
food resources. In a sense, fish moving
back and forth in a thermal gradient can
"consume" or "waste" calories as their body
temperatures come into equilibrium with
the external temperature. This process
would alter the thermal habitat for other
fishes without direct interaction between
the individuals. For example, a fish moving
back and forth between two thermal
habitats could reduce the temperature of
one and increase the other. If the temperatures differed by 10cC, each habitat was
only 50 liters, and the fish had a mass of
about 10 g, then the initial sojourn back
and forth would alter temperatures by
0.002°C. Such alterations, while real, must
be negligible in natural environments
owing to the large mass of water and the
relatively small mass of fishes.
Other analogies to exploitation are more
nebulous. Animals can occupy temperature space and lower the availability of
other resources such as food in that space.
This decreases the utility of that temperature space to competitors with similar food
habits. They can also occupy temperature
space that optimizes their growth and
other fitness parameters and thereby obtain more of or make better use of food
resources in that space than can potential
food competitors (Kitchell et al., 1977;
MacLean and Magnuson, 1977).
Optimum utilization of resources
Success of a fish in achieving its fundamental thermal niche under field conditions can contribute to its fitness. The expectations are clear for the three species in
Figure 9 chosen from warm, cool, and cold
water thermal guilds. In each case,
maximum growth is achieved at temperatures delineated by data on behavioral
10
15
20
25
30
TEMPERATURE (°C)
339
35
FIG. 9. Relation between thermal niche and body
growth with excess ration as a function of temperature for a warm, cool, and cold water fish. Diagram
based on the following data sources are: bluegillthermal niche (Magnuson and Beitinger, 1979),
growth (Lemke, 1977; Beitinger, Stuntz, and Magnuson, unpublished data), yellow perch-thermal niche
(McCauley and Read, 1973), growth (Huh, 1975;
Huh etal., 1976; Kitchell et al., 1977), sockeye salmon
(Onchorhynchus nerka (Walbaum)) (Brett, 1971).
thermoregulation. Approximations of the
percentage of maximum growth achieved
by the three species 2°C from the center of
their fundamental niches is 98 and 93 percent on the cool and warm side respectively. For those 5°C from the center of the
fundamental niche, growth would be
about 82 and 54 percent of maximum.
These are large reductions in fitness. Also,
growth declines more rapidly on the warm
side as all three growth curves are skewed
towards the cooler temperatures.
Huey and Slatkin (1976) have presented
a cost/benefit model for behavioral thermoregulation in lizards which is quite
similar to cost/benefit models for optimal
diet (summarized by Pyke et al., 1977).
Both models were independently based on
a cost/benefit approach to optimal use of
resources, though Huey and Slatkin (1976)
consider food abundance in addition to
temperature. Both approaches seem appropriate for fishes. As we interpret the
predictions of Huey and Slatkin (1976) vs.
those of Pyke et al. (1977), we see the following parallels. (1) When the cost of behavioral thermoregulation is low, lizards
should carefully maintain body tempera-
340
MAGNUSON ET AL.
tures near optimum. When benefits of
foraging are high compared to costs, one
would expect greater food specialization
on optimally sized prey. (2) Lizards will
thermoregulate more carefully if the productivity of the habitat is raised. If food
abundance increases, diet breadth declines. (3) Exploitation competition between lizards with similar temperature
optima will lead to less careful thermoregulation by both species. Species with similar food optima will reduce the prey available to them and widen their choice of
foods. (4) Competition between species
with different temperature optima will
lead to broader thermal niches only if both
species are active at the same time, and
such competition may lead to segregation
in time or space. This is exactly analogous
to diet predictions—diets need not broaden if segregation can occur on other niche
axes. Other parallels are more subtle, but
it is clear that both groups working on the
theory of optimal resource use based on
cost/benefit would enjoy a careful comparison of their predictions.
COMPARISON OF VIEWPOINTS
When we consider temperature in the
context of niche theory and competition,
we find that thermal resources can usually
be treated in the same way as food resources. In fact, when Hutchinson (1957)
first developed his hypervolume-niche
concept he began with physical/chemical
factors. We have expanded and quantified
properties of a particular niche axis, temperature. Obviously, other physical/
chemical factors can be treated similarly.
The analogy between temperature and
food resources weakens when we consider
exploitation competition. Animals do not
consume thermal resources in the same
sense that they consume food resources.
However, animals can consume other
physical/chemical resources such as' oxygen. And, of course, animals can use (occupy) temperature-space to the exclusion
of other species.
An alternative ecological approach to
temperature is that tolerance to particular
temperature, bounds the region of niche
space an animal can occupy. From this
viewpoint organisms do not compete for
temperature. Hutchinson (1978) has referred to such niche axes as scenopoetic. He
differentiates these from axes such as food
for which competition is more easily envisioned, which he refers to as bionomic.
The status of habitat axes such as branch
density (Hutchinson, 1978, Fig. 99) are
difficult to classify into one of his two
categories. Clearly, animals do not directly
consume habitat resources, but they do
consume food resources associated with
the habitat. Since food and habitat are so
tightly interrelated, it is often impossible to
separate competition for food from competition for habitat patches in which the
food is distributed.
Fry (1971) and Brett (1970) from still
another viewpoint, have classified environmental factors with respect to their
effects on fish ecology. Their classification
tends to be more detailed than Hutchinson's (1978). In general at least three
somewhat distinct classes of niche axes are
apparent. The first class imposes bounds
on the distribution of a species. Lethal
factors (Fry, 1971), tolerance factors
(Brett, 1970) and limiting factors (Fry,
1971) are like Hutchinson's (1978) scenopoetic niche axes. While bounds on these
axes may be modified by other factors,
they are not thought to be influenced
significantly by biological interactions.
Hutchinson (1978) places temperature in
this class. A second class is resources for
which animals are likely to interact (or
compete). Directive factors (Fry, 1971) appear to fall into this class as do Hutchinson's (1978) bionomic niche axes. Hutchinson (1978) puts food here. The final class
may act to modify the bounds of species
distributions and mediate interactions
between species. Fry (1971) refers to these
factors as controlling and masking.
Hutchinson (1978) does not provide a distinct classification for such factors. In Fry's
(1971) classification of factors, a single
factor such as temperature can fall into
several categories, i.e., it can be lethal, directive, and controlling. The classification
is on the organism's response to the factor.
A potential problem with any sort ot
TEMPERATURE AS AN ECOLOGICAL RESOURCE
classification system for niche axes is that it
emphasizes the differences among factors
influencing the ecology of interacting
species. This tends to leave us "comparing
apples and oranges" when we jointly consider food and temperature. Ecologists
have tended to treat temperature and food
separately and differently despite numerous analogies between food and temperature as resources such as those raised in
our paper.
Our conclusion is that fish do compete
for thermal resources and that considerations of temperature as a resource are generally consistent with the characteristics of
food as a resource. Even if our point of
view is too extreme we can at least be sure
that temperature is a mediating factor for
other bounding and interactive niche axes
and that biological interactions alter temperature distributions of fish as well as reptiles (Magnuson and Beitinger, 1979).
Brett (1970) gives a detailed evaluation of
the temperature modification of various
niche axes related to tolerance, limits,
metabolism, activity, growth, reproduction
and geographic distribution. Alderdice
(1972) summarizes temperature factors as
a part of multi-factor environmental
studies. Temperature can alter the outcome of competition for food (Birch,
1953).
We suggest that treating the variety of
niche axes as Werner (1977) has considered food size and we have considered
temperature here would be useful. The
distribution of a species along the axis
under controlled laboratory conditions describes the fundamental niche. An evaluation of the "trade offs" along multiple
niche axes reveals actual interrelations
between axes. For example, will an animal
shift its thermal niche to exploit an abundant food resource in colder or warmer
water? This has been observed in the laboratory (Neill and Magnuson, 1974). Fish in
field situations will also swim into anoxic
waters (Hasler, 1945) and warmer waters
(Engel and Magnuson, 1971) for short
periods to forage.
In addition, the effect of an animal's use
of the multiple dimensions of niche space
on measures of fitness seems important to
341
us. Food, temperature, habitat structure,
oxygen and other niche axes influence the
utility (Werner, 1977) of a particular portion of niche space. The implication of the
theory of optimal foraging is that animals
should feed in areas that maximize their
fitness (Schoener, 1971; Charnov, 1973;
Krebs, 1973). For fishes, at least, growth is
likely to be a reasonable measure of fitness
(Bagenal, 1967). Animals may well utilize
other resources in niche space such as
temperature (Huey and Slatkin, 1976) so
as to maximize net energy intake and thus
growth. For example, a fish might be expected to feed at temperatures different
from preferred if net energy return were
greater than obtained by feeding at preferred temperatures. Conversely, a fish
may feed on lower utility foods (Werner,
1977) in a zone of preferred temperature
rather than feed on a higher utility diet
where temperatures are extreme. If a
number of niche axes are considered concurrently in terms of utility of a particular
region of niche space, apparent "tradeoffs" actually made by animals in resource
use may be more clear.
In summary, we need to consider (1)
how animals position themselves along
niche axes of various classes acting together, (2) how this influences the utility of
various regions of niche space to a particular organism, and (3) how interspecific
competition for "interactive niche axes"
alters these utility rankings.
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