r- and K-Strategists in Endemic Host-Parasitoid Communities
By
Department
DON
C.
FORCE
of Biological Sciences, California State Polytechnic
Pianka (1970), Gadgil and Solbrig (1972), and others
have thoroughly reviewed and analyzed the concept of
r- and K-selection as proposed by MacArthur and Wilson
(1967). Pianka listed some of the attributes characteristic of r- and K-strategists.
To mention the most important, r-strategists
often exist in unpredictable environments, have high capacities for population increase
(high r values), and are poor competitors, whereas
K-strategists exist in fairly constant environments, have
relatively low capacities for population increase, and are
good competitors.
Pianka also categorized vertebrates
as being relatively K-selected and most of the insects
and other terrestrial
invertebrates
as being relatively
r-selected.
Other workers (e.g., Gadgil and Bossert
1970) suggested that r-strategists exist in environments
where resources are readily available, K-strategists
in
environments where they are not. Still others (e.g., King
and Anderson 1971) considered the genetic aspects of
r- and K-selection in relation to constant and variable
environments.
For the past five years I have studied an insect-hostparasitoid community consisting of a midge, Rhopalom:yia
cali/arnica Felt (Diptera:
Cecidomyiidae),
that forms
galls on the plant Baccharis pilularis De Candolle (coyote
brush), and numerous hymenopterous
parasitoids
(at
least 10 species) that attack the midge and/or one
another. All the members of this community are endemic
to the northern and central coastal region of California
and extend inland to the Sierra Nevada foothills in some
areas. There are both primary parasitoids (attacking
only the host insect) and facultative secondary parasitoids
(attacking either the host insect or other parasitoids)
present in this complex .. Four species of primaries are
fairly consistent in their presence within the community,
as are three species of facultative secondaries, but the
remaining species are sporadic. The results of detailed
laboratory analyses of the reproductive capacities and
competitive abilities of the four consistently present
sllecies of primary parasitoids were reported elsewhere
(Force 1970). Briefly, I found that the parasitoid having
by far the highest reproductive capacity was the poorest
competitor for a host attacked by one or more other
parasitoid species. Each of the four species appeared to
have a reproductive capacity inversely related to its
competitive abilities, although this relationship was somewhat less clear in some cases than in others. It has
occurred to me that endemic populations of host insects
may evolve, over long periods, parasitoid complexes
which represent a sequential series of r- and K-strategists
when considered relative to one another. Obviously, very
few if any insects could be called K-strategists
when
compared with vertebrates, but relative to another insect, one could very well be so called (Gadgil and
Solbrig 1972). The purpose of this paper is to consider how a sequential r- K-strategist
series of parasitoids might evolve with a host insect and how one
might apply insight into such systems to problems of
biological control.
University,
Pomona
degree of species packing, many ecologists have tended
to favor the explanation that parasitoids in general must
be very specialized organisms, since their ecological
niches are so apparently restricted. Consequently, it is assumed that anyone species possesses little potential for
adapting to changing conditions or controlling hosts over
broad geographical areas. Indeed, many parasitoids do
appear to be rather specialized. On the other hand, numerous introduced pest insects have been controlled over
wide geographical areas by only one or two species of
their introduced parasitoids, even though the pest may
have had a large complex of parasitoids in its endemic
environment (Turnbull and Chant 1961, and others). The
latter observation substantially weakens the hypothesis
that all parasitoids are more or less specialized organisms incapable of wide niche dominance. An alternative
explanation that would provide for wide niche dominance
by certain introduced parasitoids and at the same time
account for large endemic parasitoid complexes would be
the evolution of host-parasitoid communities in which the
host gradually accrues parasitoids that create niches
which did not previously exist.
Goulden (1969), in discussing temporal changes in
community diversity, suggested that immature or disturbed ecosystems are characterized by few species, one
of which is often dominant. These early dominants are
opportunists
("weeds")
able to adapt to unstable environments because of their great physiological tolerances. Goulden then speculates that as the environment
stabilizes (via ecological succession), specialized, relatively more intolerant species which are better competitors replace the opportunists. Slobodkin and Sanders
(1969) suggested a sucessional mechanism somewhat
similar to that of Goulden to explain community diversity, although they limited their discussion primarily to
changes in diversity in stable or benign (opposed to
harsh) environments. And Whittaker (1969) speculated
that although bird communities (and probably those of
other vertebrates)
often become saturated because of
territorial behavior displayed by the inhabitants, insect
and plant communities do not, and that therefore there
may be no ceiling on their diversity.
Following the ideas of these authors, the evolution of
host-parasitoid
communities could logically proceed as
follows. It is reasonable to assume from past evidence
that early successional stages of newly formed or reforming communities are situations where density-independent mortality is high. Gadgil and Solbrig (1972)
identified three types of environments where this is the
case: (1) environments which are permanent in both
space and time and have continuously occurring factors
causing high-density-independent
mortality; (2) environments permanent in space but with discontinuously occurring factors causing high-density-independent
mortality
periodically, e.g., an adverse season during the year; (3)
environments temporary in time and space such as disturbed areas where density-independent mortality during
the periods of disturbance is high. Most agricultural
areas would be good examples of this type of environment. Let us assume that the host insect is able to adapt
to one of these environments.
Any parasitoid species
It is well known that endemic insects often have large
complements of parasitoids. As many as 30 or 40 species
11er host species is not uncommon. To account for this
135
species) commumtles are more stable (having fewer
population number fluctuations and of lesser magnitude)
than those of less diversity, is still debatable. The commonly held belief that tropical communities, for example,
are more stable than temperate communities may be a
somewhat tenuous conclusion. Smith (1970) suggested
from his own observations that tropical terrestrial community relationships are complex, but not necessarily
stable or well "buffered." Perhaps we have assumed too
much from too little evidence gathered from tropical
communities and other diverse situations. The portion
of the host-parasitoid complex I am studying would appear to occur under circumstances that should lead to
stability. The sampling sites are situated along the central California coast under rather benign physical conditions - no great fluctuations in temperature or humidity, although there is a summer period' relatively free
of rain: the conditions would have to be considered
largely subtropical. All the insects are multivoltine and
continue to reproduce the year around with the possible
exception of one species, which appears to diapause for
a period during the cooler months. Also, as already
indicated, there are numerous insect species interacting
with one another in various ways, certainly the kind of
situation where stability might be expected. Yet the
populations within this community are not stable in any
sense of the word. Yet unpublished data suggest great
irregular fluctuations in host insect numbers, in numbers
of the different species of parasitoids, and in percent
parasitization of the host.
that is to successfully parasitize the host under the circumstances must have one or more of the following
attributes:
(1) great physiological tolerances to withstand regular or irregular physical perturbations;
(2)
great dispersal capabilities so that new host populations
can be located quickly in case of local disaster; (3) a
high potential rate of increase so that normal population
numbers can be established quickly between perturbations
or disturbances.
Two of these attributes (physiological
tolerances of unpredictable perturbations and high potential rates of increase) are characteristic of opportunists
or r-strategists
as noted earlier, and the third (great
dispersal capabilities) is characteristic of colonizers or
fugitive species. It is apparent that r-strategists which
have efficient dispersal mechanisms are preadapted to
becoming colonizing species. However, as pointed out
by Gadgil and Solbrig (1972), dispersibility is not necessarily a quality of all r-strategists.
As the ecosystem ages, more physical stability is likely
to result (under normal succession conditions), allowing
the possibility of invasion and establishment by less
physiologically tolerant and adaptable species of parasitoids. However, any parasitoid capable of invading a
system dominated by a successful r-strategist must either
itself be a better r-strategist than the other, or more
likely, it must have some of the characteristics
of the
K-strategist, i.e., competitive mechanisms, whereby it can
usurp part of the niche (a certain number or percent of
hosts) that formerly was utilized by the r-strategist.
Usurpation can be accomplished provided the invader is
not behaviorally restrained in parasitizing hosts already
parasitized by the r-strategist, and that the progeny of
the latter are killed in most cases of multiple parasitism.
A whole series of invasions can be visualized in time,
each new invasion accomplished by a parasitoid species
which is more K-selected than one or more of the
preceding group and therefore able to outcompete the
others for hosts, thus creating its own smaller niche out
of the larger niches of its predecessors. Theoretically at
least, more K-strategists
can eventually become established in a host-parasitoid
community situated in a
benign climate, since there are fewer problems involving
their limited physiological tolerances here than in harsh
climates. Those that become established under more
severe conditions apparently
rely on mechanisms to
escape periods of severity, e.g., diapause. The K-strategists finally become the dominant parasitoids under the
more stable conditions of advanced community succession
because of their competitive abilities for hosts, leaving
the r-strategists to face a rather precarious survival dependent upon hosts that the complex of more K-selected
parasitoids do not happen to find, or at least do not
utilize. The r-strategists
may also find refuge at the
outer range limits of the host where physical factors may
be more stringent. In the more centrally situated range,
only if the environment is periodically disturbed in some
manner, thereby decimating the K-strategists,
do the
r-strategists find sufficient unparasitized hosts to increase
their population numbers before the recovery of their
competitors.
Perhaps there is a lesson here for people active in
the field of biological control. These workers are interested in controlling insect pests, particularly those that
are established in geographical areas other than their
native habitat. Often the insects are very destructive
under these conditions, because their natural enemies are
not normally introduced with them. Biological control
workers attempt to establish the pest's native parasitoids
(and other natural enemies) in the new area in the
hope of controlling the pest. Most often the parasitoids
imported for establishment are probably those that are
most numerous in the endemic environment because these
are the parasitoids that can be most easily collected.
However, it is very likely also that the parasitoids that
are most numerous under natural conditions in the
endemic environment are those that have become dominant over a long period, in other words, the K-strategists. The problem with attempting to establish Kstrategists in new areas is that they tend to have characteristics which are just the opposite of those needed
by good colonizers. The attributes of good colonizers
are not altogether easy to discern, but Mayr (1965)
suggested a few: they need (1) an intrinsic "toughness"
to survive unfavorable conditions; (2) adequate dispersal
or searching abilities to find a suitable host or habitat;
(3) a high reproductive capacity; (4) an adaptiveness
to a variety of conditions. Apparently many introduced
parasitoids do not have these characteristics,
since the
record of establishment of natural enemies in new areas
has been rather dismal. Only about 22% of the colonization attempts by biological control workers have been
successful according to Carter (1970). In addition, the
degree of control of the pest by those imports that do
become established is often insignificant (Turnbull and
Chant 1961, Carter 1970). What would appear to be
needed to control pests in situations where they have
invaded new, often disturbed (agricultural, etc.) environments are natural enemies that are r-strategists.
One might assume that the successful invasion of the
r-strategist
niche by K-strategists
results in a more
diverse, presumably more stable host-parasitoid
community. Under conditions of multiple parasitoids, one
might further expect more predictable regulation of host
numbers than that produced by a single parasitoid
species. The assumption held by many population and
community ecologists that more diverse (having more
136
But how does one find the r-strategists in an endemic
parasitoid complex? Perhaps those species that are least
dominant numerically in undisturbed situations but are
found consistently over a wide geographical or ecological
range are the r-strategists.
In newly disturbed situations,
on the other hand, they may be relatively much more
abundant. At least this appears to be the case in the
host-parasitoid community I am studying. The parasitoid
(Tetrasticlllls sp.) having the highest reproductive capacity, the greatest heat tolerance (cold tolerance has not
been studied), and the poorest competitive abilities is the
least dominant of the four consistently present species
of primary parasitoids, yet it is widely distributed. This
species is rarely responsible for more than 1% of the
total host lJarasitization, and often less. The highest percent parasitization I have recorded under normal conditions over a five-year period from this species was 12%.
However, an interesting abnormal situation developed
recently at one of the collecting sites. Most of the
Barrharis plants in the area (several acres) were removed or cut low to the ground, leaving only scattered
and cut plants instead of the thick, normal growth.
Immediately, parasitization
of the host insect in this
area by Tetrastie/ills sp. increased from 1 to 46%, and
it stayed at this general level for several months. Total
parasitization of the host rose from 81 to 97%, indicating
that even though the host insects were more scattered
under the disturbed conditions, they were being found
and parasitized. Such evidence indicates that Tetrastie/ills
sp. is an efficient host searcher in addition to its other
attributes which are those of an opportunist or r-strategist. The plant growth at this collecting site is now
beginning to return, and host parasitization by Tetrastirhlls sp. has dropped at the last sampling to 30% as the
K-strategist parasitoids are beginning to resurge. I am
presently beginning a sampling project for these insects
in a large area that was denuded by fire several months
ago. IIarrharis 111antsare now returning to parts of this
area. The data taken from these samples should give
more conclusive evidence of the succession of r- K-strategist parasitoids as proposed in this paper.
referred to was supported by National Science Foundation
Grant GB-5446.
REFERENCES
CITED
I thank Drs. W. D. Edmonds, P. W. Price, and L. J.
Szijj for critically reading the manuscript. The project
Carter, W. 1970. The dynamics of entomology. Bu]l.
Entomol. Soc. Am. 16: 181-85.
Force, D. C. 1970. Competition among four hymenopterous parasites 0.£ an endemic insect host. Ann.
Entomol. Soc. Am. 63: 1675-88.
Gadgil, M., and W. H. Bossert. 1970. Life historical
consequences of natural selection. Am. Natur. 104:
1-24.
Gadgil, M., and O. T. Solbrig.
1972. The concept of
r- and K-selection: evidence from wild flowers and
some theoretical considerations.
Ibid. Am. Natur.
106: 14-31.
Goulden, C. E. 1969. Tempora] changes in diversity,
p. 96-102. !II Diversity and Stability in Ecological
Systems. Brookhaven Symposia in Biology, no. 22.
Brookhaven National Laboratory.
King, C. E., and W. W. Anderson.
1971. Age-specific
selection. II. The interaction between rand
K
during population growth. Am. Natur. 105: 137-56.
MacArthur, R. H., and E. O. Wilson. 1967. The theory
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Princeton, N.]. 203 p.
Mayr, E. 1965. Summary. III Baker and Stebbins [ed.].
The Genetics of Colonizing Species. Academic Press,
Inc., New York.
Pianka, E. R. 1970. On rand K selection. Am. Natur.
100: 592-7.
Slobodkin, 1. B., and H. 1. Sanders. 1969. On the
contribution of environmental predictability to species diversity, p. 82-95. III Diversity and Stability
in Ecological Systems. Brookhaven
Symposia in
Biology, no. 22. Brookhaven National Laboratory.
Smith, N. G. 1970. Book review of: Diversity and
Stability in Ecological Systems. Ibid., no. 22. Science 170: 312-3.
Turnbull, A. 1., and D. A. Chant. 1961. The practice
and theory of biological control of insects ;11 Canada.
Can. ]. Zoo I. 39: 697-753.
Whittaker, R. H. 1969. Evolution of diversity in plant
communities, p. 178-96. In Diversity and Stability
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Errata in Fundamentals
of Applied Entomology,
ond Edition (New York: Macmillan, 1971).
These errors as well as a few less serious ones will
be corrected in the first reprinting.
ACKNOWLEDGMENT
Sec-
Robert E. Pfadt, editor of this introductory
text,
wishes to call attention to several errors that went unnoticed until the book was received.
SYMPOSIUM ON
"PEST MANAGEMENT
FOR THE 21ST CENTURY"
The solar constant is approximately
2 cal/cm2/min
and not 2 cal/cm"/sec as found on page 140 line 34
and page 142 line 7.
October 13-14, 1972
University of Idaho, Moscow
The picture of the grasshopper nymph emerging from
the egg pod in Fig. 9:9 page 257 (extreme
left)
should not have the antennae outstretched as the young
grasshopper is still contained in the serosal sack at this
time.
Five 60 minute papers have been scheduled in the areas
of Legal, Social and Economic Restraints
(W. E.
Waters), Conceptual Organization of Research and Development (R. W. Campbell), Measurement of Social
and Economic Impact (N. E. Johnson), Future Tech·
niques (c. B. Huffaker and R. F. Smith), and Decision
Making Techniques (G. C. Mott). For further information write A. R. Gittins, Head, Department of Entomology, University of Idaho, Moscow, Idaho 83843.
Recent studies indicate that Anaplasma is not a protozoan but belongs with the bacteria in the order
Rickettsia]es.
Thus anaplasmosis should not have been
lumped with piroplasmosis, a protozoan-caused
disease
(See page 554, line 17 & 18. See also page 652, line 21).
137