BioI. Jb. Dodonoea. 42. 1974. 1/7-120.
A NOTE ON THE LIFE-SPAN
OF r-STRATEGISTS
by
Carlo HElP
ABsTRACT.
- The idea is presented that the short life-span of r-strategists may
in some cases be an adaptation to avoid predation. Evidence supporting this
hypothesis is presented by examination of the predation of the polyp Protohydra
leuckarti on harpacticoid copepods.
INTRODUCTION
The idea of two different kinds of selection probably originates with DaBZHANSKY
(1950) who argued that selection in the tropics should work in a
different way than in temperate regions. McARTHURand WILSON(1967)
laid the foundations of a more rigorous treatment of the problem and termed these two kinds of selection r- and K-selection, thereby defining two
kinds of population genetics, one where fitness is defined by the malthusian
parameter r and one where it is defined by the logistic parameter K. During
recent years an ever increasing amount of theoretical research has confirmed
the central role of the concepts of r- and K-selection in population biology
(see GILL, 1974, for a lucid review).
PIANKA(1970) gives an account of some of the correlates of r- and Kselection and states that r-strp.tegists should have a shorter life-span than
K-strategists. GADGlLand SoLBRIG(1972) on the other hand see no obvious reason why r-strategists should have this shorter life-span, though
they emphasize the positive correlation which exists between life-span and
the age of sexual maturity, and r-selection obviously favours early sexual
maturity.
THE LIFE-SPAN OF R-STRATEGlSTS
This paper presents the idea that the shorter life-span of r-strategists
could in some cases be an adaptation to the lower ability of r-strategists to
117
avoid predation. Indeed, r-strategists devote a greater part of their resources
into reproduction than K-strategists (GADGlLand BoSSERT,1970; GADGlL
and SoLBRIG, 1972); the consequence is the absence in r-strategists of
more or less elaborate mechanisms to avoid predation. In this situation, the
probability of an individual prey organism of being captured by a predator
may be considered as being dependent mostly on the chance of meeting the
predator, which in turn depends on predator density and the life-span of the
prey organism; the longer it lives, the greater its chances of meeting the
predator. A long-living animal with late sexual maturity is obviously in a
disadvantageous position if the fact of living longer does not involve the
development of escape mechanisms, which would mean devoting a greater
part of resources into non-reproductive activities, hence a more K-strategy.
The hypothesis that a shorter life-span is an adaptation to predation
could be tested by looking at what happens if predator densities become so
high that the chance of an individual prey organism to meet the predator
becomes large, i.e. involves a times shorter than its life-span. As no adaptations to avoid the predator physically exist, mortality in the prey
population should be very large.
THE PREDATION OF PROTOHYDRA ON COPEPODS
We examined the following situation: in the meiobenthos of a brackish
water pond in northern Belgium (see HElP, 1972 for description of the
habitat and methods) the dominating group after the nematodes are harpacticoid copepods, probably typical r-strategists with a short generation
time (11-12 days in optimum conditions), a high reproductive potential
(observed r-values in the field between 0.04 and 0.06 per day (HElP,
1972» and a short life-span (I -2 months). The habitat, being a shallow
brackish water, can be regarded as a harsh environment, so populations may
be essentially physically controlled and r-strategists should be favoured in
these conditions. The most important predator of the meiobenthos is the
polyp Protohydra leuckarti, which captures the copepods by releasing the
nematocysts upon contact (Muus, 1966); there is no active mechanism to
avoid the contact, though once captured the prey will make frantic
movements to escape, sometimes succesfully in the case of adults.
The probability of a copepod to meet Protohydra can be very roughly
calculated as follows: if we assume that the copepod makes a horizontal
displacement of 5 cm per hour, the surface covered by the organism during
a 12 hour active period is 0.1 mm (breadth) x 60 cm (displacement)
= 0.6
cm2. If we assume this figure to be the density of the polyp (about 17000
per or), the copepod has a great chance of meeting a polyp during one day.
118
These predator densities are frequently encountered in summer, maximum
density being somewhat near 40000 per m2 (Muus, 1967; HElP, 1971).
Here we can assume that on the average each copepod meets one polyp
during a day.
Numb..
0' COp'pOds IIOC,,!
,-"
50
/'1
,
/
'\\.
,
/,'" 'I
I
\~\
I
"
'\.1
,\
250
"'
.o" .
"'"
,;'
\
'. \~
"
,
1972
\
\,
\
'.'"
\
'
0
.-"d
' I9 70
1971
1969
Tim.
FIG. I. - Total number of copepodsper 10 cm' in the course of the spring bloom during four consecutiveyears. Time intervalis fourteendays. Only in 1972 did the decline not coincidewith high densities of Protohydraleuckarti.
As can be expected we observed a catastrophic decline in the population
density of the copepods once the polyp attained a high density (fig. I).
Moreover, the decline was largest when the numbers before it where largest
too. The reasons for this enormous mortality may be found in two characteristics of r-strategists, in the first place their inability to propagate in
crowded conditions, in the second place their inability to withstand
predation; as there is no avoidance mechanism, the bigger the animal, the
greater its chances of being captured.
The biggest animals in the population are egg-carrying females, the most
important members of the population with regard to reproduction; on the
other hand, tJ'toseare the most likely to escape due to their greater strength,
but this can result in leaving the egg sac with the predator (Muus, 1966),
thereby making the escaping individual useless for the population for some
time. As the polyp eats more naupliar and copepodite stages than adults.
119
this will result in a population with an older age-distribution, and, given the
short life-span of the adults, a marked decline of population density will
reinforce the effect after some time.
A further confirmation of the hypothesis can be obtained with the data of
1972 (fig. 1). That year the polyp did not attain high densities, maximum
density during the spring bloom of the copepods was only 3500 per or, well
below the assumed criticalleveI. In 1972 we did not observe a steep decline
in population density of the copepods. When predator densities are beneath
a certain critical level, the whole life cycle of most of the individual prey
organisms can take place without meeting the predator.
ACKNOWLEDGMENTS
I acknowledge a grant as aspirant of the Belgian National Foundation for Scientific Research (N.F.W.O.).
LITERATURE CITED
DoBZHANSKY,T. (J 950). Evolution in the tropics. Amer. Sci. 38: 209-221.
GADGlL,M. and W. H. BOSSERT(J 970). Life historical consequences of natural
selection. Amer. Natur., 104: 1-24.
GADGIL, M. and O. T. SoLBIRG (J 972). The concept of r- an K-selection:
evidence from wild flowers and some theoretical considerations. Amer. Natur.,
106: 14-31.
GILL, D. E. (J 974). Intrinsic rate of increase, saturation density and competitive
ability. 11.The evolution of competitive ability. Amer. Natur., 108: 103-116.
HElP, C. (J 97 I). The succession of benthic micrometazoans in a brackish water
habitat. Bioi. Jrb. Dodonaea, 39: 191-196.
HElP, C. (1972). The reproductive potential of copepods in brackish water. Marine
Biology, 12: 219-221.
McARTHUR,R. H. and E. O. WILSON(J 967). The Theory of Island Biogeography.
Princeton Univ. Press, Princeton, N. J. 203 pp.
Muus, B. J. (J967). The fauna of Danish estuaries and lagoons. Meddr. Dann.
Fisk og Havunders., N.S. 5: 1-316.
Muus, K. (1966). Notes on the biology of Protohydra leuckarti Greeff (Hydroidea,
Protohydridae). Ophelia, 3: 141-150.
Rijksuniversiteit Gent,
Laboratorium voor Morfologie en Systematiek der Dieren.
(Dir. Prof. Dr. L. De Coninck en Prof. Dr. A. Coomans)
K. L. Ledeganckstraat 35, B-9000 Gent (Belgium).
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