Tradeoffs between offspring size and number

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Tradeoffs between offspring size and
number Individuals have a choice between making a
few large offspring or making numerous smaller
ones, using the same energy, and at the same time
in the life history etc. This is part of
parent-offspring conflict. The parentwould
increase fitness by making larger numbers the
offspring would be better off if they were
larger, and fewer were produced. Offspring (or,
in plants, seed) size is likely critical to
individual fitness it should be strongly
optimized by selection, i.e. there should be
little additive genetic variation left. But in
plants offspring disperse, and seed size
influences dispersal distance. As a result,
there's the potential for interesting compromises
between seed size and seed number, a condition
more evident as compromise in plant life history
than for animals.
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Is there evidence that there is little variation
left in seed size? A plant stressed by
competition (or abiotic conditions) seems first
to drastically reduce allocation (absolute amount
of biomass, at least) to reproduction and the
number of seeds produced before additionally
reducing seed or fruit size. The relative
plasticity in components of reproduction is
indicated in wheat (data from Harper) when
ratios of performance at high and low densities
are compared Ratio ears
per plant 56
total seeds per plant 833
grains per ear
1.43 mean grain weight
1.04 How do plants achieve this?
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Harper (1977) points out an observation of some
importance, since studied also by Stephenson
(1984). Most plants initiate many more flowers
(and seeds) than they develop, aborting the
excess. Thus the number aborted can be adjusted
at the times of energetic drain (flower
development, seed maturation) with much less
error than inherent in a time lag process which
would make these decisions at the outset of
reproduction (initiation) or alternatively
building a fixed number into the genotype.
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Size-Number tradeoffs in goldenrods In a study of
goldenrods (Werner and Platt. 1976)
found variation in size, but almost absolute
complimentarity between size and number, i.e.
Number x Weight K This
implies a strict allocation pattern, but
flexibility ( based in the differing ecology of
the species) in the balance between size and
number. The following graph shows a log-log plot
of the number of propagules per basal stem on the
mean weight of the propagules. It follows a
straight line quite well. The regression equation
which fits this line is log N 5.29 -
1.19 log W. The constant 1.19 does not differ
significantly from 1.
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It is interesting and important to recognize that
goldenrods in differing habitats differ
significantly in size. How can they allocate the
same total biomass to seeds (called achenes),
varying only the balance between size and number,
when they are different sized plants? They must
have evolved adjustments to the balance in
allocation among growth, maintenance, and
reproduction. Here well look at compromises
between achene size and number. Later well look
at biomass allocations, and the compromises
between growth and reproduction.
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Look again at the graph Prairie and old field
populations were studied. The prairie
populations came from northwestern Iowa the old
field populations came from the Kellogg
Biological Station, near Kalamazoo, MI oak
woods populations were also from western
Michigan. Old field populations produce the
largest numbers of smallest seeds (this
environment corresponds to dry, open, disturbed
sites). Prairie populations are intermediate in
both size and number, and oak woods populations
produce smaller numbers of larger seeds. For
individual species from old field and prairie
(where we can find the same species in both
habitats)...
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Species Old field
Prairie Wt.
loading Wt.
loading S. nemoralis 26.7 2300 7.319
104. 200 9.968 S. missourensis
17.6 4200 2.862 39.3 1100
4.485 S. speciosa 19.5 9100 5.345
146.3 500 13.193 S. canadensis
27.3 13000 3.385 58.3 1100
8.965 S. graminifolia 24.5 17700 3.92
10.6 7800 1.509 Achene weights are in
?g. Loading is a ratio of achene weight to the
area of the dispersal accessory structure, called
a plummule. It should be closely correlated to
dispersal distance. Weights are higher from
prairie samples for each species except S.
graminifolia. The same is true for wing loading.
Numbers are lower for each species.
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S. nemoralis S. rigida S.
missouriensis
S. speciosa S. graminifolia
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Something else should have been evident The
product of weight and number was, according to
the larger scale hypothesis, supposed to be a
constant. Is it? Species
Old field Prairie
Wt. total
Wt. total S.
nemoralis 26.7 2300 61,410 104.
200 20,800 S. missourensis 17.6 4200
73,920 39.3 1100 43,230 S. speciosa
19.5 9100 177,450 146.3 500
73,150 S. canadensis 27.3 13000 354,900
58.3 1100 64,130 S. graminifolia 24.5
17700 433,650 10.6 7800
82,680 Clearly not. But it is also evident that
there is a gradient among these species.
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These differences are due to the species being
distributed along a soil moisture gradient. S.
nemoralis, typically found in open and
relatively disturbed sites, occurs in both old
fields and prairies, but it occupies the driest
sites in both places. In both habitats it
produces the smallest total biomass of
achenes. The other species are arrayed along a
gradient in soil moisture which, on the prairie,
corresponds to walking down from the drier ridge
tops into the moist valleys between.
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There are 2 parallel conditions intertwined in
the comparisons in the size-number data.
Successional status (diversity, intra- and
interspecific competition) clearly influences
size-number tradeoffs, and we can hypothesize
that the diverse, climax prairie should expose
component species to higher biotic
stress. However, there is also water stress
caused by the moisture gradient. We can, at
least in a primitive way, separate those factors.
How does the allocation to seeds (the product of
size and number for each species) vary along the
moisture gradient in each habitat?
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If the slopes of these two lines were identical,
we could dismiss competitive stress as a
significant factor. However, the slopes differ
markedly. The slope is much lower on the prairie
(2966 µg/ moisture) than in old field (21,117
µg/ moisture). The biotic stress (mostly
competition) on the prairie has already limited
the potential response to improving moisture
conditions.
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Other Examples of size-number tradeoffs 1) egg
size and egg number in European alpine char.
The char are found in a series of lakes in Sweden
which lie along a latitudinal gradient. The
lakes at both extremes of the gradient had
sparse populations, but the mean size of
adult fish in these lakes was larger, and the
broods produced consisted of larger numbers
of smaller eggs (and a greater proportion of
female biomass allocated to eggs) than found
in central lakes. Egg sizes in the two
groups of lakes did not overlap. In lakes
with low densities of adults, eggs ranged
from 40.3 - 57.2 mg in lakes with high densities
eggs weighed from 58.0 - 66.4 mg.
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Interpretation pure Lack. Low intraspecific
competition (low density) favors individuals
that produce large numbers of young, while a
greater density, and thus intensity of
intraspecific competition, would favor
individuals producing fewer, larger
young. Another example in fish reproduction We
can be sure that a similar pattern in lake
whitefish in Alberta is due to effects of
intraspecific competition one lake studied
lacked predators and interspecific competitors
for the whitefish. Fish population size in the
lake increased dramatically, intraspecific
competition increased. Egg volume increased by
about 10 there.
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Egg numbers and their relationship to female body
size are even more interesting. Not only do
whitefish allocate more to reproduction in the
lake with lower density, the effect of body size
on egg number is greater.
low density and low intraspecific competition
high density and high intraspecific competition
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Intraspecific competition evidently reduces the
potential to respond to altered conditions (here
female size rather than soil moisture) with
increased reproduction.
In some species, temporal patterns in density
over a single season may lead to evolution of
temporally different reproductive
output. Calanoid copepods in Polish lakes produce
resting, sexual eggs (ephippia) to overwinter,
and effectively recolonize an empty lake each
spring. During the later spring and summer
reproduction is by parthenogenesis, and both
numbers and density increase, until, as winter
approaches, a sexual generation once more
produces the resting eggs. This pattern of
reproduction is called cyclical parthenogenesis.
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Density (and the intensity of intraspecific
competition) is low in spring and high in summer.
Food abundance may also change seasonally, with
more food and less, hard-to-consume blue green
algae present in spring than in summer in these
lakes. The result egg number, egg size, and
allocation to reproduction change seasonally.
Eudiaptomus graciloides in 3 lakes  Lake
number size
total
spring summer spring summer spring
summer Biale 6.4 4.5
.83 .97 5.3
4.4 Rajgrodskie 9.5 6.8 .75
.90 7.1 6.1 Drestwo 11.0
6.0 .65 1.05 7.2
6.3  Mean 8.96 5.76
.74 .97 6.4 5.6
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Though the lakes differ, in each number is lower,
size of the average egg is larger, and the total
biomass of eggs is lower in summer than in
spring. The apparent reason is increased
intra- specific competition for a declining food
resource (which may be a quantitative or
qualitative change). In considering how many
babies (eggs) is appropriate, we have inevitably
run up against the question of how much and
when total energy should be committed energy .
That is the next subject to be considered in
greater detail