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.

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. Therelative 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?

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.

Size-Number tradeoffs in goldenrods

In a study of goldenrods (Werner and Platt. 1976) foundvariation 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 betweensize 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.

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 we’ll look at compromises between achene size and number.

Later we’ll look at biomass allocations, and the compromises between growth and reproduction.

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)...

Species Old field Prairie Wt. # loading Wt. # loadingS. nemoralis 26.7 2300 7.319 104. 200 9.968S. missourensis 17.6 4200 2.862 39.3 1100 4.485S. speciosa 19.5 9100 5.345 146.3 500 13.193S. canadensis 27.3 13000 3.385 58.3 1100 8.965S. 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.

S. nemoralis S. rigida S. missouriensis

S. speciosa S. graminifolia

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 20,800S. missourensis 17.6 4200 73,920 39.3 1100 43,230S. speciosa 19.5 9100 177,450 146.3 500 73,150S. canadensis 27.3 13000 354,900 58.3 1100 64,130S. 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.

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.

There are 2 parallel conditions intertwined in the comparisonsin 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?

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.

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.

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.

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.

high density and highintraspecific competition

low density and lowintraspecific competition

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.

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 summerBiale 6.4 4.5 .83 .97 5.3 4.4Rajgrodskie 9.5 6.8 .75 .90 7.1 6.1Drestwo 11.0 6.0 .65 1.05 7.2 6.3 

Mean 8.96 5.76 .74 .97 6.4 5.6

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…