The Measurement and Meaning of Reproductive Effort in Plants

MacArthur and Wilson (1967) recognized that the way in which organisms allocate resources to competing "sinks" could be used to determine the position of a species on a selective continuum. This recognition has stimulated a spate of experimental studies on resource allocation. The proportion of resources devoted to reproduction, as opposed to the development of a competitive growth form or defense against predators, has rightly been seen as the character of greatest importance. It is, in any case, the one which appears to be easiest to measure. Reproduction in this sense has usually been taken to mean sexual reproduction, especially since "vegetative reproduction" is an imprecise term capable of varied interpretations (Harper 1977). Herbaceous plants have proved to be ideal subjects for study in this respect and numerous attempts have been made to determine the reproductive effort of species and even of whole communities. An examination of table 1, which lists the important features of a number of such attempts, clearly reveals that no consensus of exactly what is being measured or how it is to be measured has yet been reached. There exist, therefore, both conceptual and methodological problems. Here we attempt to examine these problems and suggest possible ways in which they might be overcome. Perhaps the most striking conceptual dichotomy is that which divides the approach of Harper and Ogden (1970) from that of Gadgil and Solbrig (1972). The former have chosen to measure the proportion of resources devoted to seeds while the latter have measured allocation to "all reproductive structures." These quantities will be referred to subsequently as seed output and total reproductive ffort, respectively. What makes this division so remarkable is that, with very few exceptions, no one on either side has felt it necessary to provide any satisfactory justification for his chosen method, nor does any other approach seem to have been seriously considered. This is not by any means a sterile academic argument; there are fundamental ecological differences between the quantities being measured by the two schools. We contend that total reproductive effort is a more realistic estimate of the resources actually committed to reproduction than is seed output. This usage conforms to the normally accepted meaning of the word "effort." Production of the whole floral apparatus and perhaps supporting structures as well is all part of the "effort" involved in reproduction. Seed output, on the other hand, may be viewed as the result of the interaction of this effort with a number of environmental variables (pathogens, climate, predators, pollinators, etc.) over which the plant has little or no control. It should be made clear at this point that there is no objection to the measurement of seed output per se, but that this quantity should not be confused with reproductive effort. Unfortunately the conceptual problems do not end with the decision to measure total reproductive effort. Nearly all the workers who have used this approach have measured only the allocation to the actual flowers; all supporting structures

[1]  J. L. Doust.  Experimental manipulation of patterns of resource allocation in the growth cycle and reproduction of Smyrnium olusatrum L. , 1980 .

[2]  D. Raynal.  Population Ecology of Hieracium florentinum (Compositae) in a Central New York Limestone Quarry , 1979 .

[3]  R. Benton,et al.  The reproductive strategies of five perennial Compositae , 1979 .

[4]  J. Harper.  Population Biology of Plants , 1979 .

[5]  P. B. Cavers,et al.  Resource allocation in young plants of two perennial species of Plantago , 1978 .

[6]  E. Tramer,et al.  REPRODUCTIVE STRATEGIES IN HERBACEOUS PLANT COMMUNITIES DURING SUCCESSION , 1978 .

[7]  J. Andel,et al.  Reproductive Allocation in Senecio Sylvaticus and Chamaenerion Angustifolium in Relation to Mineral Nutrition , 1977 .

[8]  James C. Hickman,et al.  Environmental unpredictability and plastic energy allocation strategies in the annual Polygonum cascadense (Polygonaceae) , 1975 .

[9]  Roderick Hunt,et al.  Relative growth-rate: its range and adaptive significance in a local flora. , 1975 .

[10]  M. Gaines,et al.  Reproductive Strategies and Growth Patterns in Sunflowers (Helianthus) , 1974, The American Naturalist.

[11]  J. Ogden.  The Reproductive Strategy of Higher Plants: II. The Reproductive Strategy of Tussilago Farfara L. , 1974 .

[12]  M. Gadgil,et al.  Growth Form and Reproductive Effort in Goldenrods (Solidago, Compositae) , 1973, The American Naturalist.

[13]  Madhav Gadgil,et al.  The Concept of r- and K-Selection: Evidence from Wild Flowers and Some Theoretical Considerations , 1972, The American Naturalist.

[14]  J. Harper,et al.  The reproduc-tive strategy of higher plants. 1. The concept of strategy with special reference to Senecio vulgaris L. , 1970 .

[15]  R. Macarthur,et al.  The Theory of Island Biogeography , 1969 .

[16]  R. F. Parsons.  The Significance of Growth-Rate Comparisons for Plant Ecology , 1968, The American Naturalist.

[17]  T. Hutchinson.  COMPARATIVE STUDIES OF THE ABILITY OF SPECIES TO WITHSTAND PROLONGED PERIODS OF DARKNESS , 1967 .

[18]  D. Clarkson.  PHOSPHORUS SUPPLY AND GROWTH RATE IN SPECIES OF AGROSTIS L. , 1967 .

[19]  M. Cody,et al.  A GENERAL THEORY OF CLUTCH SIZE , 1966, Evolution; international journal of organic evolution.

[20]  J. P. Grime.  Comparative Experiments as a Key to the Ecology of Flowering Plants , 1965 .

[21]  O. W. Richards.  Biological Flora of the British Isles , 1942 .

[22]  W. Abrahamson.  PATTERNS OF RESOURCE ALLOCATION IN WILDFLOWER POPULATIONS OF FIELDS AND WOODS , 1979 .

[23]  F. Bazzaz,et al.  PHOTOSYNTHETIC CONTRIBUTION OF FLOWERS AND SEEDS TO REPRODUCTIVE EFFORT OF AN ANNUAL COLONIZER , 1979 .

[24]  M. Gadgil,et al.  Growth form and reproductive effort in goldenrods , 1973 .

[25]  R. Lewin,et al.  Biological flora of the British Isles , 1948 .