Age (years)
C- S-R selection
Because many plants may exhibit a ‘com- promise’ of r/K-selected attributes, a modi- fied theory of plant strategy and selection was developed (Grime, 1977, 1979). Grime used characteristics of the established phase of the life cycle to characterize plants based
on their ability to withstand competitors, disturbance and stress. In his triangular con- ceptual model, the corners represent ruder- als (disturbance tolerators) (R), competitors (C) or stress-tolerators (S) (Fig. 3.12a). C- strategists maximize resource capture in undisturbed but productive habitats by increasing vegetative production and reduc- ing allocation to reproduction. R-strategists maximize reproduction and growth, and are
adapted to disturbed but potentially pro- ductive environments. These two strategies are somewhat analogous to K- and r-selec- tion, respectively. The S-strategists are adapted to stressful, harsh environments where disturbance is rare and competition is unimportant. By reducing vegetative growth and reproduction they maximize their sur- vival.
Characterization of C, S and R species is C
S R
C-S-R
S-R C-S C-R
Annual herbs Biennial herbs Perennial herbs and ferns
Trees and shrubs Lichens Bryophytes
Relative importance of stress
Relative importance of disturbance
Relative importance of competition (a)
(b)
Fig. 3.12.The C-S-R model showing: (a) the location of the three main strategy types (C= competitors, S= stress tolerators, R= disturbance tolerant ruderals) and secondary strategies, and (b) the placement of various types of vascular and non-vascular plants along the three axes (redrawn from Grime, 1977).
based on a plant’s morphology, physiology, life history and other traits (Table 3.4).
Intermediate species are shown in the cen- tral region of the triangular model (Fig.
3.12b). Weeds are usually classified as rud- erals (R), or competitive ruderals (CR). Both strategies are adapted to productive habitats, but CR-strategists would be found in less fre- quently disturbed habitats than R-strategists
who have short life spans which allow species to re-establish after disturbance.
While Grime’s strategies have been dis- cussed widely in reference to weed species, some have pointed out its limitations (Tilman, 1987). Grime’s model relies on a narrow definition of competition (Grace, 1991). This will be dealt with in the next chapter on competition.
Table 3.4.Characteristics of competitive, stress-tolerant and ruderal plants (adapted from Grime, 1977).
Competitive C Stress-tolerant S Ruderal R Morphology
Life forms Herbs, trees, shrubs Lichens, herbs, trees, Herbs shrubs
Morphology Leaves form high, dense Variable Small stature, little canopy, extensive lateral spread lateral spread of roots
and shoots
Leaf form Robust Often small, leathery or Various needle-like
Life history
Longevity of Variable Long Short
established phase
Longevity of leaves Relatively short Long Short and roots
Frequency of Usually every year Variable Produced early in life
flowering history
Annual production Small Small Large allocated to seeds
Structures persisting Dormant buds and Stress-tolerant leaves Dormant seeds in unfavourable seeds and roots
conditions
Regeneration Vegetative growth, Vegetative growth, Small seeds, strategies small seeds, persistent persistent seedling persistent seed
seed bank bank bank Physiology
Maximum potential Rapid Slow Rapid relative growth rate
Response to stress Rapid response to Slow, limited response Rapid response to maximize vegetative divert from vegetative growth growth to flowering Storage of mineral Into vegetative Storage in leaves, Seeds
nutrients from structures, some stored stems, and/or roots photosynthesis for new growth in
following season Other
Litter Copious, often Sparse, sometimes Sparse, not usually persistent persistent persistent Palatability to Variable Low Variable, often high
unspecialized herbivores
Summary
Describing population dynamics, population structures, life cycles and life history strate- gies is difficult because of genetic and envi- ronmental variation and the complex inter- actions and combinations that can occur.
This complexity is the reason why our con- venient measures and descriptions of popu- lations are often not adequate even if they do a reasonable job of approximating the real world. This complexity explains why:
• simple logistic and exponential equations do not adequately describe populations;
• spatial isolation within metapopulations influences survival and conservation decisions;
• classifying plant population structure by age, growth stage, size and life cycle can be difficult; and
• life history strategies are good rules of thumb but not all that accurate in pre- dicting the population dynamics and impact of plants, especially weeds.
Population dynamics and structure are good concepts to understand, but they need to be developed and studied in the context of ecological interactions and genetic varia- tion. This means it is not enough to under- stand the general patterns of populations.
We should also understand how popula- tions change with genetic diversity, varia- tion in reproduction, and with the presence of competitors, herbivores and disease. In short, population dynamics and structure influence and are influenced by many other factors that we will be discussing in future chapters.
Questions
1. What is known about the population structure and dynamics of your selected species of weed? Suggest ways that that your species can be structured, i.e. by age, size, phenology. Describe the life history strat- egy of your species. Is it an r-or K-selected species – or somewhere in between? Place your species on Grime’s C-S-R model and explain why you placed it there.
2. Describe the size distributions of the four populations shown in Fig. 3.11. Assuming that age is corre- lated with size, what is the likely fate of each of these populations? Explain why. Would your answer change if age were not correlated with size? Explain why.
3. How might metapopulation dynamics be considered in controlling a recently introduced invasive weed?
4. Explain what it means to have a Type I, II or III survivorship curve.
5. How might the carrying capacity (K) of a weed be modified by changes in management practices?
6. Explain why a plant’s population size does not increase indefinitely.
General References
Beeby, A. (1994) Applying Ecology. Chapman and Hall, New York.
Cousens, R. and Mortimer, M. (1995) Dynamics of Weed Populations. Cambridge University Press, Cambridge.
Hanski, I. and Gilpin, M. (1991) Metapopulation dynamics: brief history and conceptual domain.
Biological Journal of the Linnaean Society42, 3–16.
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Introduction
Plants have two general means by which they reproduce: asexually and sexually.
Since humans only reproduce sexually, asexual reproduction (= vegetative repro- duction) is not as familiar to us, but it is rather common in plants. Asexual repro- duction involves the replication of chromo- somes without the production of gametes or the need for sex. Asexual reproduction pro-
duces offspring that are genetically identical to their parents. Typical examples of this form of reproduction are the stolons (‘run- ners’) produced by strawberries (Fragaria species), and root sprouting (‘suckering’) by aspens (Populusspecies). We will discuss asexual reproduction in the next chapter; in this chapter, we focus on sexual reproduc- tion.
As in any organism, plant sexual repro- duction requires the fusion of two gametes (a
© 2003 CAB International. Weed Ecology in Natural and Agricultural Systems 49 (B.D. Booth, S.D. Murphy and C.J. Swanton)