Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

The Theory of Island Biogeography

SUMMARY
1According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities.
2Most plant communities are longer-lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all-important to the argument presented.
3Several mechanisms not involving regeneration also contribute to the maintenance of species-richness:


(a). differences in life-form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence-relationships,
(b) differences in phenology coupled with tolerance of suppression,
(c) fluctuations in the environment coupled with relatively small differences in competitive ability between many species,
(d) the ability of certain species-pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition,
(e) the production of substances more toxic to the producer-species than to the other species,
(f) differences in the primary limiting mineral nutrients or pore-sizes in the soil for neighbouring plants of different soecies, and
(g) differences in the competitive abilities of species dependent on their physiological age coupled with the uneven-age structure of many populations.


4The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species-rich communities known.
5In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance.
6The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2.
7The definition of a plant's niche requires recognition of four components:


(a) the habitat niche,
(b) the life-form niche,
(c) the phenological niche, and
(d) the regeneration niche.


8A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche.
9All stages in the regeneration-cycle are potentially important and examples of differentiation between species are given for each of the following stages:


(a) Production of viable seed (including the sub-stages of flowering, pollination and seed-set),
(b) dispersal, in space and time,
(c) germination,
(d) establishment, and
(e) further development of the immature plant.


10In the concluding discussion emphasis is placed on the following themes:


(a) the kinds of work needed in future to prove or disprove that differentiation in the regeneration niche is the major explanation of the maintenance of species-richness in plant communities,
(b) the relation of the present thesis to published ideas on the origin of phenological spread,
(c) the relevance of the present thesis to the discussion on the presence of continua in vegetation,
(d) the co-incidence of the present thesis and the emerging ideas of evolutionists about differentiation of angiosperm taxa, and
(e) the importance of regeneration-studies for conservation.

The maintenance of species-richness in plant communities: the importance of the regeneration niche

Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.

/pdf/seed-dormancy-and-the-control-of-germination-4tdhm2985s.pdf

Seed dormancy and the control of germination

The proposal is made that seed scientists need an internationally acceptable hierarchical system of classification for seed dormancy. Further, we suggest that a modified version of the scheme of the Russian seed physiologist Marianna G. Nikolaeva be adopted. The modified system includes three hierarchical layers – class, level and type; thus, a class may contain levels and types, and a level may contain only types. The system includes five classes of dormancy: physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY) and combinational dormancy (PY + PD). The most extensive classification schemes are for PD, which contains three levels and five types (in the non-deep level), and MPD, which contains eight levels but no types. PY is not subdivided at all but probably should be, for reasons given. Justifications are presented for not including mechanical dormancy or chemical dormancy in the modified scheme. PD (non-deep level) is the most common kind of dormancy, and occurs in gymnosperms (Coniferales, Gnetales) and in all major clades of angiosperms. Since, first, this is the class and level of dormancy in seeds of wild populations of Arabidopsis thaliana and, secondly, Type 1 (to which seeds of A. thaliana belong) is also common, and geographically and phylogenetically widespread, it seems that biochemical, molecular and genetic studies on seed dormancy in this model species might have rather broad application in explaining the basic mechanism(s) of physiological dormancy in seeds.

/pdf/a-classification-system-for-seed-dormancy-33n2carxeu.pdf

A classification system for seed dormancy

Maternal effects in plants were recognized as long ago as 1909 (32). Recent evidence, primarily over the last 15-20 years, shows that maternal effects can contribute substantially to the phenotype of an individual, and as we show, this has important consequences for the interpretation and' design of both ecofogical and genetic studies. Following a discussion of the consequences of maternal effects and an analysis of the different ways these effects can be estimated, we review the evidence for maternal effects from the fields of physiological ecology, crop science, and quantitative genetics. We do not review all of the literature because that would be a monumental task; rather, we focus on representative studies from each of these fields. It is our contention that despite evidence that maternal effects can have a large in­ fluence on offspring phenotype, few detailed studies have identified the specific causes of maternal effects, particularly in natural populations.

Maternal effects in plants

Dormancy in seeds of Dodonaea viscosa is due to a water-impermeable seed coat (physical dormancy, PY). Thus, mechanically scarified seeds imbibed water (c. 95% increase in mass) and germinated to high percentages over a wide range of temperature regimes in both white light and darkness, whereas non-scarified seeds did not take up water. Dry heat at 80‐160°C and dipping in boiling water for 1‐60 s also broke dormancy in a high percentage of the seeds, and continuous far-red light was not inhibitory to germination. However, dry storage in the laboratory for >1 year did not overcome dormancy. Seeds made water-permeable by boiling imbibed water, and thus germinated, at a much slower rate than those made water-permeable by mechanical scarification. We suggest that boiling opened the ‘water gap’ in the seed coat (not yet described in Sapindaceae but present in other taxa with PY) and that water entered the seed only through this small opening, thereby accounting for the slow rate of imbibition and subsequent germination. Physical dormancy has now been shown to occur in seeds of this polymorphic, worldwide species from Australia, Brazil, Hawaii, Mexico and New Zealand. The low level of dormancy reported for seed lots of D. viscosa in China, India and Pakistan is probably due to collection of seeds before they dried to the critical moisture content for development of water-impermeability of the seed coat. Germination of non-dormant seeds over a wide range of temperatures and in white light, far-red (leaf-canopy shade) light and darkness are part of the germination strategy of D. viscosa and of other taxa whose seeds have PY at maturity.

Physical dormancy in seeds of Dodonaea viscosa (Sapindales, Sapindaceae) from Hawaii

Buried seeds of Arabidopsis thaliana exposed to natural seasonal temperature changes exhibited an annual dormancy/nondormancy cycle. The majority of freshly matured seeds were dormant. During burial in summer, fresh seeds and those that had been buried for 1 and 2 yr afterripened. During afterripening, seeds first germinated at 15/6 C and then at progressively higher temperatures until they were nondormant by October, when they germinated to high percentages at all thermoperiods except 35/20 C. Light was required for germination of both conditionally dormant and nondormant seeds. During autumn and winter, seeds reentered dormancy, first losing the ability to germinate at high and then at low temperatures. Thus, seeds can germinate in the field only in autumn, and germination is restricted to those seeds on or near the soil surface where both light and moisture can be nonlimiting In spring, seeds are dormant; in summer, temperatures are too high for germination, and in winter, they are too low. Consequentl...

Seasonal changes in the germination responses of buried seeds of arabidopsis thaliana and ecological interpretation

http://sourcedb.ib.cas.cn/cn/ibthesis/200811/P020090427606372677096.pdf

Germination of dimorphic seeds of the desert annual halophyte Suaeda aralocaspica (Chenopodiaceae), a C4 plant without Kranz anatomy.

(1) Wild parsnip (Pastinaca sativa L.) is a monocarpic perennial herb which grows as a weed in North America in grassy habitats that are usually mown in summer. Flowering occurs in May and June, and seeds are mature by early July. Seeds remain attached to the dead erect shoots until autumn, unless the plants are disturbed, as by mowing. (2) At maturity small numbers of seeds can germinate at summer temperatures. As the seeds after-ripen during summer their ability to germinate at high temperatures increases, but there is little increase in ability to germinate at autumn temperatures. Stratification (moist chilling) over winter lowers the temperature requirement for germination, and therefore the majority of seeds in the field germinate in early spring. (3) Seedling mortality is high; of 3980 seedlings marked in a natural population in Kentucky, only thirty-three (0.83%4) flowered before dying. Cohorts of seedlings marked in August, November, December, January and April had concave survivorship curves; cohorts marked in February and March initially had concave survivorship curves, but the curves subsequently became linear. (4) Plants produce rosettes and grow until the second, third, fourth or fifth growing season before producing flowers. Vernalization is required for flowering, and plants reach the flowering stage quicker under long than under short days. (5) An old-field population showed a wide range of individual plant sizes in late May: flowering plants had root-crown diameters (RCDs) of 5-39 mm, and non-flowering plants had RCDs of < 1 (first-year seedlings) to 10 mm. It is suggested that plants must reach a critical size before they can respond to the environmental stimuli of vernalization and photoperiod.

Studies on the autecology and population biology of the weedy monocarpic perennial, Pastinaca sativa

Freshly matured seeds of Eriogonum abertianum Torr. and Eriastrum diffusum (Gray) Mason and those of both species exhumed after various periods of burial in the Chihuahuan Desert in Arizona, USA, were tested for germination in light and darkness at five thermoperiods. Seeds of Eriogonum were nondormant (ND) at maturity in autumn 1988, and thus 87% of them germinated in light at 15/6 and at 20/10°C. Seeds entered conditional dormancy (CD) in winter, germination at 20/10°C decreased to 6% in light. However, a high percentage of the seeds were again ND by autumn 1990. Thus, like seeds of facultative winter annuals in humid, temperate regions, those of Eriogonum have an annual CD/ND cycle, which is characteristic of annuals in unpredictable habitats (...)

https://www.jstor.org/stable/info/2261533

Jerry M. Baskin

Papers

Physical dormancy in seeds of Dodonaea viscosa (Sapindales, Sapindaceae) from Hawaii

Seasonal changes in the germination responses of buried seeds of arabidopsis thaliana and ecological interpretation

Germination of dimorphic seeds of the desert annual halophyte Suaeda aralocaspica (Chenopodiaceae), a C4 plant without Kranz anatomy.

Studies on the autecology and population biology of the weedy monocarpic perennial, Pastinaca sativa

Annual seed dormancy cycles in two desert winter annuals