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

In light, fresh seeds of Barbarea vulgaris germinated to 33–62% at 30:15 and 35:20 °C, and 0–5% at 15:5, 20:10, and 25:15 °C; in darkness, they germinated to 0–5% at all thermoperiods. Seeds were b...

Seasonal changes in the germination responses of buried seeds of Barbarea vulgaris

In southeastern United States, a region where the zonal vegetation is deciduous forests, shallow soils over limestone and dolomitic bedrock support edaphic climax herbaceous plant communities known as cedar glades. The flora includes over 400 species of vascular plants; 79 of these are winter annuals and 47 are summer annuals. This chapter reviews the life cycle ecology of both groups of therophytes in the cedar glade habitat in relation to their tolerances, requirements and adaptations. Temperature, through its influence on seed dormancy, dormancy break and germination, is the single most important environmental factor regulating the timing of the life cycle of annual plants of cedar glades.

Life Cycle Ecology of Annual Plant Species of Cedar Glades of Southeastern United States

Summary: Resume: Zusammenfassung

Seedling emergence of Lamium amplexicaule L. and L. purpureum L. was monitored in field plots tilled at various times during the growing season, and the number of viable seeds in the soil was determined. In plots tilled in early spring, only seeds of L. amplexicaule, germinated, but seeds of both species germinated in the same plots in autumn without further disturbance. Additional seeds of L. amplexicaule, but not of L. purpureum, germinated the following spring. In plots tilled in late spring and summer, seeds of L. amplexicaule germinated in autumn and the following spring, whereas seeds of L. purpureum germinated only in autumn. The number of viable seeds in the top 13 mm layer of soil ranged from 189 to 1216 m−2 for L. amplexicaule and from 131 to 854 for L. purpureum. These field results support those obtained in previous glasshouse-laboratory physiological studies on the annual dormancy cycle in the two Lamium species.



Levee au champ de Lamium amplexicaule L. et L. purpureum L. par rapport au cycle annuel de la dormance des graines



La levee de jeunes plants de Lamium amplexicaule L. et L. purpureum L. a ete observee sur des parcelles cultivees a differentes epoques de la saison de vegetation et le nombre de graines viables au sol a ete determine. Sur les parcelles cultivees en debut de printemps, seules les graines de L. amplexicaule ont germe, mais des graines des deux especes ont germe en automne sur les memes parcelles, sans cultivation ulterieure. Au printemps suivant, une nouvelle germination a ete constatee chez L. amplexicaule mais pas chez L. purpureum. Sur les parcelles cultivees en fin de printemps et en ete, des graines de L. amplexicaule ont germe en automne et aussi au printemps suivant, tandis que les graines de L. purpureum n'ont germe qu'en automne. Le nombre de graines viables dans les premiers 13 mm de la surface du sol allait de 189 a 1216 m−2 pour L. amplexicaule et de 131 a 854 pour L. purpureum. Ces resultats sur le terrain confirment ceux obtenus dans des etudes physiologiques sur le cycle annuel de dormance chez les deux especes de Lamium, menees prealablement en serre et au laboratoire.



Ueber dus Auflaufen von Lamium amplexicaule L. und L. purpureum L. in Bezug auf den Jahrlichen Zyklus der Samenruhe unter Feldbedingungen



In Parzellen, die zu verschiedenen Zeitpunkten wahrend der Vegetationsperiode einer Boden-bearbeitung unterworfen worden waren, wurde sowohl das Auflaufen der Keimlinge von Lamium amplexicaule L. und L. purpureum L. registriert, als auch die Anzahl lebenfahiger Samen festgestellt. Nach Bodenbearbeitung im fruhen Fruhling keimten nur Samen von L. amplexicaule; im Herbst keimten in denselben Parzellen jedoch Samen beider Arten, sofern der Boden nicht mehr bewegt worden war. Weitere Samen von L. amplexicaule, nicht aber von L. purpureum, keimten im folgenden Fruhling. In Parzellen, die im spaten Fruhling und im Sommer bearbeitet worden waren, keimte L. amplexicaule im Herbst und im folgenden Fruhling. Wahrend L. purpureum nur im Herbst keimte. Die Anzahl lebensfahiger Samen m−2 in den obersten 13 mm des Bodens bewegte sich zwischen 189 und 1216 fur L. amplexicaule und zwischen 131 und 854 fur L. purpureum. Diese Feldresultate bestatigen Ergebnisse von Gewuchshausstudien uber den jahrlichen Zyklus der Samenruhe der beiden Lamium-Arten.

Field emergence of Lamium amplexicaule L. and L. purpureum L. in relation to the annual seed dormancy cycle

Although seed dormancy of temperate legumes is well understood, less is known about it in species that grow in subalpine/alpine areas. This study investigated dormancy and germination of four Vicia species from the Tibetan Plateau. Fresh seeds of V. sativa were permeable to water, whereas those of V. angustifolia, V. amoena and V. unijuga had physical dormancy (PY). One year of dry storage increased the proportion of impermeable seeds in V. angustifolia, but showed no effect on seed coat permeability in V. amoena or V. unijuga. Seeds of all four species also had non-deep physiological dormancy (PD), which was especially apparent in the two annuals at a high germination temperature (20 degrees C). After 1 year of storage, PD had been lost. The hydrotime model showed that fresh seeds obtained a significantly higher median water potential [Psi(b)(50)] than stored seeds, implying that PD prevents germination in winter for seeds dispersed without PY when water availability is limited. After 6 months on the soil surface in the field, a high proportion of permeable seeds remained ungerminated, further suggesting that PD plays a key role in preventing germination after dispersal. Addition of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis, evened-out the differences in germination between fresh and stored seeds, which points to the key role of ABA biosynthesis in maintaining dormancy. Further, fresh seeds were more sensitive to exogenous ABA than stored seeds, indicating that storage decreased embryo sensitivity to ABA. On the other hand, the gibberellic acid GA(3) increased germination rate, which implies that embryo sensitivity to GA is also involved in seed dormancy regulation. This study showed that PY, PD or their combination (PY + PD) plays a key role in timing germination after dispersal, and that different intensities of dormancy occur among these four Vicia species from the Tibetan Plateau.

Seed dormancy in four Tibetan Plateau Vicia species and characterization of physiological changes in response of seeds to environmental factors

Fourteen seedlots of five species of Rhus were surveyed for presence/absence of physiological dormancy and/or for germination requirements of non-dormant seeds. Physiological dormancy was present in the four seedlots of R. aromatica studied, but not in either of the two seedlots of its close relative R. trilobata, which is in contrast to previous reports. Neither were seeds of R. glabra, R. typhina, nor R. virens physiologically dormant. Stratification at 5oC for 1 week or incubation in 500 or 1000 mg/l solutions of gibberellic acid broke physiological dormancy in > 90% of the R. aromaticaseeds. Maturation desiccation acted as a switch from a developmental to a germinative mode in R. aromatica embryos, whereas it was not required for germination of R. glabra or R. virens (R. trilobata and R. typhinanot tested). Seeds of all five species incubated on a moist substrate became fully imbibed in 2 d, at which time moisture content was approx. 70–80%of their initial weight. In general, germination of non-dormant seeds was rather insensitive to temperature and light. Seeds germinated equally well in light and in darkness over a daily (12 h/12 h) temperature range of 15/6–35/20oC. Over a 4 week period, the best germination percentages were obtained at 25/15 and 20/10oC, whereas 35/20oC appeared to be supraoptimal, though not always significantly so. If the incubation period was extended to 30 weeks, germination percentages were as high at 15/6oC as at 25/15 and 20/10oC.

Jerry M. Baskin

Papers

Seasonal changes in the germination responses of buried seeds of Barbarea vulgaris

Life Cycle Ecology of Annual Plant Species of Cedar Glades of Southeastern United States

Field emergence of Lamium amplexicaule L. and L. purpureum L. in relation to the annual seed dormancy cycle

Seed dormancy in four Tibetan Plateau Vicia species and characterization of physiological changes in response of seeds to environmental factors

Physiological dormancy and germination requirements of seeds of several North American Rhus species (Anacardiaceae)