Barbara Rice

Predicting Dispersal Spectra: A Minimal Set of Hypotheses Based on Plant Attributes

1 The dispersal mode adopted by a plant species is frequently associated with other attributes of the plant and its habitat. In this paper we review these associations and present a set of hypotheses which, when considered together, make a probabilistic prediction of the dispersal mode adopted by a plant species. When applied to a species list, the hypotheses can be used to generate a prediction of its dispersal spectrum, i.e. the percentages of different dispersal modes that have been adopted. 2 The formulation of such a set of hypotheses has several purposes: (i) to summarize existing knowledge about dispersal adaptations and their interrelations with other attributes of plants and their habitats; (ii) to couch that knowledge in such a way that falsifiable predictions can be made; (iii) to arrive at provisional conclusions about which factors are the most important in shaping the evolution of dispersal mode in different plants or different environments. 3 The review of relationships between dispersal mode and other attributes of plants and their habitats lead to the following provisional conclusions; (i) seeds larger than 100 mg tend to be adapted for dispersal by vertebrates while those smaller than 0.1 mg tend to be unassisted; most seeds, however, are between 0.1 and 100 mg, and in this range all of the dispersal modes are feasible; (ii) plant growth form and stature (sometimes in relation to the canopy height of the vegetation) seem to exclude certain dispersal modes; (iii) the availability of specific dispersal vectors seems rarely to be an important determinant of dispersal mode; (iv) attributes of the physical environment also seem rarely to be important, except indirectly through their influence on plant stature and seed size.

Seed dispersal spectra: a comparison of temperate plant communities

We compare the dispersal spectra of diaspores from varied plant communities in Australia, New Zea- land, and North America, assigning dispersal mode to each diaspore type on the basis of apparent morpholo- gical adaptations. Species with ballistic and external dispersal modes were uncommon in most communities we surveyed. Ant dispersal was also rather uncommon, except in some Australian sclerophyll vegetation types. The frequency of vertebrate dispersal ranged up to 60% of the flora, the highest frequencies occurring in New Zealand forests. Wind dispersal ranged as high as 70% of the flora, with the highest values in Alaska, but usually comprised 10-30% of the flora. Many species in most communities had diaspores with no special morpholo- gical device for dispersal. Physiognomically similar vegetation types in different biogeographic regions usually had somewhat dissimilar dispersal spectra. The fre- quency of dispersal by vertebrates often increased and the frequency of species with no special dispersal device decreased along gradients of increasing vertical diversity of vegetation structure. Elevation and moisture gradi- ents also exhibited shifts in dispersal spectra. Within Australia, vertebrate- and wind-dispersal increased in frequency along a soil-fertility gradient, and dispersal by ants and by no special device decreased. Habitat breadths (across plant communities) and microhabitat breadths (within communities) for species of each major dispersal type did not show consistent differences, in general. Ant- dispersed species often had lower cover-values than other species in several Australian vegetation types. We discuss the ecological bases of these differences in dispersal spectra in terms of the availability of dis- persal agents, seed size, and other ecological constraints. Seed size is suggested to be one ecological factor that is probably of general relevance to the evolution of dispersal syndromes.

Evolution of the seed plants and inclusive fitness of plant tissues

Here we put forward a new interpretation of the significance of some major features of vascular plant reproductive biology. These features are the integuments of maternal tissue surrounding the offspring, which distinguish seed plants from vascular cryptogams, and the (usually) triploid endosperm produced by double fertilization, which characterizes angiosperms. Maternal integuments and double fertilization are generally regarded as key innovations in plant evolution, and are usually given as the most fundamental characters which distinguish seed plants and angiosperms respectively (Cronquist, 1968; Takhtajan, 1969; Bold, 1973; Foster and Gifford, 1974; Stebbins, 1974; Sporne, 1975; Darlington, 1976). Our interpretation draws together an evolutionary-ecology model on maternal investment, inclusive-fitness calculations from population genetics, and information from comparative plant morphology and embryology. Each of these elements of our interpretation is well-known within the relevant discipline, but the elements have not previously been combined and applied to interpreting plant evolution.1 In this paper we first summarize how mother plants support their offspring in different groups of vascular plants. We show that this support is committed later, with respect to meiosis and fertilization, in more advanced groups of plants. Second, we describe available interpretations, to which our interpretation is an alternative, of the integuments and of double fertilization. Deficiencies of the available interpretations are pointed out, and the points to be explained by our interpretation are defined more exactly. Third, we argue that the advantage achieved by deferring investment in more advanced groups is that mothers can cause a limited total investment to be directed selectively to better offspring genotypes. Fourth, we show that the pattern of investment allocation which maximizes inclusive fitness for the mother is not the same as that which maximizes inclusive fitness for any one offspring. Fifth, we put forward our interpretation, which briefly is that the integuments and endosperm allow the mother to retain control of how her investment is allocated, while deferring the investment until offspring with genotypes different from her own (and therefore with potentially conflicting interests) have been created. Finally, we consider some relatively minor features of comparative plant reproductive biology, which are relevant to assessing our interpretation and other published interpretations.

Seed Size and Plant Growth Form as Factors in Dispersal Spectra

All vascular plant species were listed on eight sites near Sydney, Australia. Four sites were on infertile and four on fertile soil. Each of the 355 species was classified according to seed mass (measured as fresh mass of embryo plus endosperm), morphological adaptations for dispersal by different vectors, and growth form. The infertile—soil sites had more species and more cover adapted for dispersal by ants, and the fertile—soil sites had more species and more cover with fleshy fruits adapted for dispersal by vertebrates. We tested the hypothesis that this difference could be interpreted as a secondary correlate of seed size or of growth form. Species with smaller seed mass of growing to <2 m tall were significantly more likely to be adapted for dispersal by ants relative to vertebrates. An indirect association via growth form was capable of accounting for up to 84% of the relationship between soil type and dispersal mode, and indirect association via seed mass for up to 23% of the relationship; the two together could account for a maximum of 85%. The indirect association association via seed mass relatively weaker because seed masses proved not to be very different between these soil types. There remained a significant residual tendency for species in any given seed size class and growth form to be more likely to be dispersed by vertebrates relative to ants on fertile soils than on infertile soils.

Evidence Against the Hypothesis that Ant-Dispersed Seeds Reach Nutrient-Enriched Microsites

It has been hypothesized that seeds of species adapted for dispersal by ants (myrmecochores) obtain an adaptive advantage because they reach microsites for germination that are nutrient—enriched relative to background levels. This hypothesis has some direct support, mostly from Northern Hemisphere studies, and could also explain the high incidence of myrmecochory in sclerophyll shrubs in Australia and South Africa, which characteristically are found on low—nutrient soils. We therefore tested the hypothesis for a cross section of species in Australian sclerophyll vegetation. Soil close to roots of emerging seedlings of myrmecochores was not higher in total N or available P than soil around emerging seedlings of nonmyrmecochores, or than soil not occupied by any seedling. We therefore reject the nutrient—enriched microsite hypothesis as an adaptive explanation for these species, in this vegetation. See full-text article at JSTOR

Plant species richness at the 0.1 hectare scale in Australian vegetation compared to other continents

New data are reported, and literature data compiled, for species richness in 0.1 ha plots in Australian vegetation. We conclude that on present evidence the same vegetation types are rich, and the same types poor, at a 0.1 ha scale, in Australia as elsewhere. Tropical rainforest averages 140 species per 0.1 ha in permanently humid types. Temperate sclerophyll shrub-dominated types on low-nutrient soils are generally in the range 50–100 species, with open woodlands somewhat richer than scrublands. Warm semi-desert shrublands can have 50–80 species, counting ephemerals both of summer and of winter. Temperate closed forests generally have fewer than 50 species per 0.1 ha. For none of these types is there clear evidence that they are richer or poorer in species at a 0.1 ha scale than types in similar environments with similar growth-form mixes on other continents. We give data for grassy woodlands and sclerophyll scrublands in the monsoonal tropics; the fragments of data on such types available from other continents suggest there may be a wide range of species richness in sub-types of this very broad grouping. Generally, available data do not support the idea that floristic evolutionary history is a strong influence on the species richness of vegetation at the 0.1 ha scale, relative to the influence of the present-day climatic and soil environment.

SPECIES IN THE TAIL OF RANK–ABUNDANCE CURVES

At focal sites within dry sclerophyll woodland and temperate rain forest, species were identified that were of low local abundance and hence in the tail of the rank–abundance curve. We then asked the question: What proportion of tail species within a given community are constitutive members of the tail everywhere throughout their geographical range, versus what proportion are found as substantially more abundant somewhere within their range? Out of 55 tail species identified from dry sclerophyll woodland and 116 tail species identified from temperate rain forest, 91% and 95%, respectively, were significantly more abundant at other locations (“somewhere-abundant” species), versus 9% and 5% “everywhere-sparse” species. Among eight attributes in dry sclerophyll woodland and nine attributes in temperate rain forest compared between somewhere-abundant and everywhere-sparse species, none discriminated consistently between the two groups of species. The size and dispersal morphology of seeds, flowering and fruiting durations and seasons, regeneration strategy after fire, size of geographical ranges, maximum plant height, and size class revealed no consistent distinctions. For the small minority of species that are everywhere-sparse, some general explanation may exist as to why they are in the tail of rank–abundance curves, though none was located among the attributes compared in this paper. For the majority of tail species that are somewhere-abundant, any explanation as to why they are in the tail will need to account for different outcomes in different places.

Plants’ use of ants for dispersal at West Head, New South Wales

Of the world’s known species of myrmecochores, plants which provide food bodies to induce ants to disperse their seeds, many are found in the dry sclerophyll vegetation of Australia. Here we present data and observations on myrmecochory on the West Head, and area of dry sclerophyll vegetation near Sydney, and in the light of these we discuss possible explanations for the distribution of myrmecochory. In most stands myrmecochores made up about 30% of the overall plant species complement, but few of the dominant species were myrmecochores. Myrmecochores tended to occur in a wider range of stands than non-myrmecochores. Within a stand, they tended to occur more of their own diameters from their nearest conspecific neighbours, but otherwise did not occupy detectably different microsites. Many proposed explanations for myrmecochory could explain either its commonness in Australia, or why it should be adaptive in sclerophyll shrubs, but none explain why it should be adaptive in sclerophyll shrubs in Australia but not in California or the Mediterranean. Australian dry sclerophyll vegetation, unlike that of California or the Mediterranean, is delimited by low-phosphorus soils; empirically, the correlation of myrmecochory with low-phosphorus soils is good. Evidence is given to suggest that in the vegetation of the West Head the limiting currency for seed production is phosphorus. Food bodies cost little phosphorus, so that myrmecochory is cheap in terms of the effective currency. However, wings and hairs are also cheap in terms of phosphorus.

Vegetative responses of some Great Basin shrub communities protected against jackrabbits or domestic stock.

We surveyed the vegetation at 19 locations inside and outside 12 exclosures built at various times in Curlew Valley, northern Utah. The exclosures were in semidesert shrub vegetation and included several communities definable by a dominant perennial shrub distribution having sharp boundaries. At the level of the individual quadrat, there was no correlation between the density of any of the abundant annuals and the percentage of the soil surface that was bare, or covered by rock, dead plant matter, or cryptogam crust. The communities as defined by dominants arranged themselves in the order winterfat, shadscale, shadscale and perennial grasses, sagebrush, black sage. These communities are known to be found on progressively less xeric sites. The changes which resulted from protecting samples of these communities from grazers were fairly consistent within each community, but differed among communities; and moreover these changes were not correlated with a trend from more to less xeric sites. Protection against sheep, with or without protection against jackrabbits, did not have very many effects even over 15 years: halogeton generally decreased; peppergrass increased where present; winterfat increased in vigor but not in density where it was dominant. Other dominant shrubs and perennial grasses, did not respond to protection. Protection against jackrabbits had no consistent extra effect on the parameters studied. The classical concept of range succession is that recovery from overgrazing moves a community through secondary succession parallel to a gradient towards relatively more mesic conditions. On the whole, this concept has not been useful in interpreting the results of excluding grazers from these semiarid shrublands.

Evolutionary divergence of leaf width and its correlates

UNLABELLED • PREMISE OF THE STUDY The question why leaf dimensions vary so much between species has long puzzled ecologists. Presumably, variation arises from selective forces acting on leaf function but which selective forces and which leaf functions? This investigation assesses the consistency of divergence in plant traits and habitat variables in association with leaf width divergence in the flora of NSW, Australia.• METHODS More than 80 traits and habitat variables were measured for 25 independent evolutionary divergence events (PICs). Each PIC was represented by two related plant species that had diverged substantially in leaf width. Outgroup species provided indications of the direction of divergence. Most PICs were within genus, so divergences represent relatively recent evolutionary events.• KEY RESULTS No plant traits or habitat variables were 100% consistently associated with a divergence in leaf width, and surprisingly few diverged in a consistent direction significantly more than what might be expected by chance. This surprising lack of consistent divergence with leaf width contrasted with the result that many of these traits and habitat variables were correlated with leaf width across all species in our data set and in line with correlations reported from other studies. Subcategorizing PICs according to the probable direction of leaf width divergence did not improve consistency.• CONCLUSIONS These results indicate that evolutionarily recent leaf width divergence events are not tightly tied to divergences in other leaf traits or in environmental situations, despite the broad correlations that have been observed across many species. Rather, cross species correlations are underpinned by earlier divergence events in the phylogeny.