Avi Shmida

Biological determinants of species diversity

We consider four categories of biological mechanisms of deter- minants which cause and maintain species diversity: niche relations, habitat diver- sity, mass effects and ecological equivalency. Two of these determinants are origi- nal to this paper: mass effect, the establishment of species in sites where they cannot be self-maintaining; and ecological equivalency, the coexistence of species with effectively identical niche and habitat requirements. The mode of action and ecological implications of each biological determinant are discussed using a schematic method for measuring alpha (community), beta (differentiation), and gamma (regional) diversities. The importance of mass effects and ecological equivalency to species richness is documented with several types of field data from Israel and California, U.S.A. Floristic richness and, in particular, the richness of floristic transitions, are discussed and interpreted by use of the biological determinants of diversity. Contact transitions between distinct floras are rich predominantly because of mass effects. Transitions induced by marked environmental changes are rich because of the combined influences of habitat diversity and mass effects. The rate at which species richness increases with sample area is related to the combined effects of all four biological determinants. This complexity explains the failures of simple species-area models. The relative intensity of each determinant is related to area: niche relations are most important at within-community scales, habitat diversity most important at both within-community and land- scape scales, and ecological equivalency most important at regional scales. We suggest that understanding patterns of species diversity will be enhanced by the partitioning of total species richness into the richness caused by each of the four ecologically distinct determinants of diversity.

Coexistence of plant species with similar niches

In the context of a simple mathematical model, we derive several mechanisms whereby plant species can coexist in a community without differing in their trophic niches (their relations with habitats, resources and exploiters). The model is based on the dynamics of species turnover in microsites, and incorporates localized competition, non-uniform seed dispersal and aspects of spatiotemporal environmental heterogeneity. These factors, which are not included in most standard competition models, allow stable coexistence of trophically equivalent species due to: (a) Differences in life-history ‘strategy’. (b) Input of seeds from nearby habitats (spatial Mass Effect). (c) Differences in demographic responses to environmental fluctuations (temporal Mass Effect). (d) Turnover in species composition between different habitat patches.

Ultraviolet as a Component of Flower Reflections, and the Colour Perception of Hymenoptera

Based on the measurements of 1063 flower reflection spectra, we show that flower colours fall into distinct clusters in the colour space of a bee. It is demonstrated that this clustering is caused by a limited variability in the floral spectral reflectance curves. There are as few as 10 distinct types of such curves, five of which constitute 85% of all measurements. UV reflections are less frequent and always lower in intensity than reflections in other parts of the spectrum. A further cluster of colour loci is formed in the centre of the colour space. It contains the colour loci of green leaves, several other background materials and only very few flowers. We propose a system to classify the reflection functions of flowers, and a set of colour names for bee colours.

Why are adaptations for long-range seed dispersal rare in desert plants?

SummaryThe rarity of long-range seed dispersal (telechory) and commonness of antitelechory in desert plants are examined in light of contemporary mathematical theories of the evolution of dispersal and germination behaviors. Analysis of dispersal 3-habitat relationships in the flora of Israel supports the general trend towards atelechory in deserts; in particular epizoochory and tumbleweeds are practically absent from the desert and heterocarpy is centered in the Mediterranean region. In contradiction to the accepted “mother-site” theory, we find that (a) there is a high turnover in microscale spatial pattern among antitelechoric species; (b) antitelechoric (especially basicarpic) species are widespread and dominant in the desert vegetation of Israel; (c) amphicary and geocary are rare in the desert flora of Israel.We argue that the openness of desert vegetation and the patterns of climatic variation favor atelechory while antitelechory is generally a side-effect of mechanisms whose adaptive value is not directly related to dispersal. Thus for example the desert plants of Israel have evolved a variety of dispersal-restricting seed-containers that protect the seed from predation and flooding, regulate the within-season timing of germination, and spread dispersal and germination over several years.

Pattern and Biological Microsite Effects in Two Shrub Communities, Southern California

Strip transects of 100 contiguous, aligned quadrats were sampled during the spring blooming in a creosotebush (Larrea tridentata) semidesert and a chamise (Adenostomafasciculatum) chaparral, with species covers in quadrats recorded. Techniques of pattern analysis included recip- rocal averaging ordination of quadrats, measurements of pattern diversity and pattern periodicity, and species association and contagion. In both communities the first ordination axis expressed a strong pattern of differentiation of the herb flora from shrub centers to openings; second and third axes expressed other responses of herb species to shrub species and shrub cover. The pattern axis represents a principal direction of niche difference to which most species responded and along which several pairs of congeners were separated. Pattern periodicities were 6 m (weakly defined) in the semidesert and 9 m in the chaparral; pattern diversities were 1.4 and 1.8 half-changes. Quadrat species richness was highest in the transitions between shrub clumps and openings in the semidesert, but higher in the openings and lower under the shrubs of the chaparral. Overall alpha diversity resulted from roughly comparable contributions of point or small-quadrat diversity, pattern diversity of more common species, and rare species. The importance of biological modification of microsites for pop- ulation function and niche relationships is suggested by species responses to the primary pattern axis, the responses of some herb species to particular shrub species, and the indicated allelopathic effects in the chaparral.

Adaptive phenology of desert and Mediterranean populations of annual plants grown with and without water stress.

SummaryThe dynamics of vegetative and reproductive growth were compared in matched pairs of Mediteranean and desert populations of three unrelated annual species, Erucaria hispanica (L.) Druce, Brachypodium distachyon (L.) Beauv. and Bromus fasciculatus C. Presl., under high and low levels of water availability in a common-environment experiment. Plants of all desert populations showed earlier switches to reproductive development and to subsequent phenophases, and the transition to flowering occurred at smaller plant sizes. Water stress had no effect (E. hispanica) or slightly accelerated the transition to flowering in B. fasciculatus (by 1–2 days) and in B. distachyon (by 4–6 days). Plant senescence was strongly enhanced by water stress, and this enhancement was greater in desert populations than in corresponding Mediterranean ones. Duration of life cycle was greatly shortened by water stress in all three species. Desert and Mediterranean populations of the three species exhibited small differences in their relative response, i.e. phenotypic plasticity, to water stress for phenological and plant size parameters. In E. hispanica and B. fasciculatus the population x water regime interaction amounted to less than 3% of total variance. By contrast, the Mediterranean population of B. distachyon was much more plastic in its response to water stress than the desert population in its transition to plant senescence. Plants from the desert populations appeared to be adapted to shorter, more compact growth cycles, culminating in earlier dates of seed maturation and plant senescence. In addition, they showed larger phenotypic plasticity in the transition to plant senescence, which trait was enhanced or magnified by sustained or repeated lack of water. By contrast, plants from Mediterranean populations delayed switchover from one phenophase to the next, seeming thus to “bet” on more water being forthcoming.

Spatial scale and the determinants of plant species richness

Traditional ecological theory has stressed the importance of competitive interactions in regulating species richness. Recent research has transcended this viewpoint by considering the role of stochastic processes, mosaic phenomena and nonequilibrium conditions in the regulation of richness. This growing body of work indicates that the determinants of plant species richness may vary predictably over different spatial scales.

WHITTAKER'S PLANT DIVERSITY SAMPLING METHOD

ABSTRACT A nested vegetation sampling method which was developed by R.H. Whittaker for comparison of plant species diversity is described. The species richness of an area of 0.1 ha (1 dunam; 20 × 50 m) is analyzed by a special procedure in which different parameters of diversity such as differential diversity, point diversity, equibility and dominance are recorded. Normal supplementary observations such as plant coverage, growth forms, phenology and vertical foliage profile are added to the sample. By standardization of data collection for diversity samples, different plant communities from different regions of the world can be compared.

RED BOWL-SHAPED FLOWERS: CONVERGENCE FOR BEETLE POLLINATION IN THE MEDITERRANEAN REGION

ABSTRACT Anemone coronaria, Papaver rhoeas, Ranunculus asiaticus, and Tulipa agenensis are pollinated primarily by scarabaeid beetles (Amphicoma, Glaphyridae) and secondarily by bees (Lasioglossum marginatum Br., Halictidae, and Synhalonia plumigera Kohl, Anthophoridae). The four plant species have large bowl-shaped flowers which are orange-red in color with a black center, radial symmetry, weak scent (to humans), and filamentous stamens. It is suggested that there is a convergent evolution of red bowl-shaped flowers in the East Mediterranean region, concordant with a center of diversity in the genus Amphicoma. The floral phenology of these “Poppy guild” species correlated positively with the amount of edible pollen produced by each of them and with the visiting frequencies of their pollinators. Field experiments showed that Amphicoma beetles also preferred red, odorless flower models over odorless models of different colors, and models with a dark center over plain red ones. This evidence extends the cla...

The Evolution of Flower Display and Reward

The evolution of the allocation of resources for display and reward in flowers is critically analyzed. We construct models which take into account the foraging behavior and preferences of pollinators and their effects on the pollination success of the plants. The resulting equilibrium ESS (evolutionary stable strategy) of the flowers—pollinators community is analyzed at both the short-term ecological level and the long-term coevolutionary level.