Genetic diversity across a vertebrate species' range: a test of the central-peripheral hypothesis

Although it has been long presumed that population genetic variability should decrease as a species' range margin is approached, results of empirical investigations remain ambiguous. Sampling strategies employed by many of these studies have not adequately sampled the entire range. Here we present the results of an investigation of population genetic diversity in a vertebrate species, the Italian agile frog, Rana latastei, sampled comprehensively across its entire range. Our results show that genetic variability is not correlated with population location with respect to the range periphery. Instead, the model that best explains the genetic variation detectable across the range is based on an east-to-west gradient of declining diversity. Although we cannot state definitively what has led to this distribution, the most likely explanation is that the range of Rana latastei expanded postglacially from a Balkan refugium.     

Tree species diversity in small, tropical riparian forest fragments in Belize, Central America

Tree species diversity was measured in a network of very small galleryforests within the Mountain Pine Ridge savanna in Belize. Research focussed onforest patches smaller than 1 ha in size (micro-forests) and linearstrips of trees along creeks lacking interior core zones with low understoreylight levels (tree thickets). Twenty-five micro-forests and 51 tree thicketsites were sampled throughout the savanna. A total of 144 morphospecies 5cm dbh (106 in micro-forests and 117 in tree thickets) werefound, which represents 1/5 of the approximately 700 native tree species in Belize.Most (85.3%) of the species encountered are typically found in tropical rainforests and few are restricted to savanna or riparian environments. Speciesaccumulated at a much faster rate in micro-forests than in tree thickets. Onlyone species, the palm Acoelorraphe wrightii, was extremelyabundant, accounting for almost 30% of all stems. Many of the species werepresent in very low densities: 19% of all species found in micro-forests and 42%of those found in tree thickets had on average one or fewer stems per hectare. Alarge proportion of species were also found infrequently across the landscape,being present in only 36% of micro-forests and at 52% of tree thicket sites. Theresults indicate that networks of very small forest patches can contain highnumbers of species and could therefore contribute to the maintenance of regionalbiodiversity.     

Japanese maple (Acer palmatum var. Matsumurae, Aceraceae) recruitment patterns: seeds, seedlings, and saplings in relation to conspecific adult neighbors

We analyzed the spatial patterns among seeds, seedlings, saplings, and conspecific adult trees of the cool-temperate tree species Acer palmatum var. Matsumurae in a conifer-hardwood mixed forest in northern Japan, using two models that consider the influence of each adult within the neighborhood of the offspring. The results showed that recruitment patterns of each stage could be characterized and that significant shifts occur between successive stages. Sound seeds were more widely dispersed than unsound seeds; the mean dispersal distance (MDD) was 41.5 m for sound seeds, but only 12.6 m for unsound seeds. Most seedlings were located near conspecific adult trees, with a MDD of 14.3 m. Saplings, however, were more dispersed away from conspecific adult trees, with an MDD of more than 35 m. Light and gap distributions did not strongly affect the spatial distribution of the offspring; most saplings were located under nonconspecific canopies. These results suggest that the recruitment pattern of Japanese maple offspring is strongly affected by conspecific adult neighbors, rather than by light and gap distributions, with close proximity to conspecific adult trees reducing the growth and survival of seedlings during the transition to saplings.     

Clonal structure and patterns of allozyme diversity in the rare endemic Cycladenia humilis var. jonesii (Apocynaceae)

The rare endemic Cycladenia humilis var. jonesii (Jones cycladenia) has low levels of sexual reproduction. Enzyme electrophoresis was used to explore possible causes of low seed set and high fruit abortion by assessing the clonal structure and genetic diversity in populations. The seven populations studied were composed of multiple, highly interdigitated clones; thus low fruit set is not likely to be due to a scarcity of mates. Genotype frequencies did not differ significantly from Hardy-Weinberg proportions, suggesting that populations are not highly inbred. Jones cycladenia exhibited high levels of genetic diversity at both the population level (A = 1.7; P = 37; He = 0.14) and the taxon level (A = 2.7; P = 60) in comparison to other plants. These data suggest that genetic drift is unlikely to have left this taxon genetically depauperate. Furthermore, we detected little divergence among geographically disjunct populations of Jones cycladenia (FST = 0.10). In comparison, Jones cycladenia populations were highly differentiated from a population of the taxon's close relative, C. h. var. humilis (mean genetic identity = 0.76). Our study suggests that other reasons for low fruit set in Jones cycladenia, such as resource or pollinator limitation, or genetic load, should be explored in future research.     

Cryptic clades, fruit wall morphology and biology of Andira (Leguminosae: Papilionoideae)

Andira comprises 29 species distributed throughout tropical America, with two subspecies in Africa. Its fruits are unusual for a papilionoid legume because they are drupes. The majority of species have fruits dispersed by bats, but eight species have larger fruits dispersed by-rodents. Some fruits of both dispersal types are secondarily dispersed by water. Cladistic analysis of chloroplast DNA (cpDNA) restriction site characters discovered four well-supported clades of Andira species. None of these 'cryptic' clades had been recognized by previous workers, because they are not apparently marked by any known morphological innovations. This prompted a search for new characters that might support these groupings. An anatomical study of fruit walls of 25 Andira species revealed the presence of three principal endocarp types, dominated by (1) parenchyma, (2) fibres, or (3) stone cells. These features arc best coded as a single unordered multistate character. When incorporated into a simultaneous cladistic analysis of previously gathered molecular and morphological data, states of this endocarp character are shown to be apomorphies for two of the well-supported clades evident in the cpDNA restriction site data. The most likely plesiomorphic state for the endocarp is parenchyma-dominated. Thicker, stronger endocarps of fibres and stone cells may have evolved in response to the need to protect the seed from predators.     

Application of Live Monocells from Macroalgae to Shellfish Seed Production

Monocells were isolated from several macroalgae, Porphyra yezoensis, Undaria pinnatifida, and Laminaria japonica, by digestion with alga-tool enzymes. The monocells were then used to feed the parents or larvae of bay scallop Argopecten irradians, blood cockle Arca inflata, and abalone Haliotis discus juveniles. Results showed that the parents of bay scallop and blood cockle fed with Porphyra monocells could mature and discharge eggs and spermatozoa and their larvae could metamorphose; the survival rate of abalone juveniles fed with isolated cells from Laminaria and Undaria increased by 100% compared with that of those fed with artificial food. Received October 2, 1997; accepted December 18, 1998.     

Local colonization and extinction of field layer plants in a deciduous forest and their dependence upon life history features

Abstract. In this study we used species inventory data collected in 1970 and 1993 from 132 plots in a 14.5ha deciduous forest to examine local extinction and colonization processes among 45 field layer species. Local colonization rate was positively related to both seed size and seed dispersal features. Local extinction rate was negatively related to seed size. Growth form (clonal vs. non-clonal) and presence of a seed bank were not found to be associated with local dynamics. Despite an overall constancy in species composition during this period, plants exhibited a considerable mobility among the 132 plots. This pattern conformed to a suggested ‘carousel model’ of species mobility in grasslands. A tentative suggestion is that this mobility acts on a comparatively broader spatio-temporal scale in deciduous forests as compared with grasslands. Additional data are presented indicating that species abundance (frequency) among field layer plants in deciduous forest communities is consistent among forest fragments, and when comparing local and regional scales. The main conclusion is that life history features of the seed dispersal/recruitment phase, particularly seed size, are causally associated with abundance patterns at least at a local scale, but possibly also on a broader regional scale.     

Recruitment and survival of native annual Trifolium species in the highlands of Ethiopia

Germination and survival of indigenous annual Trifolium species were studied. Seedlings naturally emerging in the field and from sown seeds in pots were regularly counted and uprooted, and survival was studied by monitoring colour‐coded seedlings. Differences in recruitment of Trifolium species were strongly related to the rainfall pattern. In the fallow (crop) lands, no seedling survived the dry season between the short and main rainy periods during the year, while in the natural pasture, 8% of the seedlings survived into the main growing (rainy) season. Occasional rains occurring in the dry period (between the two rainy periods) also induced successions of germination and seedlings death, and therefore depleted the soft seed reserve in the soil by the beginning of the main growing season. As temperature fluctuations were minimal during the main rainy season, the rate of seed softening was low, affecting new germinations. This had a significant impact on the quality of the natural pastures on which livestock in the highlands are dependent.