Explaining patterns of spatial variation in species composition is a major challenge facing ecologists. While many studies have focused on patterns of species richness and their causal factors, the spatial change in species composition has received far less attention (but see Harrison et al. ABSTRACTAim The aim of this paper is to evaluate the combined effects of geographical distance and environmental distance on patterns of species similarity (similarity in species composition between sites), and to identify factors affecting the rate of decay in species similarity with each type of distance.Location Israel.Methods Data on species composition of land snails and land birds were recorded in 27 sites of 1 · 1 km scattered across a rainfall gradient in Israel. Matrices of similarity in species composition between all pairs of sites were computed and analysed with respect to corresponding matrices of geographical distance and rainfall distance (defined as the difference in mean annual rainfall between sites, and used as a measure of environmental distance). Mantel tests were applied to determine the correlation between species similarity and each type of distance. Factors affecting the decay in species similarity were investigated by comparing different subsets of the data using randomization tests.Results Both rainfall distance and geographical distance had negative effects on species similarity. The effect of rainfall distance was statistically significant even after controlling for differences in geographical distance, and vice versa. The perunit effect of rainfall distance on species similarity decreased with increasing geographical distance, indicating that the two types of distances interacted in determining the similarity in species composition. Snails showed a higher rate of decay in species similarity with geographical distance than birds, and large snails showed a higher rate of decay than small snails, which are better passive dispersers. The per-unit effects of both rainfall distance and geographical distance on species similarity were higher in the desert region than in the Mediterranean region. Analyses focusing on a grain size of 10 · 10 m showed a lower similarity in species composition and a lower rate of decay in species similarity with rainfall distance than analyses carried out at a grain size of 1 · 1 km.Main conclusions Patterns of similarity in species composition are influenced by the combined effects of environmental variation, the position of the area along environmental gradients, the dispersal properties of the component species, and the scale (both spatial extent and grain size) at which the patterns are examined.
The rough periwinkle Littortnu scrxutilis is considered, in current literature. to consist of various sympatric "subspecies" and "varieties". These, it has been suggested, are incipient species and are evidence for sympatric speciation.This paper shows that in Wales the "saxatilis" aggregate consists of four separate. fully sympatric species: L. rudis. putufu, nigrolineuru and neglecta. Differences between these species include proportions, sculpture and size of the shell, range of colour patterns. the anatomy of the genitalia, and isozyme patterns. The possible evidence for incipient species is discussed and rejected.The diversity of penis form amongst the British winkles is remarkable, and it is suggested that in these sympatric species the structure of the penis could be of importance in species recognition.The four species occupy different zones on the shore and three certainly differ also in their methods of reproduction: L. rudir is ovoviviparous and contains embryos throughout the year, purulu is ovoviviparous but contains embryos only in winter, nigrolineuzu is oviparous. The reproduction and dispersal methods of all British winkles are reviewed. The evidence available supports the view that non-planktonic dispersal is an adaptation which, in species whose shell colour varies considerably from one locality to another, enables each population to become adapted to its local environmental. conditions. Ovoviviparity occurs in high3hore non-planktonic species, and may be an adaptation to avoid the dangers of egg desiccation.Planktonic dispersal is found in species whose shell d o u r docs not vary as much as in non-planktonic ones. In these species, it is argued that a common planktonic pool enables a rapid re-colonization in the case of local decrease in population numbers, and also the rapid dispersal of successful mutations.
The Red Queen hypothesis predicts that sex should be more common in populations heavily infested with parasites, than in those without. This hypothesis was investigated in the aquatic snail Melanoides tuberculata, in which both sexual and parthenogenetic individuals exist in natural populations, and some populations are heavily infested by trematodes. The presence of fertile males and the higher genetic diversity of bisexual populations are indicative of sexual reproduction. We compared sites in 1990, 1999, and 2001, and we looked for a positive correlation between male and parasite frequencies. Male frequency was not correlated with the frequency of individuals infected by trematodes. This lack of correlation was reconfirmed in a retrospective power analysis. In a period of 9 years, male frequencies decreased but infection levels increased. These results do not support the Red Queen hypothesis. In samples with high male frequency the number of embryos was low, perhaps indicating that males may have a negative effect on embryo numbers. This effect of males on fitness could perhaps suggest that the cost of sex is fewer embryos. The reduction in embryo numbers may also represent a trade‐off between mating and egg production costs.
Abstract. This study describes and exemplifies a multivariate approach based on geographical information system (GIS) technology for the analysis of faunal responses to climatic gradients. The approach is particularly suitable for the analysis of museum collections, and it combines the data reduction capabilities of multivariate techniques (ordination and classification) with GIS tools for the manipulation of geographically referenced data. The specific steps of the analysis are (1) construction of a grid covering the study area at an appropriate resolution, (2) overlaying the grid with a data base of spatially referenced species records, (3) determining the number of times each species was recorded in each cell, (4) constructing a matrix of grid cells by species where each entry indicates the number of records of a particular species in a particular cell, (5) removing cells whose total number of records is lower than some threshold (to be determined) from the matrix, (6) analysis of the reduced matrix using ordination and classification techniques, (7) construction of maps representing the results of the multivariate analyses, and (8) analysis of these maps with respect to digital maps of relevant climatological factors. The applicability of this approach was evaluated by analysing the response of the land snail fauna of Israel to regional variation in mean annual rainfall. As was expected, patterns of faunal variation were significantly correlated with underlying variation in rainfall. However, the per‐unit effect of rainfall on the composition of the studied fauna was much greater in dry regions than in more rainy areas. Above 450 mm, no relationships could be detected between the observed patterns of faunal variation and rainfall. These patterns were consistent over a wide range of grid cell sizes (5–20 km), and were robust to outliers. The overall results indicate that the integration of GIS tools with standard multivariate techniques may serve as a valuable methodology for the identification and interpretation of regional patterns of faunal variation.
In a review of recent challenges in conservation planning, Ferrier (2002) proposed the incorporation of models of similarity in species composition as a means for prioritizing areas for biodiversity conservation. A key assumption of this approach is that estimates of compositional similarity derived from models of similarity in species composition can be used as effective surrogates for real similarity data. We used data on snail distribution in Israel to test this assumption. We used two types of models to analyze patterns of similarity in species composition: one based on presence/absence data and the second based on abundance data. Both models accounted for large amounts of the observed variation in compositional similarity. Variation-partitioning analysis indicated that a considerable amount of the variation in compositional similarity could be separated into "pure" geographical versus "pure" environmental components, indicating that reserve selection procedures should take into account spatial considerations in determining priorities for conservation. The relative effects of geographical versus environmental factors varied between the two types of models, indicating that different indices of similarity should be used if one wishes to represent species composition per se or ecological communities including their relative species abundances. A comparison of distribution patterns of land snails and land birds in a subset of the study sites revealed a high degree of congruence in compositional similarity between the two groups. Moreover, compositional similarity in snails was a better predictor of compositional similarity in birds compared with all environmental and geographical distances taken together. Models calibrated based on data collected in small plots explained a considerable amount of the variation observed at larger scales, suggesting that sampling efforts required for conservation planning might be lower (and thus, more feasible) than assumed previously. Models of similarity in species composition may serve as an important tool for conservation planning.Resumen: En una revisión de los recientes retos de la planificación de la conservación, Ferrier (2002) propuso la incorporación de modelos de similitud en la composición de especies como una manera de priorizaŕ areas para conservación de la biodiversidad. Una suposición clave de este método es que se pueden utilizar las estimaciones de similitud en la composición derivadas de modelos de similitud en la composición de especies como sustitutos efectivos de datos reales de similitud. Utilizamos datos de la distribución de caracoles terrestres en Israel para probar esta suposición. Utilizamos dos tipos de modelo para analizar patrones de similitud en la composición de especies: uno con base en datos de presencia/ausencia y el segundo con base en datos de abundancia. Ambos modelos explicaron gran parte de la variación observada en la similitud en la composición. El análisis de partición de la variación indicó que una parte considerable de la variación en ...
Land snails are subject to daily and seasonal variations in temperature and in water availability and depend on a range of behavioral and physiological adaptations for coping with problems of maintaining water, ionic, and thermal balance. Heat shock proteins (HSPs) are a multigene family of proteins whose expression is induced by a variety of stress agents. We used experimental desiccation to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desiccation-resistant, desert species Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species Sphincterochila cariosa. We examined the HSP response in the foot, hepatopancreas, and kidney tissues of snails exposed to normothermic desiccation. Our findings show variations in the HSP response in both timing and magnitude between the two species. The levels of endogenous Hsp72 in S. cariosa were higher in all the examined tissues, and the induction of Hsp72, Hsp74, and Hsp90 developed earlier than in S. zonata. In contrary, the induction of sHSPs (Hsp25 and Hsp30) was more pronounced in S. zonata compared to S. cariosa. Our results suggest that land snails use HSPs as part of their survival strategy during desiccation and as important components of the aestivation mechanism in the transition from activity to dormancy. Our study underscores the distinct strategy of HSP expression in response to desiccation, namely the delayed induction of Hsp70 and Hsp90 together with enhanced induction of sHSPs in the desert-dwelling species, and suggests that evolution in harsh environments will result in selection for reduced Hsp70 expression.
The functional significance of hermaphroditism is, as yet, not well understood. This study attempts to examine the relevance of some theories on this topic to reality, by reviewing the literature on hermaphroditism among molluscs. The study reveals that about 40% of the 5600 mollusc genera are either simultaneous or sequential hermaphrodites. Hermaphroditism occurs in loo%, of the Solenogastres, 99% of the Opisthobranchia, 100°/o of the Pulmonata, 3% of the Prosobranchia and go//, of the Bivalvia.Sequential hermaphroditism, whatever its theoretical advantages over gonochorism, is nbt common among molluscs.Simultaneous hermaphroditism is common among the Euthyneura. Over 80% of the hermaphroditic genera belong to the Euthyneura (about 2000 recent genera), and over 99% of the Euthyneura are hermaphrodites. The almost ubiquitous occurrence of hermaphroditism throughout the Euthyneurans, coupled with this group's very widespread dispersal in marine, terrestrial and freshwater habitats (and corresponding exposure to a very wide range of predators, competitors or parasites); and also its wide range of population densities, suggests that in this group hermaphroditism is not an adaptive response to selective forces of the environment and it may well be a phylogenetic constraint.A few stylommatophorans have broken this constraint by possessing genitalia in which the male system is absent (aphally) or extremely reduced (hemiphally). They reproduce by self-fertilization. In theory this is advantageous in that these species save the cost of developing two sets of reproductive apparatus. In reality there is no record of aphally occurring throughout an entire species, as one may expect if this saving of cost were so overwhelmingly advantageous. There is also no record of stylommatophoran individuals developing only the male system, as one may expect if there was any evolutionary trend from hermaphroditism towards gonochorism.Simultaneous hermaphroditism offers a (theoretical) option to selfing. Selfing by self-fertilization is widespread among freshwater basommatophorans, amphibious stylommatophorans and freshwater bivalves. Selfing by parthenogenesis is widespread among freshwater prosobranchs.Altogether, of 48 genera which self, 60% are dwellers of freshwater. Certain freshwater habitats may perhaps be less stable than most marine or terrestrial ones are.Beyond the Euthyneura, hermaphroditism is common among parasitic, deep sea and Cnidaria-eating molluscs. Perhaps these three associations should be slightly rearranged and combined into a single generalization: that hermaphroditism (beyond the Euthyneura) is widespread among molluscs that live in a close and permanent intimacy with live marine invertebrates, whether as parasites, commensals or predators.Gonochorism but with dwarf males is common among the Eulimoidea, which are parasites, and Galeommatoidea, which are commensals. This situation of gonochorism, in which there is a very close association between members of the two sexes, is functionally a hermaphroditic type. A...
With 4 figures in the text)Land snails are subject to desiccating conditions in their terrestrial habitat. Our previous studies have revealed significant difercnces in resistance to desiccation among closely related species and among genera that share a similar life style. suggesting that the distribution pattern is correlated with the abiotic regime in its specific habitat. Our present study extends the scope of comparison to the intraspecific level, by examining the resistance to desiccation in five populations of Xeropicm rv.s/u/i.s. a Mediterranean-dwelling species that ranges from the 1000 mm to the 200 mm isohyet.The resistance to desiccation varied among populations in correlation with the specific habitat of each population and with the north-to-south and the west-to-east climatic gradients. Furthermore. in one case it exceeded the resistance of other. desert-dwelling species (Trochoideu ,sin~u/uiu. Spkincrc~rochi/u zonuiu). We suggest that. in spite of its physiological capacity to invade deserts, X. twsrulis is prevented from doing so because it is an annual, semelparous species. The Negev Desert is an unpredictable environment. susceptible to year-to-year fluctuations to such an extent that one rainless year would wipe out all its populations.
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