Historical and Ecological Factors in the Evolution, Adaptive Radiation, and Biogeography of Freshwater Mollusks

SYNOPSIS. There are some 36 families that are wholly freshwateror with representative species in freshwater. There are virtuallyno phylogenetic analyses for these families. Zoogeographic analysesof freshwater molluscan faunas are hindered by a lack of significantsystematic studies of these faunas. Such studies are essentialif one hopes to develop hypotheses about phylogeny or biogeography. It is clear from a phylogenetic analysis of the Pomatiopsidaethat phylogenetic, vicariance, dispersal, and ecological factorsall have significant effects on the patterns of distributionof this family. At one stage in history vicariance may be adominant factor while during another stage of history, dispersalmay be a dominant factor. At every stage, ecological considerationsare necessary to understand the phenotypes seen and the spatialrelationships among taxa. In examining the distribution patternsof dominant freshwater families with regard to their biological,ecological, and overall phylogenetic relationships it is evidentthat ecology plays a major role along with dispersal and vicariance.Clearly a synthesis is needed in biogeographical studies thatincorporates vicariance dispersal, ecology, and geology-paleontology.     

Nonsignificant isolation by distance implies limited dispersal

Understanding the scale of dispersal is an important consideration in the conservation and management of many species. However, in species in which the high‐dispersal stage is characterized by tiny gametes or offspring, it may be difficult to estimate dispersal directly. This is the case for many marine species, whose pelagic larvae are dispersed by ocean currents by several days or weeks before beginning a benthic, more sedentary, adult stage. As consequence of the high‐dispersal larval stage, many marine species have low genetic structure on large spatial scales (Waples 1998 ; Hellberg 2007 ). Despite the high capacity for dispersal, some tagging studies have found that a surprising number of larvae recruit into the population they were released from (self‐recruitment). However, estimates of self‐recruitment are not informative about mean dispersal between subpopulations. To what extent are limited dispersal estimates from tagging studies compatible with high potential for dispersal and low genetic structure? In this issue, a study on five species of coral reef fish used isolation by distance (IBD) between individuals to estimate mean dispersal distances (Puebla et al. 2012 ). They found that mean dispersal was unexpectedly small (<50 km), given relatively low IBD slopes and long pelagic durations. This study demonstrates how low genetic structure is compatible with limited dispersal in marine species. A comprehensive understanding of dispersal in marine species will involve integrating methods that estimate dispersal over different spatial and temporal scales. Genomic data may increase power to resolve these issues but must be applied carefully to this question.     

Ranging and nesting behaviour of the dormouse Muscardinus avellanarius, in coppice-with-standards woodland

Radiotracking was used to investigate the movements of dormice at different times during the summer, in an area of coppice-with-standards woodland, a habitat traditionally associated with this species. The dormice were shown to be wholly arboreal in their activity, foraging in different strata of their three-dimensional habitat, as different food sources became seasonally available. Nest sites were located and both sexes were found to travel only short distances from them, implying that this species probably has a low potential for dispersal. Males normally travelled further than females each night and had larger home ranges, within which activity was usually concentrated in small areas associated with a particular food resource. The findings are discussed in relation to woodland management and distribution surveys.     

The importance of metacommunity ecology for environmental assessment research in the freshwater realm

Most bioassessment programs rest on the assumption that species have different niches, and that abiotic environmental conditions and changes therein determine community structure. This assumption is thus equivalent to the species sorting perspective (i.e. that species differ in their responses to environmental variation) in metacommunity ecology. The degree to which basing bioassessment on the species sorting perspective is reasonable is likely to be related to the spatial extent of a study and the characteristics of the organism groups (e.g. dispersal ability) with which the effects of anthropogenic changes are assessed. Recent findings in metacommunity research have stressed that community structure is determined not only by local abiotic environmental conditions but also by biotic interactions and dispersal‐related effects. For example, dispersal limitation may prevent community structure recovery from the effects of a putative stressor, as organisms may not be able to disperse to all sites in a region. Mass effects (i.e. the presence of species in environmentally suboptimal sites due to high dispersal rates from environmentally suitable sites) may, in turn, obscure the effects of a stressor, as dispersal from source sites (e.g. an unaltered site) allows persistence at sink sites (e.g. an anthropogenically altered site). Better bioassessment should thus take both niche‐ and dispersal‐related processes simultaneously into consideration, which can be accomplished by explicitly modelling spatial location as a proxy for dispersal effects. Such an integrated approach should be included in bioassessment programs using general multivariate approaches, predictive modelling, and multimetric indices.     

食肉质果鸟对种子的传播作用

食肉质果鸟对种子的传播作用鲁长虎常家传（东北林业大学,哈尔滨１５００４０）ＥｆｆｅｃｔｏｆｔｈｅＦｌｅｓｈｙ＿ｆｒｕｉｔｓＥａｔｉｎｇＢｉｒｄｓｏｎＳｅｅｄＤｉｓｐｅｒｓａｌ．ＬｕＣｈａｎｇｈｕ,ＣｈａｎｇＪｉａｃｈｕａｎ（ＮｏｒｔｈｅａｓｔＦｏ...     

Time to abandon the loss of dispersal ability hypothesis in island plants: A comment on García‐Verdugo,Mairal, Monroy,Sajeva and Caujapé‐Castells (2017)

García‐Verdugo et al. (2017) recently tested the loss of dispersal ability hypothesis in a wind‐dispersed shrub from Southern Europe. Although the hypothesis has guided research for over 150 years, García‐Verdugo et al. (2017) results failed to substantiate its central prediction—the loss of seed dispersal potential in island populations. Here, I highlight several additional limitations of the hypothesis. First, García‐Verdugo et al. (2017) results are not unusual. Empirical support for the hypothesis is equivocal. Second, when reduced dispersal potential is documented, it may often evolve as a passive by‐product of selection for large seeds, for reasons that are wholly unrelated to their dispersal. Third, the hypothesis does not readily apply to all plant dispersal modes, particularly plants that produce fleshy fruits. These issues advocate a fresh approach to the study of how selection shapes the evolution of dispersal potential on islands.     

Pythons in the Eocene of Europe reveal a much older divergence of the group in sympatry with boas

Extant large constrictors, pythons and boas, have a wholly allopatric distribution that has been interpreted largely in terms of vicariance in Gondwana. Here, we describe a stem pythonid based on complete skeletons from the early-middle Eocene of Messel, Germany. The new species is close in age to the divergence of Pythonidae from North American Loxocemus and corroborates a Laurasian origin and dispersal of pythons. Remarkably, it existed in sympatry with the stem boid Eoconstrictor. These occurrences demonstrate that neither dispersal limitation nor strong competitive interactions were decisive in structuring biogeographic patterns early in the history of large, hyper-macrostomatan constrictors and exemplify the synergy between phylogenomic and palaeontological approaches in reconstructing past distributions.     

Emus as non-standard seed dispersers and their potential for long-distance dispersal

Long-distance seed dispersal may have important consequences for species range, migration rates, metapopulation dynamics, and gene flow. Plants have evolved various adaptations for seed dispersal by standard agents, with typical dispersal distances associated with them. Seeds may also be dispersed by non-standard agents for which they do not show any apparent adaptation and may reach long distances. By sampling the droppings of emus Dromaius novaehollandiae at three localities in Western Australia, we investigated their potential to act as long-distance dispersers of seeds with adaptations for dispersal modes other than endozoochory, such as unassisted, ant, wind and exozoochory, for which they act as non-standard agents. Seventy-seven plant species with five types of dispersal syndromes were found in the 112 droppings analysed, with at least 68 having viable seeds. Although endozoochory was the most frequent syndrome, the presence of other syndromes was important in terms of number of species (61%) and seeds (50%). Estimates of species richness indicated that an increase in sampling effort would increase the number of species observed, especially among non-endozoochores. As a consequence of their long gut retention times and high mobility, emus can provide long-distance dispersal opportunities that may be especially relevant for species with dispersal modes of typically short distances (unassisted, ant).
Our results suggest that the role of emus as non-standard agents for long-distance dispersal should be taken into account for understanding current geographic ranges, gene flow and metapopulation dynamics of some plant species, as well as for predicting their future responses to climate change and fragmentation.     

Postglacial dispersal patterns and mitochondrial genetic structure of the Pyrenean desman (Galemys pyrenaicus) in the northwestern region of the Iberian Peninsula

The genetic structure of small semiaquatic animals may be influenced by dispersal across both rivers and land. The relative importance of these two modes of dispersal may vary across different species and with ecological conditions and evolutionary periods. The Pyrenean desman (Galemys pyrenaicus) is an endemic mammal of the Iberian Peninsula with a strong phylogeographic structure and semiaquatic habits, thus making it an ideal model to study the effects of river and overland dispersal on its genetic structure. Thanks to different types of noninvasive samples, we obtained an extensive sampling of the Pyrenean desman from the northwestern region of the Iberian Peninsula and sequenced two mitochondrial DNA fragments. We then analyzed, using an isolation‐by‐distance approach, the correlation between phylogenetic distances and geographical distances measured along both river networks and land to infer the relative importance of river and overland dispersal. We found that the correlations in the whole area and in a large basin were consistent with an effect of overland dispersal, which may be due to the postglacial colonization of new territories using terrestrial corridors and, possibly, a more extensive fluvial network that may have been present during the Holocene. However, in a small basin, likely to be less influenced by the impact of ancient postglacial dispersal, the correlations suggested significant overall effects of both overland and river dispersal, as expected for a semiaquatic mammal. Therefore, different scales and geographical regions reflect different aspects of the evolutionary history and ecology of this semiaquatic species using this isolation‐by‐distance method. The results we obtained may have crucial implications for the conservation of the Pyrenean desman because they reinforce the importance of interbasin dispersal for this species in the studied area and the need to protect the whole riverine ecosystem, including rivers, upland streams and terrestrial corridors between basins.     

Dispersal restricts local biomass but promotes the recovery of metacommunities after temperature stress

Landscape connectivity can increase the capacity of communities to maintain their function when environments change by promoting the immigration of species or populations with adapted traits. However, high immigration may also restrict fine tuning of species compositions to local environmental conditions by homogenizing the community. Here we demonstrate that dispersal generates such a tradeoff between maximizing local biomass and the capacity of model periphyton metacommunities to recover after a simulated heat wave. In non‐disturbed metacommunities, dispersal decreased the total biomass by preventing differentiation in species composition between the local patches making up the metacommunity. On the contrary, in metacommunities exposed to a realistic summer heat wave, dispersal promoted recovery by increasing the biomass of heat tolerant species in all local patches. Thus, the heat wave reorganized the species composition of the metacommunities and after an initial decrease in total biomass by 38.7%, dispersal fueled a full recovery of biomass in the restructured metacommunities. Although dispersal may decrease equilibrium biomass, our results highlight that connectivity is a key requirement for the response diversity that allows ecological communities to adapt to climate change through species sorting.     

Active dispersal is differentially affected by inter‐ and intraspecific competition in closely related nematode species

Competition is one of the main drivers of dispersal, which can be an important mechanism to achieve permanent or temporal coexistence of multiple species. This coexistence can be achieved by a dispersal‐competition tradeoff, spatial store effects or neutral dynamics. Here we test the effect of inter‐ and intraspecific competition on dispersal of four species of the marine nematode species complex Litoditis marina. A previous study in closed microcosms without a possibility for dispersal had demonstrated pronounced interspecific competition, leading to the exclusion of one species. We now investigated whether 1) the dispersal is affected by interspecific interactions, by intraspecific competition (density) or by food availability, 2) the dispersal dynamics influence assemblage composition and can lead to co‐occurrence of the species, and 3) the abiotic environment (here salinity) can affect these dynamics. We show that density is the main driver for dispersal in two of the four species. Dispersal of a third species always started at the same time irrespective of density, whereas in the fourth species interspecific interactions accelerated dispersal. Remarkably, this fourth species was not a strong competitor, suggesting that a dispersal–competition tradeoff does not explain the observed coexistence. Salinity did not alter the timing of dispersal when interspecific interactions were present but did affect assemblage composition. Consequently, spatial store effects may influence coexistence. All four species co‐occurred in fairly stable abundances throughout the present experiment indicating the importance of species specific dispersal strategies for coexistence. Co‐occurrence can be facilitated because competition is postponed or avoided by dispersal. Neutral dynamics also played a role as intra‐ and interspecific competition were of similar importance in three of the four species. We conclude that dispersal is a driver of the coexistence of closely related nematode species, and that population density and interspecific interactions shape these dynamics.     

Dynamical mechanism for coexistence of dispersing species.

Dispersal of organisms may play an essential role in the coexistence of species. Recent studies of the evolution of dispersal in temporally varying environments suggest that clones differing in dispersal rates can coexist indefinitely. In this work, we explore the mechanism permitting such coexistence for a model of dispersal in a patchy environment, where temporal heterogeneity arises from endogenous chaotic dynamics. We show that coexistence arises from an extreme type of intermittent behavior, namely the phenomenon known as on-off intermittency. In effect, coexistence arises because of an alternation between synchronized and de-synchronized dynamical behaviors. Our analysis of the dynamical mechanism for on-off intermittency lends strong credence to the proposition that chaotic synchronism may be a general feature of species coexistence, where competing species differ only in dispersal rate.     

Reserve mass and dispersal investment in relation to geographic range of plant species: phylogenetically independent contrasts

Recent studies have reported conflicting evidence about correlations between seed size and plant species geographic range sizes. Using phylogenetically independent contrasts (PICs) within genera, we found no consistent differences in reserve mass between species with similar dispersal morphology and «wide>> versus «narrow>> geographic ranges. There was also no tendency within genera for broad ranged species to be those that allocate a larger percentage of the resources invested in each diaspora to dispersal structures. PICs were also constructed between species having a tenfold difference in seed size. In these PICs, the larger seeded species often occupied a greater number of regions than species with smaller seed sizes. This result was generated primarily through the comparison of species from different genera, families or higher level taxa which differed not only in seed mass but also in dispersal modes and growth forms. Finally, comparing species within Acacia and Eucalyptus having similar seed size but different dispersal modes, we found that bird dispersal (in Acacia ) and possession of a wing for wind dispersal (in Eucalyptus ) was associated with wider geographic range compared to lower-investment dispersal modes. Taken together, these comparisons indicate that seed size is not itself important as a factor influencing breadth of geographic range. Dispersal mode and growth form may have an influence, however, and seed size differences may be associated with contrasts in dispersal mode or growth form.     

Human-mediated dispersal of seeds over long distances

Human activities have fundamental impacts on the distribution of species through altered land use, but also directly by dispersal of propagules. Rare long-distance dispersal events have a disproportionate importance for the spread of species including invasions. While it is widely accepted that humans may act as vectors of long-distance dispersal, there are few studies that quantify this process. We studied in detail a mechanism of human-mediated dispersal (HMD). For two plant species we measured, over a wide range of distances, how many seeds are carried by humans on shoes. While over half of the seeds fell off within 5m, seeds were regularly still attached to shoes after 5 km. Semi-mechanistic models were fitted, and these suggested that long-distance dispersal on shoes is facilitated by decreasing seed detachment probability with distance. Mechanistic modelling showed that the primary vector, wind, was less important as an agent of long-distance dispersal, dispersing seeds less than 250 m. Full dispersal kernels were derived by combining the models for primary dispersal by wind and secondary dispersal by humans. These suggest that walking humans can disperse seeds to very long distances, up to at least 10 km, and provide some of the first quantified dispersal kernels for HMD.     

Differences in Time until Dispersal between Cryptic Species of a Marine Nematode Species Complex

Co-occurrence of closely related species may be achieved in environments with fluctuating dynamics, where competitively inferior species can avoid competition through dispersal. Here we present an experiment in which we compared active dispersal abilities (time until first dispersal, number and gender of dispersive adults, and nematode densities at time of dispersal) in Litoditis marina, a common bacterivorous nematode species complex comprising four often co-occurring cryptic species, Pm I, II, III, and IV, as a function of salinity and food distribution. The experiment was conducted in microcosms consisting of an inoculation plate, connection tube, and dispersal plate. Results show species-specific dispersal abilities with Pm I dispersing almost one week later than Pm III. The number of dispersive adults at time of first dispersal was species-specific, with one dispersive female in Pm I and Pm III and a higher, gender-balanced, number in Pm II and Pm IV. Food distribution affected dispersal: in absence of food in the inoculation plate, all species dispersed after ca four days. When food was available Pm I dispersed later, and at the same time and densities irrespective of food conditions in the dispersal plate (food vs no food), suggesting density-dependent dispersal. Pm III dispersed faster and at a lower population density. Salinity affected dispersal, with slower dispersal at higher salinity. These results suggest that active dispersal in Litoditis marina is common, density-dependent, and with species, gender- and environment-specific dispersal abilities. These differences can lead to differential responses under suboptimal conditions and may help to explain temporary coexistence at local scales.     

Dispersal potential impacts size clines of grasshoppers across an elevation gradient

Body size is a life history trait that determines the reproductive success of a variety of organisms. Changes in body size may have a genetic component when persistent conditions such as season length and climate select for individuals of an optimal body size and an environmental component when it is influenced on an ecological scale by factors such as weather, food availability, or maternal effects. Along elevational gradients that experience seasonality, insects commonly become smaller with increases in elevation. In this study we test the hypothesis that dispersal potential, an indicator of gene flow, impacts the type of size clines exhibited by insects along elevational gradients and that these differences in local adaptation should lead to predictable changes in their reproductive potential and output. Using two short winged grasshopper species, Aeropedellus clavatus and Melanoplus boulderensis, and two long winged species, Camnula pellucida and Melanoplus sanguinipes, we showed that species with low dispersal potential are associated with significant declines in body size with increases in elevation while species with high dispersal potential displayed no size clines. Consistent with short winged species being more locally adapted, we show that reproductive potential, as measured by the proportion of ovarioles that become functional, do not differ among populations of short winged species, but decline with elevation in the long winged species. While our study failed to show that dispersal potential impacts reproductive output in a consistent and predictable manner (as measured by clutch and egg sizes), we address the possibility that clutch size may not reflect changes in total reproductive output and that changes in egg size may be a plastic trait. We concluded that studies exploring the evolution of body size, the reproductive capacity and species level responses to environmental change should note the importance of dispersal potential in influencing these patterns.     

Myrmecochory in some plants (F. chenopodiaceae) of the Australian arid zone

Summary Several common plants (Chenopodiaceae) of the Australian arid zone produce diaspores that bear small and inconspicuous food bodies and are adapted for dispersal by ants. For these species, myrmecochory probably represents an adaptation for highly directional dispersal of diaspores to favorable microsites where nutrients are concentrated and possibly more accessible. Dispersal of diaspores by ants can have a pronounced effect on plant dispersion. In habitats characterized by red, crusty alluvial loam soils, myrmecochorous species grow almost exclusively on ant mounds; these same species grow in relatively continuous stands in sandy soil habitats. The flora of the Australia arid zone may contain many plant species that are adapted to use ants as dispersal agents. We consider several factors that may have promoted or facilitated the evolution of myrmecochory in arid zone plants.     

Polygamy slows down population divergence in shorebirds

Sexual selection may act as a promotor of speciation since divergent mate choice and competition for mates can rapidly lead to reproductive isolation. Alternatively, sexual selection may also retard speciation since polygamous individuals can access additional mates by increased breeding dispersal. High breeding dispersal should hence increase gene flow and reduce diversification in polygamous species. Here, we test how polygamy predicts diversification in shorebirds using genetic differentiation and subspecies richness as proxies for population divergence. Examining microsatellite data from 79 populations in 10 plover species (Genus: Charadrius) we found that polygamous species display significantly less genetic structure and weaker isolation‐by‐distance effects than monogamous species. Consistent with this result, a comparative analysis including 136 shorebird species showed significantly fewer subspecies for polygamous than for monogamous species. By contrast, migratory behavior neither predicted genetic differentiation nor subspecies richness. Taken together, our results suggest that dispersal associated with polygamy may facilitate gene flow and limit population divergence. Therefore, intense sexual selection, as occurs in polygamous species, may act as a brake rather than an engine of speciation in shorebirds. We discuss alternative explanations for these results and call for further studies to understand the relationships between sexual selection, dispersal, and diversification.     

Decreased relatedness between male prickly forest skinks (<Emphasis Type="BoldItalic">Gnypetoscincus queenslandiae</Emphasis>) in habitat fragments

In species with low levels of dispersal the chance of closely related individuals breeding may be a potential problem; sex-biased dispersal is a mechanism that may decrease the possibility of cosanguineous mating. Fragmentation of the habitat in which a species lives may affect mechanisms such as sex-biased dispersal, which may in turn exacerbate more direct effects of fragmentation such as decreasing population size that may lead to inbreeding depression. Relatedness statistics calculated using microsatellite DNA data showed that rainforest fragmentation has had an effect on the patterns of dispersal in the prickly forest skink (Gnypetoscincus queenslandiae), a rainforest endemic of the Wet Tropics of north eastern Australia. A lower level of relatedness was found in fragments compared to continuous forest sites due to a significantly lower level of pairwise relatedness between males in rainforest fragments. The pattern of genetic relatedness between sexes indicates the presence of male-biased dispersal in this species, with a stronger pattern detected in populations in rainforest fragments. Male prickly forest skinks may have to move further in fragmented habitat in order to find mates or suitable habitat logs.     

Disturbance alters habitat isolation's effect on biodiversity in aquatic microcosms

Isolated habitats generally have fewer species at local spatial scales than more connected habitats. However, over larger spatial scales, the response of species richness to variation in the degree of isolation is variable. Here, we hypothesized that the effects of habitat isolation on patterns of regional level species richness may depend at least in part on the level of disturbances those habitats receive. We tested this hypothesis in a microcosm experiment using an aquatic community consisting of container dwelling protists and rotifers by manipulating disturbance and dispersal to experimental regions factorially. In disturbed regions, regional species richness was lower in regions with isolated patches compared to regions where patches were experimentally connected by dispersal. A likely mechanism for this result is that dispersal from adjacent undisturbed local patches allowed disturbance-intolerant species a temporary refugia, thereby allowing regional coexistence of disturbance-tolerant and intolerant species. In contrast, without disturbances (and thus no temporal heterogeneity) it is likely that dispersal homogenized communities, leading to overall lower richness with higher dispersal. Our results emphasize the importance of simultaneously considering multiple limiting factors, disturbance and dispersal in this case, as well as the spatial scale of the response, in order to fully understand factors that control biodiversity.