Experimental quantification of long distance dispersal potential of aquatic snails in the gut of migratory birds

Many plant seeds and invertebrates can survive passage through the digestive system of birds, which may lead to long distance dispersal (endozoochory) in case of prolonged retention by moving vectors. Endozoochorous dispersal by waterbirds has nowadays been documented for many aquatic plant seeds, algae and dormant life stages of aquatic invertebrates. Anecdotal information indicates that endozoochory is also possible for fully functional, active aquatic organisms, a phenomenon that we here address experimentally using aquatic snails. We fed four species of aquatic snails to mallards (Anas platyrhynchos), and monitored snail retrieval and survival over time. One of the snail species tested was found to survive passage through the digestive tract of mallards as fully functional adults. Hydrobia (Peringia) ulvae survived up to five hours in the digestive tract. This suggests a maximum potential transport distance of up to 300 km may be possible if these snails are taken by flying birds, although the actual dispersal distance greatly depends on additional factors such as the behavior of the vectors. We put forward that more organisms that acquired traits for survival in stochastic environments such as wetlands, but not specifically adapted for endozoochory, may be sufficiently equipped to successfully pass a bird's digestive system. This may be explained by a digestive trade-off in birds, which maximize their net energy intake rate rather than digestive efficiency, since higher efficiency comes with the cost of prolonged retention times and hence reduces food intake. The resulting lower digestive efficiency allows species like aquatic snails, and potentially other fully functional organisms without obvious dispersal adaptations, to be transported internally. Adopting this view, endozoochorous dispersal may be more common than up to now thought.

Space-for-time substitution to assess likely response of aquatic ecosystems to future climate-related change

Predicting and managing ecological response to a changing climate is often limited by an incomplete understanding of response thresholds and biogeographic differences. For example, step changes in rainfall and runoff, and threshold dynamics and hysteresis in ecological response make projection of future conditions difficult. To combat these constraints we propose that biophysical data across exiting climatic gradients can be used in a space-for-time substitution to predict climate-related ecological response elsewhere. This method builds on previous attempts at space-for-time substitution by using patterns in physical and physicochemical data to explain biological differences across the spatial gradient, then using those patterns to formulate hypotheses of temporal ecological response and finally testing those hypotheses on temporal data available in a second, similar region of interest. 

How did this snail get here? Several dispersal vectors inferred for an aquatic invasive species

1. How species reach and persist in isolated habitats remains an open question in many cases, especially for rapidly spreading invasive species. This is particularly true for temporary freshwater ponds, which can be remote and may dry out annually, but may still harbour high biodiversity. Persistence in such habitats depends on recurrent colonisation or species survival capacity, and ponds therefore provide an ideal system to investigate dispersal and connectivity. 2. Here, we test the hypothesis that the wide distributions and invasive potential of aquatic snails is due to their ability to exploit several dispersal vectors in different landscapes. We explored the population structure of Physa acuta (recent synonyms: Haitia acuta, Physella acuta, Pulmonata: Gastropoda), an invasive aquatic snail originating from North America, but established in temporary ponds in Doñana National Park, southern Spain. In this area, snails face land barriers when attempting to colonise other suitable habitat. 3. Genetic analyses using six microsatellite loci from 271 snails in 21 sites indicated that (i) geographically and hydrologically isolated snail populations in the park were genetically similar to a large snail population in rice fields more than 15 km away; (ii) these isolated ponds showed an isolation-by-distance pattern. This pattern broke down, however, for those ponds visited frequently by large mammals such as cattle, deer and wild boar; (iii) snail populations were panmictic in flooded and hydrologically connected rice fields. 4. These results support the notion that aquatic snails disperse readily by direct water connections in the flooded rice fields, can be carried by waterbirds flying between the rice fields and the park and may disperse between ponds within the park by attaching to large mammals. 5. The potential for aquatic snails such as Physa acuta to exploit several dispersal vectors may contribute to their wide distribution on various continents and their success as invasive species. We suggest that the interaction between different dispersal vectors, their relation to specific habitats and consequences at different geographic scales should be considered both when attempting to control invasive freshwater species and when protecting endangered species.

Gut travellers : internal dispersal of aquatic organisms by waterfowl

Aim: Patterns of high biodiversity among less mobile organisms throughout isolated locations suggest that passive dispersal importantly contributes to biodiversity. We examined the contribution of waterbirds to the dispersal of plant seeds and macroinvert

Subfossils of extinct and extant species of Simuliidae (Diptera) from Austral and Cook Islands (Polynesia): anthropogenic extirpation of an aquatic insect?

Subfossil head capsules of Simuliidae larvae have been recovered from swamps on Tubuai and Raivavae of the Austral Islands, and Atiu and Mangaia of the southern Cook Islands. For Tubuai and Raivavae it is likely that the simuliids are extinct, but a single simuliid species is extant on nearby Rurutu. For Atiu and Mangaia, extant simuliids have not been reported, but are known on Rarotonga. Well-preserved head capsules indicate that the Cook Islands subfossils are those of Simulium (Inseliellum) teruamanga Craig and Craig, 1986. For the Austral Islands, the simuliid from Tubuai is considered a variant of Simulium (Inseliellum) rurutuense Craig and Joy, 2000. That from Raivavae is morphologically distinct and is described here as a new species, Simulium (Inseliellum) raivavaense Craig and Porch. Humans arrived in Eastern Polynesia ca. 1,000 years ago resulting in the widespread destruction of lowland forest and conversion of wetlands to agriculture with implied consequences for the indigenous biota of these habitats. Here we consider that one such result was loss of freshwater aquatic biodiversity.

Climate-driven impacts of prey abundance on the population structure of a tropical aquatic predator

In the present study we explore how annual variation in climate (late wet-season rainfall) affects population demography in a gape-limited obligate piscivorous predator, the Arafura filesnake Acrochordus arafurae in the Australian tropics. These aquatic snakes display extreme sexual dimorphism, with body sizes and relative head sizes of females much larger than those of males. Two consecutive years with low rainfall during the late wet season reduced the abundance of small but not large sized fish. Although snake residual body mass (RBM, calculated from a general linear regression of ln-transformed mass to ln-SVL) decreased after the first year with low prey availability, it was not until the second year that reduced prey abundance caused a dramatic decline in filesnake survival, and hence in population numbers. Thus, our results suggest that most snakes survived the first year of reduced prey abundance, but a successive year with low prey availability proved fatal for many animals. However, the effects of prey scarcity on RBM and survival fell disproportionately on some size classes of snakes. Medium-sized animals (large males and intermediate-sized females) were affected more dramatically than were small or large snakes. We attribute the higher survival of small snakes to their lower energy needs compared to medium-sized individuals, and the higher survival of large snakes to the continued abundance of large prey (mainly large catfish). Two successive years with low abundance of smaller sized prey thus massively modified the size-structure of the filesnake population, virtually eliminating large males and intermediate-sized females. Our field data provide a clear demonstration of the ways in which stochastic variation in climatic conditions can have dramatic effects on predator population demography, mediated via effects on prey availability.