Seed dispersal by waterbirds in southern Africa: comparing the roles of ectozoochory and endozoochory

相似文献   

Do birds of a feather disperse plants together?

1. Dispersal of propagules by waterbirds is thought to be important for wetland plants because of the abundance of birds and their frequent movements among aquatic habitats. Differences in bird characteristics (size, movement, feeding ecology) were expected to lead to different outcomes for plant dispersal. 2. We investigated heterogeneity in plant dispersal by ducks (Anas superciliosa, Anas gracilis, Anas castanea). We calculated the probability of transport of viable seeds by germinating propagules retrieved from feathers and feet (epizoochory) and the contents of the oesophagus, gizzard and lower gut (endozoochory). 3. The abundance and richness of seeds carried internally and externally did not differ among sympatric bird species. We used estimates from the literature of movements of Anas species to approximate dispersal kernels for the transport of plant propagules. 4. Heterogeneity in the abundance and movement ecology of disperser species will result in differing patterns and degrees of connectivity for wetland plant metacommunities. Sedentary waterfowl are likely to have an important role in replenishing propagules and connecting aquatic metacommunities over small distances. Nomadic waterfowl may facilitate long‐distance dispersal. We discuss the implications of differences between duck species in movement patterns for connectivity of aquatic plant metacommunities across landscapes.     

Field evidence of dispersal of branchiopods, ostracods and bryozoans by teal (Anas?crecca) in the Camargue (southern France)

Invertebrate propagules may survive internal and external transports by waterbirds, thus facilitating their dispersal between aquatic habitats. However, field data on such transport remain limited, especially for exozoochory. We quantified and compared the rates of internal and external invertebrate transports simultaneously in a wintering population of teal (Anas crecca) in the Camargue (southern France). We inspected lower gut (rectum) contents of birds that had been shot (N = 366) and washed birds that had been live-trapped (N = 68) during the winters 2006–2007 and 2007–2008. At least one propagule was recorded in 2.5% and 10.3% of internal and external samples, respectively. Cladoceran, ostracod, plumatellid bryozoan and anostracan propagules were all recorded in both internal and external samples. Hatching confirmed their viability, except for anostracan eggs. At least three cladoceran species and two ostracod species were recorded, none of which had previously been shown to be dispersed by birds. Amongst external samples, we recorded a significant seasonal trend in 1 year with most propagules recorded before December, keeping with a seasonal decline in the availability of propagules on the water surface. This study provides evidence that teal may be important vectors of invertebrate dispersal both within the Camargue and along migratory flyways.     

The potential role of waterbirds in dispersing invertebrates and plants in arid Australia

1. The role of waterbirds as vectors of plants and invertebrates within and between arid‐zone wetlands is poorly understood. We present the first detailed study of passive dispersal by nomadic birds in Australasia. We investigated the numbers and types of invertebrate and plant propagules within freshly collected faecal samples as well as their viability. We compared dispersal among Grey Teal (Anas gracilis), Eurasian Coot (Fulica atra) and Black Swan (Cygnus atratus) in the Macquarie Marshes, a complex of temporary to semi‐permanent wetlands in New South Wales. 2. When faecal samples (n = 60) were inundated in the laboratory and monitored over 3 weeks, ciliates (75% of samples), nematodes (22%), ostracods (13%) and rotifers (5%) were recorded, with higher taxon richness in coot samples. Faecal samples (n = 71) were also sieved to quantify intact propagules, and ostracod eggs (70% of samples), large branchiopod eggs (31%) and bryozoan statoblasts (31%) were the most abundant invertebrates. Diaspores of 19 plant taxa were recorded, 14 of which were germinated in the laboratory or shown to be viable at the end of germination trials. The abundance and diversity of invertebrate propagules was highest in coot samples, whereas the abundance and diversity of diaspores was highest in teal samples. 3. One Australian Pelican (Pelecanus conspicillatus) sample was obtained and found to contain more taxa and far more propagules than any sample from other waterbirds, suggesting that piscivorous birds might have an important role in the indirect dispersal of propagules ingested by fish. 4. Our results support a role for birds in explaining the distributions of cosmopolitan plant genera such as Lemna, Typha, Myriophyllum and Nitella. The alien plants Ranunculus sceleratus, Medicago polymorpha and Polygonum arenastrum were recorded, demonstrating the potential role of waterfowl in the spread of exotic species. As the frequency and duration of flooding of arid‐zone wetlands decreases owing to human activities, the importance of waterbirds in facilitating recolonisation of temporary wetlands is likely to increase.     

Crayfish invasion facilitates dispersal of plants and invertebrates by gulls

相似文献   

Plant dispersal by teal (Anas crecca) in the Camargue: duck guts are more important than their feet

1. Migratory waterbirds are major vectors for the dispersal of aquatic plants. However, quantitative field studies of the frequency of transport are scarce, and the relative importance of internal and external transport remains unclear. 2. We quantified and compared the rates of internal and external transport of aquatic plant propagules by teal (Anas crecca) in the Camargue (southern France), inspecting the lower gut contents of birds that had been shot (n = 366) and washing birds that had been live‐trapped (n = 68) during the winters of 2006–2007 and 2007–2008. 3. Intact propagules (n = 902) of 21 plant taxa were recorded in the rectum of teal, of which 16 germinated or were shown to be viable. Intact propagules were recorded in the rectum of 20% of teal, with up to 171 propagules per individual bird. Chara oogonia were most abundant (60% of intact propagules), suggesting that small size favours internal transport. Eleocharis palustris, Juncus spp. and Potamogeton pusillus (17, 7 and 6% of intact propagules, respectively) were also very abundant. 4. Intact propagules (n = 12) of 10 plant taxa were found on the outside of live teal, and four of these taxa later germinated. Intact propagules were found on 18% of teal. No teal was found to carry more than one propagule externally. There was no difference in size between propagules transported internally and externally. 5. Teal are major dispersers of plants within the Camargue, despite being highly granivorous. Contrary to widespread assumptions in the literature, endozoochory by ducks appears to be a much more important mode of dispersal for aquatic plants than exozoochory. We found no evidence of changes in the probability of plant propagule dispersal at a landscape scale over the course of the winter, so propagule production and zoochory appear to be decoupled over time in aquatic systems.     

Dispersal of aquatic organisms by waterbirds: a review of past research and priorities for future studies

1. Inland wetlands constitute ecological islands of aquatic habitat often isolated by huge areas of non-suitable terrestrial habitats. Although most aquatic organisms lack the capacity to disperse by themselves to neighbouring catchments, many species present widespread distributions consistent with frequent dispersal by migratory waterbirds.
2. A literature review indicates that bird-mediated passive transport of propagules of aquatic invertebrates and plants is a frequent process in the field, at least at a local scale. Both endozoochory (internal transport) and ectozoochory (external transport) are important processes.
3. The characteristics of the dispersed and the disperser species that facilitate such transport remain largely uninvestigated, but a small propagule size tends to favour dispersal by both internal and external transport.
4. We review the information currently available on the processes of waterbird-mediated dispersal, establishing the limits of current knowledge and highlighting problems with research methods used in previous studies. We also identify studies required in the future to further our understanding of the role of such dispersal in aquatic ecology.     

Analysis of trophic structure of two carnivore assemblages by means of guild identification

We evaluated the existence of trophic guild structure, considering seasonal and annual variation, in two terrestrial carnivore assemblages: one from Santa Cruz province (Argentinean Patagonia, composed by six carnivore species), and the other from Doñana National Park (SW Spain, composed by five carnivore species). To identify trophic guilds, we first studied seasonal and annual diets of predators, calculated trophic overlap among species pairs, and then constructed overlap matrices (similarity matrices). We determined guild membership objectively by entering the similarity matrices into the clustering technique unweighted pair-group method with arithmetic averaging. Carnivores from both assemblages were grouped, respectively, into four feeding guilds. Lagomorphs and rodents promoted the formation of two feeding guilds in both study sites, although the taxonomic composition of predator species that composed them was different. The ungulates-edentates feeding guild was only present at Santa Cruz, whereas the birds and reptiles feeding guild was only present at Doñana. Invertebrates and fruits were the base for the formation of a guild composed by species of the same taxonomic origin both in Santa Cruz and Doñana. Guild structure of Santa Cruz and Doñana assemblages did not exhibit seasonal or annual variation, although the specific guild composition changed over the two studied periods for both assemblages. This structure probably responded to discontinuities in resource spectra in Santa Cruz and fluctuations in rabbit abundance in Doñana. Our results support the hypothesis that establishes that guilds are originated by opportunistic convergence of species on abundant and energetically rewarding resources.     

Recent advances in the study of long-distance dispersal of aquatic invertebrates via birds

Although Darwin pioneered the study of long‐distance dispersal (LDD) of aquatic invertebrates via waterbirds, it remains in its infancy as a modern discipline. A handful of recent studies have quantified internal or external transport in the field, confirming that a variety of long‐distance migrants carry invertebrates both internally and externally. These studies show that variation in the morphology of vectors influences the frequency and size of propagules transported, and suggest that more invertebrate groups disperse via birds than was previously thought. Dispersal limitation has mainly been investigated for zooplankton in small experimental systems from which waterbirds were effectively excluded, and the extent of such limitation for invertebrate populations in wetlands interconnected by waterbird movements remains unclear. We expect that the spatial and temporal scales at which dispersal limitation constrains geographical ranges, species richness and genetic structure of invertebrates depends partly on the density of migratory birds using the area. Birds may have a major role in the expansion of exotic species. We propose several avenues for future research. There is a particular need for more quantitative studies of LDD by birds that will enable modellers to assess its role in maintaining invertebrate biodiversity among increasingly fragmented wetlands and in the face of climate change, as well as in the spread of invasive species.     

Primary moult of continental Black-tailed Godwits Limosa limosa limosa in the Doñana wetlands,Spain

Capsule: Most Continental Black-tailed Godwits Limosa limosa limosa using the Doñana wetlands during post-breeding migration appear to begin moult before they arrive and suspend moult before they migrate onwards to West Africa.

Aims: We aim to describe the primary moult strategies and patterns in the Continental Black-tailed Godwits using the Doñana wetlands, a major passage and wintering area for waterbirds in southern Spain.

Methods: Individual godwits were captured, marked and their primary moult was scored in Doñana during the non-breeding season (June–March) in 2011 and 2012. Data from resightings of colour-marked godwits and birds equipped with satellite transmitters were used to estimate stopover duration during post-breeding migration (June–September) to determine if godwits move to West Africa before completing their primary moult.

Results: Average primary moult duration was estimated to be 84 days?±?9 se, during 29 June–21 September and did not differ between sexes. Only 2% of individuals were observed with suspended moult. We estimated stopover duration in Doñana to be 13 days?±?2 se before migrating to West Africa.

Conclusions: Most godwits stage for about two weeks in the Doñana wetlands during southward migration, moult their primaries and appear to suspend moult before crossing the Sahara. Others may complete their primary moult in Doñana, or elsewhere in Europe and overwinter in Doñana where increasing numbers of godwits have been detected in recent years. A few individuals may finish the moult in Doñana and migrate to West Africa late in the post-breeding season.     

Responses of a small-mammal community to habitat management through controlled burning in a protected Mediterranean area

Fire is widely used as a management tool to achieve conservation goals. However, the consequences of such management on non-target species are frequently neglected and unknown. This study examines the effects of traditional management practices of scrubland clearance by controlled burning to improve menaced carnivores on non-target species: rodent and insectivores in Doñana National Park (SW of Iberian Peninsula). We used capture–recapture methods to examine changes in abundance in areas that were burnt one and three years ago, compared with unburnt areas. Results showed that burnt areas had higher species abundances, but mainly on the ecotonal boundaries. Species abundances showed dramatic seasonal differences with high abundances in autumn and winter, and very low abundance in summer. Our study revealed that scrubland management by controlled fires increases the abundance of small mammal species, mainly Mus spretus and Apodemus sylvaticus. We found only four small mammal species between the different treatments. However, some species that were formerly abundant in Doñana, such as Elyomis quercinus, were found only in burnt areas. Our results suggest that controlled burning is not contributing to the current loss of biotic diversity in this community.     

Overseas seed dispersal by migratory birds

Long-distance dispersal (LDD) promotes the colonization of isolated and remote habitats, and thus it has been proposed as a mechanism for explaining the distributions of many species. Birds are key LDD vectors for many sessile organisms such as plants, yet LDD beyond local and regional scales has never been directly observed nor quantified. By sampling birds caught while in migratory flight by GPS-tracked wild falcons, we show that migratory birds transport seeds over hundreds of kilometres and mediate dispersal from mainland to oceanic islands. Up to 1.2% of birds that reached a small island of the Canary Archipelago (Alegranza) during their migration from Europe to Sub-Saharan Africa carried seeds in their guts. The billions of birds making seasonal migrations each year may then transport millions of seeds. None of the plant species transported by the birds occurs in Alegranza and most do not occur on nearby Canary Islands, providing a direct example of the importance of environmental filters in hampering successful colonization by immigrant species. The constant propagule pressure generated by these LDD events might, nevertheless, explain the colonization of some islands. Hence, migratory birds can mediate rapid range expansion or shifts of many plant taxa and determine their distribution.     

Waterbirds as endozoochorous dispersers of aquatic organisms: a review of experimental evidence

It is commonly assumed that waterbirds act as dispersal vectors of aquatic organisms. In this paper we review experimental work focusing on the endozoochorous transport of propagules by waterbirds with the aim to determine what aspects of this mechanism have been investigated. We discuss (i) the main issues addressed in propagule feeding experiments, (ii) aspects of the gut structure that affect the retrieval, retention time and viability of propagules, and (iii) the importance of assessing propagule retention time in the gut. A total of 26 experimental studies exist, which represent a very low number when compared to studies of seed dispersal in terrestrial systems. The diversity of questions dealt with in these articles was low, with the majority focusing exclusively on whether propagules survive gut passage. More comparative studies involving batches of disperser and dispersed species should be carried out, also to establish whether they possess characteristics that favour endozoochorous transport. It is important to assess the effect of intra- and inter-specific variation in the different sections of the waterbird gut on the fate of propagules. Additionally, experimental work dealing with the mechanism of endozoochorous dispersal can be separated into a sequence of events. Each event is characterised by its own probability of occurrence. In most studies, complete data sets relating to every step of the process have not been collected. In order to quantify dispersal probabilities of propagules such data are necessary.     

Egg masses on the move: corixid oviposition on terrapin shells

In a very dry year, when aquatic habitats were scarce, we observed exceptionally massive corixid oviposition on terrapin shells in Doñana National Park, southwestern Spain. Most terrapins inhabiting a large pond devoid of aquatic vegetation exhibited a dense cover of corixid eggs, whilst terrapins from vegetated ponds had no egg cover. Corixid oviposition had also been observed on terrapin shells in previous periods of severe drought. Considering that terrapins frequently alternate between aquatic and terrestrial environments, we experimentally assessed the viability of corixid eggs in varying degrees of dry-exposure. A significant proportion of corixid eggs did not hatch when they were occasionally dried under sun exposure, but egg mortality was lower when they were dried in the shade. The massive oviposition on terrapin shells was probably a consequence of the limited availability of adequate oviposition sites, and it might increase the odds of egg survival when terrapins migrate to other ponds.     

Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Species Richness and Distribution of Copepods and Cladocerans and their Relation to Hydroperiod and Other Environmental Variables in Doñana, South-west Spain

Twenty-five sites located in five wetland zones within Doñana, south-west Spain were studied for copepod and cladoceran species richness and composition in relation to habitat characteristics from January to March 2004. The γ-diversity of copepods and cladocerans combined varied between wetland zones, which differed significantly in conductivity, surface area and abundance of vegetation. However, there were no significant differences between zones in local species richness of either copepods or cladocerans or the two combined. Species richness was significantly higher in sites with intermediate hydroperiods (duration between 3 and 5 months) than in sites with shorter or longer hydroperiods. CCA analyses performed separately for copepods and cladocerans extracted conductivity, surface area, and vegetation cover as the main factors structuring species composition of both groups. Ours is one of the first studies of zooplankton in the natural marshes of the Doñana World Heritage Site, and we recorded five species new to the Doñana area.     

Diversity of waterbirds wintering in Douz wetlands (south Tunisia): factors affecting wetland occupancy and species richness

Understanding the environmental factors shaping wetland attractiveness for waterbirds is an important question in wetland ecology and for conservation purposes. However, detailed data from numerous North African wetlands, notably those situated in inland areas, are still lacking. Thus, the aim of this study was to identify the factors influencing wetland use by waterbirds wintering in one of such poorly known habitat systems, namely the Saharan wetland complex of Douz, in south-western Tunisia. Thirty-four waterbird species (20 wading birds and 14 waterfowl species) were found to winter in this area. Using discriminant function analyses, we found that wetland size was the unique variable discriminating between occupied and unoccupied sites for total waterbirds and wading birds, while waterfowl distribution was related to both wetland size and water depth. Wetland size also provided the strongest predictor of species richness of wading birds, waterfowl and total waterbirds. Overall, our findings highlight the importance of wetland size as a key factor determining the attractiveness of wetlands for waterbirds wintering in the Saharan wetland complex of Douz. The possible explanations of this wetland size effect are discussed.     

Temporal variation in the drift of plant litter and propagules in a small boreal river

SUMMARY 1. Rivers are linear ecosystems across landscapes with an effective transport of organisms, sediment and organic matter. Dispersal is studied mostly during single events and for single species, and there is little knowledge on how the drift of plant litter and propagules varies within and between years for entire communities.
2. We used floating traps for collecting waterborne plant litter and propagules in a small boreal river over 2 years. We installed the traps at four different locations along the river, and emptied them at least once a week during the ice-free season. We analysed propagule content by sorting and identifying species and through germination tests on bare soil.
3. In total, we recorded at least 54 taxa in the samples, and the highest density recorded in one sample was 5000 propagules per 100 g litter (dry weight). Large temporal variations in litter and propagule transport were revealed, both within and between years.
4. The longitudinal pattern was consistent between years, with an increasing mass of litter and number of propagule taxa downstream. The results highlight the importance of the temporal and longitudinal dimensions in river management.     

Rainfall stochasticity controls the distribution of invasive crayfish and its impact on amphibian guilds in Mediterranean temporary waters

Invasive crayfish have severely impacted invaded aquatic ecosystems worldwide. We studied temporal and spatial variation in the range expansion of the red swamp crayfish at one of the first European localities to which it was introduced: Doñana National Park (SW Spain). In contrast to the rapid range expansion witnessed in other areas, this invasive crayfish has not spread across the entire park. Instead, its distribution has expanded during wet periods, but contracted during drought periods. The red swamp crayfish has caused steep amphibian declines in other invaded areas. However, after approximately 35 years of crayfish presence in Doñana National Park, we have yet to detect a reduction in the number or occurrence of amphibian species. Amphibians may thus be protected by the large abundance of temporary ponds in the area, which provides them with an effective refuge network. We show that natural fluctuations in annual rainfall and in the number of ponds filled can temporarily eliminate invasive crayfish from particular areas. This fact should be taken into account when attempting to reduce the impact of crayfish on aquatic communities, intensifying crayfish removal during particularly dry years, when it is most effective.     

Passive external transport of freshwater invertebrates by elephant and other mud‐wallowing mammals in an African savannah habitat

1. Recent findings hint at the potential importance of mammals affecting the spatial dynamics of aquatic organisms in areas where mammals live in close association with water. Perhaps the most iconic example of such an environment is the African savannah. 2. We investigated dispersal patterns of freshwater organisms among a set of temporary ponds in SE Zimbabwe to test the hypothesis that large mammals, and particularly African elephants (Loxodonta africana), can be important vectors of aquatic organisms. Dispersal kernels were reconstructed by hatching mud collected from ‘rubbing’ trees located at increasing distances from a set of isolated ponds. To assess the relative importance of other mammalian vectors, the vertical distribution of mud on rubbing trees was mapped and related to the body size of candidate vector species. 3. Laboratory hatching of mud samples revealed large numbers of propagules of 22 invertebrate taxa as well as some aquatic macrophytes. Dispersing communities reflected source communities and diverged with increasing distance from the source. Both dispersal rates and richness of transported taxa decreased significantly with dispersal distance. No indications for differences in dispersal capacity among propagule types were detected. Instead, common propagules were more likely to travel greater distances. Most mud was attached to trees at heights >1.5 m, implicating elephants as the dominant vector. Vertical distributions of tree mud, however, also revealed clustering at heights up to 50 cm and 90–120 cm corresponding to the height of warthog, rhinoceros and buffalo, respectively. Finally, variation in the vertical distribution of mud on trees in combination with differences in vector vagility suggests that local differences in vector species composition may affect passive dispersal dynamics of aquatic organisms. 4. Based on vagility and vector load, mud‐wallowing mammals emerge as highly effective vectors that, in some areas, may be more important in transporting aquatic organisms than traditionally recognised vectors such as waterbirds. Since most large‐ and medium‐sized mammals currently have restricted geographic distributions, it is likely that mammal‐mediated dispersal was more important in the past.