Comparing the potential for dispersal via waterbirds of a native and an invasive brine shrimp

1. Migratory waterbirds are likely to have a major role in the spread of many exotic aquatic invertebrates by passive dispersal. However, in the field, this has so far only been confirmed in the case of the American brine shrimp Artemia franciscana, which is spreading quickly around the Mediterranean region. 2. We compared experimentally the capacity of A. franciscana and the native brine shrimp Artemia parthenogenetica to disperse via migratory shorebirds. After Artemia resting eggs (cysts) were fed to Redshank Tringa totanus and Dunlin Calidris alpina, we compared the proportion that survived gut passage, their hatchability and their retention time within the gut. We also tested the ability of cysts to stick to the feathers of Black‐tailed Godwit Limosa limosa. 3. The proportion of ingested cysts retrieved from faeces was the same for each Artemia species (8%), and there were no significant differences in retention time (mean 1.2 h and maximum 10 h for A. parthenogenetica, 1.4 and 12 h for A. franciscana) or hatchability (11% versus 14%). The two shorebird species showed similar retention times and retrieval rates, but cysts recovered from Dunlin had a significantly higher hatchability. Only one of the 1000 A. parthenogenetica cysts and three of the 1000 A. franciscana cysts stuck to feathers. 4. These results indicate that both non‐native and native brine shrimps have a similar high capacity for endozoochory via birds, and that the invasiveness of A. franciscana is probably explained by its competitive superiority owing to high fecundity and release from cestode parasitism. Owing to their different migratory behaviour, Redshank and Dunlin are likely to have different roles as brine shrimp vectors. Brine shrimps provide a suitable model for understanding the role of birds in the dispersal of exotic aquatic invertebrates.     

Did fleshy fruit pulp evolve as a defence against seed loss rather than as a dispersal mechanism?

Relatively few studies have examined the evolution of the mutualism between endozoochorous plants and seed dispersers. Most seed dispersal studies are ecological and examine the role of fruit pulp in promoting seed dispersal. This interaction is often assumed to have originated due to selection stemming from seed dispersers. Here I suggest a "defence scenario" wherein fleshy fruits originated as mechanisms to defend seeds and secondarily became structures to promote seed dispersal. I suggest that frugivory followed from herbivores that specialized on consuming seed defensive tissues and that enhanced seed dispersal was initially a consequence of seed defence. The proposed defence scenario is not posited as an explanation for the sequence that led to all modern frugivores. However, it is suggested that seed predation was the initial source of selection that led to fleshy fruits; the necessary precursor to frugivory. Support is described from the fossil record and from modern structures and interactions. Testable predictions are made in hope that greater interest will be focused on the defensive role of fleshy fruit pulp both in modern interactions and historically.     

Endozoochorous seed dispersal by cattle and horse in a spatially heterogeneous landscape

Seed dispersal has become an important issue in plant ecology and restoration management. In this paper we examined dung germinating seed content and seed deposition patterns of horses (Shetland and Konik breeds) and Scottish Highland cattle grazing two coastal dune nature reserves. Two times 2.5 l of fresh dung from each type of herbivore were collected during seven sessions in the main fruiting season. Dung samples were placed under greenhouse conditions after drying and cooling. Animal defecation patterns were derived from a study of herbivore activities during 6 h observation sessions 8 times a month. One hundred and seventeen plant species i.e. 27% of all species occurring in the study area, were recorded as seedlings emerging from the dung samples. The most abundantly and frequently recorded plant species were Urtica dioica, Juncus spp. and different species of Poaceae and Caryophyllaceae. In general seedling density is high (1158 seedlings/dung sample). Seedling density and species richness were further analysed in order to detect temporal variability and possible animal and site related characteristics. Dung deposition patterns reflect a non-random use of habitats and hence a non-random seed deposition among habitats. Calculated seed deposition per square meter ranged from a few (<10 germinating seeds) to more than 100 in the most frequently selected habitats. From the herbivores’ selective habitat use and their estimated mean retention time we can further assume their ability for inter-habitat endozoochorous seed dispersal. This characteristic of large herbivores is further discussed in the light of nature management and restoration.Indra Lamoot is an aspirant of FWO-Flanders.     

Testing the potential seed availability in dung samples: comparison of two seedling emergence methods

To optimize the estimation of species composition and viable seed content of herbivore faeces and to make different approaches comparable, two seedling emergence methods are evaluated. The Ter Heerdt method (TH) employs concentrated samples, potentially increasing and accelerating seedling emergence, as shown for soil samples (95% of all seedlings emerged within 6 weeks). Samples are kept under controlled conditions (glasshouse or climate room). Secondly, a common garden method (CG) using unconcentrated samples so that seasonal changes could fulfill the germination requirements of a broad species spectrum (experiment duration approximately 15 months) was applied. The methods were tested by the use of sheep faeces samples, collected during a six-day grazing period in a threatened dry grassland (Allio-Stipetum capillatae).Both methods proved largely similar in species composition (QS=0.81) and viable seed content (QS=0.69). More species (e.g. monocotyls) and a higher seedling emergence of hard-seeded species (Fabaceae and Cistaceae) were found in the CG method. Besides a higher emergence of some small-seeded winter annuals, few other species emerged exclusively by use of the TH method. Nevertheless, all species detected by only one method were found in low individual numbers (4).Depending on research interest and availability of space and time, the most appropriate method can be chosen. If the main focus is on the species composition, unconcentrated faeces samples can be studied by CG. In case the overall viable seed content is more important and/or a shorter time period is available, TH serves as a suitable alternative.     

Impact of different‐sized herbivores on recruitment opportunities for subordinate herbs in grasslands

Abstract. Potential effects of herbivores on plant species diversity depend on herbivore size, species and density. In this study we examine the effect of different‐sized herbivores (cattle and rabbits) on recruitment of subordinate herbs in grasslands. We show that in a grazed floodplain, grassland plant species richness is mainly determined by the presence of many species of subordinate herbs. These herbs experience high colonization and extinction rates. We conclude that the creation of colonization opportunities for subordinate herbs plays a crucial role in maintaining plant species richness in productive grasslands. We found that cattle disperse large amounts of seeds via their dung, over ten times more than rabbits. Rabbits create more and on average larger bare soil patches than cattle. In a field experiment artificial disturbances improved germination success tremendously for four tested herb species. We found that bare soil is the best regeneration site, while cattle dung gave a too strong nutrient stimulus, resulting in tall vegetation and therefore light limitation. These results can be confirmed with results from field monitoring plots where plant species richness was positively related to the occurrence of bare soil patches. Therefore both large and small herbivores have a major impact on dispersal and colonization, but for different reasons. Cattle are identified as most important for seed dispersal whereas rabbits have a main effect as creators of disturbances. These results emphasize the importance of distinguishing between herbivore species in assessing their (potential) effects.     

The effect of seeds of exotic species transported via horse dung on vegetation along trail corridors

It has been suggested that exotic species will colonize within forests more frequently by the continual introduction of seeds through horse dung deposited along trails. Whether or not these exotic species have the ability to spread into and establish in the forest interior has been disputed. To address this, horse dung and soil samples were collected from trails during Autumn 1994 and Summer 1995 from three areas in southern Illinois, USA open to recreational horse travel. In addition, deer dung samples were collected from each of the study areas. Vegetation data were collected from each of the trail systems as well as from a trail along which horse travel was prohibited. The density of vascular plants in 0.25 m² quadrats placed at varying distances from the trail center to 5 m into the forest interior were recorded. Finally, dung samples were placed in situ along horse trails at one site to examine seedling germination in natural conditions. While 23 exotic species germinated from samples of horse dung placed out in a greenhouse, only one of these exotic species was also found in trail plots (Kummerowia striata). Similarly, while there were empirically more exotic species found along the trails allowing horse travel than there were on the trail lacking horse travel, the relative importance of those species was negligible along both trails. These results suggest that the emigration of exotic species via horse dung does not pose an immediate threat to the plant communities adjacent to trails in these forest systems. Nevertheless, the large number of exotic species in horse dung reflects the constant threat to any system from these species. Care must be taken, when allowing horseback use in areas, to anticipate invasion by exotic species from horse dung     

Effects of bird ingestion on seed germination of four woody species of the temperate rainforest of Chiloé island, Chile

We study the effect of ingestion by birds on seed germination and theconsequences of absence of dispersal, with the persistency of the seedsinside the fruit. We collected seeds of four woody species ofthe temperate rainforest of Chiloé: Gaultheriamucronata, Luma apiculata, Myrteolanummularia, and Myrceugenia planipes. The seedstested had the following origins: 1) Ingested seeds: seeds collected fromthe feces of birds, 2) Extracted seeds: seeds obtained directly from thefruits, and 3) Intact fruits: fruits collected directly from the plants.Germination of Myrceugenia planipes under greenhouseconditions, Luma apiculata, and Myrteolanummularia under laboratory conditions, and Gaultheriamucronata under both conditions was analyzed. We found that the seedsreach their maximum germination between 15–20 days after sowing, withthe exception of those of G. mucronata sown in the greenhouse,which showed a low germination rate. In the greenhouse assay, seeds ofG. mucronata ingested by birds, seeds extracted manuallyfrom the fruits, and seeds inside the fruits did not show significant differencesin their germination percentages. In the laboratory assays, the seeds ofG. mucronata and M. nummulariaingested by birds and the seeds extracted manually from the fruits also did not show anysignificant difference in germination. Under laboratory conditions, theseeds of L. apiculata ingested by birds presented astatistically greater percentage of germination than the seeds extracted manually.Under greenhouse conditions, seeds of M. planipes ingestedby birds did not present a statistically different germination percentage fromthose seeds extracted from the fruits. The seeds of M.planipes, and L. apiculata inside the intactfruits did not germinate. We conclude that birds do not affect the seedviability of any of the four species studied.     

Herbivorous mammals as seed dispersers in a Mediterranean dehesa

Endozoochrous seed dispersal by herbivorous mammals has been verified repeatedly and its possible influence on the structure and function of herbaceous communities has been suggested. Quantitative studies, however, are lacking in the field of seed dispersal via the dung of herbivore guilds in little-altered environments. The present paper analyses seed dispersal via rabbit, fallow deer, red deer and cow dung in a Mediterranean dehesa (open woodland used for hunting and ranching) during the seeding season. Dung seed content was determined by the glasshouse cultivation of eight dung samples from each herbivore, collected fortnightly between February and August. The four herbivores disperse many seeds (spring averages are 6–15 seeds per gram of dry dung and maxima of 25–70) from a large number of species (totals between 52 and 78). Dispersal seems to be mainly determined by seed production of the plant comminity. This is reflected in (i) the dissemination of a high percentage of the species present in the dehesa, (ii) great seasonal variability, related to seed production, in the amount of seeds and number of species dispersed, and (iii) a high semi-quantitative similarity of seed content in the four types of herbivore dung throughout the year. There is also important quantitative variation that depends on animal traits and feeding habits. These results and the characteristics of species found in dung suggest the adaptation of plant species to the dispersal of their seeds via herbivore gut. This process may well have profound implications for vegetation dynamics and the evolution of plant traits.     

When condition trumps location: seed consumption by fruit‐eating birds removes pathogens and predator attractants

Seed ingestion by frugivorous vertebrates commonly benefits plants by moving seeds to locations with fewer predators and pathogens than under the parent. For plants with high local population densities, however, movement from the parent plant is unlikely to result in ‘escape’ from predators and pathogens. Changes to seed condition caused by gut passage may also provide benefits, yet are rarely evaluated as an alternative. Here, we use a common bird‐dispersed chilli pepper (Capsicum chacoense) to conduct the first experimental comparison of escape‐related benefits to condition‐related benefits of animal‐mediated seed dispersal. Within chilli populations, seeds dispersed far from parent plants gained no advantage from escape alone, but seed consumption by birds increased seed survival by 370% – regardless of dispersal distance – due to removal during gut passage of fungal pathogens and chemical attractants to granivores. These results call into question the pre‐eminence of escape as the primary advantage of dispersal within populations and document two overlooked mechanisms by which frugivores can benefit fruiting plants.