Unburied Seeds Research Articles

Dung beetles as secondary seed dispersers: impact on seed predation and germination

Dispersal of seeds away from the parent plant may facilitate escape from density-dependent seed mortality. However, many post-dispersal events can have a profound influence on the survival of dispersed seeds. By incorporating seeds in the dung that dung beetles process for consumption and oviposition, dung beetles could enhance seed survival if they remove seeds from areas of high predation risk and place them in locations that avoid subsequent predation and that are suitable for germination. The role of dung beetles in seed survival was investigated over 15 mo in Kibale National Park, Uganda. Depths of seeds buried by beetles, levels of predation on buried and unburied seeds, and germination success of seeds buried to different depths were examined. Results suggest that by burying seeds dung beetles increase the probability that seeds will escape predation and germinate. Of seeds placed in dungpiles, 69% remained at the surface, while 25% were buried from 1–3 cm in depth. Larger seeds were buried more shallowly than smaller seeds. Buried seeds were less likely to be removed by predators than seeds at the surface. Germination of seeds buried at 1- and 3-cm depths was significantly higher than seeds buried at 10 cm. For the species tested, many seeds were buried by dung beetles between 1 and 3 cm and at this depth there was a high probability of escaping predators and germinating. This demonstrates the potential ecological importance of dung beetles in facilitating seed survival and provides data to consider the role of dung beetles in the evolution of seed attributes.

Sexual Dimorphism of Dormancy and Survivorship in Buried Seeds of Silene Latifolia

1 To test for differences in survivorship between buried male and female seeds in the dioecious perennial, Silene latifolia (Caryophyllaceae), the behaviour of large numbers of seeds buried under field conditions and unearthed after 5 and 10 months was compared with unburied seeds. 2 After 5 months of burial there was significant mortality of seeds and a large fraction of the remaining seeds displayed induced dormancy, i.e. they did not germinate under glasshouse conditions. The sex ratio of germinating seeds was not significantly different from the sex ratio of seeds germinated without burial (0 months). 3 Seed survivorship was constant from 5 to 10 months. However, by 10 months levels of dormancy had decreased so that most seeds germinated when unearthed and tested in a growth chamber. Furthermore, the proportion of females in germinating seed was significantly higher after 10 months than after 5 months. 4 Sex differences in mortality were therefore only revealed after 10 months, when dormancy had been broken. This result suggests that female seeds were more likely to become dormant during the first 5 months of burial, whereas males suffered greater mortality. Thus, sex differences in both survivorship and dormancy of buried seeds affected the sex ratio of germinating seeds over time. Such sexual dimorphism in seed bank dynamics may have an important effect on sex ratio variation in natural populations of dioecious plants.

Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes

The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhone delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure. In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged. In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity. The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years.

Evidence for secondary seed dispersal by rodents in Panama.

The data presented show thatVirola nobilis (Myristicaceae), a bird/mammal-dispersed tree species in Panama, may also be dispersed by a terrestrial rodent, the agouti (Dasyprocta punctata). Using a thread-marking method, we observed that agoutis scatterhoardedV. nobilis seeds that they found both singly or in clumps. Seed removal and seed burial rates were strongly affected by features of forest habitats, such asV. nobilis tree richness (rich vs poor) and/or forest age (old vs young), but not by seed dispersal treatment (scattered vs clumped). Predation (mostly post-dispersal) of unburied seeds by weevils was independent of habitat and dispersal treatment. Seeds artificially buried in aVirola-rich area were more likely to escape predation and become established than unburied seeds under natural conditions. The food reward for agoutis is in the germinating seedlings. The seed dispersal syndrome ofV. nobilis involves long- and short-distance dispersers which both appear important for tree recruitment.

Seed-dispersal ofVouacapoua americana(Caesalpiniaceae) by caviomorph rodents in French Guiana

ABSTRACTA possible mutualistic dispersal system between a large-seeded tree of French Guiana,Vouacapoua americana(Caesalpiniaceae), and caviomorph rodents,Myoprocta exilisandDasyprocta leporina, is described. Mast fruiting ofVouacapouaat the beginning of the wet season coincides with scatter-hoarding seed dispersal. During the wet season, almost 100% of marked seeds on three sites were removed: nearly 70% were buried and the rest were eaten by mammals. Unburied seeds were attacked by insects and/or lost their ability to germinate. Rodents preferred ungerminated seeds, and had no interest in germinated seeds. Seeds were buried individually near natural objects such as palms, branches, logs, lianas, roots and trees. After predation by rodents, seedling distribution did not differ from seed distribution. Most seeds were transported less than 5 m from the feeding plots but some were carried as far as 22.4 m. Between 40 and 85% of dispersed seeds were retrieved during the following month by rodents and eaten. The disinterest of caviomorph rodents in germinated seeds, because of rapid exhaustion of endosperm reserves, prevents feeding from hoardedVouacapouaduring the long dry season when resources are scarce. Seedlings emerging from forgotten or abandoned cached seeds appear to increase the recruitment ofVouacapoua americana.

Efficacy of Explosive vs. Hygroscopic Seed Dispersal by an Annual Grassland Species

Seeds of Erodium moschatum (Geraniaceae) were moved on average 56 cm by primary, explosive dispersal and then traveled another 7 cm across the soil surface through activity of the hygroscopic awn, although not necessarily away from the source. In a 10-day field experiment, 68% of the diaspores buried themselves through activity of the awn. The percentage of seeds becoming buried per day was not correlated with rainfall. More seeds became buried on plant-littered ground, with litter cover from 8 to 61%, than on bare soil. As soil compaction increased, burial on littered surfaces declined. In laboratory experiments using straw to simulate sparse densities of plant litter, the percent of burial was enhanced, but sticks simulating plant stems did not facilitate burial. In a greenhouse experiment, more buried seeds germinated than unburied seeds. Buried seeds produced seedlings that developed more quickly, had higher survivorship and had greater shoot biomass than those from unburied seeds. Self-burial also ensured that the radicle was positioned downward, which probably contributed to the observed higher survivorship. Overall, in semiarid grasslands, plant-littered microhabitats are probably more favorable sites for the placement of seeds than bare areas (e.g., compacted gopher mounds). Littered sites promote self-burial and burial promotes seedling establishment, survival and growth.

EFFICACY OF EXPLOSIVE VS. HYGROSCOPIC SEED DISPERSAL BY AN ANNUAL GRASSLAND SPECIES

Seeds of Erodium moschatum (Geraniaceae) were moved on average 56 cm by primary, explosive dispersal and then traveled another 7 cm across the soil surface through activity of the hygroscopic awn, although not necessarily away from the source. In a 10‐day field experiment, 68% of the diaspores buried themselves through activity of the awn. The percentage of seeds becoming buried per day was not correlated with rainfall. More seeds became buried on plant‐littered ground, with litter cover from 8 to 61%, than on bare soil. As soil compaction increased, burial on littered surfaces declined. In laboratory experiments using straw to simulate sparse densities of plant litter, the percent of burial was enhanced, but sticks simulating plant stems did not facilitate burial. In a greenhouse experiment, more buried seeds germinated than unburied seeds. Buried seeds produced seedlings that developed more quickly, had higher survivorship and had greater shoot biomass than those from unburied seeds. Self‐burial also ensured that the radicle was positioned downward, which probably contributed to the observed higher survivorship. Overall, in semiarid grasslands, plant‐littered microhabitats are probably more favorable sites for the placement of seeds than bare areas (e.g., compacted gopher mounds). Littered sites promote self‐burial and burial promotes seedling establishment, survival and growth.