Critical phases in the seed development of common juniper (Juniperus communis)

Common juniper (Juniperus communis L.) populations in northwest European lowlands are currently declining in size and number. An important cause of this decline is a lack of natural regeneration. Low seed viability seems to be one of the main bottlenecks in this process. Previous research revealed a negative relation between seed viability and both temperature and nitrogen deposition. Additionally, the seeds of common juniper have a variable ripening time, which possibly influences seed viability. However, the underlying mechanisms remain unresolved. In order to elucidate this puzzle, it is important to understand in which phases of seed production the main defects are situated, together with the influence of ripening time. In this study, we compared seed viability of populations with and without successful recruitment. We examined three seed phases: (i) gamete development; (ii) fertilisation and early‐embryo development; and (iii) late‐embryo development. After the first two phases, we found no difference in the percentage viable seeds between populations with or without recruitment. After late‐embryo development, populations without recruitment showed a significantly lower percentage of viable seeds. These results suggest that late‐embryo development is a bottleneck in seed development. However, the complex interaction between seed viability and ripening time suggest that the causes should be in the second seed phase, as the accelerated development of male and female gametophytes may disturb the male–female synchrony for successful mating.     

The effect of seed morphology on the potential dispersal of aquatic macrophytes by the common carp (Cyprinus carpio)

1. The potential for seed dispersal by fish (ichthyochory) will vary among aquatic plants because of differences in seed size and morphology. 2. To examine how seed morphology influences the probability of dispersal by the common carp (Cyprinus carpio), we studied seed ingestion, retention time and subsequent egestion and germination of seeds of Sparganium emersum and Sagittaria sagittifolia, two aquatic plant species with similar sized but morphologically different seeds. 3. We compared dispersal probabilities between the two plant species, in which the probability of dispersal is assumed to be a function of the probabilities of seed ingestion, egestion and germination, and the dispersal distance is assumed to be a function of seed egestion rate over time. 4. We found that, although the soft seeds of S. sagittifolia had an approximately 1.5 times higher probability of being ingested by the carp than the hard seeds of S. emersum (83.15% ± 1.8% versus 56.16% ± 2.7%, respectively), the latter had an almost twofold higher probability of surviving the passage through the digestive tract (38.58% ± 2.7% versus 20.97% ± 1.5%, respectively). Patterns of seed egestion over time did not differ between the two plant species, despite the difference in seed morphology. Gut passage had a different effect on seed germination between plant species. Compared with non‐ingested controls, seeds of S. emersum showed a 12.6% increase in germination and a 2.1 day acceleration in germination rate, whereas seeds of S. sagittifolia displayed a 47.3% decrease and 5.1 day delay, respectively. 5. Our results suggest that seed morphology affects the dispersal probability and postdispersal establishment, but not the dispersal distance, of aquatic plants that are dispersed by fish.     

鼠类对山杏（Prunus armeniaca）种子扩散及存活作用研究

虽然有关鼠类搬运森林种子的证据已很清楚,但这些初移走种子的存活情况却知之甚少。提出了一个新的标记和跟踪种子的方法－－标签法,即将种子拴一带有编码的细长金属片,研究了北京东灵山地区山杏（Prunus armeniaca）种子的扩散距离和存活率。于1998年6月19－20日,7月3日和10月23日共在24个样点释放1440粒山杏种子。几科所有释放的种子在10d内被鼠类取走,夏天释放的种子比秋天释放的种子消失的速度快。大多数种子的扩散距离在20m以内,小于鼠类的活动距离,鼠类吃掉种子的速度很快,但当种子变得稀少时,种子存活率有所提高。山杏种子6、7月份的每日存活率小于其它月份的每日存活率。     

Splash seed dispersal by raindrops

Splash seed dispersal by raindrops was investigated for plants in southern Japan. Nine families, 10 genera and 19 species were confirmed as raindrop-dispersed plants. The 10 genera were Gentiana, Gratiola, Chrysosplenium, Mazus, Mitella, Ophiorrhiza, Sagina, Sedum, Trigonotis and Veronica. The method of splash rain dispersal in these species was clarified. Raindrop-dispersed species were all small herbaceous plants with a vertical pedicel and an apically opening fresh capsule when the seeds mature. Open capsules were cup-shaped or boat-shaped and can accommodate raindrops easily. The raindrops splashed the seeds from the capsule. In general, the seeds weighed very little, but they were heavier than powder or dust seeds dispersed by wind. A strong negative correlation was found between seed weight and the number of seeds per capsule. In the case of Trigonotis brevipes (Maxim.) Maxim., raindrops were received into the cup-shaped calyx-tube and dispersed the fruitlets. Some species, such as Gentiana thunbergii (G. Don) Griseb., Gentiana zollingeri Fawcett and Ophiorrhiza japonica Blume, had hydroscopic movement capsules that opened widely only when wet. Raindrop-dispersed plants were found in various habitats. For example, some plants grew together on rocks along the mountain torrents where splash water could easily be caught. The results of the laboratory and field experiments indicated that the dispersal distance of seeds by raindrops was 1m or less. For small herbaceous plants, splash dispersal by rain might be an effective and advantageous method of seed dispersal because dispersal is not affected by plant height.     

Temporal and spatial patterns of seed dispersal in two Cistus species (Cistaceae)

Cistus species are obligate seeding, early colonizers that follow disturbance, particularly fire, in Mediterranean ecosystems. We studied seed release, seed dispersal and soil seed populations in stands of Cistus ladanifer and C. libanotis. Seed release started in mid- to late summer (C. ladanifer) or in early autumn (C libanotis), and continued for a very extended period: 8-10 months in C. ladanifer, and for a mean of 16 months in C. libanotis. The xerochastic capsules of both species released seeds by successive dehiscence of the locules. All capsules begin to dehisce simultaneously at the start of the seed release period, but in C. libanotis capsule fragmentation replaced dehiscence early in the seed release period. In plants of both species, seed shadows were characterized by a peak of density beneath the plant canopy and a very short tail of much lower densities, indicating that seeds are concentrated beneath mother plants when dispersed. Nevertheless, in late May, at the onset of the fire season, soil seed densities beneath plant canopies were low compared with densities expected from seed shadows, but were apparently high enough to allow recovery of the stands if a disturbance, such as fire, had taken place. Seed-eating Bruchidae in summer and granivorous ants during the seed release period were apparently the main causes of seed losses. Results suggest that in both Cistus species, the staggered seed release could constitute an efficient risk-reducing trait. The plant pool of seeds existing throughout most of the year could be a relevant component of Cistus seed banks.     

Endozoochory by beetles: a novel seed dispersal mechanism

Background and Aims

Due in part to biophysical sized-related constraints, insects unlike vertebrates are seldom expected to act as primary seed dispersers via ingestion of fruits and seeds (endozoochory). The Mediterranean parasitic plant Cytinus hypocistis, however, possesses some characteristics that may facilitate endozoochory by beetles. By combining a long-term field study with experimental manipulation, we tested whether C. hypocistis seeds are endozoochorously dispersed by beetles.

Methods

Field studies were carried out over 4 years on six populations in southern Spain. We recorded the rate of natural fruit consumption by beetles, the extent of beetle movement, beetle behaviour and the relative importance of C. hypocistis fruits in beetle diet.

Key Results

The tenebrionid beetle Pimelia costata was an important disperser of C. hypocistis seeds, consuming up to 17·5 % of fruits per population. Forty-six per cent of beetles captured in the field consumed C. hypocistis fruits, with up to 31 seeds found in individual beetle frass. An assessment of seeds following passage through the gut of beetles indicated that seeds remained intact and viable and that the proportion of viable seeds from beetle frass was not significantly different from that of seeds collected directly from fruits.

Conclusions

A novel plant–animal interaction is revealed; endozoochory by beetles may facilitate the dispersal of viable seeds after passage through the gut away from the parent plant to potentially favourable underground sites offering a high probability of germination and establishment success. Such an ecological role has until now been attributed only to vertebrates. Future studies should consider more widely the putative role of fruit and seed ingestion by invertebrates as a dispersal mechanism, particularly for those plant species that possess small seeds.     

Effects of black howler monkey (Alouatta caraya) seed ingestion on insect larvae

Fig (Ficus monckii) and laurel (Ocotea puberula) seeds were obtained from Alouatta caraya feces (ingested seeds) and from trees (noningested) in northeastern Argentina. Seeds were examined to detect the presence (infested) or absence (noninfested) of larvae. Sixty percent (N = 315) of noningested fig seeds were galls with insect larvae inside, while 23% (N = 331) were encountered in the ingested group. Eighty-two percent (N = 28) of noningested laurel seeds were infested, and only 19% (N = 63) of ingested seeds were infested. According to the present data, the insects' larvae are digested by howlers intaking animal protein, but the laurel seeds were not destroyed. Am. J. Primatol. 45:411–415, 1998. © 1998 Wiley-Liss, Inc.     

A seed predator drives the evolution of a seed dispersal mutualism

Although antagonists are hypothesized to impede the evolution of mutualisms, they may simultaneously exert selection favouring the evolution of alternative mutualistic interactions. We found that increases in limber pine (Pinus flexilis) seed defences arising from selection exerted by a pre-dispersal seed predator (red squirrel Tamiasciurus hudsonicus) reduced the efficacy of limber pine's primary seed disperser (Clark's nutcracker Nucifraga columbiana) while enhancing seed dispersal by ground-foraging scatter-hoarding rodents (Peromyscus). Thus, there is a shift from relying on primary seed dispersal by birds in areas without red squirrels, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents in areas with red squirrels. Seed predators can therefore drive the evolution of seed defences, which in turn favour alternative seed dispersal mutualisms that lead to major changes in the mode of seed dispersal. Given that adaptive evolution in response to antagonists frequently impedes one kind of mutualistic interaction, the evolution of alternative mutualistic interactions may be a common by-product.     

Effect of seed passage through vertebrate frugivores'' guts on germination: a review

The capacity of seeds to germinate after ingestion by frugivores is important for the population dynamics of some plant species and significant for the evolution of plant-frugivore interactions. In this paper the effects of different vertebrates on seed germination of nearly 200 plant species are reviewed, searching for patterns that predict the circumstances in which germination of seeds is enhanced, inhibited, or unaffected by the passage through the digestive tract of a seed disperser. It was found that seed dispersers commonly have an effect on the germinability of seeds, or on the rate of germination, or both, in about 50% of the plants they consume, although the diversity of animal species tested so far is still rather low (42 bird species, 28 non-flying mammals, 10–15 bats, 12 reptiles, 2 fishes). Enhancement of germination occurred about twice as often as inhibition.

In spite of the morphological and physiological differences in their digestive tracts, the different animal groups tested have similar effects on seed germination, although non-flying mammals tend to influence germination slightly more often than the other groups. Data on fishes are still too scarce for any generalization. Seed retention time in the dispersers' digestive tract is one factor affecting germination, and helps to explain the variation in seed responses observed among plant species, and even within a species. However other factors are also important; for example, the type of food ingested along with the fruits may affect germination through its influence on chemical or mechanical abrasion of the seed coat. Seed traits such as coat structure or thickness may themselves be responsible for some of the variation in seed retention times. Seeds of different sizes, which usually have different transit times through frugivores, and seeds of either fleshy or dry fruits, show often similar germination response to gut passage.

Seeds of different plants species differ strongly in their germination response after ingestion, even by the same frugivore species. Congeneric plants often show little consistency in their response. Even within a species variation is found which can be related to factors such as the environmental conditions under which germination takes place, seed morphology, seed age, and the season when the seeds are produced.

The effect of gut passage on germination differs between tropical and temperate zones. Seed germination of both shrubs and trees (data on herbaceous species are still scarce) in the temperate zone is more frequently enhanced than in the tropics. This result supports the hypothesis that enhanced germination may be more advantageous in unpredictable or less constant environments. Significant differences in frugivore-mediated germination are also found among different life forms. In both tropical and temperate zones, trees appear to be consistently more affected than shrubs or herbs. This might be due to an overall higher thickness of the seed coats, or to a higher frequency of seed-coat dormancy in tree species.

The influence of frugivory upon the population dynamics of a species has to be evaluated relative to other factors that influence germination and seedling recruitment at a particular site. Whether seed ingestion by dispersers is really advantageous to a plant (as has commonly been assumed) can only be assessed if we also determine the fate of the ingested seeds under natural conditions, and compare it to the fate of seeds that have not been ingested.     

The influence of temperature during seed development on the germination characteristics of millet seeds

Abstract. The cardinal temperatures, rate of germination and final percentage germination of pearl millet seeds were measured for seeds raised in greenhouses maintained at mean air temperatures of 19, 22, 25, 28 and 31°C. The results showed that cardinal temperatures for germination are unaffected by the temperature during seed development and growth. However, the conditions during seed growth did affect seed size and, subsequently, germination rate and seed viability.     

The ecology of seed dispersal in two species of callitrichid primates (Saguinus mystax and Saguinus fuscicollis)

Data collected during a 12-month field investigation of mixed species troops of Saguinus mystax and Saguinus fuscicollis in the Amazon Basin of north-eastern Peru indicate that callitrichid primates play an important role in tropical forest seed dispersal. Moustached and saddle-back tamarins were observed to ingest seeds from a variety of tree and liana species and pass them unharmed. These seeds tended to be large and heavy, and passed through the tamarin digestive tract in one to three hours. Experimental plantings of defecated seeds yielded a germination success rate of 70%. The specific gravity of these seeds (weight/volume) was inversely correlated with passage time and apparently had an indirect influence on the distance that seeds were dispersed from the parent tree. In the case of three preferred fruiting species, Leonia glycycarpa, Pourouma sp., and Hippocrateaceae #283, the present distribution of adult trees closely resembled the pattern of the seed shadow created by Saguinus. Moustached and saddle-back tamarins appeared to be reliable and high-quality dispersal agents for a number of tree and liana species. In this role, they are likely to exert an important influence on the composition, distribution, and regeneration patterns of Amazonian rain forest.