槲寄生的生物学特征及鸟类对其种子的传播

2001年9月至2002年1月在黑龙江省尚志市帽儿山林场研究了槲寄生(Viscum coloratum)的生物学特征及鸟类对其种子的传播。槲寄生寄主共有9种阔叶树,杨树占比例最大,达73．3％。78．0％的槲寄生个体以杨树为寄主。多数槲寄生着生在杨树主干侧枝(43％)和细侧枝(42％)上,主干上仅l5％。寄主和槲寄生本身都呈聚集分布。槲寄生有两种色型果实,在量度上无明显差异。果实由果皮、果肉和种子构成,种子外面包裹一层胶质层。在帽儿山秋、冬季常见的7种食果实鸟类中,只有太平鸟（Bombycilla garrulus)在野外觅食槲寄生果实,斑鸫（Turdus naumanii)在笼养无其它食物时才食,其余5种鸟类拒食。太平鸟在11月以后觅食槲寄生果实频率增大。除根寄生果实外,太平鸟共可食帽儿山常见l7种肉质果中的12种。太平鸟在吞食果实后,种子外包胶质层随粪便排出,形成“种子线”粘在树枝上,槲寄生种子被传播到适宜的生境,在春天萌根生长。笼养太平鸟时,种子在消化道的滞留时间约11min,这意味着在野外很长的扩散距离。槲寄生和太平鸟之间形成了特定的互利互生系统。     

Predicting mistletoe seed shadow and patterns of seed rain from movements of the mistletoebird,Dicaeum hirundinaceum

Abstract At the scale of an individual host, mistletoes are aggregated in space, resulting from the preferential perching of mistletoe dispersers on previously infected plants. We hypothesized that the landscape scale movement patterns of mistletoe dispersers will also promote the aggregation of mistletoes. This hypothesis was tested by predicting the seed shadow for box mistletoe Amyema miquelii (Loranthaceae) and patterns of seed rain, by combining radiotelemetry data of mistletoebird Dicaeum hirundinaceum (Dicaeidae) movements with existing gut passage time data. Thirteen adult mistletoebirds had a mean home range of 20 ha, with core activity areas of approximately 1 ha, and birds more often used areas with high levels of mistletoe infestation. The predicted seed shadow of box mistletoe was leptokurtic, with a 12%, 35% and 23% probability of mistletoe seed being deposited 0 m (same host tree), 1–50 m and 51–100 m from the host tree, respectively. Although rare (3% probability), long distance dispersal of mistletoe (>500 m) can occur. The predicted patterns of seed rain were strongly aggregated, with birds dispersing large amounts of seed (>66 000 per ha) in areas with higher mistletoe infestation levels. The movements of mistletoe dispersers will therefore promote mistletoe aggregation at a landscape scale.     

Primate seed dispersal: Coevolution and conservation implications

Early studies of primates have demonstrated that many species rely heavily on fruit, and that primates constitute a large component of the frugivore biomass in tropical forests. Consequently, primates have long been thought to be important seed dispersers. It is only recently that studies have been conducted that have illustrated the complex nature of the interactions between fruit-eating primates and their food trees. Such studies have raised questions as to the causes and conse-quences of the intriguing differences between primate communities, the importance of other animals in the interactions (such as dung beetles and rodents that secon-darily disperse seeds), how primate-plant interactions evolve, and the significance of primates in forest regeneration and conservation. Since subsistence and com-mercial hunting of primates has heavily impacted frugivore communities, but left the physical structure of the forest relatively unaltered, studies of primate seed dispersal have important implications for the future of forests where seed dispersers have declined or disappeared.     

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.     

Consequences of frugivore-mediated seed dispersal for the spatial and genetic structures of a neotropical palm

The idiosyncratic behaviours of seed dispersers are important contributors to plant spatial associations and genetic structures. In this study, we used a combination of field, molecular and spatial studies to examine the connections between seed dispersal and the spatial and genetic structures of a dominant neotropical palm Attalea phalerata. Field observation and genetic parentage analysis both indicated that the majority of A. phalerata seeds were dispersed locally over short distances (<30 m from the maternal tree). Spatial and genetic structures between adults and seedlings were consistent with localized and short-distance seed dispersal. Dispersal contributed to spatial associations among maternal sibling seedlings and strong spatial and genetic structures in both seedlings dispersed near (<10 m) and away (>10 m) from maternal palms. Seedlings were also spatially aggregated with juveniles. These patterns are probably associated with the dispersal of seeds by rodents and the survival of recruits at specific microsites or neighbourhoods over successive fruiting periods. Our cross-cohort analyses found palms in older cohorts and cohort pairs were associated with a lower proportion of offspring and sibling neighbours and exhibited weaker spatial and genetic structures. Such patterns are consistent with increased distance- and density-dependent mortality over time among palms dispersed near maternal palms or siblings. The integrative approaches used for this study allowed us to infer the importance of seed dispersal activities in maintaining the aggregated distribution and significant genetic structures among A. phalerata palms. We further conclude that distance- and density-dependent mortality is a key postdispersal process regulating this palm population.     

胡杨种子散布的时空分布格局

以额济纳胡杨为研究对象,对种子雨的散布时间、强度、散布距离以及种子雨和空气湿度、风之间的关系进行了研究。胡杨种子雨可以分为初始期、高峰期和消退期3个阶段,大部分的种子集中在高峰期落下。种子的散布主要受湿度和风的影响。湿度对种子雨的强度起主要作用,在一天之中,种子在湿度较低的中午和下午集中散落。应用一元线性回归模型对种子雨强度和相对湿度进行分析后表明二者之间存在显著的负相关关系。对种子的散播距离进行研究后发现,大部分种子落在母树附近,少部分种子能够进行长距离传播。风对种子的传播的方向和距离起决定性的作用,不同方向上的种子传播距离和强度相差很大。在顺风方向上,种子的传播距离最远,所有的长距离传播现象几乎都发生在这一方向上;而在主风向的垂直方向和逆风方向上,种子的散布距离较小,很少有种子能够进行长距离传播。对风的观测表明中午后和下午初的风力较强,而此时种子雨强度又最大,有利风力条件和高种子雨强度出现的同步性可能是促进胡杨种子进行长距离传播最有效的生物控制机制。由于胡杨种子在自然条件下的存活时间非常短暂,所以研究中不同胡杨母树林间种子散播时间的差异可能是胡杨种群内部为适应不同洪水期所表现出的风险分摊机制所造成的。     

Scale and macroecological patterns in seed dispersal mutualisms

Although some studies of seed dispersal mutualisms have documented adaptive relationships between fruits and frugivores, others have shown that adaptive patterns are constrained by phylogenetic, historical or climatic effects. Variable results among studies have thwarted attempts to find a paradigm to unite the field and direct research. Two recent studies in Global Ecology and Biogeography exemplify this dichotomy. One paper reported adaptive relationships between abundances of birds and fruits, while the other study found that bird‐fruit abundance patterns were constrained by climatic effects. Almost paradoxically, both studies were conducted at the same locale. However, they focused on different spatio‐temporal scales. These results are surprisingly consistent with several other recent studies that have taken a macroecological approach. They also indicate that mutualistic relationships between fruits and frugivores are scale dependent. When viewed together, recent work suggests that the conflicting results of previous studies may result from spatio‐temporal variability of mutualistic relationships. This paper briefly reviews the emerging field of seed dispersal macroecology. A growing appreciation for scale appears to be leading the field in a new direction.     

Ant seed removal in a non-myrmecochorous Neotropical shrub: Implications for seed dispersal

This study investigated ant seed removal of Piper sancti-felicis, an early successional Neotropical shrub. Neotropical Piper are a classic example of bat-dispersed plants, but we suggest that ants are underappreciated dispersal agents. We identified eleven ant species from the genera Aphaenogaster, Ectatomma, Paratrechina, Pheidole, Trachymyrmex, and Wasmannia recruiting to and harvesting P. sancti-felicis seeds in forest edge and secondary forest sites at La Selva, Costa Rica. We also tested for differences in ant recruitment to five states in which ants can commonly encounter seeds: unripe fruit, ripe fruit, overripe fruit, bat feces, and cleaned seeds. Overall, ants harvested more seeds from ripe and overripe fruits than other states, but this varied among species. To better understand the mechanisms behind ant preferences for ripe/overripe fruit, we also studied how alkenylphenols, secondary metabolites found in high concentrations in P. sancti-felicis fruits, affected foraging behavior in one genus of potential ant dispersers, Ectatomma. We found no effects of alkenylphenols on recruitment of Ectatomma to fruits, and thus, these compounds are unlikely to explain differences in ant recruitment among fruits of different maturity. Considering that P. sancti-felicis seeds have no apparent adaptations for ant dispersal, and few ants removed seeds that were cleaned of pulp, we hypothesize that most ants are harvesting its seeds for the nutritional rewards in the attached pulp. This study emphasizes the importance of ants as important additional dispersers of P. sancti-felicis and suggests that other non-myrmecochorous, vertebrate-dispersed plants may similarly benefit from the recruitment to fruit by ants.     

Contemporary pollen and seed dispersal in a Prunus mahaleb population: patterns in distance and direction

Pollination and seed dispersal determine the spatial pattern of gene flow in plant populations and, for those species relying on pollinators and frugivores as dispersal vectors, animal activity plays a key role in determining this spatial pattern. For these plant species, reported dispersal patterns are dominated by short-distance movements with a significant amount of immigration. However, the contribution of seed and pollen to the overall contemporary gene immigration is still poorly documented for most plant populations. In this study we investigated pollination and seed dispersal at two spatial scales in a local population of Prunus mahaleb (L.), a species pollinated by insects and dispersed by frugivorous vertebrates. First, we dissected the relative contribution of pollen and seed dispersal to gene immigration from other parts of the metapopulation. We found high levels of gene immigration (18.50%), due to frequent long distance seed dispersal events. Second, we assessed the distance and directionality for pollen and seed dispersal events within the local population. Pollen and seed movement patterns were non-random, with skewed distance distributions: pollen tended moved up to 548 m along an axis approaching the N-S direction, and seeds were dispersed up to 990 m, frequently along the SW and SE axes. Animal-mediated dispersal contributed significantly towards gene immigration into the local population and had a markedly nonrandom pattern within the local population. Our data suggest that animals can impose distinct spatial signatures in contemporary gene flow, with the potential to induce significant genetic structure at a local level.     

杂草种子传播研究进展

种子传播将母株生殖周期的末端与它们后代种群的建立连结了起来,广泛认为,其对植被结构具有深刻的影响。种子传播的整个过程称为种子传播循环。研究表明,杂草种子传播的因子多种多样,包括仅依赖自身来完成的主动传播,以及依赖风、水、动物、人类等外界媒介的被动传播。其中,人类传播杂草种子是影响最广泛的一种,对现代植物的分布格局产生了深刻的影响。杂草种子的传播,对杂草种子库的数量和空间动态影响很大。研究种子传播的主要方法有荧光染料标记法、放射性同位素标记法、稳定同位素分析、分子遗传标记等。结合近几年国内外的研究进展,作者就杂草种子传播对种子库数量和空间动态影响的精确直接研究、杂草种子传播的过程及传播后的命运、杂草种子适应传播的机理、生态控草措施研究、外来杂草入侵蔓延与其种子传播的关系等方面提出了展望。     

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.     

Host spatial structure and disperser activity determine mistletoe infection patterns

What processes and factors are responsible for species distribution are long-standing questions in ecology and a key element for conservation and management. Mistletoes provide the opportunity to study a forest species whose occurrence is expected to be constrained by multiple factors as a consequence of their life form. We studied the mistletoe Tristerix corymbosus (Loranthaceae) on its most common hosts species in northwest Patagonia. The seeds of this mistletoe are almost exclusively dispersed by the small arboreal and endemic marsupial Dromiciops gliroides (Microbiotheridae). We assessed the underlying causes of plant spatial patterns through point pattern analysis and we used different variables that characterize the neighborhood around each host to analyze the relative effect of host availability, potential for disperser movement and canopy light conditions. We found that potential hosts were strongly aggregated and that the three most common host species were distributed independent of each other. Considering all host species together, infected and non-infected host were individually aggregated but segregated from each other. The aggregated pattern of infected hosts could be explained in part by the template of potential hosts distribution, but was subsequently modulated by the activity of the mistletoe disperser. Potential for disperser movement, the proximity to reproductive mistletoes and habitat complexity, increased mistletoe infection probability. However, neighboring host availability decreased mistletoe infection probability, and tree DBH (used as surrogate for light conditions) had no detectable effect. Our results suggested that the distribution of mistletoe infection was determined by the structure of potential host populations and by the marsupial disperser activity. Compared to bird dispersed mistletoes, the scale of the infection was smaller and the proximity to reproductive mistletoes and habitat complexity were important for seed arrival and infection. The interplay between landscape structure and disperser activity determine the spatial structure of mistletoe future generations.