Seed dispersal spectra: a comparison of temperate plant communities

Abstract. We compare the dispersal spectra of diaspores from varied plant communities in Australia, New Zealand, and North America, assigning dispersal mode to each diaspore type on the basis of apparent morphological adaptations. Species with ballistic and external dispersal modes were uncommon in most communities we surveyed. Ant dispersal was also rather uncommon, except in some Australian sclerophyll vegetation types. The frequency of vertebrate dispersal ranged up to 60% of the flora, the highest frequencies occurring in New Zealand forests. Wind dispersal ranged as high as 70% of the flora, with the highest values in Alaska, but usually comprised 10–30% of the flora. Many species in most communities had diaspores with no special morphological device for dispersal. Physiognomically similar vegetation types indifferentbiogeographic regions usually had somewhat dissimilar dispersal spectra. The frequency of dispersal by vertebrates often increased and the frequency of species with no special dispersal device decreased along gradients of increasing vertical diversity of vegetation structure. Elevation and moisture gradients also exhibited shifts in dispersal spectra. Within Australia, vertebrate- and wind-dispersal increased in frequency along a soil-fertility gradient, and dispersal by ants and by no special device decreased. Habitat breadths (across plant communities) and microhabitat breadths (within communities) for species of each major dispersal type did not show consistent differences, in general. Ant-dispersed species often had lower cover-values than other species in several Australian vegetation types. We discuss the ecological bases of these differences in dispersal spectra in terms of the availability of dispersal agents, seed size, and other ecological constraints. Seed size is suggested to be one ecological factor that is probably of general relevance to the evolution of dispersal syndromes.     

Conflicting selection on diaspore traits limits the evolutionary potential of seed dispersal by ants

Conflicts of selection on diaspore traits throughout the dispersal cycle can limit the evolutionary consequences of seed dispersal. However, these conflicts have never been investigated in directed dispersal systems. We explored conflicts of selection through life stages of dispersal in the myrmecochorous herb Helleborus foetidus. Seeds are subject to two contrasting partial selective scenarios. Undispersed seeds are subject to positive directional selection on seed size characters, whereas seeds dispersed are subject to stabilizing selection for size. In both scenarios, seedling establishment determined the magnitude and direction of selection. This does not reflect ant preferences for seed size. However, total selection still depends largely on ant activity, as ants control the relative importance of each selective scenario. We advocate the use of analytical approaches combining multiplicative fitness and microenvironment‐specific selection to more realistically estimate the realized selection on traits functional during several life stages. This approach may be extended to any organism dispersing offspring to different environments.     

Soil seed banks near rubbing trees indicate dispersal of plant species into forests by wild boar

Current knowledge about processes that generate long-distance dispersal of plants is still limited despite its importance for persistence of populations and colonization of new potential habitats. Today wild large mammals are presumed to be important vectors for long-distance transport of diaspores within and between European temperate forest patches, and in particular wild boars recently came into focus. Here we use a specific habit of wild boar, i.e. wallowing in mud and subsequent rubbing against trees, to evaluate epizoochorous dispersal of vascular plant diaspores. We present soil seed bank data from 27 rubbing trees versus 27 control trees from seven forest areas in Germany. The mean number of viable seeds and the plant species number were higher in soil samples near rubbing trees compared with control trees. Ten of the 20 most frequent species were more frequent, and many species exclusively appeared in the soil samples near rubbing trees. The large number of plant species and seeds – more than 1000 per tree – in the soils near rubbing trees is difficult to explain unless the majority were dispersed by wild boar. Hooked and bristly diaspores, i.e. those adapted to epizoochory, were more frequent; however, many species with unspecialized diaspores occurred exclusively near rubbing trees. As opposed to plant species closely tied to forests species which occur both in forest and open vegetation and non-forest species were more frequent near rubbing trees compared with controls. These findings are consistent with previous studies on diaspore loads in the coats and hooves of shot wild boars. However, our method allows to identify the transport of diaspores from the open landscape into forest stands, where they might especially emerge after disturbance, and a clustered distribution of epizoochorically dispersed seeds. Moreover, accumulation of seeds of wetness indicators near rubbing trees demonstrates directed dispersal of plant species inhabiting wet places among remote wallows.     

Closing the gaps for animal seed dispersal: Separating the effects of habitat loss on dispersal distances and seed aggregation

Habitat loss can alter animal movements and disrupt animal seed dispersal mutualisms; however, its effects on spatial patterns of seed dispersal are not well understood. To explore the effects of habitat loss on seed dispersal distances and seed dispersion (aggregation), we created a spatially explicit, individual‐based model of an animal dispersing seeds (SEADS—Spatially Explicit Animal Dispersal of Seeds) in a theoretical landscape of 0%–90% habitat loss based on three animal traits: movement distance, gut retention time, and time between movements. Our model design had three objectives: to determine the effects of (1) animal traits and (2) habitat loss on seed dispersal distances and dispersion and (3) determine how animal traits could mitigate the negative effects of habitat loss on these variables. SEADS results revealed a complex interaction involving all animal traits and habitat loss on dispersal distances and dispersion, driven by a novel underlying mechanism of fragment entrapment. Unexpectedly, intermediate habitat loss could increase dispersal distances and dispersion relative to low and high habitat loss for some combinations of animal traits. At intermediate habitat loss, movement between patches was common, and increased dispersal distances and dispersion compared to continuous habitats because animals did not stop in spaces between fragments. However, movement between patches was reduced at higher habitat loss as animals became trapped in fragments, often near the parent plant, and dispersed seeds in aggregated patterns. As movement distance increased, low time between movements and high gut retention time combinations permitted more movement to adjacent patches than other combinations of animal traits. Because habitat loss affects movement in a nonlinear fashion under some conditions, future empirical tests would benefit from comparisons across landscapes with more than two levels of fragmentation.     

Causes and consequences of animal dispersal strategies: relating individual behaviour to spatial dynamics

Knowledge of the ecological and evolutionary causes of dispersal can be crucial in understanding the behaviour of spatially structured populations, and predicting how species respond to environmental change. Despite the focus of much theoretical research, simplistic assumptions regarding the dispersal process are still made. Dispersal is usually regarded as an unconditional process although in many cases fitness gains of dispersal are dependent on environmental factors and individual state. Condition-dependent dispersal strategies will often be superior to unconditional, fixed strategies. In addition, dispersal is often collapsed into a single parameter, despite it being a process composed of three interdependent stages: emigration, inter-patch movement and immigration, each of which may display different condition dependencies. Empirical studies have investigated correlates of these stages, emigration in particular, providing evidence for the prevalence of conditional dispersal strategies. Ill-defined use of the term 'dispersal', for movement across many different spatial scales, further hinders making general conclusions and relating movement correlates to consequences at the population level. Logistical difficulties preclude a detailed study of dispersal for many species, however incorporating unrealistic dispersal assumptions in spatial population models may yield inaccurate and costly predictions. Further studies are necessary to explore the importance of incorporating specific condition-dependent dispersal strategies for evolutionary and population dynamic predictions.     

Multiple pathways in diaspore dispersal, exemplified by studies of Noogoora Burr (Xanthium occidentale Bertol., Compositae)

LIDDLE, M. J. & ELGAR, M. A., 1984. Multiple pathways in diaspore dispersal, exemplified by studies of Noogoora Burr ( Xanthium occidmtale Bertol., Compositae). Some of the ways in which the burrs are dispersed and some of the factors that may cause changes between the different dispersal pathways were examined. Populations of burrs were marked on the plants in an ungrazed paddock, a paddock grazed by horses, and a paddock grazed by sheep. Eleven percent of the marked populations were recovered from the hair of horses and 26% from the sheeps' wool. A single flood removed 80% of one population. Burrs moved a mean distance of 30 cm on paths used by horses and 18% were buried in the path soil. Burrs were placed in animals' hair by hand. After 16 days, sheep retained 76% of the burrs in their wool, while only 2% remained in the hair of horses and cattle. An experiment showed that burrs may be catapulted up to a distance of 3.3 m from the plants.
A model is presented which outlines the alternative or sequential pathways that may be followed by the burrs. Occasional events, such as flooding and presence or absence of stock, were the main causes of changes between pathways. In the model, these dispersal mechanisms are considered as release-phase and substrate-phase processes. Each process derives its energy from different sources and may be modified by inputs of energy from other sources.     

Estimating wind dispersal potential in Ailanthus altissima: The need to consider the three-dimensional structure of samaras

Plant dispersal is a very important ecological phenomenon, as it can enable species to move away from the parent plant. This contributes to shaping communities, determining patterns of distribution, landscape configuration, plant invasions and evolutionary processes. Measuring dispersal distance directly is difficult and thus, diaspore morphology can be used to make estimates. Previous research on the topic often resorts to analysing the diaspore’s morphology as if it was a bi-dimensional structure; when in many cases, diaspores have three-dimensional qualities. In this study, we show how estimates of wind dispersal potential of Ailanthus altissima can be considerably improved using morphological variables that succeed in describing the three-dimensional nature of samaras. We suggest that this reasoning could be extensively applied to research involving not only other species, but also multi-specific scenarios with a wide range of diaspore morphologies.     

树鼩作为丙型肝炎动物模型的HCV受体研究进展

丙型肝炎病毒(hepatitis C virus,HCV)全球流行、危害严重,合适的小动物模型的缺乏严重阻碍了药物和疫苗的研发。该文介绍丙型肝炎危害与病毒复制特点,以HCV入胞受体为重点,通过比较现有丙型肝炎动物模型,从分子水平探讨树鼩作为丙型肝炎动物模型的可能性。     

Radiation following long‐distance dispersal: the contributions of time,opportunity and diaspore morphology in Sicyos (Cucurbitaceae)

Aim  To infer the most plausible explanations for the presence of 14 species of the Neotropical cucurbit genus Sicyos on the Hawaiian Islands, two on the Galápagos Islands, two in Australia, and one in New Zealand. Location  Neotropics, the Hawaiian and Galápagos archipelagos, Australia and New Zealand. Methods  We tested long‐problematic generic boundaries in the tribe Sicyoeae and reconstructed the history of Sicyos using plastid and nuclear DNA sequences from 87 species (many with multiple accessions) representing the group’s generic and geographic diversity. Maximum likelihood and Bayesian approaches were used to infer relationships, divergence times, biogeographic history and ancestral traits. Results  Thirteen smaller genera, including Sechium, are embedded in Sicyos, which when re‐circumscribed as a monophyletic group comprises 75 species. The 14 Hawaiian species of Sicyos descended from a single ancestor that arrived c. 3 million years ago (Ma), Galápagos was reached twice at c. 4.5 and 1 Ma, the species in Australia descended from a Neotropical ancestor (c. 2 Ma), and New Zealand was reached from Australia. Time since arrival thus does not correlate with Sicyos species numbers on the two archipelagos. Main conclusions  A plausible mechanism for the four trans‐Pacific dispersal events is adherence to birds of the tiny hard fruit with retrorsely barbed spines found in those lineages that underwent long‐distance migrations. The Hawaiian clade has lost these spines, resulting in a lower dispersal ability compared with the Galápagos and Australian lineages, and perhaps favouring allopatric speciation.     

Wind dispersal of alpine plant species: A comparison with lowland species

Question: The prominent role of wind dispersal in alpine habitats has been recognized early but has rarely been quantified. The aim of this study is to compare wind dispersal under alpine and lowland conditions and to analyse whether differences are caused by species traits, e.g. terminal velocity of seeds (V_term) or weather conditions. Location and Methods: We characterized wind dispersal potential of > 1100 Central European species using measured V_term To quantify the habitat effect on wind dispersal, we measured meteorological key‐parameters and simulated dispersal distance spectra of nine selected species under typical alpine conditions (foreland of the Scaletta‐glacier, Switzerland) and typical lowland conditions (grassland in Bad Lippspringe, Germany). Results: Lowland species had higher V_term compared to alpine species. However, this difference is absent when only species of species of open habitats are concerned. The meteorological measurements showed that the alpine habitat was mainly characterized by higher frequency and strength of updrafts. The simulations showed that under alpine conditions long distance dispersal occurred much more frequent. Conclusions: More than 50 % of the alpine species have a fair chance to be dispersed by wind over long distances, while this proportion is less than 25 % for species from open habitats in the lowland. The more prominent role of wind dispersal in alpine habitats is mainly a result of differences in environmental conditions, namely more intense vertical turbulence in the alpine habitat, and does not result from prominent differences in plant traits, namely V_term, between alpine and lowland species.     

Estimating dispersal rate of the silky cane weevil (Coleoptera: Curculionidae)*

Abstract: The objective of this study was to estimate the silky cane weevil rate of dispersal under near‐natural conditions inside a screened enclosure where an array of buckets was baited with cut sugarcane stalks. One hundred weevils were released and weevils inside the buckets were counted hourly for 8 h, and then 24 and 48 h after release. A passive diffusion model was used to estimate the weevil's dispersal and disappearance rates, within and between rows of buckets with sugarcane. The weevils concentrated around the release point and slowly moved towards the boundaries of the experimental plot over time with an overall average dispersal rate of 2.8 ± 3.58 cm²/h. Dispersal and disappearance rates within and between rows were not significantly different among the time intervals considered (1–8, 8–24 and 24–48 h after release) except for the 1–8 time interval on the array representing the release point when the dispersal rate, D, was significantly higher than those at other time intervals. Continuum of the substratum to disperse from one side of the array to another via a wooden bridge may explain the higher dispersal rate through this array. The number of buckets exposed to the sun during the morning hours was significantly higher on those rows exposed to the sun (south side of the screen enclosure) than on the shaded side. Longer times of bucket exposure to the sun may explain the predominant distribution of weevils in that area suggesting that the weevil population is constantly expanding and retracting according to micro environmental conditions.     

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.     

Spread pathways of the invasive weed Parthenium hysterophorus L.: The potential for water dispersal

Parthenium hysterophorus L. (Asteraceae) utilises multiple mechanisms to facilitate its dispersal. It has been speculated that the cypsela, the propagule of this species, can be dispersed by water under varying environmental conditions. Four experiments were conducted to test this hypothesis, using simulated shaking and immersion to test floating ability and viability of the propagule in water. The influence of the acidity of the immersion medium on cypsela viability was also examined. Our results revealed that the freshly harvested cypselae could float on river water for at least 20 days, although around 80% sank within a week if moderate or severe turbulence was applied. Sinkage was observed to be more rapid in naked seeds (within a day) than in cypsela (within a week). On still water surfaces, germination occurred within a week but extended to 1.5 weeks under turbulent conditions due to sinkage. In river water, initial germination of floating cypselae was greater (70%) under illuminated conditions as compared to dark conditions (20%). The viability of immersed cypselae was found to remain high in distilled water for 45 days, when immersion was in cool conditions (10 or 15°C). However, in moderate (20 and 24°C) or warm (25 and 30°C) conditions, the rate of viability loss increased, and at 34°C, around 50% of the cypselae died after 20 days of immersion. Similar trends for cypselae longevity were observed in studies using river and pond water; viability loss was faster, especially in pond water. In summary, a proportion of cypselae will float in turbulent water and could be carried significant distances in river systems. Immersed cypselae can remain viable for weeks and can germinate on contact with soil. Water bodies or floods are therefore considered as important pathways in parthenium weed dispersal; hence, post‐flood monitoring is strongly recommended to minimise its spread.     

Neurobiology of secure infant attachment and attachment despite adversity: a mouse model

Attachment to an abusive caregiver has wide phylogenetic representation, suggesting that animal models are useful in understanding the neural basis underlying this phenomenon and subsequent behavioral outcomes. We previously developed a rat model, in which we use classical conditioning to parallel learning processes evoked during secure attachment (odor‐stroke, with stroke mimicking tactile stimulation from the caregiver) or attachment despite adversity (odor‐shock, with shock mimicking maltreatment). Here we extend this model to mice. We conditioned infant mice (postnatal day (PN) 7–9 or 13–14) with presentations of peppermint odor and either stroking or shock. We used ¹⁴C 2‐deoxyglucose (2‐DG) to assess olfactory bulb and amygdala metabolic changes following learning. PN7‐9 mice learned to prefer an odor following either odor‐stroke or shock conditioning, whereas odor‐shock conditioning at PN13‐14 resulted in aversion/fear learning. 2‐DG data indicated enhanced bulbar activity in PN7‐9 preference learning, whereas significant amygdala activity was present following aversion learning at PN13‐14. Overall, the mouse results parallel behavioral and neural results in the rat model of attachment, and provide the foundation for the use of transgenic and knockout models to assess the impact of both genetic (biological vulnerabilities) and environmental factors (abusive) on attachment‐related behaviors and behavioral development .     

Parasitism and dispersal potential of Sirex noctilio: implications for biological control

1 Sirex noctilio F. (Hymenoptera: Siricidae) is a wood‐boring wasp that attacks many pine species, including commercial trees planted throughout the world. Management of its populations is largely based on biological control using the nematode Beddingia siricidicola. Adult females are sterilized by the nematode, but are free to move and attack new trees, promoting nematode dispersal. Although generally successful, wasp management through nematode introductions has sometimes been inadequate. 2 We evaluated the effect of parasitism by B. siricidicola on flight performance of woodwasps under laboratory conditions. Using flight mills, we recorded a total of 46 flight trials over 23 h, obtained from infected and control (uninfected) females. 3 Although all wasps lost weight during flight, parasitized females were significantly smaller and suffered larger weight losses than uninfected females. In addition, total flight distance and velocity were lower in parasitized females. 4 Because nematode infection transmission relies on healthy wasps attacking trees previously visited by nematode‐bearing females, differential dispersal capacity could limit biological control success.     

The potential of hydrodynamic damage to animal cells of industrial relevance: current understanding

Suspension animal cell culture is now routinely scaled up to bioreactors on the order of 10,000 L, and greater, to meet commercial demand. However, the concern of the ‘shear sensitivity’ of animal cells still remains, not only within the bioreactor, but also in the downstream processing. As the productivities continue to increase, titer of ~10 g/L are now reported with cell densities greater than 2 × 10⁷ cells/mL. Such high, and potentially higher cell densities will inevitably translate to increased demand in mass transfer and mixing. In addition, achieving productivity gains in both the upstream stage and downstream processes can subject the cells to aggressive environments such as those involving hydrodynamic stresses. The perception of ‘shear sensitivity’ has historically put an arbitrary upper limit on agitation and aeration in bioreactor operation; however, as cell densities and productivities continue to increase, mass transfer requirements can exceed those imposed by these arbitrary low limits. Therefore, a better understanding of how animal cells, used to produce therapeutic products, respond to hydrodynamic forces in both qualitative and quantitative ways will allow an experimentally based, higher, “upper limit” to be created to guide the design and operation of future commercial, large scale bioreactors. With respect to downstream hydrodynamic conditions, situations have already been achieved in which practical limits with respect to hydrodynamic forces have been experienced. This review mainly focuses on publications from both the academy and industry regarding the effect of hydrodynamic forces on industrially relevant animal cells, and not on the actual scale-up of bioreactors. A summary of implications and remaining challenges will also be presented.