Greater Dispersal Distances Research Articles

Scatterhoarder abundance and advantages of seed burial drive dynamics of a tree–rodent interaction

Abstract Theoretical models suggest that species abundance plays a crucial role in mutualism; high densities can lead to overexploitation. Additionally, mutualistic benefits are expected to increase under abiotic stress. We investigated the interplay between density dependence and abiotic factors in conditional mutualism, focussing on the interactions between yellow‐necked mice (Apodemus flavicollis) and common oaks (Quercus robur) across 3 years. We controlled for seed abundance, while mouse densities varied. Our approach included monitoring mouse abundance, measuring seed removal, determining the fate of harvested seeds and conducting seedling emergence trials to assess the advantages of seeds caching. We found that mouse abundance exerted a strong influence on all studied aspects of seed dispersal. Higher densities led to increased seed removal and greater dispersal distances. However, they also decreased the probability of seed caching and increased seed consumption, degrading dispersal quality and shifting the interaction towards antagonism. Furthermore, the reliance of seedling recruitment on burial varied over time, likely becoming more critical during dry conditions. This indicates that plants face the worst conditions when high abundance of mice coincides with abiotic stress. Synthesis: Our results supports the notion that increased rodent abundance reduces plant recruitment, revealing the conflicting interests of the interacting species. However, the dynamics of rodent–oak interactions are shaped not only by the density‐dependent foraging decisions of rodents, but also by the benefits of seed burial under stress. These findings illustrate how the interplay between population density and abiotic factors jointly dictate the costs and benefits of mutualistic interactions.

Effects of Landscape Heterogeneity and Disperser Movement on Seed Dispersal

The primacy of endozoochory for the maintenance and expansion of many woody plant populations is well known, but seed dispersal is not well understood for most species. This is especially true for rare species, where small population size and low fruit production can limit field- or observation-based experiments. Additionally, the effect of environmental heterogeneity on disperser movement is rarely investigated but has been shown to improve estimates of plant population spatial patterns and dynamics. We used simulation experiments to explore the effects of environmental heterogeneity and disperser movement on Lindera subcoriacea seed dispersal, a rare shrub from the southeastern United States with avian-dispersed seeds. Our experiments incorporated environmental heterogeneity and simulated disperser movement for five bird species, based on either landscape permeability or straight path rules. We anticipated that permeability-based movement would result in greater dispersal distances and seed dispersal effectiveness, which characterizes both quantity and quality. Generally, we did not find differences in seed dispersal between permeability and straight path experiments. However, we did find that permeability-based experiments had greater deposition into suitable habitat during flight (23 vs. 1%). These rare but longer distance depositions may be especially important for plants that are influenced by gap or interpopulation dynamics. We also found consistently greater dispersal into high quality habitats regardless of disperser species in permeability experiments, implying that incorporating species-specific assessments of landscape utilization (occupancy) could influence the effectiveness of seed dispersal. Our study suggests that including environmental heterogeneity in seed dispersal models can provide additional insights not provided by avian parameters (e.g., gut capacity, seed retention time, and flight speed) commonly used to inform dispersal models.

Extinction risk of the world's freshwater mammals.

The continued loss of freshwater habitats poses a significant threat to global biodiversity. We reviewed the extinction risk of 166 freshwater aquatic and semiaquatic mammals-a group rarely documented as a collective. We used the International Union for the Conservation of Nature Red List of Threatened Species categories as of December 2021 to determine extinction risk. Extinction risk was then compared among taxonomic groups, geographic areas, and biological traits. Thirty percent of all freshwater mammals were listed as threatened. Decreasing population trends were common (44.0%), including a greater rate of decline (3.6% in 20years) than for mammals or freshwater species as a whole. Aquatic freshwater mammals were at a greater risk of extinction than semiaquatic freshwater mammals (95% CI -7.20 to -1.11). Twenty-nine species were data deficient or not evaluated. Large species (95% CI 0.01 to 0.03) with large dispersal distances (95% CI 0.03 to 0.15) had a higher risk of extinction than small species with small dispersal distances. The number of threatening processes associated with a species compounded their risk of extinction (95% CI 0.28 to 0.77). Hunting, land clearing for logging and agriculture, pollution, residential development, and habitat modification or destruction from dams and water management posed the greatest threats to these species. The basic life-history traits of many species were poorly known, highlighting the need for more research. Conservation of freshwater mammals requires a host of management actions centered around increased protection of riparian areas and more conscientious water management to aid the recovery of threatened species.

Independent and Interactive Effects of Environmental Conditions on Aerosolized Surrogate SARS-CoV-2 — Beijing, China, June to September 2020

What is already known about this topic?Environmental factors such as temperature and humidity play important roles in the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via droplets/aerosols.What is added by this report?Higher relative humidity (61%–80%), longer spreading time (120 min), and greater dispersal distance (1 m) significantly reduced SARS-CoV-2 pseudovirus loads. There was an interaction effect between relative humidity and spreading time.What are the implications for public health practice?The findings contribute to our understanding of the impact of environmental factors on the transmission of SARS-CoV-2 via airborne droplets/aerosols.

Global biogeography of marine dispersal potential.

The distance travelled by marine larvae varies by seven orders of magnitude. Dispersal shapes marine biodiversity, and must be understood if marine systems are to be well managed. Because warmer temperatures quicken larval development, larval durations might be systematically shorter in the tropics relative to those at high latitudes. Nevertheless, life history and hydrodynamics also covary with latitude-these also affect dispersal, precluding any clear expectation of how dispersal changes at a global scale. Here we combine data from the literature encompassing >750 marine organisms from seven phyla with oceanographic data on current speeds, to quantify the overall latitudinal gradient in larval dispersal distance. We find that planktonic duration increased with latitude, confirming predictions that temperature effects outweigh all others across global scales. However, while tropical species have the shortest planktonic durations, realized dispersal distances were predicted to be greatest in the tropics and at high latitudes, and lowest at mid-latitudes. At high latitudes, greater dispersal distances were driven by moderate current speed and longer planktonic durations. In the tropics, fast currents overwhelmed the effect of short planktonic durations. Our results contradict previous hypotheses based on biology or physics alone; rather, biology and physics together shape marine dispersal patterns.

Quantifying Aedes aegypti dispersal in space and time: a modeling approach

AbstractThe dispersal of the invasive mosquito Aedes aegypti in urbanized areas has received attention due to the hazard that this species poses to human health. However, we know little about this process at temperate latitudes, especially in recently colonized semi‐arid regions of the southwestern United States that differ ecologically from more typical habitats in the tropics. We collected data on Ae. aegypti dispersal through a mark–release–recapture (MRR) study in Central California. We employed stable isotopes of carbon and nitrogen to mark released mosquitoes. We characterized Ae. aegypti dispersal capacity using both traditional measures of central tendency and dispersal kernel theory coupled with space–time models, allowing for effects of environmental factors to provide more reliable dispersal estimates. Dispersal of Ae. aegypti was similar between females and males, and we found the mean distances traveled to be 224–240 m from the release location when estimated by conventional methods. Model‐based inference allowed for a more nuanced interpretation, with even greater dispersal distances possible depending on direction and environmental context. Our results showed that experimental conditions, such as spatial arrangement of traps and duration of the recapture period, as well as environmental conditions, such as wind speed and direction, altered the probability of recapturing marked mosquitoes, and therefore the observed dispersal pattern. This represents the first assessment of Ae. aegypti dispersal in the semi‐arid southwestern United States, which is unique among published studies in that the period of peak annual Ae. aegypti abundance coincides with cumulative precipitation close to zero. Results show that this species dispersed on average farther than commonly documented for other areas of the world. Our findings provide information for defining spatial control strategies, such as incompatible insect technique, which rely on a clear understanding of dispersal rates among neighboring areas.

The influence of larval migration and dispersal depth on potential larval trajectories of a deep-sea bivalve

Many fundamental questions in marine ecology require an understanding of larval dispersal and connectivity, yet direct observations of larval trajectories are difficult or impossible to obtain. Although biophysical models provide an alternative approach, in the deep sea, essential biological parameters for these models have seldom been measured empirically. In this study, we used a biophysical model to explore the role of behaviorally mediated migration from two methane seep sites in the Gulf of Mexico on potential larval dispersal patterns and population connectivity of the deep-sea mussel “Bathymodiolus” childressi, a species for which some biological information is available. Three possible larval dispersal strategies were evaluated for larvae with a Planktonic Larval Duration (PLD) of 395 days: (1) demersal drift, (2) dispersal near the surface early in larval life followed by an extended demersal period before settlement, and (3) dispersal near the surface until just before settlement. Upward swimming speeds varied in the model based on the best data available. Average dispersal distances for simulated larvae varied between 16km and 1488km. Dispersal in the upper water column resulted in the greatest dispersal distance (1173km ± 2.00), followed by mixed dispersal depth (921km ± 2.00). Larvae originating in the Gulf of Mexico can potentially seed most known seep metapopulations on the Atlantic continental margin, whereas larvae drifting demersally cannot (237km ± 1.43). Depth of dispersal is therefore shown to be a critical parameter for models of deep-sea connectivity.

The Effects of Anthropogenic Structures on Habitat Connectivity and the Potential Spread of Non-Native Invertebrate Species in the Offshore Environment.

Offshore structures provide habitat that could facilitate species range expansions and the introduction of non-native species into new geographic areas. Surveys of assemblages of seven offshore oil and gas platforms in the Santa Barbara Channel revealed a change in distribution of the non-native sessile invertebrate Watersipora subtorquata, a bryozoan with a planktonic larval duration (PLD) of 24 hours or less, from one platform in 2001 to four platforms in 2013. We use a three-dimensional biophysical model to assess whether larval dispersal via currents from harbors to platforms and among platforms is a plausible mechanism to explain the change in distribution of Watersipora and to predict potential spread to other platforms in the future. Hull fouling is another possible mechanism to explain the change in distribution of Watersipora. We find that larval dispersal via currents could account for the increase in distribution of Watersipora from one to four platforms and that Watersipora is unlikely to spread from these four platforms to additional platforms through larval dispersal. Our results also suggest that larvae with PLDs of 24 hours or less released from offshore platforms can attain much greater dispersal distances than larvae with PLDs of 24 hours or less released from nearshore habitat. We hypothesize that the enhanced dispersal distance of larvae released from offshore platforms is driven by a combination of the offshore hydrodynamic environment, larval behavior, and larval release above the seafloor.

Reduced dispersal and survival in the sweet potato weevil (Euscepes postfasciatus) after irradiation

AbstractIn southern Japan, sweet potato weevils Euscepes postfasciatus (Fairmaire) are being eradicated using the sterile insect technique because of their damaging effects on sweet potato crops. However, the effect of irradiation on the dispersal and survival of this species needs to be evaluated. We observed the dispersal of normal and irradiated weevils for 21 days after their release in a sweet potato field during all four seasons, and estimated their survival using the Weibull function. Normal weevils generally moved further than irradiated weevils, and all weevils had greater dispersal distances in the summer than in the winter, with spring and autumn intermediate between these two extremes (summer: 0.23 m/day versus 0.29 m/day; winter: 0.15 m/day versus 0.07 m/day; spring: 0.20 m/day versus 0.17 m/day; autumn: 0.20 m/day versus 0.15 m/day, respectively). Normal weevils had low variation in mean life expectancy (approximately 8 days throughout the year), whereas the life expectancy of irradiated weevils was drastically reduced in the summer (2.20 days). The implications of these results for the efficiency of this eradication technique are discussed.

Post-floral Erection of Stalks Provides Insight into the Evolution of Fruit Orientation and Its Effects on Seed Dispersal.

That stalks reorient after flowering to face upwards is a common phenomenon in many flowering plants, indicating the potential importance of fruit orientation on seed dispersal. But this idea has not been subject to an empirical test. We examined this hypothesis by analysing the evolutionary correlation between fruit orientation and other characters and by investigating the effects of fruit orientation on seed dispersal. We found that 1) in a sub-alpine plant community, upward fruit orientation strongly correlates with fruits that act as seed containers, which are often of dry type and are dispersed by non-animal vectors; 2) as exemplified by the Campanulaceae s. str., fruit orientation strongly correlates with dehiscence position. Upwardly-oriented capsules dehisce at the apex, whereas pendent ones dehisce at the base, in both cases ensuring that seeds are released from an upright position; 3) in manipulation experiments on Silene chungtienensis, upward fruits (the natural state) exhibit much greater dispersal distances and more dispersive pattern than pendent ones, and have a more even distribution of dispersal direction than horizontal ones. Our results suggest that fruit orientation may have important function in seed dispersal, which may be the reason why the phenomenon that stalk erection after flowering occurs widely.

Factors affecting the dispersal of large‐scale releases of the Queensland fruit fly, Bactrocera tryoni

AbstractThis study investigated two factors potentially affecting the spatial distribution of Queensland fruit fly Bactrocera tryoni (Froggatt) after release from a single point. First, the effect of age at release was investigated using a single cohort of 205 000 males from a mass‐rearing strain used for sterile insect releases. Approximately half were released as immature males and the remainder as sexually mature males (1 week later). Males were collected over 3 weeks from a grid of 135 traps, each containing a pheromone/insecticide bait, positioned between 4 and 500 m from the release point. Variation in the distribution of fly density around the release point was assessed by regressing trapped fly counts against distance. Unexpectedly, no significant differences were found in the spatial distribution of the flies. Second, the effect of inbreeding on spatial distribution was investigated using replicated simultaneous releases of two strains of B. tryoni. One strain was the existing (inbred) mass‐rearing strain that has been selected for high productivity in a mass‐rearing facility. The second strain was deliberately outbred but also selected for high productivity. Almost 100 000 males of each strain were released over the two experiments. Regression of trappings against distance differed significantly between strains in only one of five releases, but in all cases the outbred strain had a greater dispersal distance. As our trapping grid was not regular but contained gaps of up to 100 m, a small preliminary experiment investigated whether flies move faster along tree rows or across open fields. At distances up to 100 m, we found no detectable difference in fly distributions. These results are primarily relevant to the large‐scale point releases carried out as part of an existing B. tryoni sterile insect programme and are discussed in that context.

Shapes of ballistic seed dispersal distributions: a comparison of Oxalis corniculata with a theoretical model

Rezvani M, Cousens RD, Zaefarian F, Karimmojeni H & Robinson AP (2010). Shapes of ballistic seed dispersal distributions: a comparison of Oxalis corniculata with a theoretical model. Weed Research 50, 631–637. Summary A previously published ballistic dispersal model, based on fluid dynamics and a point source, predicts that the probability of dispersing a given distance has maxima close to the origin and at the greatest dispersal distance. We show that in plants of two taxa of Oxalis corniculata, the maximum probability of dispersal is very close to the parent, with a very minor secondary peak at intermediate distances. Using generalised additive mixed-effects modelling, we show that seed mass increases with distance from the parent and that distances dispersed by the two taxa differed. We conclude that although prediction from physical first principles is a justifiable goal, the complexity of plants as seed sources and in-flight obstacles means that predictions from very simple models may only rarely mimic reality. Until more realistic models for ballistic dispersal are developed, considerable care should be taken in the selection of dispersal functions in population models. Wherever possible, model structure should be supported by the collection of empirical data.

Is farther seed dispersal better? Spatial patterns of offspring mortality in three rainforest tree species with different dispersal abilities

The paradigm that tropical trees with farther seed dispersal experience lower offspring mortality is currently based on within‐species studies documenting higher survival of offspring located farther from conspecific adults and/or closer to light gaps. We determined whether the paradigm also holds among species by comparing spatial patterns of offspring mortality among three sympatric Neotropical rainforest tree species with the same seed dispersers but with different dispersal abilities. First, we assessed spatially non‐random mortality for each species by measuring spatial shifts of the population recruitment curve (PRC) with respect to conspecific adults and light gaps across three early life stages: dispersed seeds, young seedlings and old seedlings. Then, we determined whether PRC shifts were greater for the species with short dispersal distances than for the species with greater dispersal distances. We found that the PRC shifted away from conspecific adults consistently across life stages, but we found no consistent PRC shifts towards gaps, suggesting that mortality was related more to the proximity of conspecifics than to absence of light gaps. PRC shifts away from adults were greatest in the species with the lowest dispersal ability, supporting the paradigm. Differential PRC shifts caused the spatial distribution of offspring to become almost independent with respect to adult trees and gaps in all three species, despite large differences in seed dispersal distance among these species. Our results provide direct empirical support for the paradigm that among tropical trees, species with farther seed dispersal are less impacted by spatially non‐random mortality than are similar species with shorter dispersal distances. Thus, greater dispersal effectiveness merits extra investments of trees in seed dispersal ability, even at the cost of fecundity.

THE ROLE OF ANTS IN SEED DISPERSAL OF GLOBBA LANCANGENSIS AND THE SPATIAL DISTRIBUTION OF ITS SEEDLINGS

Seed dispersal is a key process determining the spatial pattern of plant populations. Myrmecochory is a term referring to a plant_ant mutualism in which ants disperse plant seeds while using the nutritionally valuable elaiosomes as food. In this study, our goal was to understand how ants affect seed dispersal and seedling distribution patterns of a myrmecochorous plant, Globba lancangensis. G. lancangensis is a small perennial herb of Zingiberaceae mainly distributed in southwest Yunnan Province. It propagates primarily by seeds but also by a small number of bulbils. There are 31.2±2.13 (Mean±SE, n =32) seeds in each fruit. The seeds are 2-3 mm in length and bear a white_colored elaiosome. We conducted an observational study on the seed dispersal of G. lancangensis in October 2002 in its natural habitat in Xishuangbanna, Yunnan Province of China (21°59′ N, 100°16′ E; 1 180 m asl; annual mean temperature, 18.3 ℃; annual mean rainfall, 1 339 mm). A total of ten species of ants were identified that transported the seeds of G. lancangensis and their activities were recorded. The mean dispersal distance of seeds was (0.47±0.03) m ( n =216), ranging from 0.01 m to 3.35 m. Odontoponera transversa , Pheidole sp. and Pachycondyla luteipes were the most important seed dispersing ant species, occurring 61%, 50% and 28% of the time, respectively, with mean dispersal distances of (0.60±0.09) m, (0.20±0.01) m and (0.32±0.05) m, respectively. As a whole, ants promoted a decrease in the degree of aggregation of G. lancangensis seeds. Ants of Pheidole sp. collected a total of 43% of the seeds of G. lancangensis but consumed most of the seeds collected, suggesting that Pheidole sp. is a seed harvester ant O. transversa did not remove seeds with artificially removed elaiosomes and appeared to be the most sensitive ant species to seed elaiosomes. Overall, O. transversa made the most significant contribution to seed dispersal, as demonstrated by the highest visiting frequency (61%) and the greatest mean dispersal distance (0.60±0.09) m of all ants. These results imply that a tight mutualism exists between O. transversa and G. lancangensis. To understand whether ants influence the spatial pattern of seedlings, we compared the spatial distribution of seedlings of G. lancangensis in its natural habitat with that of two other species in the Globba genus, G. barthei and G. racemosa; the latter two species propagate mainly by bulbils which are dispersed by abiotic means. Nine 25 m×25 m plots, three plots for each species, were established and the location of every Globba seedling inside the plots was recorded. The mean distance to the nearest neighbor of G. lancangensis was significantly higher than both G. barthei ((36.8±1.45) cm vs. (29.8±2.70) cm, Mean±SE; t 73,33 = 2.11, p =0.037) and G. racemosa ((36.8±1.45) cm vs. (28.7±3.16) cm, t 73,31 =2.33, p = 0.022 ). We used Z _values to describe the degree of aggregation of the populations of the three species. The Z_value of G. lancangensis (-1.70±0.19, Mean±SE) was significantly larger than the Z_value of both G. barthei (-2.58±0.37, t 73,33 =2.36, p =0.020) and G. racemosa (-3.28±0.53, t 73,31 =3.54, p = 0.001), indicating that G. lancangensis was significantly less aggregated. These results suggest that ants make a significant contribution to dispersing G. lancangensis seedlings by increasing the distance to its nearest neighbor and decreasing the degree of aggregation in its natural habitat.

Do Some Bird‐Dispersed Fruits Contain Natural Laxatives? A Comment

Murray et al. (1994) proposed that fruits of Witheringia solanacea (Solanaceae), a Costa Rican cloud forest shrub, contain laxative chemicals that function to speed passage of seeds through fruit-eating birds in order to minimize deterioration of seed viability from exposure to digestive processes. They postulated that a natural chemical laxative in Witheringia fruits mediates an adaptive trade-off: i.e., balancing the presumed positive and negative effects of prolonged retention of seeds by fruit-eating birds. The positive effect of longer seed retention by frugivorous birds is presumed to be greater dispersal distance, whereas the negative effect is reduced seed viability. Their study presents a novel ecological construct that has broad implications for our understanding of evolutionary interactions between fruit-bearing plants and their seed dispersers. Here, I present an alternative explanation of their results, based on how digestive function of frugivorous birds is influenced by nutrient concentration of foods.

Dismigration und Brutarealexpansion der TürkentaubeStreptopelia decaocto

The range extension of the Collared Dove to Europe began in the 1930s, when the population in the Balkans and Western Turkey was very low. This is a typical situation for genetic drift, which appears to have favoured a higher frequency of dispersal to the northwest. The interbreeding of individuals colonising areas northwest of the original area further increased the preference for northwestern dispersal and resulted in greater dispersal distances. However, these individuals were eliminated when the North Sea and the Atlantic Ocean were reached. The recovery of ringed birds has shown that dispersal distances have significantly decreased in Europe since the 1970s and that there has been no significant preference for dispersal direction since the period of range extension.