Plant Dispersal Ability Research Articles

Elevational patterns of plant dispersal ability in Southern Europe

Dispersal ability is a functional trait crucial to understand the distribution and spatial dynamics of plant species. This trait influences the abilities of a species to colonize, or thrive, in a region and thus play a central role on its distribution. We investigated the relationship between plant dispersal ability and elevational distribution (within southern Europe). Algorithms combining different plant dispersal traits, such as seed mass, release height, terminal velocity, growth form and dispersal syndrome, were used to estimate dispersal kernels of 929 vascular plant species along elevational gradient. Finally, we calculated Rao’s functional diversity (FD) by analysis of standardized effect size and community mean (CM) for the dispersal distances. The main results show that species, along the elevational gradient, show different dispersal capacity. The greatest dispersal ability was detected at intermediate elevation, while a reduction in dispersal ability along with a convergence in functional diversity was observed with increasing elevation. These results can be explained by variation in environmental constraints along the elevational gradient. Thus, we conclude that short-distance dispersal is crucial in a stressed environment, while long-distance dispersal ability is advantageous to colonize new habitats where environmental constraints are weak.

Costs and benefits of non‐random seed release for long‐distance dispersal in wind‐dispersed plant species

The dispersal ability of plants is a major factor driving ecological responses to global change. In wind‐dispersed plant species, non‐random seed release in relation to wind speeds has been identified as a major determinant of dispersal distances. However, little information is available about the costs and benefits of non‐random abscission and the consequences of timing for dispersal distances.We asked: 1) to what extent is non‐random abscission able to promote long‐distance dispersal and what is the effect of potentially increased pre‐dispersal risk costs? 2) Which meteorological factors and respective timescales are important for maximizing dispersal? These questions were addressed by combining a mechanistic modelling approach and field data collection for herbaceous wind‐dispersed species.Model optimization with a dynamic dispersal approach using measured hourly wind speed showed that plants can increase long‐distance dispersal by developing a hard wind speed threshold below which no seeds are released. At the same time, increased risk costs limit the possibilities for dispersal distance gain and reduce the optimum level of the wind speed threshold, in our case (under representative Dutch meteorological conditions) to a threshold of 5–6 m s–1. The frequency and predictability (auto‐correlation in time) of pre‐dispersal seed‐loss had a major impact on optimal non‐random abscission functions and resulting dispersal distances.We observed a similar, but more gradual, bias towards higher wind speeds in six out of seven wind‐dispersed species under natural conditions. This confirmed that non‐random abscission exists in many species and that, under local Dutch meteorological conditions, abscission was biased towards winds exceeding 5–6 m s–1.We conclude that timing of seed release can vastly enhance dispersal distances in wind‐dispersed species, but increased risk costs may greatly limit the benefits of selecting wind conditions for long‐distance dispersal, leading to moderate seed abscission thresholds, depending on local meteorological conditions and disturbances.

The Robustness of Plant-Pollinator Assemblages: Linking Plant Interaction Patterns and Sensitivity to Pollinator Loss

Most flowering plants depend on pollinators to reproduce. Thus, evaluating the robustness of plant-pollinator assemblages to species loss is a major concern. How species interaction patterns are related to species sensitivity to partner loss may influence the robustness of plant-pollinator assemblages. In plants, both reproductive dependence on pollinators (breeding system) and dispersal ability may modulate plant sensitivity to pollinator loss. For instance, species with strong dependence (e.g. dioecious species) and low dispersal (e.g. seeds dispersed by gravity) may be the most sensitive to pollinator loss. We compared the interaction patterns of plants differing in dependence on pollinators and dispersal ability in a meta-dataset comprising 192 plant species from 13 plant-pollinator networks. In addition, network robustness was compared under different scenarios representing sequences of plant extinctions associated with plant sensitivity to pollinator loss. Species with different dependence on pollinators and dispersal ability showed similar levels of generalization. Although plants with low dispersal ability interacted with more generalized pollinators, low-dispersal plants with strong dependence on pollinators (i.e. the most sensitive to pollinator loss) interacted with more particular sets of pollinators (i.e. shared a low proportion of pollinators with other plants). Only two assemblages showed lower robustness under the scenario considering plant generalization, dependence on pollinators and dispersal ability than under the scenario where extinction sequences only depended on plant generalization (i.e. where higher generalization level was associated with lower probability of extinction). Overall, our results support the idea that species generalization and network topology may be good predictors of assemblage robustness to species loss, independently of plant dispersal ability and breeding system. In contrast, since ecological specialization among partners may increase the probability of disruption of interactions, the fact that the plants most sensitive to pollinator loss interacted with more particular pollinator assemblages suggest that the persistence of these plants and their pollinators might be highly compromised.

Quantification of plant dispersal ability within and beyond a calcareous grassland

AbstractQuestionMany calcareous grasslands in Europe have declined in species richness in recent decades. This loss of species could be partly due to habitat loss and fragmentation, leading to increased distances and reduced connectivity through seed flow between calcareous grassland patches resulting in increased local extinction risk related to small population size, and partly due to abandonment of traditional management practices that fostered dispersal within and between patches. Here, we quantify short‐ and intermediate‐distance dispersal ability of dry calcareous grassland species and relate these to dispersal traits.LocationSchaffhauser Randen, Switzerland.MethodsWe studied wind dispersal of diaspores under natural conditions within and beyond two replicate calcareous grassland patches. Funnel traps (n = 230) were set up at heights of 0.2 m and 0.7 m along ten transects traversing the calcareous grassland and extending 40 m into the surrounding landscape. We developed a new method for quantifying short ‐ (0–1 m) and intermediate‐distance (1–40 m) dispersal ability, related these to species traits, and tested whether they were able to explain dispersal rates into the adjacent landscape.ResultsWhile grasses could be categorized as good dispersers over short or intermediate distances, or both, forbs were generally poor dispersers over both distance classes. Only small numbers of diaspores were found in the adjacent landscape, and these were predominantly grasses. Diaspore traits, such as terminal velocity or diaspore mass, contributed little to explaining dispersal ability, whereas release height was an important predictor, especially for intermediate‐distance dispersal.ConclusionsUnder natural field conditions, dispersal into the adjacent landscape depends on release height rather than terminal velocity, and is heavily biased towards grasses, so that seed rain does not reflect the species composition of the calcareous grassland community. Thus natural regeneration of species richness of degenerated calcareous grassland communities even over short distances should not rely on wind dispersal alone.

Why are small seeds dispersed through animal guts: large numbers or seed size per se?

Long‐distance dispersal of seeds is an important process in metapopulation dynamics and in plant migrations, but at the same time extremely difficult to observe or quantify directly. If seed dispersal ability were related to attributes of seeds or motherplants, long‐distance seed dispersal would be predictable by indirect approximation using easy‐to‐measure traits. Seed size has been suggested to be such a key trait in seed dispersal ability. However, having smaller seeds also implies having more numerous seeds per plant individual (given equal reproductive effort), and consequently increases the probability of seeds being ingested accidentally. The question is whether small‐seeded species are more abundant in herbivore dung because smaller seed size increases survival rate during gut passage or because they are produced (and ingested) in greater numbers than larger seeds. We investigated endozoochorous seed dispersal via cattle grazing a meadow, and related seed abundance in dung samples to seed attributes. We found that seeds were ingested and passed through the bovine intestinal tract in proportion to the numbers produced per unit area in the grazed vegetation. In contrast, no relationship could be found between endozoochorous dispersal potential (measured as abundance of seeds in dung samples corrected for seed output in the grazed vegetation) and seed attributes such as seed mass, seed shape (roundness), and thickness of the seed coat. This finding underlines the importance of seed number in plant dispersal ability. In addition, it shows that grazing mammals may constitute an important dispersal vector for many plant species conventionally classified as ‘unspecialised’.

Recruitment trade-offs and the evolution of dispersal mechanisms in plants

In this study we place seed size vs. seed number trade-offs in the context of plant dispersal ability. The objective was to suggest explanations for the evolution of different seed dispersal mechanisms, in particular fleshy fruits, wind dispersal and the maintenance of unassisted dispersal. We suggest that selection for improved dispersal may act either by increasing the intercept of a dispersal curve (log seed number vs. distance) or by flattening the slope of the curve. 'Improved dispersal' is defined as a marginal increase in the number of recruits sited at some (arbitrary) distance away from the parent plant. Increasing the intercept of the dispersal curve, i.e. producing more seeds, is associated with a reduction in seed size, which in turn affects the recruitment ability, provided that this ability is related to seed size. If recruitment is related to seed size there will be a recruitment cost of evolving increased seed production. On the other hand, a flattening of the slope by evolving dispersal attributes is likely to be associated with a fecundity cost. An exception is wind dispersal where smaller (and hence more numerous) seeds may lead to more efficient dispersal. We derive two main predictions: If recruitment is strongly related to seed size, selection for improved dispersal acts on the slope of the dispersal curve, i.e. by favouring evolution of dispersal attributes on seeds or fruits. If, on the other hand, recruitment is only weakly related to seed size (or not related, or negatively related), selection for improved dispersal favours increased seed production. Despite its simplicity, the model suggests explanations for (i) why so many plant species lack special seed dispersal attributes, (ii) differences in dispersal spectra among plant communities, and (iii) adaptive radiation in seed size and dispersal attributes during angiosperm evolution.

ANT‐HILL VEGETATION IN ACIDIC GRASSLANDS IN THE GOWER PENINSULA, SOUTH WALES

SUMMARYThe vegetation of ant‐hill and adjacent pasture was compared in five acidic grasslands in the Gower peninsula, South Wales. The acrocarpous mosses Dicranum scoparium Hedw., Polytrichum juniperinum Hedw. and P. piliferum Hedw. were almost confined to the mounds but several perennials, including Anthoxanthum odoratum L., Luzula campestris (L.) DC, Potentilla erecta (L.) Rausch and Sieglingia decumbens (L.) Bernh., were much less abundant on ant‐hills than in the surrounding vegetation. The factors which influence the distributions of the plant species are discussed briefly. In considering the responses of plants to ant‐hills soil chemical and physical factors are probably less important than a plant's dispersal ability and capacity to withstand burial.