Pollen limitation and Allee effect related to population size and sex ratio in the endangered Ottelia acuminata (Hydrocharitaceae): implications for conservation and reintroduction

Small populations may suffer more severe pollen limitation and result in Allee effects. Sex ratio may also affect pollination and reproduction success in dioecious species, which is always overlooked when performing conservation and reintroduction tasks. In this study, we investigated whether and how population size and sex ratio affected pollen limitation and reproduction in the endangered Ottelia acuminata, a dioecious submerged species. We established experimental plots with increasing population size and male sex ratio. We observed insect visitation, estimated pollen limitation by hand‐pollinations and counted fruit set and seed production per fruit. Fruit set and seed production decreased significantly in small populations due to pollinator scarcity and thus suffered more severe pollen limitation. Although frequently visited, female‐biased larger populations also suffered severe pollen limitation due to few effective visits and insufficient pollen availability. Rising male ratio enhanced pollination service and hence reproduction. Unexpectedly, pollinator preferences did not cause reduced reproduction in male‐biased populations because of high pollen availability. However, reproductive outputs showed more variability in severe male‐biased populations. Our results revealed two component Allee effects in fruit set and seed production, mediated by pollen limitation in O. acuminata. Moreover, reproduction decreased significantly in larger female‐biased populations, increasing the risk of an Allee effect.     

When bigger is not better: intraspecific competition for pollination increases with population size in invasive milkweeds

One of the essential requirements for an introduced plant species to become invasive is an ability to reproduce outside the native range, particularly when initial populations are small. If a reproductive Allee effect is operating, plants in small populations will have reduced reproductive success relative to plants in larger populations. Alternatively, if plants in small populations experience less competition for pollination than those in large populations, they may actually have higher levels of reproductive success than plants in large populations. To resolve this uncertainty, we investigated how the per capita fecundity of plants was affected by population size in three invasive milkweed species. Field surveys of seed production in natural populations of different sizes but similar densities were conducted for three pollinator-dependent invasive species, namely Asclepias curassavica, Gomphocarpus fruticosus and G. physocarpus. Additionally, supplemental hand-pollinations were performed in small and large populations in order to determine whether reproductive output was limited by pollinator activity in these populations. Reproductive Allee effects were not detected in any of the study species. Instead, plants in small populations exhibited remarkably high levels of reproductive output compared to those in large populations. Increased fruit production following supplemental hand-pollinations suggested that the lower reproductive output of naturally pollinated plants in large populations is a consequence of pollen limitation rather than limitation due to abiotic resources. This is consistent with increased intraspecific competition for pollination amongst plants in large populations. It is likely that the invasion of these milkweed species in Australia has been enhanced because plants in small founding populations experience less intraspecific competition for pollinators than those in large populations, and thus have the ability to produce copious amounts of seeds.     

Pollen limitation in invasive populations of Solanum rostratum and its relationship to population size

Aims Small plant populations may be more likely to suffer more severe pollen limitation due to the lower number of potential mates or suitable pollinators. For invasive species, this phenomenon may be more common when an invading population colonizes a new habitat. Here, we investigated whether pollen limitation occurs in invasive populations of Solanum rostratum during its invasion from North America to China and evaluated the patterns between pollen limitation and population size.Methods Pollen addition experiments were performed on six invasive populations of S. rostratum. By comparing fruit set and seed production with open pollination treatment, we calculated the index of pollen limitation and regressed it to population size and density.Important findings Among the six sampled invasive populations of S. rostratum, the fruit set and seed production per fruit were 0.346±0.014 and 52.38±9.29, respectively, with open pollination treatment and 0.572±0.022 and 56.28±10.79, respectively, with pollen addition treatment. Compared with open pollination, pollen addition significantly increased fruit set and seed production by 65.3 and 7.4%, respectively. The standardized index of pollen limitation ranged from 0.022 to 0.125, with an average of 0.065, suggesting that invasive populations of S. rostratum do suffer from pollen limitation. The index of pollen limitation was negatively correlated with population size, which is consistent with the pattern that smaller populations suffer from more severe pollen limitation.     

Seed production in a threatened Aloe is not affected by bird exclusion or population size

Small populations of many plant species have been shown to exhibit ecological Allee effects. These effects are expected to be pronounced in plants which are obligate outcrossers and rely on pollinators which forage preferentially in larger populations with greater nectar availability. We examined the breeding and pollination systems, level of pollen limitation and seed production in populations of a threatened “ornithophilous” species, Aloe pruinosa. Experimental hand-pollinations showed that A. pruinosa is genetically self-incompatible and thus an obligate outcrosser. Experimental exclusion of birds from inflorescences did not affect seed production, suggesting that insects are effective pollinators. Supplemental hand-pollinations in several populations showed that seed production in A. pruinosa is not pollen limited. Further, there were no significant relationships between population size and any measure of reproductive success in this Aloe species. Small populations of A. pruinosa are thus viable in terms of pollination processes and should be protected from more direct threats, such as habitat alteration.     

A piece of the puzzle: a method for comparing pollination quality and quantity across multiple species and reproductive events

? Understanding how pollination affects plant reproductive success and how changes in pollination service affect plant populations, communities and ecosystems is of increasing concern. Yet supplemental hand-pollination traditionally used to assess pollen limitation is prohibitive for large-scale comparative work. Moreover, it does not differentiate between quality and quantity aspects of pollen limitation, and it may suffer from confounds of post-pollination processes such as resource availability to fill seeds. ? Here, we highlight pollen tubes as the functional link between pollen arrival and seed production and suggest that consideration of pollen tubes leads to a better depiction of limitation at the pre-zygotic (pollination) phase of sexual reproduction. ? We assessed the rigor of piecewise regression to analyze the relationship between the numbers of pollen grains and pollen tubes observed in nonmanipulated wilted flowers. We illustrate how parameters obtained from this analysis provide quantitative insight into the relative relevance of the quantity and quality of pollen receipt in limiting natural pollination success, and can facilitate comparisons among data sets. ? This nonmanipulative method opens up new opportunities for rigorous assessment of the relative importance of the quantity and quality of pollination in limiting plant reproduction, especially from a community-wide perspective.     

Density-dependent seed set in the Haleakala silversword: evidence for an Allee effect

Plant species may be subject to Allee effects if individuals experience a reduction in pollination services when populations are small or sparse. I examined temporal variation in reproductive success of the monocarpic Haleakala silversword (Argyroxiphium sandwicense subsp. macrocephalum) over five years, to determine if plants flowering out of synchrony with most of the population (i.e., in low flowering years) exhibited lower percent seed set than synchronously-flowering plants (i.e., those flowering in high flowering years). Through two pollination experiments conducted over multiple years, I also measured pollen limitation and self-incompatibility in this species. The number of flowering plants varied greatly among years, as did reproductive success. Percent seed set was significantly correlated with the number of plants flowering annually, such that plants flowering in high flowering years (1997 and 2001) exhibited significantly higher percent seed set than did plants flowering in low flowering years (1998-2000). In the 3-year pollen limitation study, plants flowering asynchronously were pollen-limited, whereas plants flowering synchronously were not. This species is strongly self-incompatible. Results of this study demonstrate that the Haleakala silversword experiences reduced reproductive success in low flowering years, and suggest that this Allee effect is pollinator-mediated. Allee effects in plants are an understudied yet potentially important force with implications for the population dynamics and conservation of rare species.     

Quantitative and qualitative consequences of reduced pollen loads in a mixed‐mating plant

Greater pollination intensity can enhance maternal plant fitness by increasing seed set and seed quality as a result of more intense pollen competition or enhanced genetic sampling. We tested experimentally these effects by varying the pollen load from a single pollen donor on stigmas of female flowers of Dalechampia scandens (Euphorbiaceae) and measuring the effects on seed number and seed mass. Seed set increased rapidly with pollen number at low to moderate pollen loads, and a maximum set of three seeds occurred with a mean pollen load of 19 pollen grains. We did not detect a trade‐off between the number of seeds and seed mass within a fruit. Seed mass increased with increasing pollen load, supporting the hypothesis of enhanced seed quality via increased pollen‐competition intensity or genetic sampling. These results suggest that maternal fitness increases with larger pollen loads, even when the fertilization success is already high. Our results further highlight the importance of high rates of pollen arrival onto stigmas, as mediated by reliable pollinators. Comparing the pollen‐to‐seed response curve obtained in this experiment with those observed in natural populations suggests that pollen limitation may be more severe in natural populations than predicted from greenhouse studies. These results also indicate that declines in pollinator abundance may decrease plant fitness through lowered seed quality before an effect on seed set is detected.     

野慈姑花粉限制评估的影响因素

以野慈姑为材料,研究补充授粉的处理水平(单花、花序)和花粉质量(自花花粉、异花花粉)对花粉限制评估的影响.结果表明: 补充授粉的处理水平会造成花粉限制的程度不同(P=0.0021),单花水平的授粉会导致花朵之间的资源重新分配,从而影响结实;补充授粉的花粉质量会对结实造成影响,补充异花花粉比自花花粉能产生更多的种子(P=0.0497),且萌芽率更高(P=0.0428),表明该体系的花粉限制不是数量的限制,而是优质花粉的限制.不同的试验水平和花粉质量可能对花粉限制的评估造成影响,相关研究应尽量对整个花序或者植株进行补充授粉,材料允许时还应考虑自异花粉的对比试验,有利于更客观地认识和检测花粉限制.     

Reproductive variability and pollen limitation in three Betula taxa in northern Sweden

This 3-yr study documents large variation in pollination and seed quality in birches (Betula) in northern Sweden between taxa, high and low altitude populations, and among years Seed quality was positively correlated with pollination degree and improved by pollen supplementation in B pubescens ssp pubescens in the mountain area, indicating pollen limitation Betula pendula showed the same tendency, but less pronounced Reduced sexual reproduction of B pendula in the mountain area might more often be due to limited resources for seed maturation and defense of the seeds against predispersal seed predation In contrast, coastal populations of both species display relatively high pollination rates and supplemental pollination had no effect on seed quality, suggesting that pollen limitation is rare or absent here The mountain birch Betula pubescens ssp tortuosa showed generally lower pollination and seed quality compared with B pubescens ssp pubescens , but not compared with mountain populations of B pendula It is concluded that pollen limitation may occur in Betula , but to what extent varies with taxon, altitude and year The seed quality is also strongly affected by other factors such as infections of gall midges (Semudobia spp, Diptera Cecidomyndae) and incomplete seed maturation     

Erosion of a pollination mutualism along an environmental gradient in a south Andean treelet,Embothrium coccineum (Proteaceae)

The limit of a species’ distribution can be determined biotically if an environmental gradient causes the loss of critical mutualists such as pollinators. We assessed this hypothesis for Embothrium coccineum, a self‐incompatible red‐flowered treelet growing along a strong west‐east precipitation gradient from rainforest to forest‐steppe ecotone in the rain shadow of the southern Andes in northwestern Patagonia. For 16 populations along this gradient, we quantified composition of the pollinator assemblage, pollination efficiency and limitation, and reproductive output. The treelet has a generalized pollination system, but the hummingbird Sephanoides sephaniodes was the most effective pollinator. The relative importance of this hummingbird as a flower visitor within populations influenced pollen transfer and fruit set more strongly than local precipitation. As hummingbirds and other pollinators, including passerine birds and nemestrinid flies, were replaced by bees towards the dry eastern range limit, pollen limitation increased and reproduction eventually failed. These results support the hypothesis that pollinators can act as important biotic filters influencing plant distribution, and warn against predictions of geographical range shifts based solely on climatic variables.     

Spatio-temporal variation in pollen limitation and reproductive success of two scape morphs in Primula farinosa

In plant populations where reproductive output is limited by pollinator visitation, plants with attractive floral displays should have a selective advantage. We examined the effect of inflorescence height on pollination success in Primula farinosa, which is dimorphic for scape length. To test the hypothesis that fruit and seed initiation are more strongly pollen-limited in the short-scaped than in the long-scaped morph, and that this difference is affected by spatio-temporal variation in pollen limitation, we conducted a hand-pollination experiment in four populations over 2 yr. Pollen limitation of fruit initiation varied among populations and years, and was stronger in the short-scaped than in the long-scaped morph. The results suggest that interactions with pollinators will need to be considered for a full understanding of the maintenance of this striking polymorphism. The study also shows that, although pollen limitation is likely to vary in space and time in many plant species, such variation is not necessarily associated with variation in selection on floral characters.     

Pollen limitation or mate search need not induce an Allee effect

When a process modelling the availability of gametes is included explicitly in population models a critical depensation or Allee effect usually results. Non-spatial models cannot describe clumping and so small populations must be assumed very diffuse. Consequently individuals in small populations experience low contact rates and so reproduction is limited. In Nature invasions into new territory are unlikely to be as diffuse as those described by non-spatial models. We develop pair approximations to a probabilistic cellular automata model with independent pollination and seed setting processes (equivalently mate search and reproduction processes). Each process can be either global (population-wide) or local (within a small neighbourhood) or a mixture of the two. When either process is global the resulting model recaptures the Allee effect found in non-spatial models. However, if both processes are at least partially local we obtain a model in which Allee effects can be avoided altogether if individuals are suitably strong pollinators and colonisers. The Allee effect disappears because small populations are dramatically more clumped when colonisation is local and less wasteful of pollen when pollination is local.     

Pollen limitation of reproductive effort in willows

Summary Pollen limitation of seed set differs from resource limitation in its implications for the evolution of floral traits. Willow flowers attract insects, but also abundantly produce wind-dispersed pollen. I demonstrated pollen limitation in single branches bearing 2–4 inflorescences (catkins) in a field experiment with five species by artificially increasing or decreasing the pollen load. Because the responses by single branches might be explained by diversion of resources to better-pollinated branches within a plant, a second experiment with one species tested both pollen limitation of whole plants and the autonomy of catkins. Seed set of single willow catkins is unaffected by experimental alterations of seed set in other catkins on the same plant. Hand-pollination of single catkins and of whole plants increased seed set to the same degree, suggesting there is little or no competition for resources between catkins only 5–10 cm apart. Thus, seed set in willows appears to be pollen limited, favoring insect pollination and the evolution of entomophilous traits. The data support previous views that willows have a dual pollination system utilizing wind and insects.     

A test for Allee effects in the self‐incompatible wasp‐pollinated milkweed Gomphocarpus physocarpus

It has been suggested that plants that are good colonizers will generally have either an ability to self‐fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self‐incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self‐pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp‐pollination system, contributes in part to the colonizing success of G. physocarpus. The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed.     

Variation of pollinator assemblages and pollen limitation in a locally specialized system: the oil-producing Nierembergia linariifolia (Solanaceae)

Background and Aims

Few studies have examined the dynamics of specialist plant–pollinator interactions at a geographical scale. This knowledge is crucial for a more general evolutionary and ecological understanding of specialized plant–pollinator systems. In the present study, variations in pollinator activity, assemblage composition and pollen limitation were explored in the oil-producing species Nierembergia linariifolia (Solanaceae).

Methods

Pollen limitation in fruit and seed production was analysed by supplementary hand pollination in five wild populations. Pollinator activity and identity were recorded while carrying out supplementary pollination to assess the effect of pollinators on the degree of pollen limitation. In two populations, pollen limitation was discriminated into quantitative and qualitative components by comparing supplementation and hand cross-pollination in fruit set and seed set. The effect of flower number per plant on the number of flowers pollinated per visitor per visit to a plant was examined in one of these populations as a possible cause of low-quality pollination by increasing geitonogamy.

Results and Conclusions

Although pollen limitation was evident along time and space, differences in magnitude were detected among populations and years that were greatly explained by pollinator activity, which was significantly different across populations. Floral display size had a significant effect on the visitation rate per flower. Limitation by quality clearly affected one population presumably due to a high proportion of geitonogamous pollen. The great inter-population variation in plant–pollinator interaction (both in pollinator assemblages composition and pollinator activity) and fitness consequences, suggests that this system should be viewed as a mosaic of locally selective processes and locally specialized interactions.Key words: Nierembergia linariifolia, Centris, Chalepogenus, pollen limitation, pollen quality, oil-producing flowers, specialized pollination, floral display, assemblage composition, geographic variation, Solanaceae, tests of equivalence     

Allee effects within small populations of Aconitum napellus ssp. lusitanicum, a protected subspecies in northern France

Plants growing at low density can suffer from Allee effects as a result of pollen limitation. Previous studies of Allee effects have focused on the effects of variation among populations in size or density on reproduction. Here, the effects of plant distribution within populations on fitness components are explored in a rare plant, Aconitum napellus ssp. lusitanicum, and ecological and genetic mechanisms underlying these effects are identified. To detect pollen limitation, seed production was compared under natural versus hand-supplemented pollinations on inflorescences of different sizes in natural patches differing both in flower density and in isolation from other patches. Germination rate and juvenile survival of seeds produced in low- and high-density patches were also compared. Pollen-supplemented flowers always produced more seeds than open-pollinated flowers, especially among small plants and plants growing at low density. Offspring produced in low-density patches exhibited lower fitness that those produced in high-density patches. This could have been caused by post-fertilization mechanisms, including inbreeding depression or differential maternal resource allocation. These results show that Allee effects on fitness components (ecological and genetic Allee effects) occur within A. napellus populations at different spatial scales. The spatial distribution of plants seems to be a crucial factor affecting reproductive output and fitness.     

Testing for ecological and genetic Allee effects in the invasive shrub Senna didymobotrya (Fabaceae)

For an introduced plant species to become invasive, it must be able to reproduce even in initially small populations. We tested for Allee effects (reduced reproductive performance of individuals in small populations) in the nonclonal, buzz-pollinated shrub Senna didymobotrya in its invasive range in South Africa. The species is self-compatible, but we found that in its invasive range in South Africa it requires pollinators to set seed. Nearly all stigmas (90%) received pollen, but natural fruit set was very low (3-20%). Pollen receipt and fruit set were not significantly correlated with population size. We thus found no evidence for an ecological Allee effect arising from pollen limitation in small populations. Offspring seedling performance, measured in terms of stem volume and leaf area, was also not significantly correlated with the number of plants in the source population, indicating that genetic Allee effects, such as inbreeding depression, are either absent or of such a small magnitude that they would be unlikely to limit further spread of S. didymobotrya in South Africa.     

Reproductive interactions mediated by flowering overlap in a temperate hummingbird–plant assemblage

Pollinator‐mediated competition through shared pollinators can lead to segregated flowering phenologies, but empirical evidence for the process responsible for this flowering pattern is sparse. During two flowering seasons, we examined whether increasing overlap in flowering phenology decreased conspecific pollination, increased heterospecific pollination, and depressed seed output in the seven species composing a hummingbird–plant assemblage from the temperate forest of southern South America. Overall trends were summarized using meta‐analysis. Despite prevailing negative associations, relations between phenological overlap and conspecific pollen receipt varied extensively among species and between years. Heterospecific pollen receipt was low and presumably of limited biological significance. However, our results supported the hypothesis that concurrent flowering promotes interspecific pollen transfer, after accounting for changes in the abundance of conspecific flowers. Seed output was consistently reduced during maximum phenological overlap during the first flowering season because of limited fruit set. Responses varied more during the second year, despite an overall negative trend among species. Relations between estimated effects of phenological overlap on pollination and seed output, however, provided mixed evidence that conspecific pollen loss during pollinator visits to foreign flowers increases pollen limitation. By flowering together, different plant species might benefit each other's pollination by increasing hummingbird recruitment at the landscape level. Nevertheless, our results are mostly consistent with the hypothesis of pollinator‐mediated competition shaping the segregated flowering pattern reported previously for this temperate plant assemblage. The mechanisms likely involve effects on male function, whereby pollen‐transport loss during heterospecific flower visits limit pollen export, and more variable effects on female function through pollen limitation.     

An experimental study of the S-Allee effect in the self-incompatible plant <Emphasis Type="Italic">Biscutella neustriaca</Emphasis>

Homomorphic self-incompatibility (SI) evolved in many plant families to enforce selfing avoidance, and is controlled by a single multiallelic locus (the S-locus). In a fragmented landscape, strong variation in population size and in local density is expected to cause strong variation in allelic diversity at the S-locus, which could generate an Allee effect on female reproductive success by constraining compatible pollen availability. In this experimental study, we aimed at detecting this SI-specific Allee effect (or S-Allee effect) in the endangered species Biscutella neustriaca. We demonstrated the occurrence of a SI mating system in the species and determined compatibility relationships among genotypes through a large set of controlled pollinations. For the experiment, we chose three different pollen receptor genotypes, each compatible with respectively 100, 75 and 25% of four other genotypes, which constituted the pollen sources. We placed different ramets of each receptor at different distances from the pollen sources to control for pollen limitation due to low local density, and we measured the seed set on each receptor plant three times consecutively. Analyses performed with generalized linear mixed models showed that both the distance to the pollen sources and the mate availability due to SI had a significant effect on seed set, with a strong reduction observed when mate availability was limited to 25%. Our results suggest that pollen limitation due to a restriction in compatible mate availability could occur in small or scattered populations exhibiting low allelic diversity at the S-locus.     

Depressed pollination in habitat fragments causes low fruit set

In central New South Wales, Australia, flowers of Acacia brachybotrya and Eremophila glabra plants growing in linear vegetation remnants received less pollen than conspecifics in nearby reserves. Pollen supplementation increased fruit production by both species, indicating pollen limitation of fruit set. Together these observations explain why fruit production by these species was depressed in linear-strip populations relative to nearby reserves. This study confirms that habitat fragmentation can lead to decline in pollination and subsequent fruit set in wild plant populations. Disrupted pollination interactions of the kind documented in this study may offer a substantial challenge to the conservation of biodiversity in fragmented landscapes.