Orchid fruiting success is unrelated to surrounding floral resources in South Australian plant communities

Abstract

To maintain recruitment in orchid populations in an ecosystem setting, we must understand how surrounding floral resources affect fruiting success. We studied fruiting success in two endemic Australian species, Diuris pardina and Glossodia major, in relation to surrounding floral resources. Diuris pardina has a visually deceptive pollination strategy via mimicry of pea flowers, attracting pollinators associated with co-flowering plants of Pultenaea. Glossodia major displays dummy anthers and has a more generalist pollination strategy. We expected fruiting success of both species to positively correlate to conspecific and heterospecific floral density because orchid pollination should be enhanced by the attraction of higher densities of native bees. We expected fruiting success of D. pardina to positively correlate with abundance of Pultenaea flowers. Surveying 18 plots in South Australia, we counted species, individuals and flowers of conspecifics and heterospecifics and returned to count flowers that set fruit. We conducted Pearson correlations between fruiting success and density of conspecific flowers, richness, abundance and Shannon index of surrounding floral resources and floral abundance of individual species. Fruiting success was correlated with conspecific floral density for D. pardina but not G. major. No relationship was found between fruiting success and heterospecific floral resources. Fruiting success of D. pardina was not correlated with abundance of Pultenaea; instead it was positively correlated with the invasive species Lavandula stoechas.

摘要

为保证生态系统不断补充兰花的种群数量，了解兰花周围的花卉资源如何影响兰花的结实成功率是很有必要的。为此，本论文研究了澳大利亚的两种特有物种Diuris pardina和Glossodia major的结实成
功率与其周围花卉资源之间的关系。D. pardina模仿豌豆花，通过视觉欺骗手段吸引Pultenaea共花植物的专
化传粉者；G. major展现假花药，吸引泛化传粉者。因为吸引密度更高的本地蜜蜂可以促进兰花传粉，
我们假设这两个物种的结实成功率与其周围同种花和异种花的密度呈正相关。我们还假设D. pardina的成功结实与其周围Pultenaea花的丰度呈正相关。我们调查了澳大利亚南部的18个地块，统计了同种花和异种花的物种数、个体数和花朵数，以及成功结实的花朵数。对结实成功率与同种花的密度、周围花资源的多度、丰度和香农指数以及单个物种的花卉丰度进行了Pearson相关分析，发现D. pardina的结实成功率与周围同种花的密度相关，但G. major的结实成功率与周围同种花的密度无关，未发现结实成功率与异种花资源之间存在关联。D. pardina的结实成功率与Pultenaea的丰度无关，而与入侵物种法国薰衣草(Lavandula stoechas)呈正相关。

INTRODUCTION

The orchid family includes over 27 000 species (Chase et al. 2015; Govaerts 2016), but their rapid decline over the past decades and threatened status in the IUCN Global Red List point to the need for urgent conservation measures (IUCN 2021; Wraith and Pickering 2018). Due to the complexity of their symbiotic interactions with other species and their dependence on specific association with pollinators (Hutchings 2010), orchids constitute a complicated group from conservationist and management perspectives (Martín-Forés et al. 2022; Reiter et al. 2016; Swarts and Dixon 2017). Therefore, to design successful conservation strategies for this plant family, we need to fully understand orchid reproduction and pollination deficits that might contribute to orchid decline (Fay 2018); for that, comprehensive orchid flowering and pollination data are of particular relevance, as they reflect the complexity and maintenance of underlying ecosystem interactions and allow study of the activity of key insect groups (Brundrett 2019).

Orchid reproduction depends on pollination success, normally by insects, although there are some orchid species that can undergo auto-pollination or asexual propagation by forming clonal populations (Roberts and Dixon 2008). Orchid pollination is often one sided and specialized, with the orchid relying more on its pollinator than the pollinator on the orchid (Joffard et al. 2019; Johnson and Steiner 2003; Roberts and Dixon 2008; Tremblay 1992). Orchid pollination can be based on nectar rewards or partial rewards (Shrestha et al. 2020), although one-third of orchid species have developed deceit strategies, including food and sexual deception, in which orchids mimic a reward (mimicry of floral features and production of a copy of the insect pheromone or even resembling insect forms, respectively; Jersáková et al. 2006; Roberts and Dixon 2008).

Australia is a diversity hotspot for nectarless plants with deceptive pollination strategies (Dafni and Bernhhardt 1990; Herberstein et al. 2013). Endemic Australian species from the genus Diuris are an example of orchid species resembling legumes and pollinated by a visually deceptive mechanism (i.e. via mimicry of orange and yellow peas flowers from the genus Pultenaea; Beardsell et al. 1986; Indsto 2009); therefore, they attract a suite of pollinators associated with co-flowering plants of similar appearance (Scaccabarozzi et al. 2018, 2019). Thus, floral mimicry such as the one displayed by Diuris is expected to be more effective with a higher ratio of model to mimic flowers (Anderson and Johnson 2006). Species from the genus Glossodia have a more generalist pollination strategy (Bates and Weber 1990; Beardsell et al. 1986; Faast 2010), and they display dummy anthers (Jersáková et al. 2006). Pollination success of Diuris and Glossodia rely upon communities of native bees, which vary in space and time according mainly to the local availability of floral resources (Bates and Weber 1990; Faast 2010). The ‘magnet species effect’ states that rewardless and deceptive orchids benefit from an abundance of neighbouring rewarding plants because they attract potential pollinators (Thompson 1978). Thus, fruiting success for these two genera (and especially for species whose specific pollinators have a large nectar component in their diet) is expected to be impacted by the surrounding vegetation, with pollinator activity typically enhanced by greater density and diversity of floral resources (of both, conspecifics and heterospecifics mimicked by the deceptive orchid; Faast 2010; Johnson et al. 2003).

The aim of this study was to explore how species composition and relative abundance of surrounding floral resources influenced reproductive success (i.e. fruiting success) of Diuris pardina Lindl. and Glossodia major R.Br. in South Australian orchids communities. These species are representative of orchid communities in the region and are frequent and locally abundant, making them suitable subjects for scoring fruiting success. Additionally, we chose these species because they have contrasting degrees of reproduction specialization (i.e. D. pardina is more specialist whereas G. major is more generalist), yet relatively little is known about their reproductive biology. For example, the specific pollinators and their effect on the reproduction of these species are unknown to date. We tested the following hypotheses:

1. Fruiting success of both species is correlated with the density of conspecific flowers.

2. Diverse and abundant heterospecific floral resources increase orchid pollination by attracting higher densities of bees to the local patch. Fruiting success in orchid species is correlated with heterospecific floral abundance and diversity.

3. Pollination of D. pardina is increased by deceptive mimicry in the presence of Pultenaea flowers in the surrounding vegetation. Fruiting success of D. pardina is correlated with the abundance of Pultenaea flowers.

MATERIALS AND METHODS

Study area

Australia harbours a high diversity of endemic orchid species (approximately 1800 species, 95% endemic; Backhouse 2007; Wraith and Pickering 2019), with species from this family being over-represented among Australia’s threatened species (Faast and Facelli 2007). The study was conducted in the Spring Gully Conservation Park, in the northern area of the Mount Lofty Ranges, South Australia. The area of study is a large (4 km²), unique vegetation remnant protected to conserve the only occurrence of Eucalyptus macrorhyncha F.Muell. ex Benth. in South Australia, embedded in an agricultural landscape. The Mount Lofty Ranges are considered a climatic refugium at the continental scale (Byrne 2008; Crisp et al. 2001; Guerin et al. 2016; Guerin and Lowe 2013). The area has a Mediterranean climate. Mean annual precipitation ranges from 460 to 990 mm, whereas minimum and maximum annual mean temperatures range from 7.8 to 10.3 °C and from 18.1 to 21.7 °C, respectively (Harwood et al. 2016).

Data collection

To explore the effect of floral resources on fruiting success of two common orchid species, we related the proportion of recorded flowers that developed into fruits, to both the density of flowers and the abundance and diversity of floral resources in the plot. For this, we surveyed 18 plots 30 m × 30 m size, with a minimum distance of 100 m from each other, located at Spring Gully Conservation Park. In early–mid September 2020, during the peak flowering period, we counted the number of open flowers of the orchid species D. pardina Lindl. and G. major R.Br. (counting ~3000 individuals; Fig. 1). We returned to the same plots in late October to count the orchid flowers that had set fruit. Fruiting success was then expressed as the percentage of flowers that had set fruit.

To test the idea that fruiting success relates to the floral resources provided to pollinators in the surrounding vegetation at the time of flowering, we set up a floral transect in each plot to quantify the diversity and species composition of other floral resources available to insect pollinators. The transect consisted of a 1.8-m wide strip across the plot (54 m²); for all surrounding vegetation except for non-insect pollinated species such as grasses and sedges, we recorded floral abundance data per species. Hence, for each plant species, we recorded number of individuals and the number of open flowers. Flowers were either counted or, where necessary, estimated by counting the flowers on branchlets and multiplying by number of branchlets.

Data analyses

To determine the fruiting success, we calculated the proportion of recorded flowers that develop into fruit. We related the fruiting success of D. pardina and G. major to conspecific floral density (i.e. number of individuals and number of flowers) through Pearson correlations.

We also related the fruiting success of D. pardina and G. major to heterospecific floral density (i.e. number of individuals and number of flowers of other species in the surrounding vegetation), and to the overall species richness, abundance and Shannon diversity index of heterospecific floral resources in the plot through Pearson correlations.

Finally, we explored plant–plant relationships of surrounding floral resources affecting fruiting success by conducting pairwise correlations for D. pardina and G. major with the floral abundances of each of the recorded species (Supplementary Tables S1 and S2). For those species that we observed significant correlation coefficients (Supplementary Table S3), we conducted linear models between D. pardina and G. major fruiting success rates and the species abundance to check for the significance of the effect of that particular flowering species on enhanced pollination.

All statistical analysis and calculations were performed using R (R Core Team 2020) employing the package corrplot (Wei and Simko 2021).

RESULTS

Fruiting success in D. pardina and G. major was low (0%–18%). Glossodia major and D. pardina exceeded 5% of fruiting success only in five and two plots, respectively (Fig. 1). Fruiting success for D. pardina was especially low, with eleven plots showing flowers but not fruits (Fig. 1; Table 1). Diuris pardina fruiting success was correlated with conspecific 
floral density (i.e. number of individuals and 
number of flowers per plot; adjusted r-squared = 0.39, P < 0.01; Fig. 2), but there was no apparent trend between flowering and fruiting G. major individuals (adjusted r-squared = −0.05, P = 0.71; Fig. 2).

Fruiting success for both species was not significantly correlated to overall heterospecific floral resources (i.e. number of flowers from all the species present in the plot), nor to overall floral diversity in terms of both, species richness and Shannon diversity (Fig. 3).

Contrary to our predictions, fruiting success for D. pardina was not significantly correlated with the abundance of Pultenaea largiflorens (adjusted r-squared = 0.02, P = 0.28), although it was positively correlated with the abundance of Lavandula stoechas (adjusted r-squared = 0.23, P < 0.05). Fruiting success for G. major was significantly correlated to Wurmbea dioica flowers (adjusted r-squared = 0.24, 
P < 0.05), and marginally to Drosera auriculata (adjusted r-squared = 0.14, P < 0.1).

DISCUSSION

According to our results, fruiting success for D. pardina and G. major is mainly stochastic and not obviously controlled by heterospecific local floral resources and surrounding vegetation, contradicting a priori predictions. Fruiting plants of both species were often observed in clusters, which suggests chance encounters with pollinators that visited neighbouring plants. It is also possible that developing fruits could have been grazed by deer, kangaroos and euros, although grazing pressure on vegetation in the area and palatable plants in the plots was not high and many spent orchid flowers were observed during fruiting surveys, confirming that they had not been pollinated. Overall, the success rate of the studied species appeared to be low; however, for food deceptive orchids, it is common that the reproductive success rate does not reach 20% (Jacquemyn and Brys 2010). Fruiting success for D. pardina was positively associated with conspecific floral resources, whereas fruiting success for G. major cannot be predicted from conspecific flower density. Deceptive species which typically have low fruiting success, typically compensate this by displaying greater seed output per fruit than non-deceptive, nectar-rewarding orchids (Sonkoly et al. 2016). In this sense, populations of the species studied here might rely on a small number of individuals producing seed to replenish the population or even long-lived individuals that are rarely pollinated, contrary to other less abundant species, such as orchids from the genus Thelymitra for which developing fruits were observed more consistently in the vegetation.

Contrary to our expectation, fruiting success of D. pardina was not significantly correlated with the native pea it mimics, P. largiflorens, despite bees being observed visiting P. largiflorens individuals during sampling. This trend was also observed in Australia for Diuris magnifica, which did not increase its reproductive success associated with the presence of native pea plants but appeared to be influenced by another non-model plant (Scaccabarozzi et al. 2018, 2019). These findings might be related to the likelihood of the pollinators also feeding on a range of other species apart from the model plant. In addition, when relying on floral mimicry, the reproductive success of the deceptive orchid might be diminished with an increased ratio of deceptive orchid/model to mimic (Anderson and Johnson 2006). Unexpectedly, fruiting success of D. pardina was positively correlated with the abundance of L. stoechas, an introduced labiate woody species which strongly attracts honeybees (Kantsa et al. 2018). This might be related to the production of essential oils by L. stoechas, and the fact that this aromatic plant is known to serve as hubs in plant–pollinator networks in other Mediterranean systems (Raguso 2020). The fact that G. major was positively correlated with the abundance of W. dioica might be due to the fact that male individuals of this dioecious lily attract certain species of Australian native bees, and, to a lesser extent, honeybees (Dyer et al. 2016; Vaughton and Ramsey 1998). Previous studies focussing on the pollination and fruiting success of the Australian orchid Caladenia versicolor also observed in their plots of study co-flowering plants of W. dioica and two species from the genus Drosera (Reiter et al. 2019).

Contrary to our expectations and to previous studies stating that the presence of neighbouring flowering plants played an essential role for maintaining pollinator visits in rewardless orchid communities (e.g. Sakata et al. 2014), orchid fruiting success for D. pardina and G. major was not related to overall hetersopecific floral resources in terms of species richness, abundance and Shannon diversity. This is surprising, as facilitated pollination enhanced by co-flowering surrounding vegetation through pollinator sharing and ‘the magnet effect’ has commonly been observed for deceptive orchids (Johnson et al. 2003; Juillet et al. 2007; Pellegrino et al. 2008). The lack of relationship between orchid fruiting success and heterospecific floral resources, could be related to divergent flowering phenologies (Internicola et al. 2008), or to contrasting pollination syndromes between the species of study and the surrounding vegetation recorded in the study. Previous literature suggested that the effect of the pollination syndrome displayed by the different surrounding species and the specificity of orchid species with their pollinators can play a key role in orchids reproduction success and subsequent orchid diversity within the targeted communities (Newman et al. 2013). In this sense, elucidating pollination syndromes of orchid species and the surrounding vegetation could shed light into this matter.

We would like to highlight that the results reported in this short communication are subjected to logistic limitations and therefore should be interpreted conservatively. Ideally, fruiting success in orchids should be sampled across several years, as it can vary considerably from 1 year to another, and some orchid species can undergo masting events (Sakai 2002; Xiong et al. 2015). This was a single survey within one flowering season and not a longitudinal study. Sampling was deliberately located within the same geographic area (Spring Gully Conservation Park) and broad vegetation type (E. macrorhyncha woodlands) in elevated landscape positions to control for potentially confounding environmental and soil factors. Thus, the conclusions here should not be interpreted as a generalization, neither for South Australian orchid communities, nor across the full geographic range of these two species. In future, experimental designs encompassing several years of sampling and a wider range of vegetation systems, and in which surrounding co-flowering plants can be manipulated, may provide further clarity.

Further studies aiming to inform conservation practices should aim to control for the species’ pollination syndrome by including the specific pollinator for each species and its whole potential diet including co-flowering plants. Unfortunately, the existing literature does not cover the pollinators for the species recorded in the present study; e.g. Kuiter (2016) presented the only pollinator data existing to date for Glossodia. Therefore, tackling this knowledge gap would be necessary as a preliminary step to be able to best inform management guidelines aiming to preserve orchid diversity. Finally, the unexpected positive association between the invasive species L. stoechas and the fruiting success of D. pardina deserves further studies targeting plant–plant interactions of L. stoechas within the community, which will help designing adaptive management strategies for the control of this weed. If it is proved that L. stoechas might be enhancing the pollination regulating service for certain Australian orchid species, the eradication of this species should be ideally accompanied by the restoration of the pollination service it provides via planting bee-attracting native species.

Supplementary Material

Supplementary material is available at Journal of Plant Ecology online.

Table S1: Number of counted individuals for each plant species within the surrounding vegetation in each of the sampled plots.

Table S2: Number of counted flowers for each plant species within the surrounding vegetation in each of the sampled plots.

Table S3: Pearson correlation coefficients between floral abundance in plant species of the surrounding vegetation and fruiting success of Diuris pardina and Glossodia major.

Acknowledgements

We thank the TERN Ecosystem Surveillance team, Rosalie Lawrence, Robert Lawrence, Penny McLachlan, Katie Irvine, Michael Starkey, Sally O’Neill, Candy Guerin and the South Australian Department for Environment and Water.

Funding

This work has been funded by the grant 327-2018—Australian Orchid Foundation—Climate and habitat condition controls on orchid populations—research outcomes associated with a citizen science program. Likewise, it has been possible thanks to the grant from the Department of Industry, Innovation and Science—Inspiring Australia, Citizen Science Grants—Wild Orchid Watch.

Conflict of interest statement. The authors declare that they have no conflict of interest.

Authors’ Contributions

I.M.-F. and G.R.G. conceived the ideas; G.R.G. and S.L.B. collected the data; I.M.-F. analysed the data and drafted the paper. All authors reviewed the final version of the manuscript and approved its submission.

REFERENCES