Could Pollination Networks be Applied for Biological Conservation and Monitoring of Threatened Plant Communities? – Examples from an Endangered Ecological Community, the Howell Shrublands
Plant-pollinator interactions play a pivotal role in the structure and persistence of biological communities. Despite this, plant-pollinator relationships are rarely considered in applied conservation, potentially resulting in counterproductive or ineffective management practices. By allowing interactions between plants and pollinators to be quantitatively assessed, network analyses may offer valuable information for applied conservation. To demonstrate the potential utility of network analyses in conservation, this study will attempt to inform management for an endangered ecological community in eastern Australia using a multi-year pollination network. Over six seasons, 1,633.4 hours of floral visitation data were collected for 103 plant species to construct a network of plant-pollinator interactions. Asymptotic richness estimates were used to evaluate sampling efficacy, and predict the amount of effort required to complete sampling. Network analyses were conducted using the Bipartite package for RStudio, allowing species of high conservation value to be identified in terms of interaction strength, and the pollinator service index (PSI). Network specialisation (H2’), nestedness (wNODF), robustness, and modularity were assessed as potential indicators of community stability. Sampling was estimated to detect up to 90.05% of floral visitors, and 66.66% of interactions, with floral species predicted to require an average of 18.03 hours observation to sample a majority of interactions. This was also found to coincide with stable asymptotic richness estimates for a separate sample of species subject to relatively intensive sampling efforts and is therefore recommended as a baseline for future testing and sampling. Exotic honeybees (Apis mellifera) were identified as an important pollinator within the network, which was generally more specialised, modular, and robust to floral extinctions than could be explained by null models. Dividing the network into temporal sub-networks revealed late winter and early summer flowering plants were significantly less robust to pollinator extinction in spite of high interaction nestedness. This level of risk is likely to increase significantly if honeybees become locally extinct due to the parasite Varroa destructor. Therefore, conservation efforts should focus on late winter pollinators as a means of preserving the floral community.
Fatal attraction - the predation of pollinators leads to reproductive conflict in pollen limited populations of Drosera hookeri (Droseraceae) - Dataset
Pollinator-prey conflict in carnivorous plants occurs when plants that benefit from pollinators are pollen-limited and pollinators are snared by trap-leaves. To date a strong pollinator-prey conflict has not been found and is attributed to spatial and/or cue differences between flowers and traps abating conflict. Here we test whether pollinator-prey conflict occurs for Drosera hookeri (Droseraceae), which produces entomophilous flowers adjacent to leaf-traps. We also test the hypothesis that flowers have a dual role of pollinator attraction and deception by tumbling pollinators and other floral visitors into leaf-traps. From 2007-2021, in a drought-prone habitat in eastern Australia, pollinator diversity and visitation rates were scored at flowers using camcorders while pan-traps provided an estimate of pollinator abundance in the community. Hand-pollination experiments were used to determine pollinator dependence and pollen limitation. We also measured arthropod abundance in trap-leaves before, during and after flowering. In a paired experiment we compared leaf captures between plants with and without flowers to determine if flowers are complicit in the leaf captures of pollinators. Although self-compatible, with a delayed selfing-mechanism, outcrossed flowers produced more seed than self-pollinated flowers, indicating that pollinators are beneficial. Plants were pollen-limited in both years tested. Flies (non-pollinating and pollinating) were the most common prey with the greatest numbers during peak flowering. Pollinators, predominantly Melangyna virdiceps (Syriphidae, Diptera), contributed c. 57% of prey captures, and experimentally we show that they are not attracted to plants without flowers. Pollinators are often deposited into trap-leaves from flowers, which tip under their weight, to nearby leaf traps. A strong pollinator-prey conflict was detected with an overlap in prey and pollinators in a pollen and pollinator-limited system. Flowers attract pollinators and non-pollinators to plants. Leaf entrapment for pollinators occurs as a mishap in foraging rather than attraction to trap-leaves. The flowers in D. hookeri have the dual role of pollination and prey provision.
Interactions between two species of recently-sympatric invasive honeybees: Apis cerana induces aggression in Apis mellifera during foraging
Honeybees Apis mellifera (European honeybee) and Apis cerana (Asian honeybee) are cosmopolitan, having colonized continents beyond their natural ranges. In tropical Australia, these alien species have recently become sympatric. The environmental and economic impacts of these species when in sympatry remain to be seen; however, any interspecific competition may be of significance. We examined conspecific and heterospecific interactions between honeybees foraging at the nectar- and pollen-providing flowers of Antipogon leptopus (Polygonaceae). We cross-classified 554 encounters by three variables; incoming bee species, resident bee species, and one of four potential responses: (1) incoming defers to resident; (2) incoming procures the flower from resident; (3) incoming and resident share the flower; or (4) both incoming and resident abandon the flower. We also measured aggression and foraging rates of workers at flowers. Both species visited similar numbers of flowers in a foraging bout and spent similar foraging times on individual flowers. Incoming A. mellifera were more likely to procure flowers from resident A. cerana, and incoming A. cerana were more likely to defer to resident A. mellifera. A. mellifera were more aggressive toward heterospecifics than conspecifics, with heterospecifics 4.5 times more likely to provoke an aggressive response. However, no significant difference between conspecific and heterospecific aggression was observed for incoming A. cerana. A. mellifera were less abundant, yet overall more likely to acquire flowers and use aggression to do so. Costs of aggression may help explain the population-scale dominance of A. cerana over A. mellifera in this study.
Unsuccessful introduced biocontrol agents can act as pollinators of invasive weeds: Bitou Bush ('Chrysanthemoides monilifera ssp. rotundata') as an example
The extent of self-compatibility and reliance on pollinators for seed set are critical determinants of reproductive success in invasive plant species. Seed herbivores are commonly used as biocontrol agents but may also act as flower visitors, potentially resulting in pollination. However, such contrasting or potentially counterproductive interaction effects are rarely considered or evaluated for biological control programs. We investigated the breeding system and pollinators of Bitou Bush ('Chrysanthemoides monilifera ssp. rotundata'), an invasive species in Australia that has been the subject of biocontrol programs since 1987. We found the species to be obligate outcrossing in all six populations tested. From 150 video hours, we found 21 species of potential pollinators, including Mesoclanis polana, the Bitou Seedfly, native to South Africa and released in Australia as a biocontrol agent in 1996. Mesoclanis polana transferred pollen to stigmas and was the most common pollinator (52% of pollinator visits), followed by the syrphid fly 'Simosyrphus grandicornis' (9%) and introduced honeybee, 'Apis mellifera' (6.5%). Fruit-to-flower ratios ranged from 0.12 to 0.45 and were highest in the population with the greatest proportion of 'Mesoclanis polana' visits. In an experimental trial, outside the naturalized range, the native bee 'Homalictus sphecodoides' and the native syrphid 'Melangyna viridiceps' were the primary pollinators, and fruit-to-flower ratios were 0.35, indicating that Bitou Bush would have ready pollinators if its range expanded inland. 'Synthesis'. Invasive Bitou Bush requires pollinators, and this is effected by a range of generalist pollinators in eastern Australia including the Bitou Seedfly, introduced as a biocontrol agent, and the major pollinator detected in this study. Fruit-to-flower ratios were highest when the Bitou Seedfly was in high abundance. This study underscores the importance of evaluating the pollination biology of invasive species in their native ranges and prior to the introduction of biocontrol agents.