Ecology of plants and pollinators in calcareous grasslands along a latitudinal gradient in France : species diversity and the structure of plant-pollinator interaction networks

Dans le contexte actuel de crise de la biodiversité et des risques associés de dégradation des services écosystémiques, les réseaux plantes-pollinisateurs sont parmi les réseaux mutualistes les plus étudiés. Sans pollinisateurs, de nombreuses plantes sauvages et cultivées ne pourraient pas se reproduire. Il est important de comprendre les réseaux plantes-pollinisateurs si nous souhaitons empêcher la destruction des interactions de pollinisation et des services écosystémiques connexes. Dans ces réseaux, les espèces doivent être présentes au même endroit et au même moment pour interagir. En France, l'abondance, la richesse des plantes et des pollinisateurs diffèrent le long du gradient latitudinal, qui correspond aux variations naturelles de la biodiversité. Ces variations pourraient potentiellement affecter la structure du réseau. En outre, les réseaux d'interactions sont souvent décrits sur la base de données agrégées dans le temps, mais en réalité les interactions varient dans le temps. Les grands ensembles de données sur les interactions plantes-pollinisateurs, couvrant toute la saison de floraison ou plusieurs années et permettant des comparaisons pertinentes entre les réseaux le long du gradient environnemental, sont rares. En raison de leur complexité et de leur variation interannuelle, la plupart des études sur les réseaux mutualistes se sont concentrées sur la comparaison des mesures classiques des réseaux qui sont toutes influencées par la taille des réseaux (le nombre d'espèces). De plus, la plupart de ces réseaux sont fondés sur des interactions observées sur le terrain, ce qui fait que certains liens ne sont pas observés. Ainsi, ces réseaux représentent un sous-échantillon d'interactions possibles, ce qui nécessite l'élaboration de nouvelles approches méthodologiques pour mieux explorer les processus écologiques qui déterminent les interactions entre les espèces. L'objectif général de cette étude est de comprendre et d'aider à prédire les effets des changements environnementaux sur les communautés de plantes et de pollinisateurs en étudiant les associations plantes-pollinisateurs selon un gradient environnemental. Je fournis et analyse ici une nouvelle base de données constituée de données géolocalisées caractérisant les associations plantes-pollinisateurs au niveau de l'espèce, la variation spatiale de la structure des communautés et l'assemblage des traits, en me concentrant sur six prairies calcaires le long d'un gradient latitudinal en France. J'ai d'abord comparé la variation de la diversité taxonomique dans l'espace (inter et intra-région) et dans le temps (tout au long de la saison). Ensuite, j'ai utilisé une nouvelle approche méthodologique pour comparer des réseaux de tailles différentes et pour étudier les conséquences des gradients environnementaux sur la probabilité d'interaction plante-pollinisateur. Pour comprendre à quel point notre vision des réseaux échantillonnés selon des méthodes classiques est déformée, j'ai construit des réseaux d'interaction plus complets en utilisant le pollen trouvé sur les insectes. Enfin, j'ai étudié le mécanisme de variation géographique des odeurs florales et les différences entre les populations liées à la variation de la communauté des pollinisateurs. En raison de la complexité et variabilité des interactions plantes-pollinisateurs, notre étude a souligné l'importance de tenir compte de toute la saison de floraison et de la période de vol des insectes, en utilisant des identifications au niveau de l'espèce, afin de démêler le rôle écologique des espèces et les variations du réseau. L'utilisation d'une nouvelle approche méthodologique nous a permis de faire des comparaisons de réseaux le long du gradient environnemental et de diversité en évitant des problèmes de circularité. La grande quantité de données fournies dans cette thèse a permis d'effectuer des comparaisons à différents niveaux, du réseau dans son ensemble aux variations entre populations d’une même espèce.In the current context of biodiversity crisis and the associated risks of ecosystem service failure, plant-pollinator networks are among the most studied mutualistic networks. Without pollinators, many plants could not reproduce and set seed, and 70% of agricultural production directly depends on them. However, pollinating insects constitute some of the terrestrial taxa most affected by global changes. As such, understanding plant-pollinator networks is of particular relevance if we are to prevent catastrophic disruption of pollination interactions and associated ecosystem services. In plant-pollinator networks, species need to be present in the same site and at the same moment for interactions to occur. In France, plant and pollinator abundance, richness and presence differ along the latitudinal gradient, which correspond to natural variations in biodiversity, and these variations could potentially affect network structure. Moreover, interaction networks are often reported based on temporally aggregated data, but in truth pollination interactions are not static and vary in time, since different plant and pollinator species display different phenologies. Large datasets on plant-pollinator interactions which comprise the entire flowering season or multiple years and allow relevant comparisons among networks along environmental gradient are rare. Due to their complexity and variation among years, most studies of mutualistic networks have focused on predicting and comparing classic network metrics which are all influenced by network size, i.e. the number of plant and insect species. Furthermore, most of these networks are based on interactions observed in the field, and thus some existing links between species remain unobserved. As such, visit-based networks represent a subsample of possible interactions, which call for the development of new methodological approaches to better explore the ecological processes determining species interactions. The general aim of this study is to understand and help predict the effects of environmental changes on plant and pollinator communities by studying plant-pollinator associations along an environmental gradient. Here, I provide and analyse a new database made of geo-localized data characterizing plant-pollinator associations at the species level, spatial variation in community structure and trait assemblage, focusing on six different calcareous grasslands along a latitudinal gradient in France. I first compared the taxonomical diversity variation in space (between and within region) and time (along the season). Then, I used a new methodological approach to compare networks of different size and to study the consequences of environmental gradients on plant-pollinator interaction probability. To understand how much distorted is our vision of plant-pollinator networks sampled following classic methods, I built more complete interaction networks using the pollen found on insects. Finally, I studied the mechanism behind geographical variation of floral scents and among-populations differences linked to the variation in the pollinator community. Because of the complexity and variation of plant-pollinator interactions, our study highlighted the importance to consider the entire flowering season and insects flying period, using species-level identifications, to disentangle the ecological species' role and the network variations. The use of new methodological approach allowed us to make networks comparison along the environmental and diversity gradient avoiding data circularity. The high amount of data provided in this thesis permitted to make comparisons at different level, from the entire network to species-specific variation among-populations

Warming of experimental plant-pollinator communities advances phenologies, alters traits, reduces interactions and depresses reproduction.

Climate change may disrupt plant-pollinator mutualisms by generating phenological asynchronies and by altering traits that shape interaction costs and benefits. Our knowledge is limited to studies that manipulate only one partner or focus on either phenological or trait-based mismatches. We assembled communities of three annual plants and a solitary bee prior to flowering and emergence to test how springtime warming affects phenologies, traits, interactions and reproductive output. Warming advanced community-level flowering onset, peak and end but did not alter bee emergence. Warmed plant communities produced fewer and smaller flowers with less, more-concentrated nectar, reducing attractiveness, and warmed bees were more generalized in their foraging, reducing their effectiveness. Plant-bee interactions were less frequent, shorter and peaked earlier under warming. As a result, warmed plants produced fewer, lighter seeds, indicating pollinator-mediated fitness costs. Climate change will perturb plant-pollinator mutualisms, causing wide-ranging effects on partner species and diminishing the ecosystem service they provide

Effects of queen mating status, pre-diapause weight and pupae\u2019s sex on colony initiation in small-scale rearing of Bombus terrestris

Diapause control and colony initiation are among the major problems encountered in the rearing of bumble bee colonies in small-scale rearing. In this study, we used Bombus terrestris queens obtained from commercial colonies to investigate (1) the diapause survival in virgin and mated queens, (2) the diapause and colony initiation performance ofmated bumble bee queens in relation to the pre-diapause weight and (3) the effect of pupae\u2019s sex on colony initiation. We found that diapause survival is negatively affected by mating and by the low pre-diapause weight, but first egg deposition and development of the first adult worker were delayed in heavy queens. We found no significant differences in the egg-laying success in relation to pupae\u2019s sex; however, queens stimulated with queen pupae laid more eggs per cell and developed a first brood larger than those stimulated with male pupae. Our results can be useful in small-scale rearing, including the rearing of wild queens for conservation purposes

Lack of partner preference system for incest avoidance in the bumble bee Bombus terrestris

Inbreeding is caused by the mating of closely related individuals and produces a decrease in the offspring fitness and deleterious consequences for adults. In haplodiploid social Hymenoptera inbreeding has a further negative effect because of their particular mechanism of sex determination (sl-CSD), that causes the emergence, in half of the founded colonies, of unviable or sterile diploid males. When these males are able to develop until adult stage, as in bumble bees, they represent a huge cost for the colony. With respect to these high inbreeding consequences, a selection for mechanisms of inbreeding avoidance would be expected in bumble bees. Social recognition is one of the most common and efficient system to avoid inbred mating in social insect, but it is poorly studied in bumble bees. In this study we investigated the mating choice between siblings or non-siblings in queens and males of Bombus terrestris reared in laboratory. To investigate the role of mating behaviour in mating choice, the tests were performed both in cage and in tunnel. As opposed to what would be expected, we found that B. terrestris males and gynes do not show a mating preference for non-siblings compared to siblings (49.3% in non-siblings and 50.7% in siblings) and the mating latency was even shorter for sibling matings than for non-siblings ones

Integrated conservation of bee pollinators of a rare plant in a protected area near Bologna, Italy

An integrated approach was proposed for the conservation of the bee pollinators of the locally rare plant dittany Dictamnus albus. Based on previous studies that revealed the most efficient pollinators, we performed three related actions to improve their presence in the area: (i) we provided artificial nests for bumblebees and solitary bees; (ii) we added bee plants to support local populations of pollinators throughout their life cycle, and (iii) we reared and released bumblebee colonies from wild queens collected in the area. Artificial nests were occupied at high rates by cavity nesting species such as mason bees, leafcutter bees and carpenter bees, while we did not observe any ground nesting bees. Artificial nests for bumblebees did not attract any wild queens. The bee plants established at different rates: transplanted adult individuals survived better than seeds directly sown at the site. In three consecutive years we reared and released several colonies of buff-tailed bumblebees, which survived through the flowering season but only one developed new gynes

Spécificité et flexibilité dans la ponte de Zerynthia polyxena sur différentes espèces du genre Aristolochia en région méditerranéenne française / Specificity and flexibility of egg laying by Zerynthia polyxena on different species of Aristolochia in Mediterranean France

Butterflies of the genus Zerynthia often have different host plants in the genus Aristolochia in the different parts of their distribution range. In Mediterranean France the Southern Festoon reaches its North Western distribution limit, where it generally lays eggs on A. rotunda in either coastal wet meadows or riparian and garrigue habitats in the backcountry. In the study area (Hérault and Gard departments), its distribution range overlaps with that of the Spanish Festoon Zerynthia rumina at its north-eastern limits. The Spanish Festoon lays its eggs on A. pistolochia plants growing in dry garrigues in the backcountry. Three types of field observations were conducted to determine : (1) whether habitat variation influences the rates of egg deposition by the Southern Festoon, (2) the extent to which A. rotunda is the preferential host plant and (3) if both Southern and Spanish Festoons are specialised if they co-occur with both their host plants in proximity. We found that A. rotunda is the main host plant of the Southern Festoon in both habitats where it grows in Mediterranean France although egg laying was observed on A. clematitis and more rarely on A. paucinervis in coastal sites. Finally, in parapatry, Southern and Spanish Festoons exchange their host plants but remain both more specialised on a different host. Despite their protection status, destruction of their habitats by urbanisation and infrastructures are a major threat to these specialised butterflies.Dans leurs différentes aires de distribution, les papillons du genre Zerynthia présentent des plantes-hôtes diverses au sein du genre Aristolochia. En France méditerranéenne, où Z. polyxena, la Diane, est en limite nord-ouest de distribution, elle pond généralement sur A. rotunda qui pousse à la fois dans les prairies humides littorales et certaines garrigues d’arrière-pays. Dans la zone d’étude (départements de l’Hérault et du Gard), son aire de distribution chevauche celle de Z. rumina, la Proserpine, dans sa limite orientale. Cette dernière pond sur A. pistolochia qui pousse dans les garrigues sèches d’arrière-pays. Trois axes d’observations de terrain ont été définis afin de voir (1) si la fréquence de ponte de Z. polyxena varie dans ses différents habitats, (2) dans quelle mesure A. rotunda est réellement la plantehôte préférentielle de Z. polyxena et (3) si Z. polyxena et Z. rumina demeurent spécialistes lorsqu’elles partagent le même habitat avec leurs deux plantes-hôtes en mélange ou en étroite proximité. L’ensemble de ces travaux a bien conforté qu’A. rotunda est l’hôte privilégié de Z. polyxena dans ses deux grands habitats de France méditerranéenne bien que des pontes puissent être observées sur A. clematitis et plus rarement sur A. paucinervis sur les sites littoraux. En parapatrie, Z. polyxena et Z. rumina peuvent ponctuellement changer de plante-hôte même si elles conservent une préférence pour la leur. Malgré leur statut de protection, ces papillons spécialistes sont principalement menacés par la destruction de leurs habitats par l’urbanisation et l’implantation d’infrastructures.Gauthier Perrine, Manincor Natasha de, Picquenot Manon, Pons Virginie, Schumpp Ugo, Thompson John D. Spécificité et flexibilité dans la ponte de Zerynthia polyxena sur différentes espèces du genre Aristolochia en région méditerranéenne française / Specificity and flexibility of egg laying by Zerynthia polyxena on different species of Aristolochia in Mediterranean France. In: Ecologia mediterranea, tome 46 n°2, 2020. pp. 47-63

No evidence for an inbreeding avoidance system in the bumble bee Bombus terrestris

Inbreeding is caused by the mating of closely related individuals and may produce a decrease in the fitness of offspring and have deleterious consequences for adults. In haplodiploid social Hymenoptera inbreeding has a further negative effect due to the production of unviable or sterile diploid males. As a consequence, mechanisms of inbreeding avoidance would be expected to evolve. In this study, we investigated the mating choice between related (inbred) or unrelated (outbred) gynes and males of Bombus terrestris reared in laboratory conditions by performing cage and tunnel experiments. Not only did we find no mating preference for related or unrelated partners (mating success 41.55 ± 3.7 and 39.69 ± 4.4%, respectively), but the mating latency was even shorter in inbred (6.97 ± 0.6 min) than in outbred matings (8.74 ± 0.8 min). We hypothesize that in wild populations of B. terrestris, the lack of incest avoidance could be compensated by tolerance of high levels of inbreeding

Does phenology explain plant–pollinator interactions at different latitudes? An assessment of its explanatory power in plant–hoverfly networks in French calcareous grasslands

International audienceFor plant–pollinator interactions to occur, the flowering of plants and the flying period of pollinators (i.e. their phenologies) have to overlap. Yet, few models make use of this principle to predict interactions and fewer still are able to compare interaction networks of different sizes. Here, we tackled both challenges using Bayesian structural equation models (SEM), incorporating the effect of phenological overlap in six plant–hoverfly networks. Insect and plant abundances were strong determinants of the number of visits, while phenology overlap alone was not sufficient, but significantly improved model fit. Phenology overlap was a stronger determinant of plant–pollinator interactions in sites where the average overlap was longer and network compartmentalization was weaker, i.e. at higher latitudes. Our approach highlights the advantages of using Bayesian SEMs to compare interaction networks of different sizes along environmental gradients and articulates the various steps needed to do so