Abstract :
[en] Today, our societies are increasingly concerned about the alarming disappearance
of insects. Yet they are essential to the proper functioning of our ecosystems, our food
and our health. Pollinating insects such as bees, hoverflies and apoid wasps are also
affected by this collapse. These insects generate significant ecosystem services for
agriculture and the maintenance of natural areas through the pollination of flowering
plants and the biological control of crops. A series of causes for these collapses have
been widely documented in scientific literature, and it appears that it is mainly
landscape degradation that is causing the greatest damage to biodiversity. Agricultural
intensification and urbanization are converting complex natural areas into
homogeneous, anthropogenic zones, thereby destroying floral and nesting resources,
which are key components in the development and maintenance of pollinating insect
communities. The homogenization of the landscape also alters the foraging behavior
of our pollinating insects. It is in this context that this research thesis will focus,
through 4 case studies, on the impact of landscape fragmentation on pollinator
communities and foraging ecology in temperate zone of agricultural and urban
landscapes.
This thesis is structured around a general introduction (Chapter I), a general
methodology (Chapter II) which introduces the technical tools shared between the
different case studies, and four chapters constituting the core of this thesis (Chapter
III-VI), the first two of which focus on agricultural landscapes, while the last two are
more concerned with urban environments. The thesis concludes with a discussion, an
outlook on future research and a general conclusion (Chapter VII).
Chapter III looks at the influence of ecological diversification on the biodiversity of
pollinators, particularly wild bees and hoverflies, on two farms in the early stages of
ecological transition. This study has the merit of providing quality pollinator
occurrence data based on standardized sampling techniques established over two
years (2018-2019) and reports certain species that may be of conservation concern
given their critical conservation status. In view of the specific diversity of wild bees
present (101 species) and hoverflies (31 species), the ecological diversification of
environments within the two sampled farms (e.g. wetlands, flower strips...) and
agroecological practices may constitute new favorable environments for rare,
oligolectic or endangered pollinator species.
Building on the methodology and results of the previous chapter, in chapter IV we
looked at the impact of a more specific floral resource supply practice: flower strips
in intercropping systems with winter wheat. We found that a multifloral composition
of flower strips attracted a greater diversity of hoverflies but not wild bees than
monofloral oleaginous strips composed of Dimorphoteca pluvialis (Asteraceae) and
Camelina sativa (Brassicacea). Finally, the ecological and economic benefits
provided by these oleaginous flower strips are organized on three levels: (i) they
provide alternative floral resources to pollinators; (ii) farmers are rewarded by the
Agro-Environmental and Climate Measures (MAEC) of the European Union's Common Agricultural Policy; (iii) and farmers can generate additional income from
the resale of harvested oleaginous seeds.
In chapter V, we investigate urban landscapes. Pollen availability is a key factor
influencing the population dynamics of pollinating insects in urban disturbed
landscapes. In this study, we analyzed the foraged floral resources of honeybee
colonies, Apis mellifera L., along an urban-rural gradient in a mega-city Tokyo
(Japan). After a genomic analysis (i.e., metabarcoding) to identify the floral species
foraged on the pollen samples, we showed that the landscape factor explains the
differences in composition of the flowers visited and not their specific richness,
whereas this richness is significantly dependent on the temporal factor. Honeybees
forage more on woody species in spring, and more on herbaceous species in autumn.
This study provides us with a better understanding of the urban needs of honeybees in
a mega-city.
Urban surfaces are commonly perceived as "ecologically impermeable" and can
therefore represent a considerable obstacle to nesting opportunities for ground-nesting
insects. In Chapter VI, we looked at the nesting alternative that pavements could offer
for a range of ground-nesting Hymenoptera in the city of Brussels-Capital. Our study
identified 22 species of wild bees and apoid wasps capable of nesting on Brussels
sidewalks. These species mainly nested in old Brussels sidewalks made of sandstone
pavers or concrete slabs with unbonded joints averaging one centimeter in width, and
where the construction layers beneath the sidewalks were sandy in texture. At the end
of this experiment, we were able to design technical recommendations and
management practices to encourage these pollinating insects.
The relevance of this research project and the general context are discussed at the
end of this thesis in Chapter VII. Taken as a whole, these four studies underline the
importance of food and nesting resources for insect pollinator communities and
constitute milestones of knowledge providing further arguments in favor of the
conservation and restoration of insect pollinators in disturbed environments We
concluded this discussion with an open-minded outlook for the continuation scientific
work in these topics of agricultural and urban ecology.
