Weiner, Christiane Natalie
Diversity and resource choice of flower-visiting insects in relation to pollen nutritional quality and land use.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2016)
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|Item Type:||Ph.D. Thesis|
|Title:||Diversity and resource choice of flower-visiting insects in relation to pollen nutritional quality and land use|
Recent declines in honey bee colonies as well as in the diversity and abundance of native pollinators generated widespread concern about the future of pollination and set off a discussion about a general “pollination crisis”. Partly, this is due to the functional importance of this ecosystem service not only for plant reproduction but above all for crop production and thus human welfare. Consequently, the matter gained particular attention in current research and a rising number of studies focus on the stability of plant-pollinator interactions in relation to biodiversity and ecosystem change.
My dissertation focuses on the mutualistic interactions between flowering plants and flower-visiting insects and their interactive response to land-use intensity as well as resource choice of flower-visitors in relation to pollen quality. The innovative approach of my project is to use quantitative interaction networks to provide knowledge on how species respond to land use and how their responses may influence their interaction partners. We show that information from ecological networks may help to predict natural community responses to disturbance and possible secondary extinctions in systems that undergo agricultural intensification, if the identity of each species’ partners and relative interaction strengths are considered. This project presents large-scale investigations on the characteristics and fragility of multi-species networks in real landscapes. During two seasons we recorded the diversity, species composition and specialization of plant-pollinator networks along a gradient of increasing land-use intensity. The study was conducted within the framework of the Biodiversity Exploratories, which are located in the Schorfheide-Chorin (NE Germany), Hainich-Dün (Central Germany) and Schwäbische Alb (SW Germany) regions. Each Exploratory contains 50 experimental grassland plots which comprise near natural, protected sites as well as intensively fertilized, mown or grazed meadows and pastures.
Comparing meadows of high and low land-use intensities we found that species richness of plants, bees and butterflies was significantly higher on low intensity meadows. However, in terms of Shannon diversity and abundance only butterflies responded negatively to land-use intensification (Chapter 2). Nevertheless, the analysis of plant and flower-visitor composition revealed crucial differences between grassland types with species overlaps of just 43 % in plants and 42 % in insects. This pointed to the fact that investigation of biodiversity and abundance alone may not detect biotic homogenization e.g. a loss in functional diversity. Moreover, resource impoverishment had stronger effects on the land-use response of highly specialized flower-visitor groups than on little or unspecialized ones. We examined if mutual specialization could explain the accelerating parallel declines observed in plants and pollinators. Focusing on their stability, we analyzed 162 plant-pollinator networks from 119 meadows and pastures managed at different intensities. The fate of a flower visitor was predicted by the land-use response of its associated plant species and vice versa. Furthermore, we detected a disproportional impact of land-use intensification on the abundance of more specialized pollinator species (Chapter 3). Land-use intensification seems to set off losses in flower diversity, which leads to resource-mediated declines in pollinator species. While the mean land-use response of the pollinators visiting a plant species also influenced its abundance, this effect was weaker. Network analyses provide a valuable tool for characterizing mutualisms in a community context and may be used to predict community responses to disturbance and possible consequences of species loss. To further illuminate which land-use practices most fundamentally influence plant-pollinator associations, we analyzed the isolated effects of fertilization, mowing and grazing intensity on plant – flower-visitor networks (Chapter 4). I found that these three components of land use strongly differed in their effects on the species richness and composition of flower-visitor networks. While increases of fertilization and mowing intensity in two out of three bioregions were accompanied by a decrease in plant species richness, concerning pollinator species richness, abundance and composition trends were even more conflictive between taxonomically different pollinator groups and between regions. Thus, the results showed that it is not possible to readily transfer results and management recommendations from one region to another. Yet, I found that across all three regions pollinator fate was determined by the average land-use response of the plant species they visited and vice versa (Chapter 4). Moreover, in pollinators – but not in plants – specialized species were disproportionately affected by land-use intensification. Specialized pollinators such as oligolectic bees (bees that collect pollen only from one plant family or even just from one single species) are often expected to be more prone to disturbance and thus more vulnerable to ecosystem change. This is recognized as the cost of specialization and was reinforced by my results (Chapter 3 and 4). On the other hand it has frequently been proposed that benefits from resource specialization may outweigh the costs. In pollinators, benefits of specialization so far were presumed to result from higher foraging efficiency. Among the various adaptations is assumed specialization on very nutrient-rich pollen. We therefore analyzed hand-collected pollen from 142 plant species for its quantitative and qualitative amino acid composition. The composition of amino acids varied strongly among plant species, but taxonomically related species had similar compositions. Surprisingly, the concentration of free- and protein-bound amino acids – also of the essential ones – was significantly lower in pollen sources used by oligolectic bees than in other pollen sources (Chapter 5). Moreover, pollen sources of oligoleges deviated more strongly from the ideal composition of essential amino acids as determined for honey bees than plants not hosting oligolectic bees. This leads to the assumption that competitive avoidance or in other words an advantage in terms of the available pollen quantity might have led to oligolectic bees being specialized on pollen that is deficient in amino acids. This hypothesis still needs to be tested in detail.
Several of the results presented in this thesis shed new light on patterns and processes within plant-pollinator interactions. We found that – contrary to the prevailing contemporary opinion – plant-pollinator networks are highly specialized systems in which the diversity of plants and pollinators is strongly related to each other.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||Pollinators, bees, land use, biodiversity, pollination, plants, plant-animal interaction networks|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
500 Naturwissenschaften und Mathematik > 580 Pflanzen (Botanik)
500 Naturwissenschaften und Mathematik > 590 Tiere (Zoologie)
|Divisions:||10 Department of Biology > Synthetic Ecological Networks|
|Date Deposited:||27 May 2016 07:28|
|Last Modified:||27 May 2016 07:28|
|Referees:||Blüthgen, Prof. Nico and Jürgens, Prof. Andreas|
|Refereed:||29 April 2016|