| 1. |
- Burian, Alfred, et al.
(author)
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A community-based evaluation of population growth and agro-pastoralist resilience in Sub-Saharan drylands
- 2019
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In: Environmental Science and Policy. - : Elsevier BV. - 1462-9011 .- 1873-6416. ; 92, s. 323-330
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Journal article (peer-reviewed)abstract
- Human population growth is considered together with climate warming as major driver of change in Sub-Saharan Africa. Research on the implications of increased population densities often utilises community knowledge but without incorporating the view of local stakeholders. In this study, we applied a community centred approach to assess direct and indirect consequences of population growth in drylands of north-western Kenya. Combined social, agricultural and geo-spatial analyses allowed us to identify major system transitions, determine their linkage to population growth and deduce consequences for local livelihoods and community resilience. Community-members reported positive and negative consequences of fourfold population growth since 1974 but evaluated its overall effect as clearly beneficial. This overall positive effect was based on both, positive developments and the successful mitigation of potential system stressors. First, food security was maintained despite high growth rates because a shift from migratory pastoralism to a more labour-intensive agro-pastoralist system helped to increase agricultural productivity. Additionally, land-use changes were linked to land privatisation and improved erosion protection on private land, decoupling population growth from environmental degradation. We detected, however also early warning signs of reduced community resilience as households were unable to fully recover livestock densities after catastrophic events. A population-growth driven reduction in household land-sizes and the decreased monetary value of agricultural production were identified as drivers of this development. The extrapolation of our results to establish a general relationship between population densities, land-use and household resilience in Sub-Saharan drylands suggest that further system transformations will be required to ensure regional food-security.
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| 2. |
- Burian, Alfred, 1984-, et al.
(author)
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Benthic-pelagic coupling drives non-seasonal zooplankton blooms and restructures energy flows in shallow tropical lakes
- 2016
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In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 61:3, s. 795-805
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Journal article (peer-reviewed)abstract
- Zooplankton blooms are a frequent phenomenon in tropical systems. However, drivers of bloom formation and the contribution of emerging resting eggs are largely unexplored. We investigated the dynamics and the triggers of rotifer blooms in African soda-lakes and assessed their impact on other trophic levels. A meta-analysis of rotifer peak densities including abundances of up to 6 × 105 individuals L−1 demonstrated that rotifer bloom formation was uncoupled from the food environment and the seasonality of climatic conditions. A time series with weekly sampling intervals from Lake Nakuru (Kenya) revealed that intrinsic growth factors (food quality and the physicochemical environment) significantly affected rotifer population fluctuations, but were of minor importance for bloom formation. Instead, rotifer bloom formation was linked to sediment resuspension, a prerequisite for hatching of resting-eggs. Population growth rates exceed pelagic birth rates and simulations of rotifer dynamics confirmed the quantitative importance of rotifer emergence from the sediment egg-bank and signifying a decoupling of bloom formation from pelagic reproduction. Rotifer blooms led to a top-down control of small-sized algae and facilitated a switch to more grazing-resistant, filamentous cyanobacteria. This shift in phytoplankton composition cascaded up the food chain and triggered the return of filter-feeding flamingos. Calculations of consequent changes in the lake's energy budget and export of aquatic primary production to terrestrial ecosystems demonstrated the large potential impact of nonseasonal disturbances on the functioning of shallow tropical lakes.
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| 3. |
- Burian, Alfred, et al.
(author)
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Food quantity-quality interactions and their impact on consumer behavior and trophic transfer
- 2020
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In: Ecological Monographs. - : Wiley. - 0012-9615 .- 1557-7015. ; 90:1
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Journal article (peer-reviewed)abstract
- Food quantity-quality interactions determine growth rates and reproductive success of consumers and thereby regulate community dynamics and food web structure. Predator-prey models that shape our conceptual understanding of foraging ecology typically rely on the parametrization of fixed consumer responses to either food quantity or food quality. In nature, however, consumers optimize their fitness by responding simultaneously to changes in food quantity and quality. Therefore, we assessed consumer responses to changing food environments using a new fitness optimization model that accounted for food quality-quantity interactions to better capture the regulatory flexibility of consumers. Our simulations demonstrated that the impact of food quality on important consumer traits can be altered or even reversed by changes in food quality. Low food quality, for example, affected feeding rates negatively at low food concentrations but triggered surplus feeding at high food concentrations. The scope of surplus feeding was thereby mainly dependent on dynamics of nutrient digestion and in contrast to previous assumptions, energy costs of feeding played a minor role. Further, the regulation of digestive enzyme production, a crucial factor determining assimilation efficiencies, was strongly dependent on whether nonessential or essential nutrients were limiting growth. Consequently, not only the degree but also the type of nutrient limitation mediated the impact of the food environment on consumers' fitness. At the community level, food quality was key in shaping predator-prey biomass ratios. High food qualities resulted in top-heavy systems with larger consumer than prey biomass. Decreases of prey digestibility or the availability of essential nutrients, however, triggered a switch from inverted to classical pyramid shapes of bi-trophic systems. The impact of food quantity on trophic transfer and emerging structural ecosystem properties thus critically hinges on behavioral and physiological responses of consumers. The inclusion of the regulatory flexibility of consumers is therefore an essential next step to improve predator-prey models and our conceptual understanding of trophic interactions.
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| 4. |
- Burian, Alfred, 1984-
(author)
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Impact of food quality on aquatic consumers : Behavioral and physiological adjustments
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Food quantity and quality together determine growth rates of consumers and the utilisation efficiencies of available resources in aquatic and terrestrial ecosystems. The effect of food quality on the performance of consumers is dependent on both, its direct influence on ingestion and assimilation rates, and on the behavioural and physiological adjustments of consumers to their food environment. The main target of this thesis was to investigate the nature and scope of behavioural and physiological adjustments in consumers and assess the resulting consequences for consumers’ fitness and ecosystem-wide nutrient flows.In paper I, we investigated the extent of elemental homeostasis across several taxonomic groups of planktonic herbivores. We found that adjustments in elemental ratios (C:N:P) in body tissues are an important physiological response of heterotrophic flagellates, but that in ciliates and multi-cellular organisms C:N:P ratios varied much less than in their algal prey. Hence, alternative regulatory mechanisms determine the reactions of metazoan zooplankton to decreases in food quality. In paper II, we developed a theoretical model to explore regulation in behaviour and digestive physiology of consumers to changes in the food environment. Our results demonstrate that feeding and digestion of consumers are determined by trade-offs between benefits and costs of investments in these processes. We revealed that the flexibility in consumers’ behaviour and physiology had strong influences on assimilation rates and efficiencies and thereby affected growth rates and a wide range of ecosystem functions. In paper III, we investigated the scope and consequences of adjustments in feeding and assimilation rates of copepods exposed to different diets. An important finding was that consumers can use resources, which are available in surplus, to increase the uptake of a limiting nutrient. Such nutrient interconversion led to co-limitation, the simultaneous limitation of copepods by two different nutrients. Finally, in paper IV, we aimed to test the effect of food quality on population dynamics in the field. We investigated zooplankton populations in tropical soda-lakes, an environment with a surplus of planktonic food sources that thus provides an ideal setting for investigations of food quality. However, we found that the hatching of resting eggs from lake sediments was the main driver of zooplankton bloom formation resulting in non-cyclical dynamics that were not related to food quality.These findings contributed to our understanding under which circumstance and by which mechanisms food quality affects the performance of consumers. My results highlight that food quality has not only direct effects on consumers’ growth but also triggers behavioral and physiological responses in consumers to maximize their fitness.
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| 5. |
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| 6. |
- Burian, Alfred, et al.
(author)
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Nutrient deficiencies and the restriction of compensatory mechanisms in copepods
- 2018
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In: Functional Ecology. - : Wiley. - 0269-8463 .- 1365-2435. ; 32:3, s. 636-647
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Journal article (peer-reviewed)abstract
- 1. The flexible regulation of feeding behaviour and nutrient metabolism is a prerequisite for consumers to grow and survive under variable food conditions. Thus, it is essential to understand the ecological trade-offs that restrict regulatory mechanisms in consumers to evaluate the consequences of nutrient limitations for trophic interactions.2. Here, we assessed behavioural and physiological adjustments to nutrient deficiencies in copepods and examined whether energy limitation, food digestibility or co-limitation with a second nutrient restricted compensatory mechanisms.3. A combination of C-13-labelling and compound-specific stable isotope measurements revealed that copepods compensated nitrogen deficiencies by raising retention efficiencies of amino acids (AA). The costs of higher retention efficiencies were reflected in the doubling of structural fatty acids (FA), probably required for morphological adaptations of the gut. A depletion of highly unsaturated FA in storage lipids and their selective retention suggested that these FA became co-limiting and restricted a further increase in AA retention efficiencies.4. Copepods feeding on phosphorus-limited algae showed a marked increase in ingestion rates but were not fully able to compensate dietary deficiencies. The increase in ingestion rates was thereby not restricted by higher foraging costs because energy storage in copepods increased. Instead, thicker cell walls of nutrient-limited algae indicated that algal digestion resistance restricted the extent of surplus feeding.5. The strongly nutrient-specific response of copepods had large implications for recycling rates, growth efficiencies and the potential top-down control at the plant-animal interface. Compensatory mechanisms to mitigate nutrient deficiencies are therefore an essential aspect of trophic interactions and have the potential to alter the structure of food web.
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| 7. |
- Burian, Alfred, et al.
(author)
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Species-specific separation of lake plankton reveals divergent food assimilation patterns in rotifers
- 2014
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In: Freshwater Biology. - : Wiley. - 0046-5070 .- 1365-2427. ; 59:6, s. 1257-1265
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Journal article (peer-reviewed)abstract
- The analysis of functional groups with a resolution to the individual species level is a basic requirement to better understand complex interactions in aquatic food webs. Species-specific stable isotope analyses are currently applied to analyse the trophic role of large zooplankton or fish species, but technical constraints complicate their application to smaller-sized plankton. We investigated rotifer food assimilation during a short-term microzooplankton bloom in the East African soda lake Nakuru by developing a method for species-specific sampling of rotifers. The two dominant rotifers, Brachionus plicatilis and Brachionus dimidiatus, were separated to single-species samples (purity >95%) and significantly differed in their isotopic values (4.1 parts per thousand in delta C-13 and 1.5 parts per thousand in delta N-15). Bayesian mixing models indicated that isotopic differences were caused by different assimilation of filamentous cyanobacteria and particles A main difference was that the filamentous cyanobacterium Arthrospira fusiformis, which frequently forms blooms in African soda lakes, was an important food source for the larger-sized B.plicatilis (48%), whereas it was hardly ingested by B.dimidiatus. Overall, A.fusiformis was, relative to its biomass, assimilated to small extents, demonstrating a high grazing resistance of this species. In combination with high population densities, these results demonstrate a strong potential of rotifer blooms to shape phytoplankton communities and are the first in situ demonstration of a quantitatively important direct trophic link between rotifers and filamentous cyanobacteria.
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| 8. |
- Burian, Alfred, et al.
(author)
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The potential of fatty acid isotopes to trace trophic transfer in aquatic food-webs
- 2020
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In: Philosophical Transactions of the Royal Society of London. Biological Sciences. - : The Royal Society. - 0962-8436 .- 1471-2970. ; 375:1804
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Journal article (peer-reviewed)abstract
- Compound-specific isotope analyses (CSIA) of fatty acids (FA) constitute a promising tool for tracing energy flows in food-webs. However, past applications of FA-specific carbon isotope analyses have been restricted to a relatively coarse food-source separation and mainly quantified dietary contributions from different habitats. Our aim was to evaluate the potential of FA-CSIA to provide high-resolution data on within-system energy flows using algae and zooplankton as model organisms. First, we investigated the power of FA-CSIA to distinguish among four different algae groups, namely cyanobacteria, chlorophytes, haptophytes and diatoms. We found substantial within-group variation but also demonstrated that delta C-13 of several FA (e.g. 18:3 omega 3 or 18:4 omega 3) differed among taxa, resulting in group-specific isotopic fingerprints. Second, we assessed changes in FA isotope ratios with trophic transfer. Isotope fractionation was highly variable in daphnids and rotifers exposed to different food sources. Only delta C-13 of nutritionally valuable poly-unsaturated FA remained relatively constant, highlighting their potential as dietary tracers. The variability in fractionation was partly driven by the identity of food sources. Such systematic effects likely reflect the impact of dietary quality on consumers' metabolism and suggest that FA isotopes could be useful nutritional indicators in the field. Overall, our results reveal that the variability of FA isotope ratios provides a substantial challenge, but that FA-CSIA nevertheless have several promising applications in food-web ecology. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.
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