Beyond the here and now : herbivore ecology in a spatial-temporal context

Ecological phenomena at a certain location often not only depend on the current characteristics of that location itself, but also on characteristics of the landscape surrounding the site or influences from the past. In other words, in order to be able to understand ecological processes here and now, we often need information that is beyond here and now. This thesis investigates the role of such spatial-temporal context on the relationships between species and their environment. The focus is on large mammalian herbivores, an in particular the factors determining the movement and distribution of African elephants (Loxodonta africana) in Kruger National Park (KNP), South Africa. This thesis covers four main themes. First, it is shown that spatial-temporal variation in the biotic and abiotic environment creates predictable seasonal and daily cyclic patterns in the distribution of elephants in KNP. Then, herbivores are shown not only to respond to environmental heterogeneity: they also can create distinct spatial patterning in savanna vegetation when foraging initiates a positive plant-herbivore feedback and when the foraging process is spatially explicit. This leads to specific sites being revisited often, and therefore the patterns of site revisitation by elephants in KNP are studied, mainly in relation to surface water availability and vegetation characteristics. Lastly, this thesis focuses on the scale-dependent response of organisms to environmental heterogeneity, showing the consequences of a mismatch in the scale of analysis on statistical inference, and then focusing on finding the appropriate spatial scales to analyse and predict the spatial distribution of elephants in KNP. Together, the chapters presented in this thesis highlight the importance of explicitly considering the scale and context dependency of species-environment relationships, and demonstrate methods to take issues of spatial-temporal scale and context into account. These methods have the potential to increase our understanding of ecological phenomena and therefore may lead to better management of natural resources. <br/

Seasonal diet changes in elephant and impala in mopane woodland

Elephant and impala as intermediate feeders, having a mixed diet of grass and browse, respond to seasonal fluctuations of forage quality by changing their diet composition. We tested the hypotheses that (1) the decrease in forage quality is accompanied by a change in diet from more monocots in the wet season to more dicots in the dry season and that that change is more pronounced and faster in impala than in elephant; (2) mopane (Colophospermum mopane), the most abundant dicot species, is the most important species in the elephant diet in mopane woodland, whereas impala feed relatively less on mopane due to the high condensed tannin concentration; and (3) impala on nutrient-rich soils have a diet consisting of more grass and change later to diet of more browse than impala on nutrient-poor soils. The phosphorus content and in vitro digestibility of monocots decreased and the NDF content increased significantly towards the end of the wet season, whereas in dicots no significant trend could be detected. We argue that this decreasing monocot quality caused elephant and impala to consume more dicots in the dry season. Elephant changed their diet gradually over a 16-week period from 70% to 25% monocots, whereas impala changed diets rapidly (2-4 weeks) from 95% to 70% monocots. For both elephants and impala, there was a positive correlation between percentage of monocots and dicots in the diet and the in vitro digestibility of these forage items. Mopane was the most important dicot species in the elephant diet and its contribution to the diet increased significantly in the dry season, whereas impala selected other dicot species. On nutrient-rich gabbroic soils, impala ate significantly more monocots than impala from nutrient-poor granitic soils, which was related to the higher in vitro digestibility of the monocots on gabbroic soil. Digestibility of food items appears to be an important determinant of diet change from the wet to the dry season in impala and elephants

Unveiling the roles of temporal periodicity, the spatial environment and behavioural modes in terrestrial animal movement

Background: Animal movement arises from complex interactions between animals and their heterogeneous environment. To better understand the movement process, it can be divided into behavioural, temporal and spatial components. Although methods exist to address those various components, it remains challenging to integrate them in a single movement analysis. Methods: We present an analytic workflow that integrates the behavioural, temporal and spatial components of the movement process and their interactions, which also allows for the assessment of the relative importance of those components. We construct a daily cyclic covariate to represent temporally cyclic movement patterns, such as diel variation in activity, and combine the three components in a multi-modal Hidden Markov Model framework using existing methods and R functions. We compare the trends and statistical fits of models that include or exclude any of the behavioural, spatial and temporal components, and perform variance partitioning on the model predictions that included all components to assess their relative importance to the movement process, both in isolation and in interaction. Results: We apply our workflow to a case study on the movements of plains zebra, blue wildebeest and eland antelope in a South African reserve. Behavioural modes impacted movement the most, followed by diel rhythms and then the spatial environment (viz. tree cover and terrain slope). Interactions between the components often explained more of the movement variation than the marginal effect of the spatial environment did on its own. Omitting components from the analysis led either to the inability to detect relationships between input and response variables, resulting in overgeneralisations when drawing conclusions about the movement process, or to detections of questionable relationships that appeared to be spurious. Conclusions: Our analytic workflow can be used to integrate the behavioural, temporal and spatial components of the movement process and quantify their relative contributions, thereby preventing incomplete or overly generic ecological interpretations. We demonstrate that understanding the drivers of animal movement, and ultimately the ecological phenomena that emerge from it, critically depends on considering the various components of the movement process, and especially the interactions between them.</p

Tamoxifen use and potential effects on liver parenchyma:A long-term prospective transient elastographic evaluation

Tamoxifen is a commonly prescribed drug in both early and metastatic breast cancer. Prospective studies in Asian populations demonstrated that tamoxifen‐related liver steatosis occurred in more than 30% of the patients within 2 years after start of treatment. No well‐designed prospective studies on potential tamoxifen‐related liver steatosis have been conducted in Caucasian patients so far. Therefore, our prospective study aimed to assess the incidence of tamoxifen‐related liver steatosis for a period of 2 years in a population of Caucasian breast cancer patients treated with tamoxifen. Patients with an indication for adjuvant treatment with tamoxifen were included in this study. Data were collected at 3 months (T1) and at 2 years (T2) after start of tamoxifen treatment (follow‐up period of 21 months). For the quantification of liver steatosis, patients underwent liver stiffness measurement by transient elastography with simultaneous controlled attenuation parameter (CAP) determination using the FibroScan. A total of 95 Caucasian breast cancer patients were included in this evaluation. Liver steatosis was observed in 46 of 95 (48%) and 48 of 95 (51%) of the patients at T1 and T2, respectively. No clinically relevant increase in liver steatosis was observed during the treatment period of 2 years with tamoxifen (median CAP = 243 ± 49 dB/m (T1) and 253 ± 55 dB/m (T2), respectively; p = 0.038). Conclusion: In this prospective longitudinal study in Caucasian breast cancer patients, no clinically relevant alterations in liver steatosis in terms of CAP values and liver/lipid parameters were observed after 2 years of tamoxifen treatment. This study therefore demonstrates an absence of tamoxifen‐related adverse events such as steatosis and (early) development of fibrosis or cirrhosis during a treatment period of at least 2 years

Adherence to a plant-based, high-fibre dietary pattern is related to regression of non-alcoholic fatty liver disease in an elderly population

Dietary lifestyle intervention is key in treating non-alcoholic fatty liver disease (NAFLD). We aimed to examine the longitudinal relation between well-established dietary patterns as well as population-specific dietary patterns and NAFLD. Participants from two subsequent visits of the Rotterdam Study were included. All underwent serial abdominal ultrasonography (median follow-up: 4.4 years) and filled in a food frequency questionnaire. Secondary causes of steatosis were excluded. Dietary data from 389 items were collapsed into 28 food groups and a posteriori dietary patterns were identified using factor analysis. Additionally, we scored three a priori dietary patterns (Mediterranean Diet Score, Dutch Dietary Guidelines and WHO-score). Logistic mixed regression models were used to examine the relation between dietary patterns and NAFLD. Analyses were adjusted for demographic, lifestyle and metabolic factors. We included 963 participants of whom 343 had NAFLD. Follow-up data was available in 737 participants. Incident NAFLD was 5% and regressed NAFLD was 30%. We identified five a posteriori dietary patterns (cumulative explained variation [R2] = 20%). The patterns were characterised as: vegetable and fish, red meat and alcohol, traditional, salty snacks and sauces, high fat dairy & refined grains pattern. Adherence to the traditional pattern (i.e. high intake of vegetable oils/stanols, margarines/butters, potatoes, whole grains and sweets/desserts) was associated with regression of NAFLD per SD increase in Z-score (0.40, 95% CI 0.15–1.00). Adherence to the three a priori patterns all showed regression of NAFLD, but only the WHO-score showed a distinct association (0.73, 95% CI 0.53–1.00). Hence, in this large elderly population, adherence to a plant-based, high-fibre and low-fat diet was related to regression of NAFLD

Immunosuppressive drug withdrawal late after liver transplantation improves the lipid profile and reduces infections

BACKGROUND: Treatment with immunosuppressive drugs (IS) after transplantation is accompanied by severe side effects. A limited number of studies have investigated the effect of IS withdrawal on IS-related comorbidities after liver transplantation (LTx) and the results are contradictory. PATIENTS AND METHODS: We determined in a retrospective case-control study the clinical effects of complete IS withdrawal in operationally tolerant (TOL) LTx recipients who discontinued IS 10.8 ± 5.1 years after LTx (n = 13) compared with a completely matched control (CTRL) group with a regular IS regimen (n = 22). TOL recipients have been IS and rejection free for 4.0 ± 2.8 years. RESULTS: IS withdrawal in TOL recipients resulted in lower low-density lipoprotein levels (P = 0.027), whereas this was not observed in the CTRL group. Furthermore, persistent infections in individual recipients were resolved successfully by IS withdrawal. TOL recipients also had significantly fewer de novo infections after IS withdrawal (TOL pre vs. post withdrawal P = 0.0247) compared with recipients continued on IS during the same follow-up period (post withdrawal TOL vs. CTRL P = 0.044). Unfortunately, no improvement in kidney function, and lower rates of de novo occurrences of diabetes, hypertension, cardiovascular diseases, and malignancies were observed in the TOL group after IS withdrawal compared with the CTRL group during the same follow-up time period. CONCLUSION: IS withdrawal late after LTx reduces infection rates and low-density lipoprotein levels, but other IS-related side effects persist late after LTx. An accurate tolerance immune profile enabling identification of tolerant LTx recipients eligible for safe IS withdrawal earlier after transplantation is needed to prevent the development of irreversible IS-related side effects

Modeling elephant-mediated cascading effects of water point closure

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Beyond the here and now : herbivore ecology in a spatial-temporal context

Ecological phenomena at a certain location often not only depend on the current characteristics of that location itself, but also on characteristics of the landscape surrounding the site or influences from the past. In other words, in order to be able to understand ecological processes here and now, we often need information that is beyond here and now. This thesis investigates the role of such spatial-temporal context on the relationships between species and their environment. The focus is on large mammalian herbivores, an in particular the factors determining the movement and distribution of African elephants (Loxodonta africana) in Kruger National Park (KNP), South Africa. This thesis covers four main themes. First, it is shown that spatial-temporal variation in the biotic and abiotic environment creates predictable seasonal and daily cyclic patterns in the distribution of elephants in KNP. Then, herbivores are shown not only to respond to environmental heterogeneity: they also can create distinct spatial patterning in savanna vegetation when foraging initiates a positive plant-herbivore feedback and when the foraging process is spatially explicit. This leads to specific sites being revisited often, and therefore the patterns of site revisitation by elephants in KNP are studied, mainly in relation to surface water availability and vegetation characteristics. Lastly, this thesis focuses on the scale-dependent response of organisms to environmental heterogeneity, showing the consequences of a mismatch in the scale of analysis on statistical inference, and then focusing on finding the appropriate spatial scales to analyse and predict the spatial distribution of elephants in KNP. Together, the chapters presented in this thesis highlight the importance of explicitly considering the scale and context dependency of species-environment relationships, and demonstrate methods to take issues of spatial-temporal scale and context into account. These methods have the potential to increase our understanding of ecological phenomena and therefore may lead to better management of natural resources