Näringsdistribuering av herbivorer i Hluhluwe-Imfolozi Park (Sydafrika)

African megaherbivores are ecosystem engineers, which are under threat of extinction caused by, among others, poaching and climate change. Their current rates of loss means that there’s a need to better analyse their impact on the environment, in order to predict the consequences of losing this group. One of the ways herbivores are influencing their habitat is by defecating in a certain spot, altering the nitrogen to phosphorus (N:P) ratio of the soil and therefore changing the nutrient availability to different plant species, which could change the structure and/or the composition of the vegetation. Therefore, the main objective of this thesis is to determine how body mass, feeding strategy and digestive type of various mammalian herbivores influences the spatial distribution of dung, nitrogen and phosphorus. I focused on two aspects in which species influence nutrient dynamics differently: (1) the variation among species in terms of dung nutrient content and (2) the way species differ in distributing dung and thus overall faecal nutrients. From January to April 2018 I collected dung from all abundant mammalian herbivore species in Hluhluwe-Imfolozi Park (HiP). The nitrogen and phosphorus concentration in the dung samples were determined. I found that the N:P ratio in dung increases with body mass as a result of a decrease in the concentration of phosphorus. The N:P ratio in browser and mixed-feeder dung is significantly higher than the ratio in grazer dung, driven by higher concentrations of nitrogen in browser dung versus grazer dung. Also, the results showed that, even though the average N:P ratio in dung of ruminants and non-ruminants did not differ significantly, the concentration of nitrogen and phosphorus were significantly higher in ruminant dung. In order to find the pattern of dung distribution through HiP, I combined my qualitative data with quantitative data from a long term study where herbivore dung was counted along 24 line transects. The impact of herbivores on the distribution of nitrogen and phosphorus is found to be highly complex. Differences between species in body mass, feeding strategy and digestive type all contribute to the concentrations of nitrogen and phosphorus in dung and therefore the distribution of these nutrients over an area. This leads to the conclusion that the effect of megaherbivore extinction will differ per area depending on the identity of the megaherbivores living in that area. Possible consequences for nature reserves are changes in abundance of smaller herbivore and vegetation structures. In HiP the expected result of megaherbivore extinction, when looking at the nutrient distribution, will be a decrease in the amount of phosphorus distributed through herbivore dung. The nitrogen distribution through herbivore dung is expected to show minimal change. The composition of the African herbivore community is changing and to predict the full consequences of these changes it is important to conduct additional research which combines the effects of herbivory with studies on nutrient distribution

The Ecosystem Resilience Approach (ERA) to control the invasive Australian swamp stonecrop (Crassula helmsii)

The invasive Australian swamp stonecrop (Crassula helmsii) threatens species characteristic of shallow soft water lakes and pools in Europe. Eradication of this plant is, however, difficult and costly once a large population has established. For this reason, we here explore a novel ecosystem resilience approach (ERA) to control this species. These datasets contain information about the first field performances of the ERA. Effectiveness of the ERA is measured by determining the cover (%) of the invasive species and the introduced and naturally established native vegetation after the introduction of a competing species in field plots. Additionally we’ve observed and measured field characteristics (soil- and water nutrient content by AA- and ICP analysis) to describe difference between the treated locations. *The data appropriate to this article may contain Dutch notes. For an English explanation, please contact the author of the data Abstract of paper: The invasive Australian swamp stonecrop (Crassula helmsii) threatens species characteristic of shallow soft water lakes and pools in Europe. Anthropogenic disturbances of these ecosystems cause open niches in their littoral zones and allow C. helmsii to form dominant stands, especially under nutrient enrichment. Eradication of this amphibious and clonal plant is, however, difficult and costly once a large population has established. For this reason, we here explore a novel ecosystem resilience approach (ERA) to control this species. This approach includes supressing the species by facilitating the occurrence and expansion of native vegetation. This requires a set-back of C. helmsii’s abundance by actively reducing its biomass, and the rehabilitation of optimal environmental conditions for native species. Our ERA study in four nature areas shows that restoring abiotic conditions, a one-time reduction of C. helmsii’s biomass and the introduction of native species makes the ecosystem more resilient against invasions, as shown by less recolonization of this invasive plant species. Therefore, we state that ERA can effectively be applied in practice to decrease the invasibility of ecosystems by C. helmsii. Effectiveness, costs and benefits, and recommendations for application in practice are discussed. Overall, we argue that incorporating ERA in nature- and water management will provide more sustainable solutions in terms of biodiversity as well as economics for invasive species prevention and control

Low Resource Competition, Availability of Nutrients and Water Level Fluctuations Facilitate Invasions of Australian Swamp Stonecrop (Crassula helmsii)

Australian swamp stonecrop (Crassula helmsii (Kirk) Cockayne) is invasive in Western Europe. Its small size and high potential for regeneration make it difficult to eliminate. Short-term experiments have demonstrated that the growth of C. helmsii depends on nutrient availability and resource competition. In order to confirm those mechanisms in the field, we studied the abundance of C. helmsii in Northern Europe over a longer period of time in relation to nutrient availability and co-occurring plant communities and plant species. C. helmsii impacted native species mainly by limiting their abundance. The native plant species present indicated that previous or periodic elevated nutrient availability were likely responsible for the proliferation of C. helmsii. When growing in submerged conditions, the dominance of C. helmsii depended on a high availability of CO2. A series of exceptionally dry summers allowed C. helmsii to increase in cover due to weakened biotic resistance and a loss of carbon limitation. Only Littorella uniflora (L.) Asch. and Juncus effusus L. were able to remain dominant and continue to provide biotic resistance. Based on our findings, minimizing nutrient (C and N) availability and optimizing hydrology provides native species with stable growth conditions. This optimizes resource competition and may prevent the proliferation of C. helmsii

Animal body size distribution influences the ratios of nutrients supplied to plants

Herbivores influence nutrient cycling by depositing feces across the landscape. Where herbivores go in the landscape is governed by factors such as food requirements and vulnerability to predation, traits that are related to body size. We show that mammals that differ in body size not only use the landscape differently but also differ in the amount of nitrogen relative to phosphorus that they release through their feces. This ensures that plants that grow in areas used by predominantly larger herbivores (such as areas of higher predation risk or areas of greater food availability) will receive lower amounts of fecal phosphorus relative to fecal nitrogen, potentially impacting plant nutrient availability. This finding is noteworthy considering the extinction bias toward larger animals.Nutrients released through herbivore feces have the potential to influence plant-available nutrients and affect primary productivity. However, herbivore species use nutrients in set stoichiometric ratios that vary with body size. Such differences in the ratios at which nutrients are used leads to differences in the ratios at which nutrients are deposited through feces. Thus, local environmental factors that affect the average body size of an herbivore community (such as predation risk and food availability) influence the ratios at which fecal nutrients are supplied to plants. Here, we assess the relationship between herbivore body size and the nitrogen-to-phosphorus ratios of herbivore feces. We examine how shifts in the average body size of an herbivore community alter the ratios at which nitrogen and phosphorus are supplied to plants and test whether such differences in the stoichiometry of nutrient supply propagate through plants. We show that dung from larger-bodied herbivores contain lower quantities of phosphorus per unit mass and were higher in N:P ratio. We demonstrate that spatial heterogeneity in visibility (a proxy for predation risk and/or food availability) and rainfall (a proxy for food availability), did not affect the overall amount of feces deposited but led to changes in the average body size of the defecating community. Feces deposited in areas of higher rainfall and reduced visibility originated from larger herbivores and were higher in N:P ratios. This indicates that processes that change the size distribution of herbivore communities, such as predation or size-biased extinction, have the potential to alter the nutrient landscape for plants.Data files and the analysis script have been deposited in the Mendeley Data repository (DOI: 10.17632/jwc4rr6kwr.3) (50)

Invasive Australian swamp stonecrop (Crassula helmsii) negatively affects spawning but accelerates larval growth of the endangered natterjack toad (Epidalea calamita)

C. helmsii often invades the breeding habitat of endangered amphibians, such as Epidalea calamita. However, effects on natterjack population were unclear. The spawning rate, egg survival and the speed of larval growth of this toad species in relation to the presence of C. helmsii were studied. For this study we’ve measured temperature (by temperature data loggers) and chemical properties (by ICP- and AA analysis) of the water, as affected by C. helmsii. Additionally we’ve measured numbers of spawning, egg survival and larval sizes to investigate larval growth rate performing an exclosure experiment at a field location (Gijzenrooise zegge). *The data appropriate to this article may contain Dutch notes. For an English explanation, please contact the author of the data Abstract of the paper: The invasive Crassula helmsii is expanding in Europe. Several ecological effects are described, most of which focus on ecosystem functioning and native vegetation but rarely on fauna. In North-western Europe, C. helmsii often invades the breeding habitat of endangered amphibians, such as Epidalea calamita. The spawning rate, egg survival and the speed of larval growth of this toad species in relation to the presence of C. helmsii were studied. In order to unravel causal mechanisms, the outcome is related to temperature and chemical properties of the water, as affected by C. helmsii. Spawning and egg survival were significantly lower in case of C. helmsii dominance compared to bare soil conditions, and negatively affected the population size of E. calamita. However, larval growth rate was significantly higher in C. helmsii dominated treatments, which could be beneficial for tadpoles. Differences in water temperature and chemistry were a possible explanation for these effects. It remains unclear whether the population viability of E. calamita is negatively affected when C. helmsii is present. In many areas, however, this plant species completely overgrows and causes desiccation of waterbodies. Therefore, appropriate management measures will be required to protect this toad against this invader

The Ecosystem Resilience Approach (ERA) to control the invasive Australian swamp stonecrop (Crassula helmsii): Innovative control of C. helmsii

The invasive Australian swamp stonecrop (Crassula helmsii) threatens species characteristic of shallow soft water lakes and pools in Europe. Eradication of this plant is, however, difficult and costly once a large population has established. For this reason, we here explore a novel ecosystem resilience approach (ERA) to control this species. These datasets contain information about the first field performances of the ERA. Effectiveness of the ERA is measured by determining the cover (%) of the invasive species and the introduced and naturally established native vegetation after the introduction of a competing species in field plots. Additionally we’ve observed and measured field characteristics (soil- and water nutrient content by AA- and ICP analysis) to describe difference between the treated locations. *The data appropriate to this article may contain Dutch notes. For an English explanation, please contact the author of the data Abstract of paper: The invasive Australian swamp stonecrop (Crassula helmsii) threatens species characteristic of shallow soft water lakes and pools in Europe. Anthropogenic disturbances of these ecosystems cause open niches in their littoral zones and allow C. helmsii to form dominant stands, especially under nutrient enrichment. Eradication of this amphibious and clonal plant is, however, difficult and costly once a large population has established. For this reason, we here explore a novel ecosystem resilience approach (ERA) to control this species. This approach includes supressing the species by facilitating the occurrence and expansion of native vegetation. This requires a set-back of C. helmsii’s abundance by actively reducing its biomass, and the rehabilitation of optimal environmental conditions for native species. Our ERA study in four nature areas shows that restoring abiotic conditions, a one-time reduction of C. helmsii’s biomass and the introduction of native species makes the ecosystem more resilient against invasions, as shown by less recolonization of this invasive plant species. Therefore, we state that ERA can effectively be applied in practice to decrease the invasibility of ecosystems by C. helmsii. Effectiveness, costs and benefits, and recommendations for application in practice are discussed. Overall, we argue that incorporating ERA in nature- and water management will provide more sustainable solutions in terms of biodiversity as well as economics for invasive species prevention and control