TY  - JOUR
A1  - Wiegand, K.
A1  - Jeltsch, Florian
A1  - Ward, D.
T1  - Minimum recruitment frequency in plants with episodic recruitment
N2  - There is concern about the lack of recruitment of Acacia trees in the Negev desert of Israel. We have developed three models to estimate the frequency of recruitment necessary for long-term population survival (i.e. positive average population growth for 1,000 years and <10% probability of extinction). Two models assume purely episodic recruitment based on the general notion that recruitment in and environments is highly episodic. They differ in that the deterministic model investigates average dynamics while the stochastic model does not. Studies indicating that recruitment episodes in and environments have been overemphasized motivated the development of the third model. This semi-stochastic model simulates a mixture of continuous and episodic recruitment. Model analysis was done analytically for the deterministic model and via running model simulations for the stochastic and semi-stochastic models. The deterministic and stochastic models predict that, on average, 2.2 and 3.7 recruitment events per century, respectively, are necessary to sustain the population. According to the semi-stochastic model, 1.6 large recruitment events per century and an annual probability of 50% that a small recruitment event occurs are needed. A consequence of purely episodic recruitment is that all recruitment episodes produce extremely large numbers of recruits (i.e. at odds with field observations), an evaluation that holds even when considering that rare events must be large. Thus, the semi- stochastic model appears to be the most realistic model. Comparing the prediction of the semi-stochastic model to field observations in the Negev desert shows that the absence of observations of extremely large recruitment events is no reason for concern. However, the almost complete absence of small recruitment events is a serious reason for concern. The lack of recruitment may be due to decreased densities of large mammalian herbivores and might be further exacerbated by possible changes in climate, both in terms of average precipitation and the temporal distribution of rain
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Tews, Jörg
A1  - Schurr, Frank Martin
T1  - Seed dispersal by cattle may cause shrub encroachment of Grewia flava on southern Kalahari rangelands
N2  - Shrub encroachment, i.e. the increase in woody plant cover, is a major concern for livestock farming in southern Kalahari savannas. We developed a grid-based computer model simulating the population dynamics of Grewia flava, a common, fleshy-fruited encroaching shrub. In the absence of large herbivores, seeds of Grewia are largely deposited in the sub-canopy of Acacia erioloba. Cattle negate this dispersal limitation by browsing on the foliage of Grewia and dispersing seeds into the grassland matrix. In this study we first show that model predictions of Grewia cover dynamics are realistic by comparing model output with shrub cover estimates obtained from a time series of aerial photographs. Subsequently, we apply a realistic range of intensity of cattle-induced seed dispersal combined with potential precipitation and fire scenarios. Based on the simulation results we suggest that cattle may facilitate shrub encroachment of Grewia. The results show that the severity of shrub encroachment is governed by the intensity of seed dispersal. For a high seed dispersal intensity without fire (equivalent to a high stocking rate) the model predicts 56% shrub cover and 85% cell cover after 100 yr. With fire both recruitment and shrub cover are reduced, which may, under moderate intensities, prevent shrub encroachment. Climate change scenarios with two-fold higher frequencies of drought and wet years intensified shrub encroachment rates, although long-term mean of precipitation remained constant. As a management recommendation we suggest that shrub encroachment on rangelands may be counteracted by frequent fires and controlling cattle movements to areas with a high proportion of fruiting Grewia shrubs
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Tews, Jörg
A1  - Moloney, Kirk A.
T1  - Modelling seed dispersal in a variable environment : a case study of the fleshy-fruited savanna shrub Grewia flava
N2  - In ecology much attention has been paid towards seed dispersal of fleshy-fruited plants, however, knowledge is lacking about the Iona-term demographic consequences of variation in dispersal distance and fruit removal rate, particularly given the natural variability of the environment the organism lives in. In this study we used a spatially explicit, two-level stochastic computer model to simulate population dynamics of a fleshy-fruited shrub living in the sub-canopy of solitary savanna trees. On the landscape level we implemented three realistic scenarios of savanna landscape dynamics for a period of 500 years with equal inter-annual mean of environmental variables. The first scenario is representative of a relatively constant environment with normal variability in precipitation, constant tree density and random tree recruitment pattern. The second and third scenarios represent positive auto-correlated, cyclic patterns with alternating phases of tree cover increase and decrease corresponding with favorable and unfavorable rain phases. Our simulation experiments show that when fruit removal rate is extremely low, population persistence is enhanced under relatively constant rain conditions, while alternating rain phases of the cyclic scenarios lead to a significant population decrease. This result confirms previous findings that periodically fluctuating environments may increase local extinction risk. However, when dispersal distance is a limiting factor (whilst removal rate was sufficiently high), tree clumps typically forming in wet phases of both cyclic scenarios compensated for the negative effect of low dispersal distances, while the constant scenario with random tree pattern and larger inter-tree distances resulted in a significant population decline. (C) 2003 Elsevier B.V. All rights reserved
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Hansen, Frank
A1  - Tackmann, K.
A1  - Staubach, C.
A1  - Thulke, Hans-Hermann
T1  - Processes leading to a spatial aggregation of Echinococcus multilocularis in its natural intermediate host Microtus arvalis
N2  - The small fox tapeworm (Echinococcus multilocularis) shows a heterogeneous spatial distribution in the intermediate host (Microtus arvalis). To identify the ecological processes responsible for this heterogeneity, we developed a spatially explicit simulation model. The model combines individual-based (foxes, Vulpes vulpes) and grid- based (voles) techniques to simulate the infections in both intermediate and definite host. If host populations are homogeneously mixed, the model reproduces field data for parasite prevalence only for a limited number of parameter combinations. As ecological parameters inevitably vary to a certain degree, we discarded the homogeneous mixing model as insufficient to gain insight into the ecology of the fox tapeworm cycle. We analysed five different model scenarios, each focussing on an ecological process that might be responsible for the heterogeneous spatial distribution of E multilocularis in the intermediate host. Field studies revealed that the prevalence ratio between intermediate and definite host remains stable over a wide range of ecological conditions. Thus, by varying the parameters in simulation experiments, we used the robustness of the agreement between field data and model output as quality criterion for the five scenarios. Only one of the five scenarios was found to reproduce the prevalence ratio over a sufficient range of parameter combinations. In the accentuated scenario most tapeworm eggs die due to bad environmental conditions before they cause infections in the intermediate host. This scenario is supported by the known sensitivity of tapeworm eggs to high temperatures and dry conditions. The identified process is likely to lead to a heterogeneous availability of infective eggs and thus to a clumped distribution of infected intermediate hosts. In conclusion, areas with humid conditions and low temperatures must be pointed out as high risk areas for human exposure to E. multilocularis eggs as well. (C) 2004 on behalf of Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Tews, Jörg
A1  - Blaum, Niels
A1  - Jeltsch, Florian
T1  - Structural and animal species diversity in arid and semi-arid savannas of the southern Kalahari
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Tews, Jörg
T1  - Climate change impacts woody plant population dynamics in arid savanna
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Wiegand, K.
A1  - Ward, D.
T1  - Seed dispersal by cattle may cause shrub encroachment of Grewia flava on southern Kalahari rangelands
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Tews, Jörg
A1  - Brose, Ulrich
A1  - Grimm, Volker
A1  - Tielbörger, Katja
A1  - Wichmann, Matthias
A1  - Schwager, Monika
T1  - Animal species diversity driven by habitat heterogeneity/diversity : the importance of keystone structures
N2  - In a selected literature survey we reviewed studies on the habitat heterogeneity-animal species diversity relationship and evaluated whether there are uncertainties and biases in its empirical support. We reviewed 85 publications for the period 1960-2003. We screened each publication for terms that were used to define habitat heterogeneity, the animal species group and ecosystem studied, the definition of the structural variable, the measurement of vegetation structure and the temporal and spatial scale of the study. The majority of studies found a positive correlation between habitat heterogeneity/diversity and animal species diversity. However, empirical support for this relationship is drastically biased towards studies of vertebrates and habitats under anthropogenic influence. In this paper we show that ecological effects of habitat heterogeneity may vary considerably between species groups depending on whether structural attributes are perceived as heterogeneity or fragmentation. Possible effects may also vary relative to the structural variable measured. Based upon this, we introduce a classification framework that may be used for across-studies comparisons. Moreover, the effect of habitat heterogeneity for one species group may differ in relation to the spatial scale. In several studies, however, different species groups are closely linked to 'keystone structures' that determine animal species diversity by their presence. Detecting crucial keystone structures of the vegetation has profound implications for nature conservation and biodiversity management.
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Jeltsch, Florian
A1  - Wichmann, Matthias
A1  - Dean, W. R. J.
T1  - Global change challenges the Tawny Eagle (Aquila rapax) : modelling extinction risk with respect to predicted climate and land use changes
Y1  - 2004
ER  - 
TY  - JOUR
A1  - Schurr, Frank Martin
A1  - Dean, W. R. J.
A1  - Milton, Sue J.
A1  - Jeltsch, Florian
T1  - A conceptual model linking demography of the shrub species Grewia flava to the dynamics of Kalahari savannas
N2  - Environmental heterogeneity is a major determinant of plant population dynamics. In semi-arid Kalahari savannas, heterogeneity is created by savanna structure, i.e. by the spatial arrangement and temporal dynamics of woody plant and open grassland microsites. We formulate a conceptual model describing the effects of savanna dynamics on the population dynamics of the animal-dispersed shrub Grewia flava. From empirical results we derive model rules describing effects of savanna structure on several processes in Grewia's life cycle. By formulating the model, we summarise existing information on Grewia demography and identify gaps in this knowledge. Despite a number of such gaps, the model can be used to make certain quantitative predictions. As an example, we apply the model to investigate the role of seed dispersal in Grewia encroachment on rangelands. Model results show that cattle promote encroachment by depositing substantial numbers of seeds in open areas, where Grewia is otherwise dispersal-limited. Finally, we draw some general conclusions about Grewia's life history and population dynamics. Under natural conditions, concentrated seed deposition under woody plants appears to be a key process causing the observed association between Grewia and other woody plants. Furthermore, low rates of recruitment and high adult survival result in slow-motion dynamics of Grewia populations. As a consequence, Grewia populations interact with savanna dynamics on long temporal and short to intermediate spatial scales.
Y1  - 2004
ER  -