@article{SchurrDeanMiltonetal.2004, author = {Schurr, Frank Martin and Dean, W. R. J. and Milton, Sue J. and Jeltsch, Florian}, title = {A conceptual model linking demography of the shrub species Grewia flava to the dynamics of Kalahari savannas}, year = {2004}, abstract = {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.}, language = {en} } @article{SchurrBondMidgleyetal.2005, author = {Schurr, Frank Martin and Bond, William J. and Midgley, Guy F. and Higgins, Steven I.}, title = {A mechanistic model for secondary seed dispersal by wind and its experimental validation}, issn = {0022-0477}, year = {2005}, abstract = {1 Secondary seed dispersal by wind, the wind-driven movement of seeds along the ground surface, is an important dispersal mechanism for plant species in a range of environments. 2 We formulate a mechanistic model that describes how secondary dispersal by wind is affected by seed traits, wind conditions and obstacles to seed movement. The model simulates the movement paths of individual seeds and can be fully specified using independently measured parameters. 3 We develop an explicit version of the model that uses a spatially explicit representation of obstacle patterns, and also an aggregated version that uses probability distributions to model seed retention at obstacles and seed movement between obstacles. The aggregated version is computationally efficient and therefore suited to large-scale simulations. It provides a very good approximation of the explicit version (R-2 > 0.99) if initial seed positions vary randomly relative to the obstacle pattern. 4 To validate the model, we conducted a field experiment in which we released seeds of seven South African Proteaceae species that differ in seed size and morphology into an arena in which we systematically varied obstacle patterns. When parameterized with maximum likelihood estimates obtained from independent measurements, the explicit model version explained 70-77\% of the observed variation in the proportion of seeds dispersed over 25 m and 67- 69\% of the observed variation in the direction of seed dispersal. 5 The model tended to underestimate dispersal rates, possibly due to the omission of turbulence from the model, although this could also be explained by imprecise estimation of one model parameter (the aerodynamic roughness length). 6 Our analysis of the aggregated model predicts a unimodal relationship between the distance of secondary dispersal by wind and seed size. The model can also be used to identify species with the potential for long-distance seed transport by secondary wind dispersal. 7 The validated model expands the domain of mechanistic dispersal models, contributes to a functional understanding of seed dispersal, and provides a tool for predicting the distances that seeds move}, language = {en} } @article{BuchmannSchurrNathanetal.2011, author = {Buchmann, Carsten M. and Schurr, Frank Martin and Nathan, Ran and Jeltsch, Florian}, title = {An allometric model of home range formation explains the structuring of animal communities exploiting heterogeneous resources}, series = {Oikos}, volume = {120}, journal = {Oikos}, number = {1}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0030-1299}, doi = {10.1111/j.1600-0706.2010.18556.x}, pages = {106 -- 118}, year = {2011}, abstract = {Understanding and predicting the composition and spatial structure of communities is a central challenge in ecology. An important structural property of animal communities is the distribution of individual home ranges. Home range formation is controlled by resource heterogeneity, the physiology and behaviour of individual animals, and their intra- and interspecific interactions. However, a quantitative mechanistic understanding of how home range formation influences community composition is still lacking. To explore the link between home range formation and community composition in heterogeneous landscapes we combine allometric relationships for physiological properties with an algorithm that selects optimal home ranges given locomotion costs, resource depletion and competition in a spatially-explicit individual-based modelling framework. From a spatial distribution of resources and an input distribution of animal body mass, our model predicts the size and location of individual home ranges as well as the individual size distribution (ISD) in an animal community. For a broad range of body mass input distributions, including empirical body mass distributions of North American and Australian mammals, our model predictions agree with independent data on the body mass scaling of home range size and individual abundance in terrestrial mammals. Model predictions are also robust against variation in habitat productivity and landscape heterogeneity. The combination of allometric relationships for locomotion costs and resource needs with resource competition in an optimal foraging framework enables us to scale from individual properties to the structure of animal communities in heterogeneous landscapes. The proposed spatially-explicit modelling concept not only allows for detailed investigation of landscape effects on animal communities, but also provides novel insights into the mechanisms by which resource competition in space shapes animal communities.}, language = {en} } @article{EstherGroeneveldEnrightetal.2008, author = {Esther, Alexandra and Groeneveld, J{\"u}rgen and Enright, Neal J. and Miller, Ben P. and Lamont, Byron B. and Perry, George L. W. and Schurr, Frank Martin and Jeltsch, Florian}, title = {Assessing the importance of seed immigration on coexistence of plant functional types in a species-rich ecosystem}, issn = {0304-3800}, doi = {10.1016/j.ecolmodel.2008.01.014}, year = {2008}, abstract = {Modelling and empirical studies have shown that input from the regional seed pool is essential to maintain local species diversity. However, most of these studies have concentrated on simplified, if not neutral, model systems, and focus on a limited subset of species or on aggregated measures of diversity only (e.g., species richness or Shannon diversity). Thus they ignore more complex species interactions and important differences between species. To gain a better understanding of how seed immigration affects community structure at the local scale in real communities we conducted computer simulation experiments based on plant functional types (PFTs) for a species-rich, fire-prone Mediterranean-type shrubland in Western Australia. We developed a spatially explicit simulation model to explore the community dynamics of 38 PFTs, defined by seven traits - regeneration mode, seed production, seed size, maximum crown diameter, drought tolerance, dispersal mode and seed bank type - representing 78 woody species. Model parameterisation is based on published and unpublished data on the population dynamics of shrub species collected over 18 years. Simulation experiments are based on two contrasting seed immigration scenarios: (1) the 'equal seed input number' scenario, where the number of immigrant seeds is the same for all PFTs, and (2) the 'equal seed input mass' scenario, where the cumulative mass of migrating seeds is the same for all PFTs. Both scenarios were systematically tested and compared for different overall seed input values. Without immigration the local community drifts towards a state with only 13 coexisting PFTs. With increasing immigration rates in terms of overall mass of seeds the simulated number of coexisting PFTs and Shannon diversity quickly approaches values observed in the field. The equal seed mass scenario resulted in a more diverse community than did the seed number scenario. The model successfully approximates the frequency distributions (relative densities) of all individual plant traits except seed size for scenarios associated with equal seed input mass and high immigration rate. However, no scenario satisfactorily approximated the frequency distribution for all traits in combination. Our results show that regional seed input can explain the more aggregated measures of local community structure, and some, but not all, aspects of community composition. This points to the possible importance of other (untested) processes and traits (e.g., dispersal vectors) operating at the local scale. Our modelling framework can readily allow new factors to be systematically investigated, which is a major advantage compared to previous simulation studies, as it allows us to find structurally realistic models, which can address questions pertinent to ecological theory and to conservation management.}, language = {en} } @article{KuparinenSchurr2008, author = {Kuparinen, Anna and Schurr, Frank Martin}, title = {Assessing the risk of gene flow from genetically modified trees carrying mitigation transgenes}, issn = {1387-3547}, doi = {10.1007/s10530-007-9129-6}, year = {2008}, language = {en} } @article{HigginsFloresSchurr2008, author = {Higgins, Steven I. and Flores, Olivier and Schurr, Frank Martin}, title = {Costs of persistence and the spread of competing seeders and sprouters}, issn = {0022-0477}, doi = {10.1111/j.1365-2745.2008.01391.x}, year = {2008}, language = {en} } @article{SchiffersSchurrTielboergeretal.2008, author = {Schiffers, Katja and Schurr, Frank Martin and Tielb{\"o}rger, Katja and Urbach, Carsten and Moloney, Kirk A. and Jeltsch, Florian}, title = {Dealing with virtual aggregation : a new index for analysing heterogeneous point patterns}, issn = {0906-7590}, doi = {10.1111/j.0906-7590.2008.05374.x}, year = {2008}, language = {en} } @article{SchleicherMeyerWiegandetal.2011, author = {Schleicher, Jana and Meyer, Katrin M. and Wiegand, Kerstin and Schurr, Frank Martin and Ward, David}, title = {Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants}, series = {Journal of vegetation science}, volume = {22}, journal = {Journal of vegetation science}, number = {6}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {1100-9233}, doi = {10.1111/j.1654-1103.2011.01310.x}, pages = {1038 -- 1048}, year = {2011}, abstract = {Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species? Location: Semi-arid savanna in the Kimberley Thorn Bushveld, South Africa. Methods: We developed a two-step protocol for the statistical differentiation of association-promoting mechanisms in plants based on the Acacia erioloba-Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non-spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non-spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter. Results: We first showed a significant positive spatial association of the two species. Thereafter, the non-spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred. Conclusions: Our two-step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).}, language = {en} } @article{ThuillerMuenkemuellerSchiffersetal.2014, author = {Thuiller, Wilfried and Muenkemueller, Tamara and Schiffers, Katja H. and Georges, Damien and Dullinger, Stefan and Eckhart, Vincent M. and Edwards, Thomas C. and Gravel, Dominique and Kunstler, Georges and Merow, Cory and Moore, Kara and Piedallu, Christian and Vissault, Steve and Zimmermann, Niklaus E. and Zurell, Damaris and Schurr, Frank Martin}, title = {Does probability of occurrence relate to population dynamics?}, series = {Ecography : pattern and diversity in ecology ; research papers forum}, volume = {37}, journal = {Ecography : pattern and diversity in ecology ; research papers forum}, number = {12}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0906-7590}, doi = {10.1111/ecog.00836}, pages = {1155 -- 1166}, year = {2014}, abstract = {Interestingly, relationships between demographic parameters and occurrence probability did not vary substantially across degrees of shade tolerance and regions. Although they were influenced by the uncertainty in the estimation of the demographic parameters, we found that r was generally negatively correlated with P-occ, while N, and for most regions K, was generally positively correlated with P-occ. Thus, in temperate forest trees the regions of highest occurrence probability are those with high densities but slow intrinsic population growth rates. The uncertain relationships between demography and occurrence probability suggests caution when linking species distribution and demographic models.}, language = {en} } @article{SarmentoBondMidgleyetal.2011, author = {Sarmento, Juliano Sarmento and Bond, William J. and Midgley, Guy F. and Rebelo, Anthony G. and Thuiller, Wilfried and Schurr, Frank Martin}, title = {Effects of harvesting flowers from shrubs on the persistence and abundance of wild shrub populations at multiple spatial extents}, series = {Conservation biology : the journal of the Society for Conservation Biology}, volume = {25}, journal = {Conservation biology : the journal of the Society for Conservation Biology}, number = {1}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0888-8892}, doi = {10.1111/j.1523-1739.2010.01628.x}, pages = {73 -- 84}, year = {2011}, abstract = {Wildflower harvesting is an economically important activity of which the ecological effects are poorly understood. We assessed how harvesting of flowers affects shrub persistence and abundance at multiple spatial extents. To this end, we built a process-based model to examine the mean persistence and abundance of wild shrubs whose flowers are subject to harvest (serotinous Proteaceae in the South African Cape Floristic Region). First, we conducted a general sensitivity analysis of how harvesting affects persistence and abundance at nested spatial extents. For most spatial extents and combinations of demographic parameters, persistence and abundance of flowering shrubs decreased abruptly once harvesting rate exceeded a certain threshold. At larger extents, metapopulations supported higher harvesting rates before their persistence and abundance decreased, but persistence and abundance also decreased more abruptly due to harvesting than at smaller extents. This threshold rate of harvest varied with species' dispersal ability, maximum reproductive rate, adult mortality, probability of extirpation or local extinction, strength of Allee effects, and carrying capacity. Moreover, spatial extent interacted with Allee effects and probability of extirpation because both these demographic properties affected the response of local populations to harvesting more strongly than they affected the response of metapopulations. Subsequently, we simulated the effects of harvesting on three Cape Floristic Region Proteaceae species and found that these species reacted differently to harvesting, but their persistence and abundance decreased at low rates of harvest. Our estimates of harvesting rates at maximum sustainable yield differed from those of previous investigations, perhaps because researchers used different estimates of demographic parameters, models of population dynamics, and spatial extent than we did. Good demographic knowledge and careful identification of the spatial extent of interest increases confidence in assessments and monitoring of the effects of harvesting. Our general sensitivity analysis improved understanding of harvesting effects on metapopulation dynamics and allowed qualitative assessment of the probability of extirpation of poorly studied species.}, language = {en} }