Eckart Winkler

A simulation model of plant invasion: long-distance dispersal determines the pattern of spread

Mechanisms and consequences of biological invasions are a global issue. Yet, one of the key aspects, the initial phase of invasion, is rarely observed in detail. Data from aerial photographs covering the spread of Heracleum mantegazzianum (Apiaceae, native to Caucasus) on a local scale of hectares in the Czech Republic from the beginning of invasion were used as an input for an individual-based model (IBM), based on small-scale and short-time data. To capture the population development inferred from the photographs, long-distance seed dispersal, changes in landscape structures and suitability of landscape elements to invasion by H. mantegazzianum were implemented in the model. The model was used to address (1) the role of long-distance dispersal in regional invasion dynamics, and (2) the effect of land-use changes on the progress of the invasion. Simulations showed that already small fractions of seed subjected to long-distance dispersal, as determined by systematic comparison of field data and modelling results, had an over-proportional effect on the spread of this species. The effect of land-use changes on the simulated course of invasion depends on the actual level of habitat saturation; it is larger for populations covering a high proportion of available habitat area than for those in the initial phase of invasion. Our results indicate how empirical field data and model outputs can be linked more closely with each other to improve the understanding of invasion dynamics. The multi-level, but nevertheless simple structure of our model suggests that it can be used for studying the spread of similar species invading in comparable landscapes.

Population dynamics along a primary succession gradient: do alpine species fit into demographic succession theory?

BACKGROUND AND AIMS Understanding processes and mechanisms governing changes in plant species along primary successions has been of major importance in ecology. However, to date hardly any studies have focused on the complete life cycle of species along a successional gradient, comparing pioneer, early and late-successional species. In this study it is hypothesized that pioneer species should initially have a population growth rate, lambda, greater than one with high fecundity rates, and declining growth rates when they are replaced by late-successional species. Populations of late-successional species should also start, at the mid-successional stage (when pioneer species are declining), with growth rates greater than one and arrive at rates equal to one at the late successional stage, mainly due to higher survival rates that allow these species to persist for a long time. METHODS The demography of pioneer- (Saxifraga aizoides), early (Artemisia genipi) and late-successional species (Anthyllis vulneraria ssp. alpicola) was investigated together with that of a ubiquitous species (Poa alpina) along the Rotmoos glacier foreland (2300-2400 m a.s.l., Central Alps, Austria) over 3 years. A matrix modelling approach was used to compare the main demographic parameters. Elasticity values were plotted in a demographic triangle using fecundity, individual growth and survival as vital rates contributing to the population growth rates. KEY RESULTS The results largely confirmed the predictions for population growth rates during succession. However, high survival rates of larger adults characterized all species, regardless of where they were growing along the succession. At the pioneer site, high mortality rates of seedlings, plantlets and young individuals were recorded. Fecundity was found to be of minor relevance everywhere, but it was nevertheless sufficient to increase or maintain the population sizes. CONCLUSIONS Demographically, all the species over all sites behaved like late-successional or climax species in secondary successions, mainly relying on survival of adult individuals. Survival serves as a buffer against temporal variation right from the beginning of the primary succession, indicating a major difference between primary and secondary succession.

Long-term control of species abundances in a dry grassland: a spatially explicit model

. The long-term spatio-temporal dynamics of a sparse dry-grassland community (Thymo-Festucetum) is investigated by a spatially explicit individual-based simulation model and by analytical model equations. The community (investigated over 15 yr in permanent plots) is characterized by a permanently low cover (30–50 %), mainly of the perennial tuft grass Festuca cinerea. Seedling establishment and the fate of juveniles are strongly dependent on weather conditions. The simulation programme focuses on the mechanism of clonal growth of grasses and the reproduction of tufts by fragmentation. Questions answered by the modelling approach were (1) which life-history features of the species are responsible for their persistence and for the low vegetation cover of the community and (2) what are the main mechanisms of species interactions. Different sets of simulation runs, together with the evaluation of the analytical models, show: (1) long-term persistence of the main species is possible only by a combination of sexual and clonal reproduction; the low cover is due to low germination rate, low mortality and limited growth of tufts; (2) the intra and interspecific control of the community is performed mainly via a reduction by already established individuals; (3) persistence of uncommon species relies on a diaspore buffer in, or around, the community (‘spatial mass effect’).

Clonal plant species in a dry-grassland community: A simulation study of long-term population dynamics

Abstract An individual-based spatially explicit grid-based model is developed to simulate the simultaneous development of two clonally growing grassland plant species. The model is based on long-term permanent-plot observations in a low-cover Thymo-Festucetum grassland community which is characterized, besides of some low-abundant (‘sparse’) species, by the interaction between the perennial tuft grass Festuca cinerea and the stoloniferous rosette plant Hieracium pilosella . The grid-based model includes rules for seedling recruitment, diaspore exchange, mortality, tuft growth and rosette establishment, local interactions, and considers the dependence of population parameters on environmental factors (weather). The simulations give the characteristic spatial features of the community (aggregations of grass tufts and of rosettes) and the highly fluctuating species abundances. They allow the assessment of the importance of different factors which control the abundances in the community: reproduction modes, intra- and interspecific interactions, dependence of life-history processes on weather conditions, and overall climatic conditions. The mechanisms of the spatially explicit model are concentrated by averaging over local interactions into analytical models (stochastic difference equations) which form a basis for a general insight into community dynamics and for a large-scale study of heterogeneous grassland communities containing clonal plant species.

Demographic Consequences of the Two Reproductive Modes in Poa alpina L. along a Primary Succession Gradient in the Central Alps

Abstract Poa alpina grass plays a predominant role across the entire range of primary succession on alpine glacier forelands. One demographic factor that reacts clearly to changing environmental conditions is reproduction. Using permanent plot data, the complete life cycle of Poa alpina was studied along a successional gradient of the Rotmoos glacier foreland (2300–2400 m a.s.l., Central Alps, Austria) over a period of three years. We used matrix modeling to study the importance of the generation of plantlets and seedlings along the successional gradient and their ability to form adult individuals, and we hypothesized that plantlets develop faster to adults than seedlings because they start already with 3–4 developed leaves. The study showed that plantlet and seedling fecundities of Poa alpina changed differently in the course of succession: seedling establishment was observed over the entire range of the successional stages, whereas plantlet establishment almost vanished with ongoing succession. In the pioneer stage, plantlets were more important than reproduction by seedlings. But we found neither a higher survival rate nor a significant advantage in development to adults for plantlets compared to seedlings. Opportunistic reproduction—plantlets under harsh abiotic conditions, seeds under increasing density—may therefore explain the fact that the species is ubiquitous along the whole glacier foreland.

Long-term Population Development and Spatial Pattern of Carex curvula Subspecies

Abstract A study of shoot populations of Carex curvula subspecies (Carex curvula ssp. rosae and C. curvula ssp. curvula) compared predictions from a 3-year observation period with renewed observations after 10 years. The study sites were located in the Dolomites (Mount Latemar), Italy, at 2390–2580 m a.s.l. The first observation period (1990–1992) produced findings on shoot numbers in different plots at five sites. These shoot numbers allowed the finite rates of population increase λ (characterizing the dynamic state of the populations) to be calculated. The values of λ varied modestly within and among populations. The long-term rates of population increase remained close to the equilibrium value of 1.0. The spatial pattern was calculated by means of the patchiness index and patch occupancy. The smaller the plot population, the higher the patchiness and the lower the proportion of occupied patches. To some extent, patchiness can serve as a measure of a populations dynamic state. A correlation between the structural measures (patchiness, patch occupancy) and the rate of population increase λ showed that strongly aggregated populations were small and also decreasing in size. The extinction risks for the next 50 years were found to be very low for populations on the pioneer grassland stand whereas most other populations will decrease and probably reach critical thresholds.