Tomáš Herben

Small-scale spatial dynamics of plant species in a grassland community over six years

. In a species-rich mountain grassland in the Krkonosse Mts., Czechoslovakia, data from four permanent plots of 50 cm x 50 cm were recorded annually from 1985 to 1990 to study the spatial dynamics of the species. Plots were divided into 15 x 15 subplots and the number of vegetative units of all plants within each subplot was determined. There was not much net change at the plot level, but the subplots were very dynamic. Two aspects of the spatial dynamics of the species were followed: (1) persistence, i.e. the tendency of the species to remain in the same subplot, and (2) long-distance spreading, i.e. movement to subplots beyond the immediate neighbourhood. Species differed widely in their persistence and longdistance spreading and were classified into mobility types: long-range guerrilla, short-range guerrilla, phalanx and sitting’. The mobility types were, to a certain extent, correlated with the growth form of plants, but some species of one growth form showed different types of small-scale dynamics and some species with different growth forms had the same spatial dynamics.

INVASIBILITY AND SPECIES RICHNESS OF A COMMUNITY: A NEUTRAL MODEL AND A SURVEY OF PUBLISHED DATA

A widespread but controversial idea in ecology states that the number of invaders of a species assemblage depends on its species richness. Both negative and positive relationships have been reported. We examined whether a simple neutral model where assemblages are generated by drawing individuals from two pools of identical species (native and alien) can predict this relationship. We performed a meta-analysis of published data on this relationship. The neutral model showed that in communities with low and fixed numbers of indi- viduals, the relationship between the number of aliens and native species is strong and negative. This becomes weaker as the proportion of species from both pools already present in the community increases. The relationship between alien and native species richness becomes positive when the number of individuals is allowed to vary, because the richness depends on number of individuals and/or area sampled. The meta-analysis showed that scale-dependence of the relationship between alien and native species richness is universal and compatible with the neutral model. Unless more evidence is available to refute it, the relationship between native and alien species richness should be considered a result of a neutral processes due to constraints on the number of individuals in the community.

Small‐scale variability as a mechanism for large‐scale stability in mountain grasslands

. Data from a 7-yr permanent plot study of grassland dynamics were used to address the relationship between processes at two levels of resolution (3.3 cm x 3.3 cm, 50 cm x 50 cm). Grasslands mown and manured in the traditional way in the Krkonose Mts. (Riesengebirge) were used as a model system. Spatial dynamics at the finest scale were very high, as demonstrated by turnover of individual species in 3.3 cm x 3.3 cm subplots and year-to-year transition matrices of the same subplots. The direction of these dynamics was not correlated with grassland treatment, although there was some correlation within years. An extrapolation of such year-to-year dynamics to larger time scales would result in big large-scale changes on the community level, and large shifts in species composition of the whole sward. However, dynamics at larger spatial or temporal scales were generally small. Some directional change occurred in manured plots, whereas little change occurred in unmanured plots. Large-scale dynamics were not correlated with small-scale dynamics in plots without manuring, but some correlation was detectable in manured plots. There are probably several processes that drive small-scale dynamics, such as non-linear interactions and environmental fluctuations. We argue that within certain limits these forces act on species composition so as to make small-scale dynamics non-directional. This results in both large-scale species diversity and apparent large-scale stability of these grasslands. However, if these forces are beyond these limits, the small-scale dynamics may become directional, resulting in rapid changes at larger spatial scales.

Within population genetic differentiation in traits affecting clonal growth: Festuca rubra in a mountain grassland

Festuca rubra , a clonal grass of mountain grasslands, possesses a considerable variability in traits related to spatial spreading (rhizome production, length and branching; tussock architecture). Since these traits highly influence the success of the species in a spatially heterogeneous system of grasslands, a combined field and growth chamber approach was adopted to determine the within‐population variation in these parameters. Clones were sampled in a mountain grassland (The Krkonoše Mts., Czech Republic); the environment (mean neighbour density) of individual clones varied highly. Before the clones were collected, shoot demography and tussock architecture within these clones were recorded in the field for four seasons. Their clone identity was determined using DNA RAPD. Vegetatively propagated plants from these clones were cultivated in a common garden experiment to demonstrate variation in tussock growth and architecture. Their response to change in red/far red light ratio was determined in the growth chamber.

Non-manipulative estimates of competition coefficients in a Montane grassland community

1. A method is given for estimating competition coefficients in multispecies grassland communities without manipulation, using the natural turnover of individuals at a fine spatial scale. The method requires a detailed map of the spatial distribution of plants documented at several points in time, and uses non-linear regression of the local density of the plants on their densities in small neighbourhoods at a previous time. 2. The method is tested by generating realizations of two spatiotemporal stochastic processes for which the true parameter values are known. It is shown that non-linear regression successfully recovers the major features of the competition matrices. 3. The method is applied to a montane grassland dominated by four species of grass, for which spatial data on four plots are available for an 11-year period, and for which results of manipulation experiments are also available. 4. The results show that competitive interactions between species are as strong as interactions within species. There are strong asymmetries in the competition coefficients of species pairs, but little sign of species specificity. Intransitivities of the interaction matrix are not evident. The competition coefficients obtained show a good measure of agreement with the results of manipulation experiments that have been carried out on the community. 5. It is argued that non-manipulative methods of the kind described here hold a useful key to understanding interactions in plant communities.

SPATIOTEMPORAL DYNAMICS IN MOUNTAIN GRASSLANDS: SPECIES AUTOCORRELATIONS IN SPACE AND TIME

Permanent plots with a fine scale recording system were used to trace the spatiotemporal process within two mountain grasslands in the Krkonoše Mts., Czech Republic. The analysis used autocorrelation over increasing lags in space and/or time. MoransI was used to measure the autocorrelation. There was a lot of variation between species both in spatial and temporal correlograms. The spatiotemporal pattern of species correlated well with the growth form of the species and the degree of its clonality. Clonally-growing species tended to have high clumping at distances of a few cells, whereas rosette species often did not show any clumping. The type of clonal growth (compact vs. long spacers) is well corrlated, with the temporal correlogram (species mobility). There is a relation between low mobility and high clumping at low distances. Attempts to explain the mechanisms of species coexistence in these grasslands should take into account the particular structure of the fine-scale dynamics of these communities of predominantly clonal plants. *** DIRECT SUPPORT *** A02DO006 00007

Physiological integration affects growth form and competitive ability in clonal plants

Clonal plants translocate resources through spacers between ramets. Translocation can be advantageous if a plant occurs in heterogeneous environments (‘division of labour’); however, because plants interact locally, any spatial arrangement of ramets generates some heterogeneity in light and nutrients even if there is no external heterogeneity. Thus the capacity of a clonal plant to exploit heterogeneous environment must operate in an environment where heterogeneity is partly shaped by the plant growth itself. Since most experiments use only simple systems of two connected ramets, plant-level effects of translocation are unknown. A spatially explicit simulation model of clonal plant growth, competition and translocation is used to identify whether different patterns of translocation have the potential to affect the growth form of the plant and its competitive ability. The results show that different arrangements of translocation sinks over the spacer system can completely alter clonal morphology. Both runners and clumpers can be generated using the same architectural rules by changing translocation only. The effect of translocation strongly interacts with the architectural rules of the plant growth: plants with ramets staying alive when a spacer is formed are much less sensitive to change in translocation than plants with ramets only at the tip. If translocation cost is low, translocating plants are in most cases better competitors than plants that do not translocate; the difference becomes stronger in more productive environments. Key traits that confer competitive ability are total number of ramet, and their fine-scale aggregation.

Spore establishment probability and the persistence of the fugitive invading moss, Orthodontium lineare : a spatial simulation model

Spore establishment probability and the persistence of the fugitive invading moss, Orthodontium lineare : a spatial simulation model

Which habitat parameters are most important for the persistence of a bryophyte species on patchy, temporary substrates?

The effects of changes in habitat parameters important for conservation (abundance and size of habitats and distance between them) on both short-lived fugitive and long-lived resident bryophyte species was studied. Persistence of a species in a system of temporary and patchily distributed habitats was simulated both for stable and fluctuating environments. The values of the parameters in the model were derived from estimates made in two colonist species, Orthodontium lineare and Ptilidium pulcherrimum. Persistence of a resident species is much more enhanced by variable environment that persistence of a fugitive species. Both species are also more often limited with regard to dispersal/establishment in variable environments, especially the resident species. Reducing the habitat parameters (decreased area, fewer localities/substrate patches and increased distance between them) generally results in lower persistence of the species. In variable environments, fugitive species are more sensitive than the resident species to reduction of any of the habitat parameters. Both fugitive and resident species are most sensitive to reduced habitat density (i.e. to increased distance between available patches). Increased isolation of habitat patches is therefore concluded to be the most serious effect of habitat fragmentation.

POPULATION VIABILITY AND REINTRODUCTION STRATEGIES: A SPATIALLY EXPLICIT LANDSCAPE-LEVEL APPROACH

The effect of habitat destruction on the likelihood of species survival is often estimated based on the assumption that colonization and extinctions are in balance. This assumption is not sustainable in species where the dynamics of colonization and extinctions is slow in relation to landscape changes, such as in most plants. Here we use an alternative approach, a realistic, dynamic landscape-level model that does not rely on this assumption. It enables estimation of the effect of habitat destruction using field data on the biology of a species and on real landscape structure. Because our approach relies on direct comparisons of changes in population size and survival probability due to habitat changes, it can be easily extended to other conservation questions, such as assessing the effects of events causing the extinction of populations but allowing for re- colonization, or identifying optimal reintroduction strategies. We applied this method to a perennial herb, Succisa pratensis, that is a typical grassland species. We combined detailed demographic data with information on the spatial distribution of suitable habitats to model species dynamics in the landscape under different scenarios. The results show that habitat destruction alone has little effect on regional survival. How- ever, the effect of habitat destruction increases when combined with factors causing ex- tinctions of the existing populations that are expected to play a significant role in the study system. Our results further show that an optimal reintroduction strategy at the landscape level depends on the number of available seeds. The approach presented here was designed for studying systems where species colo- nization-extinction dynamics is slow compared with landscape changes. Such time lags and nonequilibrium dynamics have been suggested to be important features of many eco- systems and life forms, and this approach is thus likely to be useful for a wide range of future studies. The approach also allows the estimation of short-term effects of habitat destruction, i.e., situations that are nonequilibrium by definition. This is never possible with equilibrium models, giving the model a wide applicability for all types of organisms.