Florian Jeltsch

Ecological buffering mechanisms in savannas: A unifying theory of long-term tree-grass coexistence

Despite the large spatial extent and the obvious importance of the savanna biome, not to mention several decades of savanna research, the origin, age, nature, and dynamics of savannas are not well understood. Basically, the question surrounding the presence or existence of savannas focuses on the long-term coexistence of the dominating life forms – trees and grasses. How do these two very different components coexist, without one of them dominating the other, and what mechanisms determine the proportion of each? Earlier equilibrium concepts have recently been replaced by non-equilibrium concepts, and the current view is that tree-grass interactions in savannas cannot be predicted by a simple model. Instead, many interacting factors operating at various spatial and temporal scales contribute to creating and maintaining savanna physiognomy. In this paper we analyse a number of studies from savannas in different parts of the world and discuss whether a general pattern can be perceived behind the numerous factors influencing the presence of savannas systems. On the basis of this analysis we propose a new unifying concept of savanna existence, i.e., the concept of ecological buffering mechanisms. In contrast to previous approaches to explain tree-grass coexistence in savannas, the concept of buffering mechanisms does not focus on equilibria or non-equilibria, steady states of the system or domains of attraction. Instead, in the concept of ecological buffering mechanisms we suggest that it is much more useful to focus on the boundaries of savanna existence itself and to investigate the mechanisms that allow a savanna to persist in critical situations where this system is driven to its boundaries, e.g., pure grasslands or tropical forests. The concept of ecological buffering mechanisms integrates both earlier concepts of ecological theory and general ideas on savanna dynamics as well as specific studies of savannas in different parts of the world.

Tree Spacing and Coexistence in Semiarid Savannas

1 In the debate on the stability of savanna vegetation, spatial processes are often neglected. A spatial simulation model based on a cellular automata approach was constructed to identify the factors and processes crucial to the coexistence of trees and grass, and their effects on the spatial arrangement of trees in arid and semiarid savannas. 2 The simulation shows that the traditional key determinants of savannas rain, fire and grazing generate and sustain a coexistence of trees and grasses only under specific conditions. 3 An increase in the rainfall (improved tree establishment) or in grazing (reduced competition from grass), led to an increase in the woody component in the model. Where this trend was reversed by occasional fires, the simulation indicated that trees would be patchily distributed in thickets that excluded fire. 4 For an intermediate range of fire, grazing and rainfall variables this strongly clumped distribution pattern of trees represented a stable tree-grass mixture for more than 20 000 simulated years. 5 The hypothesis is formulated that factors or processes other than competition for moisture, herbivory and fire are needed in addition to induce a long-term persistence of scattered trees. 6 By exploring the long-term and spatial consequences of altering the variables that were thought to be key determinants of savanna vegetation, this spatiotemporal model provides a novel insight into the understanding of savanna dynamics.

Pattern-oriented modelling in population ecology

Abstract Ecological modelling should take its orientation more from real patterns observed in nature, than has been the case up to now, to overcome the deficiencies of the present strategies. Firstly, the orientation towards patterns provides guidelines about the manner and extent of the aggregation of biological information in the model. Modelling thereby loses much of its arbitrariness; secondly, pattern-oriented models are not ‘scale-free’, i.e. they relate explicitly to spatial and temporal scales; and finally, they produce comparative predictions which are better suited for testing than the predictions achieved by models, which are only either complex or generalizing. To demonstrate the strategy of pattern-oriented modelling and its advantages, three examples from population ecology are presented: (1) In a model concerning density dependence and individual variability, the orientation towards a pattern in weight distributions requires explicit inclusion of the within-generation time scale in the model. (2) Orientation towards a pattern in the dispersal capabilities of small organisms leads to a metapopulation model, where environmental correlations about certain distances are taken into account. The survival of the metapopulation depends mainly on the ratio of the correlation distance to the range of dispersal of the organisms. (3) The wave-like pattern of the spread of rabies is reproduced by an extremely simple one-dimensional model, which is based on an extended cellular automaton approach. From this basic model, a description on finer spatial and temporal scales can be developed with the aim of constructing a model which allows for the investigation of spatial barriers against the spread of rabies. A comparison of the three example models shows that the main features of pattern-oriented models are generic. In producing comparative predictions which are related explicitly to scales, pattern-oriented modelling seems to be a strategy well suited to the ‘scaling up’ from population ecology to community and ecosystem ecology.

Analysing shrub encroachment in the southern Kalahari: a grid-based modelling approach

Shrub encroachment is reducing the carrying capacity of arid grasslands in southern Africa for cattle. Although shrub-encroachment is known to occur as a result of the selective overgrazing of grasses by cattle, the interactions between rainfall and grazing are not well understood. Both the quantity and sequence of rainfall events are likely to influence the growth rates and competitive abilities of shrubs and grasses. Shrub encroachment is a slow process and animals are stocked at low densities in arid regions. Thus, field experiments for determining stocking rates that avoid shrub encroachment under various rainfall scenarios are almost impossible to replicate. We used a grid-based simulation model to investigate the shrub-grass dynamics of the southern Kalahari under various realistic rainfall scenarios and stocking rates of domestic livestock. The simulation experiments addressed the following questions: Does simulated cattle grazing lead to shrub encroachment? Over what time scale does the process take place? Are the dynamics of vegetation-change continuous in relation to grazing pressure or do they show a threshold behaviour? Simulation results indicated that the answers to all three questions depended on the quantity and sequence of rainfall. Simulated cattle grazing led to shrub encroachment under all rainfall scenarios, once stocking rates exceeded a threshold determined by long-term mean annual rainfall. The stocking rate threshold for shrub encroachment was less distinct (i.e. shrub cover in different simulation experiments had a higher coefficient of variation) under xeric than mesic climatic scenarios. This is because either competition from the herbaceous layer or rain may limit shrub establishment. In relatively mesic scenarios, where shrub encroachment was limited mainly by grass competition, the grazing of grasses beyond a certain threshold led to an almost deterministic increase in shrub cover. However, under xeric climates, where rainfall was lower and more stochastic, the rate of shrub encroachment in response to a given intensity of grazing became less predictable. The most significant finding of the simulation experiments was that, although the stocking rates currently recommended by pasture scientists are unlikely to lead to shrub encroachment within 20 years, they have a high probability of bringing about shrub encroachment within a century. These findings applied to most of the rainfall scenarios found in the southern Kalahari and are therefore of particular interest to rangeland policy makers in this semi-arid region.

Simulated pattern formation around artificial waterholes in the semi‐arid Kalahari

. Sinking boreholes to tap groundwater supplies facilitated expansion of all-year round livestock production into the semi-arid Kalahari. Increased grazing and trampling pressure around the boreholes often caused vegetation changes and range degradation. The long-term influences of cattle grazing on vegetation pattern around watering points in the southern part of the semi-arid Kalahari are investigated using a grid-based simulation model. Shrub-grass dynamics are modelled for two regimes with high and low rainfall and under various stocking rates. Results indicate the formation of distinct vegetation zones (‘piosphere’ zones) at the high rainfall site. Under all tested stocking rates distinct zones of bare soil, woody shrubs and a mixed grass-shrub savanna develop. The piosphere zones expand outwards at a rate correlated with the grazing pressure. At the lower-rainfall site zone development is limited and influenced by rainfall. Under abnormally high stocking rates an increase in shrub cover occurs within 50 yr under the low rainfall regime, leading to less distinct zones than under the high rainfall scenario. Modelling results suggest that the recovery potential of shrub-encroached piosphere zones after withdrawal of cattle is negligible in a time span of 100 yr.

Detecting process from snapshot pattern: lessons from tree spacing in the southern Kalahari

The spatial distribution of plants is often thought to be an indicator of underlying biotic and abiotic processes. However, there are relatively few examples of spatial patterns being analysed to detect an underlying ecological process. Using the spacing patterns being analysed to detect an underlying ecological process. Using the spacing of savanna trees in the southern Kalahari as an example, we applied methods of computer simulation modelling and point pattern analysis in an evaluation of their potential for identifying relevant pattern generating processes from snapshot pattern. We compared real tree patterns from the southern Kalahari, derived from aerial photographs, with patterns produced from computer simulation experiments in an investigation of the following questions: does the present pattern of tree distributions allow us to characterize (1) the relative importance of the major driving forces (e.g., competition for moisture, grass fire, herbivory), (2) the spatial dimensions and structures of the underlying processes, and (3) the actual dynamic status of the ecological system (a phase of decline, increase or constancy with respect to tree abundance)? The simulation experiments are based on a well established, spatially explicit, grid-based model that simulates the vegetation dynamics of the major life forms under a realistic rainfall scenario of the southern Kalahari and under the impact of grass fires, herbivory and the formation of localized clumps with increased tree seed availability. For a realistic range of parameters the simulation model produces long-term coexistence of trees and grasses with tree densities that correspond with long-term coexistence of trees and grasses with tree densities that correspond with densities observed in the field. Both real tree distributions derived from acrial photographs and tree pattern produced by the model are characterized by a tendency towards even spacing at small scales, clumping at intermediate scales and randomness or clumping at large scales. However, increasing the spatio-temporal correlation in the formation of seed patches in the model caused an increase in the tendency towards clumping in the tree distribution whereas an increase in seed patch numbers led to a decrease in clumping. Within single simulation runs the tree pattern could change in response to the variable rainfall sequences and the corresponding differences in grass fire frequency: periods of slightly increasing tree numbers caused by higher precipitation were characterized by an increase in tree clumping whereas periods of slightly decreasing tree numbers showed a tendency towards random or even tree spacing. Simulating the transition of an open savanna to a savanna woodland showed that the tree pattern in the transitional phase can be diagnostic of the underlying process: If the transition was caused by improved moisture conditions the transitional phase was characterized by increased clumping in the tree pattern. In contrast, a transition caused by an increase in the number of localized tree seed patches led to a characteristic even spacing of trees. Even though the simulated savanna clearly showed non-equilibrium dynamics, simulation results indicate that the tree population in the simulated area of the southern Kalahari is in a state of long-term tree-grass coexistence with the persisting structure of an open savanna system.

Pattern formation triggered by rare events: lessons from the spread of rabies

Understanding of large–scale spatial pattern formation is a key to successful management in ecology and epidemiology. Neighbourhood interactions between local units are known to contribute to large–scale patterns, but how much do they contribute and what is the role of regional interactions caused by long–distance processes? How much long–distance dispersal do we need to explain the patterns that we observe in nature? There seems to be no way to answer these questions empirically. Therefore, we present a modelling approach that is a combination of a grid–based model describing local interactions and an individual–based model describing dispersal. Applying our approach to the spread of rabies, we show that in addition to local rabies dynamics, one long–distance infection per 14000km2 per year is sufficient to reproduce the wave–like spread of this disease. We conclude that even rare ecological events that couple local dynamics on a regional scale may have profound impacts on large–scale patterns and, in turn, dynamics. Furthermore, the following results emerge: (i) Both neighbourhood infection and long–distance infection are needed to generate the wave–like dispersal pattern of rabies; (ii) randomly walking rabid foxes are not sufficient to generate the wave pattern; and (iii) on a scale of less than 100 km times 100 km, temporal oscillations emerge that are independent from long–distance dispersal.

Does red noise increase or decrease extinction risk? Single extreme events versus series of unfavorable conditions.

Recent theoretical studies have shown contrasting effects of temporal correlation of environmental fluctuations (red noise) on the risk of population extinction. It is still debated whether and under which conditions red noise increases or decreases extinction risk compared with uncorrelated (white) noise. Here, we explain the opposing effects by introducing two features of red noise time series. On the one hand, positive autocorrelation increases the probability of series of poor environmental conditions, implying increasing extinction risk. On the other hand, for a given time period, the probability of at least one extremely bad year (“catastrophe”) is reduced compared with white noise, implying decreasing extinction risk. Which of these two features determines extinction risk depends on the strength of environmental fluctuations and the sensitivity of population dynamics to these fluctuations. If extreme (catastrophic) events can occur (strong noise) or sensitivity is high (overcompensatory density dependence), then temporal correlation decreases extinction risk; otherwise, it increases it. Thus, our results provide a simple explanation for the contrasting previous findings and are a crucial step toward a general understanding of the effect of noise color on extinction risk.

Phytomass and fire occurrence along forest–savanna transects in the Comoé National Park, Ivory Coast

In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest-savanna transects in the Comoe National Park, the change in the amount and composition of non-woody phytomass was investigated from savanna to forest interior. This was correlated with the cover of vegetation strata above, soil depth, and the occurrence of seasonal surface fires. Phytomass mainly consisted of leaf litter in the forests (about 400 g m -2 at the end of the rainy season, and about 600 g m -2 at the end of the dry season) and of grasses in the savanna (about 900 g m -2 ). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.

Integrating movement ecology with biodiversity research - exploring new avenues to address spatiotemporal biodiversity dynamics

Movement of organisms is one of the key mechanisms shaping biodiversity, e.g. the distribution of genes, individuals and species in space and time. Recent technological and conceptual advances have improved our ability to assess the causes and consequences of individual movement, and led to the emergence of the new field of ‘movement ecology’. Here, we outline how movement ecology can contribute to the broad field of biodiversity research, i.e. the study of processes and patterns of life among and across different scales, from genes to ecosystems, and we propose a conceptual framework linking these hitherto largely separated fields of research. Our framework builds on the concept of movement ecology for individuals, and demonstrates its importance for linking individual organismal movement with biodiversity. First, organismal movements can provide ‘mobile links’ between habitats or ecosystems, thereby connecting resources, genes, and processes among otherwise separate locations. Understanding these mobile links and their impact on biodiversity will be facilitated by movement ecology, because mobile links can be created by different modes of movement (i.e., foraging, dispersal, migration) that relate to different spatiotemporal scales and have differential effects on biodiversity. Second, organismal movements can also mediate coexistence in communities, through ‘equalizing’ and ‘stabilizing’ mechanisms. This novel integrated framework provides a conceptual starting point for a better understanding of biodiversity dynamics in light of individual movement and space-use behavior across spatiotemporal scales. By illustrating this framework with examples, we argue that the integration of movement ecology and biodiversity research will also enhance our ability to conserve diversity at the genetic, species, and ecosystem levels.