Uta Berger

A new approach to spatially explicit modelling of forest dynamics: spacing, ageing and neighbourhood competition of mangrove trees

This paper presents a new approach to spatially explicit modelling that enables the influence of neighbourhood effects on the dynamics of forests and plant communities to be analysed. We refer to this approach as ‘field of neighbourhood’ (FON). It combines the ‘neighbourhood philosophy’ of grid-based models with the description of individual spacing in the ‘zone of influence’ (ZOI) approach. The novel feature of FON is that modelling of local competition between neighbouring trees is based on the notion of a field of neighbourhood exerted by each tree. This field is defined only on the ZOI of a tree and depends on the distance to the stemming point. For the demonstration of FONs power, a simulation model (KiWi) was implemented that focuses on the dynamic of mangrove forests. The realistic self-thinning behaviour of modelled stands of Avicennia germinans and Rhizophora mangle confirms the suitability of the FON approach for the description of intra- and inter-specific competition. In KiWi, mortality is modelled in terms of a ‘memory function’, i.e. the yearly stem increment of each tree is stored in its ‘memory’ over a certain time period and determines — as a sign of vitality — tree mortality. The results of KiWi demonstrate that this description is sufficient to keep the maximum age of the trees within a reasonable limit. The model thus manages without a description of individual tree age. This is an important feature considering the fact that a direct relationship between tree age and mortality is questioned and there is no established method as yet for determining the age of mangrove trees.

Fish communities in central Amazonian white- and blackwater floodplains.

In Amazonian floodplains, the flood cycle of the river is becoming the dominant seasonal factor, and fish communities are found to fluctuate greatly over the year. During inundation, fish migrate into floodplain forests to feed on fruits and seeds, in an area more than 300 000 km2 in size. To document patterns of species diversity, distribution, abundance and temporal dynamics and in order to describe the ecological importance of the inundated forest, floodplain fish were captured using variously sized gill nets in white and black water areas inside and outside the floodplain forests during low, rising, high and falling water level in 1990 and 1991. Dominance varies to some extent in white water between floodplain forest (0.06) and open water (0.11) while it is unchanged in black water (0.04). Black water fish communities were more diverse. Most abundant among white water fish were Liposarcus pardalis, Pygocentrus nattereri, and Pellona flavipinnis, for example, or Plagioscion squamsissimus, Serrasalmus rhombeus, and Serrasalmus manueli in black water. Among the most abundant white water fish, Colossoma macropomum, Mylossoma duriventre and Osteoglossum bicirrhosum occurred almost exclusively in inundated forests. Of the black water species there were a large number of species which were captured only in inundated forest, such as Geophagus cf. altifrons, Hoplias malabaricus, Osteoglossum bicirrhosum and Uaru amphiacanthoides. Catches varied with sample site, water level and direction of water level change. The average CPUE in white and black water was 190 and 41 g fish m−2 and day, respectively, with maximum yields at low water and minimum yields at high water. Comparing rising and falling water levels, a significantly higher quantity of fishes was captured at falling water level. In black water, fish catches from the floodplain forest exceeded the open water catch by 183 to 550%, depending on season. Differences in respect of white water are smaller (106–281%). Fish communities in the area under investigation seem to be stochastically assembled, with significant differences between white and black water only. Many fishes move into the floodplain forest not only to feed but probably also for other reasons – to seek shelter, for example.

Growth Strategies of Tropical Tree Species: Disentangling Light and Size Effects

An understanding of the drivers of tree growth at the species level is required to predict likely changes of carbon stocks and biodiversity when environmental conditions change. Especially in species-rich tropical forests, it is largely unknown how species differ in their response of growth to resource availability and individual size. We use a hierarchical Bayesian approach to quantify the impact of light availability and tree diameter on growth of 274 woody species in a 50-ha long-term forest census plot in Barro Colorado Island, Panama. Light reaching each individual tree was estimated from yearly vertical censuses of canopy density. The hierarchical Bayesian approach allowed accounting for different sources of error, such as negative growth observations, and including rare species correctly weighted by their abundance. All species grew faster at higher light. Exponents of a power function relating growth to light were mostly between 0 and 1. This indicates that nearly all species exhibit a decelerating increase of growth with light. In contrast, estimated growth rates at standardized conditions (5 cm dbh, 5% light) varied over a 9-fold range and reflect strong growth-strategy differentiation between the species. As a consequence, growth rankings of the species at low (2%) and high light (20%) were highly correlated. Rare species tended to grow faster and showed a greater sensitivity to light than abundant species. Overall, tree size was less important for growth than light and about half the species were predicted to grow faster in diameter when bigger or smaller, respectively. Together light availability and tree diameter only explained on average 12% of the variation in growth rates. Thus, other factors such as soil characteristics, herbivory, or pathogens may contribute considerably to shaping tree growth in the tropics.

Mangrove vegetation in Amazonia: a review of studies from the coast of Pará and Maranhão States, north Brazil

O presente estudo apresenta uma compilacao da literatura sobre a vegetacao dos manguezais da costa norte do Brasil, apresentando uma sintese do conhecimento e listando a literatura disponivel. O estudo se concentra na costa dos estados do Para e Maranhao que formam um cinturao continuo de manguezais . Foram contabilizadas seis especies arboreas exclusivas de mangue e varias outras associadas. A altura e o diâmetro das arvores de mangue variam em funcao de parâmetros abioticos locais. As variacoes sazonais do regime de chuvas e da salinidade afetam a fenologia das especies e a producao de serapilheira. A populacao costeira utiliza a flora do manguezal para diferentes fins (ex: combustivel, medicinal, construcao rural). O aumento da ocupacao costeira inicia um processo de impacto para as florestas de mangue e a disponibilidade de seus recursos.

Making predictions in a changing world: The benefits of individual-based ecology

Ecologists urgently need a better ability to predict how environmental change affects biodiversity. We examine individual-based ecology (IBE), a research paradigm that promises better a predictive ability by using individual-based models (IBMs) to represent ecological dynamics as arising from how individuals interact with their environment and with each other. A key advantage of IBMs is that the basis for predictions—fitness maximization by individual organisms—is more general and reliable than the empirical relationships that other models depend on. Case studies illustrate the usefulness and predictive success of long-term IBE programs. The pioneering programs had three phases: conceptualization, implementation, and diversification. Continued validation of models runs throughout these phases. The breakthroughs that make IBE more productive include standards for describing and validating IBMs, improved and standardized theory for individual traits and behavior, software tools, and generalized instead of system-specific IBMs. We provide guidelines for pursuing IBE and a vision for future IBE research.

Asymmetric competition as a natural outcome of neighbour interactions among plants: results from the field-of-neighbourhood modelling approach

Numerous attempts have been made to infer the mode of competition from size or biomass distributions of plant cohorts. However, since the relationship between mode of competition and size distributions may be obscured by a variety of factors such as spatial configuration, density or resource level, empirical investigations often produce ambiguous results. Likewise, the findings of theoretical analyses of asymmetric competition are equivocal. In this paper, we analyse the mode of competition in an individual-based model which is based on the new field-of-neighbourhood approach. In this approach, plants have a zone of influence that determines the distance up to which neighbours are influenced. Additionally, a superimposed field within the zone of influence defines phenomenologically the strength of influence of an individual on neighbouring plants. We investigated competition at both individual and population level and characterised the influence of density and of the shape of the field-of-neighbourhood on occurrence and degree of competitive asymmetry. After finding asymmetric competition emerging in all scenarios, we argue that asymmetric competition is a natural consequence of local competition among neighbouring plants.

Evaluation of movement parameters in insects – bias and robustness with regard to resight numbers

Abstract Because of the low number of observation points per animal attainable in insect movement studies, linear parameters are frequently used to quantify the data. These linear parameters are evaluated, as well as several home range estimators, by means of an empirical study of the bush cricket Phaneroptera falcata (Insecta: Ensifera) and a Monte Carlo simulation model. The examination of differences between complete and artificially reduced data sets, as well as between the “real” (i.e. simulated) data set and the data recorded by a simulated observer, allows us to quantify robustness and bias of the evaluated parameters. We show that nearly all tested methods are strongly influenced by the resight number of the investigated individuals. Hence, those parameters should be used cautiously in studies with few resights, i.e. in insect studies as well as studies on vertebrates. Results of earlier studies should be reconsidered and some comparisons between different studies are questionable. Estimates of home range using a 95% ellipse with fewer than five locations are extremely unreliable (overestimation). The minimum convex polygon leads to a clear underestimation. The robustness of both parameters is low. Among the home range parameters the kernel method is most robust, but it leads to an overestimating bias. The harmonic mean method is the only home range parameter whose results are comparable to the area actually used. However, this method requires a minimum number of 11 observation points per individual. Evaluating the linear parameters, the mean daily movement and the total recorded movement are inappropriate for statistical analyses, because of their high sensitivity to the resight number. Maximum activity radius and dispersal range are much more robust. They are not as sensitive to sample size and deviate little from the “real” values of the parameters. However, this bias is statistically significant. The mean activity radius of an individual is the most useful linear parameter. This measure is very robust down to a sample size of four individual locations and compares well with the real parameter values.

Cyclic dynamics in simulated plant populations

Despite the general interest in nonlinear dynamics in animal populations, plant populations are supposed to show a stable equilibrium that is attributed to fundamental differences compared with animals. Some studies find more complex dynamics, but empirical studies usually are too short and most modelling studies ignore important spatial aspects of local competition and establishment. Therefore, we used a spatially explicit individual–based model of a hypothetical, non–clonal perennial to explore which mechanisms might generate complex dynamics, i.e. cycles. The model is based on the field–of–neighbourhood approach that describes local competition and establishment in a phenomenological manner. We found cyclic population dynamics for a wide spectrum of model variants, provided that mortality is determined by local competition and recruitment is virtually completely suppressed within the zone of influence of established plants. This destabilizing effect of local processes within plant populations might have wide–ranging implications for the understanding of plant community dynamics and coexistence.

Implications of mangrove dynamics for private land use in Bragança, North Brazil: a case study

This work analyses effects of recent variations in the tidal inundation frequency in a mangrove ecosystem in the Bragança peninsula, North Brazil, and its implications for land occupation and use. Field data, time series of remote sensing images and local legislation were analysed focusing on the potential socio-economic impact of a changing environmental setting due to a rise in relative sea level. In the investigated period (1972–1997), vegetation changes along the coastline indicate net losses of mangrove coverage. In the central part of the peninsula, a topographically higher herbaceous plain constituting part of a farm presents an active progression of mangrove forest into an area previously dominated by grasses and herbs. This area measured 8.8 km2 in 1972 but was gradually reduced to 5.6 km2 in 1997, while progressively replaced by a monospecific stand of the black mangrove,Avicennia germinans. A linear extrapolation indicates that the elevated plain may be completely covered by mangrove by 2035. Current Brazilian legislation prohibits the extraction of mangrove trees without an officially approved management plan. Thus, the usable area of the farm has suffered a reduction by ca. 36% over 25 yr and we predict that is could be entirely replaced by mangroves in the next 35 yr. In this case study, legislation and ecosystem characteristics are analysed and a management plan discussed which could represent income alternatives for affected resource users at the local and regional level.

Virtual biologists observe virtual grasshoppers: an assessment of different mobility parameters for the analysis of movement patterns

An individual-based model was developed to investigate the accuracy of three mobility indices commonly used in field studies: mean daily movement, maximal distance and mean activity radius. Although the model is based on a field study of the mobility of the grasshopper Oedipoda germanica (Saltatoria: Acrididae) in Thuringia (Germany), the results are valid more generally. We show that the observational interval is the key factor influencing the accuracy of all three indices. Moreover, there is an optimal observational interval that minimises the absolute error. The influence of individual mobility, recapture probability, accurate registration of the capture and recapture points and the effect of the observer on the movement of the animals, are also considered.