Robert Lingeman

Metapopulation dynamics of a persisting predator-prey system in the laboratory: time series analysis

The scarcity of experimental evidence for the persistence of predator–prey systems at the metapopulation level inspired us to develop a simple predator–prey experiment that could be used for testing several theoretical predictions concerning persistence and its causes. The experimental system used consisted of one or several islands with small bean plants, the phytophagous mite Tetranychus urticae and the predatory mite Phytoseiulus persimilis. In the first experiment, one large system was used consisting of 90 small bean plants, prey and predators. The system persisted for only 120 days. Second, a system was used consisting of eight islands with ten plants each where the islands were connected by bridges. Two replicate experiments showed persistence for at least 393 days. The difference between the first and the second experiments suggests that the longer persistence is caused by a limited migration between the eight islands. Despite efforts to start both replicates of the second experiment with similar initial conditions, the dynamics of both replicates varied substantially. In one replicate the prey and predator numbers showed a trend through time, whereas the numbers fluctuated around a fixed value in the other replicate. A time series analysis of the data of the prey and predators showed the presence of periodicity with a lag of 8.5 weeks in one replicate, whereas such cyclic behaviour was not found in the other replicate. The differences between the two replicates suggest that it is difficult to perform experiments where one replicate is perturbed and the other serves as an undisturbed control. We suggest using a longer time series, where a system is disturbed only during the second half of the experiment. The data from the first and second halves can subsequently be used to estimate the effect of the perturbation. The advantages and disadvantages of this method are discussed.

A characterization and description of the Maarsseveen Lake system

The Maarsseveen Lakes system, located near the City of Utrecht, The Netherlands, consists of two major bodies of water, a larger lake (Maarsseveen I) and a smaller lake (Maarsseveen II), interconnected by a maze of channels known as a ‘petgaten’ area. The hydrodynamics of the lake region coupled with this ‘petgaten’ area are responsible for the maintenance of the relatively oligomesotrophic character of Lake Maarsseveen I, as compared with the more eutrophic lake maarsseveen II. Both lakes are plagued by problems of summer anoxia, although this phenomenon is far more pronounced in the more eutrophic small lake. Atmospheric deposition is responsible for contributions of significant quantities of trace metals to both lakes, and the River Vecht acts as an additional source for Lake Maarsseveen II.

A demonstration of asynchronous local cycles in an acarine predator-prey system

Population fluctuations in a continuous predator-prey system consisting of the spider mite,Tetranychus urticae Koch and its predator,Typhlodromus occidentalis Nesbitt, were assessed for almost 2 years (ca. 50 prey generations), on six mini-orchards of young apple trees in a climatically controlled glasshouse. During the first half of the experiment the plants were either ‘connected’ to each other with dowel rods or ‘unconnected’ and separate. In the second half, the plants were all ‘connected’. Population densities of both prey and predator on the unconnected mini-orchards were always higher than on the connected ones. Within the mini-orchards (=local scale), prey and predator populations go through a pattern of large amplitude cycles which continued throughout the experimental period. At the regional scale (mean over all mini-orchards) two types of fluctuations were observed. Large amplitude fluctuations associated with synchrony of the local cycles and small amplitude fluctuations associated with periods when local population cycles are proceeding out of phase. Transition from synchronous to asynchronous cycles took place very fast (within a few weeks), suggesting a mechanism generating asynchrony and possibly also partial refuges, which in turn trigger stabilising mechanisms at a regional scale. Causes of the synchrony are unclear. They may relate to the application of a pesticide (pirimicarb) used to control aphids, but there may just as well be an indirect form of causation, e.g. whereby the factors that promote aphid outbreaks also promote spider-mite population growth and temporary escape from control by the predators. Despite the synchronising factors the predator-prey system persisted, without external inputs, at a fairly small spatial scale, the size of a small glasshouse. Assessing the spatial scale and heterogeneity whereby decoupling of local cycles is sufficient for the cycles to proceed out of phase, remains an important area of experimental research. The experiment reported here provides some first clues and may form a standard for forthcoming experiments to assess the mechanisms promoting asynchrony, persistence and possibly also stability.

Time series of physical, chemical and plankton parameters in Lake Maarsseveen I: 1980–1986

In 1983, an unexpected bloom of the cyanobacteriaMicrocystis aeruginosa was observed in Lake Maarsseveen I. It was supposed that this phenomenon might be an indication of accelerated eutrophication of the lake. However, data on physical, chemical and phytoplankton parameters, collected over the last 6 years do not support this contention. Phytoplankton total phosphate and physical characteristics did not change. Annual levels of dissolved nutrients such as silicate and nitrate were even observed to show significant decreases over the period of observation.

Local and global cycles in an acarine predator-prey system: a frequency domain analysis

Techniques of time series analysis are applied to the dynamics of the phytophagous miteTetranychus urticae Koch and its predatorTyphlodromus occidentalis Nesbitt in six experimental mini-orchards, sampled weekly during two years (van de Klashorst et al., 1992). Autocovariance and crosscovariance functions characterize local and global behaviour in the time domain. Spectral density function and cross amplitude spectrum provide important information of the systems behaviour in the frequency domain. Because of a strikingly different behaviour, the total record was cut into two distinct periods. During the first period, all local systems oscillate with a frequency of four cycles per year and all in phase, resulting in a strongly periodical global behaviour at a high mean level. Since the mean local power spectra coincide with the global power spectrum over a wide range of frequencies, it is concluded that the total system of six orchards is homogeneous. During the second period, all local power spectra are mutually different, more or less smoothed with no apparent peak pointing to a periodical component. The resulting global power spectrum is almost flat at a low mean level. The system is heterogeneous with a quasi-stable global behaviour.From the cross-amplitude spectrum it became clear that the dynamics of the two species within the orchards remained strongly coupled over a wide range of frequencies.Since the experimental circumstances had not been intentionally changed, the origin of the drastic change in behaviour could not be identified. If high rates of dispersal have been the synchronizer during the first period, it is not clear why asynchrony suddenly occured in the second period.A comparison of the overall record with the results of Nachmans (1987) stochastic simulation model suggests that the change could possibly be of stochastic origin.

Experiments on movement of DNA regions in Escherichia coli evaluated by computer simulation

During the cell cycle of Escherichia coli DNA is replicated and segregated over two prospective daughter cells. Nucleoids as a whole separate gradually in line with cell elongation, but sub-nucleoid DNA regions may behave differently, separating non-gradually. We tested the ability of three models to predict the outcome of a fluorescent in situ hybridisation (FISH) experiment. We did this by comparing computer-simulated data with experimental data. The first model predicts gradual separation in line with cell elongation. The second model predicts that origins stick together for some time after duplication before one copy jumps to the other side of the cell (non-gradual separation). The simulated data of these models are very similar, indicating that FISH is not a suitable method to distinguish between these two models. The third model predicts that origins may be anywhere within the nucleoid(s). We found that simulated data using the third model resemble the experimental data most. However, DNA regions are not randomly localised in the cell, although their localisation is fuzzy. We propose that movement of DNA regions is the result of a combination of factors. Nucleoid segregation (or the forces behind it) dictates the overall direction of movement. Other factors, of which we show that diffusion could be an important one, move DNA in other directions giving rise to non-gradual movement in individual cells and contributing to variation in intracellular position per cell length in a population of cells.

Diel and annual oxygen regimes in a small lake

An extensive ecosystem analysis program was conducted at the “Barlosche Kolk”, a small eutrophic lake in the eastern Netherlands, from September 1971 to September 1973. As part of this program, the oxygen concentration at different depths was determined continuously over several 24 hour periods. Although no well defined thermocline was observed, a marked oxygen stratification persisted during summer. Detailed pictures of the diel oxygen regime were constructed, showing strong dynamics especially during early autumn. Diel column-mean ranges were highly correlated with time of the year and diel patterns showed to be highly predictable.

Abundancy estimations of aDaphnia magna population in a small artificial pond

Practical difficulties in the application of correct statistical methods in field ecology are illustrated by a plankton abundancy estimation in a small pond. Random sampling was performed over a limited number of previously chosen sample places and times. A cylindric sampler was used, which caught the plankton within a whole water column (1 m). Relatively high numbers of samples were needed to achieve reliable estimates of population parameters, which was demonstrated by simultaneous sampling. The probability distribution of the number of daphnids per sample was shown to be approximately lognormal. However, the variances of the log-transformed data were not uniformly homogeneous, which reduces the applicability of familiar tests for parameter comparison. Arguments are presented to make comparisons of population abundancies through transformed data only.