Seite 108 [108]
OCR
106 |V. The dynamics of the animal communities There is no easy answer to this guestion. All coenological studies so far, as we have seen, analysed the fauna, and the results can only conclude that different physiognomic units have different animal communities. We hardly know of studies that lasted for several years in the same biotope or oecus, and followed the composition of the same zoocoenosis. Ihe guestion, though, of whether a species combination first found remains unchanged during subseguent years, can only be answered by such studies. Knowledge from plant protection offers few clues, because a negative conclusion only means that a corrumpent was not a pest, which is far from implying that it is not present in the zoocoenosis. Precise plant protection censuses only exist for a few pests, and the information only indicates a forceful fluctuation in the densities of certain corrumpents (Schwerdtfeger, 1951). Due to theoretical speculations, we have to assume that characteristic species combinations are constant. It must be the case, because we cannot assume that a latent phase (Thalenhorst, 1951) of an animal would have disappeared from the area, and will colonise it again in an eruptive phase. More likely its density failed to reach the detection threshold. ‘The detection threshold is the relaxed state of a population dispersion, below which our current methods cannot detect is presence. We cannot exclude, though, that a species in the state of gradological latency, has indeed disappeared from a large area, and its presence is only “island-like”. The knowledge of catenae can provide certainty as to whether a member of a species combination, currently not detected, is still present. Species of concealed activity can be ascertained through members of its food chain. For example, the presence of Tetramesa spp. can be identified from the obstant Homoporus, or the root-living Pseudococcus from their encyrtid parasitoids. Change in the population size of species is incessant; the density, the number of individuals per unit of space is, consequently, in constant flux. At the start of every generation, density reaches a peak, followed by gradual decline, reaching its nadir when the semaphoront performs the last step of species continuation, egg laying. This intracyclic fluctuation can be repeated once or several times a year, depending on the tocogenetic features (Hennig, 1950) of the species. Over and above the intracyclic fluctuation, there is another movement of bigger amplitude, extending over years, in which generations with small population size alternate with those of very high population sizes. This hypercyclic fluctuation appears because of the direction of movement of the intracyclic fluctuations. If this increases in several generations, the population size of the species will noticeably grow, and the lower the pre-reproductive within-generation mortality, the steeper this increase will be. The conspicuous peaks of these hypercyclic waves are called gradations. Their study fits in to gradology - from the point of zoocoenology this is relevant because this mechanisms creates strong dominance of certain populations, or at least increases their density above the detection threshold.
