Interspecific competition in metapopulations

The assumptions and predictions of metapopulation models for competing species are discussed in relation to empirical studies of colonization and extinction in metapopulations. In three species of Daphnia in rockpools, interspecific competition increased local extinction rates, while no effects on colonization rates were detected. Distributional patterns were consistent with several predictions of the competition model; for example, the number of species on an island increased with the number of pools and the proportion of pools occupied by each species decreased with increasing species number. It is concluded that interspecific competition is important for the distributional dynamics of Daphnia species in rockpools, but the question whether the coexistence of these species depends on metapopulation dynamics is still unresolved. Other studies on the effects of interspecific competition on colonization and extinction rates are discussed.     

The origin of animal phyla and the new phylum Procoelomata

BergstrÖm, Jan 1989 07 15: The origin of animal phyla and the new phylum Procoelomata. Lethaia , Vol. 22, pp. 259–269. Oslo. ISSN 0024–1164.
A model of metazoan evolution presented previously (BergstrÖm 1986 in Zoologica Scripta 15 ) explains deuterostomian characters as derived from protostomian ones through loosening of the constraints in the spiralian type of morphogenesis. This fits phylogenies derived from studies of molecular sequences. The model helps explain (1) the well-known mixture of proto- and deuterostomian features in several groups; (2) the difficulties in making a phylogeny based on comparative anatomy, and (3) the fossil explosion in the Cambrian. Since protostomian features such as a ciliated locomotory sole and a pelagic larva with ciliary bands are widely distributed in branches of the phylogenetic tree, they must have been present in the stem of the tree. Most probably the stem forms were pseudosegmented, which helps explain how segmentation, oligomery and non-segmentation could evolve repeatedly in derived groups. Origination of new phyla involved macroevolutionary changes primarily in the mode of feeding and locomotion. The stem phylum, from which most other phyla appear to have been derived directly, is here named the Procoelomata. Machaeridian-type animals are referred to it. The Ediacaran-type Precambrian fossils Cannot be placed in the metazoan evolutionary tree. * Biochemical evolution, Cambrian fossil explosion, Deuterostomia, eukaryote evolution, Machaeridia. macroevolution, Precambrian fossils. Procoelomata, Protostomia .     

The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China

New specimens of Paucipodia inermis Chen, Zhou & Ramsköld, 1995, are described from the Lower Cambrian Chengjiang Lagerstätte in Haikou, Kunming. Details not previously seen in the Chengjiang material appear to be caused by early diagenetic processes. Some features not previously observed in Palaeozoic lobopodians include details of the dermomuscular sac, body cavities, contents of the gut, possible paired ventral nerve ganglia, and a rasping or biting apparatus with teeth. The latter implies a fundamental difference from onychophorans and rules out an ancestral position for Palaeozoic lobopodians. The supposed tail is shown to be the head, and it is shown that this animal possessed nine pairs of lobopods rather than six, as originally stated. The family Paucipodiidae n. fam. is introduced.     

Does the tsetse parturition rhythm have a circadian basis?

Abstract. Under an LD 12:12h photoregime at constant temperature, parturition in Glossina morsitans centralis is a gated event occurring late in the afternoon. When flies are switched to continuous light the rhythm quickly dampens, but its persistence for at least two 24-h cycles beyond the final scotophase suggests the rhythm has a circadian basis. A weak rhythm appears after 7 days of continuous light, perhaps in response to the daily disturbance caused by feeding. Return of the flies to LD 12:12h restores the rhythm after exposure to a single scotophase.     

The immigration of spiders (Araneida) into a new polder

Abstract. (1) The immigration of spiders into the Lauwerszeepolder (constructed in 1969) was studied during four years in four areas. The spider fauna was sampled weekly and 65 000 specimens of eighty-four species were caught using strip traps, window traps, simple pitfall traps and fences.
(2) About sixty species from all kinds of habitats were caught rarely; nineteen species were caught in numbers that suggest that they had established populations in the study areas.
(3) The four successful pioneer species, plus two marsh-dwelling species, were most abundant during 1969 and 1970. The 1971 catch was dominated by species from saline habitats; they became less abundant in 1972 when species from non-saline habitats increased. Only halotolerant species established populations.
(4) No evidence was found that dispersing spiders select certain areas. The differences between the spider fauna of the differennt study areas arise from degrees of success in colonization by immigrants. Pioneer species are least influenced by the abiotic environment.
(5) A greater aeronautic dispersal power is found in species inhabiting unstable habitats than in species from stable habitats.
(6) Aeronautic activity in adult linyphiids (s.l.) is not restricted to a special season, but is related to definite phases of the phenology.
(7) Male and female erigonids are equally active in aeronautic behaviour. In erigonids and linyphiids intense aeronautic activity coincides with great ground activity; on the ground males are more active than females.     

Updating rDNA Restriction Enzyme Maps of Tetrahymena Reveals Four New Intron-Containing Species1

The extrachromosomal rDNA molecules from a number of Tetrahymena strains wered racterized by restriction enzyme mapping using three different restriction enzymes combined with gel blotting and hybridization analysis. Strains from four out of six recently described species were found to contain an intron in the 26s rRNA coding region. The evolutionary relationship among the species of the T. pyriformis complex was examined on the basis of the rDNA maps with emphasis on similarities between two of the new species and the widely studied T. thermophila and T. pigmentosa. Examination of a large number of T. pigmentosa strains showed this species to exhibit an unusual polymorphism with respect to its rDNA. It is suggested that recombinational cross-over events play a role in the formation of new rDNA alleles in this species.     

Predation risk affects diapause induction in the spider mite Tetranychus urticae

Whenever diapause induction triggers movement into another microhabitat or the development of protective morphological structures, this may also alter predation risk. If the risk of being eaten is lower in the diapause phase, then there may be selection favouring diapause induction in response to predators or their cues. In this article, we studied the effect of the predatory mite Typhlodromus pyri on diapause induction in the spider mite Tetranychus urticae. We used a Greek strain because under long-night photoperiods and low temperature only part of the population enters diapause, thereby leaving room for the impact of another factor. In spider mite groups under predation, the percentage diapause induction increased whenever night-lengths were such that diapause was induced (13–16 h of night). Given this diapause induction in response to predation risk, the question arises whether entering diapause helps spider mites to escape from predation and contribute more offspring to the spring generation next year.     

Artificial mixing prevents nuisance blooms of the cyanobacterium Microcystis in Lake Nieuwe Meer, the Netherlands

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m² was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake.
2. The phytoplankton shifted from a cyanobacteria-dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non-buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy.
3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m^–2) increased in the artificially mixed lake.
4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification.
5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.     

Seasonal variation in growth of Mastomys natalensis (Rodentia: Muridae) in Morogoro, Tanzania

The paper describes seasonal growth variation in a free living population of Mastomys natalensis (Smith, 1834) from Morogoro, Tanzania. Growth patterns are associated with rainfall patterns. Young animals enter a growth stop at the end of the dry season (October-November) and growth is only resumed after the first heavy rains. Reproduction is more related to size than to age. If the heavy rains come early in the rainy season, animals start reproducing in the beginning of the year. The resulting young reach their maximal size without growth stop and start reproducing immediately. Possible causes for the growth stop are discussed. Growth phenomena are thought to explain at least partially the relation between rainfall and reproduction in Mastomys .

Résumé

Cet article décrit la variation de croissance saisonnière d'une population de Mastomys natalensis (Smith, 1834) vivant en libertéà Morogoro, en Tanzanie. Les graphiques de croissance sont associés à ceux des chutes de pluie. Les jeunes animaux connaissent un arrêt de la croissance à la fin de la saison sèche (octobre-novembre) et la croissance ne reprend qu'après les premières fortes pluies. La reproduction est plus liée à la taille qu'à l'âge. Si les fortes pluies surviennent tôt en saison des pluies, les animaux commencent à se reproduire au début de l'année. Les jeunes atteignent alors leur taille maximale sans avoir connu d'arrêt de croissance et commencent à se reproduire immédiatement. On pense que les phénomènes de croissance expliquent au moins partiellement la relation entre les chutes de pluie et la reproduction chez Mastomys .     

Trilobite taphonomy and ecology in Upper Ordovician carbonate buildups in Dalarna, Sweden

A major ecological grouping of the trilobites in the Upper Ordovician Boda Limestone buildups in Dalarna is outlined, based primarily on the modes of occurrences in the Jutjärn quarry. The so-called pockets are analysed with regard to their trilobite contents and distribution within a carbonate buildup. The pockets are of two general types. One consists of narrow opened fissures along bedding planes. The matrix of these may contain either of the peculiar blind Isocolus sjoegreni or Ityophorus undulatus , both of which appear to have been cavernous species, possibly belonging to the Bathyuracea (or Leiostegiacea). In addition, small proetids used the fissures for moulting. The other kind of pocket is a cavity largely filled with sediment from the surface of the buildup. The size is considerable, in rare cases up to many tens of cubic metres. Although the buildups housed about 90 trilobite species, each of these pockets is dominated by one or two out of seven large species. Of these, six are illaenids, the seventh the illaenid-like cheirurid Holotrachelus punctillosus. Usually only pygidia and cranidia are found. These species may have lived gregariously around the rims of the cavities. The remaining species, about 85% of the trilobite fauna, appear to have lived on the surface of the buildups. The largest group is the proetids with some 20 species (in addition to those known from pockets). Cheirurinids and lichids contribute with some 15 species each, and there are six odontopleurids. The distinctness of the 'community' is indicated by the circumstance that only three species are shared with contemporaneous level-bottom faunas in Sweden.