Community and population-level responses to disturbance in a sandflat community

The responses of soft sediment infauna were investigated in an intertidal sandflat to determine patterns of recolonization and succession at the community and population level. Experimental disturbance plots, 1 m², were initiated in August and sampled for 4.5 months along with ambient sediments. Sediment grain-size was used as a general indicator of the physical state of the disturbance patches, and grain-size distributions among disturbance and ambient patches became similar after ∼2.5 months. Recolonization varied among the dominant infaunal taxa. Densities of infauna that were most abundant in the habitat, primarily syllid polychaetes, did not recover to ambient levels until 3-4 months after disturbance, when ambient densities were falling to winter lows. Multivariate analysis indicated that community recovery occurred by the end of the study period after 4.5 months. Although community structure recovered by the end of the study, the population structure of the dominant species Parapionosyllis longicirrata remained significantly different among ambient and disturbed patches. On all sampling dates except one, disturbance patches had a higher number of larger individuals than ambient sediments. Previous studies have shown that bedload transport of juveniles and adults, and other processes, can cause recolonization to be relatively rapid on intertidal sandflats. However, our results indicate that recovery times may be on the order of months at large disturbance sizes. Therefore, rapid responses may occur primarily in the case of small-scale (<1 m²) disturbance patches. Secondly, recovery at the community level does not necessarily mean that population-level characteristics of species comprising the community have recovered. Population-level differences may be longer lasting than indicated by community level indicators of recovery.     

Genetic divergence in nuclear genomes between populations of <Emphasis Type="Italic">Fagus crenata</Emphasis> along the Japan Sea and Pacific sides of Japan

Genetic diversity and structure in Fagus crenata were studied by analyzing 14 nuclear microsatellite loci in 23 populations distributed throughout the species’ range. Although population differentiation was very low (F _ST = 0.027; R _ST = 0.041), both neighbor-joining tree and Bayesian clustering analyses provided clear evidence of genetic divergence between populations along the Japan Sea (Japan Sea lineage) and Pacific (Pacific lineage) sides of Japan, indicating that physical barriers to migration and gene flow, notably the mountain ranges separating the populations along the Japan Sea and Pacific sides, have promoted genetic divergence between these populations. The two lineages of the nuclear genome are generally consistent with those of the chloroplast genome detected in a previous study, with several discrepancies between the two genomes. Within-population genetic diversity was generally very high (average H _E = 0.839), but decreased in a clinal fashion from southwest to northeast, largely among populations of the Japan Sea lineage. This geographical gradient may have resulted from the late-glacial and postglacial recolonization to the northeast, which led to a loss of within-population genetic diversity due to cumulative founder effects.     

Recolonization pattern of the polychaete Lanice conchilega on an intertidal sand flat following the severe winter of 1995/96

The recolonization process of Lanice conchilega was studied on an intertidal sand flat in the northern Wadden Sea after an intertidal population had been wiped out during the severe winter of 1995/96. After very low recruitment in the first two years, the population fully recovered in the third year after its destruction. By then, the distribution pattern closely resembled the pattern before the severe winter with >95% of all Lanice occurring below –0.5 m mean tidal level and dominance of the density categories 6–50 individuals (ind) m^–2 and 51–200 ind m^–2. Lanice larvae have been observed to attach preferentially on the tubes of adults. However, the high recruitment in the third year was also seen in areas devoid of adult Lanice, indicating that settlement facilitation by adults was of little importance for the recolonization process. Instead, we suggest that the observed recruitment pattern was a consequence of a large-scale decline in the metapopulation in the tidal basin and the onshore coastal subtidal which resulted in insufficient larval supply onto the tidal flats and low recruitment in the first two years following the severe winter. Received in revised form: 7 May 2001 Electronic Publication     

Re-establishment of epibiotic communities in reforested mangroves of Gazi Bay, Kenya

Recolonization of epibiotic flora and fauna in two fringing Sonneratia alba reforestation plots was investigated and compared to a natural mangrove stand and a denuded site in Gazi Bay, Kenya. The reforested sites differed with respect to land history and planting density. Habitat availability in the form of pneumatophore surface differed among forested sites (P<0.001), and between landward and seaward zones (P<0.05). Eighteen algal species were found in the natural area compared to 23 and 10 in replanted sites. Only one species was encountered in the denuded area. SIMPER analysis distinguished Enteromorpha ramulosa, Polysiphonia sp., Hypnea sp. and Caloglossa leprieuri as the main algal species responsible for differences between sites. Algal biomass was positively correlated to pneumatophores area (P<0.001). Total algal biomass differed markedly between forested sites: 1.4 (matrix replantation), 28.6 (natural stand) and 44.3 g m⁻² (integrated replantation) in the seaward zones. The matrix replantation showed strong differences in algal community assemblages compared to the other forested sites, and this site also had significantly lower biomass of sessile benthic fauna (P<0.001). Statistical differences in algal (P<0.01) and sponge (P<0.05) community composition between landward and seaward zones were observed in all sites and trunk fouling fauna was distinctly different between sites. Reasons for the above patterns are discussed and it is suggested that zonation patterns affecting pneumatophore surface and inundation time, in combination with proximity of sites to natural seeding areas, are the most likely explanations for observed patterns of epibiotic community distribution in this study.     

Flood drift and propagule bank of aquatic macrophytes in a riverine wetland

Abstract. Drift of aquatic macrophyte propagules was investigated in a wetland along the River Rhône, during the first flood after the growing season (i.e. in the winter of 1995–1996). Input and output drift were studied at the beginning, around the top, and at the end of the river overflow in the upper reach of a cut-off channel. The soil propagule bank was sampled along the study area before and after the flood. The amount and composition of viable propagule drift and bank were determined, analysed and compared. Drift densities and richness were on average higher at the outlet of the channel than at the inlet (respectively: 23.2 vs 13.1 viable propagules/100 m³ of water and 8.7 vs 2.6 taxa per sample). Immigrating taxa were mostly in the form of helophyte seeds, whereas numerous resident hydrophyte species left the disturbed area rather as vegetative propagules. Temporal variability in propagule bank structure was weak, and mean bank densities did not change before and after the flood (respectively: 33 047 ± 10 510 vs 35 653 ± 15 070 viable propagules/m² of ground, including Chara). However, the density of Elodea canadensis significantly increased after the flood while that of Eleocharis acicularis decreased. This contrast suggests that flood responses vary among species. Despite a broad overlap in the taxa (18 out of 25 were common both to drift and bank collections), no significant relationship occurred in composition or structural changes between flood drift and propagule bank. Flood acted as a means of distribution of existing propagules and also as a provider of new vegetative dispersal units.     

Development of mass blooms of Thiocapsa roseopersicina on sheltered beaches on the Orkney Islands

Abstract Mass developments of the purple sulfur bacterium Thiocapsa roseopersicina in the surface layers of sandy beaches on the Orkney Islands were examined with respect to microcolony formation on sand grains, vertical distribution of viable cells and the ability to colonize beach surfaces. It was observed that microcolonies of the non-motile phototrophic bacterium cemented individual sand grains to each other and that the resulting aggregates could withstand severe wave action and may play a decisive role in the stabilization of these sandy beaches.
After removal of the top layer similar population densities of T. roseopersicina were recorded within seven days. It was calculated that the net specific growth rate initially was 0.53 day⁻¹ (0.022 h⁻¹).
Laboratory studies strongly suggest that the populations of T. roseopersicina on sheltered beaches on the Orkney Islands were growing phototrophically in the light even when the microenvironment was oxic. Bacteriochlorophyll a synthesis was repressed by oxygen and occurred during periods with low light intensities when the microenvironment was anoxic and contained sulfide.     

Genetic structure of American black bears in the desert southwest of North America: conservation implications for recolonization

American black bears (Ursus americanus) have recolonized parts of their former range in the Trans-Pecos region of western Texas after a >40-year absence. Assessment of genetic variation, structuring, gene flow, and dispersal among bear populations along the borderlands of Mexico and Texas is important to gain a better understanding of recolonization by large carnivores. We evaluated aspects of genetic diversity and gene flow for 6 sampling areas of black bears in southwestern North America using genotypic data from 7 microsatellite loci. Our results indicated that genetic diversity generally was high in the metapopulation of black bears in northern Mexico and western Texas. The episodic gene flow occurring via desert corridors between populations in northern Mexico and those in western Texas has permitted the establishment of only moderate levels of genetic structuring. Bayesian clustering analyses and assignment testing depicted the presence of 3 subpopulations among our 6 sampling areas and attested to the generally panmictic nature of bear populations in the borderlands region. The potentially ephemeral nature of the small populations in western Texas and genotypic characteristics of bears recolonizing these habitats attest to the importance of linkages along this portion of the borderlands of the United States and Mexico to effectively conserve and manage the species in this part of its range.     

Seedling emergence and survival in contrasting soil microsites in Patagonian Monte shrubland

Abstract. The potential of two perennial species (Larrea divaricata and Stipa tenuis) to colonize different soil microsites was analyzed in the Patagonian Monte shrubland. We hypothesize that the short-lived grass S. tenuis is more able to colonize the soil of microsites beneath vegetation patches where N-fertility is higher than those in bare soil, while the long-lived shrub L. divaricata colonizes different soil microsites irrespective of their N fertility. A greenhouse experiment was carried out to evaluate the emergence and survival of both species in different soil microsites at different water, inorganic N and seed densities. In all cases soil microsites were seed limited since the addition of viable seeds increased seedling emergence. Both species showed, however, different abilities to emerge and survive in different soil microsites. Microsites of bare soil were more favourable for seedling emergence and survival of L. divaricata than those beneath vegetation patches, independent of their water status. This ability of L. divaricata can not be explained on the basis of increased water or N availability, but probably because of lower salt content of bare soil. The addition of inorganic N reduced the survival of L. divaricata in both microsites but increased individual plant performance. The emergence and survival of S. tenuis was not different in both types of soil microsites but the addition of inorganic N increased seedling emergence and plant biomass. According to these results, emergence and plant performance of S. tenuis may be promoted during humid years by increased concentration of inorganic N. Since N mineralization occurs at a higher rate in soil microsites beneath vegetation patches than in those of bare soil, higher plant performance and probably establishment of S. tenuis is to be expected. These results are consistent with an existing conceptual model of plant dynamics under various grazing intensities in the Patagonian Monte shrubland based on previous field observations.     

Loss of historical immigration and the unsuccessful rehabilitation of extirpated salmon populations

Comprehensive evaluations of multiple genetic factors are rarely undertaken in rehabilitation attempts of extirpated populations, despite a growing need to address why some rehabilitation projects succeed and others fail. Using temporally-spaced samples of microsatellite DNA, we tested several genetic hypotheses that might explain an unsuccessful attempt to re-establish Atlantic salmon populations (Salmo salar) in two rivers of the inner Bay of Fundy, Canada. Census sizes (N) in both populations plummeted to near zero from initial increases after reintroduction/human-mediated recolonization occurred. Over the same period (1974–1996), both populations were characterized by low or relatively low effective sizes (N _e ) and temporally unstable genetic structuring, whereas neighbouring populations, known historically for their significant salmon production, were not. Despite evidence for genetic bottlenecking and continual linkage disequilibrium over time in both populations, neither exhibited detectable inbreeding or a significant loss of allelic diversity or heterozygosity relative to known donor/source populations. Ratios of N _e to N also increased with decreasing N in both populations, implying a buffering capacity against losses of genetic diversity at depressed abundances. Most significantly, multiple lines of evidence were consistent with the hypothesis that there has been substantial and recurrent asymmetric migration (migration rate, m) from neighbouring areas into both populations even after initial rehabilitation. This included migration from a historically productive population that became extirpated during the course of rehabilitation efforts, indicating that both populations might have naturally depended on immigration from neighbouring areas for persistence. Our results highlight the value of incorporating temporal genetic data beyond commonly used metrics of neutral genetic diversity (F _ST, allelic richness, heterozygosity) to evaluate rehabilitation successes or failures. They also illustrate how the joint evaluation of multiple genetic concerns in rehabilitation attempts, at spatial scales beyond donor and rehabilitated populations, is useful for focusing future rehabilitation efforts.