Long-term dynamics of a tropical savanna bird community

Complete two-month censuses of diurnal raptors and strip transect counts of other bird species were compared between 1968–1972 and 1996 to assess changes in the bird community of a 2700ha guinean savanna in Ivory Coast. Among 117 non-raptor species recorded at least once on the transects, 25 mostly uncommon species were absent in 1996 (but only two of them were likely to be extinct in the study area), 4 were first seen in 1996, 14 decreased and 4 increased significantly. All six large forest raptors and resident savanna eagles disappeared before the mid-1980s. Among the 12 remaining breeding raptors, 4 species increased (including the 2 seasonal migrants), 4 species remained stable (including the 2 forest accipiters) and 4 species decreased, but not significantly (including the 2 largest taxa). All 14 non-breeding raptors (Palearctic and African migrants) remained stable or increased. With on average 63 breeding pairs per 1000ha, this area still supports one of the highest densities of diurnal raptors ever recorded. The geographic distributions, seasonal movements and ecological requirements of species involved suggest the most likely origin of specific changes observed. They were attributed, in roughly similar proportion, to: (i) decreasing rainfall (southward shifts leading to appearance or increase of some resident or migrant species); (ii) deforestation, agricultural development and hunting pressure around the reserve (extinction of larger species following habitat fragmentation or degradation, increase of some species); and (iii) habitat modifications inside the reserve (increasing tree cover, invasion of dense thickets, illegal exploitation of palms). The overall community of this small protected area, however, exhibited a remarkable stability during the last 30 years, in spite of dramatic habitat changes outside its limits.     

植物群落动态的模型分析

植物群落的动态是植物群落学的中心问题之一,包括更新、波动、演替、进化等主要内容。空间格局对种群和群落的动态起着至关重要的作用,种群空间格局和群落空间结构是群落中各种过程相互作用的产物。模型是描述群落动态、认识植物群落组建和维持机理的有效工具。本文阐述和比较了描述群落动态的四种具有代表性的经验模型,即镶嵌循环模型、随意游走模型、同资源种团比例模型、空间抢先占有模型及其机理。四种经验模型的空间性及缺陷分别是:(1)“镶嵌循环模型”考虑到了相邻斑块之间的植被空间结合在群落动态中的作用,而另外三种模型没有考虑到这一点;(2)在一定程度上,四种植物群落动态模型对各自针对的植物群落可能是适合的,但要作为描述群落动态发展的一般性模型还需要不断完善和发展;因为四种模型均没有考虑到自然干扰和人类干扰对植物群落动态的影响。作者对将来植物群落动态的研究及实践意义做出以下展望:(1)在不同空间尺度上,更加有效地评价控制群落动态变化的各种过程的相对重要性,并进一步将它们之间的复杂相互作用整合到群落动态模型中;(2)充分认识植物群落中存在的各种自然环境条件和生物群体的结构配置对植物群落动态发展的重要性;(3)重视植物群落动态发展中自然干扰过程和人类干扰过程的整合以     

Nonmanipulative determination of plant community dynamics

Knowledge of the strengths of interactions between species in plant communities is of fundamental importance to our understanding of how communities are structured, although they are notoriously difficult to quantify. Techniques have recently been developed that allow the detailed enumeration of the strength of interactions between plant species within unmanipulated multispecies communities. Nonlinear regression analysis is used to fit competition models to long-term census data using natural variations in plant densities in lieu of manipulation. The models generated have been used to infer the intensity and importance of interactions as well as to analyse the effects of spatial and temporal variability. Theoretical work has begun to look at how different techniques for measuring competition perform in a range of systems, highlighting the importance of spatial scale. The lessons learned from applying these methods will enable improved estimation of the strength of competition in natural communities.     

Bacterial community dynamics in a functionally stable pilot-scale wastewater treatment plant

To determine whether functional stability was correlated with a stable microbial community structure in a functionally stable pilot-scale wastewater treatment plant, bacterial communities in the system were monitored over a one-year period. Bacterial community dynamics was characterized by the terminal restriction fragment length polymorphism (T-RFLP) of 16S rRNA genes. During the study period, the effluent BOD concentrations were very stable, with the average BOD concentration below 10 mg/L. The effluent TN concentrations were always below 20 mg/L, except for the first 40 days. T-RFLP results showed that, during the test period, the bacterial community structures were not stable, with an average change rate (every 15 days) of 20.4% ± 11.2%. Based on Lorenz distribution curves, it was observed that 20% of the species corresponded with 40-77% of cumulative relative abundances. Results clearly revealed that, in the pilot-scale wastewater treatment plant, functional stability did not correlate with stable bacterial communities.     

Ammonia oxidizing bacteria community dynamics in a pilot-scale wastewater treatment plant

Background

Chemoautotrophic ammonia oxidizing bacteria (AOB) have the metabolic ability to oxidize ammonia to nitrite aerobically. This metabolic feature has been widely used, in combination with denitrification, to remove nitrogen from wastewater in wastewater treatment plants (WWTPs). However, the relative influence of specific deterministic environmental factors to AOB community dynamics in WWTP is uncertain. The ecological principles underlying AOB community dynamics and nitrification stability and how they are related are also poorly understood.

Methodology/Principal Findings

The community dynamics of ammonia oxidizing bacteria (AOB) in a pilot-scale WWTP were monitored over a one-year period by Terminal Restriction Fragment Length Polymorphism (T-RFLP). During the study period, the effluent ammonia concentrations were almost below 2 mg/L, except for the first 60 days, indicting stable nitrification. T-RFLP results showed that, during the test period with stable nitrification, the AOB community structures were not stable, and the average change rate (every 15 days) of AOB community structures was 10%±8%. The correlations between T-RFLP profiles and 10 operational and environmental parameters were tested by Canonical Correlation Analysis (CCA) and Mantel test. The results indicated that the dynamics of AOB community correlated most strongly with Dissolved Oxygen (DO), effluent ammonia, effluent Biochemical Oxygen Demand (BOD) and temperature.

Conclusions/Significance

This study suggests that nitrification stability is not necessarily accompanied by a stable AOB community, and provides insight into parameters controlling the AOB community dynamics within bioreactors with stable nitrification.     

Six years of submerged plant community dynamics in a freshwater tidal wetland

1. The dynamics of a submerged plant community were studied for 6 years in a freshwater tidal wetland. The degree and nature of change at several spatial scales (quadrat, transect and overall community) was determined, and the implications for community stability were assessed. 2. A high degree of change was recorded in 1 m² quadrats, and this was reflected in 10 m² transects as well. In quadrats, mean species richness changed every year. Species richness changed in >60% of quadrats each year. Stem number changed by as many as several 100 stems per quadrat from one year to the next. 3. Richness varied more among quadrats than among transects and varied less at the community level than among either quadrats or transects. Greater stability at the spatial scale of the whole community was reflected in high scores on the Jaccard and Morasita–Horn indices and Kendall's coefficient of concordance. 4. Although most of the submerged species were perennials, persistence at the local scale was low, and 4‐year persistence exceeded 50% for only one species. Change in abundance was largely independent among the species. 5. In the face of great small‐scale changes, species remain in the community (and the community persists) because of high recruitment rates.     

Long-term dynamics of a high-latitude coral reef community at Sodwana Bay,South Africa

Dynamics in reef cover, mortality and recruitment success of a high-latitude coral community in South Africa were studied over 20 yr with the aim to detect the effects of climate change. Coral communities at this locality are the southernmost on the African continent, non-accretive, attain high biodiversity and are dominated by soft corals. Long-term monitoring within fixed transects on representative reef was initiated in 1993 and has entailed annual photo-quadrat surveys and hourly temperature logging. Although sea temperatures rose by 0.15 °C p.a. at the site up to 2000, they have subsequently been decreasing, and the overall trend based on monthly means has been a significant decrease of 0.03 °C p.a. Despite this, minor bleaching was encountered in the region during the 1998 El Niño–Southern Oscillation event, again in the summer of 2000/2001 and in 2005. A significant decreasing trend of 0.95% p.a. in soft coral cover has been evident throughout the monitoring period, attributable to significant decreases in Sinularia and Lobophytum spp. cover. In contrast, hard coral cover gradually and significantly increased up to 2005, this being largely attributable to increases in cover by Acropora spp. Recruitment success and mortality of both soft and hard corals has displayed high inter-annual variability with increasing but non-significant trends in the last 5 yr. The reduction in soft coral cover has been more consistent and greater than that of hard corals, but it is difficult at this stage to attribute this to changes in water quality, acidification-linked accretion or temperature.     

Small-scale spatial dynamics of plant species in a grassland community over six years

Abstract. In a species-rich mountain grassland in the Krkonosse Mts., Czechoslovakia, data from four permanent plots of 50 cm x 50 cm were recorded annually from 1985 to 1990 to study the spatial dynamics of the species. Plots were divided into 15 x 15 subplots and the number of vegetative units of all plants within each subplot was determined. There was not much net change at the plot level, but the subplots were very dynamic. Two aspects of the spatial dynamics of the species were followed: (1) persistence, i.e. the tendency of the species to remain in the same subplot, and (2) long-distance spreading, i.e. movement to subplots beyond the immediate neighbourhood. Species differed widely in their persistence and longdistance spreading and were classified into mobility types: long-range guerrilla, short-range guerrilla, phalanx and 'sitting’. The mobility types were, to a certain extent, correlated with the growth form of plants, but some species of one growth form showed different types of small-scale dynamics and some species with different growth forms had the same spatial dynamics.     

Multi-scale regulated plant community dynamics: mechanisms and implications

Plant competition is not a direct interaction, but operates via environmental feedback loops, which interconnect population densities and environmental regulating variables. It is suggested that due to scale dependent elements of these feedback loops, competition may occur eventually on very different scales, necessitating a cross-scale extension of plant competition theory. After introducing the concept of cross-scale competition, we incorporate its elements into a metacommunity model and study its implications on community organization. It is found that both the equilibrium community composition, regarding coexisting functional types, and its stability depend on scale dependent attributes of environmental feedback loops and disturbance regimes. We argue that plant communities are likely to exhibit properties, which are in line with the hierarchical ecosystem concept. Environmental feedback loops on different scales act as distinct organizational levels, what can be affected by disturbances of corresponding spatial extent.     

Switching roles from antagonist to mutualist: a harvester ant as a key seed disperser of a myrmecochorous plant

1. Harvester ants are major seed predators in arid environments. However, given that many harvester ants are partly omnivorous and therefore potentially attracted to the elaiosomes of myrmecochorous seeds, it is unclear if these ants act as predators or dispersers when removing myrmecochorous seeds. 2. We describe the outcomes of interactions between the harvester ant, Pogonomyrmex naegelii, and myrmecochorous plant, Microstachys serrulata, in a Brazilian savanna. We: (i) evaluated the role of elaiosome in seed removal by P. naegelii; (ii) investigated the fate and viability of removed seeds; (iii) tested if soils associated with P. naegelii nests are nutrient-enriched; (iv) compared seedling survival; and (v) the density of seedling and adult M. serrulata nearby to P. naegelii nests compared with those away from these nests (i.e. controls). 3. Rates of removal of M. serrulata seeds were two-fold higher with elaiosomes than without. The ant attractant oleic acid was the dominant fatty acid in elaiosomes, but it was absent from seeds. Removed seeds are taken into nests, and Tetrazolium tests indicated that 95% of seeds remain viable. Soils associated with P. naegelii nests were not nutrient-enriched, and seedling survival was similar nearby to P. naegelii nests compared with control areas. However, densities of both seedling and adult M. serrulata were higher nearby to P. naegelii nests than in control areas. 4. Our findings show that P. naegelii switches its role from seed predator for most plant species to be the dominant seed disperser for M. serrulata, playing a key role in the distribution of adult plants.     

Genetic diversity of Stemona parviflora: A threatened myrmecochorous medicinal plant in China

The plant species Stemona parviflora, which is endemically distributed in Hainan and Guangdong Provinces of China, is an important constituent of Chinese traditional medicine “Baibu”. Based on AFLP analysis, at the species level, the percentage of polymorphic bands (PPB), gene diversity H, and Shannon's information index I of S. parviflora are 100%, 0.2826, and 0.4458, respectively. The genetic differentiation among populations is low (G_st = 0.0799), while the gene flow among these populations is high (N_m = 5.754). Actually, no distinguishable genetic structure is inferred according to the AFLP analysis in this study. There was no significant positive correlation between genetic distance and geographic distance either. Based on its medicinal use, seed dispersal pattern (myrmecochory), mating system, fragmentation of the natural habitat by rubber and eucalyptus planting, and the historical formation and transition of Hainan island and Guangdong provinces, we discuss the current threatened condition of the traditional medicinal plant S. parviflora. In/ex situ conservation tactics are recommended to save this important species.     

Long-term dynamics of catabolic plasmids introduced to a microbial community in a polluted environment: a mathematical model

The long-term dynamics of mobile plasmids in natural environments are unclear. This is the first study of the long-term dynamics of introduced plasmids with xenobiotic degradation abilities using a mathematical model that describes the horizontal gene transfer (HGT) of plasmids into indigenous bacteria via conjugation. We focussed on negative feedback between the spread of plasmids and their selective advantage, i.e. the severe competition between plasmid-bearing and plasmid-free bacteria resulting from a decrease in xenobiotic concentration caused by the gene expression of plasmids, favoring plasmid-free bacteria. Two types of HGT enhanced the persistence of plasmids and the degradation of the xenobiotic in different conditions: a relatively low rate of 'intergeneric HGT' from introduced to indigenous bacteria and a high rate of 'intraindigenous HGT' from indigenous to indigenous bacteria. In addition, when the indigenous resource supply rate was high and when the cost of bearing plasmids was low, both types of HGT made large contributions to xenobiotic degradation compared to the contribution of vertical transfer via plasmid replication within the introduced host population. Initial conditions were also important; a higher initial density of introduced plasmid-bearing bacteria led to a lower degradation rate over a long time scale.     

Hormonal dynamics contributes to divergence in seasonal stomatal behaviour in a monsoonal plant community

The plant hormone abscisic acid (ABA) is a primary regulator of plant transpiration, but its influence in determining seasonal stomatal behaviour in natural plant communities is poorly understood. We examined distantly related vascular plants growing together in a seasonally dry, monsoonal environment to determine whether ABA dynamics contributed to contrasting water use patterns in this natural setting. Regular sampling of angiosperm, cycad, conifer and fern species revealed characteristic seasonal patterns in ABA production, but these were highly distinct among species. Although no general relationship was observed between ABA levels, plant hydration or stomatal conductance among species, the seasonal dynamics in stomatal behaviour within species were predictable functions of either ABA or leaf water potential. Strong divergence in the seasonal role of ABA among species suggests that modification in ABA–stomatal interactions represents an important evolutionary pathway for adaptation in plant water use.