Neighborhood interactions in a natural population of the perennial bunchgrassBouteloua gracilis

We analyzed neighborhood interactions in a natural population of the perennial bunchgrass blue grama (Bouteloua gracilis). Space occupation by individual plants was characterized in terms of neighborhood size. Neighborhood size was defined as the area potentially ‘available’ to an individual, which included the basal area of the plant and the bare area closer to the edges of the plant than to any others. Geographic Information Systems (GIS) were used to describe space partitioning. Growing season performance was evaluated as a function of neighborhood area and neighbor size, controlling for focal plant size. The area of the neighborhood was significant in explaining the remaining variation of allometric relationships between basal area and current vegetative and reproductive performance. In contrast, current performance of focal individuals was not related to the average basal area or the sum of basal areas of adjacent neighbors. Growing season performance was apparently affected by plant spacing, suggesting that competition for spatially distributed resources occurs. The presence of relatively small plants in neighborhoods with a high proportion of bare soil is consistent with the view of a community composed of patches undergoing their own successional dynamics. Competition and disturbances seem to play an important role in this semiarid grassland.     

The comparative phosphorus requirements of some temperate perennial legumes

The phosphorus (P) responses of seven temperate perennial pasture legumes and two species of lupins were compared in a field trial over a range of nine P rates, from 0 to 800 kg ha^-1. The two lupins produced more than 5 t ha^-1 of dry matter in the absence of added P and showed no response to the fertiliser. In contrast, the pasture legumes initially failed to grow without added P and responded to applications of between 200 and 800 kg ha^-1. At the higher rates of P, dry matter production of the pasture legumes was equivalent to that of the lupins.In the first 2 years of the trial; the most productive pasture legume species at the higher rates of added P were also the most productive at the lower rates. Phosphorus requirements for 90% of maximum yield varied greatly between species, but were closely related to maximum yield. Thus species with low P requirements for maximum yield were not necessarily P-efficient species. In the third and subsequent years of the trial Lotus corniculatus performed better than the other pasture legumes at the lower rates of added P. In contrast to other studies Lotus pedunculatus showed no ability to outyield Trifolium repens at low rates of P. Critical P concentrations of the pasture species for the late spring-early summer period declined in the order Trifolium repens (0.34%) > Lotus pedunculatus (0.30%) > Triofolium pratense (0.28%) > Trifolium hybridum (0.27%) > Trifolium ambiguum (0.26%) > Lotus corniculatus (0.23%).Mineralisable nitrogen (N) levels were determined in soils under three species in the 7th year of the trial. At the lowest rates of added P, mineralisable N levels were much higher under Lupinus polyphyllus than under Trifolium repens or Lotus corniculatus. With increasing P rate, levels under the latter species increased, and at 100 kg P ha^-1 were equivalent to those under the lupin with no added P.     

Effects of two types of herbivores on the population dynamics of a perennial herb

The joint effects of multiple herbivores on their shared host plant have received increasing interest recently. The influence of herbivores on population dynamics of their host plants, especially the relative roles of different types of damage, is, however, still poorly understood. Here, we present a modelling approach, including both deterministic and stochastic matrix modelling, to be used in estimating fitness effects of multiple herbivores on perennial plants. We examined the effects and relative roles of two specialist herbivores, a pre-dispersal seed predator, Euphranta connexa, and a leaf-feeding moth, Abrostola asclepiadis, on the population dynamics and long-term fitness of their shared host plant, a long-lived perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). We collected demographic data during 3 years and combined these data with the effects of natural levels of herbivory measured from the same individuals. We found that both seed predation and leaf herbivory reduced population growth of V. hirundinaria, but only very high damage levels changed the growth trend of the vigorously growing study populations from positive to negative. Demographic modelling indicated that seed predation had a greater impact on plant population growth than leaf herbivory. The effect of leaf herbivory was weaker and diminished with increasing level of seed predation. Evaluation of individual fitness components, however, suggested that leaf herbivory contributed more strongly to host plant fitness than seed predation. Our results emphasize that understanding the effects of a particular herbivore on plant population dynamics requires also knowledge on other herbivores present in the system, because the effect of a particular type of herbivory on plant population dynamics is likely to vary according to the intensity of other types of herbivory. Furthermore, evaluating herbivore impact from using individual fitness components does not necessarily reflect the long-term effects on total plant fitness.     

Computing a population budget from sequentially sacrificed,replicated cultures

When a population budget must be obtained from censuses based on replicated, sacrificed cultures, it is difficult to obtain estimates of transition probabilities and of the errors of such estimates, because there is no logical basis for pairing successive census counts. In a study of this nature estimating a population budget of immature stages of the housefly, the problem was solved by a randomization treatment of the original census results obtained at two densities. One hundred randomly generated census matrices over all census times for each density were smoothed to remove the effects of sampling error and a population budget constructed according to defined rules. Transition probabilities computed from the population budget were plotted on triangular coordinate paper and mean probabilities, 95% confidence regions for these means, and 95% equal frequency ellipses computed. All computations and the graphing of the results were carried out on a digital computer. The computer program, available from the authors, is written in FORTRAN IV and could be easily modified for similar studies.     

The role of rock weathering in the phosphorus budget of terrestrial watersheds

Residual soils (saprolites) developed on crystalline rocks appear to form by an essentially isovolumetric process (i.e. without dilation or compaction). Isovolumetric geochemical analysis of a suite of saprolite samples developed on a common parent rock can be used to estimate the relative rates of long-term losses of P and Si during weathering. Using the export of dissolved Si in rivers as a weathering index, one can then estimate the rate of P release due to chemical weathering by means of the P-Si loss ratio in saprolite. For three basins where data are available (Liberty Hill, SC; Amazon River, Brazil: Rio Negro, Brazil) estimated P weathering release rates are 163, 457, and 242 moles P km^–2 yr^–1 respectively. These compare to precipitation inputs of 684, 700 and 630 moles P km^–2 yr^–1 and total river exports of 256, 4490 and 820 moles P km^–2 yr^–1, respectively. The Rio Negro shows a near perfect balance between the input of P via precipitation and chemical weathering and the riverine output of dissolved and suspended P. This system, however, raised the unsolved problem of the source that supports the atmospheric P input.     

Soil phosphorus drawdown by perennial bioenergy cropping systems in the Midwestern US

Without fertilization, harvest of perennial bioenergy cropping systems diminishes soil nutrient stocks, yet the time course of nutrient drawdown has not often been investigated. We analyzed phosphorus (P) inputs (fertilization and atmospheric deposition) and outputs (harvest and leaching losses) over 7 years in three representative biomass crops—switchgrass (Panicum virgatum L.), miscanthus (Miscanthus × giganteus) and hybrid poplar trees (Populus nigra × P. maximowiczii)—as well as in no-till corn (maize; Zea mays L.) for comparison, all planted on former cropland in SW Michigan, USA. Only corn received P fertilizer. Corn (grain and stover), switchgrass, and miscanthus were harvested annually, while poplar was harvested after 6 years. Soil test P (STP; Bray-1 method) was measured in the upper 25 cm of soil annually. Harvest P removal was calculated from tissue P concentration and harvest yield (or annual woody biomass accrual in poplar). Leaching was estimated as total dissolved P concentration in soil solutions sampled beneath the rooting depth (1.25 m), combined with hydrological modeling. Fertilization and harvest were by far the dominant P budget terms for corn, and harvest P removal dominated the P budgets in switchgrass, miscanthus, and poplar, while atmospheric deposition and leaching losses were comparatively insignificant. Because of significant P removal by harvest, the P balances of switchgrass, miscanthus, and poplar were negative and corresponded with decreasing STP, whereas P fertilization compensated for the harvest P removal in corn, resulting in a positive P balance. Results indicate that perennial crop harvest without P fertilization removed legacy P from soils, and continued harvest will soon draw P down to limiting levels, even in soils once heavily P-fertilized. Widespread cultivation of bioenergy crops may, therefore, alter P balances in agricultural landscapes, eventually requiring P fertilization, which could be supplied by P recovery from harvested biomass.     

A seasonal carbon budget for a laminarian population in a Scottish sea-loch

Employing in situ SCUBA methods a seasonal carbon budget has been established for aLaminaria saccharina population in a Scottish sea-loch. Concurrent studies of photosynthesis, secretion rates, reserve fluctuations and frond growth were undertaken. Net annual production is in excess of 120 g C m^–2 yr^–1. Over 13% of gross carbon input is released as extracellular secretions (over 30% in autumn) and 40–50% is lost by distal decay, entering detrital food chains. The large concentrations of laminarin, synthesised in summer months, are nearly all lost in autumn-winter distal tissue loss and therefore not available for early spring growth.     

Applicability of phosphorus budget models to southern African man-made lakes

An investigation of the phosphorus loading characteristics of 31 southern African man-made was lakes made. The lakes were characterized by low water retention times, with most of the lakes having retention times of less than one year. Catchment phosphorus export rates showed wide variation (1–162 mg P m^-2 y^-1) with those lakes experiencing excessive municipal wastewater inputs having export rates in excess of 53 mg m^-2 y^-1. The phosphorus data were tested against the Vollenweider (1976) and Dillon & Rigler (1974) phosphorus budget models which predict in-lake steady state concentrations of phosphorus. It was found that both models displayed good potential for the prediction of steady state concentrations of phosphorus, with better results being obtained from the Dillon & Rigler (1974) model. However, because phosphorus concentrations within these lakes may not necessarily be related to trophic status the use of these models as a predictive tool for eutrophication control still requires further development.     

Stochastic LTRE analysis of the effects of herbivory on the population dynamics of a perennial grassland herb

Herbivores can have strong deleterious effects on vital rates (growth, reproduction, and survival) and thus negatively impact the population dynamics of plant species. In practice, however, these effects might be strongly correlated, for example as a result of tradeoffs between vital rates. To get better insights into the effects of herbivory on the population dynamics of the long‐lived grassland plant Primula veris population projection matrices were constructed from demographic data collected between 1999 and 2008 (nine annual transitions). Data were collected in two large grassland populations, each of which was subjected to two treatments (grazing by cattle versus a mowing treatment), yielding a total of 36 matrices. We applied a lower‐level vital rate life table response experiment (LTRE) using the small noise approximation (SNA) of the stochastic population growth rate to disentangle the contributions of changes in mean vital rates, variability in vital rates, correlations between vital rates and vital rate elasticities to the difference in the stochastic growth rate. Stochastic growth rates (a= log λ_S) were significantly lower in grazed than in mown plots (a= 0.0185 and 0.1019, respectively). SNA LTRE analysis showed that contributions of mean vital rates by far made the largest contribution to the observed difference in a between grazed and control plots. In particular, changes in sexual reproduction rates made the largest contributions to lower the stochastic growth rate in grazed plots: both adult flowering probabilities and flower and seed production were importantly lower in grazed populations, but these negative effects were largely buffered by increased establishment and seedling survival rates. Among the stochastic terms of the SNA decomposition, contributions of covariance and correlations between vital rates had the largest impact, whereas contributions of elasticities were smaller. The strongest correlation driver was the association between adult survival and seedling establishment, suggesting that environmental conditions favouring adult survival also are beneficial for seedling establishment. Overall, our results show that herbivory had a strong negative effect on the long‐term population growth rate of P. veris that was primarily mediated by differences in fecundity (flower and seed production) and germination.     

A note on the energy budget for an oyster population in a temperate estuary

There are six beds of oysters in the estuary of the Beaulieu River, Hampshire; these correspond to areas of gravel bottom. Between November, 1974 and November, 1975 the population was dominated by large individuals (>5 years) and there was no evidence of significant recruitment during the previous two to three years. During the year the population filtered material with an energy content of 15.95 × 10⁶ kcal; 30.4% of this is estimated to have been rejected as pseudofaeces; of the remainder, which was consumed, 30.7% was passed as faeces, 29.0% was respired and lost as heat, 6.4% was diverted into production of soft tissue and shell organics. and 5.5% was released as gonad output. The remaining 28.4% may be partly accounted for as excreta lost to the population as mucus and dissolved organic matter. Gonad output was proportionally large due to the presence of a high percentage of older individuals; this energy was lost from the population and there was no recruitment.     

Relationships of nitrogen and phosphorus to a tropical phytoplankton population

Summary The relationships of nitrogen and phosphorus in the plankton of tropical Lake George (Uganda) have been investigated. The data used are from analyses of samples collected from mid-lake areas during one year, and along transects of increasing plankton concentration, at various times when conditions allowed, and also from results of nutrient uptake experiments. These data are discussed in relation to previously reported information about general limnological conditions in Lake George, and in relation to physiological effects of nutrient enrichments. The features are compared with other waters.The peculiarities of the lake make difficult the precise calculations of long term primary production using oxygen exchange techniques, so that the determination of nutrient turnover rates based upon metabolic activity measured in this way could be misleading. Nevertheless, reasonable overall assessment of nitrogen and phosphorus turnover might be obtained from the proportions of these elements in the plankton. This could be done mainly due to the low variability of the limnological conditions, the apparent limitation of both nitrogen and phosphorus nutrients and the extreme predominance of algal metabolism over the remaining biota.It was also considered that the multiplicity of growth limiting factors which the nitrogen and phosphorus conditions impart contribute to an inherantly stable biomass.

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Zusammenfassung Die Beziehungen zwischen Stickstoff und Phosphor im Plankton des tropischen Sees Lake George (Uganda) wurden untersucht. Die benutzten Daten stammen aus Analysen vom Proben, die über ein Jahr hinweg aus einem Gebiet in Seemitte entnommen wurden. Des weiteren wurden Proben entlang von Profilen mit steigender Planktonkonzentration untersucht. Ferner wurden Ergebnisse von Nährstoffaufnahme-experimenten benutzt. Die Daten wurden mit denen verglichen, die bereits über die algemeinen limnologischen Bedingungen des Sees veröffentlicht wurden. Auch wurden sie in Bezug auf die physiologischen Einflüsse einer erhöhten Nährstoffzufuhr diskutiert. Die Merkmale des Sees wurden mit denen anderer Gewässern verglichen.Die charakteristischen Besonderheiten des Sees machen eine genaue Berechnung von lang anhaltender Primärproduktion unter Anwendung der Sauerstoffaustausch-Methode schwierig, so dass die Bestimmung der Raten des Nährstoffwechsels, basiert auf gemessener metabolischer Aktivität, ein falsches Bild geben könnte. Immerhin konnten annehmbare allgemeine Werte von Stickstoff und Phosphorstoffwechsel aus der Verteilung dieser Elemente im Plankton gewonnen werden. Diese Werte konnten vor allem durch die geringen Schwankungen in den limnologischen Bedingungen, durch die auffallende Beschränkung im Auftreten von Stickstoff und Phosphor und durch das extreme Überwicht vom Algenmetabolismus über die übrige Biota, gewonnen werden.Es wird angenommen, dass die Vielfalt der wachstumbegrenzenden Faktoren, die mit dem Stickstoff- und Phosphorhaushalt zusammenhängen, zu einer von Natur aus stabilen Biomasse führen.

     

Contrasting effects of different landscape characteristics on population growth of a perennial forest herb

Anthropogenic changes in landscape structure, such as habitat loss, habitat subdivision and edge increase, can strongly affect the performance of plants, leading to population declines and extinctions. Many studies to date have focused on single characteristics of landscape structure or single life‐cycle phases, but they poorly discern the different pathways through which landscape change influences plant population dynamics via different vital rates. In this study, we evaluated the effect of two structural characteristics (habitat quantity and edge length) on vital rates and population growth rates of a perennial forest plant (Primula vulgaris) in a historically managed landscape. Areas with higher amounts of forest habitat had higher population growth rates due to higher recruitment, survival and growth of seedlings, while increased forest edge length was positively associated with population growth rates primarily due to a higher survival of reproductive individuals. Effects were stronger during the first of the two transition intervals studied. The results demonstrate that changes in different landscape structural characteristics may result in opposing effects acting via different vital rates, and highlight the need for integrative analyses to evaluate the effects of rapid landscape transformation on the current and long term plant population dynamics.     

The productivity and carbon budget of a natural population of Daphnia lumholtzi Sars

D. lumholtzi in Lake Samsonvale, Queensland, Australia, is a small species (max. size approx. 7 µgC) that occurs in low abundance (max. abundance 6400 m^–3), with an average daily biomass of 3.32 mgC m^–3. Its annual rates of carbon assimilation, production and respiration, are 166, 110, and 56 mgC m^–3 y^–1 respectively. Annual biomass turnover (annual production/average daily biomass) is 33 and production efficiency is 50–66%. The population may consume 1.65–2.20 mgC m^–3 daily, equivalent to about 1% of the average daily standing crop of phytoplankton. Clutch size is small, 2 eggs, but represents 30–80% of a female's weight. A female may only produce 8–10 offspring in a full lifespan, nevertheless egg production may account for 56% of total production. The population shows autumn and spring peaks in abundance, and is believed to oversummer (4 months) as ephippia.     

Linking environmental and demographic data to predict future population viability of a perennial herb

Recent advances in stochastic demography provide tools to examine the importance of random and periodic variation in vital rates for population dynamics. In this study, we explore with simulations the effect of disturbance regime on population dynamics and viability. We collected 7 years of demographic data in three populations of the perennial herb Primula farinosa, and used these data to examine how variation in vital rates affected population viability parameters (stochastic growth rate, λ_S), and how vital rates were related to weather conditions. Elasticity analysis indicated that the stochastic growth rate was very sensitive to changes in regeneration, quantified as the production, survival, and germination of seeds. In one of the study years, all seedlings and mature plants in the demography plots died. This extinction coincided with the driest summer during the study period. Simulations suggested that a future increase in the frequency of high-mortality years due to climate change would result in reduced population growth rate, and an increased importance of survival in the seed bank for population viability. The results illustrate how the limited demographic data typically available for many natural systems can be used in simulation models to assess how environmental change will affect population viability.     

黑麦草-内生真菌共生体对磷缺乏的生理生态反应

选取感染和未感染的黑麦草为材料,在田间盆栽条件下研究内生真菌感染对宿主植物抵抗磷胁迫方面的贡献。结果表明,土壤中缺磷或内生真菌感染对黑麦草地上部生长的影响不显著,但内生真菌感染对植株地下部生长和生理指标有明显影响。缺磷条件下,内生真菌感染有助于黑麦草地下部分的生长,表现在根系总长度更长,生物量更大;同时根中酚类物质和有机酸的含量也显著高于未感染植株,但因酚类物质和有机酸总量增加的同时并未伴随着二者浓度的增加,由此推测,内生真菌在改变宿主黑麦草根系代谢活动方面的贡献有限。此外,内生真菌感染显著提高了宿主植物的磷利用效率,这可能和缺磷条件下内生真菌感染植株具有更高的酸性磷酸酶活性有关。     

普通野生稻小种群的交配系统与遗传多样性

小种群的遗传动态是保育遗传学关注的核心问题之一,而种群遗传动态又与交配系统密切相关.普通野生稻(Oryza rufipogon Griff.)是具有重要经济价值的濒危物种,目前其种群规模都较小,研究其小种群交配系统与遗传变异性对普通野生稻的保护具有重要意义.运用7对SSR引物,对采自江西东乡普通野生稻小种群的36份种茎和其中20个家系共计601份子代进行了分析.结果显示:该种群的表观异交率为0.318,多位点法估计(MLTR)的多位点异交率为0.481;50%以上的子代共享亲本,非随机交配明显;东乡普通野生稻种群交配系统属于混合交配类型.比较亲本和子代种群的遗传变异性显示:子代种群比亲本种群遗传变异性更丰富;子代种群的杂合子不足与种群变小自交比例上升有关;而亲本种群杂合子过剩可能与杂合基因型的选择优势有关.这些结果说明创造条件扩大种群规模对普通野生稻的原生境保护显得尤为重要.