基于比率的一类捕食者-食饵系统的持久性和全局吸引性

研究了基于比率的一类捕食者．食饵系统，该系统是非自治的．应用比较定理和构造Liapunov泛函的方法，得到了该系统的一致持久性和全局吸引性的充分条件，推广了已有文献的相关结果．     

Improved persistence of red clover (Trifolium pratense L.) increases the protein supplied by red clover/grass swards grown over four harvest years

UK livestock agriculture can significantly reduce its protein imports by increasing the amount of forage based protein grown on-farm. Forage legumes such as red clover (Trifolium pratense L.) produce high dry matter yields of quality forage but currently available varieties lack persistence, particularly under grazing. To assess the impact of red clover persistence on protein yield, diploid red clover populations selected for improved persistence were compared with a range of commercially available varieties. All populations were grown over four harvest years in mixed swards with either perennial ryegrass (Lolium perenne L.) or perennial plus hybrid ryegrass (L. boucheanum Kunth). Red clover and total sward dry matter (DM) herbage yields were measured in Years 1–4, red clover plant survival in Years 3 and 4 and herbage protein (CP) yield and concentration in Years 2 and 4. In general, red clover DM yield in year 4 (3.4 t ha⁻¹) was lower than in year 1 (13.9 t ha⁻¹) but the red clover populations differed in the extent of this decline. Differences in the persistence of the red clover populations in terms of plant survival and yield were reflected in the contribution of red clover to the total sward yield in Year 4, which ranged from 61% for the highest yielding population, AberClaret, to 11% in the lowest yielding, Vivi. Increased red clover DM yield was reflected in a greater CP yield (protein weight per unit area), which ranged from 1.6 t ha⁻¹ year⁻¹ to 2.9 t ha⁻¹ year⁻¹ in Year 2 and from 1.1 t ha⁻¹ year⁻¹ to 1.9 t ha⁻¹ year⁻¹ in Year 4. CP concentration (protein weight per unit herbage weight) of all of the red clover populations was within a range considered suitable for ruminant production. The implication of these results for the future use of red clover in sustainable grassland systems is discussed.     

Effects of DDT and its metabolites on soil algae and enzymatic activity

 The persistence of DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] and its metabolites in soil, their toxicity to soil algae, and effects on microbial activities were studied in laboratory microcosms for 45 days. In non-sterile soils, removal of DDD [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane] and DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene] was less than 3%, while 4–8% of applied DDMU [1-chloro-2,2-bis(p-chlorophenyl)ethylene], DDA [2,2-bis(p-chlorophenyl)acetic acid] and DDT were lost. Added DDOH [2,2-bis(p-chlorophenyl)ethanol] was more labile, as 60% was degraded during the same period. Soil microalgae were not measurably affected by the compounds tested at 10–50 mg kg^–1, but at 100 mg kg^–1 soil, DDD, DBP (p,p′-dichlorobenzophenone) and DDA significantly reduced their growth. Phosphatase activity was not affected by DDT and its metabolites at the concentrations tested (≤50 mg kg^–1), but all compounds inhibited dehydrogenase activity at concentrations of 50 mg kg^–1 soil. The toxic effects of DDT and its metabolites were dose-related. Received: 9 January 1998     

Driving forces of landscape change - current and new directions

The concept of driving forces is gaining increasing attention in landscape-change research. We summarize the state of the art of this field and present new conceptual and methodological directions for the study of driving forces of landscape changes. These new directions address four major challenges faced by landscape-change studies, i.e., studying processes and not merely spatial patterns, extrapolating results in space and time, linking data of different qualities, and considering culture as a driver of landscape change. The proposed research directions include: studying landscape change across borders and transects, focusing on persistence as well as change, investigating rates of change, considering attractors of landscape change, targeting correlation and causality, and searching for precursors of landscape change. Based on established knowledge and the new approaches we outline a standard procedure to study driving forces of landscape change. We anticipate that our analytical and systematic approach increases the relevance of studies of landscape change for science as well as for the solution of real world problems.     

Metapopulation dynamics and threatened species management: Why does the broad-toothed rat (Mastacomys fuscus) persist?

Many species that exist patchily across the landscape are declining due to incremental losses of their constituent sub-populations and increasing isolation of those that remain. For threatened species with such patchy metapopulation structure, it is particularly important to identify key habitat patches and understand what factors govern their occupancy so that their management can be targeted and effective. In this paper, we describe the spatial and temporal distribution of an endangered population of broad-toothed rats (Mastacomys fuscus) at Barrington Tops, New South Wales, and model its dynamics using metapopulation theory. The study population occurs patchily in swamps on the Barrington Plateau. Using faecal pellet searches, live-trapping and data from previous surveys, we identified 12 swamps where M. fuscus persists and 13 where there has been a history of colonisation and extinction. The species now appears to be entirely absent from seven of these latter swamps. Using logistic regression and model selection procedures, we found the strongest predictor of the presence of M. fuscus to be proximity to the nearest occupied swamp. Persistence declined strongly with swamp isolation, probably due to low success of individuals dispersing through the intervening habitat. These patterns support the interpretation that swamp patches at Barrington Tops contain a functioning but fragile metapopulation of M. fuscus. We predict continued loss of remaining sub-populations in peripheral swamps if current dynamics continue, and recommend research to identify the factors that are limiting dispersal and re-colonisation so that the species’ decline can be slowed and reversed.     

Biotoxicity and persistence of nonylphenol during incubation in a compost-sandstone mixture

Summary The persistence and biotoxicity of nonylphenol, a mixture of monoalkyl phenols that is found in relatively high concentrations in sewage sludge, were studied in an incubation experiment with a reconstituted soil system (compost + sandstone). The effect of nonylphenol (100 and 1000 ppm) on CO₂ evolution and biomass ATP were monitored. Nonylphenol depressed CO₂ production significantly only at high concentrations [1000 ppm 4-nonylphenol (4-NP)]. Biomass ATP declined progressively after the 5th day. At 100 ppm no toxic effects were detected. After a lag phase, nonylphenol disappeared readily upon incubation at the lower concentration (100 ppm), but persisted at high levels (1000 ppm). The persistence of 4-nonylphenol increased under aseptic conditions. The chromatogram profile of 4-NP changed over time, indicating that some of the isomers are more easily degraded than others.     

Driving forces of landscape change - current and new directions

The concept of driving forces is gaining increasing attention in landscape-change research. We summarize the state of the art of this field and present new conceptual and methodological directions for the study of driving forces of landscape changes. These new directions address four major challenges faced by landscape-change studies, i.e., studying processes and not merely spatial patterns, extrapolating results in space and time, linking data of different qualities, and considering culture as a driver of landscape change. The proposed research directions include: studying landscape change across borders and transects, focusing on persistence as well as change, investigating rates of change, considering attractors of landscape change, targeting correlation and causality, and searching for precursors of landscape change. Based on established knowledge and the new approaches we outline a standard procedure to study driving forces of landscape change. We anticipate that our analytical and systematic approach increases the relevance of studies of landscape change for science as well as for the solution of real world problems.