基于LUCC 的兰州城市生态系统服务价值动态分析

生态脆弱型城市生态系统服务价值评估是保育和合理开发城市生态系统的重要基础。构建典型生态脆弱型城市生态服务价值评估模型, 以兰州城市生态系统为例, 运用GIS 和遥感技术分析LUCC 特征, 计算1989 年、1999 年和2009 年6 类主要土地利用类型的9 项生态系统服务项目价值。结果表明: 1)20a 间兰州市耕地面积减少幅度最大为42.54%, 水体湿地、草地次之。总价值呈现出先增加后减少的趋势, 1999 年兰州城市生态服务价值最大为4.4377×108元, 2009 年生态服务价值是1989 年的1.5 倍。2)土壤形成与保护服务功能价值占总价值的比例最高约20%, 食物生产和娱乐文化服务功能价值占总价值比例较低为3.7%、3.4%, 气体调节、娱乐文化和原材料生态服务价值增幅最大。3)各年份林地、草地、耕地和水体湿地生态服务价值均为总价值构成的主要部分, 林地和草地生态系统对总服务价值的贡献率最高。城市扩张中应注重林草地的保育与合理规划。4)价值系数敏感性检验表明, 生态系统服务价值对其缺乏弹性, 模型能够合理评估该区域生态服务价值波动。     

胶州湾滨海湿地生态系统服务价值变化

为科学的评估青岛胶州湾滨海湿地生态系统服务功能的变化特点并明确其驱动因素,通过构建生态系统服务评价体系,综合运用生态学和经济学方法,对2005年、2010年和2015年胶州湾滨海湿地生态系统的供给、调节和文化3大类共12项生态服务的价值进行了评估。结果表明,气候调节和水产品生产是胶州湾滨海湿地生态系统的两大核心功能,二者占到服务总价值的90%以上;调节服务是其主要的服务类型,占到总服务价值的60%以上;2005年、2010年和2015年胶州湾滨海湿地生态系统服务总价值呈递减趋势,分别为1419.73×10~7元、1343.91×10~7元和1239.23×10~7元。湿地面积减少、湿地功能退化是造成胶州湾滨海湿地生态系统服务价值总体呈下降趋势的直接原因,不合理围垦、过度开发生物资源、水污染严重是造成湿地面积减少、功能退化的人为因素,气候条件的不利变化是造成湿地环境进一步恶化的自然因素。建议以生态系统服务价值为指导,将评估结果纳入到决策体系中,加强对青岛胶州湾滨海湿地现有资源的保护和生态恢复。     

长江流域生态系统格局演变及驱动力

长江流域生态系统格局复杂,多种社会经济、政策和自然因素对土地利用变化的影响使得生态环境发生变化。分析了2000年至2015年长江流域生态系统格局和演变特征,及主要驱动力对生态系统变化的贡献。15年间,共有约6.4万km~2的生态系统类型发生变化,城镇增长67.5%,农田缩减7.5%,森林增加2.1%,剧烈的生态系统变化集中于下游,以及中上游的大城市,城镇聚集区以及退耕还林区。生态系统景观破碎化程度和景观多样性提高。上、中、下游生态系统格局、构成差异较大,15年间,上游和下游森林显著增加,下游城镇显著扩张、农田和湿地显著缩减,上游湿地增加最为显著。城镇化是生态系统格局演变的首要驱动力,对生态系统变化的贡献率达48.0%,长江下游城镇化的贡献率高达64.5%。生态保护与恢复工程是第二驱动力,对生态系统变化的贡献率为32.8%,在上游高达47.8%。水资源开发和农业开发贡献率分别为8.5%和9.9%,此外,气候变化促使高原湖泊面积增大。为保护长江流域生态系统的可持续发展,需划定生态保护红线,合理规划城市化进程中的土地利用,保护优质耕地,禁止重要湿地的开发。     

滇西北高原湿地景观变化与人为、自然因子的相关性研究

人为活动的干扰与自然因子的变化共同作用于湿地生态系统,但两者对湿地生态系统作用的贡献率存在差异,目前尚缺乏进一步的研究。本研究基于面向对象分割和目视解译相结合的技术方法,研究了滇西北高原典型湿地纳帕海汇水区内28年来(1987—2015年)的湿地类型、分布及其空间格局的变化特征,并探讨其与当地人为活动的干扰(主要社会经济发展指标)、自然因子(主要气候因子)之间的相互关系。结果表明:(1)湿地总面积共计减少2456.46 hm2,其中,原生沼泽、沼泽化草甸和草甸面积分别减少了1152.07,1257.72,202.74 hm2,湖泊面积增加了156.07 hm2;(2)湿地景观多样性发生显著变化,其中,斑块数量(NP)由1987年的221增加到2005年的299,随后减少到2015年的260;香农多样性指数(SHDI)由1987年的1.81增加到1999年的1.84,随后减少到2015年的1.75;聚集度指数(contagion index)由1987年的52.82减少到1999年的52.02,随后增加到2015年的53.49;(3)湿地分布面积和香农多样性指数与第一、二、三产业值,以及年均温度呈负相关,与降水量呈正相关;斑块数量、聚集度指数均与第一、二、三产业值,以及年均温度呈正相关,与降水量呈负相关;(4)社会经济发展主要指标对湿地面积和景观多样性指数变化的解释度为63.50%,气候因子对其的解释度为36.50%。整体上,人为活动的干扰是导致该区域湿地不断萎缩、景观多样性改变的关键驱动力。减缓人为活动对湿地生态系统的过度影响,是当地保护湿地资源、维护湿地生态功能的关键。     

拉萨河源头麦地卡湿地景观格局及功能动态分析

在全球变化的大背景下,如何保护好高原湿地显得非常迫切。为了有效保护拉萨河源头,采用遥感影像解译和模型评估方法,分析拉萨河源头麦地卡湿地的土地利用格局现状特征和服务功能变化,结果表明:麦地卡湿地保护区湿地面积减少5825hm2,湿地面积减少以沼泽湿地为主,草地面积增加5727 hm2,未利用地面积仅增加98.5 hm2;近30年来因气温和地表温度升高明显,暖干化的气候变化趋势导致湿地面积萎缩,天然牧草地面积显著增加;调节服务是麦地卡湿地保护区主要的生态服务功能,占总服务价值的70%,湿地面积尤其是沼泽湿地面积的萎缩是麦地卡湿地保护区生态服务功能减弱的主要原因。     

近30年来白洋淀湿地演变驱动因子分析

湿地是水陆相互作用形成的独特生态系统,其面积极易受到气候变化和人类活动的影响。本文以华北最大的天然湖泊湿地白洋淀为例,利用1984—2013年的11期遥感影像数据,提取湿地信息,对湿地面积变化特征进行分析;采用主成分分析方法,从气候、经济、人口和农业等方面选取13个因子,分析1984—2013年这些因子对白洋淀湿地面积变化的影响,揭示影响湿地面积变化的主要驱动因子。结果表明,白洋淀湿地面积先增加再减少之后又缓慢增加,总体呈下降趋势,由1998年前的平均25008 hm2下降到1998年后的平均21573 hm2;引起白洋淀湿地面积变化的主导驱动因子是社会经济发展、农业发展和降水量减少,其中社会经济发展是最主要的因素;为了遏制湿地面积减少的势头,应该有效地控制上游水量的使用,提高水资源利用率,从根本上减少人类活动对白洋淀湿地的压力,使自然入淀水量增加。     

西部半干旱区兰州市南北两山森林生态效益评估

城市森林对城市的生态系统起着至关重要的作用, 对城市森林生态效益评估有利于市民更直观的认识到城市森林的重要性, 为城市生态保护与利用的制定提供参考。选择兰州市南北两山作为研究区, 基于谷歌地球引擎云平台获取1994、2006 和 2017 年的土地利用数据。引入资源稀缺性系数、支付意愿性系数和空间异质性系数, 采用当量因子法评估生态系统服务价值(ESV), 并以格网为研究单元对空间分布格局进行分析。研究结果表明, 1994、2006和2017年南北两山的生态系统服务价值分别为2.80、3.33和3.36亿元。就生态系统而言, 草地贡献率最高, 达到65%左右, 建设用地贡献率为负值。就生态系统服务类别而言, 调节服务贡献率最高, 达到71%左右。生态系统服务价值的空间格局存在明显的南山高北山低的空间分异特征。从损益矩阵看, 生态用地ESV的流入是总的ESV增加的主要原因, 水域ESV流出和建设用地ESV的流入减缓了总的ESV的增长速度。     

广西北部湾沿岸地区生态系统服务价值变化及其驱动力

掌握生态系统结构及其功能的时空变化规律是开展科学生态系统管理的重要前提,如何衡量人类在满足自我需求的同时对自然资源和生态系统的改变程度是当前研究值得关注的一个问题。伴随北部湾经济区的崛起,区域经济发展与生态保护的矛盾日益凸显,海水倒灌、植被退化和土地沙化等现象加剧,生态系统服务可持续供给受到严重威胁。以时序遥感数据为基础,分析北部湾沿岸地区生态系统时空演变,评估生态系统服务价值时空变化规律及其驱动因素,为区域生态系统管理提供科学基础。研究显示:1999—2014年,城市点状、离散扩张使城市生态系统面积显著增加,破碎化程度加剧;环境恶化给湿地生态系统带来毁灭性的破坏,红树林不断消失,破碎度增加;林地和果园构成的森林生态系统面积有所增加,而耕地面积急剧缩减。生态系统构成中,森林生态系统服务价值最高,约占研究区总价值的50%,且呈增长趋势;生态系统服务构成中,除食物生产、气体调节和维持养分循环价值有所减少外,其他类型生态系统服务价值均呈增长趋势。驱动力分析表明,综合城镇化率是影响北部湾沿岸地区生态系统服务价值变化的重要驱动因素,说明区域生态系统服务与社会经济发展极为相关,合理调控经济结构可有效提升区域生态系统服务价值。     

湿地生态系统服务功能及其价值核算——以福州市为例

湿地在生态保护和社会发展中占据重要地位,科学合理评估湿地生态系统价值,是湿地保护与合理开发、确保湿地资源可持续性利用的基础。如何科学有效、标准化地评估湿地生态系统生产总值(GEP)并使之纳入国民经济体系核算是科学界和政府部门关注的焦点。本研究构建了湿地生态系统生产总值核算框架体系,并以福州市为研究案例,以多源数据为基础,从功能量与价值量层面,开展了福州市湿地GEP核算研究。结果表明: 2015年,福州市湿地生态系统GEP为2392.33亿元,主要源于湿地的生态调节服务价值(1578.69亿元,占总价值的66.0%),约是产品供给服务价值和精神文化服务价值的2.2和15.6倍。湿地生态调节服务中,以水流动调节服务价值和气候调节服务价值最高,两者占生态调节服务价值的82.9%。福州市湿地单位面积生态调节价值为134.78万元·hm^-2,高于全国水平。福州市各区县中,闽清县湿地生态系统价值量最大,为888.31亿元,占福州市湿地生态系统产品供给和生态调节总价值量的40.1%。福州市滨海湿地的生态调节服务价值最大,约分别是河流湿地和人工湿地类型的2.5倍。福州市湿地生态系统GEP核算是“绿水青山就是金山银山”理念城市尺度的重要践行,为指导地方政府科学管理湿地生态系统提供了技术支撑。     

湿地生物连通性影响因素及测度方法研究进展

湿地是地球上最多样化和最具生产力的生态系统之一,但近年来由于人类活动的频繁干扰,自然环境以前所未有的速度变化,两者的协同作用导致湿地呈现破碎化趋势。湿地的退化、转化使得湿地生物连通性减弱。良好的湿地生物连通性有利于湿地生态系统服务功能的维持、生物多样性的保护以及生物对气候变化适应水平的提高。因此理解、评估和保护生物连通性对于湿地生态系统而言至关重要。综述了湿地生物连通性的定义、总结了生物连通发生与变化的四个相互作用因子,从结构连通性、功能连通性两个测定角度,论述动物、植物和微生物三类研究对象生物连通性的测度方法,最后从湿地生物连通的时空尺度,连通与物质循环的耦合关系,湿地生物连通性评估分析模型改进角度提出了未来的研究方向,从而确保湿地生物连通网络的完整有效。可为湿地生物多样性、湿地保护和管理提供新的思路。     

Rapid risk assessment of wetland degradation and loss in low-lying coastal zone of Shanghai,China

Coastal wetlands are facing an increasingly high risk of degradation and loss due to a wide variety of human activities and natural processes. Human encroachment, including land reclamation, drainage, and introduction of invasive species, has direct negative effects on wetlands, mainly through changes in hydrology and vegetation. Additionally, accelerated sea level rise (SLR) can result in flooding of wetlands in low-lying coastal zones. In this study, we present a rapid risk assessment method for coastal wetland loss and degradation. The main stress factors, i.e., urban sprawl, agriculture, coastal erosion, and SLR, have been examined and quantified through remote sensing and geographic information system spatial analysis. A weighted factor-based linear model has been used to evaluate the spatial risk levels of wetland loss. The proposed methodology is applied to the low-lying coastal wetlands of Hangzhou Bay in Shanghai, China. The results show that the regions closer to the sea have relatively higher risk levels on the landward side of the coastline, but relatively low risk levels on the seaward side of the coastline. This work emphasizes the need to sustainably use and protect wetlands in order to reduce disaster risks and contribute to the improvement of human well-being.     

水文情势与盐分变化对湿地植被的影响研究综述

湿地植被是湿地生态系统的重要组成部分。水文情势与盐分变化直接影响到湿地植被的分布与演替。目前,全球气候变化和人类活动导致的水文情势改变与盐分聚集已造成大面积的湿地退化和盐渍化,已严重威胁全球淡水湿地生态系统的稳定和健康。系统总结了水文情势与盐分变化单一环境变量及其交互作用对湿地植物生理生态、物种多样性、群落结构与演替和植被动态等诸多方面的影响研究进展,并探讨了湿地水文动态-盐分变化-植被响应的综合模型研究现状,认为发展湿地综合模型预测未来水文情势与盐分变化情景下湿地演变,是应对气候变化湿地水盐管理和生态保护的重要工具,最后指出今后亟需加强的研究方向。     

Carbon budgets of wetland ecosystems in China

Wetlands contain a large proportion of carbon (C) in the biosphere and partly affect climate by regulating C cycles of terrestrial ecosystems. China contains Asia's largest wetlands, accounting for about 10% of the global wetland area. Although previous studies attempted to estimate C budget in China's wetlands, uncertainties remain. We conducted a synthesis to estimate C uptake and emission of wetland ecosystems in China using a dataset compiled from published literature. The dataset comprised 193 studies, including 370 sites representing coastal, river, lake and marsh wetlands across China. In addition, C stocks of different wetlands in China were estimated using unbiased data from the China Second Wetlands Survey. The results showed that China's wetlands sequestered 16.87 Pg C (315.76 Mg C/ha), accounting for about 3.8% of C stocks in global wetlands. Net ecosystem productivity, jointly determined by gross primary productivity and ecosystem respiration, exhibited annual C sequestration of 120.23 Tg C. China's wetlands had a total gaseous C loss of 173.20 Tg C per year from soils, including 154.26 Tg CO₂‐C and 18.94 Tg CH₄‐C emissions. Moreover, C stocks, uptakes and gaseous losses varied with wetland types, and were affected by geographic location and climatic factors (precipitation and temperature). Our results provide better estimation of the C budget in China's wetlands and improve understanding of their contribution to the global C cycle in the context of global climate change.     

Effects of an African grass invasion on vegetation,soil and interstitial water characteristics in a tropical freshwater marsh in La Mancha,Veracruz (Mexico)

Abstract

The transformation of freshwater wetlands to pastures is a common practice in Mexico. This rapid loss of wetlands contrasts with the scarce information that exists about these ecosystems. To identify the environmental factors that control vegetation structure of a freshwater wetland invaded by the African grass Echinochloa pyramidalis, we characterized the vegetation (species composition, cover and aerial biomass), soil (moisture, redox potential, bulk density and topography) and water (water depth level, electric conductivity and pH) in two seasons of the year (dry and rainy). In addition, we analyzed the soil and water of three vegetation areas in the wetland, one dominated by E. pyramidalis, another by Sagittaria lancifolia and a third by Typha domingensis. The parameters associated with the hydrology of the wetland (water level, soil moisture, redox potential and bulk density) explained the plant species distribution. The invasive grass dominated in the relatively drier areas in the wetland while native species such as S. lancifolia, T. domingensis and Pontederia sagitatta dominated wetter sites. Introduction of E. pyramidalis has caused negative changes in the wetland, in particular a decrease of the diversity of plant species. In addition, we believe that the invader grass, as a C4 species, has more efficient use of water than the native plants, as well as a larger biomass, characteristics that can change the hydrological pattern of this wetland.     

Upland burning and grazing as strategies to offset climate-change effects on wetlands

Wetland ecosystems perform a multitude of services valued by society and provide critical habitat for migratory birds and other wildlife. Despite their importance, wetlands have been lost to different local, regional, and global drivers. Remaining wetlands are extremely sensitive to changing temperature and precipitation regimes. Management of grassland areas in wetland catchments may be an effective strategy for counteracting potentially negative impacts of climate change on wetlands. Our objective was to estimate the effects of climate changes on wetland hydrology, and to explore strategies for increasing surface-water inputs to wetlands. We coupled a field study with process-based simulation modeling of wetland-water levels. We found that climate change could decrease the number of wetlands that hold ponded water during the waterfowl breeding season by 14% under a hot wet scenario or 29% under a hot dry scenario if no upland-management actions were taken. Upland burning reduced pond losses to 9% (hot wet) and 26% (hot dry). Upland grazing resulted in the smallest loss of ponded wetlands, 6% loss under the hot-and-wet scenario and 22% loss under the hot-and-dry scenario. Overall, water inputs could be increased by either burning or grazing of upland vegetation thereby reducing pond losses during the waterfowl breeding season. While field results suggest that both grazing and burning can reduce the vegetative structure that could lead to increases in runoff in grassland catchments, our model simulations indicated that additional actions may be needed for managers to minimize future meteorologically driven water losses.

     

长沙城市斑块湿地资源的时空演变

利用GIS技术,对长沙市1955、1972和1990年地形图湿地数据及2007年长沙市湿地资源普查数据进行提取和分析,选取最具代表性的斑块湿地作为研究对象,从时间与空间、动态与静态、规模与填埋等视角,研究50年来城市斑块湿地生态系统各层次要素的时空演变过程和变化规律.结果表明:(1)时间层次上,长沙城市斑块湿地总面积呈现先增后减、总体增加的态势;斑块湿地面积变化幅度不断加大,速率逐步加快;(2)规模层次上,面积在32 hm2规模以下的斑块湿地呈增加态势,32 hm2规模以上斑块湿地呈减少态势;(3)动态空间层次上,被填埋斑块湿地的比例在建成区和郊区呈相反的演变结果;(4)静态空间层次上,斑块湿地密度在建成区范围和郊区范围演变结果相背.研究显示,伴随着城市化进程,不同时间尺度、不同规模尺度、不同空间属性、不同空间状态的城市斑块湿地常常呈现差异很大、甚至是截然相反的演变结果;无论是动态空间还是静态空间,建成区与郊区的空间分界线往往是城市斑块湿地演变态势的分水岭.     

Landscape transformation after irrigation development in and around a semi-arid wetland ecosystem

The objectives of this study are to determine the spatial and temporal land use/cover changes in a semi-arid agricultural basin (Develi Basin) after the implementation of an irrigation project and to understand how these changes affected the wetlands (Sultan Marshes) located in the basin. The changes were determined using multitemporal Landsat Thematic Mapper and Landsat 8 Operational Land Imager imagery taken in 1987, 1998, 2007, and 2013. The images were classified into six information classes (grasslands/shrublands, croplands, permanent wetlands, water bodies, barren, and urban/built-up) using a hybrid classification method. Post-classification change detection was applied to determine the changes between different years. Overall, the accuracy of the classified images ranged from 85 to 94%. Grasslands/shrublands covered the largest area in the basin (63% in 2013), followed by croplands (32% in 2013). The area covered by water bodies, permanent wetlands, barren, and urban/built-up was 5% (in 2013). From 1987 to 2013, croplands expanded by 56%, while grasslands/shrublands declined by 15%. The areas occupied by water bodies decreased by 88% and permanent wetlands decreased by 4%. Urban/built-up areas expanded by 140%. The hydrologic regime of the Sultan Marshes wetland changed, which resulted in declines in water volumes by 85% and in water inflows by 55% from 2000 to 2015. Climatic variations during the 1987–2013 period were low and there was no apparent trend in precipitation and air temperature, which ruled out climatic conditions as one of the drivers of wetland changes. Economic and institutional factors supported the expansion of irrigated agriculture and animal husbandry in the basin and accelerated the expansion of croplands and conversion to industrial and fodder crops and orchards from traditional non-irrigated crops. Expansion of croplands and irrigated agriculture were the major drivers of the changes in the Sultan Marshes.

     

Development of an Indicator to Monitor Mediterranean Wetlands

Wetlands are sensitive ecosystems that are increasingly subjected to threats from anthropogenic factors. In the last decades, coastal Mediterranean wetlands have been suffering considerable pressures from land use change, intensification of urban growth, increasing tourism infrastructure and intensification of agricultural practices. Remote sensing (RS) and Geographic Information Systems (GIS) techniques are efficient tools that can support monitoring Mediterranean coastal wetlands on large scales and over long periods of time. The study aims at developing a wetland indicator to support monitoring Mediterranean coastal wetlands using these techniques. The indicator makes use of multi-temporal Landsat images, land use reference layers, a 50m numerical model of the territory (NMT) and Corine Land Cover (CLC) for the identification and mapping of wetlands. The approach combines supervised image classification techniques making use of vegetation indices and decision tree analysis to identify the surface covered by wetlands at a given date. A validation process is put in place to compare outcomes with existing local wetland inventories to check the results reliability. The indicator´s results demonstrate an improvement in the level of precision of change detection methods achieved by traditional tools providing reliability up to 95% in main wetland areas. The results confirm that the use of RS techniques improves the precision of wetland detection compared to the use of CLC for wetland monitoring and stress the strong relation between the level of wetland detection and the nature of the wetland areas and the monitoring scale considered.     

Zooplankton dynamics in response to the transition from drought to flooding in four Murray–Darling Basin rivers affected by differing levels of flow regulation

A review of stratigraphic, radiocarbon, pollen, and aerial photographic data on the Swan Coastal Plain, south-western Australia, allows interpretation of long-term changes in climate and its effects on wetlands during the Holocene, whereas monitoring wetland hydrology and vegetation provides a measure of shorter-term changes. The information provides models for basin wetland response to changing climate. Drying climates shift wetlands to drier conditions, turning lakes into seasonally inundated or waterlogged basins, or resulting in an overall loss of wetlands, and favours more saline conditions, and development of carbonate deposits. Wetter conditions results in more frequent inundation, shifting damplands to sumplands or lakes, and resulting in fresher water conditions, and development of peat and/or organic matter enriched deposits. Examples of wetland basin responses to climate change across the Swan Coastal Plain show differential responses depending on setting, spatial distribution, hydrology, hydrochemistry and geochemistry, different temporal frameworks, and biological resilience.     

Biodiversity dynamics of freshwater wetland ecosystems affected by secondary salinisation and seasonal hydrology variation: a model-based study

Freshwater wetlands worldwide are under threat from secondary salinisation and climate change. Given that many freshwater wetlands naturally have highly variable hydrology, it is important to understand the combined effects of salinity and water regime on wetland biodiversity. Here a mathematical model has been developed to explore the biodiversity dynamics of freshwater wetland ecosystems affected by secondary salinisation and seasonal hydrology variation. The model shows that seasonal hydrological change can drive the wetland ecosystem into a stable oscillatory state of biodiversity, with the same period as the wetting and drying cycle. The initial condition of a wetland mediates the ecological response of the wetland ecosystem to salinity and seasonal variability. There are two manifestations of stability that occur in relation to wetland biodiversity: monostability and bistability. In model simulations, some wetland ecosystems may respond to the effects of seasonal change quickly, while others may do so more slowly. In ‘slow response’ wetlands, seasonal variability has a weak impact on the ecosystem properties of stability, resilience, sensitivity and the species richness–mean salinity relationship. In contrast, ‘fast response’ wetlands are seasonally controlled heavily. Seasonal variability can play a critical role in determining ecosystem properties. Changes in the strength of seasonality can induce the transition between monostability and bistability. Seasonal variability may also reduce wetland resilience, exacerbating the risk of secondary salinisation. On the other hand, seasonal variability may provide opportunities for the restoration of salinised wetlands by increasing their sensitivity to management actions and facilitating recovery processes. Model simulations show that the response of the stable biodiversity oscillation to changing mean salinity is dependent on seasonality strength (primarily for fast response wetlands) and other wetland conditions. Generally, there are two types of wetland responses to changes in mean salinity: type 1 wetlands exhibit a graded response of species richness (a surrogate for biodiversity), whereas a hysteretic response occurs in type 2 wetlands. Species diversity displays critical behaviour: regime shifts in diversity occur at the thresholds of mean salinity, strength of seasonality or initial species diversity. The predictions are consistent with previously-published field observations in salinised freshwater wetlands. Handling editor: D. Hamilton