Stream community richness predicts apex predator occupancy dynamics in riparian systems

Streams and adjacent riparian habitats represent linked terrestrial and aquatic ecosystems that exchange materials and energy. Recognized relationships among apex predators and ecosystem biodiversity led us to hypothesize that these predators in riparian‐stream systems were more likely to be found in sites with high stream quality, defined as increased ecosystem function and integrity. In our freshwater study system, river otter Lontra canadensis and mink Neovison vison play critical roles as apex predators. We used multi‐season occupancy modelling across three sampling years (2012–2014) to compare aspects of the stream communities that explain occupancy dynamics of river otter and mink, including their interactions with other semi‐aquatic mammals. We surveyed for semi‐aquatic mammals at 77 sites in 12 major watersheds in southern Illinois, USA (44 526 km²). Naïve occupancy differed among years but generally increased for river otter, and remained high (≥93.5%) for mink. Increasing substrate availability increased detectability of river otter, whereas mink detection varied by survey period. Occupancy of river otter during the initial survey period was higher in sites closer to reintroduction points. Probability of colonization of river otter was positively associated with macroinvertebrate index of biotic integrity, fish species richness, and beaver presence. Sites with high species richness of fish families preferred by river otter also had increased river otter persistence. Mink occupied sites with increased fish richness, muskrat presence and mussel community index. Taken together, our results show occupancy of both mink and river otter were predicted by aspects of prey diversity and presence, indicating the importance of community composition in occupancy dynamics of riparian predators. Ultimately, these relationships suggest that habitat heterogeneity and system stability are important to apex predator site use. However, the relative role of bottom–up and top–down forcing in stream systems remains to be resolved.     

Understanding the inter-specific dynamics of two co-existing predators in the Tierra del Fuego Archipelago: the native southern river otter and the exotic American mink

Knowledge about interactions between endangered native southern river otters (Lontra provocax) and introduced American mink (Neovison vison) is essential for effective management of both species. We evaluated competition for spatial and trophic niches between otter and mink in overlapping and non-overlapping areas, comparing distribution, habitat preference, diet and mink marking behavior. We surveyed otter and mink signs along 250 km of Beagle Channel coastline. Habitat suitability models were constructed based on species presence/absence and habitat characteristics, using generalized linear models. Feces were collected for diet analyses. Otters used forested coasts with 12°–32° shoreline slope and without human influence, and our evidence suggests they were not affected by mink presence. Mink preferred forested and shrubland coasts with 10°–28° shoreline slope. Neither human influence nor otter presence affected mink habitat occupation, but in the presence of otters, mink left fewer signs. Otters consumed more aquatic prey than mink, and mink modified their diet in the presence of otters, consuming more exotic small terrestrial mammals and less fish as well as shifting to smaller and shallower fish species that are less consumed by otters. Mink showed more plastic, generalist behavior than otters, being more tolerant of human presence, using more habitat types and having greater diet breadth. At the same time, otters apparently affect mink adversely and could help limit their invasion in sympatric areas. Conservation and recovery of otters, therefore, may produce a secondary benefit of simultaneously reducing the effect of mink, thereby providing an additional way to control this exotic predator’s population.     

A synergistic trio of invasive mammals? Facilitative interactions among beavers,muskrats, and mink at the southern end of the Americas

With ecosystems increasingly having co-occurring invasive species, it is becoming more important to understand invasive species interactions. At the southern end of the Americas, American beavers (Castor canadensis), muskrats (Ondatra zibethicus), and American mink (Neovison vison), were independently introduced. We used generalized linear models to investigate how muskrat presence related to beaver-modified habitats on Navarino Island, Chile. We also investigated the trophic interactions of the mink with muskrats and beavers by studying mink diet. Additionally, we proposed a conceptual species interaction framework involving these invasive species on the new terrestrial community. Our results indicated a positive association between muskrat presence and beaver-modified habitats. Model average coefficients indicated that muskrats preferred beaver-modified freshwater ecosystems, compared to not dammed naturally flowing streams. In addition, mammals and fish represented the main prey items for mink. Although fish were mink’s dominant prey in marine coastal habitats, muskrats represented >50 % of the biomass of mink diet in inland environments. We propose that beavers affect river flow and native vegetation, changing forests into wetlands with abundant grasses and rush vegetation. Thus, beavers facilitate the existence of muskrats, which in turn sustain inland mink populations. The latter have major impacts on the native biota, especially on native birds and small rodents. The facilitative interactions among beavers, muskrats, and mink that we explored in this study, together with other non-native species, suggest that an invasive meltdown process may exist; however further research is needed to confirm this hypothesis. Finally, we propose a community-level management to conserve the biological integrity of native ecosystems.     

Monitoring site occupancy for American mink in its native range

American mink (Neovison vison) are secretive, semi-aquatic carnivores that often require noninvasive methods based on field signs such as tracks and scat for determining their spatial distribution. Most previous assessments of survey methods for American mink have been conducted in the United Kingdom where mink are an invasive species. We evaluated survey techniques for American mink in riparian habitat in its native range in the midwestern United States. We used occupancy modeling to compare detection rates between walking surveys and mink raft surveys, and we evaluated the potential for environmental covariates and observer bias to influence detectability from walking surveys. Per-survey detection probabilities were greater for walking surveys (0.72) than for mink rafts (0.39). Walking surveys also were cheaper and easier to conduct in small streams prone to flooding when compared to mink raft surveys. However, detection probabilities from walking surveys were affected by observer bias, recent rainfall, substrate, and date. We recommend walking surveys for determining the distribution of American mink in riparian habitat in the Midwest if occupancy modeling is applied to adjust for environmental and observer effects on detectability. We used such an approach to demonstrate occupancy dynamics of mink were related to variable water depths, which has implications for how this carnivore might be influenced by climate change. Mink rafts standardize the substrate for recording mink tracks and reduce the likelihood of observer effects. For studies using many volunteers, we recommend mink rafts for determining site occupancy by American mink. © 2011 The Wildlife Society.     

Habitat occupancy by riparian muskrats reveals tolerance to urbanization and invasive vegetation

Wildlife communities are being altered by rapid environmental change including habitat loss and fragmentation, urbanization, and spread of invasive species. To predict consequences of these anthropogenic changes to landscapes, it is necessary to identify not only species that are negatively affected, but also species that are unaffected or even thrive. We used occupancy modeling to examine the spatial distribution of muskrats (Ondatra zibethicus) in riparian habitat within an agricultural region of east-central Illinois from 2007 to 2008. We examined whether site occupancy was related to local habitat conditions and anthropogenic landscape alterations including urbanization and dominance of invasive reed canary grass (Phalaris arundinacea). We sampled 90 study sites (200-m stream segments) for occupancy by muskrats based on presence of tracks, scat, and feeding sign. Per-survey detection probability was 0.79 (SE = 0.04) in 2007 and 0.76 (SE = 0.04) in 2008. Detection was related positively to Julian date and negatively to abundance of woody debris and emergent rocks. Site occupancy by muskrats was 0.59 (SE = 0.09) in 2007 and 0.69 (SE = 0.06) in 2008, a year with above-average precipitation. Occupancy was related positively to urban land cover surrounding sites, which could reflect higher baseflows and reduced risk from predation and trapping in urban areas. Occupancy was unrelated to site dominance by invasive reed canary grass, but muskrats occurred more often at larger, deeper streams and those with greater bank heights and less sandy bank soils. Turnover between years was driven by stream size and water availability. Muskrats exhibited tolerance to key aspects of environmental change, and muskrats might even be urban adapters when occupying riparian habitat that remains adequately connected in urbanizing landscapes. © 2011 The Wildlife Society.     

Differential habitat use promotes sustainable coexistence between the specialist otter and the generalist mink

Optimal foraging and habitat selection theories predict that heterogeneous environments should favour the coexistence of competitors, especially when the dominant competitor is a specialist and the sub-ordinate is a generalist. In this paper, we analysed differential habitat use as a potential mechanism for the coexistence of two competing riparian mammals, the specialist and dominant Eurasian otter ( Lutra lutra ) and the generalist and sub-ordinate American mink ( Mustela vison ). We tested three hypotheses: H1: mink coexist with otters for longer in areas with abundance of habitats hosting terrestrial prey because, by not relying on aquatic prey, mink can segregate from its competitor. H2: the characteristics of the habitat closer to the riverbank will affect the length of time the two species coexist, because mink are still tied to the water even in the presence of otters. H3: denser vegetative cover along the bank increases the duration of coexistence of mink and otters because it reduces the frequency of their encounters. The first hypothesis was supported by the data and we found that in areas where terrestrial prey was abundant mink coexisted for longer with otters. The second hypothesis was also supported by the data and the characteristics of the habitat closer to the riverbank were the most important in determining coexistence time. Finally, we did not find supporting evidence for the third hypothesis. This study provides strong evidence that habitat heterogeneity plays an important role in determining the likelihood of coexistence of American mink with Eurasian otters. This result is particularly important from a conservation standpoint. Mink are invasive and a threat to endangered species in parts of their range. The knowledge that mink have a higher chance to persist in the presence of otters when terrestrial prey is abundant should be used to target areas for preferential mink management.     

River otter and mink occupancy dynamics in riparian systems

Semi-aquatic mammals are dependent upon streams and riparian areas, which are a product of the landscapes they drain. Both local stream morphology and surrounding land use are likely to have important influences on current occupancy of semi-aquatic mammals and potentially affect future geographic distributions. We identified aspects of the riparian system and stream structure at multiple scales that relate to the presence of river otter (Lontra canadensis) and mink (Neovison vison) to better understand how changing landscapes affect occupancy dynamics of these semi-aquatic mammals and to facilitate future monitoring and management. We estimated multi-season occupancy using 103 sites sampled over 6 seasonal sampling periods in southern Illinois, USA (44,526 km²) during 2012–2014. We hypothesized river otter and mink occupancy were related to multiple aspects of landscape and local habitat attributes including land cover, water availability, human disturbance, and stream characteristics. Occupancy of river otter was predicted by large stream size, less developed area near the stream site, and proximity to areas with reintroduced or remnant populations of river otter. Mink were more likely to occupy sites with small streams and decreased water availability near the site. However, top models for both species had low weights and high uncertainty for multiple variables. Habitat-based models may not be the best predictors of occupancy for these carnivores because they are more likely to respond to prey diversity or availability, but landscape changes that decrease natural water availability and increase human disturbance to the stream at the local scale are likely to negatively affect river otter. © 2019 The Authors. The Journal of Wildlife Management published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.     

The Effect of Habitat Association and Edaphic Conditions on Tree Mortality during El Niño‐induced Drought in a Bornean Dipterocarp Forest

The effects of El Niño‐induced droughts on dipterocarp forests must be quantified to evaluate the implications of future global climatic changes for the tropical forests of Southeast Asia. We studied the mortality of trees ≥ 1 cm in diameter in a lowland dipterocarp forest in Borneo before, during, and after the 1997/1998 El Niño drought. The annual mortality rates were 1.30, 1.75, and 1.66 percent/yr for the pre‐drought, drought, and post‐drought periods, respectively. The effect of drought was tree size‐dependent being greater for larger trees. Modified logistic regression analysis revealed a significant interaction effect between species' habitat association and edaphic condition on mortality rates in all periods. For species associated with wet habitat, drought effect was greater in dry conditions than in wet conditions, in both the drought and post‐drought periods. The mortality rates of dry‐habitat species were less affected by the drought both in dry and wet conditions. A similar pattern was also found in common Dipterocarpaceae species; mortality rates increased more in species associated with wet‐habitat in the drought and post‐drought periods. Species and families with higher mortality in the pre‐drought period tended to experience greater mortality increases during the drought and post‐drought periods. These results suggest that changes in drought regimes alter the species composition and spatial distribution of dipterocarp forests.     

Effect of mink, Mustela vison Schreber, predation on cohorts of juvenile Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in three small streams

Two cohorts of Atlantic salmon parr and one of brown trout were studied in periods with and without the presence of mink, Mustela vison . In all localities a marked increase in mortality rate was observed during periods when mink were present. Mink were observed catching salmon parr, and approximately 10% of the parr had bite marks, especially on the tail fins. In the smallest stream with brown trout, the mortality rate was 0.80 during a few days with mink present; remnants of trout were found along the stream. The present study suggests that mink predation may be a major cause of mortality of salmonids in small streams. The results indicate that predation efficiency may vary with characteristics of the habitat, especially stream width and discharge, and fish density.     

Phenotypic variation of an alien species in a new environment: the body size and diet of American mink over time and at local and continental scales

Introduced species must adapt their ecology, behaviour, and morphological traits to new conditions. The successful introduction and invasive potential of a species are related to its levels of phenotypic plasticity and genetic polymorphism. We analysed changes in the body mass and length of American mink (Neovison vison) since its introduction into the Warta Mouth National Park, western Poland, in relation to diet composition and colonization progress from 1996 to 2004. Mink body mass decreased significantly during the period of population establishment within the study area, with an average decrease of 13% from 1.36 to 1.18 kg in males and of 16% from 0.83 to 0.70 kg in females. Diet composition varied seasonally and between consecutive years. The main prey items were mammals and fish in the cold season and birds and fish in the warm season. During the study period the proportion of mammals preyed upon increased in the cold season and decreased in the warm season. The proportion of birds preyed upon decreased over the study period, whereas the proportion of fish increased. Following introduction, the strictly aquatic portion of mink diet (fish and frogs) increased over time, whereas the proportion of large prey (large birds, muskrats, and water voles) decreased. The average yearly proportion of large prey and average‐sized prey in the mink diet was significantly correlated with the mean body masses of males and females. Biogeographical variation in the body mass and length of mink was best explained by the percentage of large prey in the mink diet in both sexes, and by latitude for females. Together these results demonstrate that American mink rapidly changed their body mass in relation to local conditions. This phenotypic variability may be underpinned by phenotypic plasticity and/or by adaptation of quantitative genetic variation. The potential to rapidly change phenotypic variation in this manner is an important factor determining the negative ecological impacts of invasive species. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 681–693.     

Long-term radial growth and climate-growth relationships of Quercus petraea (Matt.) Liebl. and Quercus cerris L. in a xeric low elevation site from Hungary

Sessile oak (Quercus petraea [Matt.] Liebl.) and Turkey oak (Quercus cerris L.) dominated mixed forests are common in low montane and hilly regions in Hungary. Here, we aimed to describe the long-term pattern and climatic responses of the radial growth of Q. petraea and Q. cerris in a xeric low-elevation forest, using retrospective tree-ring analysis for the period 1910–2019. We performed separate analyses with time series of full tree-ring (TRW), earlywood (EW) and latewood (LW) widths. Our results showed that the radial growth of the two species was largely synchronous over time, but became transiently divergent for a 20-year period after a drought in 1968, due to the greater-than-expected growth of Q. cerris and the supressed growth of Q. petraea. Precipitation was the major growth-limiting factor for both species, with a strong positive influence on LW particularly during the current early growing season (March-June), on EW in the previous late summer (August-September) and in the current early spring (March), and on all tree-ring traits in the previous December. The radial growth of both species was negatively related to temperature in the spring (May) and late summer (August) of both current and previous years. The climate-growth relationships showed general instability over time: the most striking temporal change was a gradual shift of positive correlations with precipitation and SPEI during the growing season from spring (March-May) to summer (June-August) since the 1980s over the analysed period. The two species had similarly low growth resistance to droughts in four studied pointer years (1968, 1993, 2002 and 2012), but Q. cerris exhibited a greater capacity to recover over the four post-drought years, and thus higher growth resilience, particularly after the drought of 1968. Our results contribute to the better understanding of the role of climate variability and droughts in the growth of the two co-existing species in transitional locations between closed forests and forest-steppes.     

Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams

Recent droughts raise global concern over potential biodiversity loss and mitigating impacts to vulnerable species has become a management priority. However, drought impacts on populations are difficult to predict, in part, because habitat refuges can buffer organisms from harsh environmental conditions. In a global change context, more extreme droughts may turn previously suitable habitats into ecological traps, where vulnerable species can no longer persist. Here, we explore the impacts of California's recent record‐breaking drought on endangered juvenile Coho salmon. We estimated the variability of cumulative salmon survival using mark–recapture of nearly 20,000 tagged fish in intermittent stream pools during a 7‐year period encompassing drought and non‐drought conditions. We then determined the relative importance of physical habitat, streamflow, precipitation, landscape, and biological characteristics that may limit survival during drought. Our most striking result was an increase in the number of pools with reduced or zero survival during drought years and a coincident increase in spatial variability in survival among study reaches. In nearly half of the stream pools, salmon survival during drought was similar to mean survival of pools assessed during non‐drought years, indicating some pools had remarkable resistance (ability to withstand disturbance) to extreme drought. Lower survival was most attributable to longer duration of disconnection between upstream and downstream habitats, a consequence of increasing drought severity. Our results not only suggest that many pools sustain juvenile salmon in non‐drought years transition into ecological traps during drought but also highlight that some pools serve as refuges even under extreme drought conditions. Projected increases in drought severity that lead to longer droughts and greater habitat fragmentation could transform an increasing proportion of suitable habitats into ecological traps. Predicting future impacts of drought on Coho salmon and other sensitive species will require identification and protection of drought refuges and management strategies that prevent further habitat fragmentation.     

Effects of a multi-year drought on a drought-adapted shrub, <Emphasis Type="Italic">Artemisia tridentata</Emphasis>

Models of climate change predict more variable precipitation for much of western North America, including more severe multi-year droughts. Droughts are known to increase mortality to trees although less is known about effects on shrubs from arid environments and about effects on reproduction. In this study, we followed a cohort of young sagebrush plants from 2010 to 2016, a period that included a severe drought from 2012 to 2015. Plants experienced little mortality preceding and during the drought. However, in the year following the drought, 14% of individuals died and 33% of branches on living plants died. There was little flowering in the years preceding the drought and flowering increased in each successive year from 2014 to 2016. Plants that produced more flowers in 2015 had more dead branches in 2016. Larger plants had fewer branches that died. Contrary to expectations, afternoon shade was not associated with greater survival or flowering, perhaps because shaded plants were in proximity to large trees which likely competed for water. Plants of the two common chemotypes had similar rates of survival and flowering. Experimental watering during the summer of 2015 did not affect survival and may have increased flowering in 2016. If multi-year droughts become more common in the future, even drought-adapted shrubs may be expected to suffer high rates of mortality.     

A comparison of the feeding biology of Mink Mustela vison and otter Lutra lutra

The feeding habits and prey selectivity of Mink Mustela vison and otters Lutra lutra were compared in two localities in Devon: a eutrophic lake and a moorland river, in which both species occurred and had access to the same prey populations. The effects of prey availability on the predators' diets were assessed by comparing prey consumed, as revealed by scat analysis, with estimates of prey abundance and size range. Otters specialized in fish at all times of year but showed seasonal variation in species taken. Selection for slow-moving fish and seasonal changes in behaviour of some fish species were the probable causes of this variation. Otters diversified more into non-fish food in summer, when fish availability was reduced. The main alternative prey in the lacustrine habitat was waterfowl, but in the riverine habitat, rabbits. Mink were more generalized carnivores, taking a variety of fish, waterside and terrestrial prey in all seasons. These three prey categories were taken to an almost equal extent in the lake but terrestrial prey dominated in the riverine habitat. Fish were taken most frequently in winter and birds and mammals in summer. Neither predator showed selection in respect of prey size. In each area, about one third of the otter and Mink diets was common to both species. Fish was the principal group of the shared component, and dietary overlap in respect of them was greatest in autumn and winter. In view of the dietary preferences of each predator, the existence of alternative prey items and limited degree of dietary overlap, it is considered unlikely that the two species competed for food to any extent. Other factors must therefore be responsible for the spread of feral Mink and the decline in otter populations in many parts of Britain.     

Drought survival is a threshold function of habitat size and population density in a fish metapopulation

Because smaller habitats dry more frequently and severely during droughts, habitat size is likely a key driver of survival in populations during climate change and associated increased extreme drought frequency. Here, we show that survival in populations during droughts is a threshold function of habitat size driven by an interaction with population density in metapopulations of the forest pool dwelling fish, Neochanna apoda. A mark–recapture study involving 830 N. apoda individuals during a one‐in‐seventy‐year extreme drought revealed that survival during droughts was high for populations occupying pools deeper than 139 mm, but declined steeply in shallower pools. This threshold was caused by an interaction between increasing population density and drought magnitude associated with decreasing habitat size, which acted synergistically to increase physiological stress and mortality. This confirmed two long‐held hypotheses, firstly concerning the interactive role of population density and physiological stress, herein driven by habitat size, and secondly, the occurrence of drought survival thresholds. Our results demonstrate how survival in populations during droughts will depend strongly on habitat size and highlight that minimum habitat size thresholds will likely be required to maximize survival as the frequency and intensity of droughts are projected to increase as a result of global climate change.     

Demography of three populations of American mink Mustela vison in Europe

• 1 Demographic data on an invasive species of management concern, the American mink, are presented. Data were obtained on three feral mink populations in Europe distinguished by differences in the time elapsed since population establishment.
• 2 Demographic data are presented in the form of life tables, age–sex distributions and sex ratios. Mink lived a maximum of 6 years, and mortality of 1‐year‐olds and adults differed substantially between populations.
• 3 The data support the hypothesis that mink populations subject to culling have a higher proportion of young (less than 1 year old) to adults compared with non‐culled populations.

     

A Comparison of the diet and distribution of southern river otter (Lutra provocax) and mink (Mustela vison) in Southern Chile

In Chile, between latitudes 39S and 4330'S, southern river otter ( Lutra provocax ) and introduced mink ( Mustela vison ) scats were collected. Mink sign was recorded in 29% of the aquatic habitat where otter sign was found. Sixty-eight per cent of mink scats were collected at otter rest sites. A significant difference between the diets and low percentage of habitat use overlap of the two species (5-22%) suggest that, in Chile, river otter and mink coexist with little competition for space or food. There are no data supporting a relationship between the introduction of mink and the decline of southern river otters.     

Impact of released Eurasian otters on a population of American mink: a test using an experimental approach

Competition is considered to be a major evolutionary driving force within assemblages of related and morphologically similar species, but it is notoriously difficult to test and quantify especially amongst species that cannot be easily manipulated in a laboratory or in the field. By exploiting a re-introduction of Eurasian otters ( Lutra lutra ) in the Upper Thames catchment (UK) in 1999 we performed an experiment to test whether there was evidence of competition between otters and American mink ( Mustela vison ). Mink and otters are semi-aquatic mustelids belonging to the same guild. Otters are expected to be the dominant competitor because they are larger and better adapted at exploiting aquatic resources. Our hypothesis was that mink declines when the density of otters increases. We measured the effect of competition at the population level, by observing whether mink distribution and densities changed in an area of 1353 km² in association with the arrival of its putative competitor. We estimated distribution and densities by means of sign surveys and trapping. The results showed that otters were associated with a significant and rapid reduction in the densities of mink, while mink occupancy remained approximately the same in an area of 2464 km² that was used as a control. We observed that the spatial distribution of the mink population throughout its decline was influenced by the yearly cycle of mink activities, with areas being temporarily re-colonized during the dispersal season. Mink is an invasive species in the UK threatening the survival of some native species. Our findings suggest that the re-establishment of otter populations is likely to lead to a decline of mink that may, in turn, be beneficial to native species threatened by this invasive.     

基于SPEI指数的长江中下游流域干旱时空特征分析

基于长江中下游流域1961—2015年129个气象站点的逐日气温和降水数据,利用标准化降水蒸散指数(SPEI),对长江中下游流域近55年年尺度及各季节干旱变化趋势、站次比、强度和频率进行了分析,并探讨了干旱和区域气温、降水变化及ENSO的关系。结果表明:(1)在区域尺度,近55年长江中下游流域年尺度、春季和秋季呈干旱化趋势,春季干旱化趋势显著;夏季和冬季呈湿润化趋势。空间变化上,对于年尺度,汉江流域、中游干流区及洞庭湖流域以干旱化趋势为主,鄱阳湖流域、下游干流区和太湖流域以湿润化趋势为主;春季和秋季分别有96.90%和92.25%的站点呈干旱化趋势;夏季和冬季分别有82.95%和72.87%的站点呈湿润化趋势。(2)年尺度、春季和秋季干旱站次比及强度均呈增加趋势,春旱站次比与强度增加趋势显著;夏季和冬季干旱站次比和强度均呈下降趋势。(3)年尺度和春季干旱频率在21世纪初均达到最高,年尺度、春季和夏季干旱频率从20世纪90年代到21世纪初均呈增加趋势。(4)春、秋季干旱化趋势与降水量的减少及气温的上升相关,夏、冬季降水量的增加使得夏、冬季呈湿润化趋势。冬季SOI和次年春季干旱相关性极显著,冬季发生拉尼娜事件时,次年春季更易发生干旱。