Determining the density dependence of immigration and emigration of benthic stream invertebrates: theoretical considerations

Simple mathematical models are formulated to describe density independent and density dependent dispersal. These models clarify hypotheses of density dependence and may be manipulated easily to suit particular applications. The models demonstrate that the initial composition of a species aggregate must be controlled before valid conclusions can be drawn about the density dependency of the aggregate's dispersal. Stochastic models of emigration are derived to assess the power of particular experimental designs and statistical techniques to discriminate a known form of density dependent emigration. Contribution No. 369, Great Lakes Research Division, University of Michigan Contribution No. 369, Great Lakes Research Division, University of Michigan     

Population genetic structure of the round goby in Lake Michigan: implications for dispersal of invasive species

Understanding subsequent dispersal of non-native species following introduction is important for predicting the extent and speed of range expansion and is critical for effective management and risk assessment. Post-introduction dispersal may occur naturally or via human transport, but assessing the relative contribution of each is difficult for many organisms. Here, we use data from seven microsatellite markers to study patterns of dispersal and gene flow among 12 pierhead populations of the round goby (Neogobius melanostomus) in Lake Michigan. We find significant population structure among sampling sites within this single Great Lake: (1) numerous populations exhibited significant pairwise F _ST and (2) a Bayesian assignment analysis revealed three distinct genetic clusters, corresponding to different pierhead locations, and genetic admixture between these clusters in the remaining populations. Genetic differentiation (F _ST) is generally related to geographic distance (i.e., isolation by distance), but is periodically interrupted at the scale of Lake Michigan due to gene flow among geographically distant sites. Moreover, average genetic differentiation among populations exhibit a significant, negative correlation with the amount of shipping cargo at ports. Our results, therefore, provide evidence that genetic structure of the round goby in Lake Michigan results from limited natural dispersal with frequent long-distance dispersal through anthropogenic activities such as commercial shipping. Our study suggests that while round gobies can undoubtedly disperse and found new populations through natural dispersal mechanisms, their spread within and among the Great Lakes is likely aided by transport via ships. We, therefore, recommend that ballast-water treatment and management may limit the spread of non-native species within the Great Lakes after the initial introduction in addition to preventing the introduction of non-native species to the Great Lakes.     

Impacts of the emerald ash borer (EAB) eradication and tree mortality: potential for a secondary spread of invasive plant species

Since the discovery of the emerald ash borer in 2002, eradication efforts have been implemented in an attempt to eliminate or contain the spread of this invasive beetle. The eradication protocol called for the removal of every ash tree within a 0.8 km radius around an infested tree. In 2005 this study was established to identify environmental changes attributed to the eradication program and measure subsequent shifts in forest community composition and structure. We conducted this study in Ohio and compared areas that received the eradication treatment (ash trees cut down), to areas that were left uncut, (ash still standing). The goal of this project was to identify how the plant community is responding in these two areas. The eradication protocol accelerated the formation and size of gaps within the forest and thus increased the duration and intensity of light penetrating through to the forest floor. In addition, the use of track vehicles for removal of cut trees resulted in significant soil compaction. The resultant plant community had greater species diversity (H′). When specific species composition differences were compared, an increase in the establishment of invasive plant species was detected in areas that received eradication efforts compared to those that did not. Invasive species accounted for 18.7% of the total herbaceous cover in this highly disturbed environment which included Cirsium arvense, Rhamnus cathartica and 2 species of Lonicera. In contrast, invasive species accounted for <1% of the total herbaceous cover in the undisturbed uncut areas.     

白蜡窄吉丁虫在中国的研究现状与分布调查

2002年在美国密执安州发现了一重大外来入侵害虫,白蜡窄吉丁Agrilus planipennis Fairmaire, 由于该虫新近在美国发现,危害白蜡树,故英文名称为Emerald Ash Borer (EAB)。初步分析显示,该虫至少在5年前就已传入美国,只是现在刚爆发被发现。目前该虫在美国密执安州和加拿大安大略省可致死各种大小的白蜡树。该虫在我国曾被定名为花曲柳窄吉丁Agrilus marcopoli Obenberger,为鞘翅目吉丁科。据资料报道分布于黑龙江、吉林、辽宁、山东、内蒙古和台湾等地;在国外分布于朝鲜,日本,蒙古,俄罗斯远东地区。危害木樨科属树木。两年一代或一年一代,因地而已。在东北三省主要危害水曲柳和花曲柳等树木,但不是主要害虫。天津市1993年在引种白蜡树上发现了白蜡窄吉丁,1998年严重爆发造成大面积白蜡树死亡。在亚洲的其他分布区,该虫还危害另外一些阔叶树,如多种蜡树、榆树等。鉴于该虫的广泛分布和寄主树种多及毁灭性危害的特点,美国一些专家认为其潜在危害不亚于1996年在美国发现的光肩星天牛。因此,美国已全面开展对该虫的研究,包括资料收集,风险性评估,研究各种控制措施等。 本文是中美对此虫合作研究的一部分,旨在对该虫在我国的分布、 危害、 研究基础作一调查与回顾, 为下一步深入研究提供信息与参考资料。白蜡窄吉丁入侵北美再一次显示随着贸易的全球化发展, 外来入侵种问题也越来越严重。国际合作共同开展研究是控制入侵种的重要举措之一。     

Patterns and Pathways in the Post-Establishment Spread of Non-Indigenous Aquatic Species: The Slowing Invasion of North American Inland Lakes by the Zebra Mussel

The zebra mussel, Dreissena polymorpha, has spread through eastern North American aquatic ecosystems during the past 15 years. Whereas spread among navigable waterways was rapid, the invasion of isolated watersheds has progressed more slowly and less predictably. We examined the patterns of overland spread over multiple spatial and temporal extents including individual lake districts, states, and multi-state regions in the USA and found that only a small proportion (<8%) of suitable inland lakes have been invaded, with the rate of invasion appearing to be slowing. Of the 293 lakes known to be invaded, 97% are located in states adjacent to the Laurentian Great Lakes with over half located in Michigan. Only six states have more than 10 invaded lakes and only in Michigan and Indiana have more than 10% of suitable lakes become invaded. At smaller spatial extents, invaded lakes are often clustered within a lake-rich region across southern Michigan and northern Indiana. This clustering appears primarily due to multiple overland invasions originating from the Great Lakes followed to a lesser extent by subsequent secondary overland and downstream dispersal. Downstream spread appears responsible for only one third of the inland invasions. Temporally, invasions peaked in the late 1990s, with only 13 new invasions (0.4% of suitable lakes) reported in 2003 in the four-state region surrounding Lake Michigan. Peak rates of invasion occurred 4–6 years earlier in Michigan relative to Indiana and Wisconsin, but this time lag is likely due to differences in the establishment of Great Lake source populations rather than ‘stepping stone’ dispersal across the landscape.     

Native Vaccinium spp. and Gaylussacia spp. infested by Rhagoletis mendax (Diptera: Tephritidae) in the Great Lakes Region: a potential source of inoculum for infestation of cultivated blueberries.

In this study, we addressed the question of whether or not native stands of blueberry (Vaccinium spp.) and/or huckleberry (Gaylussacia spp.) support populations of blueberry maggot, Rhagoletis mendax Curran, in the Great Lakes region. Infestation of commercial blueberries by the blueberry maggot, R. mendax, is a serious problem in many areas where blueberries are grown. In the past 10-20 yr, commercial bighbush blueberry, Vaccinium corymbosum L., production has expanded into places such as southern Ontario and southern Quebec where blueberry maggot had not previously been reported. In the mid-1990s, isolated infestations of commercial highbush blueberry were reported in southern Ontario. Because R. mendax was not considered endemic to that area, it was widely assumed that the pests had come into the fields via movement from exotic localities. Here we present an alternative hypothesis, that the blueberry maggots infesting newly established highbush plantations are derived from native blueberries growing in the vicinity. To test this hypothesis, in 1997-1999, we sampled potential native hosts for R. mendax (Vaccinium spp. and Gaylussacia spp.) from 31 localities in the Great Lakes region, primarily in Michigan and Ontario. R. mendax was reared from fruits of native hosts collected at four sites in Michigan and one site each in Ontario, Indiana, and Ohio. V. corymbosum was the predominant host infested, with infestation of this host observed at five of the seven sites. However, two huckleberry species [Gaylussacia baccata (Wangenheim) K. Koch, and Gaylussacia dumosa (Andersson) Torrey & Gray] had the highest rates of infestation that we observed (25.4 and 17.6%, respectively). These data represent the first published reports of R. mendax infesting native host plants in the Great Lakes region, and support the hypothesis that infestations observed in commercial fields may have originated from infested native host plants.     

Evolving Management Strategies for a Recently Discovered Exotic Forest Pest: The Pine Shoot Beetle, Tomicus Piniperda (Coleoptera)

Established populations of the Eurasian pine shoot beetle (Tomicus piniperda (L.); Coleoptera: Scolytidae) were first discovered in North America in Ohio in 1992. As of 31 December 2000, T. piniperda was found in 303 counties in 12 US states (Illinois, Indiana, Maine, Maryland, Michigan, New Hampshire, New York, Ohio, Pennsylvania, Vermont, West Virginia, and Wisconsin) and in 43 counties in 2 Canadian provinces (Ontario and Quebec). A federal quarantine imposed in November 1992 regulates movement of pine (Pinus) trees, logs, and certain pine products from infested to uninfested areas within US. The forest products, Christmas tree, and nursery industries are affected by the quarantine. This paper summarizes information on the discovery and spread of T. piniperda in North America, survey efforts, recent interception history, development and changes in the federal quarantine, development of a national compliance management program, and extension and research efforts.     

Climate connectivity of the bobcat in the Great Lakes region

The Great Lakes and the St. Lawrence River are imposing barriers for wildlife, and the additive effect of urban and agricultural development that dominates the lower Great Lakes region likely further reduces functional connectivity for many terrestrial species. As the climate warms, species will need to track climate across these barriers. It is important therefore to investigate land cover and bioclimatic hypotheses that may explain the northward expansion of species through the Great Lakes. We investigated the functional connectivity of a vagile generalist, the bobcat, as a representative generalist forest species common to the region. We genotyped tissue samples collected across the region at 14 microsatellite loci and compared different landscape hypotheses that might explain the observed gene flow or functional connectivity. We found that the Great Lakes and the additive influence of forest stands with either low or high canopy cover and deep lake‐effect snow have disrupted gene flow, whereas intermediate forest cover has facilitated gene flow. Functional connectivity in southern Ontario is relatively low and was limited in part by the low amount of forest cover. Pathways across the Great Lakes were through the Niagara region and through the Lower Peninsula of Michigan over the Straits of Mackinac and the St. Marys River. These pathways are important routes for bobcat range expansion north of the Great Lakes and are also likely pathways that many other mobile habitat generalists must navigate to track the changing climate. The extent to which species can navigate these routes will be important for determining the future biodiversity of areas north of the Great Lakes.     

Spatial spread and infestation risk assessment in the Asian longhorned beetle,Anoplophora glabripennis

The Asian longhorned beetle (ALB), Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae), is recognised as potentially one of the most damaging invasive insects in Europe and North America. International trade has increased the risk of accidental introduction of ALB. An eradication programme was initiated in Northeast Italy in June 2009, when an ALB infestation was discovered. The infestation was monitored by annual surveys of all host‐tree species growing in the eradication area. Infested trees were cut down and chipped. This study analyses the spatiotemporal distribution of infested trees for a 5‐year period from 2008 to 2012 using a generalised linear model approach. The results show that spread and infestation risk were significantly affected by (1) distance of suitable hosts from the nearest infested trees, (2) number of infested trees in the surroundings, and (3) annual variation. The significant differences in beetle dispersal between years reflect to some extent the onset of the eradication programme. The model allowed the estimation of arbitrary probability‐based management boundaries surrounding ALB‐infested trees. For example, the model estimated a 0.1% probability of attack on a suitable host tree 1 910 m from an existing attack.     

Interspecific variation in resistance to emerald ash borer (Coleoptera: Buprestidae) among North American and Asian ash (Fraxinus spp.)

We conducted a 3-yr study to compare the susceptibility of selected North American ash and an Asian ash species to emerald ash borer, Agrilus planipennis Fairmaire, an invasive wood-boring beetle introduced to North America from Asia. Because of a coevolutionary relationship between Asian ashes and emerald ash borer, we hypothesized an Asian ash species, Manchurian ash, is more resistant to the beetle than its North American congeners. Consistent with our hypothesis, Manchurian ash experienced far less mortality and yielded far fewer adult beetles than several cultivars of North American green and white ash. Surprisingly, a black ash (North American) x Manchurian ash hybrid was highly susceptible to emerald ash borer, indicating this cultivar did not inherit emerald ash borer resistance from its Asian parent. A corollary study investigated the efficacy of soil-applied imidacloprid, a systemic, neonicotinoid insecticide, for controlling emerald ash borer in each of the five cultivars. Imidacloprid had no effect on emerald ash borer colonization of Manchurian ash, which was low in untreated and treated trees. In contrast, imidacloprid did enhance survival of the North American and hybrid cultivars and significantly reduced the number of emerald ash borer adults emerging from green and white ash cultivars. We identify a possible mechanism of resistance of Manchurian ash to emerald ash borer, which may prove useful for screening, selecting, and breeding emerald ash borer-resistant ash trees.     

Post-glacial recolonization of the Great Lakes region by the common gartersnake (Thamnophis sirtalis) inferred from mtDNA sequences

Pleistocene events played an important role in the differentiation of North American vertebrate populations. Michigan, in particular, and the Great Lakes region, in general, were greatly influenced by the last glaciation. While several hypotheses regarding the recolonization of this region have been advanced, none have been strongly supported. We generated 148 complete ND2 mitochondrial DNA (mtDNA) sequences from common gartersnake (Thamnophis sirtalis) populations throughout the Great Lakes region to evaluate phylogeographic patterns and population structure and to determine whether the distribution of haplotypic variants is related to the post-Pleistocene retreat of the Wisconsinan glacier. The common gartersnake was utilized, as it is believed to have been one of the primary vertebrate invaders of the Great Lakes region following the most recent period of glacial retreat and because it has been a model species for a variety of evolutionary, ecological, behavioral, and physiological studies. Several genetically distinct evolutionary lineages were supported by both genealogical and molecular population genetic analyses, although to different degrees. The geographic distribution of the majority of these lineages is interpreted as reflecting post-glacial recolonization dynamics during the late Pleistocene. These findings generally support previous hypotheses of range expansion in this region.     

花曲柳窄吉丁的寄主植物范围、危害和防治对策

花曲柳窄吉丁(emerald ash borer, EAB) Agrilus planipennis Fairmaire是木犀科（Oleaceae）梣属Fraxinus植物树木的毁灭性蛀干害虫,目前发现其主要危害欧梣亚属Subgen. Fraxinus欧梣组(Section Fraxinus)和象蜡树组（Section Meliodes）的一些树木种类,如美国白蜡Fraxinus americana、洋白蜡F. pennsylvanica var subintegerrima、绒毛白蜡F. velutina、黑白蜡F. nigra和水曲柳F. mandshurica等。与早期文献上的记载不同,很少发现花曲柳窄吉丁对苦枥木亚属Subgen. Ornus白蜡树组(Section Ornaster)的白蜡树F. chinensis Roxb和花曲柳F. rhynchophylla造成危害。不同寄主白蜡树对花曲柳窄吉丁的敏感性不同,洋白蜡、绒毛白蜡等北美白蜡树种受其危害重于亚洲的水曲柳。这可能是由于长期的适应进化,分布在亚洲地区的梣属寄主树木发展了对花曲柳窄吉丁的抗虫性。中国部分地区发生的花曲柳窄吉丁灾害应该归属于引进的北美白蜡树种对本地害虫敏感所致。在天津、黑龙江、吉林、辽宁、河北、山东、四川、台湾、内蒙古和新疆等省市和自治区,都曾发现花曲柳窄吉丁的危害或分布。白蜡属树木广泛分布于我国除了青藏地区和海南省之外的大部分地区,其中大面积引种北美白蜡树种的北方和西北地区,以及近年来种植发展水曲柳的东北地区都是花曲柳窄吉丁这种毁灭性蛀干害虫的潜在发生地。花曲柳窄吉丁在我国部分地区为检疫性林业害虫,做好检疫是杜绝其扩散蔓延的首要方法。我国局部地区出现花曲柳窄吉丁灾害说明,北美白蜡树种在我国的适应性还需要进行更进一步科学的全面的风险评估,在我国生态建设中应重视利用我国本地的白蜡树种。在花曲柳窄吉丁灾害的治理中,可采用营造混交林以降低蔓延的速度和发挥自然控制能力,加强肥水管理以增强树木的抗虫能力,清除被害木消灭虫源,以及幼虫期树干注射、成虫期喷洒化学农药等防治措施。     

Dispersal versus climate: Expansion of Fagus and Tsuga into the Upper Great Lakes region

Pollen records for American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis) compiled from 50 sites in Michigan and Wisconsin, USA, show that both species entered the Upper Great Lakes region about 7000 yr B.P., reaching their western and southwestern boundaries between 2000 and 1000 yr B.P. Fagus advanced northward into lower Michigan as a continuous front, except where Lake Michigan posed a geographic barrier. Colonies were established on the far side of the lake after a 1000 year lag, implying that longdistance dispersal across a 100-km wide barrier can occur. The Fagus range may not have been in equilibrium with climate for one or two thousand years before this time, when seeds were dispersed across the lake to Wisconsin. Tsuga seeds may have been dispersed 150 km or more from Ontario to reach Upper Michigan. Scattered colonies were established 6000–7000 yr B.P. on either side of Lake Michigan, which did not pose a significant barrier to this wind-dispersed species, Tsuga spread rapidly over a large region prior to 5000 yr B.P. Subsequent expansion to the west occurred more slowly, and may reflect gradual climatic changes in northern Wisconsin during the second half of the Holocene. Tsuga's range may have been limited by dispersal, rather than climate, for an unknown length of time prior to 5000 yr B.P. During this period Tsuga was expanding its range rapidly. The study shows, however, that it is difficult to devise rigorous tests to distinguish between dispersal limitations and climate as factors limiting range limits in the past.This work has been supported by the U.S. National Science Foundation.This work has been supported by the U.S. National Science Foundation.     

Host range of the emerald ash borer (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae) in North America: results of multiple-choice field experiments

Emerald ash borer (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae), an invasive phloem-feeding pest, was identified as the cause of widespread ash (Fraxinus) mortality in southeast Michigan and Windsor, Ontario, Canada, in 2002. A. planipennis reportedly colonizes other genera in its native range in Asia, including Ulmus L., Juglans L., and Pterocarya Kunth. Attacks on nonash species have not been observed in North America to date, but there is concern that other genera could be colonized. From 2003 to 2005, we assessed adult A. planipennis landing rates, oviposition, and larval development on North American ash species and congeners of its reported hosts in Asia in multiple-choice field studies conducted at several southeast Michigan sites. Nonash species evaluated included American elm (U. americana L.), hackberry (Celtis occidentalis L.), black walnut (J. nigra L.), shagbark hickory [Carya ovata (Mill.) K.Koch], and Japanese tree lilac (Syringa reticulata Bl.). In studies with freshly cut logs, adult beetles occasionally landed on nonash logs but generally laid fewer eggs than on ash logs. Larvae fed and developed normally on ash logs, which were often heavily infested. No larvae were able to survive, grow, or develop on any nonash logs, although failed first-instar galleries occurred on some walnut logs. High densities of larvae developed on live green ash and white ash nursery trees, but there was no evidence of larval survival or development on Japanese tree lilac and black walnut trees in the same plantation. We felled, debarked, and intensively examined >28 m2 of phloem area on nine American elm trees growing in contact with or adjacent to heavily infested ash trees. We found no sign of A. planipennis feeding on any elm.     

Genetic analysis across different spatial scales reveals multiple dispersal mechanisms for the invasive hydrozoan Cordylophora in the Great Lakes

Discerning patterns of post‐establishment spread by invasive species is critically important for the design of effective management strategies and the development of appropriate theoretical models predicting spatial expansion of introduced populations. The globally invasive colonial hydrozoan Cordylophora produces propagules both sexually and vegetatively and is associated with multiple potential dispersal mechanisms, making it a promising system to investigate complex patterns of population structure generated throughout the course of rapid range expansion. Here, we explore genetic patterns associated with the spread of this taxon within the North American Great Lakes basin. We collected intensively from eight harbours in the Chicago area in order to conduct detailed investigation of local population expansion. In addition, we collected from Lakes Michigan, Erie, and Ontario, as well as Lake Cayuga in the Finger Lakes of upstate New York in order to assess genetic structure on a regional scale. Based on data from eight highly polymorphic microsatellite loci we examined the spatial extent of clonal genotypes, assessed levels of neutral genetic diversity, and explored patterns of migration and dispersal at multiple spatial scales through assessment of population level genetic differentiation (pairwise F_ST and factorial correspondence analysis), Bayesian inference of population structure, and assignment tests on individual genotypes. Results of these analyses indicate that Cordylophora populations in this region spread predominantly through sexually produced propagules, and that while limited natural larval dispersal can drive expansion locally, regional expansion likely relies on anthropogenic dispersal vectors.     

Effects of the emerald ash borer invasion on the community composition of arthropods associated with ash tree boles in Maryland,U.S.A.

相似文献   

Temperature,recreational fishing and diapause egg connections: dispersal of spiny water fleas (<Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>)

The spiny water flea (Bythotrephes longimanus) is spreading from Great Lakes coastal waters into northern inland lakes within a northern temperature-defined latitudinal band. Colonization of Great Lakes coastal embayments is assisted by winds and seiche surges, yet rapid inland expansion across the northern states comes through an overland process. The lack of invasions at Isle Royale National Park contrasts with rapid expansion on the nearby Keweenaw Peninsula. Both regions have comparable geology, lake density, and fauna, but differ in recreational fishing boat access, visitation, and containment measures. Tail spines protect Bythotrephes against young of the year, but not larger fish, yet the unusual thick-shelled diapausing eggs can pass through fish guts in viable condition. Sediment traps illustrate how fish spread diapausing eggs across lakes in fecal pellets. Trillions of diapausing eggs are produced per year in Lake Michigan and billions per year in Lake Michigamme, a large inland lake. Dispersal by recreational fishing is linked to use of baitfish, diapausing eggs defecated into live wells and bait buckets, and Bythothephes snagged on fishing line, anchor ropes, and minnow seines. Relatively simple measures, such as on-site rinsing of live wells, restricting transfer of certain baitfish species, or holding baitfish for 24 h (defecation period), should greatly reduce dispersal.     

Temporal dynamics of woodpecker predation on emerald ash borer (Agrilus planipennis) in the northeastern U.S.A.

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Interspecific proteomic comparisons reveal ash phloem genes potentially involved in constitutive resistance to the emerald ash borer

The emerald ash borer (Agrilus planipennis) is an invasive wood-boring beetle that has killed millions of ash trees since its accidental introduction to North America. All North American ash species (Fraxinus spp.) that emerald ash borer has encountered so far are susceptible, while an Asian species, Manchurian ash (F. mandshurica), which shares an evolutionary history with emerald ash borer, is resistant. Phylogenetic evidence places North American black ash (F. nigra) and Manchurian ash in the same clade and section, yet black ash is highly susceptible to the emerald ash borer. This contrast provides an opportunity to compare the genetic traits of the two species and identify those with a potential role in defense/resistance. We used Difference Gel Electrophoresis (DIGE) to compare the phloem proteomes of resistant Manchurian to susceptible black, green, and white ash. Differentially expressed proteins associated with the resistant Manchurian ash when compared to the susceptible ash species were identified using nano-LC-MS/MS and putative identities assigned. Proteomic differences were strongly associated with the phylogenetic relationships among the four species. Proteins identified in Manchurian ash potentially associated with its resistance to emerald ash borer include a PR-10 protein, an aspartic protease, a phenylcoumaran benzylic ether reductase (PCBER), and a thylakoid-bound ascorbate peroxidase. Discovery of resistance-related proteins in Asian species will inform approaches in which resistance genes can be introgressed into North American ash species. The generation of resistant North American ash genotypes can be used in forest ecosystem restoration and urban plantings following the wake of the emerald ash borer invasion.     

The influence of mixing on rotifer assemblages of Michigan lakes

Seasonal changes in the rotifer assemblages of 42 lakes in northern lower Michigan was closely related to lake mixing characteristics, basin morphometry, and the presence of an oxygenated coldwater refuge. Three major classes of lakes (dimictic, discontinuous polymitic, and continuous polymictic) were evident by their capacity to maintain coldwater species as seasons progressed from winter through fall. The disappearance of coldwater assemblages from dimictic lakes coincided with oxygen depletion in the hypolimnion or with erosion of the hypolimnion through mixing. Coldwater species disappeared from large discontinuous polymictic lakes when deep epilimnetic mixing occured in late summer and fall. Species assemblages of nearly all stratified lakes converged with those of continuous polymictic lakes when the hypolimnetic refuge deteriorated in summer and fall. Local weather conditions, however, between years had a pronounced effect on the persistence of cold water species through the seasons by affecting the temperature and oxygen conditions of the hypolimnion. Large lakes of the region contain many of the coldwater species of the Laurentian Great Lakes but some taxa are conspicuosly absent. Cold stenothermal rotifers persist in the lakes of the region despite adverse environmental conditions. Their life histories and ability to form resting stages permit them to escape periods of oxygen depletion and thermal stress. In contrast, the crustacean glacial marine fauna (i.e. Mysis relicta, Limnocalanus macrurus, and Scenecella calanoides) was absent from all of the study lakes even though many of the present-day basins were once connected to the Laurentian Great Lakes. These species long life cycles, lack of diapausing stages, and limited dispersal may make them vulnerable to local extinction with the deterioration and loss of the coldwater refuge.