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Barriers to longitudinal river connectivity: review of impacts, study methods and management for Iberian fish conservation
Abstract
Barriers to longitudinal river connectivity: review of impacts, study methods and management for Iberian fish conservation River connectivity is essential for the correct functioning of freshwater ecosystems at all scales. However, it has not received the necessary attention by researchers, managers and policymakers until recent years. In this review, we recap the state of knowledge in river connectivity and its applications to conservation. We describe the particular characteristics of river connec-tivity and summarise the effects of its interruption in different freshwater ecosystem elements. We then focus on the effects of the lack of segment connectivity in fish species and review the different methods developed to study it. The application of connectivity in freshwater fish conservation areas is also reviewed, which highlights the lack of studies on this subject. Finally, connectivity restoration is studied. The review addresses these topics in a general way and then focus on the Iberian Peninsula. The Iberian Peninsula is an interesting place to study river connectivity because it has one of the highest numbers of dams per square kilometre and a large number of endemic and endangered freshwater fish species. Despite the high number of fish species affected by water extraction and damming, river connectivity and its effect in Iberian freshwater fish populations have not been well studied. A small number of studies analyse the effect of small dams in nearby fish communities, but large-scale impact assessments are scarce. More connectivity analyses are needed to improve freshwater ecosystem conservation strategies. We conclude addressing some gaps in the knowledge of fragmentation and research opportunities in river connectivity and conservation.
... In freshwater ecosystems, the hierarchical linear structure of river networks limits the dispersal pathways between current and future suitable habitats, which significantly hurdles climate-driven shifts in species distributions (Crook et al., 2015;Herrera-R et al., 2020;Rodeles et al., 2020). For species with limited dispersal ability, escaping climate change is even more challenging (Archambault et al., 2018;Årevall et al., 2018;Urban et al., 2013). ...
... Gains can only be taken into account if mussels are able to reach the new areas, which is highly dependent on the ability of fish hosts to transport glochidia and release juveniles at suitable locations. Like most European water bodies, Iberian rivers are highly fragmented by artificial barriers (Belletti et al., 2020;EEA, 2018), which has a significant impact on the movement of fish species within the river network (Branco et al., 2017;Gibson-Reinemer et al., 2017;Rodeles et al., 2020). Specifically, the location and type of artificial barriers within the river network in relation to the current presence of species will play an important role in determining which future habitats are colonizable and which are not (Radinger et al., 2017). ...
... It is widely recognized that even small barriers can significantly limit the movement of individuals in rivers and streams and increase the risk of extinction (Crook et al., 2015;Hermoso, Clavero, & Filipe, 2021;Hermoso, Vasconcelos, et al., 2021;Herrera-R et al., 2020;Radinger et al., 2018;Zarfl et al., 2019). Accordingly, fish, and by association, mussels, are strongly impacted by instream artificial barriers, as evidenced by reported extinctions and steep population declines (Benson et al., 2018;Branco et al., 2017;Pires et al., 2021;Rodeles et al., 2020), despite the implementation of mitigation strategies (e.g., fish ladders) that are often inefficient (Hermoso et al., 2018). ...
Climate change may lead to range shifts, and barriers to such displacements may result in extirpations from previously suitable habitats. This may be particularly important in freshwater ecosystems that are highly fragmented by anthropogenic obstacles, such as dams and other smaller in-stream barriers. Conservation planning in freshwaters should consider the dynamic effects of climate change and the ability of species to cope with it. In this study, we developed a framework for incorporating climate-driven dispersal barriers into conservation planning taking into account the medium and long-term impacts of climate change and species with obligatory interactions. Given that freshwater mussels (Bivalvia: Unionida) are a group of highly threatened organisms dependent on fish hosts to complete their larval development and dispersal, we used Marxan to prioritize areas for their joint conservation in the Iberian Peninsula as a case study. We tested two connectivity scenarios between current and future habitats, (i) unlimited dispersal capacity and (ii) dispersal constrained by artificial barriers, and also identified priority translocation areas for species that were unable to disperse. Accounting for the effects of climate change on species distributions allowed the identification of long-term conservation areas, but disregarding artificial barriers to dispersal may lead to unrealistic solutions. Integrating the location of barriers allowed the identification of priority areas that are more likely to be colonized in the future following climatic shifts, although this resulted in an additional loss of six to eight features (~5%–7%) compared to solutions without dispersal constraints. Between 173 and 357 artificial barriers (~1.6%–3.3%) will potentially block species dispersal to irreplaceable planning units. Where removal of artificial barriers is unfeasible, conservation translocations may additionally cover up to eight additional features that do not meet conservation targets due to dispersal constraints. This study highlights the challenge of identifying protected areas to safeguard biodiversity under climate change.
... Research by Barbarossa et al. (2020), Segurado et al. (2015), and Blanchet et al. (2010) reveals that barriers severely disrupt these species' migratory routes, causing population fragmentation and isolation. Specific studies illustrate this impact in different areas; for example, Rodeles et al. (2020aRodeles et al. ( , 2020b document salmonids in Spain's Duero River basin, while van Puijenbroek et al. (2019) detail similar challenges along the Cantabrian coast. Bednarek and Mołoniewicz (2023) further highlights that impassable barriers contribute to extensive habitat fragmentation, worsening the situation for sturgeon and eel populations, which have suffered substantial declines, with European eel now limited to less than 20 % of its historic range (Rodeles et al., 2020b;Teichert et al., 2022). ...
... Specific studies illustrate this impact in different areas; for example, Rodeles et al. (2020aRodeles et al. ( , 2020b document salmonids in Spain's Duero River basin, while van Puijenbroek et al. (2019) detail similar challenges along the Cantabrian coast. Bednarek and Mołoniewicz (2023) further highlights that impassable barriers contribute to extensive habitat fragmentation, worsening the situation for sturgeon and eel populations, which have suffered substantial declines, with European eel now limited to less than 20 % of its historic range (Rodeles et al., 2020b;Teichert et al., 2022). ...
... These structures have always been used to regulate rivers and augment the ecosystem services provided by them. However, their placement fragments the natural longitudinal connectivity of rivers, impairing fish movements along river networks and imperilling fish biodiversity around the world [1,3]. ...
... This approach should now be extended to additional datasets, and focus on additional species while trying to pursue different questions. The general incremental knowledge framework should also target additional longitudinal connectivity conundrums such as fish negotiation of small instream obstacles and weirs e.g., [3,4,68]. As relevant as they are, laboratory-controlled studies should, as much as possible, be complemented by field studies. ...
Simple Summary
River barriers fragment longitudinal connectivity and limit the ability of several fish species to complete their life cycle. Vertical slot fishways (VFS) are the best technical fishway solution to enhance connectivity on artificial barriers. To (i) identify the variables affecting fish passage in VSF, and (ii) the best VSF for two freshwater fish species (Luciobarbus bocagei and Squalius pyrenaicus), we collected data from several fishway studies and applied Bayesian Generalized Mixed Models. Results show (i) that the main predictor for fish passage is fish size and (ii) from the tested configurations, the multiple slot fishway with an orifice was, overall, the best-performing fishway configuration for all fish sizes. Fishways, to be holistically effective, need to be designed with requirements for small fish.
Abstract
River artificial fragmentation is arguably the most imperilling threat for freshwater-dependent fish species. Fish need to be able to freely move along river networks as not only spawning grounds but also refuge and feeding areas may be spatially and temporally separated. This incapacity of free displacement may result in genetic depletion of some populations, density reduction and even community changes, which may in turn affect how meta-community balances are regulated, potentially resulting in functional resilience reduction and ecosystem processes’ malfunction. Fishways are the most common and widely used method to improve connectivity for fish species. These structures allow fish to negotiate full barriers, thus reducing their connectivity impairment. Among all technical fishway types, vertical slot fishways (VSF) are considered to be the best solution, as they remain operational even with fluctuating water discharges and allow fish to negotiate each cross-wall at their desired depth. In the present study, we collected both published and original data on fish experiments within VSF, to address two questions, (1) What variables affect fish passage during experimental fishway studies? and (2) What is the best VSF configuration? We used Bayesian Generalized Mixed Models accounting for random effects of non-controlled factors, limiting inherent data dependencies, that may influence the model outcome. Results highlight that fish size, regardless of the species, is a good predictor of fishway negotiation success. Generally, multiple slot fishways with one orifice proved to be the best solution. Future work should be focused on small-sized fish to further improve the design of holistic fishways.
... These structures have always been used to regulate rivers and augment the ecosystem services provided by them. However, their placement fragments the natural longitudinal connectivity of rivers, impairing fish movements along river networks and imperilling fish biodiversity around the world [1,3]. ...
... This approach should now be extended to additional datasets, and focus on additional species while trying to pursue different questions. The general incremental knowledge framework should also target additional longitudinal connectivity conundrums such as fish negotiation of small instream obstacles and weirs e.g., [3,4,68]. As relevant as they are, laboratory-controlled studies should, as much as possible, be complemented by field studies. ...
... This will be accentuated even more by the increase in urban areas (and so in water demand) and the effluents they will produce, as well as by the expansion of new hectares of irrigated land, among others (Canuto et al. 2019). Thus, the survival of the organisms composing these communities is determined by the composition of the community before the dry period, the presence or absence of residual pools and their size, the duration of the dry period, the speed of water withdrawal (Várbíró et al. 2020) and the presence of artificial barriers limiting repopulation between sites (Jones et al. 2020;Rodeles et al. 2020), despite these organisms being morphologically and physiologically adapted to the dry period through a strong selection of traits (Datry et al. 2017a;Várbíró et al. 2020). This increased isolation between communities may have negative effects as it affects both structural (increasing the distance between local sites) and functional connectivity (decreasing gene flow and dispersal between local sites) (Valenzuela-Aguayo et al. 2020). ...
Metacommunity studies have been gaining in importance in recent decades due to their relevance when interpreting community dynamics. The elements of metacommunity structure (EMS), i.e. coherence, turnover and boundary clumping, are used to assess the assembly of metacommunities. In the present study we analysed the EMS of the Guadiana Hydrographic Demarcation, a prominant seasonal basin located in the southern Iberian Peninsula characterised by a Mediterranean climate, with dry reaches and disconnected pools frequent in streams during the summer. We studied the EMS of the four different taxocoenoses used to assess the ecological status of streams and rivers according to the European Water Framework Directive (diatoms, macrophytes, macroinvertebrates and fishes), both independently of each other and taken together. These analyses were carried out using three different approaches: (1) using a gradient from reciprocal averaging analysis; (2) following a geographical gradient; and (3) following an environmental gradient. We found that the four groups of organisms analysed had either a Clementsian metacommunity structure or a similar structure. When all groups were considered together, the structure of the metacommunity was Clementsian or quasi-Clementsian. Thus, in the framework of the current global change scenario, communities in this basin may be vulnerable to increasing isolation due to more frequent and larger dry periods; consequently, management measures should be considered.
... Such delineation is mostly mediated by Iberian basin endemisms, including Cyprinids of the genera Squalius, Iberochondorstoma, Parachondrostoma, Barbus and Luciobarbus as well as endemic Cyprinodontiformes like Apricaphanius and Valencia (Fig. 5 up). Many of these endemic species are in risk of extinction , and are facing multiple threats (Maceda-Veiga, 2013), including invasive species, loss of river connectivity (Rodeles et al., 2020), and climate change (Barbarossa et al., 2021). Thus, recognizing the unique characteristics of each basin cluster will allow for the development of conservation measures that can adequately tackle the specific challenges of their freshwater communities. ...
... Similarly, MacRae and Jackson (2001) demonstrated that in small lakes ( 50 ha) in central Ontario, the presence of the non-native predatory Smallmouth Bass Micropterus dolomieu drives small cyprinids to predominantly utilise complex and specific habitats whereas, in lakes without Smallmouth Bass the cyprinids inhabited a more diverse range of habitats. As such, the presence of non-native species in river systems may alter the behaviour of the native biota (Cox and Lima, 2006;Woodford et al., 2017), and current evidence suggests that the impacts of such invasions are more prominent in freshwater ecosystems that are already modified or degraded by humans Rodeles et al., 2020). ...
Freshwater ecosystems show substantial loss of biodiversity as a result of anthropogenic induced stressors. In this study, we evaluated the distribution of freshwater fishes in the Kowie River catchment using historic fish distribution records supplemented by field survey data. Fishes were collected using a multi-method approach: seine nets, fyke nets and gill nets. Historic fish distribution data showed that 22 freshwater fishes from 11 families historically occurred, while in this study, we recorded a total of 16 freshwater fish species from 9 families. Overall, a decrease in the number of native species was recorded with a total of five species absent and two new non-native species recorded during the current survey. Coptodon rendalli constituted a new record in the Kowie River catchment while Clarias gariepinus was recorded for the first time in the mainstem of the Kowie River. The presence of these two non-native species in the Kowie River catchment may have implications for the conservation and management of the freshwater diversity in the catchment.
... Other studies have made similar inferences regarding the importance of connectivity in stream networks. For example, Osborne and Wiley (1992) found higher numbers of fish species in tributary streams located lower in a drainage network and connected to a main channel system than from similarly sized streams located in the headwaters of a drainage network (see reviews in Shao et al. 2019 andRodeles et al. 2020). ...
... Many hydrological infrastructures were built in 1960-90 to satisfy agricultural, hydropower, and domestic water demand and to assist with flood regulation. Dams and reservoirs affect potamodromous and anadromous fish species, limiting their movements along watercourses and fragmenting their populations (Rodeles et al., 2020). They also affect natural flow regimes, disrupting rainy-season floods and summer droughts, common in Mediterranean environments, creating new conditions that favor invasive species establishment. ...
Fishes constitute the most species-rich group of vertebrates. They inhabit all kind of marine and freshwater ecosystems and have concentrated a high species diversity in several hotspots all around the world. Here, we explore the conservation status of freshwater fishes in one of these hotspots, the Iberian Peninsula, in South Western Europe. Iberian fish fauna includes 20 families and the highest European percentage of endemic freshwater fish species. Unfortunately, it is highly endangered by human threats, including river regulation, water extraction and invasive fish species, resulting in a high percentage of endemic fish species threatened with extinction, according to the IUCN Red List. More than 30 invasive fish species have spread across Iberian freshwaters in the past century, replacing the endemic fauna in major rivers and reservoirs and severely compromising the viability of many native fish populations. Monitoring native and invasive species, as well as sharing these data with the scientific community, is essential to preserve this unique and endangered ichthyofauna. Moreover, regarding Spanish conservation policies, several Iberian endemisms should be included in the endangered species catalogue to allow them to benefit from conservation efforts carried out by public administrations.
... Although several attributes of fish ecology are altered due to flow modifications Tonkin et al. 2018), changes in the ability of fishes to move and the corresponding changes to distributions and abundances are disproportionately affected by water withdrawals. River fragmentation by man-made barriers can reduce movements by stream fishes and prevent completion of the life cycle Rodeles et al. 2020;Zambaldi and Pompeu 2020). Movement reduction may be related to a variety of factors including swimming endurance (Leavy and Bonner 2009;Knapp et al. 2019;, jumping ability (Prenosil et al. 2016;Rahel and McLaughlin 2018;, and simply lack of connected habitat (Marshall et al. 2016;Neufeld et al. 2018;). ...
Alterations to freshwater systems are a leading cause of reduction in diversity and abundance of freshwater fishes. Pelagophils have been negatively affected by anthropogenic decreases in the frequency and magnitude of high-flow events and changes in baseflow conditions, but prescriptive flows based on pelagophil ecology are not available. Therefore, the goal of my thesis was to determine the flow components important for pelagophil persistence in the southern Great Plains. My first objective addressed how flow regime shapes pelagophil distributions over long periods of time by relating Arkansas River Shiner Notropis girardi and Plains Minnow Hybognathus placitus occupancy to flow regime components over cool and warm climatic periods. I found that pelagophils benefit from consistency in the number of high flow events and unpredictability in seasonal timing of high flow events. The relationships of Plains Minnow occupancy and predictability of seasonal flooding was dependent of the climatic regime, and may show that Plains Minnow is better able to adjust to warmer climate conditions. Disturbance and river fragmentation were negatively related to the occupancy of Arkansas River Shiner and Plains Minnow, respectively. To examine drivers of finer spatio-temporal dynamics and reveal the underlying factors affecting distributions, my second objective quantified movement of Arkansas River Shiner and Emerald Shiner Notropis atherinoides in the non-breeding season. Arkansas River Shiner and Emerald Shiner had a downstream movement bias in the winter and early spring. Long distance upstream movements appear to be initiated in the spring by both species. I found evidence that a proportion of the Arkansas River Shiner population were residents by recapturing individuals in their tagging site after up to a year. If the goal is to conserve these species, water management strategies maintaining average annual (1983-2017) high-flow frequencies while promoting high-flow unpredictability may benefit these species. Land-use mitigation such as construction or recovery of wetland and riparian buffers may beneficial. Additionally, water management strategies improving river
connectivity in late winter through the spawning season may benefit migratory individuals. However, improving lateral river connectivity in reaches with resident individuals (near river km 250) may increase recruitment from that proportion of the population.
Variability within species is key for adaptability and biological evolution. To understand individualities in the context of animal movement, we focused on one of the most remarkable migrations—the journey of the endangered European eel from their birthplace in the Sargasso Sea to freshwater environments. Laboratory observations unveiled a continuum of diverse phenotypes of migrating eels: Some displayed a heightened tendency to swim against a constant water flow, while others a greater propensity to climb obstacles. Looking for the biological underpinnings of this migratory diversity, we characterized the eels’ individual differences in traits of four key domains: life history, physiology, behavior, and cognition, among which we found significant variance and interconnectedness. Upon reducing this variance to its primary multivariate axes, we found that these predict the migratory types. Eels with 1) low exploration, high activity, low boldness, and high lateralization; 2) strong lateralization, enhanced quantitative abilities, short problem-solving time, high boldness, and low growth rates; or 3) enhanced problem-solving, reduced spatial learning, high cognitive flexibility, and shorter time to solve the cognitive tasks were more likely to display the climbing migratory type. Field sampling revealed how specific traits’ combinations seemed to influence the distribution of eels once they begin to settle in the freshwater environment. Our study underscores the impressive diversity of individuals during this critical migration, emphasizing an intrinsic connection to multidomain trait variance. Preserving this diversity becomes paramount, as it likely contributes to the resilience and adaptability of endangered migratory species.
Human disturbances (e.g., land-use changes, dam construction, and pollutant emissions) can have severe environmental impacts on freshwater ecosystems. The main aim of this study was to detect the influences of physiochemical variables, land-cover characteristics, and river connectivity on fish assemblages in Lake Chaohu Basin, China. A cluster analysis of the river connectivity variables identified four groups of sites that were characterized by significantly different river-connectivity and longitudinal gradients in local scale. These four groups of sites showed increasing connectivity from the upper reaches to the lower reaches. At the same time, among the four groups, the values of the environmental variables generally increased from the upper reaches with less human activity towards the lower reaches with more human activities. For instance, some main physiochemical variables (e.g., river width, water depth, total phosphorus, nitrate, phosphate, and dissolved organic carbon) significantly increased among the four groups. In contrast, fish taxa richness and diversity indices were not significantly different among the four connectivity groups. However, fish assemblages showed significant variation among the connectivity groups (global R = 0.089, p = 0.026). In addition, the study determined that upper riparian land uses (e.g., woodland and grassland), flow velocity, and elevation were environmental variables regulating the variance in the fish communities, and for the connectivity variables, only river order and number of branches along a path to the left of the main stem affected the variance in the fish communities. Therefore, new practices aimed at maintaining and even increasing the riparian canopy coverage and the flow velocity of rivers should be integrated into local conservation planning for freshwater ecosystems, especially in the upper reaches of the basin.
Mediterranean endemic freshwater fish are among the most threatened biota in the world. The Mediterranean basin has experienced substantial reductions in precipitation and water availability, which will worsen with climate change. Current water policy is directed to increase water-supply demands, especially for agriculture, and not to improve water-use efficiency and implement integrated and sustainable water management. Illegal extractions are common, exacerbating problems for important protected areas. Management is needed to mitigate the conflicts between environmental water and human demand, and ensure availability of water to maintain ecological processes and Mediterranean freshwater biodiversity. Water availability is not the only threat, although it is exacerbated by pollution and invasive species. The uneven spatial distribution of threats across the Mediterranean basin requires different strategies to conserve freshwater biodiversity. Implementation of multi-national laws (e.g. Water Framework Directive in the European Union) will help future management of freshwater ecosystems. Management actions must be planned at whole-catchment scales, with collaboration among different countries and water-management authorities. The current reserve area is small compared with other areas in the world and driven by terrestrial interests, and should be evaluated for its effectiveness to protect the Mediterranean freshwater biodiversity.
By applying the framework proposed by Millennium Ecosystem Assessment, we analysed the current state and trends of 20 ecosystem services provided by Spanish rivers and riparian areas using 139 indicators. We compared the obtained results with the Europe and UK assessment. It is the first document that attempts to analyse the importance of services provided by Spanish rivers and riparian areas, and it forms part of the evaluation carried out for the Spanish Millennium Ecosystem Assessment. Among the provisioning services, freshwater, hydropower energy and genetic resources were classified as high importance, and water regulation and self-purification capacity, and natural hazard mitigation are among the regulating services, with landscape-aesthetic values and recreation and ecotourism featuring among the cultural services. About 61 % of the assessed ecosystem services are currently declining or degrading, but are higher than the percentage calculated for Europe (45 %) or for the UK (53 %). All regulating services are degrading, especially water regulation, natural hazard mitigation, soil formation and fertility and biological control, and the cultural services related to rural populations. Likewise, the biodiversity of Spanish aquatic ecosystems is decreasing rapidly. Land use changes and overexploitation
of biological and mineral raw materials have been the main direct drivers of change in Spanish rivers and riparian zones, and relate directly to increase urbanised areas and irrigated agriculture. Finally, we draw some considerations on alternative models for aquatic ecosystems management which maintain aquatic ecosystem services and their biodiversity.
Communities that live in linear systems such as streams or riparian zones may be particularly sensitive to changes in habitat connectivity. I examined the relationship between environmental variables, the number of low-head dams downstream of each reach, and fish species richness in first-order streams in Wisconsin, USA. The analysis was based on an extensive database of 13 628 localities and ∼180 000 individual collection records. The environmental variables included climate, elevation, distance from nearest town, and network-specific measures such as link number (the number of first-order tributaries upstream of the current reach), downstream link number, stream order, and fractal dimension. The link number of the downstream reach was the strongest broad-scale correlate of fish species richness in first-order streams. Path analysis of a simple directed graph suggested that, although downstream dams have a significant effect on fish species richness, this effect is small by comparison to the influence of water quantity and summer maximum temperatures. From a management perspective, the results imply that modifications of water volume and temperature are greater threats to fish communities than the decrease in connectivity that results from low-head dams.
Dam removal is gaining credibility as a viable management option for dams that have deteriorated physically and are no longer economically practical. However, the decision to remove or repair a dam is often contentious and emotionally charged. Part of the acrimony arises from our limited scientific knowledge of the effects of dam removal. We believe that the ecological consequences are best understood by viewing the removal process as a disturbance. Ecological outcomes will include changes that are both environmentally costly, such as invasion of exotic species, and environmentally beneficial, such as increasing access to spawning habitats for migratory fish. It has also become apparent that the wholesale aging of the US dam infrastructure will make dam removal even more common in the future. The challenge ahead is to better understand and manage the consequences of these removals.
Effects of fragmentation on the ecology of organisms occupying dendritic ecological networks (DENs) have recently been described through both conceptual and mathematical models, but few hypotheses have been tested in complex, real-world ecosystems. Stream fishes provide a model system for assessing effects of fragmentation on the structure of communities occurring within DENs, including how fragmentation alters metacommunity dynamics and biodiversity. A recently developed habitat-availability measure, the "dendritic connectivity index" (DCI), allows for assigning quantitative measures of connectivity in DENs regardless of network extent or complexity, and might be used to predict fish community response to fragmentation. We characterized stream fish community structure in 12 DENs in the Great Plains, USA, during periods of dynamic (summer) and muted (fall) discharge regimes to test the DCI as a predictive model of fish community response to fragmentation imposed by road crossings. Results indicated that fish communities in stream segments isolated by road crossings had reduced species richness (alpha diversity) relative to communities that maintained connectivity with the surrounding DEN during summer and fall. Furthermore, isolated communities had greater dissimilarity (beta diversity) to downstream sites notisolated by road crossings during summer and fall. Finally, dissimilarity among communities within DENs decreased as a function of increased habitat connectivity (measured using the DCI) for summer and fall, suggesting that communities within highly connected DENs tend to be more homogeneous. Our results indicate that the DCI is sensitive to community effects of fragmentation in riverscapes and might be used by managers to predict ecological responses to changes in habitat connectivity. Moreover, our findings illustrate that relating structural connectivity of riverscapes to functional connectivity among communities might aid in maintaining metacommunity dynamics and biodiversity in complex dendritic ecosystems.
The current freshwater fish fauna crisis is such that natural resource managers urgently need to identify priorities and understand the management consequences of actions aimed at maximizing the preservation of biodiversity. Freshwater research is often poorly linked to conservation ecology; and interdisciplinary studies illustrating examples of freshwater ecosystem conservation are scarce. The Iberian Peninsula has a long history of anthropogenic disturbance that has led to the poor conservation status of its ichthyofauna, with 52 % of species now catalogued as critically endangered, endangered or vulnerable, according to UICN criteria. This paper gives an overview of the main threats (habitat degradation, hydrological alterations and exotic spe-cies) that have altered the function and connectivity of Iberian rivers. Case-study examples are provided to analyse the repercussions of these threats and the management actions planned or already performed in these systems. The interaction of many threats is responsible for native fish decline. However, fresh-water managers and researchers should not let the trees prevent them from seeing the overall wood, when seeking to achieve practical solutions with the best balanced cost benefit and the collaboration of all ecosystem researchers and stakeholders. Conservation efforts should be focused on the preservation of ecological processes, in order to achieve the goals of the Water Framework Directive and guarantee the conservation of Iberian native fish species.
1. Historically, biogeographic barriers to the movement of aquatic organisms existed at multiple spatial scales and contributed to the development of unique regional faunas. At increasing spatial scales, these barriers consisted of waterfalls and cascades; catchment divides; major mountain ranges and oceans. This hierarchy of movement barriers produced increasingly distinct aquatic biotas at larger drainage units.
2. Humans have provided a variety of pathways by which aquatic species can circumvent historical biogeographic barriers. These include both authorised and unauthorised stocking, construction of canals and water conveyance systems, transport in ship ballast water, fishing and angling gear (including boats) transferred among water bodies and intentional release of ornamental and other captive species.
3. One consequence of human-aided breaching of biogeographic barriers has been the spread of noxious species that have altered aquatic ecosystems and fisheries in ways that are undesirable to humans.
4. Another consequence of human-aided breaching of biogeographic barriers has been the homogenization of aquatic biotas. Homogenization occurs when a few cosmopolitan species come to dominate communities at the expense of unique native species. Among aquatic organisms this phenomenon is best documented for fish faunas where a small set of species introduced for sport fishing, aquaculture, or ornamental purposes have become widespread throughout the world.
5. Slowing biotic homogenization will require slowing the rate at which species breach biogeographic barriers. This will involve implementing regulations that limit stocking opportunities; increasing the public's awareness about the consequences of releasing non-native species and developing technological solutions that prevent movement of aquatic organisms or eliminate them before they become established.
6. River restoration can influence homogenization of aquatic biotas through two major mechanisms: by removing barriers to movement and by restoring natural habitat conditions. Removal of movement barriers may facilitate the spread of non-native species and thus contribute to biotic homogenization. Restoration of natural flow regimes and habitat conditions may reduce biotic homogenization by favouring regional native species over cosmopolitan, non-native species.
Removing small artificial barriers that hinder upstream migrations of fish is a major problem in riparian habitat restoration. Because of budgetary limitations, it is necessary to prioritize barrier removal and repair decisions. These have usually been based on scoring and ranking procedures, which, although simple to use, can be very inefficient in terms of increasing the amount of accessible instream habitat. We develop a novel decision-making approach, based on integer programming techniques, which optimizes repair and removal decisions. Results show based on real datasets of barrier culverts located in Washington State that scoring and ranking is over 25% below the optimum on average and a full 100% below in the worst case, producing no net habitat gain whatsoever. This is compared to a dynamic programming method that was able to find optimal solutions in less than a second, even for problems with up to several hundred variables, and a heuristic method, which found solutions with less than a 1% average optimality gap in even less time.
In the course of the `Meuse Salmon 2000' programme, most weirs and dams (3–8 m in height) in the regulated River Meuse have been progressively equipped with new fishways in order to restore the free circulation of all amphibiotic fish species. Nevertheless, fish entering into major spawning tributaries are still confronted with various kinds of physical obstacles of which the overall impact on fish migration has never been investigated. In order to test their ability to negotiate physical obstacles, 128 individuals of fish (Salmo trutta, Thymallus thymallus, Salmo salar, Chondrostoma nasus, Barbus barbus and Esox lucius) were captured several weeks before their spawning migrations and tagged with radio-transmitters. They were tracked from 30 to 466 days in the River Ourthe and six spawning tributaries over the period October 1995 to June 2001. All obstacles recorded in this study have been classified according to their type and main characteristics (i.e. slope, length and height). Results indicated that most fish migrate during or outside the spawning period and that some small obstacles are not as insignificant as initially thought and can significantly disrupt and/or obstruct their upstream movements. There is a need to harmonize interests in the sustainable conservation of fish populations and the development of small-scale hydropower generation and tourism.
Low-head dams and weirs can greatly limit the distribution and abundance of Atlantic salmon and other migratory salmonids
in streams. Weirs can significantly increase the vulnerability of migratory fish to anglers, alter natural migration patterns,
and exacerbate the effects of opportunistic predators. Overcrowding of fish at downstream pools can also facilitate the spread
of parasites and infectious diseases, magnify the impact of pollution incidents, and increase the risk of mass mortalities,
particularly at low flows. Not surprisingly, augmenting the accessible stream area constitutes one of the best ways to restore
depleted salmonid populations. In this context, the removal of unused or illegal weirs can be an efficient, cheap solution
to increase stream accessibility. Here, I examine some impacts of weirs on Atlantic salmon populations, and document with
case studies the removal and breaching of weirs in several Iberian streams.
The loss of connectivity of natural areas is a major threat for wildlife dispersal and survival and for the conservation of
biodiversity in general. Thus, there is an increasing interest in considering connectivity in landscape planning and habitat
conservation. In this context, graph structures have been shown to be a powerful and effective way of both representing the
landscape pattern as a network and performing complex analysis regarding landscape connectivity. Many indices have been used
for connectivity analyses so far but comparatively very little efforts have been made to understand their behaviour and sensitivity
to spatial changes, which seriously undermines their adequate interpretation and usefulness. We systematically compare a set
of ten graph-based connectivity indices, evaluating their reaction to different types of change that can occur in the landscape
(habitat patches loss, corridors loss, etc.) and their effectiveness for identifying which landscape elements are more critical
for habitat conservation. Many of the available indices were found to present serious limitations that make them inadequate
as a basis for conservation planning. We present a new index (IIC) that achieves all the properties of an ideal index according
to our analysis. We suggest that the connectivity problem should be considered within the wider concept of habitat availability,
which considers a habitat patch itself as a space where connectivity exists, integrating habitat amount and connectivity between
habitat patches in a single measure.
Restoration of unobstructed, free-flowing sections of river can provide considerable environmental and ecological benefits. It removes impediments to aquatic species dispersal and improves flow, sediment and nutrient transport. This, in turn, can serve to improve environmental quality and abundance of native species, not only within the river channel itself, but also within adjacent riparian, floodplain and coastal areas. In support of this effort, a generic optimization model is presented in this paper for prioritizing the removal of problematic structures, which adversely affect aquatic species dispersal and river hydrology. Its purpose is to maximize, subject to a budget, the size of the single largest section of connected river unimpeded by artificial flow and dispersal barriers. The model is designed to improve, in a holistic way, the connectivity and environmental status of a river network. Furthermore, unlike most previous prioritization methods, it is particularly well suited to meet the needs of potamodromous fish species and other resident aquatic organisms, which regularly disperse among different parts of a river network. After presenting an initial mixed integer linear programming formulation of the model, more scalable reformulation and solution techniques are investigated for solving large, realistic-sized instances. Results from a case-study of the Pike River Watershed, located in northeast Wisconsin, USA, demonstrate the computational efficiency of the proposed model as well as highlight some general insights about systematic barrier removal planning.
The Guadiana estuary (Iberian Peninsula, Europe) has low agriculture, industry and urban pressure, unlike other major Iberian estuaries; however, this situation is about to change. The Alqueva dam was constructed at 150 km from the river mouth, creating one of the biggest artificial lakes in Europe, which will support 110 000 ha of irrigation areas. Moreover, the estuarine margins are suffering increased human pressure. The risks that the Alqueva dam will pose to the downstream ecosystems, mainly to the estuary and adjacent coastal area, will be reviewed in this paper. Other issues are also analysed, as increased human desertification in the Portuguese basin, real estate pressure and the impact of the construction of a public infrastructure in the Portuguese lower estuary. Increased problems of sediment load reduction, water quality deterioration and shifts in the downstream fish populations are expected due to higher river flow reduction. Irreversible changes to estuarine margins and increased risk of pollution in the estuary will result from the multiple urban pressures. Lack of forest management and increased risk of wildfires, leading to water quality degradation and increased soil erosion, are a result of human desertification in the upper Portuguese basin. Multiple solutions to tackle these problems have to be considered. Creating the International Natural Park of the Lower Guadiana would not be enough and tourism should not be regarded as the only way to develop such a poor region. An ecohydrological approach of the basin should be implemented, to establish scientific based solutions to help solving conflicts and testing scenarios. Ultimately, the creation of a Portuguese-Spanish Basin Management Council for the lower Guadiana, involving those that are interested in the sustainable development of the Guadiana (local populations and associations, private stakeholders (dam managers, tourism entrepreneurs, farmers), local and national authorities, ENGO’s and scientists) is essential to produce ecological and sustainable management decisions.
Dam removal continues to garner attention as a potential river restoration tool. The increasing possibility of dam removal through the FERC relicensing process, as well as through federal and state agency actions, makes a critical examination of the ecological benefits and costs essential. This paper reviews the possible ecological impacts of dam removal using various case studies. Restoration of an unregulated flow regime has resulted in increased biotic diversity through the enhancement of preferred spawning grounds or other habitat. By returning riverine conditions and sediment transport to formerly impounded areas, riffle/pool sequences, gravel, and cobble have reappeared, along with increases in biotic diversity. Fish passage has been another benefit of dam removal. However, the disappearance of the reservoir may also affect certain publicly desirable fisheries. Short-term ecological impacts of dam removal include an increased sediment load that may cause suffocation and abrasion to various biota and habitats. However, several recorded dam removals have suggested that the increased sediment load caused by removal should be a short-term effect. Pre-removal studies for contaminated sediment may be effective at controlling toxic release problems. Although monitoring and dam removal studies are limited, a continued examination of the possible ecological impacts is important for quantifying the resistance and resilience of aquatic ecosystems. Dam removal, although controversial, is an important alternative for river restoration.
Globally, rivers directly support a large proportion of the human population and provide ecosystem services essential for quality of life, including the provision of protein through maintenance of fisheries. Many systems have been extensively engineered to support agricultural production, urbanization and industrialization, activities that can negatively affect ecosystem processes, with some of the most profound impacts due to impoundment and flow depletion. Environmental impacts include disruption of flow and sediment regimes; degradation and fragmentation of physical habitat; alteration of water chemistry, temperature and dissolved gases; disconnection of energy and nutrient transfer in longitudinal, lateral, vertical and temporal dimensions and associated reductions in productivity; direct and indirect mortality of aquatic biota as a result of interactions with river infrastructure. Despite often well-developed and long-term recognition that impoundment and abstraction negatively impact ecology, the complexity of the mechanisms that drive these responses often remains poorly understood and requires further investigation. Similarly, despite considerable efforts to develop environmental impact mitigation strategies (e.g. methods to assign appropriate environmental flows) and technology (e.g. fish passes and screens), partial solutions frequently fail to fully compensate for negative effects experienced and in some cases are themselves damaging. This chapter reviews the impacts of impoundments and water offtakes on freshwater fish populations and discusses the mitigation options currently available. Biases and gaps in understanding are identified, and recommendations for the future made. In the face of unprecedented future demand, it is argued that mitigation alone will be insufficient to sustain important populations of fish and that there is a need for society to discuss the need to better manage demand on resources based on more realistic valuation of existing ecosystem services.
Long‐term perspectives are critical to understand contemporary ecological systems. However, historical data on the distribution of biodiversity have only rarely been used in applied environmental sciences.
Here, we use historical sources to reconstruct the historical range of the European eel, a critically endangered species. We then use this baseline range to set range targets for the recovery of the European eel, as opposed to the abundance‐based targets established by the European Union, which are constrained by the poor information on pre‐collapse stocks.
We collected over 10 000 historical freshwater fish records from Spain in the 19th and 16th centuries, as well as over 25 000 records from the global biodiversity information facility ( GBIF ) to characterize historical and current European eel distribution in the Iberian Peninsula. We converted fish records into an eel presence–absence data set using subcatchment as spatial unit of analysis and modelled eel distribution in the different historical periods.
The eel was historically widespread throughout the Iberian Peninsula, but it has lost over 80% of its original range, mainly due to river fragmentation by dams. Distribution models applied to 16th‐ and 19th‐century data showed a high agreement, supporting the use of the 19th‐century estimated distribution as a baseline range. We identified the number and identity of dams that should be made passable for accomplishing specific range recovery targets, for example showing that acting upon 20 dams would make available 60% of the baseline eel range.
Synthesis and applications . This work exemplifies how insights gained from historical ecology can support and guide present‐day management of migratory fishes. Similar approaches could be developed throughout Europe to plan the recovery of the eel, since there are large amounts of historical eel records. Historical baseline ranges for the eel range should be incorporated into the European Union legal mandates aimed at the recovery of the species.
Neither linear nor two-dimensional frameworks may be the most appropriate for fish and other species constrained to disperse within river-creek systems. In particular, the hierarchical, dendritic structures of riverine networks are not well captured by existing spatial models. Here I use a simple geometric model and metapopulation modeling to make three points concerning the ecological consequences of dendritic landscapes. First, connectivity patterns of river-creek networks differ from linear landscapes, and these differences in connectivity can either enhance or reduce metapopulation persistence compared to linear systems, depending on the details of dispersal. Second, habitat fragmentation in dendritic landscapes has different (and arguably more severe) consequences on fragment size than in either linear or two-dimensional systems, resulting in both smaller fragments and higher variance in fragment size. Third, dendritic landscapes can induce striking mismatches between the geometry of dispersal and the geometry of disturbance, and as is the case for arid-lands fishes, such mismatches can be important for population persistence.
This paper examines the usage and measurement of “landscape connectivity” in 33 recent studies. Connectivity is defined as the degree to which a landscape facilitates or impedes movement of organisms among resource patches. However, connectivity is actually used in a variety of ways in the literature. This has led to confusion and lack of clarity related to (1) function vs structure, (2) patch isolation vs landscape connectivity and, (3) corridors vs connectivity. We suggest the term connectivity should be reserved for its original purpose. We highlight nine studies; these include modeling studies that actually measured connectivity in accordance with the definition, and empirical studies that measured key components of connectivity. We found that measurements of connectivity provide results that can be interpreted as recommending habitat fragmentation to enhance landscape connectivity. We discuss reasons for this misleading conclusion, and suggest a new way of quantifying connectivity, which avoids this problem. We also recommend a method for reducing sampling intensity in landscape-scale empirical studies of connectivity.
ABSTRACT / The flow regime is regarded by many aquatic ecologists to be the key driver of river and floodplain wetland ecosystems. We have focused this literature review around four key principles to highlight the important mechanisms that link hydrology and aquatic biodiversity and to illustrate the consequent impacts of altered flow regimes: Firstly, flow is a major determinant of physical habitat in streams, which in turn is a major determinant of biotic composition; Secondly, aquatic species have evolved life history strategies primarily in direct response to the natural flow regimes; Thirdly, maintenance of natural patterns of longitudinal and lateral connectivity is essential to the viability of populations of many riverine species; Finally, the invasion and success of exotic and introduced species in rivers is facilitated by the alteration of flow regimes. The impacts of flow change are manifest across broad taxonomic groups including riverine plants, invertebrates, and fish. Despite
growing recognition of these relationships, ecologists still struggle to predict and quantify biotic responses to altered flow regimes. One obvious difficulty is the ability to distinguish
the direct effects of modified flow regimes from impacts associated with land-use change that often accompanies water resource development. Currently, evidence about how rivers function in relation to flow regime and the flows that aquatic organisms need exists largely as a series of untested hypotheses. To overcome these problems, aquatic science needs to move quickly into a manipulative or experimental phase, preferably with the aims of restoration and measuring ecosystem response.
Large portions of watersheds and streams are lost to anadromous fishes because of anthropogenic barriers to migration. The loss of these streams and rivers has shifted the distribution of accessible habitat, often reducing the diversity of accessible habitat and the quantity of high-quality habitat. We combined existing inventories of barriers to adult fish passage in the Willamette and Lower Columbia River basins and identified 1,491 anthropogenic barriers to fish passage blocking 14,931 km of streams. We quantified and compared the stream quality, land cover, and physical characteristics of lost versus currently accessible habitat by watershed, assessed the effect of barriers on the variability of accessible habitats, and investigated potential impacts of habitat reduction on endangered or threatened salmonid populations. The majority of the study watersheds have lost more than 40% of total fish stream habitat. Overall, 40% of the streams with spawning gradients suitable for steelhead (anadromous rainbow trout Oncorhynchus mykiss), 60% of streams with riparian habitat in good condition, and 30% of streams draining watersheds with all coniferous land cover are no longer accessible to anadromous fish. Across watersheds, hydrologic and topographic watershed characteristics were correlated with barrier location, barrier density, and the impacts of barriers on habitat. Population-based abundance scores for spring Chinook salmon O. tshawytscha were strongly correlated with the magnitude of habitat lost and the number of lowland fish passage barriers. The characteristics of barrier and habitat distribution presented in this paper indicate that barrier removal projects and mitigation for instream barriers should consider both the magnitude and quality of the lost habitat.
1. To provide information on the movements and localized activity of barbel Barbus barbus (Cyprinidae) in a river containing potentially obstructing weirs, 31 adult barbel were radio-tracked in the River Nidd, a tributary of the Yorkshire Ouse, North East England between June 1993 and September 1994. 2. Barbel exhibited substantial movements, ranging from 2 to nearly 20 km. Four fish are known to have moved between the Nidd and the Ouse, demonstrating that at least a part of the barbel population utilize the Nidd and Ouse at different times of the year. 3. Range of upstream movement was restricted by the presence and nature of several weirs, including Skip Bridge flow-gauging weir. Low levels of spawning downstream of Skip Bridge weir appear to have been due to a lack of suitable spawning habitat. 4. Movements followed a seasonal pattern, with males and females migrating upstream in spring to spawn on gravel beds. Females moved downstream more quickly than males over the summer months. Both sexes moved downstream in autumn and winter. Day length and water temperature were the best predictors in relation to distance moved up the River Nidd. Descriptive models, relating movement to water temperature and day length, are provided. 5. For both sexes, localized activity varied greatly on both diel and seasonal scales, and was mainly associated with foraging. During summer there was typically a bimodal pattern of diel activity with peaks usually in early morning and late evening. In winter, mean daily activity was less than 20% of peak summer levels and fish were relatively dormant. In winter, diel activity patterns exhibited a single peak towards dusk. Mean daily activity levels for each month were linearly correlated with mean monthly water temperatures, even during the months where movement to and on the spawning sites occurred. 6. The importance of natural migrations and seasonal activity patterns for barbel, and likewise many other riverine cyprinids, has probably been underestimated for a wide variety of river systems. As major components of riverine fish communities, the importance of seasonal movements of mobile cyprinid species should be considered when constructing weirs and other obstructions. Greater consideration should be given to ways of mitigating effects of existing barriers to movement of non-salmonid species.
A broad perspective on hydrologic connectivity is necessary when managing stream ecosystems and establishing conservation priorities. Hydrologic connectivity refers to the water-mediated transport of matter, energy, or organisms within or between elements of the hydrologic cycle. The potential negative consequences of enhancing hydrologic connectivity warrant careful consideration in human-modified landscapes that are increasingly characterized by hydrologic alteration, exotic species, high levels of nutrients and toxins, and disturbed sediment regimes. While connectivity is integral to the structure and function of aquatic ecosystems, it can also promote the distribution of undesirable components. Here we provide examples illustrating how reduced hydrologic connectivity can provide greater ecological benefits than enhanced connectivity does in highly developed, human-modified ecosystems; for example, in urban landscapes, “restoration” efforts can sometimes create population sinks for endangered biota. We conclude by emphasizing the importance of adaptive management and balancing trade-offs associated with further alterations of hydrologic connectivity in human-modified landscapes.
We examined the status of diadromous (migratory between saltwater and freshwater) fishes within the North Atlantic basin, a region of pronounced declines in fisheries for many obligate marine species. Data on these 24 diadromous (22 anadromous, 2 catadromous) species are sparse, except for a few high-value forms. For 35 time series, relative abundances had dropped to less than 98% of historic levels in 13, and to less than 90% in an additional 11. Most reached their lowest levels near the end of the observation period. Many populations persist at sharply reduced levels, but all species had suffered population extirpations, and many species are now classified as threatened or endangered. Habitat loss (especially damming), overfishing, pollution, and, increasingly, climate change, nonnative species, and aquaculture contributed to declines in this group. For those diadromous fishes for which data exist, we show that populations have declined dramatically from original baselines. We also discuss the consequences of these changes in terms of lost ecosystem services.
Fresh waters offer intricate diversity, from small hot springs to huge floodplain systems, from temporary pools to big lakes, each containing significant proportions of the world's stock of inland water. In this collection is a great diversity of organisms, and of interesting relationships amongst them and with traditional peoples. There is thus every reason to value the diversity of freshwater systems and reasons for conserving them, from the ethical to the economic, can be listed no less than for terrestrial systems.
The approaches currently used to conserve the diversity of terrestrial systems, however, may be counter-productive for the conservation of freshwater systems. These approaches include emphasis on individual, often charismatic species, or on areas of high quality that can be boundaried and protected (fortress conservation) and rest on arguments that all species must be maintained for the system to function. The current decline in world biodiversity under our increasing impact suggests that these are failing approaches even for terrestrial systems. They may be disastrous for freshwater ones.
Many studies have assessed the effects of large dams on fishes but few have examined the effects of small obstacles. Fishes were sampled and environmental variables were characterized at 28 sites in two Iberian streams, 14 located immediately downstream, upstream and between five small obstacles at river Muge and 14 at river Erra, considered as the reference stream. Multivariate analysis indicated that habitat variables like current velocity and depth, but not physicochemistry, were mainly responsible for site groups' discrimination in both streams. The reference stream exhibited a longitudinal gradient of current velocity that, however, was not strong enough to cause significant changes in the fish assemblage's composition and structure. By successive and drastically repeating this gradient near each structure, the obstacles stream presented differences in fish fauna between the three site types. Lentic upstream sites presented higher density of limnophilic, omnivorous and exotic species, like gudgeon Gobio lozanoi, which are well adapted to this type of habitat. Downstream and between obstacles sites were characterized by the dominance of rheophilic and invertivorous taxa, especially barbel Luciobarbus bocagei. Richness metrics did not differ among site types, but diversity was higher in sites located between the obstacles away from its direct influence, where the habitat diversity was higher. Contrarily to upstream sites, downstream and between obstacles sites were similar in many of the studied features to the reference stream, implying that this type of structures cause a higher modification in the upstream fish community. This study suggests that the effects of small obstacles on habitat and fishes are similar, in some extent, to those reported for larger dams, providing important considerations for riverine ecosystem conservation efforts. Copyright © 2009 John Wiley & Sons, Ltd.
Dam construction has serious consequences for aquatic ecosystems, and one of the most serious is the “barrier effect,” the prevention of organism migration throughout a system. We assessed the effect of habitat fragmentation by damming on the population persistence of a stream-dwelling fish, the white-spotted charr (Salvelinus leucomaenis), in streams of southwestern Hokkaido, Japan. We sampled for charr at 52 dammed-off sites by electrofishing or snorkeling and measured five habitat characteristics: isolation period, watershed area, gradient, elevation, distance from sea. Of the 52 study sites above dams, white-spotted charr were absent at 17 sites and were present at 35 sites. Because the charr occupied all undammed upstream reaches, the damming would cause the absence of charr upstream. Among five habitat characteristics examined, stepwise logistic-regression analysis showed that disappearance was promoted with increasing isolation period, with decreasing watershed area (i.e., habitat size), and with decreasing gradient. The resulting logistic model explained 82.7% of the present white-spotted charr occurrence and forecasted that 12 of 35 extant populations will disappear after 50 years. Our findings imply that extirpation of small, dammed-off populations is inevitable unless efficient fish ladders are installed or dams are removed.
Increasingly, biological reserves throughout the world are threatened by cumulative alterations in hydrologic connectivity within the greater landscape. Hydrologic connectivity is used here in an ecological sense to refer to water-mediated transfer of matter, energy, and/or organisms within or between elements of the hydrologic cycle. Obvious human influences that alter this property include dams, associated flow regulation, ground-water extraction, and water diversion, all of which can result in a cascade of events in both aquatic and terrestrial ecosystems. Even disturbances well outside the boundaries of reserves can have profound effects on the biological integrity of these ''protected'' areas. Factors such as nutrient and toxic pollution and the spread of nonnative species are perpetuated by hydrologic connectivity, and their effects can be exacerbated by changes in this property. Hydrological alterations are now affecting reserves through increasingly broad feedback loops, ranging from overdrawn aquifers to atmospheric deposition and global climate change. Such alterations are often beyond the direct control of managers because they lie outside reserve boundaries, and data on hydrologic connection between reserves and sur-rounding landscapes are scant. The subject of water has also been typically excluded from the literature pertaining to both theoretical and practical aspects of reserve size, isolation, and design. This results, in part, from early management strategies developed when the landscape matrix outside of reserves was not excessively fragmented, and when awareness of hydrologic connectivity was in its infancy. The location of a given reserve within a watershed, relative to regional aquifers and wind and precipitation patterns, can play a key role in its response to human disturbance transmitted through the hydrologic cycle. To illustrate this point, I discuss reserves of varying sizes from diverse regions throughout the world. Reserves located in middle and lower watersheds often suffer direct hydrologic alterations that cause severe habitat mod-ification and exacerbate the effects of pollution. In contrast, reserves in upper watersheds may have intact physical habitat and contain important source populations of some native biota, yet hydrologic disturbances in lower watersheds may cause extirpation of migratory species, cascading trophic effects, and genetic isolation. Worldwide, 7% of land area is either strictly or partially protected, and many reserves are in danger of becoming population ''sinks'' for wildlife if we do not develop a more predictive understanding of how they are affected by hydrologic alterations that originate outside of their boundaries.
1. Landscape ecology deals with the influence of spatial pattern on ecological processes. It considers the ecological consequences of where things are located in space, where they are relative to other things, and how these relationships and their consequences are contingent on the characteristics of the surrounding landscape mosaic at multiple scales in time and space. Traditionally, landscape ecologists have focused their attention on terrestrial ecosystems, and rivers and streams have been considered either as elements of landscape mosaics or as units that are linked to the terrestrial landscape by flows across boundaries or ecotones. Less often, the heterogeneity that exists within a river or stream has been viewed as a `riverscape' in its own right.
2. Landscape ecology can be unified about six central themes: (1) patches differ in quality (2) patch boundaries affect flows, (3) patch context matters, (4) connectivity is critical, (5) organisms are important, and (6) the importance of scale. Although riverine systems differ from terrestrial systems by virtue of the strong physical force of hydrology and the inherent connectivity provided by water flow, all of these themes apply equally to aquatic and terrestrial ecosystems, and to the linkages between the two.
3. Landscape ecology therefore has important insights to offer to the study of riverine ecosystems, but these systems may also provide excellent opportunities for developing and testing landscape ecological theory. The principles and approaches of landscape ecology should be extended to include freshwater systems; it is time to take the `land' out of landscape ecology.
Most studies analyzing patterns in biotic homogenization of fish communities have used large-scale approaches, while the community-level
effects of species introductions and local extinctions within river basins have been sparsely analyzed. In this article, we
examine patterns in freshwater fish α- and β-diversity in relation to the presence of reservoirs in a Mediterranean river
(Guadiana river; Iberian Peninsula). We used fish samples from 182 river localities and 59 reservoir ones to address two main
questions: (i) do reservoirs favor the establishment of invasive fish species?; and (ii) do reservoirs bear taxonomically
homogenized fish communities? Although total species richness was not different between rivers and reservoirs, the latter
had more invasive species and less native ones. Fish species found in reservoirs tended to be larger ones, but invasive species
of any size showed higher preferences for reservoirs. Native species that were rare or absent in reservoirs were those that
showed higher sensitivity to invasive species in rivers. Reservoir fish communities were taxonomically homogenized in relation
to river ones, both when considering all fish species and using only natives or only invasive ones. Our results suggest that
invasive species occupying reservoirs constitute an ecological filter excluding most native species from such systems. Invasive
species in the study area are often widely introduced elsewhere, while native species found in reservoirs are congeneric and
ecologically similar to those found in other Iberian studies. Thus, we conclude that reservoirs promote taxonomic homogenization
at multiple spatial scales, while could also be promoting the functional homogenization of Iberian fish communities.
KeywordsBiotic homogenization–β-diversity–Freshwater fish–Fish conservation–Mediterranean rivers–Invasive species–Regulated rivers
Because human actions alter the physical nature of aquatic ecosystems similarly worldwide, the extinction risk among many freshwater fishes that share particular life-history traits may also be similar. Determining whether taxonomic selectivity, the preferential loss (or persistence) of certain species groups, exists among the world's freshwater fish families is then a key step in predicting future species declines and triaging future conservation efforts. We use binomial statistics to look for taxonomic patterns among the world's freshwater fish families currently at risk of extinction. Families are identified as being at risk of extinction if at least one species within a given family is classified as either extinct or at risk of extinction by the International Union for the Conservation of Nature's Redlist of Threatened Animals [Groombridge, B., Baillie, J., 1997. 1996 IUCN Red list of Threatened Animals. IUCN Gland, Switzerland and Cambridge, UK]. Eighteen freshwater families have more threatened species than expected if extinction risk was evenly distributed across all families. Next, we use a series of chi-squared analyses to determine if various family-level characteristics (e.g. geographic distribution, body length, habitat preference, etc.) produce this taxonomic pattern. We find that families that inhabit well-studied regions of the world contain more threatened species. However, we find no indication of a unifying set of extinction-promoting biological or ecological traits that contribute to extinction risk among freshwater families. A possible explanation for this discrepancy is that aquatic alterations worldwide are so severe that extinction is being driven by extrinsic rather than intrinsic factors.
Economically important pests usually attack a crop or group of crops over a wide region in which there are geographic, local, and temporal variations in the environment. Effectiveness of any control program will therefore depend on the different responses of the crop, pest, and control organism to this pattern of environment. Usually the environmental heterogeneity is treated as an unavoidable complication in program evaluation, and attempts are made to work with “average” conditions.
We review the early development of metapopulation ideas, which culminated in the well-known model by Levins in 1969. We present a survey of metapopulation terminology and outline the kinds of studies that have been conducted on single-species and multispecies metapopulations. Metapopulation studies have important conceptual links with the equilibrium theory of island biogeography and with studies on the dynamics of species living in patchy environments. Metapopulation ideas play an increasingly important role in landscape ecology and conservation biology.
Habitat loss has pervasive and disruptive impacts on biodiversity in habitat remnants. The magnitude of the ecological impacts of habitat loss can be exacerbated by the spatial arrangement – or fragmentation – of remaining habitat. Fragmentation per se is a landscape-level phenomenon in which species that survive in habitat remnants are confronted with a modified environment of reduced area, increased isolation and novel ecological boundaries. The implications of this for individual organisms are many and varied, because species with differing life history strategies are differentially affected by habitat fragmentation. Here, we review the extensive literature on species responses to habitat fragmentation, and detail the numerous ways in which confounding factors have either masked the detection, or prevented the manifestation, of predicted fragmentation effects.
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