Susan B. Adams , Aquatic and Terrestrial Fauna Team, Center for Bottomland Hardwoods Research, Southern Research Station, USDA Forest Service
Climate change is predicted to affect warmwater species directly, through water temperature increases and changes in water quantity and timing, and indirectly, through increased water withdrawal during droughts and changing distributions of invasive species and pathogens, among other mechanisms. Warmwater species will respond to climate change in complex and varied ways, possibly including adaptation, migration, use of local refugia, and behavior modifications leading to altered population dynamics. Responses of entire aquatic communities are exceedingly difficult to predict because they will depend on both the responses of individual species and the altered interactions among species, including species new to the community. Managing for maximum resiliency of aquatic ecosystems may help species survive climate changes.
Warmwater species occur in many parts of the U.S.A. but dominate the aquatic fauna of the country’s southern tier. Warmwater habitats of the southeastern U.S. are global diversity hotspots for many freshwater taxa, including snails, mussels, crayfishes, and fishes (1-4), many of which are highly imperiled (5). For many aquatic groups, especially invertebrates, scientists have insufficient information to understand current species ecology, let alone to confidently predict organisms’ responses to climate change (1). Vertebrates, including fish and amphibians (latter addressed in a separate synthesis), tend to be better-studied (1), but we still lack adequate information on likely responses to climate change for most North American families and regions. Given these data gaps, as well as uncertainties about future local climates, predicting ecosystem level changes is enormously challenging.
Air and surface-water temperatures are correlated; therefore, as air temperatures increase, so will surface-water and, after a lag time, groundwater temperatures (6-9). Environmental temperature strongly influences metabolic rates, growth, fecundity, and a suite of other processes in ectothermic animals (e.g., fish and aquatic invertebrates) and also affects the distribution and virulence of pathogens and parasites (10-13). Increasing water temperatures will alter distributions of aquatic organisms, including warmwater fauna (14). Although warmwater fishes are often loosely defined as species with temperature preferences above 25 or 26 °C, every species has upper thermal limits, beyond which physiological processes break down and disease susceptibility increases (11, 15-17).
Multiple facets of water quality may become increasingly important. Higher water temperatures will typically result in decreased dissolved oxygen in surface waters (18), and low dissolved oxygen has a stronger influence than high temperatures on distributions of some aquatic species (19). At higher temperatures, many water pollutants are taken up faster and are more toxic to fishes than at lower temperatures. (11).
Climate change may also change the timing and quantity of stream flows, the salinity of surface and ground water, and the character of riparian and upland vegetation. In addition, anthropogenic factors such as water withdrawals, dam construction, species introductions, and habitat degradation are expected to exacerbate climate change impacts on warmwater fauna. The predicted changes will have direct and indirect impacts on aquatic habitats and communities, and many of the changes will have complex, synergistic effects (11-12, 20). For example, during droughts, groundwater withdrawals for human use are likely to exacerbate low stream flows (18), causing the remaining water to warm faster in response to higher air temperatures (21).
The Mississippi stream channel pictured here dried out completely during a drought. Such changes in stream flow quantity and timing are expected to occur more often as climate changes. Credit: Susan Adams.
Climate change affects temperature and precipitation patterns, which regulate many ecological processes in aquatic ecosystems. Solid arrows indicate direct responses. Credit: Center for Climate and Energy Solutions .
Changing water temperatures and rainfall patterns may pose challenges for certain species. The Jackson Prairie Crayfish (Procambarus barbiger) is native to a limited area in the southern U.S., making it particularly vulnerable. Credit: Chris Lukhaup.
Maintaining or re-creating high quality habitat for native species, for example riparian buffers, woody debris, and stable banks, could help some warmwater species withstand the effects of climate change. Credit: Chris Lukhaup.
How will climate change affect water cycles?
Climate models project increasing summer and winter temperatures across the U.S. (6). Precipitation forecasts are more complex and less certain, but generalities include increases in the frequency of severe droughts and storms, and changes in runoff timing (6). Salinization of ground and surface waters are expected in coastal, as well as some arid, inland regions (6). During droughts, groundwater recharge will decline as temperature increases and rainfall decreases. Increasing groundwater extraction will further deplete aquifers, placing additional strain on surface water resources (6).
Although warmwater fishes are not expected to face the nearly-universal declines predicted for coldwater fishes, many warmwater species face risks of local extinctions, and some face global extinction, if climate change unfolds as predicted (11, 24). As temperatures increase, some warmwater species (native and non-native) will invade more area, and indeed already have (20, 23, 25-30), but many others will fare worse (11, 14, 18, 24). The latter group includes, among others, animals with:
- Limited dispersal options because of biological or geographical constraints, including human-caused habitat fragmentation
- Small ranges restricted to specific, uncommon habitat types or to habitats with temperatures already near the species’ thermal limits
- Small populations subject to extirpation by extreme events.
Aquatic invertebrates with high dispersal rates (e.g., some zooplankton and insects), may be able to shift distributions to keep pace with climate changes, but dispersal abilities of many others (e.g., many mussels, snails, crayfish, and groundwater-restricted species) preclude migration as a viable strategy for survival (1). Habitat loss for warmwater fishes is predicted to be greatest in the southern regions of the U.S. (18), whereas suitable habitats may increase in northern areas for some species, particularly widely distributed, habitat generalists such as smallmouth bass (14, 30).
With differential shifts in species’ ranges, non-native species establishment, and modification of food web interactions, community structure and function will change in complex ways that are difficult to forecast (1, 11, 26, 31). In a coolwater lake, differential responses by plankton species to warming uncoupled the predator-prey plankton dynamics that formed the foundation of the lake’s food web (32). Changes in other low trophic levels, such as aquatic insects, will also be complex and context-dependent (33) and may strongly influence higher trophic levels. Temporary increases in species richness may occur as native and non-native warmwater species move into warming waters faster than the cool- or coldwater species disappear (16, 31); however, over longer and larger scales, aquatic communities will probably become more spatially homogeneous and dominated by fewer species (31), a process already occurring due largely to human-caused habitat alterations (34). The risk of tropical species escaped from captivity establishing wild populations will rise with increasing temperatures (20).
A number of warmwater fishes and invertebrates have small native ranges (sometimes reproducing only in one to several springs), while many others have highly fragmented distributions (e.g., 35). Narrowly distributed species have little resilience for withstanding extreme events (12). One severe drought that dries several springs may eliminate a species. Drought dramatically reduced sizes of some isolated mussel populations in the southeastern U.S., possibly to densities too low to allow for successful reproduction, and river fragmentation by reservoirs prevents recolonization (36).
Many native warmwater fishes in the southwestern and central U.S. are already living near their thermal maximum limits, so predicted temperature increases will likely cause extirpations and extinctions (18, 22, 24). Twenty species of Great Plains and southwestern warmwater stream fishes with no avenues for northern migration were predicted to be vulnerable to extinction if water temperatures rose 4 °C (24).
In regions where riverine ecosystems historically had highly variable physical conditions, such as the southwestern U.S. and Great Plains, native species are adapted to extreme fluctuations (16). Such adaptation may be beneficial in an era of increasingly erratic weather events. Yet the human response to unpredictable rainfall is often to build canals and reservoirs, thereby radically altering natural hydrologic variability, favoring nonnative fishes adapted to more stable conditions, and threatening persistence of native fishes with migratory life histories or life histories adapted to specific timing of flow events (16, 37-38). Reservoirs and canals are also conducive to the introduction and spread of non-native species, some of which will harm native species (10, 16).
Increased salinity of inland waters in arid regions of the southwestern U.S. has already occurred in some habitats and is expected to be exacerbated by climate changes due to drier conditions, more surface water diversion and groundwater extraction, and higher evaporation rates in warmer temperatures (20, 39). Higher salinity may become excessive for native species or may alter competitive advantages among species, even favoring some nonnative species with high salinity tolerances (16, 20, 39).
Losses in warmwater fauna have occurred already, due to human modifications or climate, and are expected to intensify. An estimated 95% of desert pupfish habitat was lost in the past century due to water withdrawals for irrigation, reservoir construction, and increased salinity (39). In arid portions of Mexico, water depletion (largely for irrigation) led to extinction of at least nine fish and two crayfish species (40-41). Colorado River water regulation has favored development of a non-native fish community characterized by species that are generalists -- omnivores preferring slow, warm water and not requiring riverine conditions for successful reproduction. Much of the native fish community, characterized by species adapted to the hydrologically variable rivers, is gone (16). Already, some warmwater fishes near the southern limits of their range are known to rely on cooler water refugia during peak summer temperatures (24, 42-43). Population extirpations presumably due to impoundments include 38 snail species in the Mobile River and 49 mussel species in the Tennessee River (3), numbers that will increase if climatic changes spur another era of extensive dam construction in the southeastern U.S.
Options for Management
Managing ecosystems for maximum resiliency will favor species and community persistence in the face of climate change. Management practices that can increase resiliency include:
1. Maintain natural hydrograph. Generally, more water in a system means more habitat volume for species and less susceptibility to extreme temperatures and desiccation by drought (21, 44). Maintaining the natural hydrograph, including the natural fluctuations in timing and range of flows, tends to benefit native species (45) over non-natives (16).
2. Maintain groundwater levels and identify and conserve critical groundwater-dependent ecosystems. As surface-water temperatures warm to species’ thermal tolerance limits, groundwater upwelling areas will become increasingly important as refugia during periods of extreme temperature or low water (24). Drainages with more groundwater inputs appear to be somewhat more resilient to climate change impacts (29). Groundwater levels should be maintained and discharge areas (e.g., springs) identified and protected.
3. Protect and restore habitat and water quality. Maintaining or re-creating high quality habitat for native species (e.g., riparian buffers, large woody debris, bank stabilization) may ameliorate impacts of extreme events and help keep population sizes large enough to provide resilience to such events. Ensuring adequate flows will help ameliorate water quality issues, but with rising temperatures, continued adherence to water quality standards will be essential to maintaining native warmwater fauna. Because aquatic habitats integrate upstream and upslope activities, and because we lack even basic knowledge about so many aquatic taxa, habitat protection and restoration efforts may be most effectively applied to whole ecosystems at watershed or even regional scales (1, 46). Solid understanding of a region’s physical and biological processes will facilitate prioritizing habitat protection and restoration actions for maximum effectiveness (e.g., 47).
4. Maintain or restore habitat connectivity. Migration barriers adversely affect numerous aquatic species that require multiple habitats to complete their life histories (e.g., 48- 49). or that depend on multiple populations for gene flow and recolonization after local extirpations (e.g., 36). A rapidly-changing climate makes connectivity increasingly important, because connectivity is a necessary prerequisite for a population’s distribution to shift in response to changing habitats (1, 11). With a predicted increase in extreme events, the importance of connectivity cannot be overstated. Conversely, fragmentation is sometimes beneficial when it hinders non-native species invasions. Creating new connectivity among historically isolated drainages will undoubtedly create new invasion scenarios that will further stress some native populations.
5. Manage for robust and redundant populations. Multiple, large populations provide greater resilience to extreme events, provide more effective source populations for recolonization of habitats affected by extreme events, and provide greater genetic diversity, increasing the possibility of genetic adaptation to changing conditions (11-12, 24, 36).
6. Reevaluate fisheries management strategies. Stocking, introduction, and aquaculture of species not native to a drainage carries ecological risk under stable conditions. Since recent history is no longer an appropriate guide for future ecological conditions, such practices now pose even greater risks (20). Even practices such as maintaining water gardens with tropical species isolated from natural waterways poses a greater risk under climate change scenarios; rising minimum temperatures and increasing risk of floods could eliminate barriers that have prevented the dispersal and establishment of such species (20).
7. Unite planning for conservation and human water supply.
Strayer (1) suggested that aquatic invertebrate conservation may be most successful if viewed "as part of a larger problem with the dual goals of preserving freshwater ecosystems and ensuring adequate supplies of fresh water to people". Such a holistic approach may be promising for all aquatic conservation efforts as society increasingly grapples with fundamental problems of too much or not enough fresh water.