THE CRITICAL ROLE OF WET-DRY CYCLES
Although tropical wetlands depend on seasonal periods of wet and dry conditions, these wetlands are increasingly threatened by changes in hydrology.1-5 These hydrologic alterations are due to human-related drivers including climate change and water management designed to meet agricultural, industrial, and urban water needs. The negative ecological impacts to such wetlands are expected to increase in the future despite the many societal benefits provided by these ecosystems. In addition to providing habitat for fish and wildlife, tropical wetlands provide food, store flood waters, improve water quality, and provide valuable grazing lands.
Some of the world’s most important tropical wetlands are located in wet-dry climates (Figure 1). Prime examples of large, internationally-important wetlands in tropical wet-dry climates include the Pantanal (Brazil), Okavango Delta (Botswana), Kakadu National Park (Australia), Everglades National Park (USA), Kafue Flats (Zambia), Palo Verde National Park (Costa Rica), and Keoladeo National Park (India). In these tropical wetlands, wet-dry cycles govern flooding regimes, control seasonal ecological cycles, and drive ecosystem functions (Figure 2).1,2,4,6-10 During the wet season, heavy rainfalls produce pulses of surface water that flood wetlands, resulting in rapid plant growth and the conversion of areas with brown, dried mud to green, lush landscapes. In contrast, during the dry season, plant growth wanes and “wet” wetland soils become “dry” wetland soils due to evaporation rates that exceed rainfall. Vegetation in the dry season gradually changes from green to brown due to the presence of drought-like, physiologically-stressful conditions. By the end of the dry season, the lowest elevations in these wetlands are often the last to retain water and food, which means that they serve as oases in an otherwise dry and resource-poor landscape. These last water holes are key refugia for fish and wildlife during the driest months of the year, supporting large concentrations of birds, mammals, fish, and other organisms. Due to their high productivity and tremendous concentration of resources, wetlands in the wet-dry tropics are internationally-important biodiversity hotspots that are also very important to the local communities that surround these wetlands. In recognition of their value, many tropical wetlands have been designated Ramsar Wetlands of International Importance.
PROBLEM: TROPICAL WETLAND BIODIVERSITY AND ECOLOGICAL RESILIENCE ARE AT RISK
Despite the many societal benefits provided by tropical wetlands, we live in an era of unprecedented ecological change11 that has been named the Anthropocene.12,13 Direct conversion of wetlands to other land uses is the most direct threat to tropical wetlands.14 In the absence of legal protection, wetlands across the world are often converted to uplands, which results in the loss of critical ecosystem services. However, increasing human demand for water to support agriculture, industry, and cities is also a significant driver of change in tropical wetlands. Human populations are growing, which means that the demand for freshwater is increasing so that less freshwater is available for wetlands.5,15-17 The value of the ecosystem services provided by tropical wetlands is often not incorporated into regional land use and water management decisions. Thus, the freshwater inputs and wet-dry cycles that maintain tropical wetland ecosystems are often not prioritized and protected. Water management practices designed to meet increasing human urban, agricultural, and industrial demands can affect the quality, quantity, and timing of freshwater inputs to wetlands, which can have large ecological repercussions. To make matters worse, global climate change is expected to exacerbate and potentially amplify the ecological effects of local, human-driven hydrologic change. Projections of climate change impacts to the hydrologic cycle vary by region. However, in general, seasonal rainfall regimes are expected to change, and extreme drought and flooding events are expected to intensify.18-26 Hence, in the future, tropical wetland ecosystems are expected to be increasingly affected by such hydrologic alterations.
Given the vulnerability of tropical wetlands to climate change and increasing human water demands, there is a pressing need for solutions that maintain biodiversity, maximize ecological resilience, and maintain ecosystem services for future generations. Here, we define ecological resistance as the capacity to withstand change (for example, not be affected by a climate extreme) and resilience as the capacity to recover after perturbation (for example, recover after being affected by a climate extreme).27-30 Hydrology is the master controlling variable in most freshwater wetland ecosystems6,9,14,31, and hydrology is the best starting point for managing tropical wetland resilience. We argue that to sustain tropical wetlands in the face of future change, we must protect the wet-dry hydrological cycles that have shaped and defined these ecosystems. In simple terms, we must protect both the “dry” and the “wet” phases of the wet-dry flooding cycle, and we provide examples regarding the critical role of each phase.
SOLUTION: PROTECT THE “DRY” PHASE OF THE WET-DRY CYCLE
In the wet-dry tropics, freshwater resources are typically scarce during the dry season. To meet the demands of growing human populations, some regions have developed water management practices and infrastructure that increase the amount of freshwater available for human needs during the dry season. Following their intended use, a portion of the remaining waters flow into seasonal wetlands during a period where there is typically little or no water. For example, due to upstream dams, canals, and/or inter-basin water transfers, water can be controlled and inadvertently released into downstream tropical wetlands during the dry season. At first glance, increasing the freshwater inputs to a wetland during the dry season may not seem remarkable or problematic; however, freshwater inputs to wetlands during the “dry” phase of the wet-dry cycle can have large ecological consequences.
Replacing a seasonal flooding regime with a permanent flooding regime can trigger abrupt ecological transformations known as ecological regime shifts.32 Inundation-triggered regime shifts in tropical wetlands often decrease biodiversity, promote the expansion of invasive non-native species, and reduce ecological resilience.33-35 Permanent flooding reduces biodiversity because the mosaic of different biological communities that define these wetlands are dependent upon seasonal hydrologic regimes.1,2,8,36 The organisms that thrive in tropical seasonal wetlands have life history traits that enable them to rapidly respond to and recover from changes in water availability.8 These life history traits confer a high level of ecological resilience to the system. For example, the dominant plant species in these ecosystems typically have long-lived, soil seed banks that enable new individuals to quickly regenerate in response to ideal flooding or drawdown conditions.1,3,6,7,36-38 Soil seed banks in these systems often contain a diverse mixture of wet-season and dry-season specialists (i.e., flood-tolerant and drawdown-tolerant species, respectively). However, the ecological resilience that these ecosystems possess is the result of fluctuating wet-dry cycles, and permanent flooding will typically result in reduced ecological resilience. For example, permanent flooding can deplete the pool of species in the soil seed bank, as drought-tolerant and/or drawdown specialist species disappear from the system. Conversely, recurrent drought events that are interspersed by shortened flood events that do not result in reproduction can also deplete seed banks.39 As a result, the system does not maintain the species needed to respond and recover from extreme drought or flooding. In other words, protecting the dry season conditions (i.e., drawdown) and the appropriate wet-dry oscillations can protect the wetland seedbanks that enable the wetland to be resilient to future extremes of drought and flooding.37,38
Examples of the negative effects of ill-timed freshwater inputs can be found in Zambia, Australia, and Costa Rica. In Zambia, the installation of hydroelectric dams in the Kafue Basin has modified the historic, seasonal hydrologic regime and enabled year-long releases of freshwater. The modified hydrologic regime has resulted in the conversion of seasonally-flooded wetlands to wetlands that are permanently flooded, which has reduced plant and animal biodiversity and facilitated the expansion of an invasive non-native shrub species.34 The removal of the “dry” phase of the wet-dry cycle has also reduced the habitat available for an endemic antelope species. In northwestern Australia, a dam on the Ord River has converted a seasonal, intermittent river to a perennial river, which is a change that has altered downstream vegetation dynamics40, reduced prawn populations41, and decreased estuarine production.8,42 In northwestern Costa Rica, an inter-basin water transfer has redirected surface water from an adjacent watershed for agricultural use during the dry season and hydroelectric energy production. Some of these surface waters enter a portion of Palo Verde National Park during the dry season. These dry season freshwater inputs have converted a seasonally-inundated, wet-dry wetland to a wetland that is permanently inundated throughout the year33,43,44, which has reduced diversity, decreased resilience, and removed the “dry” phase of the wetland’s wet-dry cycle. All three of these examples show that to maintain diversity and maximize ecological resilience in tropical wetlands, the dry season is critical and must be maintained.
The solution to these problems is to protect and manage the “dry” phases of the wet-dry cycle. For certain areas that receive ill-timed freshwater inputs during the dry season, the solution may be to divert the ill-timed freshwater inputs to other areas. And for areas that still have a dry season, prevention and communication are important pieces of the solution. In other words, the critical role of the wetland’s dry season must be communicated by stakeholders (e.g., fishermen, farmers, cattle grazers, ecotourism guides, naturalists, and scientists) to regional water management organizations that make decisions that affect seasonal hydrologic regimes. Stakeholders must be ready to provide solutions that would help regional water managers govern hydrologic inputs in a manner that protects the wet-dry hydrologic regimes that support the many societal benefits provided wetlands.
SOLUTION: PROTECT THE “WET” PHASE OF THE WET-DRY CYCLE
In the coming century, the timing, quality, and quantity of wet-season freshwater inputs are expected to change due to climate- and human-driven hydrologic changes. The rationale for protecting the “wet” phase of the wet-dry cycle is more intuitive than the need to protect the “dry” phase. Without the “wet” phase, tropical wetlands would cease to function as wetlands and they would not provide the many ecosystem services that society values. Wet season flooding can be used to maintain diversity and maximize ecological resilience to drought events. During the wet-season, pulses of flood water can be extremely important8,9,45, and efforts to manage freshwater inputs should aim to replicate and produce desired wet season flooding regimes that include peaks and pulses.
The Tempisque River watershed in Costa Rica provides an example of the potential negative synergistic effects of climate change and increasing human water demands.46,47 Freshwater resources in the region are becoming increasingly scarce due to population gains, agricultural expansion, and tourism growth. Central America has also been deemed a global hot spot for climate change, due to projections of drier conditions in the coming century.20-22 The combination of increasing human water use and shorter, drier wet seasons would have large effects on the hydrologic cycles that govern the regionally-important wetlands within and adjacent to Palo Verde National Park. The ecological ramifications of these expected changes in freshwater availability warrant more attention from environmental managers.
Another example of the negative effects of altered wet season flooding regimes can be found in India’s Keoladeo National Park.48-50 Due to anthropogenic water management in the main river feeding the park, the amount of water reaching the park’s wetlands has decreased. Resource managers in this part of Rajasthan suspect that lower water levels during the wet season have reduced avian habitat and decreased the coverage of flood-tolerant plant species. The solution to this situation has been to re-divert water from another river to these wetlands.49,51
In the wet-dry tropics of northern Australia, the hydrologic regimes of many river floodplains have not been modified by anthropogenic activities. This region also has a rich history of wetland and riparian ecological research, which has shown that the timing and duration of peak wet season river flows have a large effect on wildlife species and govern the structure and functioning of floodplain ecosystems.7,8 The knowledge gained in northern Australian wetlands can serve as a valuable starting point for scientists and resource managers in other regions of the world seeking to understand and better manage the ecological influence of wet-dry cycles in tropical wetlands.
Despite the challenges posed by growing human water demands and climate change, there is room for hope. Wet season flooding can be used to maintain diversity and maximize ecological resilience to drought events. With future-focused planning, water management structures can be used to ensure that freshwater inputs into tropical wetlands can be managed to replicate and produce the desired wet season flooding regimes. The challenge is to balance the needs of human populations with that of nature conservation in a time of changing water availability.
SUMMARY
In the face of climate change and increasing human water demands, the fate of wetland ecosystems in tropical wet-dry climates is threatened. To maximize biodiversity and ecological resilience, environmental planners and resource managers can work to protect and manage both the “dry” and “wet” phases of the wet-dry hydrologic cycles. Adjustable water control structures can help managers maintain both wet and dry hydrologic periods during the year. Wet-dry cycles have shaped and maintained these ecosystems in the past and they can be used to maximize biodiversity and resilience in the future.
ACKNOWLEDGMENTS
We appreciate the comments provided by Hardin Waddle and the editors of Solutions on an earlier version of this manuscript. This research was partially supported by the USGS Ecosystems Mission Area, USGS Land Change Science Program, and the USGS Greater Everglades Priority Ecosystems Science Program.
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