Across the U.S., aquatic fauna face a greater threat of extinction and population decline than their terrestrial counterparts.
Mussels, dragonflies, and damselflies are among the most imperiled aquatic macroinvertebrates in freshwater ecosystems (Richter et al. 1997). Introductions of non-native species and changes in the natural hydrology of streams and rivers have resulted in the decline of aquatic macroinvertebrates. Climate change may also pose a threat to this group of organisms in the future.
Aquatic ecosystems are usually surrounded by large contiguous areas of land, and, as a result, are extremely isolated This isolation has allowed aquatic communities to evolve with little potential for species interactions or migrations between watersheds. For example, communities along the Colorado River Basin have a much higher potential for interaction and species exchange within that watershed than they experience with communities from the Rio Grande River Basin. Because of this isolation, the effects of species introductions into aquatic ecosystems are more significant and exaggerated than they are in terrestrial ecosystems (Riccardi and Atkinson 2004).
Introductions of non-native aquatic macroinvertebrates have had detrimental effects on many native species in national parks in the Southwest. Two non-native species that have scientists concerned are the zebra mussel (Dreissena polymophia) and the quagga mussel (Dreissena bugensis). These species can reduce aquatic macroinvertebrate diversity and abundance through competition for resources, particularly FPOM, and decreasing water quality. These mussel species have been found in lentic ecosystems around the southwest. The primary mechanism for their introduction has been through contaminated boats and boat trailers entering these systems.
The Asian clam (Corbicula fluminea), native to southern and eastern Asia and Africa, has made its way to Texas, southern New Mexico, and western Arizona (U.S. Geological Survey 2001). This non-native species can alter benthic substrates and compete with native mussels for space and food.
Another aquatic invader affecting lentic communities in national parks on the Colorado Plateau is the virile crayfish (Oroconectes virilis). Unlike mussels in the lakes of the Southwest, organisms in rivers and streams on the Colorado Plateau have evolved without the presence of crayfish. The virile crayfish’s ability to feed at multiple trophic levels can cause declines in aquatic macroinvertebrates across all functional groups within a community. The resulting depauperate community is then at risk to future perturbation due to a lack of species redundancy in the community. Functional roles that were once filled by many species of aquatic macroinvertebrates are replaced by one or a few. If the remaining species cannot adapt to future perturbation experienced by the system, then overall function of that ecosystem will decline.
Altered Flow Regimes
Changes in the natural hydrology of streams and rivers have contributed to the decline of aquatic macroinvertebrates in the Southwest and across the country. Activities such as water impoundment, groundwater mining, conversion of floodplains into agricultural lands, diversion of water for irrigation and power generation, and channelization alter the natural flow regimes of streams and rivers. These changes include (1) a reduction in the amount of water flowing in a stream or river, (2) a loss of flow variation, and (3) alterations in the stream channel. An estimated 85% of rivers and streams in the U.S. have been affected by some form of flow alteration (Poff et al. 1995, National Research Council 1992).
Water impoundment through construction of a basin or obstruction of a stream depletes surface water and groundwater flow and can result in decreased habitat and food availability for aquatic macroinvertebrates, leading to a decline in species richness and alteration of community structure. Groundwater depletion can also result in a loss of nutrients vital to algal organisms, which can cause further change in community structure (Scott et al. 2005 and Dahm et al. 2003).
Water impoundment and channelization (any alteration of a stream channel) not only can alter the amount and speed of water flow, but can also reduce flow variability. This, in turn, can reduce biodiversity and abundance through the loss of native species that evolved with natural flow variability (Scott et al. 2005). Species adapted to the new static flow regimes may increase, resulting in an altered community structure. On large rivers in the Southwest, the loss of natural variability in flow regimes has also led to a decrease in the transport of nutrient-rich sediments as well as altered water temperatures. Studies have shown significant decreases in aquatic macroinvertebrate diversity in places like Grand Canyon National Park (Blinn and Cole 1991), and Flaming Gorge Dam, where temperature changes as a result of the construction of the dam caused Ephemeroptera richness to decline from 30 species to a single species (Vinson 2001).
Finally, channelization of streams to straighen, widen, divert, narrow, or otherwise change the course or size of a stream channel can significantly alter stream flow, causing increased flow velocities, turbidity, and sedimentation (Scott et al. 2005). Similar to the effects of changes in flow variability, increased flow velocities, sedimentation, and turbidity can lead to
- temperature changes in the water
- altered oxygen concentrations
- altered community structure due to the loss of habitat
- a decline in species that have adapted to the natural conditions which existed prior to channelization
Lotic ecosystems in the Southwest experience a large annual variation in flow. These flows range in extremes from large flash flood events (spates) to long periods of drying and intermittency. Although aquatic macroinvertebrates have evolved under these harsh conditions for thousands of years, it is uncertain how these communities would respond to increased spate events and prolonged periods of drying that could accompany climate change. Flash floods have the potential to decimate aquatic macroinvertebrate abundance during a single event (Fisher et al. 1982). However, communities are highly resilient to such disturbance events. Studies on streams in the Southwest have shown that populations can recover to their pre-flood abundances in as little as three weeks through aerial recolonization by adults (Fisher et al. 1982; Grimm and Fisher 1989). Increased intensity of events, combined with greater frequency, could affect how quickly communities respond in the future.
Climate change is predicted to bring an increase in drought frequency of 66-90% (Gitlin et al. 2006). Increased drought will likely decrease the amount of benthic habitat available, and therefore the abundance and diversity of the aquatic macroinvertebrate population. While the hyphoreic zone has the potential to serve as refugia during such times, not all species present will be able to take advantage of it. In addition, drought could lead to further declines in macroinvertebrate populations due to a lack of allochthonous inputs into the system. Upland vegetation could potentially experience high rates of stress and mortality, leading to a decrease of riparian-derived CPOM entering the system. As a result, shedders and collectors could experience a decline in food resources, leading to increased mortality and lower species abundance.
Prepared by Stacy Stumpf, Patty Valentine-Darby, and Evan Gwilliam, NPS Inventory and Monitoring Program, 2009.
Series: Aquatic Macroinvertebrates in the American Southwest
Last updated: May 12, 2015