Biota

Desert stream ecosystems support a diverse assemblage of riparian plants and stream biota. A unifying characteristic of desert stream organisms is the shared evolutionary history in a hydrologically extreme environment. The consequences of this extreme physical template are evident from the variety of adaptations that allow species to thrive in systems prone to flash flooding and prolonged drought. Rather than providing a list of taxonomic names for each group, we place emphasis on life history, behavioral, and morphological adaptations for living in hydrologically variable ecosystems.

Desert stream ecosystems house diverse periphyton communities, which include a variety of filamentous green algae, epilithic, epiphytic, and episammic diatoms (attached to rocks, plants, and sediments, respectively), and nitrogen-fixing cyanobacteria. Both flash flooding and prolonged drought decimate algal biomass in desert streams. Rapid drying is particularly lethal, and algae typically die within hours of exposure to the hot, dry desert environment. Algal species often have physiological adaptations that allow for some resistance to drying, however, and can withstand periods of gradual drying. Such adaptations include the production of extracellular mucilage that increases cellular water retention, and intracellular osmoregulatory solutes that also prevent water loss in drying sediments. In addition to these mechanisms, at the onset of drying, algae may also produce spores, cysts, or zygotes that can reactivate upon rewetting. Benthic algae rapidly recolonize stream sediments following floods, whereas recovery following drought is variable and depends on the degree and modes of drought resistance. In Antarctic desert streams, for example, glacial melt is the primary source of streamflow, and primary producers (cyanobacterial and other microbial assemblages) are activated by higher temperature and renewed flows which may occur seasonally or even on a diel basis. However, these organisms are also able to persist for decades in the absence of liquid water.

A productive and diverse invertebrate fauna characterizes many desert streams, consisting of insect and crustacean taxa residing in both benthic and hyporheic habitats. Life-history characteristics of desert stream invertebrates reflect an evolutionary history in a hydro-logically variable ecosystem, and are shaped by both flooding and drying disturbances (Table 1). Most stream invertebrate larvae have few mechanisms that confer resistance to either type of hydrologic disturbance. Instead, many species have short developmental times (e.g., 1-3 weeks) that increase the probability of offspring surviving to reproductive maturity in ephemeral environments, and ensure that some aerial adults are available for recolonization following floods or upon rewetting previously dry channels. In addition, organisms with longer life cycles exhibit an array of avoidance behaviors to minimize the effects of flooding and drying disturbance. These include timing reproductive activity to periods of low flood probability, as well as ovipositing eggs in sections of stream that are likely to retain water for longer periods of time (e.g., deep pools, riffles). Finally, air breathing insects (e.g., coleopterans, hemipterans) may exhibit more direct avoidance behaviors, including the use of rainfall as a cue for leaving aquatic habitats before floods.

Relative to mesic counterparts, fish assemblages of arid river systems are species poor, and are composed of taxa that also have specific adaptations to life in hydrologically variable systems. These adaptations include large reproductive efforts, multiple clutches per year, and short developmental times. Such life-history features, along with the ability to migrate long distances during periods of sufficient flow, allow native desert fish to rapidly colonize habitats after disturbances, and result in dramatic temporal fluctuations in population size. In addition, while intense flash floods can decimate fish populations, many desert fish have morphological adaptations that allow for some resistance to high flows. These include depressed skulls, keeled or humped napes, buttressed fins, narrow caudal peduncles, slim bodies, and reduced scales - all of which act to reduce drag and improve swimming ability in turbulent flow.

As desert stream ecosystems contract during drought, fish become isolated in pools where the physical environment can fluctuate dramatically. Although complete water loss is lethal, as streams contract, individuals of many fish species can survive in small pools, as well as beneath logs, stones, and within beds of algae. As a consequence, native desert fish are able to tolerate a broad range of temperatures (7-37 °C); indeed, desert pupfish of western North America can survive in temperatures that exceed 40 °C. Similarly, most desert fish are able to tolerate high salinity and low dissolved oxygen concentration. Others still, like

Table 1 Invertebrate colonization/recolonization characteristics of desert streams in relation to floods in different physiographic regions

Mesic

Endorheic cold Exorheic cold Hot desert desert desert

Chapparal Glacial

Colonization sources® Colonization distances'3 Pathways0

Refugia

Species diversity

Resilienced Flood occurrence

Spatial extent of flood Severity of flood

Numerous Close

Few Far

Abundant High

High Spring/

summer Extensive

Intermediate

Limited Intermediate/

high High Winter/

summer Variable

High

Intermediate Intermediate S

Limited Low

Low Winter

Extensive

High

Intermediate

Intermediate Intermediate

High

Winter/spring/

summer Extensive

Intermediate

Intermediate

Intermediate/ far

Intermediate Intermediate/

high High Winter

Extensive

High

Limited Low

Unknown Spring/

summer Extensive

Intermediate aRefers to sources separate from the perturbed stream. bRefers to distance from other unaffected water bodies.

cStatus at time of spate: DD/dd, downstream drift; um, upstream migration; S/s, survivors; O/o, oviposition; H/h, hyporheic. Upper and lower case indicates major or lesser importance, respectively.

dRefers to number of taxa, not individuals, following recovery.

Reproduced from Cushing CE and Gaines WL (1989) Thoughts on recolonization of endorheic cold desert spring-streams. Journal of the North American Benthological Society 8: 277-287.

the African lungfish, can burrow into the stream substrate during dry periods and survive for months by breathing atmospheric air with primitive lungs.

Streamside forests, or riparian zones, stand out as hot spots for aboveground primary productivity in arid landscapes. Arid riparian zones include assemblages of phreatophytic deciduous trees capable of accessing groundwater, as well as shrubs and annual grasses. The overall taxonomic composition of riparian zones is typically in striking contrast to that of the surrounding desert landscape. Deeply rooted riparian trees are well suited to an environment where the water table is temporally variable. Obligate wetland species appear in desert riparian areas with permanent access to shallow groundwater, whereas those found in areas with strong seasonal fluctuations in water table elevation have structures such as tap roots or root architecture that maximizes water capture during precipitation events. Many riparian tree species in arid landscapes actually require over-bank flooding at specific times of the year to induce germination. Riparian vegetation is thought to play an important role in the overall cycling of nutrients in arid landscapes by taking up nutrients present in shallow groundwater and building organic matter pools in riparian soils. Finally, riparian vegetation serves as critical habitat for invertebrate, vertebrate, and avian taxa within arid landscapes.

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