Crowl T.A.

Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico

Covich, Alan P.; Crowl, Todd A.; Heartsill-Scalley, Tamara 2006. Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico.. J. N. Am. Benthol. Soc., 25(1):99-107.

Abstract: 
Extreme events (hurricanes, floods, and droughts) can influence upstream migration of macroinvertebrates and wash out benthic communities, thereby locally altering food webs and species interactions. We sampled palaemonid river shrimp (Macrobrachium spp.), dominant consumers in headwaters of the Luquillo Mountains of northeastern Puerto Rico, to determine their distributions along an elevational gradient (274–456 m asl) during a series of disturbances (Hurricane Hugo in 1989, a drought in 1994, and Hurricane Georges in 1998) that occurred over a 15-y period (19882002).We measured shrimp abundance 3 to 6 times/y in Quebrada Prieta in the Espiritu Santo drainage as part of the Luquillo Long-Term Ecological Research Program. In general, Macrobrachium abundance declined with elevation during most years. The lowest mean abundance of Macrobrachium occurred during the 1994 drought, the driest year in 28 y of record in the Espiritu Santo drainage. Macrobrachium increased in abundance for 6 y following the 1994 drought. In contrast, hurricanes and storm flows had relatively little effect on Macrobrachium abundance.

Predator–prey interactions in river networks: comparing shrimp spatial refugia in two drainage basins

COVICH, A. P., T. A. CROWL, C. L. HEIN, M. J. TOWNSEND, AND
W. H. MCDOWELL. 2009. Predator-prey interactions in
river networks: comparing shrimp spatial refugia in two
drainage basins. Freshwater Biology 54:450–465.

Abstract: 
1. Analysis of drainage networks provides a framework to evaluate the densities and distributions of prey species relative to locations of their predators. Upstream migration by diadromous shrimp (Atya lanipes and Xiphocaris elongata) during their life cycle provides access to headwater refugia from fish predation, which is intense in estuaries and coastal rivers. 2. We postulate that geomorphic barriers (such as large, steep waterfalls >3.5 m in height), can directly limit the distribution of predatory fishes and, indirectly, affect the densities of their prey (freshwater shrimps) in headwater streams. 3. We compared densities of shrimp in pools above and below waterfalls, in four headwater tributaries in two river basins of the Luquillo Mountains of northeastern Puerto Rico. We measured shrimp densities twice a year over 8 years (1998–2005) in Prieta, Toronja, Bisley 3 and Bisley 5 streams, which differ in drainage network positions relative to steep waterfalls in Rı´o Espı´ritu Santo and Rı´o Mameyes. 4. Predatory fishes are absent in the Prieta and Toronja pools and present in Bisely 3 and in lower Bisley 5 pools. Atya lanipes and X. elongata rarely occur in the Bisley streams where predatory fishes are present but these shrimps are abundant in Prieta and Toronja, streams lacking predatory fishes. 5. The mean carapace length of X. elongata is longer in pools where fish are present (Bisley 3 and lower Bisley 5) than in pools lacking fish (Prieta, Toronja, Upper Bisley 5). The increased body size is primarily due to significantly longer rostrums of individuals in stream reaches with fish (below waterfall barriers) than in those reaches lacking fish (above waterfall barriers). Rostrum length may be an adaptation to avoid predation by visually feeding fishes. 6. Atya lanipes and X. elongata distributions and densities were predicted primarily by drainage network position relative to the presence or absence of predatory fishes. High, steep waterfalls effectively impeded fish from moving upstream and created a spatial refuge. Xiphocaris elongata may rely on size refugia (longer rostrum) to minimize predation where spatial refugia are lacking.

Linking habitat stability to floods and droughts: effects on shrimp in montane streams, Puerto Rico

COVICH, A. P., T. A. CROWL, AND F. N. SCATENA. 2000. Linking
habitat stability to floods and droughts: effects on
shrimp in montane streams, Puerto Rico. Verhandlungen
der Internationalen Vereinigung fu¨ r theorestische und
angewandte Limnologie 27:2430–2434.

Abstract: 
Most previous studies on Caribbean flood and drought frequency have examined hydrological (e.g. MORRIS & VAZQUEZ 1990, GARCIA et al. 1996) rather than ecological effects (COVICH et al. 1998). We analyzed temporal and spatial distributions of rainfall, stream flow, and mean maximum pool depth over an 8-year period (1990–1997) to evaluate effects of variable flows. We examined the response of a biotic variable (the coefficient of variation in shrimp densities) to changes in water depth (coefficient of variation of maximum pool depth) along an elevational gradient.

Linking habitat stability to floods and droughts: effects on shrimp in montane streams, Puerto Rico

Covich A.P., Crowl T.A. & Scatena F.N. (2000) Linking
habitat stability to floods and droughts: effects on
shrimp in montane streams, Puerto Rico. Verhandlungen
der Internationalen Vereinigung fu¨r Theoretische und
Angewandte Limnologie, 27, 2430–2434.

Abstract: 
Most previous studies on Caribbean flood and drought frequency have examined hydrological (e.g. MORRIS & VAZQUEZ 1990, GARCIA et al. 1996) rather than ecological effects (COVICH et al. 1998). We analyzed temporal and spatial distributions of rainfall, stream flow, and mean maximum pool depth over an 8-year period (1990–1997) to evaluate effects of variable flows. We examined the response of a biotic variable (the coefficient of variation in shrimp densities) to changes in water depth (coefficient of variation of maximum pool depth) along an elevational gradient.

Effects of extreme low flows on freshwater shrimps in a perennial tropical stream

COVICH, A.P.; CROWL, T.A.; SCATENA, F.N. 2003. Effects of extreme low flows on freshwater shrimps in a perennial tropical stream.. Freshwater Biology 48, 1199-1206.

Abstract: 
1. Long-term data on rainfall suggests that perennial rainforest streams rarely are subject to drying of riffles or pools in the wet, non-seasonal Caribbean climate of Puerto Rico. Unusually low rainfall in 1994 caused some headwater riffles to dry out completely, resulting in isolated pools, reduced pool volumes and loss of access to microhabitats by benthic invertebrates. 2. From 1992 to 1998, shrimp populations were sampled bimonthly using baited traps in six pools along 1200 m (from 305 to 480 m in altitude) of Quebrada Prieta, a second-order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). 3. Following contraction of the smaller and shallower pools in the most upstream sectionof the stream, mean densities of the dominant shrimp (Atya lanipes) increased from 22 to 75shrimp m)2 of pool area during the 1994 drought year. A second common species(Xiphocaris elongata) increased from 5 to 14 shrimp m)2. A smaller percentage of adults of both species was gravid during the drought. 4. Following the 1994 drought (1995–1998), densities of both shrimp species and reproductive activity of Atya returned to predrought (1990–1993) levels. However, the reproductive activity of Xiphocaris remained lower than in the predrought period. 5. It is suggested that prolonged droughts, even in tropical rainforest biomes, may significantly alter aquatic communities through localised crowding effects resulting from habitat contraction, and lead to prolonged decreases in reproductive output. Consequently, major alterations in aquatic populations and communities would be predicted by current climate change scenarios of decreased total rainfall and increased variability.
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