Environmental Fluctuation and Population Dynamics of Two Species of Freshwater
Crayfish ( Procambarus spp.) in the Florida Everglades
William F. Loftus
USGS-Florida Center for Water and Restoration Science
Joel Trexler
Florida International University, FL
A. Noble Hendrix
University of Washington, Seattle, WA
Much effort has been directed by theoreticians to determine the response of species to environmental fluctuations by treating the physical environment as an unpredictable or stochastic factor. Applied ecologists have increasingly sought to understand and even replicate the periodicity of natural environmental fluctuation as a management tool to attain or restore “natural” ecological regimes. Thus, understanding the role of environmental fluctuation in promoting species coexistence, or facilitating the dominance of a single species, has gained additional urgency in natural resource management. Two species of crayfish have been identified from the freshwater habitats of the Everglades, the Everglades crayfish ( Procambarus alleni ) and slough crayfish ( P. fallax ). Crayfish burrow in periodic environments, and this behavior may provide a mechanism for persistence in dry periods. Everglades crayfish typically burrow during the dry season, and they have been collected from burrows throughout their range. Slough crayfish on the other hand have been captured largely from flooded habitats. The population dynamics of each species may therefore be uniquely affected by hydrology. Hendrix and
Loftus (2000) found that species composition was a function of flooding duration in both spatial and temporal domains. Yet, the mechanisms responsible for these observations are unknown. Using field samples and a mesocosm experiment, we investigated how drought frequency affected mortality and recruitment rates of two coexisting crayfish species, and how these mechanisms shaped patterns of their density and relative abundance in the Everglades.
The time series data were obtained from crayfish collected at a short (site 50, inundated <
180 days per year), a long (site 06, inundated approximately 360 days per year ) and an intermediate hydroperiod site (site 23 affected by shifts in hydromanagement in the
1950’s) between 1985 and 1998. Species relative abundance was calculated from adult males and regressed against the number of years since drought (fig 1). Everglades crayfish were the dominant species soon after drought events (species ratio < 0.5), whereas slough crayfish were dominant at relatively longer times since drought. Crayfish relative abundance responded differently to flooding at each site. Namely, site 50 shifted from Everglades crayfish-dominated to slough crayfish-dominated communities quicker than the other two sites as the years since a drought increased.

To examine the effect of drought on each species, we simulated a two-week dry-down event in mesocosm tanks. Each tank received six crayfish of a single species, which approximated average field densities of 1.91 m -2 . Tanks were checked three times weekly to measure water depth, feed crayfish dry commercial crustacean pellet food, estimate number of burrows, and collect mortalities. In mesocosm tanks, both species had higher survival in flooded tanks than in simulated drought events. Results of the experiment also suggested that relative survival was higher for slough crayfish in wet treatments, whereas survival of Everglades crayfish was higher in dry treatments. Everglades crayfish constructed more burrows in both treatments than slough crayfish.
We estimated stock recruitment relationships for each species by examining the log –log relationship between adults and juveniles in wet (flooded for the duration of the calendar year) and dry (dry at least three days) years. Everglades crayfish recruitment was affected by the hydrologic conditions in the spawning year (best fit model includes a greater intercept term for dry than wet years, fig. 2). Conversely, slough crayfish recruitment was not affected by wet versus dry year types (the best model to describe the slough crayfish stock-recruitment relationship had a common intercept for both years, fig.
2). Both models had a common slope. The log-log linear model used here was a logtransformed Ricker recruitment function. The intercept term was associated with the slope of the Ricker function near the origin, thus larger intercept terms had steeper slopes and higher recruitment at low densities of adults. Everglades crayfish had higher recruitment at low densities in dry years than slough crayfish, however recruitment at low densities was similar for both species in wet years. The slope term of this log-log linear model is the stock density at which recruitment is maximized. The slope of the slough crayfish stock-recruitment relationship was higher than the Everglades crayfish model
(fig. 2), indicating that maximum recruitment may occur at higher adult densities for the slough crayfish.
Species coexistence may be facilitated by disturbance in some ecosystems. Two species with different, but overlapping, environmental tolerances may coexist when conditions regularly fluctuate through the optima of both taxa. In the Florida Everglades, two species of procambarid crayfish coexist and we investigated how their population dynamics were affected by drought frequency. Crayfish relative abundance was determined by local hydrological conditions. Everglades crayfish (Cambaridae;
Procambarus alleni ) were dominant when droughts were frequent, whereas slough crayfish ( P. fallax ) dominated when droughts were infrequent. Differential tolerances to drought frequency were manifested in species-specific vital rates. Everglades crayfish population dynamics were limited by low recruitment in flooded years, whereas slough crayfish population dynamics were regulated by low rates of survival through droughts as short as two weeks. Because the crayfish assemblage of the Florida Everglades is sensitive to drought frequency, hydromanagement of the region can alter species relative abundance.

Figure 1. Species composition (slough crayfish males/total males) as a function of years since drought for sites 6, 23, and 50. Site 6 had the fewest drought events (N = 3), site
23 was intermediate (N = 5), and site 50 had the most droughts (N = 9) in a14-year time series from 1985-1998. Lines were fitted using logistic regression. a
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Figure 2. (a) Everglades crayfish stock-recruitment relationship. The estimated density of juveniles was analyzed in both wet and dry years. The best-fit model had similar slopes for wet and dry years, but significantly different intercept terms. (b) Slough crayfish stock-recruitment relationship. The estimated density of juveniles was analyzed in both wet and dry years. The best-fit model had a common slope for wet and dry years, and a common intercept term.
Hendrix, A. Noble, University of Washington, Box 355020, Seattle, WA, 98195, Phone:
206-221-5457, Fax: 206-685-7471, anoble@u.washington.edu