Structure of a Tropical Stream Fish Community: A Role for Biotic Interactions

Abstract

We surveyed the fish community of a tropical watershed in the Northern Range Mountains of Trinidad and conducted experiments to test the hypothesis that species interactions structure the fish community. We censused the fish community at 86 sites, measuring each fish collected and recording physical variables for each site (pool size, substrate composition, cover, etc.). One member of the fauna, a killifish, Rivulus hartii, was widely distributed in the drainage, occurring alone in each headwater but also encountering other fish species below barrier waterfalls. This pattern allowed us to use Rivulus to test four predictions derived from the species interaction hypothesis: (I) Rivulus density (in grams per square metre) would be depressed at sites with other fish species present, when compared to the density expected from physical characteristics of each site as if Rivulus were alone; (2) Rivulus density would decline at points of contact with the piscivorous fish Hoplias malabaricus; (3) local populations of Rivulus found in pools occupied by the predator would show a smaller maximal size than in pools without the predator; and (4) the growth rate of Rivulus would be lowered in the event of a local invasion by the guppy, Poecilia reticulata, another abundant and sometimes co-occurring species in the watershed. The first three predictions were tested with data from the survey, while the fourth prediction was tested by experiments in a nearby stream.We used a factor analysis to reduce the dimensionality of the physical variables to four factors. A regression analysis of Rivulus density on the four factors, using 43 Rivulus-only pools, was used to predict Rivulus density in nine pools, each in a different tributary, in which Rivulus first encountered a second species after being the only species present upstream of this pool. The expected density was based solely on the physical characteristics of the pool and represented the expected Rivulus density purely from physical attributes, as if Rivulus were the only species present. Comparison of these expected densities with the actual Rivulus densities revealed a consistent negative impact of the presence of other species on Rivulus density, in support of Prediction 1. Overall, actual Rivulus densities at points of contact with other species were about one-third of the density expected if the other species were absent.The distribution of Rivulus was essentially complementary to that of the piscivorous fish, although only one site provided an unambiguous test of Prediction 2, since Hoplias did not ascend the tributaries in this watershed. The size distribution of Rivulus in populations occurring in pools with predators was dominated by small individuals, with none larger than the median size of Rivulus in predator-free populations, supporting Prediction 3. Finally, the competition experiments revealed that adult Poecilia depressed the growth rate of young Rivulus.We conclude that interspecific interactions are important in structuring the geographical distribution and a...

Interactions between the direct and indirect effects of predators determine life history evolution in a killifish

“…First, population density or resource availability may exert a greater influence on evolutionary change in Rivulus. Rivulus densities are inversely related to stream size and can be many times that of guppies in small tributaries but several times lower in larger streams (17). For instance, the density of Rivulus is at least seven times greater in Rivulus-only sites than in high-predation sites, whereas the density of guppies differs by only 3-fold in the same river system (17).…”

“…Rivulus densities are inversely related to stream size and can be many times that of guppies in small tributaries but several times lower in larger streams (17). For instance, the density of Rivulus is at least seven times greater in Rivulus-only sites than in high-predation sites, whereas the density of guppies differs by only 3-fold in the same river system (17). However, other facets of research on guppies have illustrated a potential role for density dependence and resource availability as an agent of selection.…”

“…Guppies from low-predation environments instead equally provisioned offspring under high and low food rations. Because low-predation guppy and Rivulus-only sites are characterized by high densities and slow rates of growth (17)(18), intraspecific competition is likely intense. Consequently, the fitness advantages of a larger egg size and/or offspring size may be favored at the expense of plasticity in these sites (see also 41).…”

“…Theory predicts that high-predation Rivulus should evolve earlier maturation and increased reproductive investment in response to higher rates of predation (5,15,16). However, population densities dramatically decrease (seven times), and rates of growth increase (double) in sites with predators (17,18). These large differences in density and growth are apparently caused by a much lower abundance of Rivulus when they co-occur with predators.…”

Composition and structure of fish assemblage from Passa Cinco stream, Corumbataí river sub-basin, SP, Brazil

“…Among the main patterns of longitudinal variation in stream fishes, there are the species additions and replacements (Gilliam et al, 1993;Petry and Schultz, 2006). Species additions are generally correlated with less severe environmental gradients, leading to smooth changes in abiotic and/or structural factors, while the species replacements occur as a result of abrupt changes in stream geomorphology or are related to abiotic conditions (Balon and Stewart, 1983;Winemiller and Leslie, 1992;Edds, 1993;Jackson et al, 2001;Wilkison and Edds, 2001;Ferreira and Petrere, 2009).…”