Douglas F. Fraser

Explaining Leptokurtic Movement Distributions: Intrapopulation Variation in Boldness and Exploration

Leptokurtic distributions of movement distances observed in field‐release studies, in which some individuals move long distances while most remain at or near their release point, are a common feature of mobile animals. However, because leptokurtosis is predicted to be transient in homogeneous populations, persistent leptokurtosis suggests a population heterogeneity. We found evidence for a heterogeneity that may generate persistent leptokurtosis. We tested individuals of the Trinidad killifish Rivulus hartii for boldness in a tank test and released them back into their native stream. Boldness in the tank test predicted distance moved in the field releases, even after effects of size and sex were removed. Further, data from a 19‐mo mark‐recapture study showed that individual growth correlated positively with movement in a predator‐threatened river zone where the Rivulus population is spatially fragmented and dispersal is likely to be a hazardous activity. In contrast, no such correlation existed in a predator‐absent zone where the population is unfragmented. These results show that a behavioral trait, not discernible from body size or sex, contributes to dispersal and that a component of fitness of surviving “dispersers” is elevated above that of “stayers,” a fundamental assumption or prediction of many models of the evolution of dispersal through hazardous habitat.

NONLETHAL IMPACTS OF PREDATOR INVASION: FACULTATIVE SUPPRESSION OF GROWTH AND REPRODUCTION'

We asked whether invasions by a predator in a patchy environment altered only the death rate of the prey, or whether there were also nonlethal effects, i.e., alterations in three other vital rates: net emigration, reproduction, and individual growth rates. Field studies documented the patch use of the guppy Poecilia reticulate and the killifish Rivulus hartii in pools of a second-order forest stream in Trinidad, before and after invasion by the piscivorous fish Hoplias malabaricus. Experiments revealed that the predator altered the within-pool use of space by Poecilia and Rivulus, and caused significant emigration of the prey from pools in which it was present. Further, intimidation by the predator in an experimental stream suppressed total egg production in Rivulus by 50%, and created spatial patchiness (more eggs laid in safer pools) and temporal patchiness (pulses of eggs) in egg production. The presence of the predator also induced shifts to shallow riffle areas and significantly reduced the growth rate of adult but not juvenile Rivulus. In contrast to the familiar paradigm that increased predation rates result in compen- satory increases in per capita reproductive rates and/or growth rates as the population is thinned, we found that the threat of predation suppressed rates of reproduction and growth in predator-occupied patches.

Local adaptation in Trinidadian guppies alters ecosystem processes

Theory suggests evolutionary change can significantly influence and act in tandem with ecological forces via ecological-evolutionary feedbacks. This theory assumes that significant evolutionary change occurs over ecologically relevant timescales and that phenotypes have differential effects on the environment. Here we test the hypothesis that local adaptation causes ecosystem structure and function to diverge. We demonstrate that populations of Trinidadian guppies (Poecilia reticulata), characterized by differences in phenotypic and population-level traits, differ in their impact on ecosystem properties. We report results from a replicated, common garden mesocosm experiment and show that differences between guppy phenotypes result in the divergence of ecosystem structure (algal, invertebrate, and detrital standing stocks) and function (gross primary productivity, leaf decomposition rates, and nutrient flux). These phenotypic effects are further modified by effects of guppy density. We evaluated the generality of these effects by replicating the experiment using guppies derived from two independent origins of the phenotype. Finally, we tested the ability of multiple guppy traits to explain observed differences in the mesocosms. Our findings demonstrate that evolution can significantly affect both ecosystem structure and function. The ecosystem differences reported here are consistent with patterns observed across natural streams and argue that guppies play a significant role in shaping these ecosystems.

PREDATION AND RISK IN FORAGING MINNOWS: BALANCING CONFLICTING DEMANDS

When foraging in habitat patches that simultaneously vary in food abundance and predation risk, foragers confront the conflicting demands of efficient foraging and predator avoidance. We hypothesized that foragers will balance these conflicting demands, taking proportionately greater risks when benefits are high. To test the balancing hypothesis we predicted that (1) prey would choose patches of high food abundances when all other variables are constant; (2) prey would avoid predator locations when all other variables are constant; and (3) when food and predators vary in combination, a significant statistical interaction would exist between the two effects. We used adult Semotilus atromaculatus as the predators and juvenile Rhinichthys atratulus as the prey, to test these predictions in a seminatural, artificial stream. When specific locations in the stream were varied in all possible combinations of food level (high, low) and predators (present, absent) we found that the prey responded positively to high food locations (prediction 1) and negatively to those containing predators (prediction 2). However, prey did not take proportionately greater risks when the benefits were high (prediction 3). As an alternative to balancing we propose the patch choice model as a working hypothesis that relates net energy intake to time spent in patches (patches vary in food level and predator risk as above) where predators, if present, reduce time spent foraging by their prey. In accord with our data, this model predicts that the relative benefit of high food patches is independent of the presence of predators.

MOVEMENT IN CORRIDORS: ENHANCEMENT BY PREDATION THREAT, DISTURBANCE, AND HABITAT STRUCTURE

Movement by stream fish is known to be strongly influenced by abiotic factors such as floods and temperature, but roles of biotic factors, such as predation threat, and interactions of abiotic and biotic factors are less clear. Predation threat is known to fragment populations of killifish, Rivulus hartii, in Trinidad rivers by rendering habitat inhospitable. We asked whether such spatial fragmentation was accompanied by reduced movement by fish in the predator-occupied zone of a river, relative to a zone free of the strong piscivore, Hoplias malabaricus, that causes the fragmentation. We used a 19-mo marking study in a river with a predator barrier, field experiments in the river, and mesocosms to evaluate four hypotheses: (1) the predator reduces prey movement in the river; (2) for the special case of prey leaving refugia, the predator increases movement; (3) movement positively cor- relates with water level in the predators presence; and (4) complex physical structure in hazardous habitat promotes prey movement. We marked 1467 Rivulus in the natural study areas and had 1015 recaptures. Contrary to Hypothesis 1 but in support of Hypothesis 2, prey showed greater movement along the river in the presence of the predator, regardless of whether the fish resided in a refuge at its previous capture. An experiment with introduced fish confirmed the findings that move- ment was elevated in the predators presence. Effects of an abiotic factor (water level, Hypothesis 3) and a phenotypic trait (body size) depended upon whether the predator was present: movement was independent of water level and body size in the absence of the predator, but positively related to both variables in the predators presence. Emigration from the river to tributaries was also independent of body size in the predators absence, but positively size-dependent in the predators presence. Complex physical structure (Hy- pothesis 4), in the form of cobble added to experimental pools, enhanced the transit of fish through hazardous pools. This study shows that spatial fragmentation does not necessarily imply that movement between fragments will be impeded (dynamical fragmentation). Rather, it is possible that movement among spatial fragments may be enhanced by the same factor, predation threat, that produced the spatial fragmentation in the first place. Because of the context-dependent effects of an abiotic factor (water level) and a phenotypic variable (body size) on movement, the study also emphasizes the need to clarify the exact role of predation as an agent promoting or retarding movement, and it suggests a need for incorporating such parameters into models of movement and metapopulation dynamics.

Experimental Evaluation of Predator-Prey Relationships in a Patchy Environment: Consequences For Habitat Use Patterns in Minnows

An experimental study, using the minnows Semotilus atromaculatus and Rhinichthys atratulus, was conducted in a seminatural artificial stream to evaluate the interaction of predator presence, habitat structural complexity, and time of day in determining habitat use patterns of prey. Predators were adult S. atromaculatus, while juveniles of both species were used as prey. Prey of both species actively avoid locations in the artificial stream that contain predators. However, structural complexity and time of day each have a mediating effect on the response of the prey fish to the presence of predators. While others have shown differential survivorship of prey, dependent upon structural complexity and time of day, this study shows how prey respond behaviorally to the presence of predators in habitat patches. They are most likely to enter those patches during daylight or when structure is present. In natural headwater streams prey fish are often found in patches of high predator density where prey density is often correlated with the degree of structural complexity. Our findings offer an explanation for this variability in relative abundances of predators and prey, by relating patch use by prey habitat structural complexity and diel activity. Resulting implications for optimal foraging and guild organization are suggested.

STRUCTURE OF A TROPICAL STREAM FISH COMMUNITY: A ROLE FOR BIOTIC INTERACTIONS'

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 hartil, was widely distributed in the drainage, occurring alone in each headwaters but also en- countering other fish species below barrier waterfalls. This pattern allowed us to use Rivulus to test four predictions derived from the species interaction hypothesis: (1) 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 pisciv- orous fish Hoplias inalabaricus; (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 reticulate, 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 up- stream 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 pop- ulations 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 abundance of Rivulus throughout this tropical watershed, and suggest that interspecific interactions can fragment Rivulus into local populations.

Feeding under predation hazard: response of the guppy and Hart's rivulus from sites with contrasting predation hazard

SummaryPopulations of guppies, Poecilia reticulata and Harts rivulus, Rivulus harti, in Trinidad experience different levels of predation hazard from piscivorous fish. Those from the larger rivers (“downstream sites’) experience chronically high predation hazard, while those from headwater streams (“upstream sites”) have few predators. Guppies and Harts rivulus, collected from downstream and upstream sites, were assayed for their feeding rate in the presence and absence of predators. We defined tenacity as the ratio of the feeding rates in the presence and absence of a predator stimulus. Thus, tenacity expresses the degree to which the forager maintains its feeding rate when a predator stimulus is present. Previous work by Seghers (1973) showed that non-feeding guppies from downstream sites responded more strongly to predators than did guppies from upstream sites. Based on this, we initially hypothesized that fish from downstream sites would show lower tenacities than fish from upstream sites. However, we found the opposite in every case. When confronted with a predator stimulus, guppies and Harts rivulus from downstream sites fed at consistently greater rates and displayed greater tenacities than did those from upstream sites. These differences were found in experiments using both live and model predators. The results suggest that upstream fish readily trade off feeding for hiding and avoiding predation hazard, a likely response when predators appear infrequently, while downstream fish appear to be selected for boldness and tenacity while foraging under chronically high hazard.

Experimental evaluation of evolution and coevolution as agents of ecosystem change in Trinidadian streams.

Evolution has been shown to be a critical determinant of ecological processes in some systems, but its importance relative to traditional ecological effects is not well known. In addition, almost nothing is known about the role of coevolution in shaping ecosystem function. Here, we experimentally evaluated the relative effects of species invasion (a traditional ecological effect), evolution and coevolution on ecosystem processes in Trinidadian streams. We manipulated the presence and population-of-origin of two common fish species, the guppy (Poecilia reticulata) and the killifish (Rivulus hartii). We measured epilithic algal biomass and accrual, aquatic invertebrate biomass, and detrital decomposition. Our results show that, for some ecosystem responses, the effects of evolution and coevolution were larger than the effects of species invasion. Guppy evolution in response to alternative predation regimes significantly influenced algal biomass and accrual rates. Guppies from a high-predation site caused an increase in algae relative to guppies from a low-predation site; algae effects were probably shaped by observed divergence in rates of nutrient excretion and algae consumption. Rivulus–guppy coevolution significantly influenced the biomass of aquatic invertebrates. Locally coevolved populations reduced invertebrate biomass relative to non-coevolved populations. These results challenge the general assumption that intraspecific diversity is a less critical determinant of ecosystem function than is interspecific diversity. Given existing evidence for contemporary evolution in these fish species, our findings suggest considerable potential for eco-evolutionary feedbacks to operate as populations adapt to natural or anthropogenic perturbations.

Empirical Evaluation of the Hypothesis of Food Competition in Salamanders of the Genus Plethodon

Two species of salamander, Plethodon cinereus, and Plethodon hoffmani were used in experiments designed to test the hypothesis that food is the object of competition. Plethodon hoffmani occurs in the Appalachian Ridge and Valley Province where it appears to exclude the closely related and contiguously allopatric P. cinereus. Detailed field observa- tions and a laboratory experiment confirm Jaegers (1972) hypothesis that the food of sala- manders could be limited in availability during dry weather and suggest that the cause of the low availability is the reduced mobility of the salamanders rather than a reduced food supply. However, because stomach content data and behavioral observations indicate that salamanders do not forage when the surface is dry, it was reasoned that food competition could occur under only two restricted conditions: (1) when more than one animal is isolated under damp refugia, such as rocks and logs, during dry weather, and (2) when many animals emerge onto the surface to forage when wet weather follows a prolonged dry period. Empirical evidence suggests that salamanders do not clump in refugia as the habitat dries out and that they do not surface simultaneously to forage after a dry period. Thus, even though food is periodically unavailable, the staggered feeding schedule prevents the density of salamanders from ever reaching competitive levels. Interference mechanisms are not ruled out as mecha- nisms of the observed spacing patterns. Competition for space is suggested as an alternative to the food competition hypothesis.