James F. Gilliam

An Experimental Test of the Effects of Predation Risk on Habitat Use in Fish

We present an experiment designed to test the hypothesis that fish respond to both relative predation risk and habitat profitability in choosing habitats in which to feed. Identical populations of three size—classes of bluegill sunfish (Lepomis macrochirus) were stocked on both sides of a divided pond (29 m in diameter), and eight piscivorous largemouth bass (Micropterus salmoides) were introduced to one side. Sizes of both species were chosen such that the small class of bluegills was very vulnerable to the bass, whereas the largest class was invulnerable to bass predation. We then compared mortality, habitat use, and growth of each size—class in the presence and absence of the bass. Only the small size—class suffered significant mortality from the bass (each bass consumed on average about one small bluegill every 3.8 d); the two larger size—classes exhibited similar mortality rates on both sides of the pond. In the absence of the bass, we found that habitat use of all size—classes was similar and that the pattern of habitat use maximized foraging return rates (Werner et al. 1983). In the presence of the bass the two larger size—classes chose habitats to maximize return rates, but the small size—class obtained a greater fraction of its diet from the vegetation habitat, where foraging return rates were only one—third of those in the more open habitats. The small size—class further exhibited a significant depression in individual growth in the presence of the bass; the growth increment during the experiment was 27% less than that for small bluegills in the absence of the bass. Because of the reduced utilization of more open habitats by the small fish in the presence of bass, resources in these habitats were released to the larger size—classes, which showed greater growth in the presence of the bass than in its absence. We develop methods to predict the additional mortality expected on a cohort due to a reduction in growth rate (because individuals are spending a longer time in vunerable sizes), and discuss and potential for predation risk to enforce size—class segregation, which leads de facto to resource partitioning.

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.

FUNCTIONAL RESPONSES WITH PREDATOR INTERFERENCE: VIABLE ALTERNATIVES TO THE HOLLING TYPE II MODEL

A predators per capita feeding rate on prey, or its functional response, provides a foundation for predator-prey theory. Since 1959, Hollings prey-dependent Type II functional response, a model that is a function of prey abundance only, has served as the basis for a large literature on predator-prey theory. We present statistical evidence from 19 predator-prey systems that three predator-dependent functional responses (Beddington- DeAngelis, Crowley-Martin, and Hassell-Varley), i.e., models that are functions of both prey and predator abundance because of predator interference, can provide better descrip- tions of predator feeding over a range of predator-prey abundances. No single functional response best describes all of the data sets. Given these functional forms, we suggest use of the Beddington-DeAngelis or Hassell-Varley model when predator feeding rate becomes independent of predator density at high prey density and use of the Crowley-Martin model when predator feeding rate is decreased by higher predator density even when prey density is high.

Experimental tests of optimal habitat use in fish: the role of relative habitat profitability.

Utilizing optimal foraging theory and laboratory estimates of foraging costs, we predict the choice of foods and use of habitats by fish in the field. These predictions are tested with the bluegill sunfish (Lepomis macrochirus) foraging in three habitats (open water, sediments, and vege- tation) in a pond. Relations describing prey encounter rates in each habitat as a function of prey size, prey density, and fish size were derived from laboratory experiments. These relations permitted us to estimate prey encounter rates based on weekly prey samples in each habitat of the pond. We then determined the optimal diet and profitability (net energy return) for each habitat through time. Predictions of optimal diet exhibited good qualitative correspondence to the actual diet of the fish in the open water and vegetation, although we consistently predicted a slightly narrower diet than the fish were choosing. The model correctly predicted the magnitude of the change in size selection on Daphnia pulex with fish size and with decline in prey density. Predictions of optimal diet in the sediments were considerably in error apparently due to a tendency for late-instar midges to burrow deep in the sediments, thereby becoming unavailable to the fish. In this case habitat profitabilities were computed simply on the basis of the actual observed diet. Predictions of optimal habitat use, i.e., when the fish should switch habitats to maximize feeding rates, showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of our predictions. The actual habitat use pattern differs dramatically from a null model of random habitat use. We indicate how this approach may be useful in studying intra- and interspecific exploitative interactions.

MODELING DIFFUSIVE SPREAD IN A HETEROGENEOUS POPULATION: A MOVEMENT STUDY WITH STREAM FISH

Using a mark–recapture technique in a small temperate stream, we described the movement of four fish species over a five-month period and developed a mathematical model that described the observed movement patterns. The movement distributions were generally leptokurtic, and two of the four species demonstrated some degree of upstream bias. There was little difference in movement among species or through time. There were no temporal correlations in probability of movement, movement direction, or distance moved. The spatial spread of the most abundant species, bluehead chubs, over a four-month period was characterized by upstream bias, diffusion-like spread, and persistent leptokurtosis. Bluehead chubs demonstrated complex relationships between probability of movement and size and growth, while creek chubs showed only an effect of size on probability of movement. Further, growth of individual bluehead chubs was correlated through time. These empirical results suggest the hypothesis that heterogeneity in phenotypic attributes, such as size and growth, is related to heterogeneity in movement behavior. A diffusion–advection model of bluehead chub movement, structured with two subgroups that dispersed at different rates (“fast fish” and “slow fish”), was parameterized and validated with the field data. This model with heterogeneity in movement rates generated the leptokurtic pattern observed in the field data, in contrast to the classic diffusion model without population heterogeneity, which produces a normal distribution. The results from this work suggest that heterogeneity in fitness-influencing attributes such as size and growth could explain heterogeneity in individual-level movement behavior and might underlie the leptokurtic patterns that have been observed at the population level in numerous field studies.

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.

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.

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.

Strong Effects of Foraging Minnows on a Stream Benthic Invertebrate Community

Behavioral responses to piscivorous fish are known to alter the spatial dis- tribution of small invertebrate-feeding fish in streams, producing concentrations of small fish in safer areas. We asked whether such variation in the local density of small juvenile creek chubs (Semotilus atromaculatus) would, in turn, produce local alterations in the stream benthic invertebrate community. Replicated treatments of 0, 2, and 6 juvenile chubs per 0.5-iM2 enclosure were established in a warm-water, soft-sediment stream. Over a 3-mo period, the presence of fish resulted in reductions of 79-90% in total invertebrate volume relative to the zero-fish treatments. Numbers were reduced 55-6 1%. The two major taxa, Oligochaeta and Isopoda, showed strong fish effects, and size distributions of these two taxa showed shifts to smaller sizes. Densities of less abundant taxa (e.g., Chironomidae, Sphaeriidae) showed no statistically significant fish effects. Evenness of the invertebrate community increased in the presence of fish. No differences in invertebrate densities were detected between the two- and six-fish treatments; however, the above-sediment activity of Oligochaeta was reduced more in the six-fish treatment than in the two-fish treatment, and growth of the fish was slower in the six-fish treatment. Unlike previous experimental studies in stony streams, this experiment shows that foraging fish can significantly alter the behavior, population structure, and community structure of stream benthic invertebrates.