Donald J. Hall

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.

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.

Foraging Efficiency and Habitat Switching in Competiting Sunfishes

A series of experiments are presented which elucidate competitive interactions among 3 congeneric sunfishes (Centrarchidae). In the absence of competitors all species rank 2 habitats (vegetation, sediments) in the same order according to profitability but differ in their relative effi- ciencies at utilizing these habitats. The ordinal ranking of foraging efficiencies of these species in the 2 habitats permits qualitative predictions of the occurrence and order of habitat shifts as resources decline (competitive pressure increases). We present an experiment demonstrating such habitat shifts which corroborate these predictions. The patterns of habitat switching indicate that the addition of a competitor which is more effective in a given habitat can change the profitability ranking for a species to the extent that the preferred habitats are abandoned. Thus these results provide experi- mental evidence for the compression hypothesis as a mechanism for the development and maintenance of resource partitioning by habitat. Further, the ecological flexibility exhibited by the sunfish indicates that overlap or co-utilization of habitats can be a very dynamic process, determined by the relation between resource levels and differences in foraging efficiencies among species. The implications of these results to competition, optimal use of a patchy environment, and species abundance relations in natural lakes are discussed.

Competition and Habitat Shift in Two Sunfishes (Centrarchidae)

The bluegill sunfish (Lepomis macrochirus) in small ponds feeds on relatively large prey associated with the vegetation. However, in the presence of the green sunfish (L. cyanellus) it shifts to feeding on smaller, less preferred prey in the open water column. The mechanisms responsible for this habitat shift were examined by experimentally confining both species, alone and together, in homogeneous patches of the preferred habitat (vegetation). When confined together in the vegetation the green sunfish exhibited higher survivorship, growth rates, and amount of food in the stomachs than the bluegill. The bluegill fed on smaller items and consumed more benthic prey than did the green sunfish. The presence of the congener did not alter the food habits of either species or the growth rates of the green sunfish in relation to species stocked alone. Presence of the congener did affect the growth rates of the bluegill. Overlap in the diet was 70o when these species were confined to the vegetation as compared to 44% in an earlier study where habitat separation was permitted. The green sunfish is more of a sit-and-wait predator and is able to utilize a wider food size spectrum than the bluegill. This results in a strong asymmetry in the competition function favoring the green sunfish in the vegetation. However, in the open water column the distribution of food sizes is truncated and this provides a competitive refuge for the bluegill which handles small foods more efficiently. The bluegill appears to be more flexible in its habitat use while the green sunfish is more aggressive and limited in the habitats utilized. Comparisons with studies of habitat shifts in salmonids suggest that the competitive mechanisms outlined are of general relevance in fish communities. Consideration of the relation between habitat structure, the correlated distribution of food sizes, and species morphology provides a framework for specifying the occurrence of habitat shifts and which species of the interactive set will shift.

Perturbation and resilience : a long-term, whole-lake study of predator extinction and reintroduction

This paper presents the results of a long-term study of changing predator densities and cascading effects in a Michigan lake in which the top carnivore, the largemouth bass (Micropterus salmoides), was eliminated in 1978 and then reintroduced in 1986. The elimination of the bass was followed by a dramatic increase in the density of planktivorous fish, the disappearance of large zooplankton (e.g., two species of Daphnia that had his- torically dominated the zooplankton community), and the appearance of a suite of small- bodied cladoceran (zooplankton) species. The system remained in this state until bass were reintroduced. As the bass population increased, the system showed a steady and predictable return to its previous state; planktivore numbers declined by two orders of magnitude, large- bodied Daphnia reappeared and again dominated the zooplankton, and the suite of small- bodied cladocerans disappeared. Within each cladoceran species there was a steady increase in mean adult body size as planktivore numbers declined. Total zooplankton biomass in- creased - 10-fold following the return of large-bodied Daphnia, and water clarity increased significantly with increases in Daphnia biomass and total cladoceran biomass. These changes in community structure and trophic-level biomasses demonstrate the strong impact of removing a single, keystone species, and the capacity of the community to return to its previous state after the species is reintroduced.

Seasonal Distribution and Abundance of Fishes in the Littoral Zone of a Michigan Lake

Abstract The seasonal distributions and abundances of 15 littoral zone fish species were determined from May to October 1976 in a small Michigan lake. A team of divers performed seven censuses along a set of transects which encompassed nearly the entire littoral region of the lake; additional dives enabled us to determine vertical distributions and migration patterns to and from overwintering areas. All species appeared to show less habitat segregation in spring when temperatures rose and food was abundant. Species distributions and abundances were quite constant during the summer and considerable habitat segregation was apparent. The bluegill (Lepomis macrochirus) was the dominant species and was studied in greater detail. Small bluegills (<80 mm total length) were confined throughout the year to the vegetation apparently to avoid predation; larger fish moved up in the water column. This species overwintered in two localized regions of tall perennial vegetation in the deep littoral apparently in response...

Scale in the Design and Interpretation of Aquatic Community Research

The scales employed in investigations of aquatic ecosystems can strongly influence interpretations of community patterns and processes. Some examples are obvious; in contrasting cladocerans and rotifers, assessments of biomass (an instantaneous time scale) provide strikingly different impressions than assessments of production (a broader time scale) (Makarewicz and Likens 1975). Other examples are more subtle but equally important; seasonal distributions of phytoplankton species over several years may indicate patterns with little predictability but distributions of functional groups of phytoplankton can indicate a periodic behavior (Bartell et al. 1978; Reynolds 1984). Explicit considerations of the scales that an observer uses in an investigation are fundamental to an understanding of aquatic systems.

Littoral zone fish communities of two Florida lakes and a comparison with Michigan lakes

SynopsisThe abundance and habitat distribution of littoral zone fishes in two small southern Florida lakes were quantified by underwater censuses. The bluegill (Lepomis macrochirus) and large-mouth bass (Micropterus salmoides) accounted for 75–80% of community biomass in both lakes; important coexisting species were predominantly benthic feeders in Lake Sirena and planktivores in Lake Annie. All species were largely confined to areas of macrovegetation which extended to a depth of 4 m in Lake Annie but only to 1.2 m in Lake Sirena. The differences in community structure were related to differences in habitat and also variation in water levels and benthic production.A comparison of community composition with that of small Michigan lakes indicated that similar numbers of species coexist in the littoral zones, despite a three-fold difference in the size of regional species pools. The majority of families and about 50% of the species were common to lakes in both regions; in addition a number of ecological analogues were noted. A major difference was that the small inshore species were members of the Cyprinodontiformes in Florida and Cyprinidae in Michigan. Together the largemouth bass and bluegill comprise similar community proportions in the two regions as do other major feeding groups. Lakes in the two regions that are similar in amount and distribution of vegetation exhibit greater similarity in fish communities than those within regions that differ in littoral vegetation.

Epistemology, Experiments, and Pragmatism

In retrospect, what we now call complex interactions encompass several of the major advances in aquatic ecology. Some examples include the trophic-dynamic concept (Lindeman 1942), the multidimensional niche (Hutchinson 1957), size selective predation and the size efficiency hypothesis (Hrbacek et al. 1961; Hrbacek 1962; Brooks and Dodson 1965), the keystone predator concept (Paine 1966), and optimal foraging theory (Werner 1977). Recent advances offering similar benefit include the microbial loop (Riemann and Sondergaard 1986; Scavia and Fahnenstiel this volume; Porter et al. this volume), the ontogenic niche (Werner and Gilliam 1984; Crowder et al. chapter 10; Stein et al. chapter 11), chemical induction of antipredator morphological, behavioral and life history traits (Havel 1987), behavioral responses to predation (Kerfoot and Sih 1987) and the trophic cascade argument (Carpenter et al. 1985). The preceding chapters offer many specific examples of complex interactions in aquatic communities. Others will certainly appear in the near future. Chapters 10–14 develop both the general state of current understanding and the specific priorities for future work in this area of ecological research.

Foraging and Habitat Use by the Bluegill Sunfish

Patterns of resource utilization by different species have provided fundamental inferences for the theory of the ecological niche and community organization (e.g., Schoener 1974). Consequently, how an animal chooses to use different resources, and how this use is related to the profitability or net energy return in various habitats are critical questions bearing on the mechanisms underlying the theory-e.g., the specific mechanism of competition between species. Our study interprets habitat utilization and switching behavior of the bluegill sunfish (Lepomis macrochirus) from laboratory measures of foraging efficiency or energetic return in different habitats. Learning or training bias appear to be an important determinant of individual specialization and habitat switching. We suggest that theoretical and empirical studies of foraging behavior that concern the utilization of different habitat and prey types must incorporate the action of learning or experience. We measured foraging rates of the bluegill in two common habitats: the open water column and sediments. Aquaria were constructed to mimic these habitats, and several combinations of prey size and density were presented to the fish. We measured search and handling times for the bluegill in each treatment combination and related search time to prey size and density using stepwise multiple linear regression. Foraging rates increased in both habitats with experience, in some cases as much as fourfold over the course of eight trials. Both prey size and density also strongly influenced foraging rates. These relationships enable us to estimate return rates for a bluegill foraging in a natural habitat whose resource abundance and size-distribution are known. Subsequently, we introduced a population of 225 bluegills into a pond (29 m in diameter, 1.8 m deep) consisting primarily of open water and exposed sediments. Both invertebrate resources and the fish were sampled at oneto two-week intervals. Habitat use of the fish was readily determined from stomach contents because three prey species obligate to specific habitats accounted for >80% of the diet. Following their introduction, the fish exhibited a rapid switch from feeding in the small amount of vegetation habitat in the pond to feeding in either the open water or sediment habitats. The majority of fish specialized on a single habitat (Figure 1). Generalists were less successful foragers than specialists; those individuals with a mixed diet (i.e., 80% of diet) on one habitat or the other.