Predator Exclusion Cages Research Articles

NONLETHAL EFFECTS OF PREDATORS ON PREY POPULATIONS: PREDATOR-MEDIATED CHANGE IN BIVALVE GROWTH

Predators affect dynamics of prey populations not only by decreasing their survivorship, but also by changing other vital rates such as growth and reproduction. However, such nonlethal effects have rarely been examined experimentally in the field. In North Carolina estuaries, the hard clam Mercenaria mercenaria generally shows lower growth rates in habitats where predation rates are higher. Predator avoidance behavior of the clams (feeding inhibition) might be one of the factors responsible for this correlation. To examine possible effects of predators on clam growth, I first compared variation in clam growth rate among habitats with different predation pressure, and between predator-exclusion cages and uncaged controls. I then carried out a predator-enclosure experiment using the clam and the whelk Busycon carica, a major predator on adult clams. Among-habitat variation in survivorship, predation rate, and growth rate was generally small or not detected in winter, while large variation was found for adult clams in spring, their major growing season. Growth rate of adults was highest in a mud bottom where survivorship was highest (>60%) due to low predation rates, and it was lowest in an oyster shell bottom where survivorship was lowest (<10%). For juveniles, on the other hand, growth rate did not differ significantly among habitats, nor did survivorship. Clams in the predator-exclusion cages showed nearly twice the growth rate of uncaged controls in spring but not in winter. Clam growth rates were thus lower in habitats with lower survivorship and higher predation rates, suggesting that the predators can affect growth rates. In the whelk-enclosure experiment, the clams were enclosed in cages with three different treatments: (1) with whelks capable of attacking clams, (2) with whelks isolated from the prey, and (3) without whelks (control). The shell and somatic growth rates of the clam were at least twice as high in the control than in the two experimental treatments. However, the growth rates did not differ significantly between the two experimental treatments. The clam growth rates were therefore reduced in the presence of whelks even with no direct contact between whelks and clams, suggesting that the decrease in growth rates is due to a behavioral response of the prey to indirect cues, such as water-borne chemical signals produced by whelks. These findings show that predatory whelks can affect clam population dynamics not only by decreasing survivorship directly, but also by decreasing future reproduction and survivorship indirectly by suppressing growth rates, and that the predator-avoidance behavior observed in previous studies has significant demographic consequences. The nonlethal effects of predation on clam growth rate may also be responsible for observed variation in growth rates among habitats.

DIRECT AND INDIRECT EFFECTS OF PREDATION BY COMMON EIDERS AND ABIOTIC DISTURBANCE IN AN INTERTIDAL COMMUNITY

Predation and disturbance have been well studied in intertidal communities. However, the impact of vertebrate predators, particularly waterfowl, has been only infre- quently determined in long-term intertidal studies. Using predator exclusion cages and simulated abiotic disturbance, I studied the direct and indirect effects of predation by Common Eiders (Somateria mollissima) on an intertidal mussel bed community in Passa- maquoddy Bay, New Brunswick, Canada. Eiders fed heavily on blue mussels (Mytilus edulis), the dominant invertebrate present, reducing their abundance in ?+duck (exposed to predation) relative to -duck (exclosure) plots by nearly 50% within eight months of initiation of the experiment. Based on counts and estimated consumption rates, eiders appeared to be responsible for most predation observed in this system. Although ducks fed heavily on the dominant intertidal invertebrate, they had little effect on species diversity or richness in the community. However, relative abundance of the most common invertebrates did vary. In particular, exclusion of eiders led to an increase in abundance of dogwhelks (Nucella lapillus) one year into the experiment. Whelks, in turn, fed heavily on mussels under cages and obscured the longer-term effect of ducks in the system. This indirect effect appears to be an example of asymmetric exploitation compe- tition, with ducks influencing the food supply of whelks, but whelks having little effect on ducks. When ducks were excluded, whelks were released from this competition and acted as compensating predators. No other indirect effects developed following duck exclusion, presumably due to the increased whelk effect. Disturbance, in contrast, did lead to an increase in species diversity, which later returned to predisturbance levels as the community recovered. Predation delayed the recovery of disturbed sites, because ducks began feeding in these plots before mussel abundance had completely rebounded. Disturbance, while initially deterring predation somewhat, ulti- mately allowed the effects of predators to persist longer. This interaction of predation and disturbance resulted from compensatory growth of mussels under exclosures in disturbed sites. Mussels protected from eider predation grew quickly after disturbance and rapidly became larger than the preferred prey for whelks. As a consequence, whelks did not feed as heavily on mussels in disturbed sites as in undisturbed sites, where more mussels were of the preferred size, and the observed effect of ducks on mussel biomass persisted. This experiment demonstrates that eiders were significant, and, probably, keystone pred- ators in this system. Eiders directly reduced abundance of mussels, thereby indirectly increasing whelk density in plots where ducks were excluded. Compensatory predation by whelks following duck exclusion blocked other potential indirect effects, thus preventing eiders from having more widespread effects in the system. Such compensatory predation may act to stabilize communities and evidently can occur in relatively simple systems as well as the more species-rich communities with which it is usually associated.

Positive and negative effects of intertidal algal canopies on recruitment and survival of barnacles

Understanding the diverse ways that species interact in communities continues to pose significant challenges to experimental ecologists. By simultaneously (1) abrading the substrate (i.e. 'whiplash'), (2) altering local hydrodynamics and abiotic conditions and (3) influencing predator densities, marine algae can alter the recruitment and survival of the organisms with which they co-occur. I used predator exclusion cages and canopy manipulations in the barnacle-algae (Semibalanus balanoides-Ascophyllum nodosum) zone in Rhode Island (USA) intertidal habitats to demonstrate how to experimentally tease apart these types of multiple effects. In the absence of algal abrasion, barnacle recruitment was greater under the algal canopy than in cleared areas. This suggests that the canopy did not reduce the delivery of larvae but, rather, created a depositional environment where larvae accumulated. The overall effect of the canopy, however, was a reduction in recruitment compared to cleared areas because algal whiplash overwhelmed these positive, hydrodynamic effects. Predators had very little effect on the recruitment of barnacles, In contrast to recruitment, found that the canopy positively influenced the early post-recruitment survival of S. balanoides, The reduction in survival caused by predators was offset by an increase in survival caused by both the physical presence of the canopy (probably by reducing thermal stress) and its subsequent mechanical abrasion of the substrate (probably by reducing the accumulation of sediments). For both recruitment and survival, the effects of algal whiplash were twice as large as those of modifying the habitat or influencing predation intensity. In general, my findings illustrate how a series of a priori, statistical comparisons within a multi-factorial experiment can be used to separate and quantify these types of multiple effects in plant-animal interactions.

Predation of Zebra Mussels by Diving Ducks: An Exclosure Study

Zebra mussels (Dreissena polymorpha) are a novel and abundant potential food source for several species of diving ducks in the Great Lakes region. Using predator exclusion cages, behavioral observations, and analyses of duck gizzard contents, we examined the predator—prey relationship between zebra mussels and their duck predators during the fall migratory period in Lake Erie at Point Pelee, Ontario. Diving ducks feeding on zebra mussels in the area reduced mussel biomass by 57% during the period of heaviest feeding, but had no measurable impact on mussel numbers. Birds were size—selective predators, preferentially taking medium and large mussels over the more common small ones, thereby altering the size structure of the mussel population. Ducks were abundant at Point Pelee only during late fall, and differences between cages and control areas had disappeared by the following spring. Overall, ducks had little lasting impact on mussel populations, but mussel abundance may have determined duck concentration in the area. Duck staging populations were higher in 1990, when mussels were more abundant, than in 1991, when mussels were much scarcer. Effective predator control of zebra mussels in the portion of the Great Lakes region that freezes over winter is unlikely because diving ducks are unable to overwinter in the area. However, mussels have become an important food source for diving ducks, and temporary reductions of mussel density through predation are likely in areas where ducks feed during migratory periods. Similarly, in regions where ducks are resident through winter, greater and longer lasting effects may be observed.

Nitrogen Content of Food Plants and Vulnerability of Pieris Rapae to Natural Enemies

We tested the commonly made assumption that reduction in leaf quality reduces the survivorship of herbivores indirectly by increasing their exposure to natural enemies. Caterpillars of the butterfly Pieris rapae were placed on collard plants that had been experimentally manipulated to have leaves either rich or poor in nitrogen. On low—nitrogen plants caterpillars both developed more slowly and devoted more of their time to feeding than on high—nitrogen plants. When exposed to large (&gt; 2 mm) predators and parasites in a natural setting, the survivorship of caterpillars on low—nitrogen collards was reduced greatly compared to caterpillars protected from predators and parasites. Survivorship of caterpillars on high—nitrogen plants was reduced significantly less dramatically when exposed to predators and parasites. Small (&lt; 1—2 mm) pupal parasites that were able to reach the caterpillars through the mesh of the predator exclusion cages emerged more frequently from individuals reared on high—nitrogen plants.

Caging experiments to examine the effects of fishes on understorey species in a sublittoral kelp community

The effects of predatory fish on the abundancesof understorey species in a sublittoral Ecklonia radiata kelp assemblage were examined using exclusion cages. Fish were excluded from replicate areas for comparison with uncaged controls and appropriate treatments to test for caging artifacts. Experiments were done under the kelp canopy and in areas cleared of kelp (simulating damage due to storms). The understorey assemblage was sampled using quadrats for macroscopic species, an underwater microscope for attached microscopic organisms, and an underwater suction sampler for mobile microscopic fauna. Significant caging artifacts were found to act on certain species early in the experiments and on most species after 8 wk. Caging led to increases in abundances of many microscopic invertibrates, microscopic silt and layers of sediment. Caging led to decreases in the abundances of encrusting algae as experimental plots became dominated by layers of sediment. For some encrusting algae, Zonaria-Lobophora, Myxilla sp. and juvenile Ecklonia radiata, caging negated natural increases in abundances, whilst for sessile animals, caging negated a natural decrease. The species richness of the assemblage was greater in areas with fish than in areas excluded from fish. More species appeared to be affected by predation by fish under the canopy than in clearings due to the greater number of species living under the canopy. Some species showed significant effects (an increase, decrease or the negation of a natural change in abundance) at all times of the year when fish were excluded, whilst others only showed effects at times when they were in large abundances. After removing certain cages, fishes quickly returned the assemblage to one dominated by encrusting species. The results are discussed in terms of alternative models and hypotheses to account for the observed patterns (particularly effects of fish predation and caging artifacts), problems with using predator-exclusion cages in such experiments, and the significance of these experiments for future studies of these interactions.

Mortality of Young Brown Shrimp Penaeus aztecus in Estuarine Nurseries

We estimated actual 2-week mortalities of postlarval and juvenile brown shrimp in a Galveston Bay salt marsh by comparing densities of cohorts throughout the spring. Mortalities ranged between 33% and 61% in 1982 and 23% and 39% in 1987. Brown shrimp mortality in predator-exclusion cages during 1987 was less than 3%. These data and published information on food requirements, diseases, and physical tolerances suggest that predation is usually the major direct cause of brown shrimp mortality in estuarine nurseries of the northern Gulf of Mexico. Southern flounder Paralichthvs lethostigina was the dominant fish predator on brown shrimp during the spring, and appeared to be responsible for a large portion of brown shrimp mortality. In laboratory experiments, the presence of smooth corcigrass Spartina alterniflora reduced predation rates of southern flounder and some of the other fish predators examined. Predation rates in general increased in proportion to increased prey densities. Thus, low water levels in the marsh, which reduce access by brown shrimp to intertidal vegetation and increase their densities on nonvegetated bottom, probably result in increased brown shrimp mortality. Mortality and growth may also interact, and predation pressure should be reduced as brown shrimp grow and exceed optimal prey size. Indeed, mortality in the marsh appeared to decline as brown shrimp size increased.

The growth and survival of Panolis flammea larvae in the absence of predators on Scots pine and lodgepole pine

Abstract. 1. The survival, growth and development of larvae of the pine beauty moth, Panolis flammea (D &amp; S) (Lepidoptera: Noctuidae) were examined as part of a study to determine (i) why damaging outbreaks of this insect in Scotland are frequent on lodgepole pine but do not occur on Scots pine, and (ii) why outbreaks are associated with areas of deep un‐flushed peat.2. Larvae were studied inside predator exclusion cages on mature pine trees in Elchies forest where a major infestation occurred in 1978–79.3. Surprisingly, larval survival was generally greatest on Scots pine and lowest on lodgepole growing in deep peat. Also, larval growth and development were greater on Scots than lodgepole pine and were unaffected by the type of soil in which the lodgepole pine was growing.4. Larval performance was generally better on lodgepole pine that had been thinned and received fertilizer.5. Larval survival was affected by pine shoot stage during egg hatch in one of the two years in which this was examined.6. Observations on P.flammea pupae were confused by disease, parasitism and probably by differences in weight loss caused by differences in larval development. Parasitism by ichneumonid parasitoids was recorded on Scots pine but not on lodgepole pine.7. It was concluded that (i) the absence of pine beauty moth outbreaks on Scots pine, and (ii) the occurrence of outbreaks on lodgepole pine growing in deep unflushed peat were not the result of lodgepole pine growing in deep peat being a more suitable host plant than lodgepole pine and Scots pine growing elsewhere. It seems more likely that the observed outbreak behaviour of P.flammea in Scotland is associated with differences in predation and parasitism in plantations of difference host plants and in plantations of lodgepole pine growing in different soils.

Population ecology of preimaginal Culex tarsalis (Diptera: Culicidae) in Kern County, California.

The effects of abiotic and biotic mortality factors on preimaginal survivorship and the production of adults were investigated for populations of Culex tarsalis Coquillett at a stable foothill breeding site during 1985 and at seven ephemeral breeding sites during 1986. Female abundance was correlated temporally with larval abundance and spatially with first instar to adult survivorship. Catastrophic events, such as fluctuations in water level and pollution, drastically reduced larval and pupal abundance. Under relatively stable conditions, the effect of natural mortality factors was estimated by comparing preimaginal survivorship rates, estimated horizontally for fed and unfed cohorts in predator exclusion cages, with survivorship, estimated vertically from the age structure of the natural population using a life table approach. Mortality for fed cohorts ranged from 8.5 to 90.0% and was attributed to abiotic factors, including temperature and water quality. Mortality caused by lack of food was comparatively low (less than 15%); however, the low nutrient level at three of six sites lengthened the duration of the fourth stadium, delayed pupation, and resulted in the emergence of small-sized adults with a reduced expression of autogeny. Predation mortality ranged from 3.7 to 84.5% and was the most important cause of larval death at five of six study sites. Based on relative abundance and correlation over time and space, zygopteran naiads were the most important predator at a stable foothill breeding site during 1985, and coleopteran larvae were the most important predator at ephemeral breeding sites sampled during 1986.

Coexistence of Sea Urchins in Seagreas Meadows: An Experimental Analysis of Competition and Predation

An experimental examination at two sites of the effects of competition and predation on Tripneustes ventricosus and Lytechinus variegatus, the common sea urchins in meadows of the Caribbean seagrass, Thalassia testudinum, suggests that intraspecific competition and predation on recruits influence their population dynamics. Grazing by Tripneustes strongly influenced the biomass of Thalassia. Density-dependent mortality caused the number of caged Tripneustes to converge on naturally occurring densities (about 1 urchin/M2), but Tripneustes mortality was not significantly affected by Lytechinus. Recruitment of Tripneustes was sparse at one site and did not increase when predators were excluded. However, recruitment at a more exposed site was an order of magnitude greater in predator exclusion cages than in an unmanipulated control plot. Tripneustes recruitment was inversely related to the density of adults at this site, but was not significantly affected by Lyte- chinus. L'techinus has a more generalized diet than Tripneustes, and had a moderate effect on the amount of Thalassia. Substantial mortality of Lytechinus occurred only at a density 16 times greater than its naturally occurring abundance; Lytechinus mortality was not significantly affected by Tripneustes. Recruitment of Lytechinus at a protected site decreased as the density of Tripneustes increased, but did not change significantly when predators were excluded. However, at a more exposed site predation reduced the density of recruits by an order of magnitude. Tripneustes had a higher growth rate than Lytechinus, and both urchins grew more rapidly at the exposed site. These differences were consistent with the sizes of the urchins; Tripneustes was larger than Lytechinus and both species were larger at the more exposed site. The growth and reproductive condition of Lytechinus were density dependent and differed between sexes; Lytechinus males grew to a larger size than females, but females had larger gonads. Naturally occurring densities of Tripneustes, which were close to carrying capacity, were some- what less variable than those of Lytechinus, although the differences were not significant. Coexistence of these urchins appears to stem from differences in their diets; there was no evidence of significant interspecific competitive effects on mortality, growth, and fecundity. Although predation substantially reduced urchin recruitment at one site, this effect appeared to be superimposed on, rather than necessary for, coexistence.

Emergence Patterns and Size Variation of Odonata in a Thermal Reservoir

Patterns of odonate emergence were examined seasonally in a reservoir receiving thermal effluents. Exuviae were routinely removed from the interior sides of vertebrate predator exclusion cages. Size, distribution, mean emergence date, and synchronization of emergence were determined for nine species at six stations along a thermal gradient (19.7 to 25.6°C). Mean emergence date varied significantly among species; the earlier emerging species were more synchronous. Temperature influenced the distribution of some of the species investigated. Most species had significant differences in emergence date among stations, and significant size differences among stations were found in three of the nine species. With the exception of the two earliest emerging species, the later in the year the emergence date, the smaller the individual emerging.

Seasonal fluctuations of benthic macrofauna in the apalachicola estuary, Florida, USA: The role of predation

Long-term monitoring studies revealed a negative correlation between the abundance of bottom-feeding fishes (Leiostomus xanthurus and Micropogonias undulatus) and benthic macrofaunal density, suggesting a causal relationship. To test this hypothesis, large (3x3 m) topless predator-exclusion cages (mesh opening=6 mm) of a design that produced minimal physical artifacts were constructed at two shallow mud-bottom oligohaline stations. Two-sided control cages and uncaged control areas were also sampled Experiments coincided with maximum abundances of bottom-feeding fishes in the spring and motile macroinvertebrates (penaeid shrimp) in the fall. During short-term (two-month) exclusion experiments in the spring and fall of 1980, large predators (those excluded by 6-mm mesh) were not a major factor in regulating the densities of most macrofaunal species. Predation may be important during other seasons, although this possibility was not tested. Densities of Leiostomus xanthurus and Penaeus setiferus during 1980 were lower than in previous years. Despite the reduced abundance of bottom-feeding fishes, macrofaunal densities still declined during late winter/early spring. It is postulated that increased river flow and lowered salinities during the late winter/early spring may contribute to the decline of macrofaunal densities by reducing larval recruitment.

Field experiments on interactions between vertebrate predators and larval midges (Diptera: Chironomidae) in the littoral zone of a reservoir.

A field experiment was designed to test a frequent assumption in the literature that vertebrate predators (in this case, fish and turtles) are capable of regulating the seasonal abundance and diversity of benthic communities in the littoral zone of lentic environments. Effects of thermal effluents from a nuclear reactor on predator-prey relationships were also examined. Benthic samples were removed after each of three, 3-month test periods from 36 predator exclusion cages (4m2) and 36 control plots located along a thermal gradient in Par Pond, an 1,100 ha freshwater reservoir in the southeastern United States.Results of our field experiments provide little evidence to suggest that either a single "keystone" species or vertebrate predators as a group were capable of regulating the abundance, diversity or productivity of chironomids in Par Pond. The relationship between predator treatment and community response (changes in density and species richness) was generally unaffected by either plot location or temperature fluctuations. When data from caged and control plots were pooled, however, both location and water temperature individually had significant impacts on the chironomid community. Alternative hypotheses are proposed to explain the lack of regulatory control of the benthic community by individual species or guilds of predators.

Field Experiments on Responses of a Freshwater, Benthic Macroinvertebrate Community to Vertebrate Predators

We examined the seasonal importance of vertebrate predators in potentially regulating the abundance and diversity of the benthic macroinvertebrates in the littoral zone of a soft—bottom reservoir that receives thermal effluent from a nuclear production reactor. Thirty—six predator (fish and turtle) exclusion cages (4 m2) were placed in shallow water at six locations along a thermal gradient in Par Pond, a 1100—ha cooling reservoir on the Savannah River Plant near Aiken, South Carolina, USA. An additional 36 control plots (4 m2) were also set up. Cages were in place during three, 3—mo test periods beginning in September 1977. Estimates of benthic density, taxon richness, and distribution within functional groups (defined by feeding mechanism) were calculated for each test period. Effects of temperature on predator—prey relationships were also determined. Experimental results of this study suggest that vertebrate predation was not the fundamental parameter organizing the benthic macrionvertebrate community in the littoral zone of this reservoir. Neither taxon richness nor density of total macroinvertebrates was conclusively related to predator treatment. Relationships between predator treatment and community response (changes in density and taxon richness) were generally unaffected by either plot locality, temperature fluctuations from thermal effluent, or seasonal changes. When data from caged and control plots were pooled, however, both location and water temperature individually had direct impacts on the benthic community. From our results and other field studies we hypothesize that individual species of "keystone" benthic predators (Paine 1969b) do not occur in the littoral zone of freshwater lentic environments with soft bottoms. This hypothesis is based on the following four arguments. First, environmental heterogeneity should reduce predator efficiency relative to that found in rocky intertidal systems where keystone predation may be present. Second, relative food web complexity should reduce the community importance of an individual predator species. Third, the apparent relatively greater resource partitioning of space and food in the freshwater littoral zone hinders monopolization of rate—limiting environmental resources by a single dominant prey species. Fourth, the effects of predation may not be extensive enough in time or area to provide sufficient resource space (and thus food) for exploitation by fugitive species. Possible regulation by a guild of predators and cases where the general hypothesis might be falsified are discussed.

Effects of Polinices duplicatus (Gastropoda: Naticidae) on infaunal community structure at Barnstable Harbor, Massachusetts, USA

Effects of the predatory naticid snail Polinices duplicatus (Say) on species composition, diversity and density, of the infauna of intertidal sand-flats at Barnstable Harbor, Massachusetts, USA, were studied using field experiments. Responses of molluscs (prey of P. duplicatus) and nonmolluscs (nonprey) were considered separately to distinguish between the effects of feeding and sediment disturbance during foraging. The fauna of 0.25 m2 predator-exclusion cages, coarse-mesh cages, sieved areas, and controls was followed for 1 yr. Species associations within cages from which predators were excluded were denser, more diverse, and richer in molluscs than those in other treatments. Larger areas (3×3 m) of natural bottom were fenced and maintained as snail exclosures and enclosures during two feeding seasons. For both molluscs and nonmolluscs, diversity (H'), number of species, evenness (SD), and density all decreased with increasing snail density. Intense predation pressure on molluscs and selective feeding on thin-shelled bivalves, which were rare, removed individuals and species of molluscs from the community. Comparison of samples taken inside and outside trails made by snails showed that disturbance of the surface sediment layers by snails decreased the abundance of spionid polychaetes and total nonmolluscs. The sipunculan Phascolopsis gouldi and the bivalve Gemma gemma dominated the community in all experimental treatments and were little affected by the activity of Polinices duplicatus. Predation and disturbance by snails lowered community diversity by removing individuals of the less abundant species, and generally maintained population densities below the level where strong competition would occur.