Effects of predation on prey populations

Size classes and species of fishes that are vulnerable to piscivores frequently show an inverse relationship between predator and prey abundances or exhibit nonoverlapping distribution patterns. Surveys of fish assemblages at 86 pool sites in tropical streams in Trinidad provided cases where the widely distributed killifish Riv-ulus hartii occurred alone, as well as in various combinations with other species (Gilliam et al. 1993). Its distribution was largely complementary to the piscivorous fish Hoplias malabaricus, and its abundance at sites with other species was only about one third of that predicted from expectations based on Rivulus-only pools. Similar efforts to document negative statistical associations of invertebrate populations with natural variation in predation have met with varying success. The sampling of three streams of differing acidity in southern England demonstrated an inverse correlation between fish and predaceous invertebrates (Hildrew et al. 1984). Reice and Edwards (1986) found no differences in invertebrate abundances in stream sections below waterfalls, where trout were present, and above waterfalls, where trout were absent. In a comparison of 18 trout streams and six troutless streams in central Finland, Baetis densities were fivefold higher in troutless streams, midge larvae showed a nonsignificant trend toward greater abundance in trout streams, and cased caddis larvae did not differ (Meissner and Muotka 2006).

Several studies comparing total prey consumption by trout to prey biomass and production have concluded that predators consumed essentially all of the available prey production (Allan 1983, Huryn 1996, Section 8.4). Although the demonstration that all of the energy produced at one trophic level is consumed by the trophic level above is not definitive evidence of either bottom-up or top-down control, it certainly indicates that consumption by predators is the principal fate of the trophic level in question. Estimated prey consumption by stoneflies, the most abundant invertebrate predators present at several sites in a Rocky Mountain stream, was roughly half that attributed to trout, suggesting that the influence of invertebrate predators was markedly less than that of fish (Allan 1981, 1983). On the other hand, when fish are absent it seems plausible that invertebrate predators consume all secondary production at lower trophic levels. Indeed, predaceous invertebrates consumed nearly all detritivore production in a coastal stream (Smith and Smock 1992), and consumption by invertebrate predators was also high in small, fishless streams in the southeastern United States (Hall et al. 2000).

Manipulations of fish abundance using small cages and by fencing off and removing fishes from large stream reaches have demonstrated effects on invertebrate abundance in some studies, and no change in others. Placing small wire baskets filled with substrate with or without tops to prevent or allow entry of fish in streams in North Carolina, Reice (1983) reported little or no change in benthic macroinvertebrate populations. Using a field enclosure design, Flecker (1984) documented a reduction in numbers of some benthic invertebrates in response to a guild of invertivores, primarily sculpins (Cottus)

and dace (Rhinichthys). Baskets of substrate were placed in enclosures containing 0, 3, 6, or 12 sculpins, while open cages were used as an additional treatment permitting free access by fish and therefore natural levels of predation. Chironomidae and the stonefly Leuctra showed a significant reduction in abundance with increasing intensity of fish predation, while other insect taxa were unaffected. Enclosure of small creek chub Semotilus atromaculatus within 0.5 m2 areas of a warm-water, soft-sediment stream reduced total invertebrate abundance (Gilliam et al. 1989). Oligochaetes and isopods were strongly affected while midge larvae and clams showed no response. When trout were reduced to about 10% of their initial abundance in a 1 km reach of a Rocky Mountain trout stream, no change in macroinvertebrate populations was detected, possibly because of high replenishment of prey by drift into the experimental reach (Allan 1982b). In addition, trout often feed selectively on infalling terrestrial invertebrates, and when these are sufficiently abundant the predation pressure on benthic invertebrates may be lessened (Nakano et al. 1999). Exclusion of trout from 100 m reaches of a small stream in Finland resulted in significant benefits to large prey, particularly preda-ceous invertebrates and cased caddis, but Baetis mayflies and chironomid larvae were unaffected (Meissner and Muotka 2006).

9.2.3.1 Nonconsumptive effects of predation

Predator avoidance adaptations typically result in lost foraging opportunity, reduced growth rate, adult body size, and fecundity, and thus an overall reduction in fitness. When Baetis was reared in laboratory microcosms containing predaceous stoneflies with glued mouthparts, nonlethal contact resulted in reduced gut fullness and smaller size at maturity relative to microcosms without stoneflies (Peckarsky et al. 1993). In the study by Cooper (1984) described earlier, female ger-rids from trout pools weighed less than those from pools without trout, suggesting that lost feeding opportunity translated into reduced growth. Juvenile coho salmon Oncorhynchus kisutch feed on stream drift by making short excursions from a holding position. Using house-flies as prey and a model of a rainbow trout as threat, Dill and Fraser (1984) asked whether risk reduced foraging and whether the reduction was proportional to risk. Exposure to the model trout reduced reaction and attack distances and shortened attack time compared to young salmon foraging in the absence of threat. When the investigators varied the frequency with which they presented the model before the salmon, thereby varying the level of risk, attack distance varied proportionally (Figure 9 9). Moreover, the responsiveness of young coho to the model was reduced by higher hunger levels and the presence of a competitor. Such behavioral flexibility evidently allows juvenile salmon to make complex adjustments in their foraging.

Baetis also shows evidence of an inducible life history shift in the presence of trout, resulting in faster maturation to escape the stream environment but causing it to mature at smaller body

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