Sublethal Consequences of Stream‐Dwelling Predatory Stoneflies on Mayfly Growth and Fecundity

Abstract

Predators can have consequences on prey populations and communities that extend well beyond direct predator-induced mortality. Predator-prey interactions often affect prey feeding rates, growth rates, or fecundities, thereby significantly affecting reproductive success of prey adults. Thus, investigation of the sublethal fitness consequences of behavioral responses of prey to predators is essential to our understanding of the total impact of predators on prey populations and communities. Feeding (algal grazing) rates, growth rates, and fecundities of Baetis bicaudatus (Baetidae) larvae were measured in replicated circular flow-through stream channels to determine the sublethal consequences on fitness correlates of the presence of predatory stoneflies (Perlodidae). Gut fullness of 24-h starved Baetis larvae was measured using fluorometry after 24 h (short-term experiments) in four treatments: (1) ambient resource rocks and no predators, (2) low resource rocks and no predators, (3) ambient resource rocks and one female perlodid stonefly (Megarcys signata), or (4) Kogotus modestus whose mouthparts were glued to prevent lethal effects of predation. Mid-instar male and female Baetis larvae were reared for 3 wk until wing pad maturation (long-term experiments) and subjected to all but the Kogotus treatment. Predator avoidance significantly reduced levels of Baetis gut fullness, over the short term, to levels intermediate between the high and low resource treatments. Longer term residence with predatory stoneflies caused larvae of both male and female Baetis to mature at significantly smaller sizes than in the treatment with ambient food but no predators. Mayflies with predators matured at sizes similar to those held with low food levels over the long term. Interestingly, both starved larvae and those avoiding predators did not grow during the experimental period in contrast to about a 50% growth rate oflarvae experiencing ambient food levels, but no predators. Similarly, egg biomass per mature female larva was significantly reduced in both starved and predator treatments. The proportion of total body mass allocated to eggs was only reduced in starved mayflies indicating that the females avoiding predators accrued less total mass, but allocated a similar proportion to eggs as did well-fed Baetis. These data convincingly demonstrate that predator avoidance can have dramatic consequences on mayfly fitness. The mechanism explaining those fitness consequences is probably a disruptive drift/swim response by Baetis to encounters with stoneflies, which reduces Baetis' feeding rates. If we accept the assumption that natural selection has shaped the foraging behavior of organisms to maximize fitness, foragers need to make decisions that maximize both survivorship (minimizing risk of predation) and fecundity. In mayflies fecundity is entirely a function of resource acquisition by the larvae, which makes them particularly vulnerable to such sublethal effects of predation. Nonetheless, we suspect that the types of sublethal costs o...

Toward a predictive theory of risk effects: hypotheses for prey attributes and compensatory mortality

“…Many species respond to predation risk by reducing their use of habitats in which they are easily detected or otherwise more vulnerable to predation (Peckarsky et al 1993, Sih 1997, Heithaus and Dill 2006. The use of different habitats can have a direct influence on diet composition , so shifts of habitat in response to risk can drive changes in diet with nutritional or energetic costs (Christianson and Creel 2010).…”

“…Even with such data, field studies have rarely (if ever) yielded direct estimates of the proportion of predation's total effect on prey dynamics that was due to risk effects vs. direct killing. Determining the demographic costs of antipredator responses remains a thorny problem for observational studies (Peckarsky et al 1993, Schmitz 1998, Boonstra et al 1998, Pangle et al 2007. To further develop and test a predictive theory of the importance of risk effects, two steps will be important.…”

“…Predators can limit prey by direct killing or by risk effects, which are the costs of antipredator responses (Peckarsky et al 1993, 2008, Boonstra et al 1998. Responses to risk can be morphological (Tollrian and Harvell 1999) or behavioral (Lima 1998), including changes in habitat use (Anholt and Werner 1999, Heithaus and Dill 2006, vigilance (Armitage 2004, Winnie and, foraging (Kotler et al 1993, Lima andBednekoff 1999), aggregation (Boesch 1991, Krause and Ruxton 2002, Caro 2005, movement patterns (Sih and McCarthy 2002), and sensitivity to environmental conditions (Winnie et al 2006).…”

“…Responses to risk can be morphological (Tollrian and Harvell 1999) or behavioral (Lima 1998), including changes in habitat use (Anholt and Werner 1999, Heithaus and Dill 2006, vigilance (Armitage 2004, Winnie and, foraging (Kotler et al 1993, Lima andBednekoff 1999), aggregation (Boesch 1991, Krause and Ruxton 2002, Caro 2005, movement patterns (Sih and McCarthy 2002), and sensitivity to environmental conditions (Winnie et al 2006). The costs of these responses can be manifest by reduced survival, growth, or reproduction (Peckarsky et al 1993, 2008, Ruxton 1997, Pangle et al 2007). Recent empirical research has shown that risk effects on prey dynamics can be as large as direct effects, or even larger (Schmitz et al 1997, Nelson et al 2004, Preisser et al 2005, Pangle et al 2007).…”

Microhabitat-scale influences of resources and refuge on habitat selection by an estuarine opportunist fish

“…Organisms are often faced with a trade-off between obtaining food and remaining safe from predators, and this conflict can be resolved through shifts in activity levels and habitat selection (Lima & Dill 1990, Peckarsky 1993, Werner & Anholt 1993, Anholt & Werner 1995, 1998, Halpin 2000, and reviewed in Gotthard 2000. Foraging habits and patch use are critical aspects of behavior that directly influence an organism's fitness (Brown 1988).…”