Consequences of reduced activity in the mayfly Baetis tricaudatus. For each meas

Question

Consequences of reduced activity in the mayfly Baetis tricaudatus. For each measure (time to adult, dry mass of adults, number of eggs produced, and dry mass of eggs), the ratio (.t-axis) of its average value in the presence of predators (fish) relative to that in the absence of predators is given. A ratio smaller than 1.0 represents a reduced average value in the presence of predators, whereas values greater than 1.0 represent a greater average value in the presence of predators. (Adapted from Scrimgeour and Culp 1994. as illustrated in Lima 1998.) INTERPRETING ECOLOGICAL DATA Based on the results of the experimental study presented in Figure 15.27, how docs the reduced activity of larval mayflies in the presence of predators influence the time required for larvae to develop into adult mayflies? How docs the presence of predators and associated reduction in activity during the larval stage influence the fitness of adult mayflies? Explain the variables you used to draw your conclusions about adult fitness.

Explanation / Answer

1) Experiments were conducted to compare the patterns, mechanisms, and costs of predator avoidance behavior among larvae of five species of mayflies that co—occur with the predatory stoneflies, Megarcys signata and Kogotus modestus in western Colorado streams. Mayfly drift dispersal behavior, use of high vs. low food (periphyton or detritus) patches, microhabitat use, positioning, and activity periodicity were observed in the presence and absence of predators in circular flow—through chambers using natural stream water. Also, distances from predators at which prey initiated escape responses were compared among prey and predator species. Costs of predator avoidance behavior were assessed by measuring short—term (24 h) feeding rates of mayflies in the presence or absence of predatory stoneflies whose mouthparts were immobilized (glued) to prevent feeding. The intensity and associated costs of predator avoidance behavior of mayfly species were consistent with their relative rates of predation by stoneflies. Megarcys consumes overwintering generation Baetis bicaudatus > Epeorus longimanus > Cinygmula = Ephemerella; Kogotus consumes summer generation Baetis > Epeorus deceptivus = Cinygmula; Megarcys eats more mayflies than Kogotus. While Megarcys induced drift by Baetis, Epeorus, and Cinygmula, this disruptive predator avoidance behavior only reduced food intake by Baetis and Epeorus. The morphologically defended mayfly species, Ephemerella, neither showed escape behavior from Megarcys, nor any cost of its antipredatory posturing behavior. Only Baetis responded by drifting from Kogotus. No mayfly species shifted microhabitats or spent less time on high—food patches in the presence of foraging stoneflies. However, predators enhanced the nocturnal periodicity of Baetis drift, which was negligible in the absence of stoneflies as long as food was abundant. Lack of food also caused some microhabitat and periodicity shifts and increased the magnitude of both day and night drift of Baetis. Thus, Baetis took more risks of predation by visual, drift—feeding fish not only in the presence of predatory stoneflies, but also when food was low or they were hungry. All other mayflies were generally nocturnal in their use of rock surfaces, as long as food was abundant. Finally, the distances at which different mayfly species initiated acute escape responses were also consistent with relative rates of predation. This study demonstrates alternative predator avoidance syndromes by mayfly species ranging from an initial investment in constitutive morphological defenses (e.g., Ephemerella) to induced, energetically costly predator avoidance behaviors (e.g., Baetis). Although the costs of Ephemerella's constitutive defense are unknown, experiments show that prey dispersal is the mechanism underlying fecundity costs of induced responses by Baetis to predators, rather than microhabitat shifts to less favorable resources or temporal changes in foraging activity. A conceptual model suggests that contrasting resource acquisition modes may account for the evolution and maintenance of alternative predator avoidance syndromes along a continuum from Baetis (high mobility) to heptageniids (intermediate mobility) to Ephemerella (low mobility). Prey dispersal (swimming) to avoid capture results in reduction of otherwise high fecundity by Baetis, which trades off morphological defense for enhanced ability to acquire resources. Thus, improved foraging efficiency is the selection pressure maintaining the highly mobile life style in Baetis, which increases resource acquisition and fecundity, offsetting the high mortality costs associated with this behavior. 2) We investigated the interacting effects of copepod body size and the presence or absence of egg masses on the risk of predation by a visual predator. We conducted selection experiments involving three-spined sticklebacks (Gasterosteus aculeatus) and copepods ranging in body mass from 0.5 to 740 µg C: Oithona similis, Corycaeus anglicus, Pseudocalanus newmani, P. moultoni, Pseudodiaptomus marinus, and Paraeuchaeta elongata. We found that sticklebacks selected ovigerous females of the two smallest-bodied species of copepods (Oithona similis and Corycaeus anglicus). In contrast, the fish showed no significant selection for ovigerous females of the remaining, larger-bodied species. Unexpectedly, egg mass position (i.e., in a ventral, dorsal or lateral location on the urosome) appeared to influence predation risk more than did body size, resulting in higher predation risk for the cyclopoid and poecilostomatoid species than for the calanoid species we tested. Although the sticklebacks showed no statistically significant preference for ovigerous females of any of the four calanoid species, for each species the overall proportion of ovigerous females ingested was slightly greater than 0.50. Thus, whether body size influences the susceptibility of egg-brooding calanoid copepods to predation remains an open question.

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.