Ecology 83(2)
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Ecology, 83(2), 2002, pp. 370±385 q 2002 by the Ecological Society of America FLASH FLOODS AND AQUATIC INSECT LIFE-HISTORY EVOLUTION: EVALUATION OF MULTIPLE MODELS DAVID A. LYTLE1 Cornell University, Department of Entomology and Field of Ecology & Evolutionary Biology, Ithaca, New York 14853, USA Abstract. In disturbance ecology there is a tension between ecological and evolutionary viewpoints, because while disturbances often cause mortality in populations (an ecological effect), populations may also evolve mechanisms that ameliorate mortality risk (an evo- lutionary effect). Flash ¯oods cause high mortality in the juvenile aquatic stage of desert stream insects, but these ecological effects may be mitigated by the evolution of life-history strategies that allow the terrestrial adult stage to avoid ¯oods. Life-history theory predicts that, to balance trade-offs between juvenile growth and mortality risk from ¯oods, (1) most individuals should emerge before the peak of the ¯ood season, (2) optimal body size at emergence should decline as ¯ood probability increases, and (3) a second decline in body size at emergence should occur as the reproductive season ends. These predictions were tested with data on body mass at and timing of emergence of the caddis¯y Phylloicus aeneus measured in three montane Chihuahuan Desert (Arizona, USA) streams over two years. P. aeneus that had not reached the adult stage were eliminated from site-years that experienced ¯ash ¯oods, suggesting that timing of emergence is an important ®tness component. On average 86% of emergence occurred before the long-term (;100 yr) mean arrival date of the ®rst seasonal ¯ood, supporting prediction 1. The presence of two consecutive declines in body mass at emergence in most site-years was congruent with predictions 2 and 3. To test whether the two declines were associated with increasing ¯ood probability and end of the reproductive season, respectively, maximum-likelihood methods were used to compare ®ve body-size models: a null model that contains no parameters related to ¯ood regime or reproductive season, a seasonal model that incorporates a reproductive time constraint, and three disturbance models that incorporate both reproductive time constraints and ¯ood dynamics. The disturbance models outperformed the other models, suggesting that at least some of the body-mass pattern was in¯uenced by ¯ood dynamics. The timing of the ®rst ¯ood of the season was the most important determinant of observed emergence patterns. Overall, this study demonstrates that aquatic insects can compensate for ¯ash ¯oods by using state-dependent emergence strategies that are synchronized with long-term ¯ood dynamics. Key words: Akaike's information criterion; body size; Chihuahuan Desert (USA) stream insects; disturbance; ¯ash ¯ooding; life-history evolution; maximum likelihood; model evaluation; phenotypic plasticity; Phylloicus aeneus; state-dependent strategy. INTRODUCTION McPeek and Brown 2000). Evolutionarily, disturbance regimes alter the organisms themselves by changing A recurring problem in ecology is understanding how population gene frequencies (Vrijenhoek 1985, Vrijen- species diversity and abundance are maintained, or hoek et al. 1985) and by in¯uencing the evolution of even enhanced, in systems where organisms experience life-history strategies (Bradford and Roff 1993, 1997, high mortality from disturbances (MacArthur and Lev- Philippi 1993a, b), behaviors (Meffe 1984, Lytle 1999), ins 1967, Connell 1978, Pickett and White 1985, Col- and morphologies (Gill 1981, Christensen 1985). There lins 2000). Ecologically, disturbance regimes affect di- is a tension between ecological and evolutionary view- versity and abundance patterns through local exclusion points because while disturbances certainly produce of taxa (Meffe 1984), mediation of competitor species measurable ecological effects, such as mortality within (Hemphill and Cooper 1983, Denslow 1985, Airoldi populations, adaptive evolution in response to distur- 2000), alteration of trophic structure (Wootton et al. bance regimes is expected to ameliorate or even elim- 1996, Wootton 1998), and exclusion of taxa from re- inate some of these effects (Cohen 1966, Iwasa and gional species pools (Diamond 1975, Drake 1991, Levin 1995, Lytle 2001). However, because the timing (date of occurrence within a season), predictability Manuscript received 17 November 2000; revised 9 February (variance in timing), frequency (expected number of 2001; accepted 26 February 2001. 1 Present address: University of Arizona, Department of disturbances per season), and severity (expected mor- Entomology, Tucson, Arizona 85721 USA. tality) of disturbances may vary from year to year it is E-mail: [email protected] not surprising that even organisms with adaptations for 370 February 2002 FLASH FLOODS AND LIFE-HISTORY EVOLUTION 371 surviving some disturbances experience mortality from the ¯ood season in order to continue growing. Thus, events that occur outside normally observed parameter when there is variation in juvenile body size within a values (Poff 1992, Turner et al. 1997, 1998). For this population, body size at emergence should decline dur- reason the magnitude of disturbance-regime parameters ing the season as disturbance probability increases. may determine when disturbances control species di- Third, with some disturbance regimes there is a ®nite versity and abundance through ecological mechanisms, probability that a disturbance might not occur during and when disturbances drive the evolution of features a given year. In this case the optimal strategy for small- that allow taxa to persist in disturbance-prone systems. er individuals is to risk the entire disturbance season The interplay between mortality from disturbances in order to continue growing and then emerge towards and adaptation to disturbances is exempli®ed by sea- the end of the reproductive seasonÐa risky strategy sonal ¯ash ¯oods in desert streams. While ¯ash ¯oods with a high payoff. In years with no disturbances, this can cause .90% mortality in the juvenile stages of strategy will produce a second decline in body size at aquatic insects (Gray 1981, Molles 1985, Grimm and emergence associated with the end of the reproductive Fisher 1989, Lytle 2000a), scoured stream reaches are season, similar to that predicted by seasonal time-con- rapidly recolonized by individuals in their aerial adult straint models. In years when disturbances do occur stage (Williams and Hynes 1976, Gray and Fisher 1981, these individuals will suffer high mortality. Fisher et al. 1982, Boulton et al. 1992). Because the timing of emergence into the adult stage relative to the Is it reasonable to expect optimal emergence timing of ¯ash ¯oods largely determines the probability strategies to evolve? of surviving to recolonize (Gray 1981, Lytle 2001), In order to maximize ®tness using these optimal timing of emergence may play a critical role in allowing emergence strategies, organisms must be able to assess many insect taxa to persist in ¯ood-prone streams. both the time of the season and their own state (body Thus, life-history events such as timing of emergence size, in most models). Furthermore, for strategies to may be an important target of natural selection in ¯ood- evolve to optima there must be (or have been) heritable prone systems. variation in the traits governing these sensory mech- Models have been developed to investigate how en- anisms. For insects, many life-history changes such as vironmental variability affects the evolution of life- pupation or diapause are under hormonal control and history traits such as bet-hedging strategies (Cohen are initiated when a sensory threshold, such as critical 1966, Venable and Lawlor 1980, Sasaki and Ellner day length or body size, is exceeded (Nijhout 1994). 1995), diapause timing (Cohen 1970, Hairston and Most insects use photoperiod to mark seasonal pro- Munns 1984), and size at and timing of maturation gression (Saunders 1981, Danks 1994, Nylin and Got- (Cohen 1971, King and Roughgarden 1982, Kozøowski thard 1998), and some taxa are sensitive to small dif- and Weigert 1987, Ludwig and Rowe 1990, Rowe and ferences in day length (Nijhout 1994). The critical day Ludwig 1991). The ``seasonal time constraint'' theory length used to initiate life-history changes can vary developed by Rowe and colleagues is particularly use- widely both among and within populations, and these ful for understanding aquatic-insect emergence strat- differences are heritable as quantitative characters in egies because it shows how individuals maximize ®t- some taxa (Danilevskii 1965, Nylin et al. 1994, Tam- ness by maturing according to a state-dependent mech- maru et al. 1999). Insects are also capable of assessing anism. The switch from growth to reproductive stage body size via specialized stretch receptors or other depends on both current body size and the amount of means (Nijhout 1994), and some possess sharply de- time remaining in the season for reproduction, which ®ned critical body sizes for molting, pupation, or dia- is determined by the onset of winter or other unfavor- pause (Nijhout 1981). Like critical day length, critical able environmental conditions. Adding parameters that body size can also be heritable (e.g., body size at dia- specify disturbance timing, predictability, frequency, pause in cricket populations along a cline in season and severity to the seasonal time-constraint model pro- length [Mousseau and Roff 1989]). Critical day length duces