WETLANDS. Vol. 16, No. 4, December 1996, pp. 583-586 © 1996, The Society of Wetland Scientists

N O T E

E F F E C T S O F AN O D O N A T E P R E D A T O R A N D H A B I T A T C O M P L E X I T Y O N S U R V I V A L O F T H E F L A G F I S H JORDANELLA FLORIDAE

Frank Jordan ~,-' and Angela C. McCreary ~ Department o f Zoology and National Biological Service

Florida Cooperative Fish & Wildlife Research Unit University o f Florida

Gainesville, FL 32611

2 Department of Biology and Marine Science Jacksonville University

2800 Universi~_ Boulevard North Jacksonville, Florida 32211

Abstract: Hydrologic variability affects the development of predator populations in freshwater marsh sys- tems, with predatory fishes typically predominating in permanently inundated marshes and predatory insects predominating in ephemeral marshes. The ability of larger predatory fishes to control the abundance of small fishes is fairly well understood, whereas control of fish populations by predatory insects has not been well studied. To address this gap in our understanding of marsh food web dynamics, we exposed groups of flagfish Jordanella floridae to larvae of the widely distributed dragonfly Anax junius. We also examined how habitat complexity affected the foraging efficiency of A. junius larvae. Dragonfly larvae reduced survival of flagfish by 40% during a ten-day experiment, Survival rates did not differ between simple and complex habitats. These results suggest that predatory insects could play an important role in regulating populations of small fishes in marsh systems that lack larger predatory fishes. Further, unlike many predatory fishes, there does not seem to be a significant decrease in the foraging efficiency of A. junius in complex habitats.

Ke,, Words: Jordanetla floridae, Anax junius, Cyprinodontidae, Aeshnidae, predation~ habitat complexity

I N T R O D U C T I O N

Traditional analyses of food web structure in fresh- water ecosystems tend to segregate predatory arthro- pods and vertebrates into lower and higher trophic lev- els, respectively (McCormick and Polls 1982, Schoen- ly et al. 1991). However, empirical studies have clearly demonstrated that predatory arthropods (e.g., insects, decapod crustaceans) can control the size and structure of aquatic vertebrate populations. The majority of these studies have focused on how predatory insects affect behavior, growth rates, survival, and community dynamics of aquatic stages of amphibians (e.g., Heyer et al. 1975, Caldwell et al. 1980, Holomuzki 1986, Morin et al_ 1990. Skelly and Werner 1990, Werner 1991). Surprisingly little research has quantified the effects of predatory insects on fishes. Most informa- tion available concerning predation on fishes by in- sects has come from fish culturists, who have noted that survival of larval and juvenile fishes is inversely related to the abundance of large aquatic insects. Lab-

oratory studies have demonstrated that certain aquatic insects can be voracious predators of larval and post- larval cultured fishes (reviewed in McCormick and Po- lls 1982). Kipling and Frost (1970) found that aquatic beetle larvae of the family Dytiscidae were dominant predators on juveniles of the pike Esox lucius in small ponds. Few other studies have assessed the effects o f predatory insects on fishes in natural systems.

Predation on fishes by aquatic insects is likely to vary with hydrologic conditions, with insect predators being more important in semi-isolated, ephemeral hab- itats. In permanent habitats, fishes gain a size advan- tage and can effectively eliminate many insect taxa (e.g., Morin 1984), thereby greatly limiting their influ- ence on fishes. However, insects may be important predators of fishes in ephemeral aquatic ecosystems such as seasonally inundated marshes. The seasonal expansion and contraction of these habitats limits the ability of large predatory fishes to achieve significant production (Kushlan 1976, 1980), although small-

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584 WETLANDS, Volume 16, No. 4, 1996

sized fishes are likely to persist through such distur- bance cycles (Loftus and Eklund 1994). Furthermore, early colonization and recruitment into ephemeral hab- itats can afford predatory insects a relative size advan- tage, which both increases their ability to handle small- sized fishes and reduces their susceptibility to gape- limited predators. The importance of predatory insects may decrease during a wet season (Morin et al. 1990), first as fish prey become too large to handle (e.g., Cro- nin and Travis 1986, Formanowicz 1986) and then as they become large enough to reverse the predator-prey roles (McCormick and Polis 1982). Habitat complexity may further mediate the effects of predation on fishes by insects in seasonally variable habitats. That is, in- creased habitat complexity typically reduces the ability of predators to detect and capture prey (reviewed in Heck and Crowder 1991), which could facilitate the coexistence of small-sized fishes and predatory insects. in general, natural heterogeneity in plant stem densi- ties is likely to create a habitat mosaic that is charac- terized by variable foraging success for predators and variable risk of predation for prey.

A wtriety of small-sized fish species belonging to the families Cyprinodontidae, Fundutidae, and Poeci- liidae are quite tolerant of the seasonally variable hy- drologic conditions found in the fringing marshes, roadside ditches, and semi-isolated wetlands of Flori- da. For example, the endemic flagfish Jordanellaflor- idae Goode and Bean (Teteostei: Cyprinodontidae) is locally abundant in these semi-isolated, ephemeral marshes (Gilbert and Burgess 1980). Although these habitats rarely support large predatory fishes, they do support high densities of potentially predatory insects (i.e., odonates, coleopterans, hemipterans). However, little is known about what role, if any, insects play in regulating the sizes of fish populations in ephemeral marsh systems. In this laboratory study, we exposed flagfish to larvae of the dragonfly Anax junius Drury (Odonata: Aeshnidae) in simple and complex habitats to determine their susceptibility to predatory insects.

M E T H O D S

Flagfish and dragonfly larvae were collected in No- vember 1995 from a network of ephemeral freshwater marshes approximately 8 km southwest of the town of Otter (?reek (Levy County, Florida). Test organisms were returned to the laboratory and held in water from the collection site for 48 h to allow acclimation to laboratory temperature. Groups of 5 flagfish were then transferrect into I0 opaque plastic t)ins (32 x 22 x 17 cm) containing 8 liters of aerated tap water. Flagfish were used without regard to sex, and body size ranged between 13 and 37 m m standard length (mean = 24.7 mm, n = 80). Flagfish were not fed during the exper-

imenl, and water temperatures ranged between 16 and 21°C. Lights were maintained on a 12L:12D cycle dur- ing the experiment.

Eight experimental bins each were randomly as- signed to either a complex or simple habitat treatment. Complex habitat was created by tying 20 bipartite strands (150 × 20 mm) of black plastic to pieces of plastic mesh that rested on the bottom of the bins. The number of strands used 0.e., 284 per meter square) was within the range of plant stem densities found in similar freshwater marshes in Florida (E Jordan, un- published data). No cover was provided in the simple habitat treatment. Two late instar A. junius larvae were randomly assigned to each of five bins in both treat- ments. Larval dragonflies ranged in size between 30 and 45 mm total length (mean = 36.7 mm, n = 20). Fewer bins were used as predator controls because pre- liminary observations indicated minimal mortality of flagfish over a 14-day period in the absence of pred- ators.

The experiment was run for 10 days, and then all surviving flagfish were counted. The percentage of survivors (survivorship) was calculated and used as the response variable in a factorial analysis of variance (ANOVA) to test for the effects o f habitat complexity (simple vs. complex) and predators (absent vs. pres- ent). Inspection of residual plots and Levene 's test (Milliken and Johnson 1992) indicated that variances were unequal, therefore survivorship values were arc- sin-transformed to meet the assumptions for the ANO- VA test (Winer et al. 1991). However, means are pre- sented for the untransformed data because they pro- duced very similar ANOVA results.

RESULTS

All but one (97%) of the flagfish survived in the bins lacking dragonfly larvae, whereas only 60% of the flagfish survived in the bins containing predators (Figure l). There was a strong predator effect (F~.~2 = 14.4, p = 0.0026), which accounted for 55% of the variation in survivorship rates of flagfish. Predation rates ranged from 20 to 80% during the ten-day ex- periment, suggesting that there was considerable inter- individual variation in predator behavior. No predators died during the experiment. Habitat complexity did not affect survivorship of flagfish (F~.~2 = 0.3, p = 0.6024), and there was no interaction between predator pres- ence and habitat complexity (F,,~2 - 0.3, p - 0.5965).

DISCUSSION

The distribution patterns of many insect taxa seem to be strongly correlated with the presence or absence of large predatory f shes (e.g., Morin 1984, McPeek

Jordan & McCreary, PREDATION ON FLAGFISH BY DRAGONFLY LARVAE 585

100-

90"

"~' 70" . .'~ 60- • _ 502 ,~ 40'

30' 20'

10" 0

no predator predator Figure 1. Mean (+ 1 standard error) survivorship of the flagfish Jordanella floridae (Teleostei: Cyprinodontidae) in the presence and absence of larvae of the predatory insect Anax junius (Odonala: Aeshnidae).

1989), and hence, the importance of aquatic insects as top predators in aquatic systems is largely determined by the presence of large predatory fishes (Schoenly et at. 1991). The relative importance of these two groups of predators tends to vary along a hydrologic gradient, with large predatory fishes prevailing in more contig- uous, permanent systems and predatory insects pre- vailing in more isolated, ephemeral systems. Large predatory fishes are likely to play a role in regulating popula t ions of smal l -s ized fishes (Kushlan 1976, 1980), whereas the importance of predatory insects is largely unknown. This study demonstrates that larval A. junius prey upon small-sized fishes (i.e., <40 mm standard length) and that predation rates can approach 40% under ideal laboratory conditions.

Quantitative sampling with 1-m 2 throw traps (n = 13) indicated that there was about one A. junius larvae/ m 2 in the dense beds of Ludwigia and Bacopa at our collection site. Flagfish densities were higher, with about 9 flagfish/m 2. Previous collections at the same and similar localities have yielded higher densities of both species. Based on this study, predation by A. jun- ius and other local predators (e.g., wading birds) could easily account for these large fluctuations in flagfish abundance, although other factors (esp., variation in hydroperiod) are also likely to be important. Anaxjun- ius is found throughout North America, and the larvae of this species can be locally abundant (e.g., Catdwell et al. 1980) in habitats that typically lack large pred- atory fishes (Werner 1991). Although there are few quantitative data available for fishes, predation by A. junlus has been shown to strongly impact larval pop- ulations of amphibians (Caldwell et al. 1980, Werner 1991, Babbitt and Jordan 1996). It seems likely that predation by this and other large predatory insects (e.g., Trarnea, Lethocercus, Dytiscus) also plays an ira-

portant role in structuring populations of small-sized fishes in habitats (e.g., semi- isola ted, ephemera l marshes) that lack large predatory fishes.

Habitat structural complexity can have a profound effect on the foraging efficiency of aquatic predators (e.g., Crowder and Cooper 1982, Gotceitas 1990, Bab- bitt and Jordan 1996). Detection rates, capture success, and predator maneuverability all tend to decrease with increasing habitat complexity (see review in Heck and Crowder 1991). Much of this research has been per- formed using visually oriented fishes as predators. It is unclear how habitat complexity affects predatory aquatic insects such as dragonfly larvae, which use visual and to a lesser extent tactile stimuli to detect their prey (Pritchard 1965, Folsom and Collins 1984). The foraging efficiency of A. junius did not differ be- tween the simple (0 stems/m 2) and complex (284 stems/m 2) habitats. Although often found clinging to a perch (e.g., the plastic plants and walls of the bins used in this experiment), the larvae of this species are very active tbragers (cf., ambush predators). Their almost cylindrical bodies were probably unaffected by the density of plant stems used in this study, although ma- neuverability would probably have been reduced in a deep-bodied predatory fish.

In general, foraging efficiency does not seem to de- crease until some threshold stem density is surpassed (Gotceitas and Colgan 1989). Therefore, foraging ef- ficiency of A. junius might be lowered at some thresh- old plant stern density greater than that used in this study. Babbitt and Jordan (1996) found that the num- ber of Bufo terrestris (Bonnaterre) tadpoles eaten by A. junius nymphs did not decrease until plant density reached 90 grams/m 2 of Ludwigia repens Forst. Qual- itatively, the density of artificial plant stems used in this study seemed to be about as complex as the me- dium complexity treatment (10 grams/m 2 of L. repens) used in Babbit and Jordan (1996), which did not affect foraging efficiency of A. junius. Additional research is needed to determine if predation on fishes by aquatic invertebrates is lowered in increasingly complex hab- itats.

ACKNOWLEDGMENTS

We appreciate the assistance of Betty Jordan in the field and laboratory. We also thank Janet Johnson and A. Quinton White of Jacksonville University for their continued support of research training for undergrad- uates. Kim Babbitt and Roxanne Conrow provided valuable comments on the manu.script. Support for this research was provided in part by the St. Johns River Water Management District (contract number 95D 164 to FJ). This study was undertaken by FJ in partial ful- fillment of a Ph.D. degree at the University of Florida.

586 W E T L A N D S , V o l u m e 16, No . 4, 1996

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Manuscript received 29 December 1995: accepted 7 June 1996.