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Ecological correlates of trophic polyphenism in spadefoot tadpoles inhabiting playas D.M. Ghioca-Robrecht, L.M. Smith, and L.D. Densmore
Abstract: Polyphenism in larval amphibians has been related to several factors, including wetland hydroperiod, food availability, competition, and predation. Spadefoot toads (genus Spea Cope, 1866) often exhibit a trophic polyphenism by developing distinct carnivore and omnivore larval morphotypes. Using a multimodel selection approach, we investigated the influence of land use (cropland vs. grassland type) and differences in annual precipitation on morphotype expression in Plains spadefoot (Spea bombifrons (Cope, 1863)) and New Mexico spadefoot (Spea multiplicata (Cope, 1863)) toads in playas. We also examined the relative importance of tadpole density, tadpole age, water-loss stress, wetland size, density of larval mole salamanders (genus Ambystoma Tschudi, 1838; a predator on Spea tadpoles), and food resources on morph occurrence. The carnivore morphotype developed almost exclusively in S. bombifrons and rarely in S. multiplicata regardless of land use. Habitat availability measured by water-loss rate, as well as predation risk and tadpole age, were the most important factors influencing carnivore proportions. Ambystoma density was positively associated, whereas water-depth loss and tadpole age were negatively associated with the proportion of carnivores. The greatest proportion of carnivores was observed in grassland playas, which had the highest density of Ambystoma predators, longest hydroperiods, and experienced water-depth gain. Fairy shrimp density was not correlated with the proportion of carnivores. Upland land use through cultivation-associated erosion is altering wetland trophic structure, which further influences morphotype expression in Spea tadpoles and playa amphibian community structure. Re´sume´ : Le polyphe´nisme des larves d’amphibiens a e´te´ explique´ par divers facteurs, dont l’hydrope´riode des terres humides, la disponibilite´ de la nourriture, la compe´tition et la pre´dation. Les crapauds pieds-en-beˆche (du genre Spea Cope, 1866) posse`dent souvent un polyphe´nisme trophique et de´veloppent des morphotypes larvaires distincts, soit carnivore et ` l’aide d’une me´thodologie de se´lection multi-mode`les, nous avons e´tudie´ l’influence de l’utilisation des terres omnivore. A (terres agricoles ou pre´s) et des diffe´rences de pre´cipitations annuelles sur l’expression du morphotype chez le crapaud des plaines (Spea bombifrons (Cope, 1863)) et le crapaud pieds-en-beˆche Nouveau-Mexique (Spea multiplicata (Cope, 1863)) dans les playas. Nous avons aussi examine´ l’influence relative de la densite´ des teˆtards, de l’aˆge des teˆtards, du stress relie´ a` la perte d’eau, de la surface des terres humides, de la densite´ des larves d’ambystomes (salamandres du genre Ambystoma Tschudi, 1838; des pre´dateurs des teˆtards de Spea) et des ressources alimentaires sur l’occurrence des diffe´rents morphotypes. Le morphotype de carnivore se de´veloppe presque exclusivement chez S. bombifrons et rarement chez S. multiplicata, quelle que soit l’utilisation des terres. La disponibilite´ d’habitat de´termine´e par le taux de perte d’eau, de meˆme que le risque de pre´dation et l’aˆge des teˆtards, sont les facteurs explicatifs les plus importants de la proportion de carnivores. Il y a une relation positive entre la densite´ des Ambystoma et la proportion de carnivores et une relation ne´gative entre la diminution de la profondeur de l’eau et l’aˆge des teˆtards et la proportion de carnivores. La proportion la plus e´leve´e de carnivores a e´te´ observe´e dans les playas herbeuses ayant la plus forte densite´ d’Ambystoma pre´dateurs et les hydrope´riodes les plus longues et ayant connu un gain de la profondeur de l’eau. Il n’y a pas de corre´lation entre la densite´ des branchiopodes anostrace´s et la proportion de carnivores. L’utilisation des terres hautes, pour des cultures associe´es a` l’e´rosion, est en train de modifier la structure trophique des terres humides, ce qui influence en outre l’expression des morphotypes chez les teˆtards de Spea et la structure de la communaute´ amphibienne des playas. [Traduit par la Re´daction]
Introduction Polyphenism, the ability of a genotype to produce more than one alternative form of a trait (i.e., phenotype) in response to environmental conditions, occurs in temporally and spatially varying environments and (or) when certain fit-
ness trade-offs are conferred to each alternative morph (Smith and Sku´lason 1996; Schlichting and Pigliucci 1998). Larval amphibians are ideal to study polyphenism because of their large suite of plastic traits such as behavioral, morphological, and developmental traits (e.g., Rose and Armentrout 1976; Relyea and Hoverman 2003). Distinct
Received 22 August 2008. Accepted 14 January 2009. Published on the NRC Research Press Web site at cjz.nrc.ca on 25 February 2009. D.M. Ghioca-Robrecht.1,2 Wildlife and Fisheries Management Institute, Texas Tech University, Lubbock, TX 79409-2125, USA. L.M. Smith. Department of Zoology, Oklahoma State University, Stillwater, OK 74078, USA. L.D. Densmore. Department of Biological Sciences, Texas Tech University, Lubbock, TX 79401-3131, USA. 1Corresponding 2Present
author (e-mail:
[email protected]). address: P.O. Box 4274, Roanoke, VA 24015, USA.
Can. J. Zool. 87: 229–238 (2009)
doi:10.1139/Z09-006
Published by NRC Research Press
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carnivore and cannibal morphs are often found in amphibians inhabiting wetlands with dynamic hydroperiods (Bragg 1965; Rose and Armentrout 1976; Pomeroy 1981; Pfennig et al. 1991; Crump 1992). Although many experiments have been conducted to explore the individual contribution of factors inducing morph expression in the laboratory, it is not known whether these factors are important in nature when a variety of forces are placed on the population. The focus of our study was to use field data, as well as a model selection and inference approach based on information theory (Burnham and Anderson 2002), to investigate how a suite of environmental factors correlate and interact with morphotype frequency in a polyphenic species inhabiting wetlands with ephemeral aquatic phases. Carnivore morphology in tadpoles, especially in spadefoot toads (genus Spea Cope, 1866), is common and often correlated with unpredictable drying, resource shortages, and high densities (e.g., Bragg 1965; Duellman and Trueb 1986; Pfennig 1992b; Pfennig and Murphy 2000). In field conditions, Spea carnivore frequency has been correlated with fairy shrimp density (order Anostraca, families Streptocephalidae and Tamnocephalidae) and inversely related to hydroperiod length (Pfennig 1990). Wetlands with short hydroperiods often have high fairy shrimp densities and carnivore morphs prey more efficiently on fairy shrimp than omnivores (Pomeroy 1981; Frankino and Pfennig 2001). Pfennig (1992a) hypothesized that omnivores were better adapted to longer hydroperiod wetlands because they can store more fat reserves, which increases survival immediately after metamorphosis. Food availability can also affect age and size at metamorphosis. The size of New Mexico spadefoot toads (Spea multiplicata (Cope, 1863)) was negatively correlated with age (i.e., number of days) at metamorphosis when food was abundant, but size and age were positively correlated when food was scarce and the hydroperiod longer (Pfennig et al. 1991). Interspecific competition has also been associated with the frequency of morphotypes. In natural ponds, Pfennig and Murphy (2003) found that when S. multiplicata and Plains spadefoot toads (Spea bombifrons (Cope, 1863)) occurred separately, each species expressed similar frequencies of both phenotypes. However, S. multiplicata may produce fewer carnivores in sympatry than allopatry (i.e., almost always produced omnivores in sympatry), whereas S. bombifrons may produce more carnivores in sympatry than allopatry (Simovich 1985; Pfennig and Murphy 2003). This divergence was hypothesized to be caused by superior competitive abilities of S. bombifrons preying on fairy shrimp, whereas S. multiplicata was the superior competitor for detritus. As a consequence, when these species occur together, fairy shrimp density would likely affect the proportion of carnivores within S. bombifrons but not within S. multiplicata, thus also affecting the carnivore proportion in the pooled Spea population. Predator presence may also influence expression of phenotypes in larval amphibians. Several larval anurans have demonstrated morphological changes in the presence of predators (e.g., Van Buskirk et al. 1997; Relyea 2002), but this has not been examined in Spea species. Studies also have documented phenotypic response to competition (competition-induced traits) or predation (predation-induced
Can. J. Zool. Vol. 87, 2009
traits), with only a few examining phenotypic responses to the interaction of competition and predation (reviewed in Relyea 2004). Moreover, few studies have investigated mechanisms of phenotypic plasticity in natural-occurring communities or confirmed results of laboratory investigations with field studies (but see Pfennig and Murphy 2002, 2003). Therefore, relating the presence of phenotypes to variable environments through field studies is essential for documenting responses to changing conditions and providing evidence for mechanisms underlying morph occurrence in natural environments. Playa wetlands present a unique opportunity to study occurrence of phenotypes, owing to their ephemeral aquatic phases and various hydroperiod lengths (Smith 2003). This variability is frequently tied to the influence of land use in watersheds surrounding playas (cultivated vs. uncultivated). Playas surrounded by cropland often have shorter hydroperiods than those surrounded by native grassland because of erosional sedimentation (Luo et al. 1997; Gray and Smith 2005; Tsai et al. 2007). These habitat differences influence demographics and community structure of playa amphibians, including the barred tiger salamander (Ambystoma tigrinum mavortium Baird, 1850 (= Ambystoma mavortium mavortium Baird, 1850); herein A. t. mavortium) predator presence (Gray et al. 2004; Gray and Smith 2005; Ghioca and Smith 2008; Ghioca-Robrecht and Smith 2008). Therefore, we expected that there would be differences in competitive pressure and predation risk on larval amphibians between these land-use types. Using a multimodel selection approach, we investigated how environmental factors associated with different playas and the surrounding land use influenced Spea morph occurrence. Specifically, we hypothesized that higher desiccation stress (e.g., playa water-depth loss) and smaller wetland area would be positively associated with carnivore proportion because carnivores are able to metamorphose faster than omnivores. We hypothesized that higher proportions of carnivores would be observed in cropland than grassland playas and in a drier year because shorter hydroperiods would increase desiccation stress on the tadpoles. Tadpole age and densities of predators (larval salamander), spadefoot tadpoles, and fairy shrimp was expected to be positively associated with an increase in the proportion of carnivores because carnivores would be able to better avoid predators and prey on fairy shrimp than omnivores, thus avoiding competition with omnivores.
Materials and methods Study area We examined morphotype expression in S. multiplicata and S. bombifrons tadpoles in playa wetlands of the Southern Great Plains (SGP). Playas are shallow, circular wetlands, identified by the presence of a hydric soil, usually Randall clay (Bolen et al. 1989). They have a mean surface area of 6.3 ha (Guthery and Bryant 1982) and represent approximately 2% of the SGP landscape (Haukos and Smith 1994). Most playas have watersheds dominated by crops and few are surrounded by native grassland (Haukos and Smith 1994). The SGP was originally short-grass prairie dominated by gramas (genus Bouteloua Lag.) and buffalograss (Buchloe dactyloides (Nutt.) Engelm.) (Smith 2003). There Published by NRC Research Press
Ghioca-Robrecht et al.
has been an 80% decline in native short-grass prairie in Texas owing to cultivation (Samson and Knopf 1996). The climate of this area is dry-steppe with hot summers and mild winters (Smith 2003). Mean annual precipitation ranges from 45 cm in the northeastern SGP to 33 cm in the southwestern SGP; 54%–72% of this precipitation occurs as localized thunderstorms from May to September (Bolen et al. 1989). Thus, playas have variable hydroperiods. Field sampling We randomly selected four cropland and four grassland playa wetlands in Crosby and Randall counties, Texas, in 2002, and four cropland and four grassland playas in Floyd County, Texas, in 2003, after the first wetland-filling precipitation events occurred in spring (27 May 2002, 12 June 2002, and 3 June 2003; Ghioca 2005: 21). For the study area, monthly precipitation was higher in 2003 than 2002 (Crosby County, May and June 2002, received 6.93 cm; Randall County, June and July 2002, received 7.26 cm; Floyd County, June 2003, received 20.37 cm; National Oceanic and Atmospheric Administration 2002, 2003) with a resultant influence on hydroperiods. However, rainfall events in 2002 late summer and fall increased the hydroperiod length of 2002 grassland playas. We used a quadrat sampling method and sampled larvae by seining and dip-netting (Shaffer et al. 1994). Using a random numbers table and GPS units, we selected four sampling locations for seining and dip-netting at each playa, which were marked with stakes (Ghioca and Smith 2007). We were able to seine in 15 of 16 playas (unable to seine one playa because of dense vegetation) and we dip-netted in all 16 playas. Spea tadpoles were captured in 15 playas. We sampled each playa twice a week (every 4 days, on average), starting 8–14 days after playas were inundated when tadpoles were large enough to be retained by the sampling devices. Tadpoles had variable larval developmental periods. We found Spea tadpoles up to 43 days since inundation, although few tadpoles were captured after 30 days following playa flooding. In the field we sampled and assessed morphotype (see below) on 1352 Spea tadpoles (2002 cropland playas: 70, 50, and 27 tadpoles, respectively; 2002 grassland playas: 42, 78, 121, and 109 tadpoles, respectively; 2003 cropland playas: 160, 71, 60, and 79 tadpoles, respectively; 2003 grassland playas: 146, 134, 103, and 102 tadpoles, respectively). In 2003, we also collected fairy shrimp using a small dip net with a 15 cm 13 cm opening and 1 mm mesh size that was submersed and swept for 1 m at each seining location. Spea multiplicata and S. bombifrons tadpoles are difficult to distinguish morphologically (Bragg 1956; Altig 1970; Pfennig 1990). Therefore, we performed biochemical analysis for species identification using a protein electrophoresis method described by Simovich and Sassaman (1986). We used two allozyme markers (IDH-1, isocitrate dehydrogenase; MDH, malate dehydrogenase) to differentiate between the two species based on their migration patterns on starch gel. When present, we collected at least 10 Spea tadpoles from each playa by seining for a total of 269 tadpoles (2002 cropland playas: 10, 28, and 19 tadpoles, respectively; 2002 grassland playas: 13, 20, 17, and 16 tadpoles, respectively; 2003 cropland playas: 23, 13, 17, and 14 tadpoles, re-
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spectively; 2003 grassland playas: 25, 19, 17, and 18 tadpoles, respectively) and euthanized them in the field using MS-222, preserved them in dry ice during transportation, and immediately placed them at –85 8C in the laboratory for later identification. Before proceeding with protein electrophoresis, a morphotype was assigned to each tadpole in the laboratory. For Spea tadpoles, two extreme morphs (carnivore and omnivore) have been described (Bragg 1965). We used a qualitative evaluation of Spea morphs (Pfennig 1992a) by visually characterizing the relative size of its orbitohyoideus muscle (the primary buccal floor depressor muscle, OH) and keratinized beak morphology. Carnivores possess a keratinized beak, in contrast with the smooth mouth plates of the omnivores (Orton 1954); carnivores also have a larger OH (when standardized against snout– vent length) than omnivores (Pfennig 1990). We calculated carnivore and omnivore proportions for each species. Modeling carnivore expression Based on the smaller electrophoresis data set (n = 269 tadpoles), we found that the carnivore morph in S. multiplicata was extremely rare (see Results) and that almost all carnivores (98.3%) were S. bombifrons. We used the larger field data set (n = 1352 tadpoles) that included only carnivore versus omnivore tadpole field identification to be able to more accurately model the occurrence of carnivores (the proportion of carnivores of all Spea tadpoles) but interpreted carnivore proportion as S. bombifrons carnivore proportion. We considered nine predictor variables that could explain how the proportion of carnivores varied in different environmental conditions: (1) land-use type (Land); (2) year (Year); (3) playa water depth (Depth); (4) daily playa water depth loss (WLoss); (5) playa hydroperiod (Hydro); (6) playa area (Size); (7) density of Spea tadpoles (Spea); (8) density of larval A. t. mavortium (ATM); and (9) age of each tadpole as described by the number of days since inundation (Days). The first two variables selected were land-use type and year. By including these factors in the modeling process, we accounted for environmental influences particular to this study that might have affected the expression of carnivores. Cultivation could directly affect amphibians through, for example, pesticide runoff, or indirectly through increased sedimentation and reduced hydroperiods (Luo et al. 1997; Ghioca and Smith 2008). Year effect incorporates timing and frequency of precipitation events, amounts of precipitation, local historical effects owing to events accumulated over previous years (e.g., dryness for several years), and even geographical differences among playas. Because of localized thunderstorms that may not fill playas every year, we selected different sets of playas in the 2 years of study and modeled playa as a random effect. Desiccation risk also may influence production of Spea carnivores by increasing the need to grow and develop faster to metamorphose (Pfennig 1992a). Thus, we measured water depth, water-depth loss, and playa hydroperiod. Water depth (cm) at the time of each collection is a measure of desiccation stress given that a lower water depth may be associated with higher water temperature, lower dissolved oxygen concentration, or overcrowding. Daily water-depth loss (cm) is a weighted measure of desiccation because it accounts for Published by NRC Research Press
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Can. J. Zool. Vol. 87, 2009 Table 1. Pearson’s correlation coefficients (r), P values, and numbers of observations (n) corresponding to 21 pairwise correlations among seven biotic and abiotic variables measured in 13 playas of the Southern Great Plains of Texas, 2002 and 2003.
Daysa
Depthb
ATMc
Spead
WLosse
Sizef
Hydrog
r P n r P n r P n r P n r P n r P n r
Days 1
Depth –0.055 0.607 89 1
ATM 0.208 0.051 89 –0.275 0.009* 89 1
Spea –0.054 0.619 88 –0.476
