Effects of density and predation on Scaphiopus couchi tadpoles in desert ponds

Summary The effects of density on growth and development of Scaphiopus couchi tadpoles in desert ponds were investigated, and sources of mortality over a three-year period were documented. In 16 of the 82 ponds monitored, predation was the principal cause of death, demonstrating that tadpoles in desert ponds may be exposed to high levels of predation, although the overall importance of predation is less here than in more mesic areas. Desiccation was the primary cause of mortality in 49 ponds. Growth and development were extremely slow in most high density ponds and as a result most tadpoles were unable to metamorphose before ponds dried. Only 8 ponds produced metamorphs, and mortality was high even in these. Food-supplementation resulted in some metamorphosis in high density ponds, although the effect was diminished by the extreme crowding in most ponds. In low density ponds, S. couchi tadpoles can develop very quickly and metamorphose. High mortality due to desiccation is largely a consequence of high density: tadpoles rarely completed development in high density ponds, regardless of pond duration.     

Do tadpoles affect leaf decomposition in neotropical streams?

1. Of the relatively few studies that have examined consequences of amphibian declines on stream ecosystems, virtually all have focused on changes in algae (or algal‐based food webs) and little is known about the potential effects of tadpoles on leaf decomposition. We compared leaf litter decomposition dynamics in two neotropical streams: one with an intact community of tadpoles (with frogs) and one where tadpoles were absent (frogless) as a result of a fungal pathogen that had driven amphibians locally extinct. The stream with tadpoles contained a diverse assemblage (23 species) of larval anurans, and we identified five species of glass frog (Centrolenidae) tadpoles that were patchily distributed but commonly associated with leaf detritus and organic sediments in pools. The latter reached total densities of 0–318 tadpoles m^?2. 2. We experimentally excluded tadpoles from single‐species leaf packs incubated over a 40‐day period in streams with and without frogs. We predicted that decomposition rates would be higher in control (allowing access of tadpoles) treatments in the study stream with frogs than in the frogless stream and, in the stream with frogs, in the control than in the tadpole exclusion treatment. 3. In the stream with frogs, Centrolene prosoblepon and Cochranella albomaculata tadpoles were patchily distributed in leaf packs (0.0–33.3 m^?2). In contrast to our predictions, leaf mass loss and temperature‐corrected leaf decomposition rates in control treatments were almost identical in our stream with frogs (41.01% AFDM lost, k_{degree day} = ?0.028 day^?1) and in the frogless stream (41.81% AFDM lost, k_{degree day} = ?0.027 day^?1) and between control and tadpole exclusion treatments within each stream. Similarly, there were no significant differences in leaf pack bacterial biomass, microbial respiration rates or macroinvertebrate abundance between treatments or streams. Invertebrate assemblages on leaf packs were similar between treatments (SIMI = 0.97) and streams (SIMI = 0.95) and were dominated by larval Chironomidae, Simuliidae (Diptera) and larval Anchytarsus spp. (Coleoptera). 4. In contrast to dramatic effects of grazing tadpoles on algal communities observed previously, tadpoles had no major effects on decomposition. While centrolenid tadpoles were common in the stream with frogs, their patchy distribution in both experimental and natural leaf packs suggests that their effects on detrital dynamics and microbes are probably more localised than those of grazing tadpoles on algae.     

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free‐swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.     

Rana cascadae tadpoles aggregate with siblings: an experimental field study

Previous laboratory studies have shown that Rana cascadae larvae preferentially associated with siblings over non-siblings in choice tests. This study, conducted during three consecutive summers, tests the hypothesis that R. cascadae larvae aggregate nonrandomly with respect to sibship in natural ponds. Pairs of sibships were reared in separate tanks or together in the same tank in the laboratory. Each sibship within a pair was then stained with neutral red or methylene blue dye and released together in a natural pond. Over a period of several days, aggregations of tadpoles within test ponds were repeatedly captured, censused for sibship composition, and released. In control tests, two groups of tadpoles from the same sibship were dyed different colors and released together. In total, 25 different tests were conducted using tadpoles from 31 sibships and 456 aggregations were sampled. The distribution of color compositions of aggregations in control tests did not differ from an expected random distribution. Color compositions of aggregations in experimental tests differed from controls and from an expected random distribution. Aggregations in these tests tended to be dominated by one of the two colors (sibships). We conclude that R. cascadae tadpoles recognize and prefer to aggregate with siblings in natural field conditions. Circumstances of early rearing (i.e., whether tadpoles were reared with siblings or in mixed sibling/non-sibling groups) had no influence on preferences to associate with siblings, but there was an inverse correlation between group size and sibling association.     

PERFORMANCE OF TADPOLES FROM THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX: INTERACTIONS WITH POND DRYING AND INTERSPECIFIC COMPETITION

The performance of three genotypes (LL, LR, RR) of tadpoles resulting from the hybrid mating system of Rana lessonae (phenotype L, genotype LL) and Rana esculenta (phenotype E, genotype LR) was determined in artificial ponds. The effects of interspecific competition and pond drying on growth, development, and survival of tadpoles were used to measure the performance of genotypes and the relative fitness of offspring. Among the three genotypes, tadpoles from the homogametic mating RR had the lowest survival, growth, and development under all environmental conditions. Body size of the LL and LR genotype tadpoles at metamorphosis was reduced by competition and pond drying. Days to metamorphosis were also higher for the LL and LR genotype tadpoles in competition ponds. The proportion of individuals metamorphosing of each genotype was differentially lowered by competition and pond drying. The LL genotype produced more metamorphs than the LR genotype in the constant water level ponds, but the LR genotype produced more in drying ponds. In competition ponds, the LR genotype produced more metamorphs than the LL genotype, but the LL genotype produced more metamorphs in ponds without competition. The RR genotype produced no metamorphs in any of the experimental environments. Increased performance of LR offspring from the heterogametic mating, in harsh conditions, and reduced performance of RR offspring from the homogametic mating, even under favorable conditions, relative to the parental genotype (LL) suggests that the population dynamics of this hybridogenetic system is strongly dependent on mate choice in mixed populations and the subsequent pond environment females select for oviposition and larval development.     

Synergistic effects of multiple mechanisms drive priority effects within a tadpole assemblage

Mounting evidence suggests that the history of species arrival to a locality can have important effects on species performance but the mechanism(s) through which priority effects are produced is not always clear. Differences in the developmental time of frog tadpoles provide an opportunity to examine mechanisms through which priority effects may influence fitness components of a late arriving taxon. Specifically, tadpoles of the southern leopard frog (Rana) can often require more than a year to complete metamorphosis so they overwinter in a pond and compete with newly deposited tadpoles in the spring. We conducted an experiment in artificial ponds to evaluate mechanisms through which overwintering Rana tadpoles influence fitness components of southern toad (Bufo) tadpoles deposited into ponds during the spring. We found that overwintered Rana reduced Bufo performance while newly hatched Rana that enter a pond simultaneously with Bufo did not. The production of this priority effect was primarily the result of Rana depleting algal resources in a pond during the winter prior to Bufo arrival. The performance of Bufo did not correspond with variation in the abundance of algae present in a pond during the spring and we found evidence to indicate that, in the absence of resource exploitation during the winter, overwintered Rana do not compete strongly with Bufo during the spring when both species co‐occur. When Rana deplete algal resources during the winter, however, interactions between Rana and Bufo during the spring became much more important as both the development rate and survival of Bufo was reduced to a greater extent than what would have been predicted by resource exploitation alone. Our results demonstrate that priority effects can result from early colonists depleting resource availability prior to the arrival of other species which can intensify behavioral/physical interactions between species when they co‐occur.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Naturally occurring variation in tadpole morphology and performance linked to predator regime

Divergent natural selection drives a considerable amount of the phenotypic and genetic variation observed in natural populations. For example, variation in the predator community can generate conflicting selection on behavioral, life‐history, morphological, and performance traits. Differences in predator regime can subsequently increase phenotypic and genetic variations in the population and result in the evolution of reproductive barriers (ecological speciation) or phenotypic plasticity. We evaluated morphology and swimming performance in field collected Bronze Frog larvae (Lithobates clamitans) in ponds dominated by predatory fish and those dominated by invertebrate predators. Based on previous experimental findings, we hypothesized that tadpoles from fish‐dominated ponds would have small bodies, long tails, and large tail muscles and that these features would facilitate fast‐start speed. We also expected to see increased tail fin depth (i.e., the tail‐lure morphology) in tadpoles from invertebrate‐dominated ponds. Our results support our expectations with respect to morphology in affecting swimming performance of tadpoles in fish‐dominated ponds. Furthermore, it is likely that divergent natural selection is playing a role in the diversification on morphology and locomotor performance in this system.     

不同程度的尾损伤对镇海林蛙(Rana zhenhaiensis)蝌蚪游泳速度的影响

无尾两栖类蝌蚪的尾巴通过产生强大的游泳速度在反捕食中起到了重要的作用。以镇海林蛙(Rana zhenhaiensis)蝌蚪为实验动物来评估断尾的运动代价。以74尾具有完整尾蝌蚪作为实验组,通过截去不同尾长片段,人为分成轻微尾损伤组(30%)和严重尾损伤组(30%)并测定两组蝌蚪在断尾前后的游泳速度。以16尾完整尾蝌蚪作为对照组在实验组断尾前后同时进行游泳速度的测定。实验结果显示断尾影响蝌蚪的游泳速度,但仅在尾损伤程度达到尾长的30%以上时才产生不利的影响。这表明轻微尾损伤并不对镇海林蛙蝌蚪的游泳速度产生严重影响。在断尾前后实验组蝌蚪的游泳速度均与尾长呈正相关。在相同尾长状态下,尾损伤蝌蚪的相对游泳速度明显快于完整尾蝌蚪。因此,尾损伤的镇海林蛙蝌蚪有可能通过改变尾和身体的摆动频次等方式在断尾后对游泳速度进行了一定的补偿。尾损伤在野外频繁发生于蝌蚪的尾远端,据此推测镇海林蛙蝌蚪在自然条件下的尾损伤并不会产生严重运动代价。     

Factors affecting the distribution and abundance of an unpigmented heterotrophic alga Prototheca richardsi

1. Numbers of the contramensal alga Prototheca richardsi were high in spring in two ponds used for breeding by anuran amphibians, but lower at other times of year and undetectable in two ponds not used by anurans. 2. Prototheca richardsi became abundant in the silt of eight experimental ponds which contained tadpoles, but remained undetectable in four otherwise identical ponds lacking tadpoles. 3. Prototheca richardsi numbers in laboratory microcosms remained stable for many days in sterile tap water, but declined with a half-life of about 6 days in pond water at 20°C. 4. Further studies with microcosms using antibiotics and electron microscopy indicated that mortality of P. richardsi was caused primarily by pathogenic bacteria.     

Competing tadpoles: Australian native frogs affect invasive cane toads (Rhinella marina) in natural waterbodies

The cane toad (Rhinella marina) is one of the most successful invasive species worldwide, and has caused significant negative impacts on Australian fauna. Experimental work in the laboratory and in mesocosms has shown that tadpoles of native frogs can affect survival, size at metamorphosis and duration of larval period of cane toad tadpoles. To test if these effects occur in nature, we conducted a field experiment using two temporary ponds where we set up enclosures with tadpoles of native green tree frogs (Litoria caerulea) and cane toads in treatments with a range of densities and combinations. The presence of green tree frog tadpoles significantly decreased the growth rate of toad tadpoles and increased the duration of their larval period in both ponds; in one pond, frog tadpoles also significantly reduced the body length and mass of metamorph toads. Toad tadpoles did not have any significant negative effects on green tree frog tadpoles, but there was strong intraspecific competition within the latter species: increased frog tadpole density resulted in increased larval period and reduced survival, growth rate and size at metamorphosis for frogs at one or both ponds. Our results are encouraging for the possibility of using native frogs as one component of an integrated approach to the biological control of cane toads.     

The role of omnivorous crayfish in littoral communities

Large omnivorous predators may play particularly important roles determining the structure of communities because of their broad diets and simultaneous effects on multiple trophic levels. From June 2001 to June 2002 we quantified community structure and ecosystem attributes of six newly establishing freshwater ponds (660 m² each) after populations of omnivorous crayfish (Orconectes virilis) were introduced to three of the ponds. Crayfish preyed heavily on fish eggs in this experiment, which reduced recruitment of young-of-year fish. This effect indirectly enhanced zooplankton biomass in crayfish ponds. Phytoplankton abundance exhibited a more complex pattern and was probably influenced by non-trophic (e.g., bioturbation) effects of crayfish. Peak dissolved oxygen levels were lower in the crayfish ponds indicating that they had lower primary production: respiration ratios. Metaphytic algae were strongly affected by crayfish presence; filamentous greens quickly disappeared and the blue-green Gleotrichia (a less preferred food item) eventually dominated the composition in crayfish ponds. Chara vulgaris and vascular macrophytes established 34% cover in control ponds by June 2002, but were not able to establish in crayfish ponds. Two important periphyton herbivores (tadpoles and gastropods) were absent or significantly reduced in the crayfish ponds, but periphyton differences were temporally variable and not easily explained by a simple trophic cascade (i.e., crayfish—snails and tadpoles—periphyton). Our results indicate that crayfish can have dramatic direct and indirect impacts on littoral pond communities via feeding links with multiple trophic levels (i.e., fish, invertebrates, and plants) and non-trophic activities (bioturbation). Although the effects of omnivorous crayfish on littoral communities can be large, their complex effects do not fit neatly into current theories about trophic interactions or freshwater community structure.     

Field Response of Tadpoles to Conspecific and Heterospecific Alarm

Many organisms use chemical cues from a variety of sources to mediate predator avoidance. Response to heterospecific alarm cues has been demonstrated for tadpoles within but not among taxa and alarm response behavior has seldom been examined under field conditions. This study examined the response of three sympatric amphibian larvae and predaceous larval Dytiscus sp. (diving beetle) to damage-release signals in natural ponds by using capture rates from treated funnel traps as an index of larval behavior. Hyla regilla (Pacific tree frog) tadpoles avoided traps treated with either crushed conspecifics or with Rana aurora (red-legged frog) tadpoles but the larger ranids and Ambystoma macrodactylum (long-toed salamander) did not respond to either treatment. H. regilla tadpoles were likely susceptible to any potential predators of ranid tadpoles in these ponds and this result is consistent with the hypothesis that a response to heterospecific alarm occurs in sympatric prey with shared predators.     

Risk of predation and behavioural response in three anuran species: influence of tadpole size and predator type

Many species alter their activity, microhabitat use, morphology and life history in response to predators. Predation risk is related to predator size and palatability of prey among others factors. We analyzed the predation risk of three species of tadpoles that occur in norwestern Patagonia, Argentina: Pleurodema thaul, Pleurodema bufoninum and Rhinella spinulosa. We sampled aquatic insect predators in 18 ponds to determine predator–tadpole assemblage in the study area. In laboratory conditions, we analysed the predation rate imposed by each predator on each tadpole species at different tadpole sizes. Finally, we tested whether tadpoles alter their activity in the presence of chemical and visual cues from predators. Small P. thaul and P. bufoninum tadpoles were the most vulnerable prey species, while small R. spinulosa tadpoles were only consumed by water bugs. Dragonflies and water bugs were the most dangerous tadpole predators. Small P. thaul tadpoles reduced their activity when they were exposed to all predators, while large tadpoles only reduced the activity in the presence of large predators (dragonfly larvae and water bugs). Small P. bufoninum tadpoles reduced the activity when they were exposed to beetle larvae and dragonfly larvae, while large tadpoles only reduced activity when they were exposed to larger predators (water bugs and dragonfly larvae). R. spinulosa tadpoles were the less sensitive to presence of predators, only larger tadpoles responded significantly to dragonfly larvae by reducing their activity. We conclude that behavioural responses of these anuran species were predator-specific and related to the risk imposed by each predator.     

Direct and indirect effects of dragonfly (<Emphasis Type="Italic">Anax imperator</Emphasis>) nymphs on green toad (<Emphasis Type="Italic">Bufo viridis</Emphasis>) tadpoles

We conducted an artificial pond experiment to assess the direct and indirect effects of predation on Bufo viridis tadpoles. We ran three treatments: free Anax (unrestrained predatory dragonfly nymph Anax imperator), caged Anax (non-consumptive effects), and control (no Anax). Anax showed both strong consumptive and non-consumptive effects on Bufo tadpoles. Free Anax eliminated all of the tadpoles within six days. Tadpoles preferred the shady side of the ponds. Caged Anax caused tadpoles to increase their spatial preferences. Tadpoles avoided the center of the pond, and in the presence of the caged predator, they were found in the center even less. Tadpoles also showed a strong preference for crowding together, and in the presence of a caged Anax, they tended to crowd more. Moreover, Bufo metamorphosed earlier and at a larger size in the caged Anax ponds, possibly by providing extra food resources due to the extra organic matter excreted by the predators. Handling editor: K. Martens     

Developmental stability and genetic architecture: a comparison within and across thermal regimes in tadpoles

We investigated homogeneity of growth and development as indices of developmental stability in sibling tadpoles from two sampling regions of the common frog, Rana temporaria. One region is characterized by relatively warm breeding ponds with a short activity season (`north'), and one by relatively cool breeding ponds and a long activity season (`south'). Tadpoles from the two regions were raised in three different temperatures selected to mimic the natural variation throughout the range. The results show that (1) north tadpoles respond with a relatively greater increase in growth with increased temperature than south tadpoles, (2) mean growth rate and its coefficient of variation were negatively correlated in the temperature regime in which a population was primarily under selection in the wild, whereas no such correlation was found at temperatures more seldom encountered in the natural populations, (3) phenotypic and genetic correlations between morphological traits within individuals were positive and were relatively higher in north than south tadpoles in the warm treatment, but higher for south tadpoles in the cold treatment and (4) across thermal environments, south tadpoles showed significant genetic correlations, whereas the correlations for north tadpoles were not significantly different from zero. South tadpoles showed only positive genetic correlations (n=30), whereas 14 of 30 correlation coefficients were negative in north tadpoles. In conclusion, developmental stability for growth and morphometry was higher at `optimal' conditions and decreased at the tail ends of the reaction norms within regions, with marked differences reflecting selection history between regions.     

Ostracoda and Amphibia in temporary ponds: who is the prey? Unexpected trophic relation in a mediterranean freshwater habitat

Small and temporary freshwater ecosystems are important biodiversity “hot spots” of the Mediterranean region, and their food webs are considered as very complex systems. Amphibians and ostracods are two highly ubiquitous classes of metazoans adapted to live in temporary ponds. Their trophic interactions are considered unidirectional, the amphibians acting as predators and the ostracods as preys. In the field, we observed the opposite interaction in few ponds in Northern Italy. To confirm this qualitative evidence, we set up laboratory experiments to investigate the predation by the Ostracod mussel shrimp (Heterocypris incongruens) on eggs and tadpoles of Common toad (Bufo bufo) and Stripless tree frog (Hyla meridionalis). Amphibian eggs of both species were offered to ostracods either as unique trophic resource or, alternatively, together with another kind of food. Similarly, tadpoles of both species were simultaneously offered to ostracods (with alternative food) to disclose their preferences. Ostracods preyed mainly on amphibian eggs and no significant differences in the rate of predation between toad and treefrog eggs were detected. However, ostracods preferred Bufo when offered along with Hyla tadpoles. Toad eggs and larvae are commonly considered highly unpalatable, but our results contrasted this view. The difference in the predation rate between the two tadpole species is discussed in the light of their swimming behaviour. We show that feeding relationships between Amphibia and Ostracoda are much more complex than expected and depend on both the ecological context and amphibian life stage. The knowledge of the trophic connections among taxa is a fundamental prerequisite to further and more exhaustive studies on community ecology.     

Pond tadpoles with generalized morphology: is it time to reconsider their functional roles in aquatic communities?

With rare exceptions, anuran larvae have traditionally been considered to occupy lower trophic levels in aquatic communities where they function as microphagous suspension feeders. This view is being challenged by studies showing that tadpoles with generalized morphology often function as macrophagous predators. Here, we review the literature concerning macrophagy by tadpoles and provide two additional examples involving generalized tadpoles. In the first, we demonstrate with laboratory and field experiments that wood frog (Rana sylvatica) tadpoles are major predators of macroinvertebrates in ponds. In the second, we show that green frog (R. clamitans) tadpoles can cause catastrophic reproductive failure of the wood frog via egg predation. These results and data from other studies challenge the assumption that generalized tadpoles function as filter-feeding omnivores, and question the general applicability of community organization models which assume that predation risk increases with pond permanence. We suggest that predation risk is greater in temporary ponds than in more permanent ponds for many organisms that are vulnerable to predation by tadpoles. This being so, a conditional model based upon interactions that are species-specific, life-stage-specific, and context-dependent may better explain community organization along hydrological gradients than models which assume that temporary ponds have few or no predators. Received: 30 November 1998 / Accepted: 2 May 1999     

COSTS AND BENEFITS OF A PREDATOR-INDUCED POLYPHENISM IN THE GRAY TREEFROG HYLA CHRYSOSCELIS

The phenotypes of gray treefrog (Hyla chrysoscelis) tadpoles vary depending on whether predators are present in the pond. Tadpoles reared in ponds with predatory dragonfly larvae are relatively inactive compared with tadpoles in predator-free ponds, and have relatively large, brightly colored tailfins with dark spots along the margins. Models for the evolution of plasticity predict that induced phenotypes such as this should confer high fitness relative to the typical phenotype when in the presence of predators, but should be costly when the predator is absent. Our study tested for the predicted fitness trade-off in H. chrysoscelis by first rearing tadpoles in mesocosms under conditions that induce the alternate phenotypes, and then comparing the performance of both phenotypes in both environments. We generated the two phenotypes by rearing tadpoles in 600-liter outdoor artificial ponds that contained either two caged dragonflies (Anax junius) or an empty cage. Tadpoles from the two environments showed significantly different behavior, tail shape, and tail color within two weeks of exposure. We compared the growth and survival of both phenotypes over four weeks in ponds where there was no actual risk of predation. Under these conditions, both phenotypes grew at the same rate, but the predator-induced phenotype had significantly lower survival than the typical phenotype, indicating that induced tadpoles suffered greater mortality from causes other than odonate predation. We tested the susceptibility of both phenotypes to predation by exposing them to dragonflies in 24-h predation trials. The predator-induced phenotype showed a significant survival advantage in these trials. These results confirm that the predator-induced phenotype in H. chrysoscelis larvae is associated with fitness costs and benefits that explain why the defensive phenotype is induced rather than constitutive.     

Summer algal communities and primary productivity in fish ponds

The paper presents data on primary productivity and phytoplankton communities in new experimental ponds which received the following treatments; ammonium nitrate and triplesuperphosphate, triplesuperphosphate, cracked corn (10% crude protein) and Auburn No. 3 fish feed (36% crude protein). Comparative data on algal communities were also obtained from production ponds which received feeds or fertilizers. Basic ecological data on macro-algae are also presented.

1. All nutrient additions to experimental ponds resulted in higher levels of gross photosynthesis and greater concentrations of chlorophyll a than were found in the control treatments. Fertilization with both nitrogen and phosphorus gave the highest values. Chlorophyll a and gross photosynthesis were higher in ponds receiving high protein content feed (Auburn No. 3) than in ponds to which low protein content feed (corn) was applied.
2. Persistent blooms of blue-green algae occurred in ponds receiving nitrogen and phosphorus fertilization. Phosphorus only fertilization produced blooms of blue-greens, but these blooms did not persist as in the ponds to which nitrogen was also added. Control ponds were dominated by green algae. Blue-green algae were seldom abundant in feed treatments.
3. Production ponds had high level of gross photosynthesis and large concentrations of chlorophyll a.
4. Many of the production ponds which received feed applications developed heavy blooms of blue-green algae.
5. The major species of blue-green algae observed in the present study were Oscillatoria sp., Raphidiopsis curvata, Anacystis nidulans, A. aeruginosa, Spirulina sp., and Anabaena circinalis. Heterocyst bearing forms, which can presumably fix nitrogen, were seldom noted in ponds that received continuous additions of nitrogen from fish feeds.
6. Macro-algae are abundant in many fish ponds. Data illustrating the competition of macro-algae with phytoplankton are presented.