William A. Dunson

The Role of Abiotic Factors in Community Organization

Very few ecological studies have attempted to fully test the integrative role of abiotic and biotic factors in interspecific interactions. There is little if any dispute about the value of such an approach in community ecology. However, there is great disagreement over the present direction of the field; we advocate a reemphasis of the integrative role of abiotic and biotic factors. By using literature examples and data on the role of salinity in reversing the competitive relations between fish of the genus Lucania, we have suggested a reemphasis on these interactions. This represents a blend of the traditional fields of ecology, physiology, and toxicology and could facilitate progress in community ecology. As such it could also be a significant part of the new direction sought for the field of physiological ecology. Finally, we are not arguing that abiotic parameters are necessarily more important than any of a group of multiple factors that may be involved in regulating community structure, only that they must be included as possible controlling variables in any complete study.

Effects of Low pH and Other Chemical Variables on the Local Distribution of Amphibians

mortality caused by low pH could be responsible for the absence of the two least tolerant species from acidic ponds. Hatching of embryos of R. sylvatica was not related to pond pH while mortality of embryos of A. jeffersonianum increased significantly as pond pH declined. H. andersoni also hatched in all experimental New Jersey ponds, but embryos of B. woodhousei suffered significantly higher mortality in ponds of lower pHs. Hatching success was variable at specific pond pHs, indicating significant interaction of pH with other chemical variables. In laboratory trials, tadpoles of B. woodhousei and H. andersoni grew significantly slower when exposed to low pH. This sublethal effect on body size has important implications for dynamics of amphibian populations in acidic ponds.

Impacts of hydroperiod on growth and survival of larval amphibians in temporary ponds of Central Pennsylvania, USA

The effects of variable hydroperiod (three levels) and initial density of amphibians (two levels) on survival, growth rate, and time to and mass at metamorphosis were studied for wood frogs (Rana sylvatica), Jefferson salamanders (Ambystoma jeffersonianum), and spotted salamanders (A. maculatum). Experiments were carried out in 260-1 mesocosms set up outdoors in a forest. These pond simulations were designed to mimic conditions that occur in palustrine temporary wetlands in central Pennsylvania. No animals reached metamorphosis in the short hydroperiod (56 days). However a greater proportion (66%) of tadpoles of R. sylvatica survived to the end of the 56-day, treatment than the 84- or 158-day treatments (29 and 14%, respectively), from which all survivors metamorphosed. In contrast, neither of the salamanders metamorphosed by 84 days; survival to metamorphosis at 158 days was 15% for A. jeffersonianum and 10% for A. maculatum. Average instantaneous growth rates for A. jeffersonianum decreased with each increase in hydroperiod. Growth of R. sylvatica was greater in the 56-day hydroperiod than in hydroperiods of 84 or 158 days. Initial amphibian density had no effect on growth or survival of any species. It appears that salamander larvae were predatory on tadpoles, since survival of R. sylvatica was negatively correlated with survival of A. jeffersonianum in 84-day treatments and with growth of A. maculatum in 158-day treatments.

Sodium Balance of Amphibian Larvae Exposed to Low Environmental pH

Sodium influx and efflux and body ionic contents of amphibian larvae were measured during acute and chronic exposures to acidic and neutral waters. Acute exposure to low pHs (2.5-4.0) depressed sodium influx and markedly accelerated sodium efflux. The resulting net loss of 50% of body sodium was fatal. Increasing the external calcium concentration extended survival time by slowing the loss of sodium. Chronic exposure to low pH caused a 21%-62% reduction in body sodium content; potassium content did not change. It also caused sodium influx to increase and efflux to decrease in comparison with fluxes measured during acute exposure at the same pH. Interspecific sensitivity differences were related to the relative magnitude of disruption in sodium balance. The three species of tadpoles studied differed in body sodium content under control conditions. Initial body sodium content was inversely correlated with acid tolerance and may represent a previously unknown mechanism for coping with physiologically stressful low-pH environments.

Anoxia and sodium loss associated with the death of brook trout at low pH

Abstract 1. 1. Brook trout exposed to pH 2·00–3·50 (HCl) showed a considerable decrease in or a complete inhibition of O2 consumption prior to death. Death from anoxia caused by KCN occurred in about the same time as death caused by exposure to very low pH (2·00, HCl). 2. 2. At low pH, trout suffered a massive loss of Na prior to death. The rate of net Na loss was inversely related to survival time. A saline medium (150 mM NaCl) had a protective effect, prolonging the life of trout at pH 3·25, but not at pH 2·00. 3. 3. The toxicity of H2SO4 (pH 3·25) was less than HCl of the same pH. Acid mine water was less toxic to brook trout than H2SO4. 4. 4. Despite differences in toxicity of solutions of HCl, H2SO4 and mine acid, survival time can be accurately predicted if the rate of net Na loss is known. 5. 5. The lethal effect of these acids appears to be due to inhibition of O2 uptake and to a large increase in net Na loss.

Genetic and environmental factors involved in increased resistance of brook trout to sulfuric acid solutions and mine acid polluted waters

Abstract Several strains of hatchery-reared brook trout, Salvelinus fontinalis (Mitchill), were exposed to low pH in the laboratory (sulfuric acid solutions) and in the field (mine acid polluted waters). Wild brook trout were also used in some field tests. Tests were both acute and chronic (up to 2 mo). Pronounced strain differences in survival ability were detected among embryonic, juvenile, and adult brook trout in laboratory tests, and among juvenile brook trout in field tests. However, in one case a strain difference in resistance times detected in field tests was not evident in several laboratory studies. A single selection of NYSV strain brook trout for high resistance to sulfuric acid solutions did not yield F1 progeny of greater resistance. Prior exposure to nonlethal acidic conditions also did not enhance resistance of brook trout. In some cases, the fish actually became more sensitive to the low test pH after such a prior exposure. Fish had longer resistance times in sulfuric acid solutions and ...

Field and Laboratory Studies of Ion Balance and Growth Rates of Ranid Tadpoles Chronically Exposed to Low pH

The effect of sublethal, chronic exposure of tadpoles to low environmental pH under natural conditions was investigated by making weekly measurements of various physiological variables. Rana sylvatica tadpoles from a low pH pond (pH 4.05-4.90) had lower body sodium, chloride and water concentrations in comparison with tadpoles from a nearby high pH pond (pH 5.74-6.37). Tadpoles from both ponds placed in the low pH pond also underwent higher sodium efflux than when placed in the high pH pond. These observations parallel similar studies on fish and demonstrate the effects of low environmental pH on larval anuran ionic regulatory mechanisms. Survival times of field-collected R. sylvatica tadpoles placed at pH 3.0 (H2SO4) did not change with age nor were there any differences between tadpoles from high and low pH ponds. However, survival times of laboratory raised R. pipiens tadpoles did increase with age, and animals raised at pH 4.4 grew slower than siblings at pH 5.8.

A Multilevel Study of Effects of Low pH on Amphibians of Temporary Ponds

Most researchers who have studied effects of low pH on amphibians that breed in temporary ponds measured direct lethal effects on single species. We summarize here results of a study conducted over 4 years in central Pennsylvania that illustrate the potential ecological importance of direct and indirect effects of low pH at lethal and sublethal levels. We argue for use of a multilevel approach (field sampling and experimentation in the laboratory, simulated ponds, and natural ponds) to study such effects on embryonic and larval amphibians of temporary ponds. A greater proportion of our study ponds was lower in pH than any reported previously in North America. Ponds were soft and often had high (Al,t,,,). Median seasonal pHs of 9 ponds were negatively associated with total amounts of rainfall over 4 years, but were not associated with H+ deposition during that same period. The pHs of many ponds declined as total rainfall increased. Embryonic mortality of the salamander Ambystoma jeffersonianum was high in ponds below pH 4.5. Less pH-associated embryonic mortality was observed among the more tolerant salamander, Ambystoma maculatum, and frog, Rana sylvatica. Laboratory-determined LC,, values for hatching of R. sylvatica, A. maculatum, and A. jeffersonianum were 4.10, 4.31, and 4.51, respectively, and predicted embryonic mortalities in ponds with good accuracy. More larval R. sylvatica survived at pH 4.1 in the laboratory when initially contained with A. jeffersonianum than at pH >6. This was due to reduced survival of, and subsequent reduced predation by, A. jeffersonianum at pH 4.1. In the same experiment, survival of A. maculatum was not different between pHs as they were eaten by A. jeffersonianum at pH >6 and suffered pH-induced mortality at pH 4.1. More larval A. jeffersonianum survived at pH 4.1 over 7 days when larval R. sylvatica and A. maculatum were available as prey, than when no prey were available. Larval A. jeffersonianum did not survive to metamorphosis in pH 4.2, and survived in low numbers in pH >6, in simulated ponds. Ambystoma maculatum metamorphosed less often, took longer to metamor- phose, and weighed less at metamorphosis in simulated ponds of pH 4.2 versus those of pH >6. Repro- duction of the newt Notophthalmus viridescens in simulated ponds was significantly lower, and adults were trapped more often exiting ponds, in pH 4.2 than in pH >6. Presence of adult N. viridescens in simulated ponds resulted in significantly lower survival of R. sylvatica at metamorphosis; survival was not significantly affected by pH 4.2. Our results in simulated ponds were likely due to a combination of lethal and sublethal effects that impinged either directly on individual larvae, or indirectly via the food web. These results suggest that studies focused only on direct lethal effects of low pH could substantially underestimate total impacts of low pH on amphibians of temporary ponds. Thus, in addition to the pronounced direct lethal effects we observed on A. jeffersonianum, the chronic and intermittent fluctu- ations in pHs in our study ponds that were associated with total rainfall could conceivably have induced less obvious effects throughout the food webs.

Some aspects of electrolyte and water balance in three estuarine reptiles, the diamondback terrapin, American and “salt water” crocodiles

Abstract 1. 1. Diamondback terrapins (Malaclemys terrapin) were caught in the wild at salinities between 11·3 and 31·8%. 2. 2. Acclimation of terrapins to sea water caused a 2·4-fold increasein the Na concentration of orbital fluid. Two species of crocodiles (Crocodylus acutus, C. porosus) showed only a 1·4-fold increase. 3. 3. Sea water acclimation in terrapins caused a rise in plasma Na concentration. The increase was relatively small over a 6-week period of acclimation. Wild-caught terrapins had a higher plasma Na concentration. 4. 4. Maximum rates of salt gland secretion in sea water-acclimated terrapins measured by a head rinse technique varied from 16·8 to 26·6 μmoles Na/100 g hr. In fresh-water-acclimated animals secretion was less than 1 μmole Na/100 g hr. The estuarine terrapin thus has a salt gland intermediate in secretory capacity between those of terrestrial and marine reptiles. 5. 5. Preliminary evidence provides little support for the concept that American and “salt water” crocodiles utilize salt glands as a major pathway of electrolyte excretion, although these crocodiles are fairly tolerant of exposure to sea water.

Effect of pH Variation of Interspecific Competition Between Two Species of Hylid Tadpoles

When species differ in their ability to cope with varying abiotic conditions, a change in an abiotic factor can potentially alter the outcome of biotic interactions. The purpose of this study was to examine this phenomenon. We tested the effects of an abiotic factor (pH) and two biotic factors (intraspecific and interspecific competition) on survival and growth to metamorphosis by larvae of two anurans (Hyla gratiosa and Hylafemoralis) in outdoor tanks. Two levels of pH (4.5 and 6.0) and three levels of density (0, 30, or 60 embryos per 580 L tank) of each species were arranged in a fractional factorial design and replicated three times. Larvae of H. gratiosa exhibited typical patterns of intraspecific density-dependent re- sponses: as density increased, survival and size at metamorphosis decreased and larval period increased. The interactions of the abiotic factor and the biotic factors were complex in this experiment. The addition of H. femoralis at high pH caused decreased survival and increased larval period of H. gratiosa, but at low pH these negative effects were not detected. Variation in pH had no effect on the size at metamorphosis of H. gratiosa. Interspecific competition from H. femoralis caused decreased size at metamorphosis of H. gratiosa only at the high initial densities. Some of the responses of H. femoralis were different than those of H. gratiosa. In this species, intraspecific competition affected only the size at metamorphosis. Low pH in- creased the survival of H. femoralis, but had no effect by itself on any other trait. Increased conspecific density and pH interacted only in their effect on the size at metamorphosis of H. femoralis. Interspecific competition resulted in decreased size at metamorphosis but only increased larval period at the high initial densities. Interspecific competition at the high density interacted with pH to increase survival and size at metamorphosis at low pH in contrast to those results at high pH. Overall, the larval period was the response most strongly affected in H. gratiosa while for H. femoralis the size at metamorphosis was the most strongly affected. Hyla gratiosa had more negative impact on H. femoralis than vice versa. This may be due to the larger body size of H. gratiosa. Low pH enabled H. femoralis to compete more effectively, because the negative impact of H. gratiosa was lessened. Even in a relatively simple simulation, complex interactions occurred between biotic and abiotic factors. This finding demonstrates the importance of multi-factor analysis in furthering our understanding of the structure of assemblages.