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Density‐Dependent Growth and Survival of Larval Ambystoma: Evidence from Whole‐Pond Manipulations
Author(s) -
Petranka James W.
Publication year - 1989
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1938109
Subject(s) - larva , biology , zooplankton , metamorphosis , ecology , biomass (ecology) , population density , zoology , invertebrate , hatchling , growth rate , population , hatching , demography , geometry , mathematics , sociology
To determine whether growth and survival of marbled salamander larvae were density dependent under natural conditions, I established hatchlings at either natural density or one—fourth natural density in paired halves of 11 natural ponds. I sampled ponds at 7, 11, and 15 wk and at the initiation of metamorphosis, comparing percentage of larvae injured, larval size, and larval survivorship. I also compared density, size, and biomass of zooplankton, the dietary staple of larvae. Reducing density increased mean larval body size on all sample dates except week 7, and mean size at the initiation of metamorphosis was negatively correlated with average larval density during the larval period. Mean size at the initiation of metamorphosis was correlated positively with average zooplankton biomass during the larval period, which suggests that food was limiting in many ponds. Surprizingly, however, reducing larval density did not increase zooplankton biomass, number, or density relative to controls. Density reductions significantly increased larval survival on all sample dates. Many populations exhibited Type II survival, but survival tended towards Type I at sites where ponds contracted severely near the end of the larval period. Mortality associated with pond contraction was due to changes in biotic or physical factors associated with pond contraction, and not to desiccation per se. Most larvae suffered tail damage caused by attacks from conspecifics and/or invertebrates. high larval mortality in contracted ponds was associated with increased larval density and injury frequency. The proportion of animals showing tail damage was significantly less in reduced—density treatments compared to natural—density treatments. Tail—damaged animals were smaller than undamaged animals, perhaps because loss of tail tissue compromised growth. Because larvae did not reduce zooplankton biomass, it is unlikely that exploitative competition for zooplankton was the primary cause of density—dependent growth and survival. My data suggest that intraspecific aggression may be more important than exploitative competition in explaining density dependence in Ambystoma larvae.