Paul E. Brunkow

Trophic control of fishless ponds by tiger salamander larvae

We examined the extent to which predation by larval tiger salamanders (Ambystoma tigrinum nebulosum) affected lower trophic levels in fishless ponds in montane eastcentral Arizona. An enclosure experiment manipulating salamander densities showed that effects associated with larval presence can cascade downward through three trophic levels in environments of low complexity. Enclosures with larvae contained comparatively lower herbivorous zooplankton biomasses and densities, higher chlorophyll a concentrations, and lower orthophosphate levels than those without larvae. Relative frequency of zooplankton species also varied significantly among enclosure treatments. Cyclopoid copepod frequencies were higher in treatments with salamanders than without, while ostracods were found only in treatments without salamanders. Salamander presence negatively affected cladoceran abundance and influenced cladoceran species composition. Proportions of both Ceriodaphnia quadrangula and Simocephalus vetulus were significantly greater in treatments without salamanders than in treatments with salamanders. Although Daphnia pulex frequencies were similar in enclosures with cladocerans across treatments, individuals were on average larger and biomasses were greater in enclosures without salamanders than they were in enclosures with salamanders. Trophic interactions analyzed from our survey of 14 ponds, however, were most often uncorrelated, and the few top-down effects exerted by salamanders were limited to contiguous trophic links below them. Densities of only one zooplankter, ostracods, and two taxa of macroinvertebrates, Baetis spp. mayflies and odonate larvae, were negatively related to salamander density. In contrast, benthic chironomid densities correlated positively with salamander density. However, in the two salamander-free ponds surveyed, we observed a distinct pattern of alternating responses of trophic levels that paralleled those in enclosure treatments without salamanders. Although modulating factors operate in ponds to complicate salamander impacts, salamanders have discernible direct and indirect effects on pond trophic webs.

Effects of Individual Variation in Size on Growth and Development of Larval Salamanders

Theoretical population models suggest that variation among individuals can affect population-level properties like stability and equilibrial density. Few studies, however, have tested the importance of individual variation to factors affecting population dynamics (e.g., average individual growth rates, developmental rates, survivorship, etc.) using natural populations. We conducted a field experiment designed to test effects of larval density and individual variability in body size on growth and development of larval tiger salamanders (Ambystoma tigrinum nebulosum). We performed our experiment in a natural sinkhole lake in central Arizona; experimental populations differing in density and initial variability in body size were held in nylon mesh enclosures. Effects of individual variability in size were examined independently of any effects due to mean body size of the population or total population biomass. Density affected growth rates and development to metamorphosis of experimental larvae in a manner consistent with other studies. Initial variability in body size did not affect larval growth rates or diets but did interact with density to affect survivorship and development: experimental larvae held with similarly sized individuals experienced lower survivorship and were metamorphosing earlier than experimental larvae held with variably sized individuals. These effects did not appear to be mediated by size- specific competitive abilities but may have been mediated by individual behavioral re- sponses to size structure. This study underscores the importance of incorporating infor- mation about interindividual variability into our understanding of processes affecting pop- ulation dynamics.

CAGE CULTURING THE ENDANGERED BONYTAIL

Abstract The bonytail Gila elegans is currently the most endangered of endemic Colorado River fishes primarily because of habitat loss and predation by introduced fishes. Captive propagation may be the only way of preventing extinction of the bonytail. We examined the effects of stocking density and initial size variability on growth and survivorship of juvenile bonytails reared in floating cages for 4 months. Three stocking densities, as well as three levels of fish size variability, were used in this experiment. We found that bonytails stocked at low and intermediate densities grew larger and had higher survivorship than those stocked at high density. We also observed that fish stocked in populations with low size variability generally grew less than fish stocked in populations with high size variability; initial size variability did not affect overall survivorship. Results of this study suggest that stocking juvenile bonytails in cages at approximately 18-22 fish/m3 and using individuals representing a...