Kim W. Kratz

EFFECTS OF POPULATION DENSITY ON INDIVIDUAL GROWTH OF BROWN TROUT IN STREAMS

Some studies suggest that lotic populations of brown trout (Salmo trutta) are regulated through density-dependent mortality and emigration to the extent that mean growth rates of resident survivors are unrelated to trout densities. To test this, we studied the relationship between density and growth, mortality, and emigration of brown trout in two alpine streams and a set of stream channels in eastern California. We sampled trout at the scale of “segments” (5–31 m long riffles, runs, and pools) and “sections” (340–500 m in length) of Convict Creek over a 3-yr period. Trout were also sampled during 6 yr in seven 90-m sections of Mammoth Creek. For 2 yr, we manipulated trout densities in Convict Creek by removing trout from two sections and adding trout to two other sections. We also manipulated densities in seven 50-m stream channels, using a natural size distribution of trout in one year and underyearlings only in a second year. In both streams, average size (body length or mass) of underyearlings in fall...

Effects of Multiple, Predator‐Induced Behaviors on Short‐term Producer‐Grazer Dynamics in Open Systems

We investigated the population consequences of multiple behavioral responses of grazers to a foraging return–predation risk trade‐off in an open system consisting of primary producers, grazers, and predators. Using a dynamical model where grazers adjust their foraging activity and emigration rate to the densities of predators and producers, we explored how changes in control variables (predator density, grazer immigration, and producer immigration and carrying capacity) affect the dynamics of producers and grazers at temporal scales shorter than consumer and predator reproduction. The model predicts that producer biomass increases and that both the density of foraging grazers and the feeding rate of predators decrease with predator density. These predictions hold although total ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Effects of an invertebrate grazer on the spatial arrangement of a benthic microhabitat

\mathrm{foraging}\,+\mathrm{nonforaging}\,

Effects of an experimental acid pulse on invertebrates in a high altitude Sierra Nevada stream

\end{document} ) grazer density may actually increase with predator density. The latter will occur whenever the benefit of higher resource density outweighs the increased risk of predation. In this case, per capita grazer emigration decreases with predator density, which might be misinterpreted as a direct “freezing” response to predators. Increased grazer immigration is predicted to result in decreased producer density and increased densities of both foraging and total grazers, as well as increased grazer emigration and predator feeding rates. Increased producer immigration or carrying capacity should increase producer and grazer densities and predator feeding rate but decrease per capita grazer emigration. Manipulation of predator (trout) densities in a set of nine large (50 m2) stream channels produced results in broad agreement with model predictions. Most notably, a positive effect of trout on benthic algal biomass was mainly mediated through grazer behavior (changes in the use of epibenthic surfaces and in emigration rate) rather than through consumptive reductions of grazer numbers by trout.

Implications of scale for patterns and processes in stream ecology

We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer rep ...

Quantifying spatial heterogeneity in streams

We demonstrated the effect of an aquatic herbivore on the spatial arrangement of benthic algal biomass within artificial stream channels. Transects of ceramic tiles were exposed to a gradient of snail (Physella) densities in a 30 d experiment. We observed positive effects of snails on the mean abundance of “overstory” algae (the filamentous chlorophyte Cladophora and associated epiphytes), an important benthic microhabitat in streams. Snails altered several aspects of the spatial arrangement of overstory algae. Snails reduced the strength of downstream gradients in overstory biomass, as well as residual variability around these gradients. Geostatistical analysis revealed that snails also reduced the strength of spatial dependence, and so reduced spatial heterogeneity of the overstory, at small scales (<40 cm). As a result, organisms inhabiting the overstory might experience a more fragmented habitat landscape at high snail densities. In addition, snails increased the scale of spatial dependence in understory algal biomass (algae remaining on tiles after overstory was removed) from 10 cm to 40 cm. Consumer effects on the spatial arrangement of a microhabitat argue for the inclusion of feedbacks between the biota and the environment in spatially-explicit models.

Influences of natural acidity and introduced fish on faunal assemblages in California alpine lakes

Research on predator impact in streams has generally ignored density-de- pendent effects. Although predator and prey behaviors are known to influence the impact of stream predators on local prey populations, no study has simultaneously addressed how predator impacts, and behavioral mechanisms contributing to those impacts, vary with prey density. I assessed the effect of a stream invertebrate predator (stonefly) on local prey (baetid mayfly) density across a gradient of six prey densities in in situ channels with natural substrata. Calculations of predator foraging rates and prey responses were based, in part, on an empirically based model assuming an exponential decline in prey densities. Total predator impact (measured as the negative natural log of the ratio of prey remaining in predator vs. predator-free channels) across prey densities followed a unimodal curve, with reduced predator effects at high and low, relative to intermediate, prey densities. Although direct predator consumption was more important than predator-induced emigration in explaining overall prey losses across all prey densities (65 vs. 35%), the relative im- portance of each loss process varied across individual prey densities. Total baetid drift increased with, and per capita drift was independent of, baetid benthic density in all predator treatments. Total and per capita drift rates of baetids were increased at least 3.5 times in the presence of predatory stoneflies. Few baetids were observed on the tops of rocks in either control or predator treatments during the day; however, baetid numbers on upper rock surfaces increased substantially at night and were significantly reduced by stonefly presence. The proportion of baetid populations on surface substrata was low and unaffected by predator presence during the day (0.3%) but was reduced in predator relative to predator- free channels at night (4 vs. 14%). Predatory stoneflies exhibited a type III functional response, and stonefly emigration decreased as initial baetid density increased. These results suggest that discrepancies in predator impacts between previous studies may be owing to prey density and reflect prey and predator behaviors that vary with prey density.

Effects of single and repeated experimental acid pulses on invertebrates in a high altitude Sierra Nevada stream

The effects of pulsed acidification on invertebrate densities and drift, and water chemistry, in a high altitude Sierra Nevada stream were measured using artificial stream channels. Water was diverted from the Marble Fork of the Kaweah River, California, U.S.A., through twelve replicate channels; however, low flow in the summer of 1985 eliminated all but four of these channels. Channels were stocked with natural substrates and organisms from the Marble Fork of the Kaweah River. After a three week acclimation period, we simulated a low pH rain event by adding acid (H2SO4 and HNO3) to two of the channels, reducing pH to 5.0 for 6 hours. The other two channels acted as controls (pH 6.4). During acid additions, Baetis spp. drift in acidified channels was ca. 7 times higher than in control channels (F = 39.02, p < 0.025; data fourth root transformed, ANOVA), and the percentage of drifting baetids that was dead was significantly higher in acidified than control channels (46% vs. 0%, F = 29.86, p < 0.05; arcsine square root transformed data, ANOVA). Other taxa showed no significant drift responses, and benthic densities of all taxa showed no effects two days after acidification, probably owing to rapid recolonization by invertebrate drift in influent waters. Stream chemistry data are presented; heavy metal concentrations did not significantly increase in the 2 m stream channels.