Predator-Driven Nutrient Recycling in California Stream Ecosystems

Nutrient recycling by consumers in streams can influence ecosystem nutrient availability and the assemblage and growth of photoautotrophs. Stream fishes can play a large role in nutrient recycling, but contributions by other vertebrates to overall recycling rates remain poorly studied. In tributaries of the Pacific Northwest, coastal giant salamanders (Dicamptodon tenebrosus) occur at high densities alongside steelhead trout (Oncorhynchus mykiss) and are top aquatic predators. We surveyed the density and body size distributions of D. tenebrosus and O. mykiss in a California tributary stream, combined with a field study to determine mass-specific excretion rates of ammonium (N) and total dissolved phosphorus (P) for D. tenebrosus. We estimated O. mykiss excretion rates (N, P) by bioenergetics using field-collected data on the nutrient composition of O. mykiss diets from the same system. Despite lower abundance, D. tenebrosus biomass was 2.5 times higher than O. mykiss. Mass-specific excretion summed over 170 m of stream revealed that O. mykiss recycle 1.7 times more N, and 1.2 times more P than D. tenebrosus, and had a higher N:P ratio (8.7) than that of D. tenebrosus (6.0), or the two species combined (7.5). Through simulated trade-offs in biomass, we estimate that shifts from salamander biomass toward fish biomass have the potential to ease nutrient limitation in forested tributary streams. These results suggest that natural and anthropogenic heterogeneity in the relative abundance of these vertebrates and variation in the uptake rates across river networks can affect broad-scale patterns of nutrient limitation

Correlates and consequences of injury in a large, predatory stream salamander (Dicamptodon tenebrosus)

Conspecific aggression is an important factor structuring population dynamics through intra- and interspecific interactions, but is rarely studied in un-manipulated populations. In this study, we evaluated rates of injury as a proxy for conspecific aggression using a depletion survey of predatory coastal giant salamanders (Dicamptodon tenebrosus) in a tributary of the South Fork Eel River, California. We tested a range of hypotheses including a suite of environmental and biotic factors for the rate of injury in a population by using an AIC model-selection approach that examined the weight of evidence for individual models. We examined both the probability of a given individual being injured, and the proportion of individuals within a given study pool being injured. We found strong support for models including salamander size, density of young-of-the-year steelhead, and density of the largest size-class of salamander as factors positively influencing the rate of injury at both the individual and habitat levels. We also found that density of older steelhead (1+ steelhead) had a strong, but highly variable positive impact on frequency of injury. This study shows that both conspecific and heterospecific factors influence intraspecific aggression for the dominant salamander throughout coastal Pacific Northwest streams. Our methodology demonstrates a non-manipulative approach to identifying correlates of natural injury in a cryptic species of amphibian. More work is needed to determine how these factors directly and indirectly influence the spatial distribution, individual fitness, and dynamics of salamander populations within streams.</jats:p

Density and biomass (abundance survey) and average wet weight elemental composition of diets (diet survey) for <i>O. mykiss</i> and <i>D. tenebrosus</i> in Fox Creek, California.

<p>Values calculated using total area of the study reach.</p

Excretion rates of <i>D. tenebrosus</i>.

<p>Nitrogen (NH₄) and phosphorus (SRP) nutrient excretion rates (ug·min⁻¹) of <i>D. tenebrosus</i>. Lines represent the fit of the top model selected by AICc for P (<i>log₁₀</i>[µg_P·min⁻¹] = −3.12+1.60(<i>log</i>₁₀[mass]), r² = 0.31, P = 0.01), and N excretion rates (<i>log₁₀</i>[µg_N·min⁻¹] = −2.04+1.41(<i>log</i>₁₀[mass]), r² = 0.79, P<<0.001).</p

Impacts on excretion of simulated tradeoffs between predator biomasses.

<p>Estimated total recycled N (NH₄) and P (SRP) excretion (g·day⁻¹) in Fox Cr. due to simulated changes in the relative abundance (by biomass) of <i>O. mykiss</i> and <i>D. tenebrosus</i>. Simulations assumed a fixed total biomass of predators (6275 g) within the study reach, and estimated total excretion rates (left y-axis) and ratios (right y-axis) by bootstrapped re-sampling of surveyed individuals. Predator relative abundance (x-axis) varies by 10% increments from 100% <i>O. mykiss</i> composition to 100% <i>D. tenebrosus</i> composition, expressed as the proportion of predator biomass (salamander:fish). Vertical line indicates the observed ratio of predators in Fox Cr.</p

Daily excretion estimates for predators in our study reach.

<p>Estimated total daily excreted N (NH₄) and P (SRP) by <i>O. mykiss</i> (filled), <i>D. tenebrosus</i> (grey), and both predators combined (open) within the Fox Cr. study reach. Bars represent mean ±95%CI. Note the log scaled y-axis.</p

Average elemental body composition (by dry mass) of common <i>O. mykiss</i> and <i>D. tenebrosus</i> diet items* by order.

<p>Elemental composition estimates from the literature for orders Lepidoptera and Hymenoptera did not include estimates of variability.</p><p>Footnotes: *Unaccounted for percentage of diet was comprised of diet items not covered by invertebrate CNP survey and for which values could not be found in literature. Contributions by these uncommon items were deemed inconsequential due to their small individual proportion of the wet mass of diets. Large and/or unique diet items (orders comprising <0.5% of total items) were discounted in diets so as not to bias elemental estimates.</p>1<p>Frost PC, Tank SE, Turner MA, Elser JJ (2010) Elemental composition of littoral invertebrates from oligotrophic and eutrophic Canadian lakes. Journal of the North American Benthological Society 22:51–62.</p>2<p>Elser JJ (2003) Biological stoichiometry: a theoretical framework connecting ecosystem ecology, evolution, and biochemistry for application in astrobiology. International Journal of Astrobiology 2:185–193.</p>3<p>Woods HA, Fagan WF, and Elser JJ (2004) Allometric and phylogenetic variation in insect phosphorus content. Functional Ecology 18:103–108.</p>4<p>Elser JJ, Fagan FF, Denno RF, Dobberfuhl DR, Folarin A, Huberty A, Interlandi S, Kilham SS, McCauley E, Schulz KL, Siemann EH, Sterner RW (2000) Nutritional constraints in terrestrial and freshwater food-webs. Nature 408:578–580.</p>5<p>Slansky Jr. F, and Feeny P (1977). Stabilization of the rate of nitrogen accumulation by larvae of the cabbage butterfly on wild and cultivated food plants. Ecological Monographs 47:209–228.</p>6<p>Cross WF, Benstead JP, Rosemond AD, and Wallace JB (2003) Consumer-resource stoichiometry in detritus-based streams. Ecology Letters 6:721–732.</p