Effects of Pacific salmon spawning and carcass availability on the caddisfly Ecclisomyia conspersa (Trichoptera: Limnephilidae)

1. The effects of spawning coho (Oncorhynchus kisutch) and chum salmon (Oncorhynchus keta) on the limnephilid caddisfly Ecclisomyia conspersa were evaluated by experimentally excluding salmon from the upper 14‐m stretch of a spawning channel by a wire‐meshed fence. Density, and development and growth rates, of larvae upstream of the fence (without salmon) were compared with those downstream (with salmon). 2. Larval density in the stretch with salmon declined during spawning, but increased again after spawning subsided and the carcasses of dead fish became available. In the stretch with salmon, larval density on salmon carcasses was seven to 37 times greater than on the adjacent channel substratum. The rate of larval development in the stretch with salmon was greater than that in the stretch without salmon. Two months after carcasses became available, 98% of larvae sampled from the stretch with salmon were in the fifth instar, compared to only 23% from the stretch without salmon. Body weight of E. conspersa in the stretches with and without salmon increased by an average of 3.04 and 2.38 mg, respectively, over a 6‐month period. 3. ¹⁵N values of larvae from the stretch with salmon increased following the arrival of the fish, suggesting that the larvae were feeding on salmon‐derived material, such as eggs and carcasses, which contain a high proportion of the heavier stable isotope. In contrast, ¹⁵N values of larvae from the stretch without salmon remained relatively constant throughout the experiment. The availability of salmon carcasses as a high‐quality food source late in larval development may increase survival and fecundity of E. conspersa. 4. These substantial differences were consistent with the view that they were due to the experimental exclusion of salmon and salmon carcasses from the upstream stretch, though the study was un‐replicated and thus precludes ascribing causation more definitely.     

Disentangling the Effects of Root Foraging and Inherent Growth Rate on Plant Biomass Accumulation in Heterogeneous Environments: A Modelling Study

Empirical evidence indicates that fast-growing species generallydisplay a higher degree of selective root placement in heterogeneousenvironments than slow-growing species. Such root foraging isaccomplished by root morphological responses, but since somemorphological responses are simply the result of enhanced growthof the roots in the enriched patch it is difficult to separatethe effects of root foraging and growth rate on the biomassaccumulation of species in heterogeneous environments. Herea simple model is presented to disentangle these effects. Rootforaging is incorporated as the selective allocation of rootbiomass per unit time to the nitrogen-rich patch. Growth ratedifferences among the model plants result from differences innitrogen utilization efficiency. In the model, the degree ofselective root placement can be varied independently of growthrate. The model shows that when plants are compared at a commonpoint in time, selective root placement and growth rate interactpositively with respect to the enhancement of plant biomassaccumulation in heterogeneous compared to homogeneous environments.However, by evaluating the model at a common plant biomass,the main and interactive effects of growth rate are eliminated.These results suggest that growth rate by itself does not conferan advantage in terms of resource acquisition and biomass accumulationin heterogeneous environments. Only the selective placementof resource acquiring structures (such as roots) leads to suchbenefits. The essential differences between foraging and growth,as well as the consequences of differences in foraging abilityand growth rate between species on competition for a limitedresource, are discussed. Copyright 1999 Annals of Botany Company Environmental heterogeneity, foraging, growth rate, model, nitrogen uptake, nitrogen utilization, patchiness, plant growth, plasticity, root placement.