Per Nystrom’s research while affiliated with Lund University and other places

In a replicated field experiment we studied the effects of natural densities of two exotic consumers, the predatory and herbivorous signal crayfish (Pacifastacus len- iusculus) and the predatory rainbow trout (Oncorhynchus mykiss), on multiple trophic levels of a pond community. The goals were to: (1) determine the individual and combined effects of predators on macroinvertebrates, macrophytes, and periphytic algae; (2) evaluate the strength of direct and indirect interactions in a food web influenced by omnivores; and (3) evaluate the relative importance of direct and indirect predator effects on mortality and growth of a native frog species, Rana temporaria. The experiment showed that both signal crayfish and rainbow trout had strong effects on multitrophic levels of a littoral pond community, through direct consumption and indirect effects on lower trophic levels. Crayfish had weak but significant negative effects on the biomass of predatory invertebrates and greatly reduced the biomass of snails, the most abundant invertebrate grazers. Although the number of active herbivorous tadpoles tended to be higher in crayfish cages relative to control cages, the proportion of surviving froglets was lower in crayfish cages than in control cages, possibly due to crayfish predation on injured tadpoles. The size of surviving froglets did not differ from controls, but tadpoles in crayfish cages often suffered tail injuries. Macrophyte coverage decreased as a result of crayfish consumption and nonconsumptive fragmentation. However, the biomass of pe- riphyton increased in crayfish cages relative to controls, probably due to reduced grazing from snails. In contrast, trout had strong negative effects on the biomass of both predatory invertebrates and insect grazers, whereas trout had less effect on snail biomass than did crayfish. Also, in contrast to crayfish cages, the number of active tadpoles in trout cages was lower than in controls, probably due to a combination of trout predation and trout- induced reduced tadpole activity. Trout had a strong negative impact on froglet survival, and froglets in trout cages metamorphosed at a smaller size and had reduced growth rates compared to froglets in crayfish and control cages. As with crayfish, the biomass of pe- riphyton increased in trout cages relative to controls, which may be due to a combination of both density and trait-mediated trout effects on tadpole grazing. In treatments with multiple predators the effects of crayfish and trout on caged com- munities were independent, and there were few interactions. Mostly effects of combined predators reflected those in single predator cages. Our results demonstrate that noninter- acting, introduced multiple predators can have strong direct and indirect effects on multiple trophic levels in pond communities. Trophic cascades may develop in aquatic food webs even with omnivores such as crayfish, and in complex habitats with trout. These strong indirect effects are mediated through both predation on important grazers (i.e., the crayfish- snail-periphyton link) and a combination of density and behavioral responses of grazers to predators (i.e., the trout-tadpole-periphyton link). When two noninteracting predators have strong but different effects on prey survival or activity, their combined effects on intermediate trophic levels reflect responses to the more dangerous predator.

Effects of ultraviolet radiation on hatching success, larval survival, and induced abnormalities were tested in three threatened (Hyla arborea, Bufo calamita, and Triturus cristatus) and two common (Rana temporaria and Bufo bufo) amphibians of Sweden. In the laboratory, all species were exposed to artificial UV-radiation (intensities corresponding to midsummer conditions in Sweden) or visible light (controls) for ten days, during which hatching occurred. The effects of ambient levels of UV-radiation on hatching success and survival of B. bufo, R. temporaria, and B. calamita were also determined during outdoor conditions when exposed to three different light regimes: Full solar: radiation, UV-filtered sunlight (exposed to photo-synthetically active radiation only) or darkness. In the laboratory experiment, neither eggs nor larvae of R. temporaria, B. bufo, B. calamita, and H. arborea were negatively affected by UV-radiation. However, UV-radiation induced darker pigmentation in tadpoles of H. arborea. Only one species, T. cristatus, showed reduced hatching frequencies and lower larval survival when exposed to UV-radiation in comparison to visible light However, since female T. cristatus wrap their eggs in macrophyte leaves, embryos are effectively protected against solar radiation. In the outdoor experiments, there were no differences between the light treatments for any of the species. Thus, none of these species are likely to be threatened by moderately increased levels of ultraviolet radiation.

The aim of this study was to compare the effects of the introduced signal crayfish (Pacifastacus) and the native noble crayfish (Astacus astacus) on a benthic food web. Wt mimicked the habitat of a pond littoral in 4.5-m(2) plastic pools stocked with natural densities of macrophytes, invertebrates and either signal crayfish, noble crayfish. or kept as crayfish free controls. After two summer months, all invertebrates and macrophytes were collected from each pool. and periphyton was sampled on one substratum exposed and tno substrata not exposed to crayfish prating. Samples for stabile isotope analysis of benthos were collected in pools with noble crayfish. N-15 ratios showed that crayfish were top consumers, and C-13 ratios indicated that they received most of their carbon from invertebrates, but less from primary producers. Crayfish did not affect the biomass of predatory invertebrates, dominated by active swimmers among Heteroptera and Coleoptera, but had a strong impact on grazers dominated by thin-shelled Lymnaea snails. Hard-shelled Bithynia snails were also reduced in numbers, but the largest of these snails were consumed less than thin-shelled Lymnaea snails. The reduced biomass of snails had an indirect positive effect on periphyton biomass on all three substrata. Crayfish grazed selectively on macrophytes and reduced the biomass of Chara, whereas Elodea as less affected. The exotic signal crayfish had, overall, a stronger impact on the biomass of macrophytes and grazers than the native noble crayfish. The results indicate that crayfish may structure food webs through consumption from many food levels. The short-term influence of crayfish on other trophic levels depends on crayfish feeding efficiency. food preferences and species-specific consumption rates.