Browsing by Subject "Invertebrates"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Evaluation of the potential effects of methoprene and BTI (Bacillus thurinqiensis israelensis) on wetland birds and invertebrates in Wright County, MN, 1988 to 1993
    (University of Minnesota Duluth, 1995) Niemi, Gerald J; Axler, Richard P; Hanowski, JoAnn M; Hershey, Anne E; Lima, Ann R; Regal, Ronald R; Shannon, Lyle J
    This report summarizes the results of a six year study (1988 to 1993) to assess the potential effects of two mosquito control materials, methoprene (applied as Altosid sand granules) and Bacillus thuringiensis var. israelensis (Bti, applied as Vectobac-G granules) on zooplankton, aquatic insects, and breeding birds in the Twin Cities metropolitan area. The study was a before-and-after design with 1988 to 1990 as pre-treatment and 1991 to 1993 as treatment years. A total of 27 wetlands in western Wright County were randomly selected and randomly placed within one of three groups of sites: 9 control, 9 Bti-treated, and 9 methoprene-treated. Selected populations of zooplankton, aquatic insects, and breeding birds were sampled within each of these wetlands. Each site was also monitored to verify the applications of Bti or methoprene to the respective sites and to verify that the control sites were not treated. In 1992, the number of study sites was reduced to 26 because of the loss of one methoprene-treated site from sampling. No effects could be attributed to treatment on zooplankton or breeding birds. Aquatic insects, however, were considerably reduced following treatments in 1992 and 1993. Chironomids comprised approximately 60% of the total individuals sampled, and were greatly reduced in both methoprene and Bti sites compared to controls. We focused on this group particularly because of their abundance, but also because they are closely related to mosquitoes and known to be susceptible to both larvicides. However, our results showed that all insect groups were similarly affected by both larvicides. Both Bti and methoprene applications to these wetlands reduced aquatic insect densities by a range of 57-83% and biomass by a range of 50-83% in the second and third years of treatment. Following the effects of treatments observed in 1992, populations of aquatic insects recovered to pre-treatment levels at the start of 1993, but quickly declined again following treatment. No food chain effects of these declines, with the possible exception of increases in the density of some copepods in methoprene-treated sites, were observed in either zooplankton or in breeding birds. High nest loss rates due to predation may have been a greater limiting factor to birds than mosquito control treatment. The carrying capacity of bird populations may also be lower than that affected by food reductions, especially since alternative foods tended to be available for Red-winged Blackbirds outside of the wetland study sites. Even though this study represents one of the largest ever conducted to assess the effects of mosquito control materials on non-target organisms, a variety of questions regarding the overall effects of mosquito control treatments remain unanswered. Among the most pressing questions are the determination of the long term effects (e.g., > 5-10 yrs) of the control program and whether populations of aquatic insects can continue to recover within these treated areas.
  • Thumbnail Image
    Item
    Seasonal Prey Resource Partitioning Among Salmonids In The Bois Brule River, Wisconsin
    (2024) McCann, Daniel
    Over the past century, intentional introductions of non-native Salmonids into Lake Superior and its tributaries have raised concern about possible interactions with native Brook Trout (Salvelinus fontinalis), which have historically been subject to local extirpations. The Upper Bois Brule River, Wisconsin currently supports a diverse and abundant assemblage of Salmonid species, including Brook Trout, Brown Trout (Salmo trutta), Rainbow Trout (Oncorhynchus mykiss), Coho Salmon (Oncorhynchus kisutch), and Chinook Salmon (Oncorhynchus tshawyscha). To investigate underlying reasons for apparent co-existence of potentially competing species, I documented the seasonal diet characteristics among coexisting Salmonids and compared stomach content data to seasonal prey abundances (macroinvertebrates) in the benthos and stream drift. The primary objective was to determine if niche partitioning reduces competitive interactions between Brook Trout, Brown Trout, Rainbow Trout, and juvenile Coho Salmon in the Bois Brule River, Wisconsin. Results indicated that Brook Trout and Brown Trout had significant diet overlap during spring, summer, and fall sampling dates, while there were no instances of significant dietary overlap between the non-native species. Several positive relationships between diet overlap and the abundances of different prey categories suggest prey resource partitioning in response to competition may underlie the co-occupation of Salmond species in my study area. All Salmonid species showed positive selection for Trichoptera larvae during spring, summer, and fall. In addition, Brook Trout and Brown Trout showed positive selection for Gastropods during every season. Brook Trout were the only species with strong preferences for Crustaceans, while Rainbow Trout were the only species with preferences for Ephemeroptera nymphs. Brook Trout and Rainbow Trout exhibited opportunistic diets, while Brown Trout tended to feed primarily from the benthos and Coho Salmon fed mainly on surface-oriented prey. It’s likely that the persistence of Brook Trout in the presence of non-native Salmonids is attributed to their flexibility in foraging modes, utilization of increased invertebrate drift rates, and consumption of less preferred prey items. This study provides new information on resource utilization, niche partitioning, and resource overlap among multiple coexisting Salmonid species and may be helpful for future research and management with sympatric populations of native and non-native Salmonid species.