As a result of their extensive home ranges and slow population growth rates, predators have often been perceived to suffer higher risks of extinction than other trophic groups. Our study challenges this extinction-risk paradigm by quantitatively comparing patterns of extinction risk across different trophic groups of mammals, birds, and reptiles. We found that trophic level and body size were significant factors that influenced extinction risk in all taxa. At multiple spatial and temporal scales, herbivores, especially herbivorous reptiles and large-bodied herbivores, consistently have the highest proportions of threatened species. This observed elevated extinction risk for herbivores is ecologically consequential, given the important roles that herbivores are known to play in controlling ecosystem function.

Predator-prey interactions including prey switching, predator swamping, and size-selectivity are important in maintaining multi-species systems. In fishes, early life stages are often recruitment bottlenecks due to high mortality partially caused by predation. High mortality is of particular concern for threatened species such as lake sturgeon (Acipenser fulvescens). Effects of different relative prey densities were examined using two predatory fishes [rock bass (Ambloplites rupestris) and hornyhead chub (Nocomis biguttatus)] and two density treatments of three prey [lake sturgeon, mayflies (Family: Heptageniidae), and suckers (Family: Catostomidae)]. Treatments consisted of prey introduced to predators in a series of pulses 30 min apart. In the initial low-density treatment, predators were offered prey at a pulse of prey at a 13:13:4 ratio of mayfly, suckers, and lake sturgeon, and a second pulse with a 1:1:1 prey ratio during the second pulse. In the equal-density treatment prey numbers were equivalent during both pulses. Larval sturgeon survival, predator preference, and size selection were measured for each trial. Lake sturgeon were the least preferred prey species while mayflies were positively selected. Hornyhead chub preference for lake sturgeon was higher in the equal-density treatment than in the low-density, indicating initial prey availability affected predator foraging behaviour. High densities of preferred macroinvertebrate prey could protect threatened lake sturgeon larvae from predation.

Aquaculture programs benefit from feeding protocols that result in large larval body size and high survival. Despite high labor, processing, and material costs relative to alternative foods, feeding live foods generally results in high larval growth and survival. For many species, studies that identify alternative food types or feeding regimens that produce larvae with high survival and size comparable with wild cohorts are lacking. In a 35-d study, the effects of alternative food types (previously frozen Artemia and trout crumble starter diet (trout diet) on TL, weight per fish, and survival of larval Lake Sturgeon Acipenser fulvescens were quantified. From days 14 to 21 postexogenous feeding, larvae were transitioned from live Artemia to one of the alternative food types or remained feeding on live Artemia as the control. At the end of the study, TL and weight per fish of larvae fed live Artemia were significantly greater than larvae fed the alternative foods. Survival of larvae fed live or frozen Artemia was higher than larvae fed the trout diet. Lower body growth and survival of larvae fed the alternative foods demonstrate that the frozen and formulated foods are not appropriate diets for Lake Sturgeon larvae.

Compromised nutritional conditions associated with dietary transitions and feeding cessation in the wild and during fish aquaculture operations are common and can impact growth and survival. These effects are especially prevalent during early ontogenetic stages. We quantified phenotypic and GI tract microbial community responses with an emphasis on protease-producing bacteria of lake sturgeon (Acipenser fulvescens) larvae, a species of aquacultural and conservational importance. To quantify responses associated with experimental food transition and feeding cessation, we performed a 36-day feeding experiment using two treatments: control and diet transition. However, larvae in the diet transition treatment failed to undergo transition and ceased feeding. Larvae in the diet transition treatment exhibited lower growth (total length and body weight) and survival than control larvae. Treatment had a greater effect than ontogenetic changes on taxonomic composition and diversity of the GI tract microbial community. Proteobacteria dominated the GI tract microbial community of the diet transition larvae whereas Firmicutes dominated the GI tracts of control larvae. Most of the 98 identified protease-producing isolates in both treatments were from genera Pseudomonas and Aeromonas: taxonomic groups that include known fish pathogens. Overall, failing to transition diets affected responses in growth and GI tract microbiome composition and diversity, with the later dysbiosis being an indicator of morbidity and mortality in larval lake sturgeon. Thus, microbiological interrogations can characterize responses to dietary regimes. The results can inform fish culturalists and microbiologists of the importance of dietary practices consistent with the establishment and maintenance of healthy GI tract microbiota and optimal growth during early ontogeny.

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