We evaluated the relative success of sockeye salmon hatchery fry stocking in two British Columbia-Alaska transboundary lakes (1,622-ha Tatsamenie Lake and 492-ha Tahltan Lake). Fry stocking began in the late 1980s and is still under way. During the study period, survival patterns in the two lakes were different. At Tatsamenie Lake, wild egg-to-fry survival was higher than hatchery egg-to-fry survival (11.3% versus 4.3%) and wild egg-to-smolt survival was higher than hatchery egg-to-smolt survival (5.8% versus 2.5%). We found no relationship between fry survival and stocking date, spawner abundance, or food availability in Tatsamenie Lake, but we did find a significant positive relationship between early-spring fry length and egg-to-fry survival. Also, we found that in net-pen experiments, larger fry survived better. From this, we concluded that hatchery fry stocked into Tatsamenie Lake may suffer from size-mediated mortality and that net-pen supplemental feeding could be used to overcome this problem. However, because eggs used for hatchery production come from the lake, fry stocking can only be justified when hatchery fry survival exceeds wild fry survival. At Tatsamenie Lake, this goal has not yet been attained. At Tahltan Lake, wild egg-to-fry survival was lower than hatchery egg-to-fry survival (1.5% versus 6.3%) and wild egg-to-smolt survival was lower than hatchery egg-to-smolt survival (3.6% versus 12.8%). Hatchery fry release date and food availability both failed to explain these differences in fry and smolt survival at Tahltan Lake. However, there was a strong negative relationship between total female escapement and wild egg-to-smolt survival. This supported the results of an earlier investigation, which suggested that Tahltan Lake was spawning-site limited. We concluded that at Tahltan Lake, hatchery fry stocking increased smolt numbers, is biologically justified, and should be continued.

Many tropical plants are defended by ants, and the costs and benefits of these mutualisms can vary across gradients of herbivory, soil fertility, latitude, and other environmental factors. Yet despite an abundant literature documenting thermal constraints on ant activity and behavior, we know little about whether temperature variation can influence the benefits conferred by ants to plants. We evaluated the effects of dawn‐to‐dusk fluctuations in temperature on patrolling and aggressive behavior in four arboreal ant mutualists of Acacia drepanolobium trees in central Kenya. We found that ant aggressive behavior significantly increased with branch surface temperature, primarily in the two most aggressive ant species: Crematogaster mimosae and C. nigriceps workers attacked a simulated herbivore at higher rates as surface temperature rose. In a browsing experiment, we found that goats browsed more frequently and for longer durations on C. mimosae‐defended trees during cooler times of day, while goat browsing on plants from which ants had been removed was not affected by temperature. Our study demonstrates temperature‐dependence in the efficacy of ant defense against herbivory and suggests that these ant‐plants may be more vulnerable to herbivory during cooler hours of the day, when many native browsers are most active.

Abstract. Understanding the consequences of anthropogenic biodiversity decline has become an increasingly urgent priority for ecologists. Biological invasions are a common result of anthropogenic habitat change, and numerous studies have established the negative impact of invasions on the diversity and abundance of native species. But fewer studies have directly examined the effect of biological invasions on ecosystem functions and services. We leveraged a recent invasion by the big-headed ant (Pheidole megacephala) in an East African savanna to evaluate the impact of invasion on arthropod diversity, and on dung decomposition, an ecosystem function provided by a subset of these arthropods. We found that invaded sites had lower overall arthropod diversity, but these diversity changes did not extend to a functional group of detritivorous insects (e.g., dung beetles and termites), which play key roles in dung decomposition. In a manipulative experiment, we found that rates of dung pile decomposition did not differ significantly in invaded vs. non-invaded sites. Our study provides evidence that these invaded savannas are undergoing large changes in arthropod diversity, while maintaining resilience in decomposition function, suggesting that diversity alone may be an insufficient impact assessment tool. By monitoring functional guilds and their attendant services, we may better understand the broader structural and functional consequences of invasion.

Invasive ants shape assemblages and interactions of native species, but their effect on fundamental ecological processes is poorly understood. In East Africa, Pheidole megacephala ants have invaded monodominant stands of the ant‐tree Acacia drepanolobium, extirpating native ant defenders and rendering trees vulnerable to canopy damage by vertebrate herbivores. We used experiments and observations to quantify direct and interactive effects of invasive ants and large herbivores on A. drepanolobium photosynthesis over a 2‐year period. Trees that had been invaded for ≥ 5 years exhibited 69% lower whole‐tree photosynthesis during key growing seasons, resulting from interaction between invasive ants and vertebrate herbivores that caused leaf‐ and canopy‐level photosynthesis declines. We also surveyed trees shortly before and after invasion, finding that recent invasion induced only minor changes in leaf physiology. Our results from individual trees likely scale up, highlighting the potential of invasive species to alter ecosystem‐level carbon fixation and other biogeochemical cycles.