Can replacement of native by non‐native trout alter stream‐riparian food webs?

1. Non‐native species can affect food web and community structure, including the flow of resources from one habitat to another. In many streams of western North America, non‐native brook trout (Salvelinus fontinalis) have replaced native cutthroat trout (Oncorhynchus clarkii). Because brook trout naturally occur at higher densities and exhibit different feeding habits, this replacement may have consequences for a range of organisms in stream‐riparian food webs.
2. We conducted a large‐scale, 2‐month field experiment to test whether, compared with cutthroat trout, brook trout reduce benthic insects, cause an increase in stream algae, and reduce emerging adult aquatic insects as well as riparian spiders that rely on emergence as prey. Twenty enclosed reaches from which trout were removed were treated by adding age‐1 and older: (i) cutthroat trout at natural density (0.15 fish m^?2); (ii) brook trout at a natural density (0.40 fish m^?2); (iii) brook trout at a low density equal to the cutthroat trout treatment (0.15 fish m^?2) or (iv) no trout added.
3. Brook trout reduced the flux of emerging insects by 55% compared with cutthroat trout, but, surprisingly, only at the lower density. This reduction in emergent insects was predicted to reduce spider abundance by 20%, provided that sufficient branches were available to support riparian spider webs. We also detected an effect of trout on large‐bodied benthic and emerging insects, but not on the entire assemblage of benthic insects, or periphyton.
4. We hypothesise that our results were influenced by trout foraging behaviour mediated by their density and size structure. Regardless of the mechanisms responsible, our results indicate that non‐native brook trout can reduce the flux of emerging insects, and further analysis showed that this could reduce riparian spiders and birds that prey on them. Similar effects of non‐native fish are likely to occur in lotic and lentic waters beyond the western United States.

     

Will phenotypic plasticity affecting flowering phenology keep pace with climate change?

Rising temperatures have begun to shift flowering time, but it is unclear whether phenotypic plasticity can accommodate projected temperature change for this century. Evaluating clines in phenological traits and the extent and variation in plasticity can provide key information on assessing risk of maladaptation and developing strategies to mitigate climate change. In this study, flower phenology was examined in 52 populations of big sagebrush (Artemisia tridentata) growing in three common gardens. Flowering date (anthesis) varied 91 days from late July to late November among gardens. Mixed‐effects modeling explained 79% of variation in flowering date, of which 46% could be assigned to plasticity and genetic variation in plasticity and 33% to genetics (conditional R² = 0.79, marginal R² = 0.33). Two environmental variables that explained the genetic variation were photoperiod and the onset of spring, the Julian date of accumulating degree‐days >5 °C reaching 100. The genetic variation was mapped for contemporary and future climates (decades 2060 and 2090), showing flower date change varies considerably across the landscape. Plasticity was estimated to accommodate, on average, a ±13‐day change in flowering date. However, the examination of genetic variation in plasticity suggests that the magnitude of plasticity could be affected by variation in the sensitivity to photoperiod and temperature. In a warmer common garden, lower‐latitude populations have greater plasticity (+16 days) compared to higher‐latitude populations (+10 days). Mapped climatypes of flowering date for contemporary and future climates illustrate the wide breadth of plasticity and large geographic overlap. Our research highlights the importance of integrating information on genetic variation, phenotypic plasticity and climatic niche modeling to evaluate plant responses and elucidate vulnerabilities to climate change.