Year in review (2021)

Species richness over time at the five montane sites. Overall, 2021 was a down year, with multiple sites seeing near record lows for the total number of recorded species. Washington has seen a severe loss of species over the past two years: about 20 fewer species than historical values. Donner, previously the most diverse site, also posted one of its worst years.

(thanks to Chris Halsch for this and all graphs on this page)

Standardized population index over time, representing average (among species) performance. This metric is calculated by taking the number of days a butterfly was seen, dividing by the number of visits to that site each year (i.e., what we have called "fractional day positives" in publications), and then scaling within that unique species/site combination. A value of 0 indicates an average year. This metric is restricted to species that have been seen at a site at least 10 years. The majority of sites had “bad” years in 2021, continuing a downward trend that first became obvious during the mega drought years of 2011-2015. Using this metric, many sites had one of, if not the worst year on record. While Donner had one of its worst years from a richness perspective (see previous), it was a more average year when restricting analysis to the more commonly seen species (present in at least 10 years).

The date of the survey that saw the most species each year. The date of the highest richness this past year occurred earlier than the previous few years at some sites mountains. The exception to this was Sierra Valley, which saw an extreme outlier survey in late September with 19 species. This may be because warm conditions continued long enough into the late season to support larger broods or prolonged emergence for some species.

The standardized date of first flight across species. This metric is calculated by determining the date a butterfly was first observed in a year and then standardizing that date within that species across years. A value of 0 represents the average date of first flight, low values are earlier in the year, and larger values are later in the year. The highest elevation sites (Castle, Donner, Lang) appeared to have slightly earlier years compared to the historical average, likely due to low snowpack in the spring of 2021. The suite of years in the mega drought are still a recent standout.

Number of monitoring surveys per year. The sample effort of 2021 was comparable to historic sampling efforts. For quirks of weather and scheduling, Donner was visited slightly less than in the past, and Washington slightly more than average; future analyses will of course encompass this variation in sampling and values shown in graphs above (richness, etc.) will be affected in typically subtle ways.

Species that had pronounced deviations from their historic average day positive rate across the entire time series. Among the 50 strongest deviations from baseline, the majority were negative (i.e., species having exceptionally bad years). Pieris rapae in particular had a bad year across the mountain sites. Adelpha bredowii had a particularly good year. Donner Pass has the most positive deviations among sites, while Castle peak had the most negative deviations.

Similar to the previous graph, but here comparing to the last 5 years rather than the entire historical baseline. Among the 50 highest deviations from the most recent five years, Pieris rapae and Vanessa cardui had particularly bad years in 2021 and Adelpha bredowii had a good year across sites.

For more information on individual species and long-term trends, visit our project page prioritizing western butterflies for conservation attention: here.