Author Cheryl Schultz talks us through a new study which highlights the importance of active management interventions in helping to buffer effects of climate change, and helping to improve population trends for at-risk butterflies.
Where did the idea come from?
With extensive coverage in academic and popular publications, the widespread decline of butterflies is well-known. Butterflies face a triumvirate of threats: the cumulative effects of habitat loss, climate change and pesticides are severely impacting butterflies – and other wildlife – around the globe.
In recent years, scientists have spent a lot of time documenting this biodiversity crisis, which is important but fundamentally a depressing direction of research.
At the local level, natural area managers make daily decisions about how to address the onslaught of threats. In some cases, the managers benefit from close partnerships with researchers actively engaged in trying to identify the best possible management actions to reduce threats while, at the same time, not impacting at-risk species. In many other cases, the managers must rely solely on their knowledge of the local systems and actions they can take given the constraints of available funding. The collective long-term effects of these actions are often unknown, and have received much less attention than large-scale patterns of biodiversity loss and population declines.
Phenological change – or the change in the seasonal timing of activity – is one of the most documented fingerprints of climate change. For many butterfly species, the timing of their adult activity period has shifted weeks earlier in association with climate-driven warming. Many ecologists and conservation biologists are concerned that phenological changes will further reduce populations of at-risk species, many of which have already been impacted by substantial habitat loss.
We decided to evaluate relationships among changes in phenology, local land management actions, and population viability (trends toward increase vs. decline) through time. We focused on at-risk species, partly because there are more resources for local management of at-risk species in the U.S., and partly because these species are often not included in broad-scale analyses because working with sparse datasets for rare species is often challenging.
So, our study differs from most work on the status of butterfly populations in two ways: we evaluated the effects of local site management, and we focused on the at-risk species that are often the targets of management actions.
What did we do?
To understand the combined effects of changes in phenology and management interventions, we sought out long-term datasets for at-risk butterflies across the United States. These included species listed under the U.S. Endangered Species Act and butterfly species considered ‘Species of Greatest Conservation Need’ (SGCN) within individual states at the time we started the study. After making calls to dozens of species experts over the course of about two years, we were able to gather datasets with enough information to understand phenology and abundance for 114 populations of 31 at-risk butterfly species or subspecies in 10 U.S. states.
For each of the populations, we then sought to document management interventions that were carried out to enhance butterfly habitat over the time period for which butterfly data were available. Through a combination of agency reports, emails and phone interviews, which involved contacting dozens more people – including contacting local site managers as well as species experts – over another 1-2 years, we documented annual management interventions for 90 of these populations.
What did we find?
The strongest signal in our analysis was that, overall, populations with frequent management interventions were doing better than those which lacked management actions. Sites used various forms of management, including controlled burning, planting, mowing and other actions to enhance the habitats depending on the needs and budget constraints of individual sites.
Because multiple actions often occurred together we did not separate the effects of different kinds of management on population growth rates. For example, land managers used a combination of fire, mowing, and grass-specific herbicides to reduce non-native grasses and restore historic disturbance regimes for Fender’s blue butterfly at USFWS Baskett Slough National Wildlife Refuge.
On average, butterfly populations in our data set were declining, which is not surprising given that we were specifically targeting at-risk species. Nearly all populations at sites that received no habitat management were declining. Populations tended to be stable or growing at sites with regular management actions (some form of active attention taken every 1-1.5 years), although this average reflects a range of population trends, from declining to rapidly recovering populations in sites with regular management.
Butterfly populations were also changing in phenology, but the impacts of these changes were relatively subtle. As expected, most populations were shifting their phenology to be earlier in the year, but some populations did not shift much, and a few shifted to be later. There were no associations between common metrics of phenology (such as change in the start of the flight period) and trends in abundance. However, butterfly populations which had little shift in phenology over time (i.e., phenological constancy) were associated with a higher frequency of habitat management and were more likely to have positive population trends.
What does this mean?
Our results imply that, at a broad level, habitat management works to reverse population declines. While this may seem like common sense, it is a strong contrast to the predominant current scientific literature on climate change, which is largely focused on how climate change is impacting species across the globe and is agnostic to local management efforts to alter population trends. In contrast, we believe that the effects of global stressors like climate change and pesticides are not independent of habitat management: a species in higher quality habitat can often tolerate a less favorable climate.
The association between changes in abundance and phenological constancy and habitat management provide one hypothesis for how management can mitigate climate change. Management interventions might create spatial heterogeneity which, in turn, allows butterfly activity to track optimal conditions by moving within a site rather than shifting phenology. There is a budding literature on approaches that might allow species to persist-in-place rather than need to shift-in-space. We do not dispute the large-scale influence of global climate change, but our findings suggest that local-scale management can mediate some of the effects of climate change and other threats of global change, at least over the short-term.
In closing, we applaud the actions of the local land managers who contributed to this study. In the academic literature, these actions are often perceived as too “local” and “specific” to be included in analyses of global biodiversity trends. Indeed, obtaining these data was one of the most labor-intensive parts of our study.
Although there is no “silver bullet” to reverse the impacts of global stressors, we see this as a positive message for the prognosis for our butterfly populations. Although individuals and local organizations can only indirectly affect stressors like climate change, we can also take on-the-ground steps to improve habitat quantity and quality and directly increase resilience at-risk butterfly populations.
This research received funding from the U.S. Strategic Environmental Research and Development Program, the U.S. Fish and Wildlife Service Center for Pollinator Conservation and the U.S. Geological Survey John Wesley Powell Center for Analysis and Synthesis.
Read the full article “Phenological constancy and management interventions predict population trends in at-risk butterflies in the United States” in Journal of Applied Ecology.