Discussion
Conservation management plans that are based on current patterns of
species distribution will become less effective under climate change
(Carroll et al. 2017), so new approaches are needed for managing natural
resources (Lawler 2009; Choe et al. 2018). We applied a climate network
analysis to examine the climate links among 48 wetlands and found five
climate-connection categories that can inform natural resource
management strategies.
Wetlands lend themselves well to a network analysis because they are
distinct landscape features occurring across multiple climates that are
composed of different species at most nodes. While wetlands themselves
are ecological islands that may have enhanced capacity to persist under
climate change (Cartwright 2019), for conservation management of the
vertebrate species using wetlands, climate dynamics will likely become
an important challenge. Shorebirds in particular are largely dependent
on the NWRS-managed network due to substantial wetland loss and their
unique habitat requirements (Schaffer-Smith et al. 2018). Therefore, the
five climate categories we found in the network analysis provide useful
context for developing climate-adaptive management strategies for
vertebrate species, particularly birds within the refuge network system,
considering its unique ecosystem and management approach.
Most small-size refuges were climatically disappeared in the future time
periods, or had only one other unit with a future analogous climate. For
refuges with only one future unit, it may be beneficial to conduct
further research into the resilience and species climate vulnerability
at both the current and future locations. For climatically isolated
units, some may have suitable climates in units beyond the boundaries of
our study area (Choe and Thorne 2019), such as in northern refuges whose
climates may shift into adjoining states (Lenoir and Svenning 2015).
Alternatively, new refuges may need to be identified to account for
species inhabiting these climatically isolated refuges, particularly if
their climates do not appear anywhere in the larger network of protected
areas.
The refuge network contains four climate hubs, where the current climate
conditions of many refuges converge in the future (Bitter Creek NWR,
North Central Valley WMA, Hopper Mountain NWR, and Desert NWR). These
nodes could be targets for managed relocation (Schwartz et al. 2012),
and target species lists can be developed from the species lists of
those refuges whose climate conditions become unsuitable, but shift to
these arriving node units in the future. Conversely, the climate in some
nodes (Desert NWR, North Central Valley WMA, San Diego NWR) appears in
many other refuges (dispersing climates), suggesting greater flexibility
in species relocation for the species found in this class.
Not surprisingly, the three refuges with enduring climates include the
two largest, whose range of climate conditions is broad enough to
contain their own current climate conditions in the future. Although the
North Central Valley WMA is only designated as a refuge area of interest
and therefore the lands are mostly not under conservation management,
its role as an important node in the climate networks shows the value
that conserving this large area could provide. The third unit to retain
its own climate, the San Diego NWR, is moderately sized (eighth
largest). It is located in coastal southern California, and maritime
influence may be a factor in the lower warming in this area than
elsewhere in our study domain (Fig. S2). Management for this climate
class of refuges, should focus on external threats such as invasive
species, habitat fragmentation, and maintaining ecosystem functions.
This study could be considered a coarse filter approach (Groves et al.
2002; Khoury et al. 2011) to support wetland climate-conservation.
Further research could consider wetland hydrological processes and
additional climate variables to better define the climate conditions of
each refuge. However, when we conducted a sensitivity analysis using up
to nine climate variables, we found more variables narrowed the areas
identified as climate analogs and rendered most refuge units
climatically isolated. We decided to use the two most fundamental
climate variables (temperature and climate). In addition, our study does
not include projected shifts in species’ ranges (Choe and Thorne 2017;
Choe et al. 2017), or their sensitivity and adaptive capacity (Thorne et
al. 2016) which are commonly used frameworks for understanding the
vulnerability of individual species to climate change (Glick et al.
2011). These approaches have their own strengths and limitations,
including assumptions about dispersal success and biotic interactions
(Perez-Garcia et al. 2017). Instead this study focuses on the climate
conditions that are part of the exposure metrics for species, but that
are useful for describing the sites species occupy.
Our discovery of five classes of climate risk to existing wetland
conservation features emphasizes the importance of including climate
information when developing management strategies for protected area
networks. It may be possible to quickly identify these classes for other
conservation areas based on size, topographic complexity, and proximity
to maritime influences. For example, the >2300 Ramsar
wetlands which cover over 2.5 million km2 globally
(https://rsis.ramsar.org/),
could be good candidates for climate network analyses. Finally, although
some of the refuges we studied are very small, they intrinsically have a
high adaptive capacity because the majority are managed wetlands into
which additional waters could be pumped, which may offset climate
impacts as projected here, at least to some degree. Although this study
was limited to the refuge network, our next steps would be to explore
the locations and time schedules for additional refuges.