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\begin{document}

\title{Network Analysis of Terrestrial Wetlands Identifies Five Classes of
Climate Risk}



\author[1]{Hyeyeong Choe}%
\author[2]{James Thorne}%
\author[2]{Allan Hollander}%
\affil[1]{Kangwon National University}%
\affil[2]{University of California Davis}%


\vspace{-1em}



  
  \date{May 08, 2024}


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\selectlanguage{english}
\begin{abstract}
Climate change puts the habitat functions of wildlife conservation areas
at risk. Conservation areas managed for wetlands can be considered a
network, permitting the tracking of current climate conditions within
the network under projected future climates. A climate classification of
the nodes in such a network can help the selection among multiple
conservation management strategies based on their relative
climate-connectedness. We examined wetlands in 48 US National Wildlife
Refuges and mapped their climate networks to permit the incorporation of
climate linkages. Using four climate projections, we found five climatic
classes of wetlands: three are climatically stable; four are climate
hubs, becoming climatically similar to current climate conditions of
many other units; three whose current climate appear in many refuges;
8-16 whose climate conditions appear in only one other unit; and 10-25
are climatically isolated. The relative isolation of wetlands makes them
particularly appropriate for network-based climate assessments.%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{introduction}}

Wetlands such as swamps, marshes, and bogs contain a wealth of
biodiversity including many endemic and rare species (McLaughlin et al.
2017; Cartwright 2019). They also provide critical ecosystem services
such as water quality improvement, flood damage reduction, and
recreation and education opportunities (Randolph 2012). The unique
features of wetland landscapes support unusual local environments and
provide opportunities for species to survive in changing conditions,
making them candidates for climate refugia (Morelli et al. 2016;
McLaughlin et al. 2017). However, more research on the impacts of
climate change and the conservation management of these unique
ecosystems is needed (Cartwright and Wolfe 2016), in order to identify
appropriate climate-change adaptation strategies. This need is
pronounced for conventional reserve networks including protected
wetlands, because these conservation areas have typically not been
designated to address climate change (Ara\selectlanguage{ngerman}újo et al. 2011; Game et al.
2011; Schneider and Bayne 2015).

As climate conditions shift across continents, much conservation
research has focused on tracking analog climates (Carroll et al. 2018;
Parks et al. 2018; Fitzpatrick and Dunn 2019). Studies have examined
landscape connectivity among reserves with climate change (Andrello et
al. 2015), and projections of species future-ranges (Ramirez-Villegas et
al. 2014; Choe et al. 2017) or vegetation shifts (Powers et al. 2018) to
model theoretical networks of future conservation that incorporate
climate change (Heller et al. 2015). However, there is a need for a more
unified approach to analyzing climate connectivity across protected
areas such as national parks, nature reserves, and multiple-use
conservation areas (Belote et al. 2017) and for insular or rare
ecosystems (Cumming et al. 2010; Cartwright 2019) to inform questions
such as to which conservation areas might species need to move (Hannah
et al. 2007; Lawler and Hepinstall-Cymerman 2010) and whether dispersal
would result in species arriving at suitable, or analogous, climates to
those being lost.

We used a discrete network-based analysis to examine climatically
suitable arrival points for species being climatically dislodged from
wetland nodes in a climatic network of conservation areas. We used
wetlands in the United States National Wildlife Refuge System (NWRS)
that span the California floristic region (Burge et al. 2016), part of
the Pacific Northwest's temperate coniferous forest region, and parts of
the desert ecosystems of Nevada to model analogous climates through
time. We considered the wetland in each NWRS unit (hereafter refuge) a
node within the network analysis, and the links are the climate
relations between nodes. The NWRS administers a network of lands and
waters for the conservation and management of wildlife species and their
habitats (U.S. Code 1997). Currently, the refuges in USA provide
habitats for over 700 bird species, 220 mammals, 250 reptiles and
amphibians, and 1000 fish species (National Wildlife Refuge System,
2016). Refuges from the NWRS system are particularly suitable for a
network analysis because they are spatially and environmentally isolated
by large intervening areas.

Climate change impacts are expected to be significant (Rannow et al.
2014) and some refuges may become climatically unsuitable for the
species the units were created to protect (Jewitt et al. 2017). This
research was motivated by the question of whether suitable
climate-conditions for dislodged protected species might occur or emerge
at other nodes in the network, thereby making those nodes candidates for
possible species relocations. In such cases, some nodes may lose their
functions as habitats, but others may become more climatically suitable
as species' habitats. Thus, developing a climate classification of nodes
within a conservation network of wetlands is useful for conservation
management.

We focused on the site-level climate conditions of refuges and examined
the climate networks among 48 existing refuges (43 national wildlife
refuges and 5 wildlife management areas) for present and future periods
from the ``climate-analog'' point of view. Climate-analog analysis has
the advantage that it does not need to make assumptions about the
tolerances of species (Veloz et al. 2012), and can help to identify the
most important or highly exposed areas among the refuges for resource
management. For example, Parks et al. (2018) identified the
climate-analog of mountainous ecoregions of the western US to evaluate
how climate change may influence fire regime and vegetation shifts. The
climate-analog approach can be applicable to other environments or
regions (Veloz et al. 2012), but we have not seen the application of
this approach to conservation area networks.

Here we ask how many types of temporal climate connections exist for a
set of wetland conservation areas? We define the current climate
conditions of each refuge and identify others with analogous climates.
Then, we identify where each refuge's current climate can be found in
the future using climate change projections to understand the climate
network of each unit. Our objectives are to identify the temporal
climate networks among the refuges under future climate projections, to
classify the units using their relative climate importance based on
their climate classes for efficient conservation management of the NWRS
system, and to consider the utility of climate analog classifications
within a conservation network.

\section*{Material and methods}

{\label{material-and-methods}}

We examined the current-climate conditions of the 48 refuges in the NWRS
Pacific Southwest Region of U.S. using two current-climate variables,
annual average of minimum temperature and annual precipitation, to
facilitate the understanding of climate change. Then, we identified the
climate networks among the refuges for current and future time periods.

\textbf{Wildlife Refuges in the Pacific Southwest Regions of U.S.}

We used 48 units located in California, Oregon and Nevada, USA (Fig. 1).
These comprise 47 refuges and one area of interest, the North Central
Valley WMA (Wildlife Management Area), which is a generalized area in
which the USFWS has interest but does not manage. The average refuge
size is 99 km\textsuperscript{2}, leaving aside the unusually large
North Central Valley WMA (19,128 km\textsuperscript{2}) and the Desert
NWR (6,619 km\textsuperscript{2}) (see Table S1 in Supporting
Information).

\textbf{Climate Space Shifts for each Wildlife Refuge}

We calculated the ranges, from minimum to maximum value, of the two
climate variables to define the two-dimensional current-climate space
for each refuge using conditions from 1981-2010. We used the annual
average of minimum temperature because it is expected to warm faster
than maximum temperature at most locations (Hartmann et al. 2013).

We identified where the current-climate space of each refuge will be
found among all the refuges for three future periods, 2010-2039,
2040-2069, and 2070-2099. For future-climate scenarios, we used four
projections that bracket a range of possible future-conditions predicted
by 12 Global Climate Models (GCMs) (Thorne et al., 2016; Thorne et al.,
2017). The four futures are derived from two GCMs that respectively are
hotter and drier (MIROC ESM), and warmer and wetter (CNRM-CM5) than
current conditions; and two Representative Concentration Pathways
(RCPs), which were defined by their total radiative forcing pathway and
level by 2100 (IPCC 2014). We used the RCP8.5 emissions scenario, IPCC's
highest rate of anthropogenic greenhouse gas emissions, already
surpassed by current emissions (Hayhoe et al. 2017); and the RCP4.5
emissions scenario, representing an attempt to limit global warming to
2°C (Thomson et al. 2011). Each emission scenario was used with each
GCM.

\textbf{Climate Networks among Wildlife Refuges}

Network analysis investigates structures by identifying networks among
component individuals (Otte and Rousseau 2002). Each individual is
called a node and links are the relations between nodes (Cumming et al.
2010). Here, refuges are the nodes and we linked refuges using the
relationships of climate space of each refuge. We used arrows to
represent the directions of links. If an initial refuge node's current
climate space is found in other refuge nodes, those nodes are final
nodes and the arrowheads point to the final nodes. The density of
vectors is an indicator of the connectedness in the network (Otte and
Rousseau 2002). We calculated the levels of connections for each refuge
unit and the number of nodes from which climate departs or arrives can
be found under future projections.

After mapping the temporal climate networks among refuges for each
climate projection, we asked how many distinct types of climate
connections could be identified. For the future-climate connections
among refuges, we report on 2070-2099 in the main text, and see Table S2
for other future periods results.

\section*{Results}

{\label{results}}

\textbf{Climate Change of Each Wildlife Refuge}

The five refuges with the largest end-of-century increase in annual
minimum temperatures under the CNRM-CM5 RCP8.5 are the Modoc NWR, Tule
Lake NWR, Ash Meadows NWR, Klamath Marsh NWR, and Lower Klamath NWR.
Under the MIROC ESM RCP8.5, the five refuges with the highest minimum
temperature increases are Ash Meadows NWR, Tule Lake NWR, Ruby Lake NWR,
Modoc NWR, and Fallon NWR (Table S1, Figures S1 and S2).

Under the CNRM-CM5 RCP8.5, annual precipitation is expected to increase
in all refuges except Hopper Mountain NWR. Under the MIROC ESM RCP8.5,
annual precipitation is expected to decrease in almost all refuges
except Modoc NWR, Lower Klamath NWR, Humboldt Bay NWR, and Upper Klamath
NWR (Table S1, Figures S3 and S4).

\textbf{Climate Networks among Wildlife Refuges}

We identified five climate categories in the refuge network and ranked
them from most- to least-at-risk for climate-change impacts:
1)\emph{Disappearing climates -} refuges whose current climate
conditions are not represented in any other refuges in the future;
2)\emph{Isolating climates -} refuges whose current climate conditions
appear in only one other refuge; 3) \emph{Dispersing climates -} refuges
whose current climate conditions appear in multiple other refuges in the
future; 4) \emph{Enduring climates -} refuges that retain their current
climate conditions in the future; and 5) \emph{Climate hubs -} refuges
that meet the current climate conditions of other refuges in the future.
See Table S3 in Supporting Information for the climate categories of
each refuge under each climate scenario for three future periods,
2010-2039, 2040-2069, and 2070-2099.

\emph{Disappearing climates -} The current climates of six of the
smaller refuges are not found in any other refuges currently (Blue Ridge
NWR, Seal Beach NWR, Coachella Valley NWR, Humboldt Bay NWR, Klamath
Marsh NWR, and Sonny Bono Salton Sea NWR; Fig. 2 and Table S2). By
end-of-century, there is no climatically isolated refuge in all four
climate scenarios, but eight are climatically isolated under three
climate scenarios (Fig. 3 and Table S2). They are Antioch Dunes NWR,
Marin Islands NWR, Seal Beach NWR, Sutter NWR, Colusa NWR, Upper Klamath
NWR, Klamath Marsh NWR, and Sonny Bono Salton Sea NWR. These represent
the units that are most climatically at risk.

However, far more refuges are at this highest level of risk when each
future climate is considered separately. Under the warmer and wetter
future, CNRM-CM5 RCP4.5 scenario, the current climate space of 21
refuges are not found in any refuges by end-of-century (Fig. 3a). Among
these 21 refuges, 10 contain arriving current climates of other units
but the other 11 do not have any connections, which means the current
climate conditions of these refuges did not appear elsewhere, nor did
the current climate of other refuges appear in these refuges. Under the
CNRM-CM5 RCP8.5, the current climate space of 32 refuges cannot be found
in any others (Fig. 3b). Among these 32 refuges, 25 do not have any
climate connections. Under the hotter and drier GCM, MIROC ESM RCP4.5
scenario, the current climate space of 25 refuges cannot be found in any
others and 10 refuges do not have any connections (Fig. 3c). Under the
MIROC ESM RCP8.5, the current climate space of 31 refuges cannot be
found in any others and 20 do not have any connections (Fig. 3d).

\emph{Isolating climates -} The current climate conditions of many
refuges appear in only one other unit by end-of-century (Fig. 3 and
Table S2), especially those of Don Edwards San Francisco Bay NWR and
Pixley NWR, which appear in only a single refuge under three climate
scenarios. Under the CNRM-CM5 RCP4.5 and RCP8.5, the current climate
conditions of 14 and 10 refuges can be found in only one other refuge,
respectively (Fig. 3a and 3b). Under the MIROC ESM RCP4.5 and RCP8.5,
the current climate conditions of 16 and eight refuges can be found in
only one other unit, respectively (Fig. 3c and Fig. 3d).

\emph{Dispersing climates} - For refuges whose current climates appear
in many other refuges, the climate space of three refuges, Desert NWR,
North Central Valley WMA, and San Diego NWR, appear in many refuges in
all four climate change scenarios (Fig. 3 and Table S2). Under the
CNRM-CM5 RCP4.5, the current climate conditions of the Desert NWR and of
the San Diego NWR can be found 15 and eight refuges, respectively (Fig.
3a). Under the CNRM-CM5 RCP8.5, the current climate conditions of the
Desert NWR and of the North Central Valley WMA can be found in eight and
five refuges, respectively (Fig. 3b). Under the MIROC ESM RCP4.5, the
current climate conditions of the Desert NWR and of the San Diego NWR
can be found in 24 and 13 refuges, respectively (Fig. 3c). Under the
MIROC ESM RCP8.5, the current climate conditions of the Desert NWR and
of the North Central Valley WMA can be found in 14 and four refuges,
respectively (Fig. 3d).

\emph{Enduring climates -} Taking all four climate change scenario
outcomes into account three refuges, San Diego NWR, Desert NWR, and
North Central Valley WMA, retain their current climate conditions by
end-of-century (Fig. 3 and Table S2). Under the CNRM-CM5 RCP4.5, seven
refuges (Blue Ridge NWR, Bear Valley NWR, Bitter Creek NWR, Ruby Lake
NWR, San Diego NWR, Desert NWR, and North Central Valley WMA) retain
their current climate conditions. Under the CNRM-CM5 RCP8.5, four
refuges (Hopper Mountain NWR, San Diego NWR, Desert NWR, and North
Central Valley WMA) retain their current climate conditions. Under the
MIROC ESM RCP4.5 and RCP8.5, four refuges (Bitter Creek NWR, San Diego
NWR, Desert NWR, and North Central Valley WMA) retain their current
climate conditions.

\emph{Climate hubs -} The North Central Valley WMA meets the current
climate conditions of 23 other refuges and its current climate space can
also currently be found in 23 other refuges, the highest number of
current climate connections (Figure 2 and Table S2). Next, the Desert
NWR meets the current climate conditions of 17 other refuges and its
current climate space can currently be found in 24 other refuges. By the
end-of-century, under the CNRM-CM5, the current climate conditions of
many refuges are present in Bitter Creek NWR under both emission
scenarios (Fig. 3a and 3b). The current climate conditions of many
refuges can be found in North Central Valley WMA under both emission
scenarios of the MIROC ESM (Fig. 3c and 3d). On the other hand, using
the RCP4.5 scenario, the current climate conditions of many refuges can
be present in Hopper Mountain NWR and San Diego NWR using the both GCMs
(Fig. 3a and 3c), while the current climate conditions of many refuges
can be found in Desert NWR in both of GCMs under the RCP8.5 emission
scenario (Fig. 3b and 3d).

When we see the climate network outcomes using each climate scenario
separately, the current climate conditions of six refuges exist in the
Desert NWR including itself and of nine refuges can be found in San
Diego NWR including itself under the CNRM-CM5 RCP4.5 (Fig. 3a). The
current climate conditions of seven refuges can be found in Hopper
Mountain NWR and Bitter Creek NWR, respectively. Under the CNRM-CM5
RCP8.5, the current climate conditions of 11 refuges exist in the Desert
NWR including itself and of three refuges can be found in North Central
Valley WMA including itself (Fig. 3b). The current climate conditions of
six refuges can be found in Bitter Creek NWR. Under the MIROC ESM
RCP4.5, the current climate conditions of four, five, 12 refuges exist
in the Desert NWR, San Diego NWR, North Central Valley WMA including
itself, respectively (Fig. 3c). And, the current climate conditions of
seven refuges can be found in Hopper Mountain NWR. Under the MIROC ESM
RCP8.5, the current climate conditions of eight refuges exist in the
Desert NWR including itself and of six refuges can be found in North
Central Valley WMA including itself (Fig. 3d). The current climate
conditions of six refuges can be found in Blue Ridge NWR.

\section*{Discussion}

{\label{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 \textgreater{}2300 Ramsar
wetlands which cover over 2.5 million km\textsuperscript{2} globally
(\emph{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.

\section*{Acknowledgements}

{\label{acknowledgements}}

Funding for this study was provided by the US Fish and Wildlife Service.

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\section*{Supporting Information}

{\label{supporting-information}}

Additional Supporting Information may be downloaded via the online
version of this article at Wiley Online Library
(www.ecologyletters.com).

Table S1. The area and current (1981-2010) climate information including
annual average of minimum temperature and total annual precipitation of
each refuge.

Table S2. The number of links of each refuge node based on climate
directions. The number of ''depart'' refers to the number of other
refuge nodes where a target refuge node's current-climate space will be
found. The number of ''arrive'' means the number of other refuge nodes
where their current-climate spaces will occur in a target node for the
time period indicated.

Table S3. The climate categories of each refuge under each climate
scenario for the time period indicated. Five climate categories are
represented using five colors (red: disappearing climates; orange:
isolating climates, yellow: dispersing climates, light green: enduring
climates, green: climate hubs).

Figure S1. Current (1981-2010) average annual minimum temperature.

Figure S2. Changes in minimum temperature by 2070-2099 under four
climate scenarios.

Figure S3. Current (1981-2010) average annual precipitation.

Figure S4. Changes in precipitation by 2070-2099 under four climate
scenarios.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1. Locations of the NWRS (National Wildlife Refuge Systems) in
the Pacific Southwest Regions of USA.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. Current (1981-2010) climate connections among refuges. We used
arrows to represent the directions of links. If an initial refuge node's
current climate space is found in other refuge nodes, those nodes are
final nodes and the arrowheads headed toward the final nodes. The size
of the green circles represents the number of arriving climate
connections to a unit or area.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3. Climate connections among refuges by 2070-2099. If an initial
refuge node's current climate space is found in other refuge nodes'
future climate (2070-2099), those nodes are final nodes and the
arrowheads headed toward the final nodes. We used projections of future
climate using two GCMs that respectively are hotter and drier (MIROC
ESM), and warmer and wetter (CNRM-CM5) than current conditions. For each
GCM we used two emission scenarios (RCP 4.5 and RCP 8.5) that represent
lower and higher levels of greenhouse gas concentration. The size of the
green circles represents the number of arriving climate connections to a
unit or area.

\selectlanguage{english}
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