4.3. Diversity Metrics in Boundary Habitat
Our discovery of geomorphically-complex boundary habitat was especially
significant in the discussion of landscape mosaics as a measure of
riverscape complexity. We defined boundary habitat in this study as
habitat found along the edges of the upstream-expanding valley plug.
Overall, we found this habitat to be the most complex, containing all of
the geomorphic units we had identified as metrics of diversity (pools,
riffles, backwaters and large woody debris structures; Bottcher, 2009)
as well as the most diverse array of inundation types. These findings
support our initial suggestions that: 1) boundary habitat is the most
diverse within this system (Remiszewski, 2022) and 2) the valley plug
has contributed to ecosystem habitat diversity, especially when compared
to our simplified and degraded reference reaches. Especially significant
is the presence of pool habitat, as prior studies have identified that
the lower SRR is incredibly pool limited (Walsworth, 2011).
Pool habitat offers refugia from predation for native fishes, as well as
thermal refugia during increasingly hot summer months (Murphy, Pavlova,
Thompson, Davis, & Sunnucks, 2015). Direct observation demonstrated
pools are frequently the only wetted portions of the lower SRR during
seasonal drying and, during spot sampling that occurred simultaneously
with seasonal drying in the SRR, the only fish remaining for many
kilometers of the lower SRR were captured in pools below habitat
features such as beaver dams and BDAs (personal communication, 2021).
The presence of large quantities of pool habitat within boundary sites,
compared to other habitat types, means that these are some of the only
reaches of river offering persistent habitat for fishes during drought.
Additionally, the large quantities of large woody debris found in
boundary reaches offer rare and vital refugia for both adult and larval
native fishes, ensuring the persistence of spawning and rearing habitat
in an otherwise extremely habitat limited system (Bottcher et al., 2013;
Walsworth et al., 2013).