Small giants: Tributaries rescue spatially structured populations from extirpation in a highly fragmented stream

Habitat fragmentation is a pervasive threat to biodiversity. Linearly
arranged habitats such as stream networks are particularly vulnerable to
fragmentation. As the landscape becomes increasingly human-dominated,
conservation values of fragmented habitat patches cannot be overlooked. It
is critical to understand the demographic mechanisms of population
persistence or extirpation in fragmented patches. We studied the dynamics
of spatially structured populations of two Japanese landlocked salmonids
persisting for > 30 years in a headwater stream network that is
highly fragmented due to low-head dams in the mainstem. We parameterised
and analysed spatial matrix population models using 9-year mark-recapture
data. Tributaries supported higher survival rates in some life stages, and
movement was asymmetrical from the tributaries to the mainstem.
Accordingly, population growth rates were higher in the tributary patches
than the mainstem in both species despite the tributaries occupying only
12 or 18% of the study stream network by surface area. The tributaries
harboured more physically and hydraulically complex instream habitats
(i.e., higher wood density and flow refugia), indicating that habitat
patch quality was more important than habitat patch size in determining
the dynamics of these spatially structured populations. Tributary
locations in the stream network were important in the trajectory of these
populations. The upstream-dwelling charr persisted in the highly
fragmented mainstem patch (i.e., six impassable infrastructures in a
< 500 m patch) due to immigration of fish from upstream including
the tributary. However, the downstream-dwelling salmon have been gradually
extirpated from the uppermost section of the fragmented mainstem patch
because they could not maintain a positive population growth rate after
the loss of emigrants was accounted for and immigration was prevented due
to fragmentation. Synthesis and applications: We conclude that small
tributaries have rescued the spatially structured populations from
extirpation (charr) or at least slowed down extirpation (salmon). Legal
protection of headwaters as aquatic habitats is weak globally. Our results
suggest that stream management plans underestimating the demographic value
of small tributaries will likely fail to conserve populations of headwater
inhabitants and therefore endanger aquatic biodiversity. We discuss
conservation implications of this study related to habitat connectivity
and fisheries management.