DOI: 10.5061/dryad.83bk3j9q4
title: Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charrs in Hokkaido, Japan

description: 1. Coexistence of species requires equalizing mechanisms that minimize
 fitness differences, which are balanced by stabilizing mechanisms that
 enhance negative intraspecific interactions versus interspecific ones.
 Here, we develop a simple theoretical framework that allows measuring the
 relative strength of intraspecific versus interspecific competition in
 dominance hierarchies. We use it to evaluate mechanisms promoting
 coexistence between congeneric charrs that compete for foraging positions,
 which strongly influence density-dependent growth and survival. 2.
 Agonistic interactions (n = 761) among 71 Dolly Varden Salvelinus malma
 and whitespotted charr S. leucomaenis were measured by snorkeling in two
 pools in the sympatric zone of a Hokkaido stream during two summers.
 Interspecific dominance hierarchies, analyzed using three methods, were
 closely correlated with fish length but the species treated each other
 equally. Ranks for the most dominant fish in each pool, determined
 directly by knockout experiments, were also virtually identical to ranks
 by length. 3. Similarly, exponential random graph modeling of the social
 networks provided no evidence that either species was dominant over the
 other. Instead, larger fish were more likely to win contests, especially
 over fish of the next lower ranks. 4. These results demonstrated that the
 two species were nearly ecological equivalents in accessing key resources
 in this sympatric zone. Nearly identical growth and stable densities over
 4 years further supported this inference, although Dolly Varden were a
 minority (29% of the assemblage), a sign of some fitness difference. 5.
 Detailed foraging observations coupled with two concurrent studies
 revealed an effective stabilizing mechanism. Dolly Varden shifted to
 feeding directly from the benthos when drifting invertebrates declined, a
 behavior enhanced by morphological character displacement, thereby
 partitioning food resources and enhancing intraspecific competition while
 avoiding agonistic encounters with whitespotted charr. 6. The plurality of
 evidence indicates that fitness differences between these ecologically
 equivalent species are small in this local assemblage, and balanced by
 resource partitioning, a modest stabilizing mechanism that promotes
 coexistence. The theoretical framework presented here is a useful tool to
 evaluate the strength of interspecific versus intraspecific competition,
 which combined with information on tradeoffs in ecological performance can
 contribute to a mechanistic understanding of species coexistence.
 26-Oct-2020
description: Please see the file: <em>Metadata for dominance
 matrices.docx</em> The data were collected by the
 three senior authors, by snorkeling in two pools of a Hokkaido stream (see
 Figure S1) during the same period in late June and early July of two
 summers.  Focal-animal observations (5 minutes each) were made of
 individual fish several times each day, typically in their order in the
 dominance hierarchies.  Observations were made by snorkelers for 4 hours
 each day, during 2-hour periods distributed nearly equally between morning
 and afternoon.  Two periods (early, late) were defined for Pool B in 1991
 because many fish had left or redistributed during a 2.5-day hiatus in
 observations. All agonistic encounters were recorded on
 plastic graph paper overlaid on scale maps of each pool (Figure S2). 
 Examples of the agonistic encounters recorded can be seen in Figure
 S4.  Fish were individually identified by species, length, and unique
 spots, parr marks, and scars, which were drawn and cataloged.  The fish
 identification codes are identical to those in Table S2. 
 The data were transcribed into a spreadsheet, and the number of
 one-sided contests, where the winner excluded the loser from its defended
 space, was totaled for each pair of individuals.  Encounters where neither
 fish won, called draws, were excluded. The dominance
 hierarchies reported here are ordered by the average dominance
 rank, estimated using three independent methods: the I&SI
 (Inconsistencies and Strength of Inconsistencies) method, David's
 score, and the randomized Elo rating method (see text of article). 
   Fish labeled as "Benthic" foraged
 frequently by picking invertebrates directly from the benthos (&gt;25%
 of foraging forays; see Table S2). 
description: Please see the file: <em>Metadata for dominance
 matrices.docx</em>