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

Please see the file: Metadata for dominance matrices.docx

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 (>25% of foraging forays; see Table S2). 

Please see the file: Metadata for dominance matrices.docx