The effect of inter-and intraspecific competition on individual and population niche widths – a four-decade study on two interacting salmonids

Competition is assumed to shape niche widths, affecting species survival
and coexistence. Expectedly, high interspecific competition will reduce
population niche widths, whereas high intraspecific competition will do
the opposite. Here we test in situ how intra- and interspecific
competition affects trophic resource use and the individual and population
niche widths of two lacustrine fish species, Arctic charr and brown trout,
covering a 40 year study period with highly contrasting competitive
impacts prior to and following a large-scale fish culling experiment.
Initially, an overcrowded Arctic charr population dominated the study
system, with brown trout being nearly absent. The culling experiment
reduced the littoral Arctic charr density by 80%, whereupon brown trout
gradually increased its density in the system. Thus, over the study
period, the Arctic charr population went from high to low intraspecific
competition, followed by increasing interspecific competition with brown
trout. As hypothesized, the relaxed intraspecific competition following
the experimental culling reduced individual diet specialization and
compressed population niche width of Arctic charr. During the initial
increase of the brown trout population, there was a large dietary overlap
between the two species. Over the subsequent intensified interspecific
competition from the population build-up of brown trout, their trophic
niche overlap chiefly declined due to a dietary shift of Arctic charr
towards enhanced zooplankton consumption. Contrary to theoretical
expectations, the individual and population niche widths of Arctic charr
increased with intensified interspecific competition. In contrast, the
diet and niche width of brown trout remained stable over time, confirming
its competitive superiority. The large-scale culling experiment and
associated long-term research revealed pronounced temporal dynamics in
trophic niche and resource use of the inferior competitor, substantiating
that intra- and interspecific competition have large and contrasting
impacts on individual and population niches.