Competing species leave many potential niches unfilled

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A cornerstone of biology is that coexisting species evolve to occupy separate ecological niches. Classical theory predicts that interspecific competition should lead to all potential niches being occupied, yet observational data suggest that many niches are unfilled. Here we show that theory can be reconciled with observational data by reconceptualizing competition in the Hutchinsonian niche space to distinguish between substitutable and non-substitutable resources. When resources are substitutable (for example, seeds of different size), the components of competition along the niche axes combine multiplicatively, leading to a densely packed niche space. However, when resources are non-substitutable (such as seeds and nest sites), we show that the components of competition combine additively. Disruptive selection therefore limits niche overlap between non-substitutable niche axes, leaving most potential niches unfilled. A key corollary is that increasing the number of niche axes may greatly increase the number of potential niches but does not necessarily increase diversity. We discuss observational data that are consistent with our model and consider implications for systems with invasive species. Our work reinforces the power of competition to drive major ecological patterns: while niche space informs on species that might exist, only a small and potentially arbitrary subset will coexist in sympatry.

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Fig. 1: Competition within and across resource types based on the overlap of resource utilization functions.
Fig. 2: Evolutionary trajectories of species in two-dimensional niche space consisting of either substitutable or non-substitutable resources.
Fig. 3: Number of occupied niches at equilibrium.
Fig. 4: Illustration of nonoverlapping allelic associations in 616 isolates of Streptococcus pneumoniae collected in Massachusetts, USA, between 2001 and 200723.


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We thank G. Barabás, T. Gross, R. Iritani, S. Nee and R. Noble for helpful comments on the manuscript. B.A. acknowledges funding from the Natural Environment Research Council (NE/N014979/1). S.G., E.W. and J.L. received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 268904-DIVERSITY. K.R.F. is funded by European Research Council grant 242670 and a Calleva Research Centre for Evolution and Human Science (Magdalen College, Oxford) grant.

Author information

K.R.F. and S.G. conceived the study. E.W. and J.L. analysed empirical data sets. B.A., E.W. and S.G. performed modelling work. B.A., E.W., K.R.F. and S.G. contributed equally to writing the manuscript.

Correspondence to Sunetra Gupta or Kevin R. Foster.

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Supplementary Figure 1

Supplementary Code

Source code for the model described in Ashby et al. ‘Competing species leave many potential niches unfilled’

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