Sex allocation promotes the stable co-occurrence of competitive species

Kazuya Kobayashi

doi:10.1038/srep43966

Abstract

Biodiversity has long been a source of wonder and scientific curiosity. Theoretically, the co-occurrence of competitive species requires niche differentiation, and such differences are well known; however, the neutral theory, which assumes the equivalence of all individuals regardless of the species in a biological community, has successfully recreated observed patterns of biodiversity. In this research, the evolution of sex allocation is demonstrated to be the key to resolving why the neutral theory works well, despite the observed species differences. The sex allocation theory predicts that female-biased allocation evolves in species in declining density and that this allocation improves population growth, which should lead to an increase in density. In contrast, when the density increases, a less biased allocation evolves, which reduces the population growth rate and leads to decreased density. Thus, sex allocation provides a buffer against species differences in population growth. A model incorporating this mechanism demonstrates that hundreds of species can co-occur over 10,000 generations, even in homogeneous environments, and reproduces the observed patterns of biodiversity. This study reveals the importance of evolutionary processes within species for the sustainability of biodiversity. Integrating the entire biological process, from genes to community, will open a new era of ecology.

Highlights

Biodiversity has long been a source of wonder and scientific curiosity[1]
The theory predicts that female-biased allocation evolves in species with declining density and that this allocation improves population growth, which should lead to an increase in the density
Previous studies[19,20,21] have shown that this negative density-dependent effect of adaptive sex allocation on population growth allows two competitive species to co-occur in a homogeneous environment that does not allow niche differentiation

Summary

Results and Discussion

Species-specific amounts of resources were sequentially set from 2 to 1.1 at an equal interval (0.1), and the results indicate that 10 species co-occurred over 2,000 generations (Fig. 1). The model indicates that the species with relatively few available resources generate a small population size and occasionally go extinct This extinction arises from interspecific competition over space and from their small population size, which reduces genetic variation and the possibility of new mutations, both of which are essential for the evolution of female-biased allocation. In a simulation that began with 1,000 species and when species-specific amounts of available resources are sequentially set from 2 to 1.001 at an equal interval, the number of species co-occurring over 10,000 generations was positively correlated with the total number of individuals in the community (Fig. 2) This result suggests that with infinite space, any species can survive, even if it does so at an infinitely small frequency, despite the homogeneity of the environment and the differences in the amount of resources allocated by each species to reproductive functions. In Hamilton’s formula, the number of individuals of the focal species is fixed in Pollen biased

Allocation to seed

Species rank in abundance

Additional Information