American Society of Naturalists

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“Species differences in phenology shape coexistence”

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Christopher Blackford, Rachel M. Germain, and Benjamin Gilbert (June 2020)

Manipulation of species’ phenologies overturns competitive hierarchies with implications for coexistence

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<i>Vulpia microstachys</i> and <i>V.&nbsp;octoflora</i> growing in the greenhouse.<br />(Credit: Christopher Blackford)
Vulpia microstachys and V. octoflora growing in the greenhouse.
(Credit: Christopher Blackford)

Species in ecological communities often show striking differences in phenology – the seasonal timing of important life events. The phenology of one species is likely to affect its ability to coexist with other species but ecological theory has contrasting predictions on if species with similar phenologies are more, or less likely to coexist with each other. On one hand, differences in phenology could reduce competition for resources. On the other hand, a species with early phenology could reduce resources available to later species and thus limit coexistence.

To resolve these contrasting predictions, researchers from the University of Toronto and the University of British Columbia performed a greenhouse experiment on two competing annual grass species (Vulpia microstachys and V. microflora) to test how phenological separation of germination timing, affected coexistence. By manipulating the relative germination timing of these species, they were able to observe how phenological separation affects coexistence. They found early germination provided a competitive advantage, and thus phenological differences reduced within-year coexistence. However, through modelling they found that when the identity of the early germinating species changed across growing seasons, it was possible for phenological separation to increase coexistence.

Overall, this research clarifies how phenological differences structure competitive interactions and highlight the need to quantify year-to-year variation in these differences to better understand species coexistence.


Abstract

Ecological theory produces opposing predictions about whether differences in the timing of life history transitions, or ‘phenology’, promote or limit coexistence. Phenological separation is predicted to create temporal niche differences, increasing coexistence, yet phenological separation could also competitively favor one species, increasing fitness differences and hindering coexistence. We experimentally manipulated relative germination timing, a critical phenological event, of two annual grass species, Vulpia microstachys and V. octoflora, to test these contrasting predictions. We parameterized a competition model to estimate within-season niche differences, fitness differences, and coexistence, and to estimate coexistence when year-to-year fluctuations of germination timing occur. Increasing germination separation caused parallel changes in niche and fitness differences, with the net effect of weakening within-year coexistence. Both species experienced a competitive advantage by germinating earlier and a four-day head start allowed the generally inferior competitor to exclude the otherwise superior competitor. The overall consequence of germination separation was to limit coexistence within a given year, although year-to-year variation in relative timing of germination was sufficient to support long-term coexistence. Our results clarify how phenological differences structure competitive interactions and highlight the need to quantify year-to-year variation in these differences to better understand species coexistence.