Evol Ecol Res 19: 707-722 (2018)     Full PDF if your library subscribes.

Evolution of reproductive phenology in annual social insects competing for floral resources

Magnus Lindh¹, Jörgen Ripa² and Jacob Johansson^2,3

¹Department of Physical Geography, Stockholm University, Stockholm, Sweden, ²Department of Biology, Lund University, Lund, Sweden and ³Department of Life Sciences, Imperial College London, Silwood Park, UK

Correspondence: M. Lindh, Department of Physical Geography, Stockholm University, SE-106 91 Stockholm, Sweden. email: magnus.lindh@natgeo.su.se

ABSTRACT

Question: How does reproductive timing in annual social insects evolve under intraspecific exploitation competition and in response to a changing environment? The latter includes shifts in resource abundance, mortality, season length, and resource peak timing, factors often associated with climate change.

Background: Climate and land use are changing. Evolutionary ecology must learn to predict how organisms are affected by, and might adapt to, such changes. Classic theory predicts how the timing of reproduction in organisms with an annual life-cycle (including many plants and social insects) optimally balances their phases of growth and reproduction with seasonal resource distributions. But theory has yet to take into account interactions within or between species.

Method: We use invasion analysis to calculate the evolutionarily stable reproduction time by extending a classic model of annual social insects (such as bumblebees) to include competition for seasonally variable resources. There are two types of (colony) production: vegetative (workers) and reproductive (queens). The initial worker density in the landscape depends on the density of the surviving queens from the previous year. Reproduction time is approximated as a sudden (bang-bang) switch from vegetative to reproductive production during the season. We compare the evolutionarily stable strategy (ESS) with the classical optimization result without competition. We also compare it with the strategy that maximizes population size under resource competition.

Results and conclusions: Under resource competition, the ESS reproduction time occurs earlier than the optimum predicted by classic theory. But the ESS is later than the strategy that maximizes population size. Thus phenological adaptation to environmental change is likely to reduce population sizes. Both the ESS and the optimal reproductive time change more slowly than do shifts in seasonal resource peaks. Hence a growing asynchrony between peak flowering times and the timing of bumblebee reproduction – often interpreted as a phenological mismatch – may actually be an adaptive response of pollinators in this system.

Keywords: adaptive dynamics, annual social insect, bumblebee, climate change, phenological mismatch, reproductive phenology, resource competition.

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