On the temporally flexible structure of plant-pollinator interaction networks

Abstract

Ecological communities consist of species that are joined in complex networks of 14 interspecific interaction. The interactions that networks depict often form and dissolve rapidly, 15 but this temporal variation is not well integrated into our understanding of the causes and 16 consequences of network structure. If interspecific interactions exhibit temporal flexibility across 17 time periods over which organisms co-occur, then the emergent structure of the corresponding 18 network may also be temporally flexible, something that a temporally-static perspective would 19 miss. Here, we use an empirical system to examine short-term flexibility in network structure 20 (connectance, nestedness, and specialization), and in individual species interactions that 21 contribute to that structure. We investigated weekly plant-pollinator networks in a subalpine 22 ecosystem across three summer growing seasons. To link the interactions of individual species to 23properties of their networks, we examined weekly temporal variation in species' contributions to 24 network structure. As a test of the potential robustness of networks to perturbation, we also 25 simulated the random loss of species from weekly networks. We then compared the properties of 26 weekly networks to the properties of cumulative networks that aggregate field observations over 27 each full season. A week-to-week view reveals considerable flexibility in the interactions of 28 individual species and their contributions to network structure. For example, species that would 29 be considered relatively generalized across their entire activity period may be much more 30 specialized at certain times, and at no point as generalized as the cumulative network may 31 suggest. Furthermore, a week-to-week view reveals corresponding temporal flexibility in network 32 structure and potential robustness throughout each summer growing season. We conclude that 33 short-term flexibility in species interactions leads to short-term variation in network properties, 34 and that a season-long, cumulative perspective may miss important aspects of the way in which 35 species interact, with implications for understanding their ecology, evolution, and conservation. 36 37 3 KEYWORDS 38 connectance, interaction turnover, nestedness, robustness, specialization, temporal ecology, 39 seasonality, subalpine 40 41 Structure of pollination networks. For all 42 weekly networks and for all three cumulative, 126 season-long networks, we investigated temporal variation in three metrics that describe different 127 aspects of the structure of interactions: (i) connectance; (ii) network-level specialization; and (iii) 128 network nestedness. For network connectance and nestedness, we calculated both binary 129 (unweighted) and frequency-based (weighted) metrics (network specialization is already a 130 frequency-based metric); because overall patterns were qualitatively similar when we calculated 131 either binary or frequency-based metrics for connectance and nestedness, we limit description and 132 ...