Abstract
Seasonal variation in the availability of floral hosts or pollinators is a key factor influencing diversity in plant-pollinator communities. In seasonally dry Neotropical habitats, where month-long periods of extreme drought are followed by a long rainy season, flowering is often synchronized with the beginning of precipitation, when environmental conditions are most beneficial for plant reproduction. In the Brazilian Cerrado, a seasonally dry ecosystem considered one of the world’s biodiversity hotspots for angiosperms, plants with shallow root systems flower predominantly during the rainy season. Foraging activity in social bees however, the major pollinators in this biome, is not restricted to any particular season because a constant supply of resources is necessary to sustain their perennial colonies. Despite the Cerrado’s importance as a center of plant diversity, the influence of its extreme cycles of drought and precipitation on the dynamics and stability of plant-pollinator communities is not well understood. We sampled plant-pollinator interactions of a Cerrado community weekly for one year and used network analyses to characterize intra-annual seasonal variation in community structure. We also compared seasonal differences in community robustness to species loss by simulating extinctions of plants and pollinators. We find that the community shrinks significantly in size during the dry season, becoming more vulnerable to disturbance due to the smaller pool of floral hosts available to pollinators during this period. Major changes in plant species composition but not in pollinators has led to high levels of turnover in plant-pollinator associations across seasons, indicated by in interaction dissimilarity (<3% of shared interactions). Aseasonal pollinators, which mainly include social bees and some solitary specialized bees, functioned as keystone species, maintaining robustness during periods of drastic changes in climatic conditions.
Highlights
Some of the most impressive natural phenomena on Earth, such as the great migrations of wild ungulates in Africa, are linked to the effects of seasonal rainfall on trophic interactions between species [1, 2]
Eleven species of semi- or eusocial bees were collected, most of which belonged to the native stingless bees tribe (Meliponini)
The top ten pollinator species with the widest niches in this community maintained a large number of interaction partners in both seasons (Fig 3; S2 Table)
Summary
Some of the most impressive natural phenomena on Earth, such as the great migrations of wild ungulates in Africa, are linked to the effects of seasonal rainfall on trophic interactions between species (e.g., between primary producers and consumers) [1, 2]. Neotropical savannas are ecosystems characterized by this bi-seasonal climatic regime comprising the Cerrado, the Llanos, and some isolated, smaller pockets of vegetation scattered throughout South and Central America [6, 7] Together these neotropical savannas originally covered a large proportion of the tropical Americas (estimated to be ~3 million km2) [7] and their conservation has been considered a global priority based on their high diversity and percentage of endemism [6, 8]. Despite their importance, these seasonal habitats are understudied and little information exists about the antagonistic effects of water stress and intense rainfall on species phenologies and subsequent temporal variation in plant-animal interactions at the community level [5, 9]. Understanding how plant-pollinator communities from such habitats are organized and how they function under such contrasting climatic conditions is important for gaining insights about factors promoting the high diversity characteristic of Neotropical ecosystems, and for estimating community responses to future changes in climate [5]
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