Life is subject to cycles, and they are visible even in our daily routines. Just as day and night alternate, so do dry and wet seasons (at least, in tropical areas). Likewise, plant species flower and fruit in cycles, from repeatedly to just once in their lifetime. The most frequent patterns are annual or supra-annual, with several years in between reproductive episodes, also known as mast reproduction events. The science dedicated to the study of the timing of life cycles of organisms and their relation with the environment is called phenology.
Human interest in phenology began a long time ago, as people’s lives depended on knowing what was the best time to cultivate a given crop (rice, corn, cherry or grape, for instance). The variation in the availability of flowers and fruits also has tremendous consequences for the animals that interact with them for pollen, nectar or fruit. There are moments of the year when resources are so scarce that animals need to change their diet or behavioural patterns. Therefore, understanding how fruit production changes both seasonally and inter-annually can help us to predict the availability of resources that frugivores can encounter.
To study the seed and fruit production of an Amazon forest in French Guiana (Nouragues National Reserve), we established (in early February 2001) a long-term monitoring project using seed traps – a 0.5 sqm mesh hanging out in the forest collecting everything that was falling from trees. Over 10 years, every single seed or fruit that fell in our trap was dried, identified by specialized botanists and kept in a seed and fruit collection by the Muséum National d’Histoire Naturelle in Brunoy, France. Thanks to this impressive dataset, we were able to analyse how seed and fruit production changed over time, for 45 abundant species of a pristine neotropical forest.
Using an original and innovative statistical approach, we were able to evaluate changes in both the timing and quantity of fruit production, separately. It is important to discern whether a given species varied in its annual fruit production, and also, whether the time of the year when fruiting peaked was subject to variation. We were able to show that seed production changed substantially both in timing and quantity across years, but variation was greater in quantity than timing. This meant that most species altered the amount of fruit they produced every cycle, but the timing of their fruit production tended to remain the same. This has relevant consequences for frugivores because they rely on a permanent availability of fruit resources for their survival. Although there was a peak of fruiting during the rainy season, fleshy fruits were available throughout the year, supporting the idea that resident frugivores could alternatively change their feeding resources according to what was fruiting at each time of the year.
Our analysis also showed a surprising fact: species which changed the most across years in the amount of fruit they produced also tended to change more in their time of fruit production. This was particularly true of masting species (40% of studied species), i.e. species which produced synchronous and irregular pulses of fruiting with intervals of more than two years. Although the mechanisms implied in masting are still not well understood, mast fruiting has the potential to increase seed survival, thanks to the satiation of seed predators.
The least variable species of our dataset was Virola michelii (Myristicaceae, the nutmeg family), which always fruited in late January and early February with a relatively constant amount of fruit. We hypothesised that this species fruits when there is low availability of other fruit species, thus attracting specialist frugivores such as spider monkeys and toucans, and increasing dispersal chances. In this case, being constant in its timing of fruiting is important to all its dispersers – spider monkeys, toucans, kinkajous and large birds, which depend on the nutritious flesh of this species at a time of the year when fruits are scarce.
Although we still need to gain more knowledge of the climatic and biotic drivers of life cycles of plants, what our study points out is that shifts in both quantity and timing of seed production might have dramatic consequences for the animals which depend on fruits and plants for survival. And this can be much worse within a climate change context.
Original paper: https://onlinelibrary.wiley.com/doi/full/10.1111/btp.12560