Development is a frequent factor in protecting ecosystems. Disturbance of habitats can disrupt partnerships between plants and pollinators. But there’s more to reproduction than pollination. Joshua Borràs and colleagues in Mallorca studied the reproductive ecology of Arum pictum. Their investigation, following the plant from pollination to seedling recruitment, published in the Annals of Botany, reveals that apparently healthy plant-pollinator relations may mask deeper problems.
Arum pictum is a sneaky plant that deceives insects into pollinating it. The flowers produce heat and a scent, including chemicals associated with cow dung to attract insects. This is exactly the sort of thing sphaerocerid flies are looking for. The aim of the plant is to persuade an insect that they’ve found a site to lay their eggs. When the fly arrives, they pick up pollen and take it to another likely brood site that might well be another Arum.
Disturbing the sites appears to boost the reproductive chances of the plants, at least at first sight, write Borràs and colleagues. “Our results show that disturbed habitats (<75% of natural habitats in the surroundings) have higher pollinator abundance and diversity compared to natural habitats (>90% of natural habitats). Pollinator abundance was three times higher in disturbed habitats, where seed production increased by ~30% compared to natural habitats.”
The increase for the pollinators might seem odd, but it depends on the disturbance. In this case, the authors note that the arrival of goats and sheep, as well as other agriculture, increases opportunities for the flies. Yet there can be too many flies for a plant, say Borràs and colleagues.
“Our results show two different scenarios. In disturbed habitats, where overall seed production is higher, an increase in Coproica abundance translated into a decrease in the reproductive output of A. pictum ssp. sagittifolium. On the contrary, in natural habitats, the increase in Coproica abundance produced an increase in the reproductive output.”
“As we found that disturbed habitats showed higher pollinator abundance, the relation between Coproica abundance and reproductive output may be bell-shaped. Therefore, at low abundances (like in natural habitats) production increases with abundance, while at high abundances (as in disturbed habitats) it decreases due to a lower efficiency in pollen transfer.”
“Alternatively, it could be that when pollinator abundance is high, seed production is negatively affected because pollinators physically bother each other when moving, or by they are accidentally damaging the plant reproductive organs…”
Another difference was in seedling recruitment. There were more seedlings and juveniles by adult plants in the natural habitats than in the disturbed habitats. It’s not entirely clear why and the authors suggest both a lack of long-distance dispersal or disruption to dispersal networks. In all cases, seedlings substantially outnumbered juveniles, indicating high mortality for young plants. It’s not certain that the seedlings were growing by their own parents, said Borràs and colleagues. “We recorded on a few occasions, birds defecating beside adult plants while eating its berries.”
Given that this new generation of plants is the ultimate goal of reproduction, the difference between seedling success and pollination is striking. Borràs and colleagues conclude that understanding reproductive ecology requires more than just studying pollination or seed set.
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Borràs, J., Lázaro, A., González-Estévez, M.A. and Cursach, J. (2022) “Effects of habitat disturbance on the reproductive ecology of Arum pictum ssp. sagittifolium: from pollination to seedling recruitment,” Annals of Botany. https://doi.org/10.1093/aob/mcac120