Life history variation across a riverine landscape: intermediate levels of disturbance favor sexual reproduction in the ant-dispersed herb Ranunculus ficaria

摘要

Global climate change can fundamentally alter disturbance regimes across landscapes, but little is known about how species adjust their life histories to shifts in disturbance regimes. In plants, dispersal by seeds may permit rapid re-colonization under frequent disturbances, but often seed-dispersing animals may be absent and local dispersal by vegetative diaspores may be a more efficient means of occupying open space in the vicinity of the plant. We tested the effect of disturbances due to inundation on the investment in seed production by the ant-dispersed plant, Ranunculus ficaria ssp. bulbifer. During seed ripening we collected 392 plants within a landscape mosaic of 39 sites with different levels of inundation. We measured the mass of fruits and other tissues (leafs, roots, bulbils, stalks) and described plant growth form. We found that fruit numbers and masses were more variable among plants than numbers and masses of other tissues. We then standardized fruit mass against the mass of other tissues and other growth-form parameters. Standardized fruit mass showed a highly significantly hump-shaped relationship with the level of inundation disturbances. This pattern was consistent across 12 small-scale transects and thus not confounded by spatial autocorrelation within landscapes. The pattern was also confirmed by analyses that simultaneously accounted for disturbance and morphological co-variables. We thus conclude that plants invested most heavily into reproduction by seeds under intermediate levels of disturbance. Under intermediate disturbance, seeds are beneficial for rapidly re-colonizing open space after disturbance while the seed-dispersers are still available. The life history of mutualists such as ant-dispersed plants and ants may thus change across a landscape, reflecting small-scale variation in the disturbance regime.