Life on a leaf: the development of spatial structure in epiphyll communities

1. The spatial structure of biotic communities can be shaped by
niche-based or stochastic processes, and the importance of both can change
through time. Niche-based processes include neighbour interactions, which
can change in intensity and quality as communities develop in dependence
of environmental conditions. Epiphylls, miniature communities of
liverworts, lichens, algae and fungi on leaves, develop only in relatively
moist forests, but their leaf-surface habitat is still characterised by
moisture stress, especially in more exposed parts of the forest. As
neighbours may alleviate moisture stress, we expected that in forest gaps
(dryer, brighter) epiphyll communities would show more positive neighbour
interactions, resulting in more clustered spatial patterns, than in closed
forest, in accordance with the stress-gradient hypothesis. 2. To
understand how the processes shaping epiphyll communities change through
time and differ between gaps and closed forest, we examined the fine-scale
spatial structure within and between epiphyll functional groups (algae,
fungi, lichens, liverworts) during 12 months on 60 leaves across twelve
sites in a tropical rainforest in Panama. We analysed changes in spatial
positions and spatial associations within and between functional groups
using spatial point pattern analysis based on repeated photography. 3.
Epiphyll densities increased through time for most epiphyll functional
groups. However, although epiphyll groups changed their positions through
time, the general types of spatial patterns remained similar for most
groups: only lichen patterns shifted from mostly random to mostly
aggregated through time. We found only random and aggregated spatial
patterns within groups, while among groups we additionally found
segregated spatial associations, but no consistent temporal trend.
Patterns did not differ between gaps and closed forest, thus not
supporting the stress-gradient hypothesis. 4. Synthesis. Our results only
weakly support the assumption that community-shaping processes change from
neutral to niche-based as communities develop. We do provide empirical
evidence that epiphyll communities are dynamic and exhibit different
spatial association patterns within than among groups. This suggests
different interactions and processes within than between groups.
Experimental studies and monitoring of individual epiphyll patches are
recommended to shed further light on what processes shape life on leaves.