Species-Area Relationship: A Mesocosm Experiment with Annual Plants

1. The increase in species richness with increased island area is
one of the most widely documented patterns in ecology and biogeography but its
mechanisms are still under debate. Disentangling these mechanisms in natural
systems is challenging due to various kinds of confounding effects.

2. Here we use a novel mesocosm experiment focusing on annual
plants to disentangle the effects of two mechanisms that may lead to higher
richness on large islands: increasing population sizes and increasing opportunities
for within-island dispersal. Theoretical studies show that both mechanisms may contribute
to higher richness on large islands, but no previous study has attempted to
separate their effects. We also test an alternative, 'null' hypothesis,
according to which the only mechanism underlying the increase in richness with increased
area is a pure sampling effect (the 'passive sampling hypothesis').

3. As expected, increasing area had a strong positive effect on
species richness. However, the main mechanism underlying the difference in
richness between small and large islands was passive sampling. Moreover, our
results indicate that dispersal had a negative rather than positive effect on
island richness.

4. Synthesis: Our results corroborate previous observational
studies that failed to reject the passive sampling hypothesis. However, in
contrast to previous studies, our findings are based on experimental manipulations
of island area, under controlled uniform conditions, at small spatial scales
that facilitate within-island dispersal, and in three independent systems that
differed in resource availability and disturbance. These findings emphasize the
need for a careful examination of sampling effects in future research of the
species-area relationship.