Mixed evidence for the small island effect in a replicated colonisation experiment

Prepared by Mikhail V. Kozlov, Vojtěch Lanta & Vitali Zverev

Examples of experimental plots: (a) Kevo, plot area 2 m2, 2 September 2008; (b) Apatity, plot area 0.25 m2, 16 August 2008; (c) Seili, plot area 0.05 m2, 5 June 2010; (d) Lisino, plot area 0.01 m2, 27 August 2009. Photo credit: Vitalij Zverev.

We studied species-area relationship in ‘small islands’ artificially created in a forested landscape. We asked whether the phenomenon called Small Island Effect (SIE), expressed as the independence of species richness from the island size, exists in these environments. So far, the occurrence of SIE has been detected only in natural ecosystems, and the absence of experimental data hampered the understanding of conditions under which SIE may emerge. This motivated us to run a colonization experiment in four sites within extensively forested landscape at the northern (Kevo – Finland and Apatity – Russia) and southern (Seili – Finland and Lisino – Russia) borders of the boreal forest zone. At each site, we created 16 experimental plots (ranging from 0.01 to 2 m2 in size) with bare ground. We monitored colonisation of these plots by vascular plants in early and late summers of three consecutive years and modelled species-area relationships for each season×year×site combination by linear and breakpoint regressions. From a statistical perspective, SIE is manifested by a zero slope below the threshold (breakpoint) and a positive slope above it.

The number of plant species was (six times) greater in the largest than in the smallest plots. Nevertheless, SIE was to our surprise observed in half of our data sets, while the other half showed a classical linear species-area relationship. Our study showed pronounced SIE on plots with a smaller species pool of plant colonisers and at the initial stages of plant succession. As a bonus, by incorporating a plant trait functional approach we identified the likely drivers of this interesting biogeographical phenomenon.

This is a plain language summary for the paper of Kozlov et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13103).