Why are so few island bryophytes endemic?

Area of Science:

• Island biogeography
• Bryophyte evolutionary biology
• Phylogeography and speciation

Background:

• Endemism is a key feature of island biodiversity, but bryophytes show lower levels compared to other land plants.
• The reasons for low bryophyte endemism are debated, including low diversification rates, extinctions, or continental (back-)colonization.

Purpose of the Study:

• To investigate the causes of low endemism in Macaronesian bryophytes.
• To differentiate between low diversification rates and loss of endemicity as explanations for low endemism.

Main Methods:

• Review of available evidence on Macaronesian bryophyte flora.
• Analysis of genetic differentiation using G_ST (gene differentiation) and N_ST (phylogenetic gene differentiation).
• Estimation of ancestral areas for endemic bryophyte species.

Main Results:

• Significant genetic differentiation (N_ST > G_ST) was found, indicating incipient speciation and higher mutation than dispersal rates.
• Incipient speciation is more likely between distant populations (Macaronesia vs. extra-European) than closer ones (Macaronesia vs. European).
• Over 50% of Macaronesian endemic bryophytes have extra-European origins, unlike spermatophytes which are predominantly European/Mediterranean.

Conclusions:

• Allopatric speciation via long-distance dispersal, not sympatric radiations, prevails in island bryophytes, leading to lower endemism rates.
• Mechanisms hindering in situ diversification include small island size, insufficient isolation, lack of key innovations, and niche conservatism.
• Frequent continental back-colonization, without subsequent mainland speciation, further reduces bryophyte endemism, invalidating endemism rates as a proxy for speciation.