Island archipelagos have caught the attention of the biologically inquisitive for centuries, including the classic case of Darwin’s voyage to the Galapagos Island where he described variation in beak form and function between finch populations on different islands as a consequence of natural selection. Ever since, scientists have reported similar observations for neighbouring island populations. For example, Stuart et al. (2014) attributed variation in micro-habitat selection and toe shape in Anolis lizards to rapid evolution on islands after the colonization of a non-native competitor.
Working on island plants, my co-author Peter Arcese previously noted that seablush (Plectritis congesta) in the southern Gulf Islands of British Columbia either formed tall, dense mats of flowers predominately with ‘winged’ fruits, or sparsely distributed, short, and relatively inconspicuous patches of flowers with a second ‘wingless’ fruit type. After being introduced to the seminal work of Ganders et al. (1977) and Carey (1983) on Plectritis 30 years previously, Peter initiated a series of studies to ask what causes these dramatic differences between island populations.
In 2014, I joined Peter to complete surveys of 285 populations with and without deer present to test the hypothesis that herbivory drives the differentiation of populations, given anecdotal observations that islands with deer tended to have shorter plants with wingless fruits, and islands without deer tended to have taller plants with winged fruits (Figure 1).
Acknowledging that factors such as climate, soil depth, and isolation might also influence plant appearance and fruit type, we also collected data on these variables across the Georgia Basin to test for interactive effects (Figure 2).
Results of our study, published in our Journal of Ecology paper, support the hypothesis that browsing by deer, or factors closely related to deer presence, contribute strongly to population-level variation in fruit phenotype and plant height in seablush, with climate having comparatively modest effects on plant height and fruit type.
We detected no effect of population isolation on phenotype, except on islands rarely visited by deer, where the fraction of wingless fruits became more similar to values observed in populations without deer as their isolation increased. Given that plant height is likely to be influenced by a number of genes (polygenic), plasticity in plant height is also expected. Our data suggest strong correlations between herbivory by deer and differences in inherited traits between populations, but do not prove causation. Additional work is under way to estimate genetic and environmental components of variation in phenotype and to estimate the effect of such traits on plant fitness.
Cora Skaien, University of British Columbia, Canada