Most years of my life I have spent the summer holidays staying at a beach. Usually the beach is at Kaka Point, a great sandy beach at the northern end of the Catlins in southern South Island. New Zealand has an enormous coastline full of beaches, most  further north and warmer than Kaka Point. So why do I drag my wife and children there? It comes down to relations. I grew up on a farm in South Otago only 15 minutes away. My grandparents lived in Kaka Point. I spent holidays staying with Molly and Jim, watching the blowhole errupt near their house, wading in rock pools, turning over rocks, watching dolphins and seals swimming while sitting on the cliffs. In short, this is what made me want to be a zoologist. Sure the water is cool, but you can throw on a wetsuit. Sure the waves come from Antarctica, but they’re big and great for boogie-boarding. Sure it’s a long way from many things, but you can access wildness and wildlife, yellow-eyed penguins and sealions often visit. I had uncles and aunties with holiday houses (cribs) at Kaka Point as well. So the Patersons can be found at this beach because of historical relations. My ancestors went there, so do I. My cousins from all around New Zealand do the same.

What about the plant life at the beach? Sand dunes and coastal habitat are a small but distinctive habitats. Plant species found at beaches could be there for a number of reasons. Perhaps they are a random collection of inland species from the local countryside that can survive in sandy soils (close relatives are found in-land). Maybe they are specialist dune species that are carried by wind and sea currents from one beach to the next (close relatives are on beaches far away). Perhaps they are specialist species that arise within the local area (close relatives are found on local beaches).

Hannah Buckley and colleagues from around the world have looked at this question and published their findings in the Journal of Plant Ecology. They compared plant species data collected from 18463 sites from coastal dunes around much of the world. For each plot they recorded the plant species found at the site, and measures of plant traits, like height and seed size. An evolutionary tree for the plants was used to identify how different species were related to one another.

Closely related species tended to be found in local regions. Like the Paterson clan, relatives were often at the beach together. The equator formed a boundary, with the northern and southern hemispheres different from one another. There was also a difference depending on the type of coastal vegetation at a site. Plants with short height (herbs and succulents) were more likely to have closely related species at sites than taller species (scrub and trees). The analysis done by Buckley and colleagues suggests that much diversification of species occurs within localised regions. Local conditions that suit a species are also likely to suit close relatives.Next time I bump into my relatives at the beach I will know that I am in good company with the plant species around me.