Changes in the large-scale distribution of plants: extinction, colonisation and the effects of climate


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1. Understanding large-scale distribution shifts is vital to predict species responses to changes in the environment. Such shifts occur as a consequence of habitat distribution, dispersal processes and the influence of environment factors.

2. Historical distribution records provide a long-term perspective on change. Here, we use presence/absence data on 1781 British plant species to examine distribution shifts in the 20th century. Our aim was to determine the importance of long- and short-distance colonisation in the spread of plant populations.

3. We consider three contrasting models of distribution change: random colonisation, where colonisation occurs independently of the established distribution, localised phalanx, where favourable local conditions and/or short-distance dispersal results in colonisation of neighbouring sites only, and phalanx-spread, which is the diffusion-like spread of a population through localised colonisations.

4. We fitted a set of four generalised linear models to contrast various forms of the three mechanisms of distribution change. Model selection was used to assess the relative fit of each.

5. Overall rates of extinction and colonisation were low, but highly variable. They were strongly linked with total occupancy, which reduced the probability of extinction and increased that of colonisation.

6. Comparison of the models indicated that the majority of distributions change through a phalanx-spread process. This indicates that local habitat distribution, as well as localised dispersal, are key driving processes. Long-distance colonisation is, in particular, very rare.

7. An additional set of three models was fitted using rainfall and temperature data. These models were of improved fit compared to the models of distribution change. A clear climate signal could be identified for c. 45% of species, while for the remainder there was a signal that could not be unequivocally attributed to rainfall or temperature.

8. Synthesis. Our analysis shows that the spread of most species is spatially restricted at a national scale. This is likely to be a consequence of habitat suitability and dispersal. Habitat changes, such as climate, occur (on a decadal scale) through relatively slow spread of changes. Dispersal is typically restricted, so that new habitat is usually colonised only if it is adjacent to already colonised patches. Our findings highlight the importance of prior knowledge of dispersal mechanisms as well as local habitat structure and availability in species distribution modelling research and species conservation initiatives.