BSBI News

Invasive Woody Plants

Tree Autecology and Biology

Temperate Forest Ecology

Tropical Forest Ecology

Plant translocation, habitat fragmentation, seed dispersal and climate change.

Published in 1994 in BSBI News 66, 12-13.

Since last century (e.g. Anon. 1897) the pros and cons of plant translocations in the British Isles have regularly been discussed, often with passion. The current debate in BSBI News fails to fully take into account the implication of habitat fragmentation in relation to dispersal ability of plant species and climate change. Throughout vegetation history climate has changed, and plants have had to migrate along climatic gradients in order to survive. Often species have been able to survive in regions of unfavourable climate (e.g. too dry, wet or cold) in small, but climatically favourable, geographical areas called plant refugia. Such refugia were common during the last ice age and modern equivalents may be found, for instance, for arctic species in the British Isles. Following further climate change these species would either become extinct or spread once again.

Modern habitat fragmentation results in the creation of islands of semi-natural vegetation which are in fact not dissimilar to plant refugia mentioned above. The difference is that whenever climate changes, either naturally or as a result of global warming, most species will be unable to migrate to climatically suitable habitats. The mode of dispersal of particular species will determine their ability to hop from island to island. Bird-dispersed and some well-adapted wind-dispersed species will be more likely to migrate whereas other species will fail to do so.

If the observed increase in CO₂ results in global warming as predicted, then in Britain the majority of plant species will have to migrate northwards or upslope. Furthermore if the climatic changes are extensive and rapid the likelihood that a species moves successfully will decrease. As a result we will be left with the choice of either loosing many species or we will have to help those which are unable to migrate across man-made landscapes.

In Britain the future distribution of most species is likely to reflect their ability to cope with modern man-made landscapes rather than natural environmental factors. If we sit and watch, we will be able to find out which species are capable of migrating through a man-made countryside and also witness a decrease in species richness as well as species extinctions. At the same time the spread of introduced species, chiefly ornamentals, into semi-natural vegetation from surrounding areas will accelerate leading to an increase in the relative importance of exotic species.

In order to maintain a diverse native flora, some degree of plant translocation may be desirable. It should involve only species unable to disperse between islands of semi-natural vegetation. Plants which can propagate vegetatively should be considered separately from obligate sexually reproducing species. In the latter group of species, individual plants can be translocated. If conditions are suitable they will produce seeds and regenerate, otherwise they will disappear. For plants which can propagate vegetatively as well as sexually, it may be advisable only to artificially disperse seeds rather than translocate plants. This would prevent the establishment of populations which are unable to produce seeds, which in natural landscapes would only spread locally and fail to migrate.

There are good justifications for limiting the translocations of exotic species, chiefly because some of them become invasive. The translocation of some native species may present similar problems. However the pros and cons of plant translocations in the British Isles are not clear-cut because of man's ever increasing impact on nature.

Anon. (1897) Ignorance and introduction. Ir. Nat. 6, 82.