Fylogenetisk Systematikk og Evolusjon


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Surprisingly high genetic diversity of common toad (Bufo bufo) populations under strong natural fragmentation

In theory, the genetic consequences of habitat fragmentation can be dramatic in animals. Decrease of habitat patch size and increase of patch isolation of local population might lead to smaller populations and more isolation. Such fragmented populations are expected to have less gene flow (i.e. sexual contact among different populations) and more genetic drift (i.e. the change in the frequency of gene variants due to random effect). It is predicted to result in the erosion of genetic diversity within and simultaneously more differentiation among local populations. The consequences for the animals are less fitness and viability. That is why, in nature conservation politics argues for a network of protected areas connected by natural corridors.

the common toad Bufo bufo
A common toad (Bufo bufo) pair in amplexus from the study area in Norway. The common toad is common along the Norwegian coast up to the Dønna island in Nordland county.
Steffen Roth


Whereas many habitat fragmentations are the result of recent human activity (i.e. road construction, rain forest destruction), there are also several thousands year old landscapes with natural fragmentations. What happens with genetic diversity on such natural fragmented habitat? In order to answer this question, Steffen Roth (University Museum in Bergen) together with his colleague Robert Jehle from the Salford University in Manchester studied population of the common toad) living on 7 islands of the archipelago of Hordaland (see figure 1 and 2). In general, amphibians are key organism for such kind of studies, since they possess limited dispersal ability. In this special case, dispersal across islands or from the mainland to islands can almost completely be excluded (except some very rare events like trough human transports on vehicles and boats).

The genetic diversity of the toad populations was characterized using 9 microsatellite markers. Microsatellites are tracts of repetitive DNA in which certain short sequences (ranging in length from 2–5 base pairs) are repeated, typically 5-50 times.

The main findings from this study are twofold. Firstly, a pattern of distinct spatial genetic variation was detected which largely reflects the geographic setting. Population differentiation was overall high; remarkably, in most cases the genetic variability of individual populations did not cluster on specific islands (see figure 3). Secondly, although genetic variation was lower at island populations compared to mainland populations, an overall rather high amount of genetic variation despite putative long-term population isolation was revealed. This overall genetic diversity is comparable to previous studies on toads elsewhere in Europe mostly carried out in landscapes less fragmented than the mainland and archipelago of Western Norway.

What do these inferences tell us about conservation concerns for populations in landscapes recently fragmented by humans? It seems that amphibian populations are able to maintain significant levels of genetic variation in naturally strongly fragmented landscapes, despite clear genetic differentiation effects through fragmentation. While genetic erosion in naturally small populations is expected to accumulate over time, it can be compensated for when the environment is stable, and also important, provided that population have sufficient time to adapt to isolation effects.

These results reinforce the notion that spatial and temporal scales of fragmentation need to be considered when studying the interplay between landscape fragmentation and genetic erosion. However, they do not allow drawing premature conclusion about species conservation and nature conservation politics.


The publication is availble from here (link).