The ecology and population genetics of the prickly forest skink in a fragmented habitat

Sumner, Joanna Mary. (2002). The ecology and population genetics of the prickly forest skink in a fragmented habitat PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Sumner, Joanna Mary.
Thesis Title The ecology and population genetics of the prickly forest skink in a fragmented habitat
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2002
Thesis type PhD Thesis
Supervisor Moritz, Craig
Total pages 178
Collection year 2002
Language eng
Subjects L
0602 Ecology
Formatted abstract
This thesis presents investigations into the molecular ecology of the prickly forest skink (Gnypetoscincus queenslandiae), a lizard endemic to the wet tropical rainforests in northeastern Queensland, Australia. The broad aim of this thesis is to use integrative studies to increase our understanding of population genetics and ecology, by applying both demographic and molecular genetic techniques to determine the effect of anthropogenic rainforest fragmentation on the prickly forest skink.

Large-scale fragmentation of rainforest occurred on the Atherton Tableland in the Australian Wet Tropics from 50-100 years ago due to logging and clearing for dairy farms, leaving numerous fragments of varying sizes. Eleven fragments (from <1 to 75 ha in area) and eight continuous-forest sites were surveyed for prickly forest skinks during seven trips between November 1995 and January 2000. Morphological, ecological and genetic data were collected and used to obtain information on the ecology of the species, then further research carried out on the effects of fragmentation on the morphology and genetic diversity of prickly forest skinks.

Dispersal, or the amount of dispersion between an individual's birthplace and that of its offspring, is of great importance in population biology, behavioural ecology and conservation; however, obtaining direct estimates from field data on natural populations can be problematic. Due to the prickly forest skink's log-dwelling habits and lack of definite nesting sites a demographic estimate of dispersal distance is difficult to obtain. Neighbourhood size, defined as 4πDσ2 (where D is the population density and σ2 the mean axial squared parent-offspring dispersal rate), dispersal and density were directly and indirectly estimated for this species, using mark-recapture and microsatellite data respectively, on lizards captured at a local geographic scale of three hectares. Mark-recapture data gave a dispersal rate of 843 m2/generation (assuming a generation time of 6.5 years), a time-scaled density of 13635 individuals * generation / km2 and, hence, a neighbourhood size of 144 individuals. A genetic method based on the multi-locus (10 loci) microsatellite genotypes of individuals and their geographic location indicated that there is a significant isolation by distance pattern, and gave a neighbourhood size of 69 individuals, with a 95% confidence interval between 48 and 184. This translates into a dispersal rate of 404 m2 / generation when using the mark-recapture density estimation, or an estimate of time-scaled population density of 6520 individuals * generation / km2 when using the mark-recapture dispersal rate estimate. The relationship between the two categories of neighbourhood size, dispersal and density estimates and reasons for any disparities are discussed.

Although traditionally thought of as asocial, solitary creatures, increasing evidence indicates that lizards possess many complex social structures. Field observations of groups of prickly forest skinks of different sizes found under the same log have generated speculation about the social structure of this species. In species that live in stable aggregations of highly related individuals inbreeding may be a potential problem; sex-biased dispersal is a mechanism that may decrease the possibility of cosanguinous mating. Fragmentation of the rainforest habitat in which the prickly forest skink lives may affect mechanisms such as sex-biased dispersal, which may in turn exacerbate more direct effects of fragmentation such as decreasing population size. Demographic data indicated that lizards were less grouped than expected by chance, and that groups most often consisted of females and sub-adults, two sub-adults or a male and a sub-adult, but not significantly more so than expected by chance. Relatedness statistics calculated using microsatellite DNA data showed that prickly forest skinks are significantly more related to animals within their own group than to those in other groups; apparently due to a significantly greater relatedness between sub-adults less than two years old and adults. Rainforest fragmentation has not affected the size and composition of groups of prickly forest skinks, but has had a major effect on the patterns of dispersal in this species. The genetic relatedness between individuals indicates the presence of male-biased dispersal in rainforest fragments, but no evidence of sex-biased dispersal in continuous forest. Male prickly forest skinks may have to move further in fragmented habitat in order to find mates or suitable logs.

Eleven fragments (from <1 to 75 ha. in area) and eight continuous-forest sites were studied to assess the effects of fragmentation on the morphology and demography of Gnypetoscincus queenslandiae. Skink abundance (number of individuals captured per hour of search effort) was significantly greater in continuous forest than in rainforest fragments. Moreover, both skink abundance and the density of decaying logs, which provide key habitat for this species, increased with fragment area. Fragments contained a smaller proportion of adults, and individuals in fragments were smaller on average for all measured morphological features, than those in continuous forest. Thus, although prickly forest skinks appear to be maintaining populations in rainforest remnants, they are nonetheless being affected by fragmentation. These demographic and morphological changes may be caused by alterations in habitat and prey availability and/or by microclimatic changes associated with edge effects. 

To further examine the effects of recent habitat fragmentation, I assayed genetic diversity in prickly forest skinks from seven forest fragments and five sites in continuous forest. I genotyped 411 individuals at nine microsatellite DNA loci and found fewer alleles per locus in prickly forest skinks from small rainforest fragments. Neither heterozygosity nor variance in allele size differed between fragments and sites in continuous forests. Considering measures of among-population differentiation, there was no increase in FST among fragments, nor, except in the smallest fragments, was the frequency of immigrant (mis-assigned) individuals lower. A significant isolation by distance pattern was identified across all twelve sites. The relationship between genetic (FST) and geographic distance was significantly stronger for continuous forest sites than for fragments, consistent with disruption of gene flow among the latter. These results emphasise the inherent difficulty in detecting genetic consequences of recent habitat fragmentation. They also indicate that the genetic effects of habitat fragmentation on prickly skinks have been minor, perhaps because local effective population size has not been greatly reduced. However, given early signs of isolation, it is likely that genetic diversity within fragments will erode gradually unless habitat linkages are restored.
Keyword Skinks

Document type: Thesis
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Created: Fri, 24 Aug 2007, 17:51:27 EST