| The biological consequences of habitat fragmentation include changes in community structure, the genetic structure of populations, and species interactions. In the kipuka system, a network of forest fragments surrounded by historical lava flows on the Big Island of Hawaii, I investigated the long-term effects of fragmentation. I examined community structure and population genetic structure of a group of spiders in the genus Tetragnatha (Araneae: Tetragnathidae), and spider parasitism by a mermithid nematode (Nematoda: Mermithidae). Spider communities changed across the boundary between forest fragments and surrounding lava habitat, with a decrease in total species richness and diversity, coupled with a change in the dominant taxon from native Tetragnatha to native Cyclosa (Aranaidae). Severe habitat restrictions for some Tetragnatha species suggested that populations in kipukas may be genetically isolated from one another. I investigated the genetic structure of three Tetragnatha species with variation in population size and habitat specialization in kipukas (T. quasimodo, abundant generalist; T. anuenue, abundant forest specialist; T. brevignatha, rare forest specialist). Based on allozymes and mitochondrial COI, population subdivision among forest fragments was greater for forest specialists than for the generalist. For the specialists, T. brevignatha and T. anuenue, minimum estimates of gene flow among isolated fragments were 0.5 and 0 migrants per generation, respectively. However for the generalist, T. quasimodo, gene flow was high (≥20 migrants per generation), regardless of habitat fragmentation. Additionally, the evolutionary relationships among COI sequences were characterized for the three Tetragnatha species, using Tajima's D, a phylogenetic tree-building method, and nested clade analysis. Geographic associations among nested clades revealed evidence of a “volcano-hopping” colonization pattern in T. quasimodo and T. anuenue, and evidence of genetic subdivision in kipuka populations in T. anuenue. Both T. anuenue and T. brevignatha showed significantly negative estimates of Tajima's D, a signature of recent population expansion. These patterns suggest that geologic processes have affected the structure of populations across the Big Island and may be an integral component in the diversification of lineages in this group. Finally, the relationship between Tetragnatha spiders and parasitic mermithid nematodes was investigated. Based on a molecular analysis, Hawaiian spider mermithids appeared to be more closely related to a mainland Aranimeris species, a spider parasite, than to insect-infecting mermithids collected on Oahu and the mainland. In kipukas, parasitism was low (ranging from 0 to 4%) and the number of infected spiders differed significantly among forest fragments. The proportion of spiders parasitized was associated with fragment area, but not with spider density or species richness, suggesting that mermithids are sensitive to habitat fragmentation, but that changes in rates of parasitism do not affect spider community structure. In conclusion, overall results suggest that habitat fragmentation affects habitat specialists and generalists differently, as evident in community structure and population genetic structure. For Hawaiian Tetragnatha , the continual reshaping of the Big Island landscape due to volcanic activity is reflected in the genetic structure of populations, suggesting a link between island geology and micro-evolutionary processes within this group. |
