Kīpuka as a model for anthropogenic fragmentation
Kīpuka are a well-suited natural experiment we can use to explore the
effect of anthropogenic habitat fragmentation on tropical forest
arthropod communities. Indeed, the stark contrast in conditions provided
by lava relative to moist tropical forest makes kīpuka systems an
interesting model for studying urbanization and road-building. The
parallels are particularly striking in terms of the abiotic conditions
created by lava versus paved surfaces. Of course, we must note that the
parallelism to urbanization and road-building is limited at the biotic
level. Whereas lava flows are themselves sterile and merely create
conditions suited to the persistence of non-natives that passively
arrive to the kipuka system, urbanization and road-building are in fact
conduits for the active introduction of non-natives to the disturbed
zone.
While tropical forests harbor the majority of global terrestrial
biodiversity (Ghazoul & Sheil 2010), the resident taxa appear to be
extremely sensitive to the effect of habitat fragmentation, at least
according to what we learn from examining Hawaiian arthropods in kīpuka.
Isolation by lava flows and resulting species-area effects means that
small kīpuka harbor significantly reduced species richness. Even the
forest edge of kīpuka is entirely avoided by many forest-specialist
species, which are dominated by a few edge-tolerant native taxa living
aside non-native colonists. The near complete isolation of kīpuka
communities across very short geographic distances, supported by
accelerated species turnover over similar distances in the kipuka
compared to the continuous forest system, suggests that even a wide road
may suffice to prevent dispersal for many arthropod taxa. Intriguingly,
the species-area effect we observed in the kipuka system was even more
pronounced at the haplotype level; i.e., genetic diversity may be
affected more strongly by isolation in small fragments than taxonomic
diversity. Hence very small kīpuka might have less evolutionary value as
the haplotype diversity they contain for any given species dwindles
under powerful genetic drift, in the process accumulating high
extinction debt relative to wet forest taxa. In this light, our results
suggest that even a network of relatively close forest fragments might
not promote the persistence of viable metapopulations of some
specialized forest taxa.