The main energy sources in Hawaiian lava tube ecosystems are tree roots, which penetrate the lava for several decameters; organic matter, which washes in with percolating rainwater; and accidentals, which are surface and soil animals blundering into the cave. Both living and dead roots are utilized, and this source is probably the most important. Furthermore, both rainwater and accidentals often use the same channels as roots to enter caves, so that root patches often provide food for a wide diversity of cave organisms. The importance of roots in the cave ecosystem makes it desirable to identify the major species. This has become possible only recently by using DNA-sequencing technology. The most important source of roots is supplied by the native pioneer tree on young lava flows; Metrosiderospolymorphs Cocculus orbiculatus, Dodonaea viscosa, and Capparis are locally important in drier habitats. Several different slimes and oozes occur on wet surfaces and are utilized by scavengers in the cave. They are mostly organic colloids deposited by percolating groundwater, but some may be chemoauto-trophic bacteria living on minerals in the lava. Cave-roosting vertebrates do not occur in Hawai'i. Native agrotine moths once roosted in caves in large colonies, but the group has become rare in historic times. The composition of the community their colonies once supported is unknown. Feeding on living roots are cixiid planthoppers (Oliarus). Their nymphs suck xylem sap with piercing mouthparts. The blind flightless adults wander through subterranean voids in search of mates and roots. Caterpillars of noctuid moths (Schrankia) prefer to feed on succulent flushing root tips, but they also occasionally scavenge on rotting plant and animal matter. Tree crickets (Thaumatogtyllus), terrestrial amphipods (Spelaeorchestia), and isopods (Hawaiioscia and Littorophiloscia) are omnivores but feed extensively on roots. Cave rock crickets (Caconemobius) are also omnivorous as well as being opportunistic predators. Feeding on rotting organic material and associated microorganisms are millipedes (Nannolene), springtails (NNeanura, Sinella, and Hawinella), and phorid flies (Megaselia). Terrestrial water treaders (Cavaticovelia aaa) suck juices from long-dead arthropods. Feeding in the organic oozes growing on wet cave walls are larvae of craneflies (.Dicranomyia) and biting midges (Forcipomyia pholeter). The blind predators include spiders (Lycosa howarthi, Adelocosa anops (Figure 3), Erigone, Meioneta, Oonops, and Theridioti), pseudoscorpions (Tyrannochthonius), rock centipedes (Lithobius), thread-legged bugs (Nesidiolestes), and beetles (NNesomedon, Tachys, and Blackburnia). Most of the cave predators will also scavenge on dead animal material.
Several invasive nonindigenous species have invaded cave habitats and are impacting the cave communities. The predatory guild is the most troublesome, with some species being implicated on the reduction of vulnerable native species. Among these, the nemertine worm (Argonemertes dendyi) and spiders (Dysdera, Nesticella, and Eidmanella) have successfully invaded the stagnant air zone within the smaller spaces. The colonies of cave-roosting moths disappeared from the depredations of the roof rat (Rattus rattus) on their roosts and from parasites purposefully introduced for biological control of their larvae. Many non-native species (such as Periplaneta cockroaches, Loxosceles spiders, Porcellio isopods, and Oxychilus snails) survive well in larger accessible cave passages, where they have some impact, but they appear not to be able to survive in the system of smaller crevices. A few alien tree species also send roots into caves, creating a dilemma for reserve managers trying to protect both cave and surface habitats since their roots support some generalist native species but not the host-specific planthoppers.
Inhabited Hawaiian lava tubes range in age from 1 month on Hawai'i Island to 2.9 million years on O'ahu Island. On Hawai'i Island colonization and succession of cave ecosystems can be observed. Crickets and spiders arrive on new flows within a month of the flow surface cooling. They hide in caves and crevices by day and emerge at night to feed on windborne debris. Caconemobius rock crickets are restricted to living only in this aeolian (wind-supported) ecosystem and disappear with the establishment of plants. The obligate cave species begin to arrive within a year after lava stops flowing in the caves. The predatory wolf spider, Lycosa howarthi, arrives first and preys on wayward aeolian arthropods. Other predators and scavenging arthropods - including blind, cave-adapted Caconemobius crickets - arrive during the next decade. Under rainforest conditions, plants begin to invade the surface after a decade, allowing the root feeding cave animals to colonize the caves. Oliarus planthoppers arrive about 15 years after the eruption and only 5 years after its host tree, Metrosiderospolymorpha. The cave-adapted moth, Schrankia species, and the underground tree cricket, Thaumatogryllus cavicola, arrive later. The cave species colonize new lava tubes from neighboring older flows via underground cracks and voids in the lava. Caves between 500 and 1000 years old are most diverse in cave species. By this time the surface rainforest community is well-developed and productive, while the lava is still young and maximal amount of energy is sinking underground. As soil formation progresses, less water and energy reaches the caves, and the communities slowly starve. In highest rainfall areas, caves support none or only a few species after 10 000 years. Under desert conditions, succession is prolonged for 100 000 years or more. Mesic regimes are intermediate between these two extremes. New lava flows may rejuvenate some buried habitat as well as create new cave habitat.
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