This document summarizes a study on the importance of earthworms in Hawaiian forests. It finds that the most common earthworm species found below 5000 feet in Hawaii is Pontoscolex corethrurus, originally from South America. This geophagous species can reach densities of up to 1000 individuals per square meter. Based on limited studies to date, P. corethrurus is the most frequently encountered species in Hawaiian forests, and its presence supports high densities of invasive feral pigs that disturb soil as they feed on earthworms. More research is needed on the ecological effects of earthworms in Hawaiian ecosystems, as most current understanding comes from studies in areas and regions where earthworms are non-native.

1. A Hidden World: How Important are
Earthworms in Forests of Hawai‘i?
Glenn Metzler
Puu Ohia
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acres
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 In many areas they are considered
“Ecosystem Engineers”
 Processes: casting, burrowing,
mixing
 Endogeic – medium size,
geophagus, often in rhizosphere
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fenced
Rare
plants
As many as 35 species in Hawaii in 6 families (Blakemore 2005) and
at least 9 species in Hawaii forests below 5000 ft (Diong 1982).
Field Study and Comparisons
Pontoscolex corethrurus
The most abundant species in many areas below 5000 ft in Hawaii based on studies to date*.
A medium-sized geophagous earthworm species originally from S. America which has invaded
many tropical areas.
 Can also digest plant debris due to a an efficient enzymatic system produced in its gut
(Lattaud et al.1998)
 Endogeic – medium size, geophagus, often in rhizosphere
 Can reach an abundance of 1000 individuals/m2 in pastures (Zou and Gonzalez 1997)
 The most commonly found sp. in HI studies at low and mid-altitudes
Study method: Sample five 25 cm square
soil pits per site to 30 cm depth; ID of
species at one site by Dr. Sam James
*Amynthas spp. also common
Field Data
cm
Modified from: http://sciencelearn.org.nz/Science-Stories/Earthworms/Niches-within-earthworms-habitat
Type most
commonly
found to date
in HI Forests
Island General Area Elev. (ft) Forest Type #/m2 Source Species Noted
Oahu Manoa Valley 600 Alien wet 42 This study Pontoscolex corethrurus?
Oahu Hawaii Loa 1600 Mesic native-dominated ridgtop 83 This study P. corethrurus
Oahu Waikane 900 Mesic uluhe slopes 138 This study P. corethrurus
Oahu Manoa Cliff 2000 Mesic/wet, cinnamon/native mix 208 This study P. corethrurus
Oahu Manoa Cliff 2000 Mesic/wet, diverse ohia forest 275 This study P. corethrurus
Maui Kipahulu Valley 2000 Roseapple-dominated 146 Diong 1982 P. corethrurus +
Maui Kipahulu Valley 3000 Koa-dominated 69 Diong 1982 P. corethrurus +
Maui Kipahulu Valley 4800 Ohia-dominated 32 Diong 1982 P. corethrurus +
Maui Hana 5000-7000 Upper rainforest 93 Andersen 1994 Dendrodrilus rubidus, Lumbricus sp.
Hawaii Volcanoes NP NR Myrica-dominated 62 Aplet 1990 Amynthas spp.
Hawaii Volcanoes NP NR Open submontaine forest 18 Aplet 1990 P. cor., Aporrec. turgida, Amyn.
Hawaii Volcanoes NP 3800 Rainforest 21 Aplet 1990 P. corethrurus
Hawaii Northeast coast NR Albizzia stand 469 Zou 1993 P. corethrurus, Amynthas gracilis
Hawaii Northeast coast NR Eucalyptus stand 92 Zou 1993 P. corethrurus, Amynthas gracilis
Earthworm Ecology
Earthworms
are not native
to Hawaii

2. Photo too
dark on
first import
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Earthworm Ecological Effects
*Most studies to date have been in areas previously colonized by EWs or in temperate regions
Modified from Frelich: http://www.mnstac.org/uploads/2/0/9/3/20933948/mnstac_12-11-14_-_frelich.pdf
Feral Pigs
Conclusions
Earthworms are very important if pigs present
If not then it is likely but not certain based on currently available studies
Earthworms are a main source of protein for feral pigs resulting in soil disturbance
Very few studies on earthworm species and ecology for Hawaiian forests
Ecological effects other than soil disturbance can be inferred from other studies
Detailed studies on the ecological effects of earthworms in Hawaii are needed
Definite Effects in Hawaii
From Feral Pigs
 EWs are the primary source of animal protein, comprising up to 29% of stomach contents examined
(Diong, 1982; Anderson
 Up to one-third of the diggable area in a mountain rain forest can be disturbed by pigs during
a one year period (Cooray and Mueller-Dombois 1981)
 EW biomass correlated to the occurrence of pig activity (Andersen 1994, Lincoln 2014)
Potential Effects in Hawaii Based on Others Research (for Endogeic spp.)*
 Increased soil N availability (Pashanasi et al.1992, Gonzalez and Zou 1999)
 Increased plant growth (Pashanasi et al. 1996, Groenigan 2014)
 Increased decomposition rates of plant litter (Pashanasi et al.1992, Liu and Zou 2002)
 Loss of seeds through ingestion and digestion of seeds, especially small seeds (Eisenhauer et al. 2009)
 Modified plant seed germination (some spp. up, some down) after passage through the gut or contact with
excreta (Eisenhauer et al. 2009)
Equivocal Effects*
 Soil compaction due to production of macroaggregates, probably only at very high EW density (Chauvel et
al.1999, Barros et al. 2001)
 Increased plant growth (Pashanasi et al. 1996, Groenigan 2014)
 Greater microbial biomass (Paz-Ferreiro et al. 2015) but some conflicting results
 Increased respiration (in carbon poor soils) (McLean et al. 2006)
 Fungal colonization decreased due to hyphal disruption