DOI: 10.5061/dryad.r4xgxd2g9
title: Impacts of abiotic and biotic factors on terrestrial leeches in

description: Haemadipsid leeches are ubiquitous inhabitants of tropical and
 sub-tropical forests in the Indo-Pacific region. They are increasingly
 used as indicator taxa for biomonitoring, yet very little is known about
 their basic ecology. For example, to date no study has assessed the
 occurrence and distribution of haemadipsid leeches across naturally
 occurring gradients within intact habitats. We analysed a long-term data
 set (2012-2020) on the closely related tiger (Haemadipsa picta) and brown
 (Haemadipsa spp.) leech species to investigate if and how abiotic and
 biotic factors influence their occurrence across a gradient of forest
 types at an undisturbed tropical rainforest site in Indonesian Borneo. We
 compared a series of negative binomial mixed models and found that, of the
 abiotic factors, soil moisture had the largest positive effect on
 encounter rates of both leech species. Among biotic factors, forest type
 had differential effects on counts of the two species: while tiger leech
 counts were greater in low elevation forest types, brown leech counts were
 greater in high elevation forest types. Additionally, we found that the
 presence of one species had a positive effect on the presence of the other
 species. Finally, our results show that the tiger leech has a narrower
 distribution, being restricted to lower elevation forest types with higher
 water retention, suggesting that the tiger leech could be more sensitive
 to lower soil moisture levels.
description: In 2000, AJM established a
 series of 14 vertebrate survey transects across the seven primary forest
 types at the Cabang Panti Research Site
 in West Kalimantan, Borneo. The mean length of survey
 transects was 3.4 ± SD 0.24 km. Some transects were contained entirely in
 a single forest type, and others spanned multiple forest types. For
 analysis, transects were divided into segments that were restricted to a
 single forest type and varied in length from 50 to 550 m. We measured the
 altitude of each forest type at its approximate midpoint using a Suunto
 Vector Altimeter. From November 2012 to September 2020, we systematically
 counted leeches along these vertebrate transects. We walked each transect
 at a constant, slow speed (~1 km per hour) twice per month (starting at
 opposite ends), beginning at 0530 h. Surveys were normally carried out by
 a single observer, although occasionally two observers would walk the same
 transect (e.g., when new staff were trained). Every 500 m along transects
 or at the beginning of each segment, observers paused to examine
 themselves thoroughly for a period of two minutes. Any leeches attached to
 their bodies were counted, identified, and removed. When two surveyors
 were present, both individuals searched for and counted leeches, and the
 recorded number of leeches was combined for the two observers.
 In addition to leech species and counts, within each segment,
 observers made a note of soil moisture and rainfall history. For soil
 moisture, observers examined the dampness of soil and categorized it as
 “wet”, “moist” or “dry”. They categorized rainfall history for the whole
 survey day as “Raining now”, “Rained last night”, “Rained yesterday”,
 “Rained day before yesterday” or “Rained over 48 hours ago” based on
 conditions at the research station. Rainfall can sometimes be highly
 localized at our site, so it is possible that occasionally a segment’s
 rainfall history assessment based on conditions at the research station
 may have been inaccurate. We have no reason to believe such rare
 mischaracterizations were biased in any particular direction, however.
 Data tidying and analysis were carried out using program R (R Core Team, 2020;
 RStudio Team,
 2021).
description: Microsoft Excel & Program R