Biodiversity loss and emerging infectious diseases represent major global challenges to human and ecosystem health (Barnosky et al., 2011, Daszak et al., 2000). Anthropogenic pressures like habitat degradation, land-use change, and climate shifts accelerate extinctions and fundamentally alter host-parasite dynamics (Gibb et al., 2020, Jones et al., 2008). These changes can influence disease transmission in wildlife, which varies with multiple interacting ecological factors (Morand, 2015). Here, we focus on three such factors likely affected by current global trends: host diversity, host density, and habitat quality. Because these factors are often interdependent, modeling their individual and combined effects is crucial to disease risk assesment.

The dilution effect hypothesis posits that increased host diversity reduces disease transmission by increasing the proportion of non-competent hosts and limiting pathogen encounters with competent hosts (Keesing and Ostfeld, 2021, Ostfeld and Keesing, 2000), though its generality is debated (Rohr et al., 2020). Host density is also a key predictor of parasite richness (Kamiya et al., 2014). Under density-dependent transmission—a core assumption in epidemiology—higher host densities and contact rates amplify transmission, although effects can be non-linear (Hopkins et al., 2020, McCallum et al., 2001). Finally, habitat quality influences disease dynamics by affecting host communities or pathogens (Sousa and Grosholz, 1991). Degraded habitats can increase host susceptibility via resource scarcity and immune stress, potentially confounding dilution effects (Fearon et al., 2023). Parasite survival and persistence, particularly for species with environmental stages, are affected through variations in environmental conditions or predation (Morley and Lewis, 2004, Thieltges et al., 2013).

Non-human primates (hereafter primates) in tropical forests are ideal models for these dynamics due to their ecological diversity and shared parasites. Strongylid nematodes like Oesophagostomum spp. are of particular interest due to their zoonotic potential and widespread primate distribution (Cibot et al., 2015, Polderman and Blotkamp, 1995, Terio et al., 2018). These roundworms have a direct life cycle: L1 larvae hatch from eggs in host feces, moult twice into infective L3 larvae in the environment, and infect hosts upon ingestion before maturing in the intestinal lumen (Taylor et al., 2015).

Prior studies in Borneo showed extensive parasite sharing among sympatric primates (Frias et al., 2021, Frias et al., 2019a), with Oesophagostomum spp. predominating and resulting in low strongylid species richness (Mason et al., 2024, Yalcindag et al., 2021). While parasite species richness is a widely used metric shown to vary with several ecological factors (Kamiya et al., 2014, Nunn et al., 2003), it suffers from imprecise taxonomic resolution, especially for strongylids that produce visually indistinguishable eggs (De Gruijter et al., 2004). Molecular approaches are therefore necessary to resolve this cryptic diversity and infer transmission risk, shifting the focus from species richness to parasite genetic diversity (Pafčo et al., 2019).

High-throughput sequencing (HTS) of ITS2 ribosomal DNA allows for comprehensive characterization of parasite communities like strongylid nematodes (Avramenko et al., 2015). While delineating cryptic species remains challenging (Chaves-González et al., 2022), HTS provides an in-depth view of parasite genetic diversity by quantifying co-infecting genetic variants, or Amplicon Sequence Variants (ASVs). Ecological drivers of parasite transmission will shape this entire community of ASVs. Then, ASV alpha diversity (richness and evenness) and beta diversity (composition) can integrate these effects, providing complementary insights into parasite genetic diversity.

This study investigates ecological determinants of strongylid nematode genetic diversity in a primate community along the Kinabatangan River in Malaysian Borneo. We test three hypotheses for strongylid alpha diversity:1.

dilution effect: increased host diversity reduces strongylid ASV alpha diversity,

2.

density-dependent transmission: higher host densities increase strongylid ASV alpha diversity, and

3.

direct habitat effects: habitat quality influences strongylid ASV alpha diversity independently of host diversity or density.

We also explored the effects of these ecological factors on strongylid beta diversity. Combining HTS of nematode ITS2 rDNA, primate survey data, and a habitat quality index derived from remote sensing, we investigated how primate host demography and habitat shape strongylid genetic diversity