Conservation value of cacao agroforestry for amphibians and reptiles in South‐East Asia: combining correlative models with follow‐up field experiments

Abstract

Summary 1.Although agricultural expansion is a primary threat to tropical biodiversity, experimental studies evaluating the conservation value of tropical agricultural habitats are scarce. In particular, little is known about the sensitivity of amphibians and reptiles to habitat disturbance in areas of very high diversity such as South-East Asia. 2. We used a two-step approach to determine the relationship between habitat complexity and conservation value of cacao agroforestry for herpetological diversity in Sulawesi (Indonesia). Indonesia is the third largest cacao-exporting country globally and forest conversion to cacao plantations is a major threat to its biodiversity. We first sampled 43 cacao plantations six times to determine the environmental variables that best explained herpetofaunal diversity patterns using a Bayesian model selection approach. Based on these results, we experimentally manipulated leaf litter thickness (LLT), number of branch piles (LOGS) and LLT + LOGS combinations in the cacao plots. The experimental data were analysed using Bayesian hierarchical regression. 3. The best supported correlative models incorporated LLT, LOGS, air temperature and the ratio between leaf litter and shrub cover, showing the importance of habitat heterogeneity and suggesting climate change sensitivity. The subsequent structural manipulation of these attributes changed amphibian and reptile species richness, and reptile abundance, but only addition of leaf litter did so in a biologically meaningful way, providing microhabitat resources. However, the main beneficiaries were common disturbance-tolerant reptiles. 4. Synthesis and applications. The different results from the correlative model and the independent manipulative experiments showed how important such a combined approach is to derive adequate conservation management recommendations. Increasing leaf litter in cacao agroforestry will work best if implemented on a landscape scale to incorporate sufficient environmental variation and species life histories. This will mainly enhance the richness and abundance of disturbance-tolerant species, which still may maintain ecosystem functions such as pest removal. Particularly for rare species, native forests remain critical for herpetological richness. The direct temperature sensitivity suggests that future climate change impacts may be severe for herpetological diversity in plantation habitats and, hence, demand further research.

Endemic predators, invasive prey and native diversity

“…Plantation management for leaf litter cover and maintenance of intact canopy structure to buffer against temperature fluctuations will benefit amphibian diversity [12]. This will also facilitate toad abundance in cacao agroforest and, hence, translate into ant biomass removal.…”

“…Amphibians and reptiles were sampled three times during day and night with visual and acoustic encounter surveys [12]. Ants were sampled with observation plates (10 on the ground and 10 in the trees) on each plantation, baited with tuna and sugar solution.…”

“…), (ii) individual large ant abundances or (iii) ground-dwelling ant species richness drive abundance patterns in the most common amphibian and reptile species. For an introduction of Bayesian model evaluation, see [14] and the supplementary material of [12].…”

“…Further, the native predator should be abundant, ground-living and, ideally, an ant specialist. The common celebes toad (Ingerophrynus celebensis) and a skink (Eutropis grandis) are the most abundant groundliving vertebrates in the cacao plantations of Sulawesi [12]. To assess their roles as invasive ant control agents, we first determined dietary preferences of these two species and then experimentally excluded ant-predating specialists (the toads) from cacao plantations.…”

Responses of terrestrial herb assemblages to weeding and fertilization in cacao agroforests in Indonesia

“…Sphaerostephanos invisus (G. and reptiles (Wanger et al 2009) and insects (Wielgoss et al 2010). A negative effect of intensive weeding is evidenced by changes in water balance, litter decomposition, and nutrient cycling of the upper soil layer (Watanabe et al 2007;Smiley and Kroschel 2010).…”