A hybrid CART-GAMs model to evaluate benthic macroinvertebrate habitat suitability in the Pearl River Estuary, China

Abstract

Benthic macroinvertebrates are an important part of the aquatic food chain in estuaries. Therefore, understanding their associated habitat requirements is critical in maintaining estuarine ecosystem stability, which has until now been evaluated using habitat suitability evaluations. Although most habitat suitability index (HSI)-based methods can determine the target species preferences for a few specific habitat factors, benthic macroinvertebrates are influenced by multiple habitat factors in estuaries. However, applying too many habitat factors can result in data redundancy, thereby reducing model efficiency. Additionally, nonlinear relationships between species and habitat factors and interactions between different habitat factors are often difficult to evaluate. Previous studies have not focused on methods that can simultaneously address these two issues. Accordingly, this study introduced CART-GAMs, a classification and regression tree model (CART) and a generalized additive model (GAM) hybrid, which can evaluate the suitability of multiple habitat factors relevant to benthic macroinvertebrates in estuaries. First, to remove irrelevant variables (i.e., data redundancy), CART was used to identify critical habitat factors that have significant impacts on benthic macroinvertebrates. GAMs were then used to quantify the nonlinear relationships between critical habitat factors and species. Finally, response curves fitted by GAMs were used to determine species habitat preferences (i.e., suitable critical habitat factor ranges). This study applied the CART-GAMs model to the Pearl River Estuary (PRE) to test its effectiveness. The results showed that CART identified four critical water quality factors, that is, salinity (SAL), total phosphorus (TP), pH, and ammonia nitrogen (NH3-N) from the 12 measured factors, subsequently reducing data redundancy by 66.7%. Using critical water quality factors, GAMs explained 66.2% of the variance in Margalef’s diversity index (dM) of benthic macroinvertebrates. Additionally, the GAM response curves showed that dM was positively correlated with SAL and negatively correlated with TP and TN. The response curve between dM and pH is unimodal. Finally, this study determined suitable water quality factor ranges for benthic macroinvertebrates (SAL = 21.15‰ ∼ 32.32‰, TP = 0.05 ∼ 0.2 mg/L, pH = 7.7 ∼ 8.2, and TN = 0.4 ∼ 1.5 mg/L). This study demonstrates that the hybrid CART-GAMs model can effectively evaluate estuarine habitat suitability with multiple habitat factors, while providing a reference for estuary habitat management.