Combined mechanistic modelling predicts changes in species distribution and increased co-occurrence of a tropical urchin herbivore and a habitat-forming temperate kelp

Abstract

Aim: Identify climate change impacts on spawning and settlement of a tropical herbivore (Tripneustes), optimal habitat of a temperate kelp (Ecklonia) and implications for these species regions of interaction under future climate. Location: Along eastern Australia and into the Tasman Sea including Lord Howe Island (LHI). Time period: A contemporary scenario (2006–2015) and future “business as usual” RCP 8.5 climate change scenario (2090–2100). Major taxa studied: The tropical sea urchin, Tripneustes gratilla, and the temperate kelp, Ecklonia radiata. Methods: We combined mechanistic models to create a predictive map of Ecklonia and Tripneustes distributions, and their future potential to co-occur. We use 3D velocity and temperature fields produced with a state-of-the-art configuration of the Ocean Forecasting Model version 3 that simulates the contemporary oceanic environment and projects it under an RCP 8.5 climate change scenario. We map the contemporary and future Ecklonia's realized and fundamental thermal niche; and simulate Tripneustes larval dispersal under both climate scenarios. Results: Based on the thermal affinity of kelp and increases in projected temperatures, we predict a poleward range contraction of ~530 km by 2100 for kelp on Australia's east coast. Climate-driven changes in dispersal of Tripneustes lead to its range expansion into Bass Strait and poleward, ~340–650 km further into Ecklonia's habitat range inducing new areas of co-occurrence in the future. We find warming decreases spawning and settlement of Tripneustes in the tropics by 43%, and causes significant connectivity changes for LHI with future reliance on self-recruitment. Major conclusions: We predict novel regions of co-occurrence between a temperate Ecklonia and tropical Tripneustes species which may lead to greater loss of kelp. Our results provide a new modelling approach for predicting species range shifts that is transferable to other marine ecosystems; it considers species response to abiotic change, predicts spatial spread and anticipates new regions for species interactions.