Climb forest, climb: diverse disperser communities are key to assist plants tracking climate change on altitudinal gradients

Abstract

Introduction

Anthropogenic climate change poses a significant threat to biodiversity, with its impacts expected to intensify in the coming decades (Thomas et al., 2004; Urban, 2015; IPCC, 2023). The redistribution of regional and global climates is forcing species to shift their ranges to higher altitudes and latitudes to track suitable conditions (Parmesan & Yohe, 2003; Chen et al., 2011). Such global redistribution of species is transforming ecosystems, leading to the assemblage of novel communities at unprecedented rates (Lurgi et al., 2012; Pecl et al., 2017). In this scenario, species potential to colonise new grounds fast enough to track their shifting climatic envelopes is critical for their long-term survival and for ecosystem resilience (Loarie et al., 2009; Perino et al., 2019).

Mountains are among the most vulnerable ecosystems to climate change, with climatic envelopes rapidly moving uphill at rates unparalleled by any other ecosystem (Trew & Maclean, 2021; Adler et al., 2022; Knight, 2022). This is particularly alarming given that mountains harbour half of the global biodiversity hotspots and a quarter of all terrestrial biodiversity, including many endemics (Hoorn et al., 2018; Rahbek et al., 2019; Perrigo et al., 2020). Additionally, uphill colonisation coupled with the reduction in available habitat at higher elevations poses significant challenges to species survival, leading to a disproportionate risk of mountain biodiversity declines (Urban, 2018; Trew & Maclean, 2021).

Islands share many characteristics with mountains, including high levels of isolation, endemicity and vulnerability to climate change (Flantua et al., 2020). Oceanic islands, in particular, are privileged systems to study complex patterns related to community structure and functioning due to their discrete boundaries, relatively simple biota and high abiotic heterogeneity (Whittaker & Fernández-Palacios, 2007; Whittaker et al., 2017; Nogales et al., 2024). Tenerife, the highest island in the Canaries archipelago, has an elevation of 3718 m above sea level (asl) and has long been a flagship in biogeography due to the role of altitude in structuring its five vertical vegetation belts (Humboldt & Bonpland, 1814; Renner et al., 2023). In Tenerife, climatic envelopes are moving uphill with higher vegetation belts warming more rapidly (0.14 ± 0.07°C per decade) than lower ones (0.09 ± 0.04°C per decade) (Martín et al., 2012; Renner et al., 2023; García-Alvarado et al., 2024). Such changes have been particularly pronounced in the last two decades (García-Alvarado et al., 2024), underscoring the role of species' upward colonisation to keep pace with their shifting climatic envelope.

The sessile nature of plants makes this uphill dispersal particularly challenging, strongly hampering responses to climatic range shifts (Corlett & Westcott, 2013; Cunze et al., 2013). Seed dispersal, either by biotic (animals) or abiotic (e.g. wind) vectors, is a crucial ecosystem function that allows plants to avoid high competition and mortality near their mother plant, to recover from local perturbations and to expand their distribution range (Howe & Smallwood, 1982; Traveset et al., 2014). In this context, the direct dispersal service provided by animal dispersers is crucial in helping fleshy fruited plants migrate to higher elevations in pursuit of favourable climates (Naoe et al., 2016; González-Varo et al., 2017b). Without animal seed dispersers, many plants will likely fail tracking climate change and embody an extinction debt (Tilman et al., 1994; Fricke et al., 2022). However, the extent to which specific plant species depend on particular animal species remains poorly understood (Gilman et al., 2010; Naoe et al., 2016; González-Varo et al., 2017b; Mendes et al., 2024). The first studies and predictions of species responses to climate change have focused on individual species, mostly ignoring the complex web of interactions that sustain functional biological communities (Tylianakis et al., 2010; Walther, 2010). Forecasting how groups of species and the biotic interactions that sustain communities will respond to climate change is thus a key conservation priority (Urban et al., 2013, 2016).

By considering species and their biotic interactions, ecological networks provide a critical framework to quantify how interacting communities respond to perturbations such as climate change (Heleno et al., 2014, 2020). However, most networks are explored either as discrete entities constrained within well-defined spatio-temporal borders or as broad aggregations of regional meta-networks, ignoring internal spatial dynamics (Heleno et al., 2014; but see Timóteo et al., 2018; González-Varo et al., 2023). Multilayer networks provide a solution to overcome this limitation by enabling the representation of interconnected networks with spatially explicit intra- and interlayer links that allow the identification of species roles as spatial ecosystem couplers (Kivela et al., 2014; Pilosof et al., 2017; Timóteo et al., 2018).

Here, we compiled a seed dispersal network for each of the five altitudinal vegetation belts on the island of Tenerife and explored how animals and plants might colonise uphill habitats in response to climate change. Specifically, we ask (1) which seed dispersers can facilitate uphill plant colonisation? (2) is there any current evidence of plants being dispersed beyond their current distribution range? (3) does seed dispersal network structure vary with altitude? (4) how might a warmer future affect community composition along the altitudinal gradient, that is can fleshy fruited plants and seed dispersers reach upper vegetation belts?