Understanding factors and processes that shape species coexistence is a central question in community ecology. The framework of modern coexistence theory has been widely used to clarify the conditions for species coexistence (Barabas et al. 2018; Chesson 2000, 2018; Pande et al. 2020), which points out the mechanisms of promoting species coexistence via minimizing average fitness differences (equalizing) and increasing niche differences (stabilizing) (Letten et al. 2017). Without niche differences, the species with higher average fitness will eventually exclude the others. However, all species coexistences occur in changing environments, temporally and spatially, as environmental variation promotes species coexistence (Chesson 1994; Kuang and Chesson 2008). To coexist, different species might respond nonlinearly to fluctuating limiting factors, or via storage effects, such as resource partitioning (Levine and HilleRisLambers 2009), differential vulnerability to predators (Chesson and Kuang 2008) or pathogens (Bagchi et al. 2014), occupation of different microhabitats (Schlägel et al. 2020; Silvertown 2004), or phenological separation (Usinowicz et al. 2017). However, our planet has been experiencing unprecedented changes due to anthropogenic activities (Steffen et al. 2015), yet it remains to be explored how this affects species coexistence and what the consequences of changes in coexistence are for entire ecosystems (Thakur et al. 2017).
Previous studies have demonstrated that the warming associated with climate change will alter phenology (Thackeray et al. 2016), distribution patterns (Parmesan and Yohe 2003), species interactions (Schaum et al. 2018; Zhang et al. 2020), and food web structure (O’Gorman et al. 2019; Schwarz et al. 2017). Climate warming can also affect species coexistence directly through physiological impacts or indirectly via alterations in food web interactions (Reuman et al. 2014). Indirect cascading effects via species interactions (e. g. phenological mismatch, changing in resource availability or predation) generally have greater impacts than direct ones (Ockendon et al. 2014). Temperature plays a key role because warming will change the vital rates of individuals, and species usually have limited thermal tolerance (Rohr et al. 2018; Sunday et al. 2012). There are two key mechanisms by which warming will alter coexistence: (i) shifting phenology and (ii) increasing metabolic demands. The first predicts that populations will differ in their temporal responses to warming: acclimation capacities to the changing temperatures will differ among (Pinsky et al. 2019; Rohr et al. 2018) and within (Dahlke et al. 2020) species. For instance, stenothermal species might be more sensitive to warming than eurythermal species (Dahlke et al. 2020; Pörtner et al. 2005), and early-season species might shift more in time than later-season species (Wolkovich et al. 2012). Therefore, warming might increase the phenological mismatch between coexisting species, thus reducing the degree of competition strength between them (Chmura et al. 2019) (Fig. 1, mechanism i). The latter mechanism refers to the greater metabolic demands which individuals experience at higher temperatures.
For ectothermic herbivores which share the same resource, warming should enhance competition, because ingestion rates of heterotrophs generally increase more rapidly with rising temperature than the growth rates in autotrophs (Grainger et al. 2018; O'Connor 2009; Schaum et al. 2018; West and Post 2016). Therefore, effects of warming on coexistence of two competing species might not only be caused by increasing phenological mismatch, but also via enhancing competition on basal resource (Fig. 1, mechanism ii).
Predators are generally assumed to have negative effects on prey abundance, and the strength of this effect can differ greatly among prey species (Karakoc et al. 2020; Paine 1966). Coexistence of competing prey species will be promoted when the predators are either specialists on the more competitive species, or generalists via density or frequency-dependent predation (Ishii and Shimada 2012; Karakoc et al. 2020; Saleem et al. 2012). However, studies also showed that predation can undermine coexistence if it is particular strong (Bonsall and Hassell 1997; Chase et al. 2002), or if the rarer species is inefficient in resource consumption (Holt 1977). Eventually, species with the highest tolerance of predation will outcompete the others. Therefore, effects of predation on coexistence of competing species might be caused by the selective feeding by the predator (Fig. 1, mechanism iii).
Warming can affect the phenology of both predators and prey with implications for the effect of the predator on prey abundance (Zhang et al. 2018). Furthermore, warming will elevate the metabolic demands of predators which might strengthen the predator-prey interactions (Thakur et al. 2018; Thakur et al. 2017) and undermine prey coexistence (Thakur et al. 2017). Therefore, in addition to shifts in phenology and selective feeding, warming and predation might interactively enhance the pressure of a predator on its prey (Fig. 1, mechanism iv), with implications for prey coexistence.
Here we test the predictions that (1) Warming will alter the phenology and competition of two primary consumers; (2) Predation will reduce the abundance of the primary consumers; (3) Warming and predation will interactively increase the competition between primary consumers, reducing coexistence. To test these hypotheses, we used freshwater gastropods and their fish predators. Gastropods play key roles in aquatic ecosystems by contributing to nutrient cycling and water quality, particularly due to their role as algal grazers (Böhm et al. 2020; Strong et al. 2008). However, to our knowledge, no studies have investigated how warming and predation interact to alter their coexistence.