Introduction
Species richness is a widely used biodiversity metric in ecological and conservation studies. It reflects the compositional and organisational structure of communities of living organisms (Hillebrand et al., 2018), and is widely used as an indicator of the conservation value of ecosystems (Shokri and Gladstone, 2013, Capmourteres and Anand, 2016). Species richness shifts relatively predictably in response to changing biotic and abiotic factors in a wide range of organisms (Gaston, 2000, Hillebrand et al., 2018). Understanding such shifts, and the ecological processes associated with them, is fundamental in appreciating spatial diversity patterns (Myers et al., 2000, Hanson et al., 2020) and consequently predicting the impacts of global environmental change (Kreft and Jetz, 2007, Lopez et al., 2022).
Several factors have been proposed to explain patterns of species richness. For example, good support exists for the water-energy hypothesis (Currie et al., 2004, Field et al., 2005, Kreft and Jetz, 2007, Hawkins et al., 2003) which states that at high latitudes, ambient energy restricts species richness, whereas at low latitudes, water limitation becomes more important (Hawkins et al., 2003, Hufnagel and Mics, 2022). Plant physiological limitations, such as tolerance to desiccation and frost, can further constrain species distributions and richness (Currie et al., 2004, Hawkins et al., 2003). Resource, environmental or topographic heterogeneity can result in higher microhabitat diversity, allowing for the coexistence of more species (Pausas and Austin, 2001, Stein et al., 2014). Nutrient availability generally shows a hump‐shaped relationship with species richness, as few species tolerate nutrient-deficient environments, more species tolerate intermediate nutrient levels, and a smaller number of competitive species become dominant and suppress other species at high nutrient levels (Graham and Duda, 2011, Mittelbach et al., 2001). Biotic interactions, such as competition, mutualism and facilitation, can also limit, or increase, species richness (Van Dam, 2009, le Roux et al., 2012, Marques Dracxler and Kissling, 2022, Stachowicz, 2001).
Given the scale-dependent nature of biodiversity (Chase et al., 2018, Spake et al., 2021), it is crucial for studies examining diversity patterns and drivers to account for scale. Indeed, patterns and drivers of species richness can be influenced by spatial grain, i.e. the measurement unit or area within which species occurrences are quantified (Whittaker et al., 2001). Species richness is the count of species per unit area, and therefore, the choice of grain size may affect the species richness measured (Bhatta et al., 2018). The underlying processes that shape species assembly of plant communities may differ between different grain sizes (Martínez-Villa et al., 2020), resulting in different biotic and abiotic factors regulating species richness at different grain sizes (Kallimanis et al., 2007). For instance, Powell et al. (2013) demonstrated that invasive plant species significantly reduce native biodiversity at small spatial grains, with the effect diminishing at larger grains. On the other hand, butterfly species richness in Borneo was minimally affected by forest disturbance from logging at smaller grains, but the effect was more pronounced at larger sampling grains (Dumbrell et al., 2008). Therefore, to enhance our understanding of these patterns, some studies argue in favour of adopting a multi-scale approach, recognising the multidimensional nature of biodiversity (Chase et al., 2018, Spake et al., 2021).
Spatial grain has a significant influence on the patterns and drivers of species richness in areas with high beta diversity. For instance, in environments with high local beta diversity, even a small reduction in grain size can lead to a reduction in species richness per sampling unit (Tuomisto et al., 2017). In contrast, when local beta diversity is low, there is more stability and consistency in species composition across spatial scales. Thus, in environmentally heterogeneous landscapes, which often exhibit higher beta diversity, there is a notable advantage to adopt a multi-scale approach to understand biotic and abiotic factors shaping species richness. For instance, the productivity-species richness relationship is notably affected by sampling grain size (Whittaker et al., 2001). Coarse grains exhibit a monotonically positive relationship, whereas finer grains show a hump-shaped pattern, with maximum species richness at intermediate productivity (Virtanen et al., 2013). This phenomenon is attributed to beta diversity likely being higher in productive environments, which explains why species richness increases with productivity at larger, but not smaller, grains. Based on these observations, the importance of spatial grain in explaining patterns of species richness can be expected to depend on the landscape’s characteristics. In relatively homogeneous landscapes with gradual environmental changes, spatial grain will have less influence. Conversely, in heterogeneous environments that offer a greater number of microhabitats, spatial grain becomes more important. Therefore, assessing drivers of richness at different scales can help explain variation in the relative importance of other variables in determining species richness in a given area (Radinger et al., 2015).
Traditionally, small grain studies investigate patterns and drivers of local variations in species richness, while larger grains take into consideration accumulation of species at local scales and are mostly used in understanding broader patterns and drivers of diversity (Tuomisto et al., 2017). At large grain sizes (several kilometres) macroclimatic variables are usually the best predictors of richness variation; at medium grain sizes (typically, kilometres to meters) topographic factors most strongly influence patterns of species richness (Keil and Chase, 2019, Kalkhan and Stohlgren, 2000). Larger grains are generally favoured for studies assessing species richness patterns over large sampling area, where the large sampling unit effectively averages over within-grain heterogeneity (Sreekar et al., 2018). At smaller grains (meters to centimetres) other factors such as soil nutrition and vegetation characteristics (including type, growth form, and trait characteristics) can become important (Kalkhan and Stohlgren, 2000, Keil and Chase, 2019). The importance of biotic interactions is also scale-dependent, typically being considered particularly influential at smaller grain sizes (Huston, 1999, McGill, 2010, Willig et al., 2003). At smaller grain, the competitive effects of dominant species can be more pronounced, leading to exclusion or suppression of subordinate species. In contrast, at large grain, the effects of competition might be diluted, allowing a more diverse array of species to coexist and exploit a broader range of resources (Wisz et al., 2013, Araújo and Rozenfeld, 2014).
Performing ecological studies at different grains increases understanding of the spatial component in the underlying drivers of species richness (He et al., 2002, Otypková and Chytry, 2006). While the choice of grain size affects the estimation of species richness (Bhatta et al., 2018, Tuomisto et al., 2017), it also potentially affects the type of factors influencing the observed species richness and the magnitude of the factors’ effects in any ecosystem (Chase and Knight, 2013). Therefore, grain size must be carefully chosen and accounted for in ecological studies as it can impact the observed patterns. Understanding the scale-dependent nature of biodiversity and accounting for spatial grain in studies are crucial for a comprehensive interpretation of species richness patterns and drivers.
Cold ecosystems are important areas of conservation, largely due to their strong vulnerability to changing environmental conditions (Olson and Dinerstein, 2002, Bennett et al., 2015). These systems generally support fewer species than warmer systems (Hawkins et al., 2003) and possess weaker ecosystem resilience to changes (Boelter and Mueller, 2016), and poorer biotic resistance to invasions (Pertierra et al., 2022). Energy and water dynamics play a large role in species richness for cold climates; as energy inputs are low, there are fewer species, allowing only low-growing plants (Walker et al., 2001, Marini et al., 2008). Thus, plant species richness patterns usually follow a simple energy-water relationship gradient (Hawkins et al., 2003). Biotic interactions, such as herbivory, also play a role in shaping species richness in cold environments (Olofsson et al., 2009). For example, reindeer decrease plant species richness in low‐productivity sites but increase richness in productive sites (Sundqvist et al., 2019). Moreover, biotic interactions between species of a community (or guild) can shift from competitive to facilitative along environmental gradients; this can significantly impact richness patterns in colder environments (Choler et al., 2001). Understanding drivers of species richness, particularly how grain size can affect such drivers, is of particular importance in colder regions because they are disproportionately affected by global climate changes, and often harbour specialised biodiversity adapted to cold environments.
Research on species richness patterns and drivers in cold environments has been strongly concentrated in the northern hemisphere, with temperate forest biomes in North America and Europe being the primary focus of most publications (Lawler et al., 2006, Wells et al., 2022, Mott and Clarke, 2018, Bennett and Classen, 2020). Less research has investigated patterns of species richness in the cold ecosystems of the southern hemisphere, particularly the sub‐Antarctic and Antarctic ecosystems (Lawler et al., 2006, Bennett and Classen, 2020), with only some exceptions (Chown et al., 1998, Griffiths and Waller, 2016, Rozzi et al., 2008). Therefore, the aim of this study was to assess the drivers of vascular plant species richness on the sub-Antarctic Marion Island, and to assess whether grain size is important in determining patterns and drivers of species richness.