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.