INTRODUCTION
One of the most widespread consequences of climate change is the global
redistribution of life on Earth (Chen et al. 2011; Poloczanskaet al. 2013; Pecl et al. 2017). The climate-mediated
movement of species into novel environments alters species assemblages
and this can have flow-on effects for ecosystem functioning and services
(Pecl et al. 2017). Ecological impacts of species redistributions
can be particularly severe when foundation species are affected, as this
can have cascading impacts on associated species and lead to regime
shifts, whereby one ecosystem state shifts to an alternative state
(Hughes et al. 2013).
Climate-driven species redistribution models generally predict net
biodiversity declines and biotic homogenisation at a global scale
(Magurran et al. 2015; Savage & Vellend 2015). However,
following changes in environmental conditions, species diversity can
initially increase at the local scale (Hiddink & Ter Hofstede 2008;
Walther et al. 2009). Emerging evidence shows that patterns of
increased diversity are often masked by biological processes not easily
observed, including lag times and transitional stages (Daskalovaet al. 2020) and/or observations reflecting a turnover of species
where numbers of “winners” and “losers” are similar and diversity
appears to be maintained where lost species are offset by new ones
(Dornelas et al. 2019; Finderup Nielsen et al. 2019). The
need for long-term datasets to uncover assemblage changes is therefore
paramount for accurately predicting ecosystem change.
There are large-scale coordinated efforts to document the arrival of
range-expanders into new areas (Robinson et al. 2015), but we
know relatively little about their interactions with and responses of
recipient communities (Bates et al. 2014; Svenning et al.2014). Additionally, we lack understanding of how climate-mediated
losses of species, and the traits associated with those species, affect
ecosystem processes (Wardle et al. 2011). Such understanding is
critical to the development of management tools for mitigating impacts
of range expanding organisms on ecosystem function and services.
Biogeographic transition zones where abiotic and biotic conditions allow
for the interaction of organisms from different geographical origins are
experiencing rapid changes in the marine realm (Horta e Costa et
al. 2014; Troast et al. 2020). Transition zones provide a unique
opportunity to understand the impact of range shifting organisms because
populations of species from the surrounding environmental extremes
already exist and interact. Transition zones allow us to examine the
processes and mechanisms that facilitate successful species expansions,
as well as those which make resident species resilient within their
native range despite the establishment of range-expanders. The
identification of mechanisms that facilitate changes in abundance for
some species under these conditions may help identify range expanders
and/or at-risk species from the surrounding systems and for similar
systems elsewhere. For example, recent evidence from tropical/temperate
transition zones has shown that, despite historical mixing of tropical
and temperate species, the increasing abundance of range-expanding
tropical herbivorous fishes can overgraze seaweeds and lead to regime
shifts by maintaining reefs in canopy-free states (Vergés et al.2014; Bennett et al. 2015).
In recipient systems where range-expanding species are becoming more
common, predicting coexistence with resident species is critical to
understand the eventual structure and functioning of those systems.
Mechanisms that underpin coexistence are complex and influenced by
multiple ecological and evolutionary processes acting simultaneously
(Pinsky 2019). However, the well-established field of invasion ecology
can be used as a starting point to make predictions about climate-driven
species redistributions (Wallingford et al. 2020) whereby the
trait space of range expanders can then be empirically quantified
(Sunday et al. 2015). For instance, trait-based analyses of
invading species are often used to predict the likelihood of competitive
interactions with resident/native species and effects on the recipient
community (van Kleunen et al. 2010). Invasive species often show
generalist traits that facilitate success in novel environments, often
to the detriment of resident specialist species (Clavel et al.2011). Dynamic environments often favour generalist species that more
readily adapt (Kassen 2002) and so we could expect that, in a range
expansion context, resident specialist species may be the most
vulnerable.
For transition zones affected by climate change, species coexistence may
continue to be possible if incoming species show unique traits (Paciogluet al. 2020), i.e. do not directly compete with residents or are
able to show plasticity in resource use in a novel setting (Jackson &
Britton 2014). However, there are cases where expression of unique
traits may not be beneficial if the system is already disturbed/degraded
(Bulleri et al. 2020). Alternatively, resident populations may be
far below carrying capacity (for example due to fishing in marine
systems), and thus even if incoming species substantially overlap in
trait (niche) space, competitive exclusion will not occur and
coexistence between migrants and residents can persist.
On the east coast of Australia, the Solitary Islands region is a
sub-tropical transition zone within a climate change hotspot (Hobday &
Pecl 2014). This region is strongly influenced by the East Australian
Current, a western boundary current that is strengthening in response to
climate change and which can facilitate the poleward range expansion of
marine species (Ridgway & Hill 2012; Castro et al. 2020). An
increasing abundance of tropical herbivorous fishes has been linked to
the decline of temperate habitat-forming kelp in this region (Vergés et
al. 2016). The kelp Ecklonia radiata is a critical component of
Australia’s temperate reefs that supports ecologically and economically
important fish and invertebrate species (Wernberg et al. 2019).
The loss of seaweeds in eastern Australia is part of a global
“flattening of kelp forests” (Filbee-Dexter & Wernberg 2018) whereby
canopy seaweeds are replaced by low biomass turfing algae or corals
(Vergés et al. 2019). How this shift in benthic cover affects
resident fish communities in a tropicalisation context is largely
unknown, however declines in kelp-dependent species may be expected
concurrent with the decline of this foundation species.
Here, we surveyed tropicalised reefs within the Solitary Islands region
to quantify changes in fish assemblages during a period of kelp loss. In
particular, the study period encompasses a shift in benthic cover where
temperate kelp Ecklonia radiata was completely lost, did not
recover, and was largely replaced by turfing algae and corals (Vergés et
al. 2016). We analysed baited remote underwater video surveys (BRUVS)
collected over 17 years from 2002-2018, with total kelp loss occurring
from 2009 (Vergés et al. 2016). Abundance and morphological
traits of individual fish species were assessed. We used the morphology
of individual fish species within the community to investigate the
potential for trait differentiation that may be indicative of meaningful
competitive interactions sensu (Azzurro et al. 2014)
between tropical and temperate associated species of similar feeding
guild (herbivores, invertivores, piscivores and planktivores). With
particular interest in the vulnerability of temperate species, we
specifically asked: 1) How has the probability of occurrence of tropical
and temperate fishes from different trophic guilds changed over 17
years, encompassing the transitional period from kelp to turf dominated
reefs? 2) Are there any species, tropical or temperate, that have
significantly increased and/or decreased in abundance over this period?
Based on the analysis of fish morphological traits, 3) is the overall
“trait space” of incoming tropical fishes different from that of
resident temperate ones, and 4) are changes in fish abundance linked to
unique morphological traits?