Current trends in trait-based ecology
In total, we identified 822 studies relevant to traits-based ecology
from >200 journals published as early as 1978. Over 50% of
the studies came from just 18 journals. The number of studies applying
traits-based approaches to describe ecological pattern and process has
grown exponentially over the past forty years, with a marked increase in
the rate at which traits-based ecological studies have been published in
the last decade (Figure 2). Most studies (59%) focused on the role of
traits at a single level of environmental filtering, while only 3.5% of
studies examined traits in the context of more than two levels of
filtering. One study, Usseglio-Polatera (2000 - Freshwater
Biology ), examined traits in the context of deriving environmental
filtering from coarsest (i.e. abiotic environment matching) to finest
(i.e. trophic interactions) filters. In total, 435 studies examined more
than one type of trait (i.e. physiological, morphological, behavioural,
or life history), 176 considered more than two trait types, and only 26
(<0.5%) studies considered traits that represented all four
categories simultaneously.
Most of the research to date focuses on relating variation in
morphological and life history features of vascular plants to abiotic
environmental filtering processes within terrestrial ecosystems using
observational techniques (Figures 3 and 4; 34% of all papers focused on
plants, 30% on plant morphology); this result that is perhaps
unsurprising given that traits-based research originated in plant
ecology community (McGill et al. 2006). In contrast, traits-based
investigations within marine and freshwater systems comprises just 30%
of the studies combined, with a focus on observational studies of traits
in the context of abiotic environmental filtering processes, primarily
for fishes (Figures 3 and 4).
Trait-based research outside of plant-based research has overwhelmingly
focused on size as the characteristic of interest (Figure 5, Table S3,
Figs S3-S14). In fact, we identified 131 different metrics of size used
across studies, representing a range of length, mass, or volume
measures—including vegetative height and cone length (plants),
snout-ventral length or instar size (animals), and biovolume (cells).
Body size is the most common trait measured in studies of animal taxa,
and also the most variable in terms of its definition and measurement,
with metrics ranging from wet or dry mass, to inference from the length
of the organism (involving multiple methods of measurement depending on
the study and taxa under consideration). In contrast, specific leaf area
(SLA; measured as leaf area per dry mass) is the most common trait used
within plant trait research and is estimated via a single s standardized
method across studies and plant taxa (Evans & Hughes 1961; Spence et
al. 1973). In fact, SLA is the most common single trait examined in the
studies reviewed here (127 occurrences over the 822 studies), with
measures of body size a close second at 108 occurrences.
Beyond size, a plethora of morphological, behavioural, life history, and
physiological features have been applied to trait-based biodiversity
research (Figure 5). In total, we identified 2,561 unique traits within
the literature we reviewed. While only 5% of these (131) represented
aspects of organism size, size-based traits were used within 405 studies
(49%) and each of these 131 traits were used by 3.7 separate papers on
average, while non-size traits were each used only 1.7 times on average
across the 822 studies. Accounting for variation in trait names (i.e.
‘time to maturity’ vs. ‘age at maturity’), and instances where a similar
trait was assessed via multiple metrics (e.g. ‘trophic guild’ could be
measured as a categorical variable with levels ‘resource’, ‘primary
consumer’, ‘secondary consumer’, or ‘tertiary consumer’, while in a
separate study ‘trophic group’ could be measured in levels of ‘primary
producer’, ‘herbivore’, ‘omnivore’, or ‘carnivore’, both relating to
‘trophic role’) allowed us to attribute traits that confer information
about the same process into conceptual groupings, revealing 196
‘secondary’ trait classifications (Supplementary Material; Table S3). Of
these, 14% are used in a single study while 34% are used in more than
10 studies. Of the 2,561 unique traits, morphological traits (Figure 5;
n = 1,163) included aspects of organisms’ physical form (e.g. body size
and shape, or the presence and form of dentition or spines) and
biochemical composition (e.g. nitrogen or carbon content). Key
behavioural traits (Figure 5, n = 626) include aspects of organisms’
activity (e.g. movement rates or nocturnality) and habitat use (e.g.
vertical habitat position within forest canopies or water columns, range
size or edge position). Life history traits (Figure 5; n = 585) describe
growth, abundance, survival, and reproduction (including reproductive
mode, timing, and frequency), while physiological traits (Figure 5; n =
187 traits) conferred information about organisms’ environmental habitat
requirements (e.g. moisture or temperature tolerances), and resource
acquisition (e.g. photosynthetic rate).
The diversity and identity of traits applied to ecological research
depend on the environmental filter under investigation (Figure 1; Figure
5A-D, Figure S7&S8) and the ecosystem of interest (Figure 5I-K, Figure
S9&S10). Interestingly, relatively narrower assemblages of traits are
used in plant compared with animal research (Fig 6A,Fig S1A&S2A), with
several potential explanations: (i) plant ecology has focused on traits
for longer, and thus may have made relatively more progress in
distilling a specific set of traits that represent key processes (versus
animal trait research, which still needs to be distilled in this way),
(ii) traits that represent key processes have more standardized
measurement methods than traits for other taxonomic groups (i.e. plant
size measured as SLA vs the variety of animal body size measurements),
and (iii) differences in size of the trait space between taxonomic
groups may simply reflect differences between sessile/mobile organisms;
Compared with mobile taxa, sessile plants and fungi (represented in
‘other’; Figure 6A, Figure S12) have a relatively more narrow set of
strategies for resource acquisition, defense, dispersal, and
reproduction. In addition, perhaps also due to the longer history of
traits approaches to plant ecology, research linking plant functional
traits to their hypothesized ecological function is somewhat more
developed, conferring a more complete understanding of the ecological
processes that can be inferred from specific traits.
We also find that relatively narrow assemblages of traits have been
applied in experiments, meta-analyses, reviews, and theoretical work
compared with observational research (Figure 6B), the latter being the
majority of studies in this review, and the likely starting place for
first identifying and linking traits to important aspects of species
distribution and interactions. Likewise, a relatively narrow assemblage
of traits are applied within multi-ecosystem studies (i.e. those where
findings are relevant broadly across ecosystem types; Figure 6C).
However, the multi-ecosystem research reviewed here primarily represent
theoretical models, which apply traits that can be estimated universally
across taxa and systems such as body size and shape, and feeding mode,
potentially explaining why the assemblage of traits applied in this body
of work is more homogenous compared with marine, freshwater, or
terrestrial focused-work (Figure 6C, Figure S14).