Main text
In a recent study (Brun et al. 2022), we used an empirical
screening approach to identify species in the French Alps and
Switzerland whose abundance (1) contributed most to ecosystem
productivity given environmental conditions and (2) was positively
associated with productivity. We then demonstrated that the top five of
these key species jointly contributed more to explaining productivity
than common measures of functional community composition and diversity,
and that key species were typically tall and showed high specific leaf
area. However, the relationships we identified are highly dependent on
environmental context, with marked deviations in low-productivity
environments.
Bruun & Ejrnaes (2022) point out that our results may allow inferring
how many species can go extinct before ecosystems collapse, and that we
missed evaluating our findings in this light. They therefore combine
data from our analysis with information on red-listed species in the
study area, and demonstrate that red-listed species have significantly
lower occurrence frequencies and local abundances than key species and
would thus contribute less to ecosystem productivity. From these
findings, they conclude that few key species may suffice to maintain
productivity, and ecosystem processes in general, and that most species
can go extinct without hampering the efficiency of ecosystem processes.
We believe that these conclusions overinterpret our results. Although
the contrasting occurrence frequencies between key species and
red-listed species are striking, they are not surprising. We focused on
the most important species at regional scale and assessed to which
degree their abundance serves as a proxy for productivity after
accounting for environmental conditions, and such species need to be
common to a minimum degree. In order to be red-listed, on the other
hand, species need to be geographically restricted or locally scarce
(UNEP-WCMC and IUCN 2016), so by definition key species and red-listed
species hardly overlap in occurrence frequency. Does this mean
red-listed species are irrelevant to productivity and ecosystem
functioning in general? Most of the red-listed species are part of the
83% least frequent species that we did not investigate. That red-listed
species’ contributions to productivity are smaller than those of key
species can be expected, and our results are not even necessary to prove
this. But this is just a ranking with regards to productivity that by no
means implies that disappearing red-listed species hardly affect
ecosystem processes in general, and especially on the long run.
A fair assessment of the contributions of rare and red-listed species
should focus on per-abundance effects rather than per-species effects,
and in this respect a large body of literature highlights that they
often are disproportionately important. Geographically restricted and/or
locally scarce species have been shown to be important drivers of
grassland multifunctionality (Soliveres et al. 2016) and
ecosystem stability (Säterberg et al. 2019). Moreover, rare and
red-listed species disproportionately shape the functional structure of
ecosystems (Leitão et al. 2016), as they often exhibit distinct
combinations of traits (Mouillot et al. 2013; Loiseau et
al. 2020). Such functionally distinct species further promote forest
productivity in harsh environments (Delalandre et al. 2022) and
ecosystem multifunctionality across biomes (Le Bagousse-Pinguet et
al. 2021). Although per-species effects of rare species may currently
be smaller than those of key species, relative abundances are dynamic
(Loreau et al. 2003), and rare species today may become key in
the changing world of tomorrow - as long as they manage to survive
(Yachi & Loreau 1999).
Research on biodiversity and ecosystem functioning advances by better
understanding the system, not by debating about the validity of
established hypotheses. In our analysis, we identified high importance
of key-species abundance to explain ecosystem productivity and we found
support for the ‘mass ratio hypothesis’ (Grime 1998), but we also found
that trait diversity has a positive association with productivity, in
line with the ‘complementary resource use hypothesis’ (Naeem et
al. 1994). A joint relevance of both hypotheses has been found
repeatedly for productivity (Sonkoly et al. 2019; Gao et
al. 2021) and other ecosystem processes (García‐Palacios et al.2017; Le Bagousse-Pinguet et al. 2021), and countless studies
exist that support each of these two hypotheses individually. It
therefore appears appropriate to view the ‘mass ratio hypothesis’ and
‘complementary resource use hypothesis’ as independent axioms in the
context of multifunctionality: some ecosystem functions are driven more
by averages of traits while others primarily respond to trait diversity
or complementarity. What matters is our ability to understand ecosystem
processes and how species are contributing to them, and this ability is
unnecessarily limited by presuming that the main question is mass ratio
or complementarity.