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Global Change Biology, FS 2017
Isabelle Helfenstein
Does biodiversity always increase the stability of ecosystems?
Date: 08.05.2017
Supervisor: Harald Bugmann
Introduction
Global change is indisputable. Anthropogenic changes, e.g. global warming, eutrophication, fire suppression, drought and predator decimation can be noticed globally (IPCC 2013) and lead to shifts in
ecosystems. Species richness enhances both the production of biomass and its temporal stability, however the impact of diversity on production is not linked to its impact on stability (Cardinale et al.
2013). The relationship of biodiversity and productivity cannot be treated analogously for the case of
diversity and stability. In contrast to the large number of syntheses focusing on the impacts of biodiversity on productivity, only few have shown how biodiversity influences the stability of ecosystems.
This may be due to the fact that stability refers to a wide variety of ecological phenomena. In order to
find out what impact anthropogenic changes have on the ecosystem, there is a need to identify how we
are affecting ecosystem stability via changes in biodiversity.
Questions
Does increased biodiversity always promote the stability of ecosystems?
What are the factors that make a varying effect?
In which cases is the relationship positive, and in which cases negative?
What are the key mechanisms, according to current knowledge, between stability and biodiversity?
Results
In order to analyze the effects of biodiversity on ecosystem stability, recent studies used different
methods and a variety of definitions of diversity and stability, which leads to heterogeneity within the
field. Ives and Carpenter (2007) synthesized different aspects of stability from different surveys. The
most common definitions of stability were invasibility, variability, resistance, and return rates. They
found that studies referring to return rates lead to negative relationships between diversity and stability, but they only constituted 9 out of 59 studies. This leads to the conclusion that the majority of studies showing positive results did so because of the used definitions of stability, which are likely to show
positive results. There could even be an aspect of stability regarding phenological processes (e.g.,
Bartomeus et al. 2013). Further, biodiversity is hard to define properly. Evenness of plant abundances
and functional traits are linked to stability as much as species richness, but only the latter is commonly
used to define biodiversity (Polley et al. 2013).
There are several of mechanisms underlying the relationship of diversity and stability. Hector et al.
(2010) evaluated them in a grassland setting. One of the mechanisms pointed out was asynchrony.
When species responses are not perfectly correlated, increases in the population size of some species
can be compensated by declines in others (and vice versa) and consequently contribute to a decrease in
the variation of the entire system. A second important mechanism is overyielding (Hector et al. 2010),
which is occurring when the productivity of species mixtures is greater than expected from the respective monocultures. The increase of productivity of species mixtures is known to occur primarily
through niche partitioning, rather than through the increase of growth rate of individuals (Jucker et al.
2014). De Mazancourt et al. (2013) developed a theory in which they add the observation error as an
important aspect to asynchrony and overyielding. Their model assumes that the more species have to
be measured within a site, the more the observation errors of species biomass get averaged out in the
biomass of the entire community. This observation is more a statistical than an ecological ‘mechanism’, but it was found to play an important role in the study.
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Global Change Biology, FS 2017
Isabelle Helfenstein
Jucker et al. (2014) identify mechanisms underlying aboveground wood production, which is getting
faster and less variable in mixed-species forests. Overyielding was observed in all sites, but the
strength of the effect varied. They stated that the diversity effects on productivity are stronger in
stressful areas, as the overyielding signal was much weaker in mid-latitude areas. They further stated
that several species did not grow faster, which shows that the effect is species-specific. Asynchrony
seems to be the best indicator of stability, but it tends to saturate, i.e. the effect is much stronger with
fewer species added to the site. This tends to be a difference compared to grasslands, where increased
short-term compensatory reactions take place. Forests already are much more decoupled from interannual variations, and asynchrony thus has a much lower impact. Regarding shifts in competitive species
interactions, it was observed that not all species combinations promoted all types of stability. In areas
where drought resistance is an important factor contributing to stability, drought had more negative
impacts in mixed forests. Competition negatively impacted the growth of drought-tolerant species.
These findings show that diversity is likely to have a weaker effect on stressful sites. Therefore, species interactions do not always promote the relationship between diversity and stability. This is in
agreement with Loreau and Mazancourt (2013), who found destabilizing effects of interspecific competition in grassland communities at both the population and community levels.
Ecological mechanisms that impact diversity, resource availability and species interactions are scale
dependent. This makes the examination of diversity–stability relationships at multiple scales very important (Raffaelli 2006). Wang and Loreau (2014) proposed a hierarchical framework including variability at different scales: diversity increased with higher levels of spatial scale, while variability decreased from local to regional level. Instead of looking at experimental data, Morin et al. (2014) used
virtual experiments based on a dynamic simulation model to test for the diversity–stability relationship
and its underlying mechanisms in forests. They pointed out that temporal stability increases with increasing species richness at the community level, but decreases at the species level. This is mostly due
to the asynchrony between species and only to a lesser extent due to overyielding, which agrees with
the finding of de Mazancourt et al. (2013). However,, Pasari et al. (2013) found that biodiversity effects on the stability of ecosystem functions are especially significant from species richness at the
local scale. This shows the importance of species being organized into communities, when regarding
landscape-scale influences of biodiversity.
Another problem of the studies is their heterogeneity in habitat (Ives & Carpenter 2007). Cusson et al.
(2015) concentrated on marine communities with algae and fauna. They focused on both variation in
species richness and structure (evenness). Focusing on four habitats in two marine systems at different
temporal and spatial scales, they found a positive relationship for variability in species richness with
increasing species richness at both scales, and with increasing evenness at the site scale. Effects of
species richness on stability therefore are slightly negative, but also depend on the scale of observation. The variability of community structure, however, was independent of biodiversity at all scales.
These results also show that there is a lot of variation between the different habitats and marine systems. Regarding temporal scale, the study showed that study duration did not affect the results. However, they did not have a look at the impacts of differing seasonality.
Conclusion
The presented results show that diversity does not always promote ecosystem stability. The effects
vary over spatial scales, while there is no strong difference visible. Differences come from the definition of biodiversity and stability, which leads to great differences of results. Furthermore, results vary
through climate regime, and even habitat. The most negative effects of species richness on variability
of species richness was found in marine systems, while one cannot tell if this is due to the definition of
stability or the ecosystem itself. Study duration did not have a significant effect, and there were no
effects of biodiversity on species structure (evenness). Another important aspect, i.e. that varying results could be due to phenological variation and even phenological aspects of stability, was not taken
into consideration here.
The key mechanisms presented here are multi-faceted, and their importance varies. According to the
general consensus, the most important mechanism underlying the relationship is asynchrony of species
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Isabelle Helfenstein
responses, but it is much more dominant in grasslands, and less important in forests. Overyielding
seems to be the second most important mechanism, as it was found in many studies, but it is often not
as strong as asynchrony. Competition/species interactions seem to explain the main differences between climate regimes and contribute to some of the observed results. Lastly, averaging out measurement errors seems to be contributing to the apparent stabilizing effect of increased species richness.
References
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Cusson, M., et al.. (2015): Relationships between biodiversity and the stability of marine ecosystems:
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de Mazancourt, C. et al. (2013): Predicting ecosystem stability from community composition and biodiversity. Ecology Letters, Vol. 16, p. 617–625.
Hector, A. et al. (2010): General stabilizing effects of plant diversity on grassland productivity through
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