Defining standardized indicator frameworks is an urgent task for our society, and is essential for achieving ecosystem management goals, such as biodiversity and soil protection. Recent initiatives have developed minimum sets of indicators of biodiversity (e.g. Convention on Biological Diversity); however, several questions remain open on how to integrate and combine different types of indicator into general frameworks that can be used to estimate various goals in ecosystem management (Noss 1990; de Bello et al. 2010; Fig. 1).
Functional traits, i.e. measurable characteristics of organisms linked to their fitness, have a relatively long tradition as indicators in ecological studies. These indicators can be effective because they are relatively easy to estimate using rapid field assessments or remote sensing (Lavorel et al. 2008; Schmidtlein et al. 2012). Moreover, whereas indicators based on a list of taxa cannot be extrapolated readily to other regions, functional indicators can. It is important to note that functional trait measurements do not necessarily imply a priori knowledge of species identity and composition. For example, average plant height, leaf nitrogen content and leaf dry matter content within a vegetation stand can be measured in the field, without knowledge of species composition (Lavorel et al. 2008; Hofmeister et al. 2012), enabling vegetation surveys not constrained by the availability and costs of taxonomic experts. In cases where the community species composition is known, community functional traits can rather accurately be reconstructed when traits for species in the local flora are known (de Bello et al. 2010; Axmanová et al. 2012). Despite the high potential of trait indicators, these tools are rarely considered in current monitoring schemes.
The study of Kachergis et al. (2013) in this issue provides a timely comparison of taxonomic vs trait-based indicators to estimate changes in species composition (i.e. vegetation status) in steppe vegetation. First, using both indicator systems, the authors determined various vegetation states in a landscape that encompassed different management regimes. Functional groups were defined by combining species with similar traits. To increase the depth of information provided by the functional groups, the authors tested three plant trait levels required to define functional groups, from less to more time-consuming assessment; these simple functional groups could be estimated in the field by non-expert taxonomists. They then determined how states defined by species and by trait-based groups were related to management and environmental site characteristics.
Among the most important results, Kachergis et al. (2013) demonstrate that functional indicators result in only a marginal loss of taxonomic information while potentially covering community relationships not accounted for by taxa. We believe that this kind of approach can indeed pave the way for successful and more widespread use of functional trait-based indicators in existing monitoring schemes. Building on this study, we propose an expansion of the ideas proposed in Kachergis et al. (2013), particularly focusing on how specific plant traits may complement taxonomic indicators to achieve multiple monitoring goals.
First, we believe that single plant traits can be very useful for defining appropriate indicators: species do not necessarily need to be combined into functional groups. For example, the review of de Bello et al. (2010) demonstrated how various plant traits (e.g. leaf characteristics and plant height) could be used as indicators for biodiversity conservation in different European grasslands (see also Hofmeister et al. 2012). Single plant traits are also known to relate to a variety of ecosystem functions and services (Lavorel et al. 2008). Moreover, plant traits related to canopy and leaf structure parameters determine the habitat suitability for different plant and animal species, including endangered ones, and can potentially be used to monitor biodiversity levels indirectly across trophic levels. Functional groups can be useful to determine different plant strategies, and enable avoidance of comparatively more expensive measurements of single traits (especially in inaccessible areas). It is, however, increasingly being recognized that their definition can often be arbitrary (but see Boulangeat et al. 2012), as the type and number of traits needed to make functional groups most often remain subjective. For this reason, the application of functional groups could be, in some cases, limited in existing monitoring schemes.
Second, taxonomic and functional indicators may have different purposes. They can be used to indicate different vegetation states (as demonstrated in Kachergis et al. 2013) but can also indicate many other ecosystem management goals (Fig. 1). In the context of grassland management, plant traits can provide reliable estimates of fodder production, soil quality and a variety of ecological functions linked to nutrient cycling. Kachergis et al. recognize that taxonomic indicators in some cases could prove less effective than functional ones. This reflects the original idea of Noss (1990), stressing the need to combine different types of indicator. If for some aims functional traits could lose information compared to taxonomic ones, for others the situation may clearly be reversed. Asking if information is lost or gained by using specific indicators requires, therefore, a clear definition of the aim of the indicator system.
In conclusion, the paper of Kachergis et al. reminds us of the fact that monitoring based on taxonomy is important, but monitoring based on traits may represent a simpler approach, providing in many cases similar and in some specific cases even better information. Most generally, assuming that ecosystem management has multiple aims, we believe that it is essential to combine both taxonomic and functional indicators, while specifically testing their power to reach different management targets.