By Jane Catford (This article was first published in the July 2013 issue of Decision Point, The Monthly Magazine of the Environmental Decisions Group)
It is all very well having an academic definition of novel ecosystems, but how do we identify them in practice?
In reality, ecosystems and communities are always in a state of flux—they are never stable—so there will be constant, continuous changes in species assemblages. Species will flicker in and out, and their relative abundance can be as dynamic as environmental conditions are. In Australia, with its great environmental variability, ecosystems can be very dynamic. Consequently, quantifying ecosystem novelty and determining whether a change in community composition is meaningful or not can be a little tricky.
Some colleagues and I grappled with this issue in the context of biological invasions (Catford et al 2012). As simple as it may seem, there is no standard way to quantify the extent or severity of invasion: people use all sorts of different measures, which makes comparisons really difficult. If two ecosystems are invaded by a different suite of species, for example, how can management resources be prioritized objectively if the net impact of invasion on the ecosystems is unknown?
To help answer this question, we identified the best way to quantify the level of invasion by non-native animals and plants by assessing the advantages and disadvantages of different metrics.
Based on our review, we recommend two invasion indices: relative alien species richness and relative alien species abundance. Relative alien species richness and relative alien species abundance indicate the contribution that alien species make to a community. They are easy to measure, can be applied to various taxa, are independent of scale and are comparable across regions and ecosystems. What’s more, historical data are often available to feed into the analysis.
The relationship between relative alien richness and abundance can indicate the presence of dominant alien species and the trajectory of invasion over time, and can help us distinguish between different sorts of invasion. It can highlight ecosystems and sites that are heavily invaded or especially susceptible to invasion.
“Relative alien species richness and relative alien species abundance indicate the contribution that alien species make to a community.”
For instance, if we are concerned about species abundance, we would conclude that Ecosystem A in figure 2 is more invaded than Ecosystem B and thus more of a concern for management. In Ecosystem A, alien species are ‘punching above their weight’ and single alien species makes up more cover than any of the native species. But in Ecosystem B, the aliens are contributing less cover relative to the natives.
Establishing a standard metric that indicates invasion level has many uses: it enables us to assess ecological and economic risk, can be used to guide management and is a prerequisite for calculating ecosystem invasibility. And, as a bonus, what we learn from the process of quantifying invasion level, we can use to help quantify ecosystem novelty.
We applied our approach to two different situations to test its value. The first looked at a number of floodplain wetlands on the River Murray (see box 1 and figure 4). This demonstrates that invasion levels can vary substantially within a single type of ecosystem in a defined geographic area.
The second made comparisons across ecosystems to gauge ecosystem invasibility. This second case study shows how relative-alien-species-richness and cover vary within and among 15 types of ecosystems across the Corangamite catchment in Victoria (Figure 3). Overall, the invasion level of these ecosystems is below the unity line, but the large standard errors in several ecosystems (e.g., Plains woodlands and Hills woodlands) highlights that there are some sites where exotics ‘punch above their weight’.
Such variation can also indicate likely trajectories of invasion. For example, if a wetland in the study region currently has 25% alien species richness but only 7% alien species cover, it would be expected that the cover would increase to at least 11% in the future, bringing it on par with neighbouring wetlands (Figure 3). To maximise management efficacy, it may be sensible to control alien abundance in this wetland before it increases and reaches a later stage of invasion.
As well as indicating ecosystem types that vary greatly in their invasion level, our second case study identifies ecosystems that experience comparatively higher levels of invasion overall. In this instance, a suitable management priority may be to limit future invasion in sites that are particularly susceptible to invasion (as indicated by their ecosystem type) but currently experience relatively low levels of invasion (e.g., sites in the Plains woodland or forest ecosystems that currently have low relative alien species richness and cover).
Box 1 Invasion of the wetlands
As an example of the utility of the invasion level index, we applied it to 24 floodplain wetlands of the River Murray. The study used floristic data to calculate relative alien species richness and relative alien species abundance. The line of best fit in Figure 4a indicates that the relative contribution that alien species make to total vegetation cover is less than their contribution to total species richness. That is, in terms of cover, alien species ‘punch below their weight’ in all but one wetland. Sites (or ecosystems) that are above the unity line should be of concern as the alien species present are contributing more cover than their native counterparts. Such an occurrence may indicate the presence of a strong, dominant invader in the community, which can be ascertained by examining specific information about species evenness.
Although we focus on introduced species in the review, the same ideas and rationale can apply to any taxa of interest. In the case of novel ecosystems, one might be interested in a change in the relative abundance of different functional groups (e.g., a shift from a shrub-dominated to a tree-dominated ecosystem) or the contribution from taxa that do not historically make up a substantial portion of the local community (e.g., sclerophyllous trees in a rainforest, chenopod shrubs in a floodplain wetland) rather than the level of invasion.
Establishing standard, transparent ways to define and quantify invasion level, and the extent of ecosystem novelty, will facilitate meaningful comparisons among studies, ecosystem types and regions, and will enable us to assess change through time.