Why record common species?

From time-to-time participants in the UK Hoverflies Facebook group question the value of reporting common species. Is it necessary or of any value? My consistent response is that it is and that we do value records of common species just as much as those of rarer species. Why might this be?

Assessing rarity for conservation status

As a starting point, we need to think about rarity - it is a relative value. If something is rare, we need to be able to quantify it. When the early Red Lists for the UK were compiled, a set of somewhat arbitrary values were used to define species of conservation concern. They were based on the numbers of 10km grid squares in which a given species was found. Broadly speaking, anything that was known to occur in more than 15 10km squares could not be assigned Red Data Book status. Those that occurred in 15 to 30 squares could be listed as Notable A (now Nationally Scarce A) and from 31 to 100 squares as Notable B (Nationally Scarce B). These latter statuses are a UK construct and are not used elsewhere.

This approach is based upon comprehensive geographic coverage, which is really only possible for plants in the UK. It becomes much more complicated for invertebrates, which are comparatively under-recorded. So there is a need to use the guidelines with a modicum of expert judgment. In the case of hoverflies, we looked at overall coverage and concluded that as the Hoverfly Recording Scheme (HRS) has data for about 90% of 10km grid squares; setting the threshold at 90 squares for the maximum for Nationally scarce is probably about right. The threshold may have to be reduced still further for other families of flies where coverage is more patchy.

Assessing changes in species abundance

Species status assessment has evolved and IUCN guidance now works on the size and vulnerability of populations. So, a species that has a small range but a stable population is probably under less threat than a species with a wider range that is showing signs of declining abundance. How do we test for declining abundance when all we have is a set of maps and data that are collected in an ad-hoc manner?

There have been a great number of attempts to develop models that interpret such data, but the developing norm works on occupancy models that take account of the recording effort. For these models to work effectively we need data for both common and rarer species so that the model can test the degree to which presence or absence is likely to be a function of biogeographic or recording factors.

Importance in legal challenges

At a simpler scale, we need to think about the ways in which proponents of development projects play down the importance of seemingly rare species. It is dead easy to undermine species statuses by simply presenting coverage maps for less well recorded taxa and emphasising the gaps. Many of those gaps might be filled with records of commoner species that have not been logged. So, at this very simple scale, logging common species can help to improve general coverage and thus reinforce the importance of rarer species.

Detecting change

Conservation policy now revolves strongly around rates of decline. There are good data for farmland birds so the declines in these species are readily detectable and as a result the BAP process placed a lot of emphasis on these species. Unfortunately, we know that about 40% of invertebrates (in those taxa that can be interpreted) have also suffered similar declines. They are not represented in Priority lists that are now enshrined in law. Why? Well, as a starting point, birds have a very strong advocate that was able to push away important invertebrate contenders. But, the critical issue lies in levels of coverage and the degree to which analysis is regarded as reliable. Fewer data and more patchy coverage undermines confidence in the analysis.

Perhaps more importantly, change is most likely to be detected at an early stage when one looks at common species. The bigger the dataset, the more likely change is going to be detected. So, if a species is recorded half a dozen times each decade any fluctuation is unlikely to be detected because a rise or fall of one individual can disproportionately alter the statistics. Conversely, if there are hundreds or perhaps thousands of records of another species each year, any increase or decline is likely to be mirrored in a comparable proportion of the records received. Thus, commoner species are actually the bellwether of change (both positive and negative).

Does it matter?

The HRS is not simply a mapping tool. Data compiled are used in a wide variety of analyses; ranging from Species Status Reviews and the Wildlife Statistics produced by the NGOs, to analysis of pollinator abundance by various university groups.

Until Stuart retired, the HRS data were used on a regular basis to develop models that examine changes in both abundance and distribution. We still get asked for access to data for similar purposes, so we must hope that such work is ongoing. The better the data, the more likely it is that the outputs will be believed.