Interpreting trends in abundance

There is an increasing interest in the trends in the abundance of invertebrates, as illustrated by recent posts about 'pollinators', so it is perhaps apposite that I look at some of the trends in hoverfly abundance and think a bit about the reasons for such trends.

I am starting to get quite concerned at the messages that are emerging and begin to think that we probably need to develop two indices: one based on photographic records, which now make up the bulk of the data arriving at the HRS, and the other based on a small number of individuals who still retain specimens.

My rationale is driven by some startling trends that I don't think can be put down to environmental issues; or, at least, I don't think we can disentangle environmental and recording pressures sufficiently to provide a reliable account.

Here are a few examples extracted from the revised trends that should appear in the updated hoverfly atlas (when it finally emerges):

Neoascia podagrica

This is a relatively common species and we see lots of photographs of Neoascia that are probably N. podagrica, but we cannot be entirely sure from a photograph. What we can say is that it is likely to be either N. podagrica or N. obliqua, and unless there is butterbur Petasites hybridus close-by the chances are that it is N. podagrica. So a question-mark hangs over the data. We can also say that we very rarely see records of Neoascia that don't have infuscated outer cross-veins and yet our own field experience tells us that they are quite widespread and abundant (even super-abundant) in the right places.

Figure 1. Trend for Neoascia podagrica with marked downturn in abundance in the past 5 years.

Melanostoma mellinum

I have always found that this species occurs in lower numbers than M. scalare and it is quite plausible that its numbers have declined somewhat. But the data can be skewed by the numbers of people who record in the uplands where M. mellinum is far more prevalent. We see comparatively few records of this species as photographs, a phenomenon that is possibly complicated by the difficulty of doing Melanostoma from photographs.

Figure 2. Trend for Melanostoma mellinum in which a clear downward trend seems to have intensified in the past 4 years for no obvious reason.

Cheilosia illustrata

This is one of the traditional 'hogweed fauna' and as such might be expected to follow a similar trend to others such as Leucozona glaucia that has declined dramatically in south-east England. Yet, the trend appears to go in completely the opposite direction! My instincts are that because photographic recorders tend to concentrate on larger and more obvious species, this is one of the beneficiaries of such recording.

Figure 3. Trend for Cheilosia illustrata in which the upturn coincides quite closely with the arrival of iSpot and more data extraction from web-based sources.

This trend contrasts strongly with Cheilosia proxima, another member of the 'hogweed fauna' whose larvae are also associated with a very common plant (creeping thistle Circium vulgare).

Figure 4. Trend for Cheilosia proxima.

 

Do differences in recording technique matter?

Dramatic changes in the apparent fortunes of our insect fauna have already elicited sensational (correctly in my view) responses in the press. So we need to be clear about our interpretation of the results. There are parallels: the change in bathymetric readings on some estuaries was quite clear when traditional 'lead lining' was replaced by sonar readings. Without this change being recognised, it was possible that an incorrect geomorphological interpretation would be placed on the change. This in turn might have had important implications for our ports industry.

So, quite simply, yes we do need to think about the ways in which data are gathered and must think about our interpretations. With hoverflies, we are just about getting to the point where the data are so dominated by photographic recording that any change will be followed by a new asymptote, after which we can follow new trend lines. BUT, critically, we need to understand and take the changes into account. Thus, it is unwise to look at trends without undertaking critical evaluation taking into account the factors beyond simple environmental parameters that might affect the animal in question.

My feeling is that for the foreseeable future it will be necessary to develop parallel indicators using two different datasets - one based on traditional recording from retained specimens and the other from the photographic record. I suspect there is an element of inevitability that the photographic record will start to become the dominant indicator; in which case we need to be aware that trends for difficult taxa may not be wholly reliable and thus conservation organisations will need to develop new ways of thinking about species' status and conservation policy.