Changes in abundancy of dutch Syrphidae Theo Zeegers

Changes in abundancy of dutch Syrphidae Theo Zeegers (Dutch Syrphid Project)

Central question: What are the changes in abundancy in dutch Syrphidae ? with a focus on long-term phenomena

Besides longterm effects: Base: yearly Meliscaeva auricollis few years Parasyrphus lineola

Examples of longterm effects (1950 – 2000) • Growth: Epistrophe melanostoma • Decline: Platycheirus peltatus • Steady: Eupeodes corollae

Epistrophe melanostoma

Platycheirus peltatus

Eupeodes corollae

Artificial causes of fluctuations in data • Learning effect • Change in recording methods (esp. collecting versus field records) • Change in recording intensity • Statistics • Species with higher abundance yields more reliable results

All records 1950 - 2000

Conventional methods of analysis: IUCN red list • Based on areal, not abundance • No statistical test • Made for extinction analysis, not for abundant species • Very sensitive to datacollecting effect Least square regression • Based on wrong statistics (normal instead of binomial) • Therefore: highly aberrant records have too much weight • Extrapolation leads inevitable to impossible results

Ordinary least squares

Therefore: New method • Input: relative abundance in records • Analysis: non-parametrical regression (Spearman’s correlation) • Works with ordinals rather than numbers • Advantages: best input, robust, works also very well for common species • Disadvantage: does not work for very rare species

Spearman’s correlation

Most significant changes growth decline • Epistrophe melanostoma • Episyrphus balteatus • Eristalis pertinax • Brachyopa pilosa • Myathropa florea • • Platycheirus peltatus Platycheirus manicatus Platycheirus scambus Neocnemodon vitripennis • Eristalis intricaria

No bias on abundancy

Results for different larval types Based on 202 species

New species and extinct species Analysis by statistical improbability relative to rareness. Global results: 17 new species 13 extinct

(possibly) New and (possibly) extinct species

Some new species with year of arrival • • Helophilus hybridus: 1936 Brachyopa pilosa: 1966 Sphegina sibirica: 1988 Chalcosyrphus eunotus: 2000

Some extinct species with year of last record • • Eristalis cryptarum: 1943 Eristalis alpina: 1969 Chalcosyrphus femoratus: 1978 Chalcosyrphus curvipes: 1978

Two examples of geografical expansion Brachyopa pilosa (new in 1966) Cheilosia illustrata

Brachyopa pilosa

Cheilosia illustrata

Conclusions • New method for trends in abundancies • Works quite well except for very rare species • On the average increase of species with xylophagous larvae • And decrease for species with aquatic larvae

Conclusions (cont. ) • Separate analysis on newcomers and extinctions • Yields similar results