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

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 few years

Meliscaeva auricollis 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 Least square regression

• Based on areal, not abundance

• No statistical test• Made for extinction

analysis, not for abundant species

• Very sensitive to data-collecting effect

• Based on wrong statistics (normal instead of binomial)

• Therefore: highly aberrant records have too much weight

• Extrapolation leads inevitable to impossible results

Ordinary least squaresKleinste kwadraten regressie

0

0,1

0,2

0,3

0,4

0,5

0,6

1950 1960 1970 1980 1990 2000

jaar

frac

tie

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 correlationEristalis pertinax

0

10

20

30

40

50

60

1950 1960 1970 1980 1990 2000

jaar

ran

gn

um

mer

Most significant changesgrowth 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 typesBased 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