changes in abundancy of dutch syrphidae theo zeegers (dutch syrphid project)
TRANSCRIPT
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