changes in abundancy of dutch syrphidae theo zeegers (dutch syrphid project)
TRANSCRIPT
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Changes in abundancy of dutch Syrphidae
Theo Zeegers(Dutch Syrphid Project)
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Central question:
What are the changes in abundancy in dutch Syrphidae ?
with a focus on long-term phenomena
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Besides longterm effects:Base: yearly few years
Meliscaeva auricollis Parasyrphus lineola
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Examples of longterm effects(1950 – 2000)
• Growth: Epistrophe melanostoma
• Decline: Platycheirus peltatus
• Steady: Eupeodes corollae
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Epistrophe melanostoma
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Platycheirus peltatus
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Eupeodes corollae
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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
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All records 1950 - 2000
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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
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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
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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
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Spearman’s correlationEristalis pertinax
0
10
20
30
40
50
60
1950 1960 1970 1980 1990 2000
jaar
ran
gn
um
mer
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Most significant changesgrowth decline
• Epistrophe melanostoma
• Episyrphus balteatus• Eristalis pertinax• Brachyopa pilosa• Myathropa florea
• Platycheirus peltatus• Platycheirus
manicatus• Platycheirus scambus• Neocnemodon
vitripennis• Eristalis intricaria
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No bias on abundancy
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Results for different larval typesBased on 202 species
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New species and extinct species
Analysis by statistical improbability relative to rareness.
Global results: 17 new species
13 extinct
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(possibly) New and (possibly) extinct species
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Some new species with year of arrival
• Helophilus hybridus: 1936
• Brachyopa pilosa: 1966
• Sphegina sibirica: 1988
• Chalcosyrphus eunotus: 2000
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Some extinct species with year of last record
• Eristalis cryptarum: 1943
• Eristalis alpina: 1969
• Chalcosyrphus femoratus: 1978
• Chalcosyrphus curvipes: 1978
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Two examples of geografical expansion
Brachyopa pilosa (new in 1966)
Cheilosia illustrata
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Brachyopa pilosa
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Cheilosia illustrata
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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
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Conclusions (cont.)
• Separate analysis on newcomers and extinctions
• Yields similar results