Electronic Supplementary information 1: Insect sampling in
forests and marshes: methods and data
With: Avian population consequences of climate change are most severe for longdistance migrants in seasonal habitats.
Christiaan Both, Chris A.M. van Turnhout, Rob G. Bijlsma, Henk Siepel, Arco J. van
Strien, Ruud P.B. Foppen
Insect sampling
The aim of this analysis is to show whether insect availability shows different
temporal patterns during the spring-summer period in forests compared to marshland.
There is already some evidence for this in literature, but we gathered additional data
to confirm this pattern. This strengthens the power of our comparison between a
habitat which we consider highly seasonal (forest) and less seasonal (marshland) in
temporal insect abundance. We do not make any statements on whether the
seasonality may have changed differently in the different habitats, but we can give a
general notion of seasonality, because in both habitats data were collected in several
replicates.
Data collection
Insect biomass was sampled in reed beds in 1992 in two peat-lands, and in 2003 in
nine forests across The Netherlands. Although methods of insect sampling necessarily
differed between peat-lands and forests, we aim to compare seasonality of insect
abundance in these two habitats, rather than comparing absolute abundances.
The lowland peat-lands were investigated regarding species composition and
abundance of invertebrates. Several vegetation types were incorporated in this
research, but here we only present data from reed beds. In total 50 sites, 18 of those
being reed beds, have been sampled using emergence traps (as described in Siepel et
al. 1989, but we used larger ones here: surface 1 m2 , height 1.4 m, in higher
vegetations with an extra base element of 1 m). Emergence traps give a good estimate
on the abundance of species. Four types of reed beds (young, dominated by herbs,
dominated by Sphagnum, and not inundated) were sampled at two sites in the northern
Netherlands and two sites in the central part. Inundated reed beds were only sampled
in the northern Netherlands (twice). From May to September traps were operated
during four periods of two consecutive weeks each, and samples were collected on a
weekly basis. The first two periods covered the main breeding season of marshland
birds. Insects were identified to species level if possible. Biomass was measured
either directly by dry weight in abundant species, or by interference using length-dry
weight ratios in less abundant species (Rogers et al. 1976; Rogers et al. 1977). We
then calculated dry weight per m² and vegetation type. As specialist marshland birds
mainly forage in reed beds, we focussed on this vegetation type, but the seasonal
pattern of relative insect abundance is very similar for other sampled marshlands (like
several types of wet grasslands and carrs). We averaged biomass for each sampling
period over all areas. Means are presented with their SD.
In 2003 we sampled forest insects in nine areas by using three methods (see
Both et al. (2006) for selection of the areas). In each area we sampled caterpillar
abundance with frass collectors under two Pendiculate oaks Quercus robur. Frass was
collected approximately once every five days during the breeding season of pied
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flycatchers, from late April until mid-June. In some areas with the highest caterpillar
biomass we started sampling at, or just after the caterpillar peak (Both et al. 2006),
hence the lack of biomass build-up in figure 1c. We also sampled ground-living
insects with five pitfall traps in each of the nine areas, located along a transect with
two meters between traps. Pitfall traps were emptied once every five days, and insects
were stored in alcohol and later sorted by family and size (<2mm, 2-5 mm, 5-10mm,
>10mm). Biomass of all insects was calculated on the basis of length-weight
relationships, and values were calculated as mg/day for all five traps. Data from the
nine areas were averaged as they produced similar patterns with insect abundance
peaking at the end of May and declining afterwards. The third method of sampling
was restricted to five areas and involved the use of single malaise traps in the main
habitat of pied flycatchers. Traps were emptied once every five days, and sorting
procedures were similar as for pitfall traps, producing values of insect biomass per
trap per 24 hr. Data from the five areas were averaged, but they all give quantitatively
similar patterns, showing a rise to a peak in insect abundance at the end of May and a
decline afterwards. No samples were collected after mid-June, when all sampling
techniques showed declining insect availability. The values for the three different
methods cannot be compared quantitatively.
Data: Seasonality of insect availability in two habitats
In this section we show that forests and marshes differ in the seasonality of insect
abundance based on data from The Netherlands, backed-up with published data from
other study sites.
Forests are known to have a short burst of mainly herbivorous insects that
forage on young leaves of deciduous trees before the production of secondary plant
compounds starts (Feeny 1970; Southwood et al. 2004). Our measurements in nine
forests in The Netherlands corroborated this pattern: insect availability peaked during
a couple of weeks in May and June, and declined thereafter (figure 1a-c) (Buse et al.
1999). Caterpillars were most numerous among the insects, but caterpillar peaks
differed between sampling areas, and areas with the earliest peak also had the highest
peak (Both et al. 2006). The peak of ground-living and flying insects occurred two to
three weeks later than the caterpillar peak (figure 1a-b).
We chose marshlands as the less seasonal habitat. Marshland is known to have
a more extended period of food abundance, reflected in the long breeding season of
marsh-inhabiting passerines (see e.g. Halupka et al. 2008). Indeed, our samples from
50 sites in Dutch marshes showed a gradual increase in insect abundance from May
until early August (figure 1d). The reason for this longer period of insect abundance is
probably because reed (Phragmites australis) continues growing during spring and
summer (Dykyjova et al. 1970), and the biomass of herbivorous insects is
consequently less peaked (Halupka et al. 2008). Additionally, insects emerge from the
water over an extended period in spring and summer (Ward 2005).
In conclusion, forests have a stronger seasonality in insect availability than
marshes (Ostendorf 1993; Schaefer et al. 2006).
Reference List
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Figure a1: Insect biomass in Dutch forests (a-c) and marshlands (d) during spring and
summer. In forests three methods of insect sampling were used: (a) pitfall traps for
capturing ground-living insects, (b) malaise traps for capturing flying insects, and (c)
frass collectors to measure caterpillar abundance. In the marshes emergence traps
were used (d), which sample insects that are present in the vegetation at the moment
of placing the trap, as well as insects emerging from the soil or water covered by the
trap. The different methods corrupt quantitative comparisons within and across
habitats, and we mainly aim to show seasonal variations in insect biomass between
habitats. Forests were sampled from late April to mid-June, and marshes between
mid-May to mid-September.
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