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The Spider and the Sea
Effects of marine subsidies on the role of spiders in terrestrial food webs
Kajsa Mellbrand
©Kajsa Mellbrand, Stockholm 2009
Illustrations and photos: Kajsa Mellbrand
ISBN 978-91-7155-877-0
Printed in Sweden by Universitetsservice, US-AB, Stockholm 2009
Distributor: Department of Botany, Stockholm University
“Jag vet var spindlarna spänna
i vassen nät över vattnet”
-Dan Andersson
Doctoral dissertation
Kajsa Mellbrand
Department of Botany
Stockholm University
SE – 10691 Stockholm
Sweden
The Spider and the Sea
Effects of marine subsidies on the role of spiders in terrestrial food webs
Abstract – Marine inflows to terrestrial ecosystems have been suggested as
an important factor for the structure and function of shore ecosystems. The
overall aim of this thesis is to increase the understanding of how marine
subsidies affect shore ecosystems, with focus on the role of spiders in shore
food webs. The thesis includes five papers. In Paper I, a bottom-up food web
is constructed based on carbon sources of coastal arthropods, in Paper II
relationships between arthropod densities and edge and dispersal effects are
examined, in Paper III top-down effects in the coastal food web is examined,
Paper IV investigates variation in spider carbon and nitrogen stabile isotope
ratios in relation to eutrophications on shores, and Paper V quantifies the
inland reach of marine subsidies in Swedish shore meadows and Australian
sandy beaches and examine processes behind the inland transport of marine
subsidies. The studied marine inflows consist of marine algae and emerging
phantom midges (Chironomidae) with marine larval stages. The results show
that spiders utilize marine subsidies to a larger extent than other arthropod
predators on shores. A large-scale removal experiment showed that spiders
had a negative effect on densities of insect predators, but no top-down effects could be measured on lower levels of the food web. Responses of density to island size differed between different arthropod groups on small islands, implicating that the relative importance of dispersal and edge effects
differ between taxa. Total niche width in wolf spiders decrease along an
eutrophication gradient, and there is an individual variation in carbon stable
isotope ratios of spiders on shores. Spiders and dipterans are important vectors for the inland transport of subsidies in both Swedish and Australian
shore ecosystems, and the inland reach of subsidies is also affected by landscape characteristics of the shores.
Keywords: Marine subsidies, food webs, stable isotopes, shore ecosystems,
predators, spiders, Pardosa, algae, emerging insects
List of papers
This thesis is comprised of a summary and five papers, which are referred to
by their Roman numerals:
I
Mellbrand, K. and Hambäck, P. A. Utilization of marine nutrients by coastal arthropod predators in the Baltic Sea area: a stable isotope study. Manuscript
II
Östman, Ö., Mellbrand, K. and Hambäck, P .A. 2009. Edge or
dispersal effects – Their relative importance on arthropod densities on small islands. Basic and Applied Ecology – In press
III
Mellbrand, K., Östman, Ö. and Hambäck, P. A. Effects of subsidized predators on coastal food webs in the Baltic Sea area.
Manuscript
IV
Mellbrand, K., Enskog, M., Kautsky, L. and Hambäck, P. A.
Individual variation between spiders on shores in the utilization
of aquatic subsidies. Submitted
V
Mellbrand, K., Lavery, P., Hyndes, G. A. & Hambäck, P. A.
Linking land and sea – Arthropod vectors for marine subsidies.
Manuscript
Paper II is reprinted in this thesis by permission of Basic and Applied Ecology.
Contents
Introduction ..................................................................................................9
Aims..............................................................................................................11
Methods .......................................................................................................12
Papers ..........................................................................................................20
Discussion ...................................................................................................31
Concluding remarks ..................................................................................36
Acknowledgements ...................................................................................37
References ..................................................................................................38
Svensk sammanfattning ..........................................................................42
Tack ..............................................................................................................43
Introduction
As early as in the 1920´s, scientists (e.g. Elton 1927) had thoughts about
the importance of cross-boundary flows of nutrients, matter and organisms
for ecosystem function (Polis et al. 1997). However, spatial processes were
rarely incorporated in food web theory before the 1990´s (Power & Huxel
2004) and these processes are still often neglected although we now know
that the ecosystem is an arbitrarily defined unit and that movements of matter and organisms across ecosystem boundaries are common (Polis & Hurd
1996; Polis et al. 1996). Such cross-boundary flows often compromise an
important nutrient and energy source for receptor ecosystems, and are suggested by Polis & Hurd (1996) to possibly be the dominant factor in shaping
the dynamics of both consumers and resources in many ecosystems. One
such ecosystem boundary where between-system processes may be of great
importance is the marine shore-line (Polis & Hurd 1996; Polis et al. 1996;
Hodkinson et al. 2001), a zone in which many organisms are almost exclusively found.
Aggregation of predators on shores is a common and widespread phenomenon, and one often suggested explanation is that terrestrial predators
are positively affected by aquatic subsidies (Polis et al. 1997; Nakano &
Murakami 2001; Murakami & Nakano 2002; Henschel 2004; Power et al.
2004). The many predators found to aggregate along coasts include both
species that naturally live close to water and species that are also found further inland. Many, if not most, of the shore-line predators belong to the latter
group, for example many wolf spiders (Lycosidae) as well as other invertebrates and many species of bats and lizards (Polis et al. 1997; Power et al.
2004). On bird-subsidized islands in the Gulf of California, terrestrial predators such as spiders, scorpions, lizards and ants were found to be 10-100
times more abundant than on the mainland (Polis 1994) and spider densities
on islands were 6 times greater in the supralittoral area than in inland areas
(Polis & Hurd 1996). In many systems, predators have also been found to
move between beaches and inland habitats, thereby acting as vectors for
marine inflows.
Subsidies from marine to terrestrial systems may consist of marine organic matter drifting ashore (Polis & Hurd 1996; Polis et al. 1997; Bastow et
al. 2002), of sea birds feeding on marine food items but defecating and
breeding on land (Polis et al. 1997, Sanchez-Piñero & Polis 2000; Anderson
and Polis 2004), and of animals moving between aquatic and terrestrial habi9
tats as part of their life history (Hodkinson et al. 2001; Murakami & Nakano
2002; Power et al. 2004). Most studies in this thesis were made in the Baltic
Sea area, and while inflows here are not truly marine since the Baltic Sea is a
brackish sea, they will still be referred to as marine subsidies in this thesis.
The Baltic Sea is one of the largest brackish water bodies on the planet and
includes elements of both a marine and a freshwater ecosystem. Species
diversity in the Baltic Sea is low compared to both freshwater and strictly
marine habitats, since few species are adapted to brackish habitats, and species composition is mostly a mix of marine and freshwater species. Species
composition also changes along the salinity gradient from south to north in
the Baltic Sea, with a decrease in marine species along the gradient (Nielsen
et al.1995).
Inflows of aquatic algae and plants are common subsidies in marine shore
ecosystems, and may in arid areas with little primary production be the only
available nutrient source on shores (Polis and Hurd 1996; Polis et al. 2004;
Catenazzi & Donnelly 2007). While inflows of aquatic algae and plants exist
in freshwater ecosystems, their role as a subsidy to terrestrial ecosystems is
not well studied. Emerging insects is on the other hand a very common and
often important subsidy to shores of freshwater ecosystems, but insects are
less common in true marine ecosystems and the few “marine” insects are
typically found in habitats in ecotones between marine and terrestrial habitats, such as estuaries, salt marches and intertidal zones (Cheng 1976). The
role of emerging insects as a subsidy to shore ecosystems have been well
studied in particular in stream ecosystems (Hodkinson et al. 2001; Murakami
& Nakano 2002; Power et al. 2004), but have been given much less attention
in for example lakes. The Baltic Sea shores thereby provide an opportunity
to study relative roles of different inflow types in the same receptor system.
10
Aims of the thesis
The overall aims of this thesis were to understand the effects of marine subsidies on the role of spiders in shore food webs, and to examine the relative
importance of two subsidy types: algal inflows and emerging insects. To
achieve this, more specific questions were asked.
In Paper I, the aims were to quantify the extent to which arthropods on
Baltic Sea shores utilize marine subsidies, and to construct a bottom-up food
web for Baltic shores including marine inflows. This knowledge would make
it possible to further examine ecosystem processes in relation to marine subsidies using Baltic Sea shores as a study system. The aim of paper II was to
investigate the relative importance of dispersal and edge effects on subsidized small islands, while in Paper III the roles of subsidized arthropod
predators and the occurrence of top-down effects in the shore food web is
examined. In Paper IV, the aim was to investigate variation in spider stable
isotope ratios along an eutrophication gradient. This paper thereby addresses
both underlying causes of variation in spider stable isotope ratios, and methodological concerns when handling large variations in stable isotope ratios.
The aim of Paper V was to quantify the inland reach of marine subsidies in
relation to factors such as inflow type, landscape characteristics and arthropod vectors. Paper V is a comparative study of two types of shore ecosystems, Baltic shore meadows and Australian sandy beaches, and important
questions here were the relative importance of different subsidy types and
the relative importance of different vectors. In addition to these aims, studies
of marine subsidies can also provide a better understanding of underlying
ecosystem processes and food web interactions in ecosystems.
11
Methods
The study system
Study area
The focal ecosystems in this thesis are coastal ecosystems in the northern
Uppland archipelago in the Baltic Sea, Sweden (Fig. 1; Fig. 2). The Australian parts of Paper V were carried out on three sandy beaches on the Indian
Ocean coast in South-western Australia (Lat: N 37 º 25.82', Long: W 122º
05.36'): Hillary´s Beach and Two Rocks north of Perth, and Sulphur Beach
on Garden island in the Shoalwater marine reserve south of Perth (Fig. 2).
Arthropods, terrestrial plants, filamentous green and perennial brown algae
were collected for stable isotope analysis (Paper I & V) on the coast of the
Swedish mainland and on the coast of the island of Gräsö (16º45’E,
67º10’N, size about 150 km2) in the northern Uppland archipelago. The studies in Paper II and Paper III were made on small islands in two areas on the
northern and north-eastern coasts of Gräsö. The study of individual variation
in spiders (Paper IV) was made in Norrtäljeviken in northern Uppland.
Figure 1. The Uppland coast, Sweden .
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Most of the Swedish study shore-lines, on both mainland shores and islands,
are shore meadows separated by rocky areas (Fig. 2). The shore meadows
are mostly kept open by flooding during parts of the year and none of the
study sites in this thesis are grazed by domestic animals. Water level fluctuations are mainly due to meteorological and hydrological factors in the atidal
Baltic Sea and change both between and over the year (Johannesson 1989;
Winsor et al. 2001). Typically, mean water levels in the Baltic Sea are highest in January, lowest in late winter-early spring, and rise again in latesummer-autumn (Jerling 1999). Many of the studied shore meadows are
flooded from autumn to early spring and occasionally during late spring and
summer, depending on wind conditions. The vegetation on shore meadows
in the Baltic Sea show a gradient inland, due to factors such as salinity and
water content in the soil, water fluctuations and duration of flooding (Johannesson 1989; Jerling 1999). Closest to the sea, reeds and sedges often form
more or less large stands. Further inland, plants less tolerant of salinity and
water content may establish, and eventually trees replace the meadow vegetation. The study shores in this thesis often have reeds (Phragmites australis)
and sedge (Bolboschoenus maritimus) closest to water. In rockier areas
without reeds, salinity tolerant herbs such as Triglochin maritima, Plantago
maritima, Aster tripolium and Glaux maritima are often found close to the
waterline. Further inland on shore meadows, usually a turf of grasses, often
dominated by Agrostis stolonifera, interspersed with plants such as Eleocharis uniglumis, Carex nigra, Carex distica, Plantago maritima, Glaux maritima, Spergularia media, Blysmus rufus, Juncus gerardii, Ophioglossum
vulgatum, Rhinantus serotinus, Pedicularis palustris, Orchis maculata, Filipendula ulmaria and Mentha aquatica. On the rocky beaches vegetation is
scarcer and typically consist of species such as Lythrum salicaria, Sedum
acre, Sedum telephium, Ophioglossum vulgatum, Matricaria maritima, Viola
tricolor, Vicia cracca and Allium schoenoprasum.
The Australian study systems in Paper V (Fig. 2) are sandy beaches and
dunes where marine subsidies to shores consist of seagrass (mainly Posidonia species) and algal detritus (mainly Sargassum spp.) washed ashore during winter. On beaches little or no vegetation is found, though sometimes
scattered succulents (mainly Tetragonia decumbens) grow in the upper part
of the beach. Further inland dunes are colonized first by grasses (mainly
Spinifex longifolius) closest to the beach and then by shrubs further inland.
Common plants are various Acacia species, Scaevola crassifolia, Leucophyta brownii, Tetragonia decumbens, Lepidospermum gladiatum, Spinifex
longifolius and veldt grass (Ehrarta spp.).
13
Figure 2. Shore meadow on Gräsö, northern Uppland, Sweden (left) and sandy
beach north of Perth, Western Australia (right).
Figure 3. Marine inflow types (left to right): Chironomid midges (Sweden), filamentous green algae (Sweden) and brown algae and seagrass (Australia).
Figure 4. Wolf spiders Pardosa amentata (Sweden) and Tetralycosa oraria (Western Australia), and web weaving spider Larinioides cornutus (Sweden).
14
Study organisms
The two types of marine subsidies included in this thesis are marine algae
and phantom midges (Chironomidae) (Fig. 3). Most of the studied shores
and islands in paper I and III are located in shallow bays, and the inflow of
algae to many of these shores is low. The algal inflow mainly consists of
filamentous green algae (Cladophora spp., Mougeotia spp., Spirogyra spp.
and Ulva spp.) which are very fast degradable, providing an ephemeral and
less stable habitat for detritivores than driftwalls consisting of perennial
slowly degrading species (Paper I, III, IV and V). In the Baltic Sea study
sites in Paper III, an inflow of brown (Fucus) algae only occurred to four of
the 19 islands in the study. In the Australian part of Paper V, wrack is the
only type of marine inflow to shores and consists of brown algae, mainly
Sargassum spp., and seagrasses, e.g. Posidonia spp., all relatively slowly
degradable.
Several insect families with aquatic larval stages are common on the
Swedish study shores, such as dragonflies (Odonata), damselflies (Zygoptera), mayflies (Ephemeroptera), caddisflies (Trichoptera) and various dipterans, but the most abundant are chironomid midges. The midges emerge in
swarms throughout summer and rest in vegetation when not flying. A dominant group of chironomids in the area are Orthocladinae species that are
generally herbivorous (Wiederholm 1989), but various Chironominae species, of which several species are detritivores, are also abundant.
In the terrestrial ecosystem my main study organisms are spiders, in particular wolf spiders (Lycosidae) in the genus Pardosa (Fig. 4), which are the
by far most common and abundant spiders (and predators) on shores in the
Baltic Sea area. Of the arthropod fauna on shores, about 80% of all groundliving arthropod predators are spiders, about 50% of the spiders are wolf
spiders, and about 80% of the wolf spiders are Pardosa wolf spiders (Fig. 4)
of which the most common species is Pardosa amentata (50% of all wolf
spiders). Other common spiders on shores are web weaving spiders in families Araneidae (Fig. 4), Linyphiidae and Tetragnathidae, though most Swedish spider families are represented. Other arthropod predators common on
Baltic Sea shores are shore bugs (Saldidae), nabid bugs (Nabidae), carabids
(mainly Dyschirius) and staphylinid beetles (Staphylinidae). Common herbivores on the shores are planthoppers (Homoptera; Cercopidae, Cicadellidae),
grasshoppers (Orthoptera), mirids (Heteroptera), butterfly larvae and beetles
such as weevils (Curculionidae) and chrysomelids. The most common detritivores are springtails (Collembola; mostly Podura aquatica). Dipterans are
also a common and abundant group on the shores.
In the Australian study sites, common wrack fauna are mainly dipterans
and weevils but also predators such as wolf spiders (in wrack only Tetralycosa oraria (Fig. 4)), small web weaving spiders and staphylinid beetles.
Spiders, in particular wolf spiders but also web weaving spiders and jumping
spiders are common predators both on the beach and in the dunes. Insect
15
predators in dunes include staphylinid beetles, carabids, predatory heteropterans, mantids and scorpions (Two Rocks only). Lizards and snakes, though
not included in this study, are also very common predators on shores. Common herbivores are mainly plant hoppers (Homoptera) and weevils (Curculionidae).
16
Methods
Data collection
For the papers in this thesis I used a variety of sampling methods for density
estimates of arthropods and for collection of samples for stable isotope
analysis: glue traps, pitfall traps, and d-vac sampling using a vacuum sampling device, a Stihl® BG85 Leaf Blower/VAC. Plants, marine algae and
often wolf spider stable isotope samples were sampled by hand.
Not included in any of the papers are several smaller studies aimed at examining intraguild predation, fitness, and movement and stable isotope turnover rates in wolf spiders. For all these studies, spiders were collected on
shore meadows on the Gräsö.
To examine intraguild predation among wolf spiders on shores, Pardosa
and Trochosa wolf spiders were collected and separated into two size classes
each; with small Trochosa being roughly the same size as large Pardosa.
Four spiders of two size classes were placed together in 2 l containers. Five
replicates were used for each of the six combinations of size classes, and
containers were continuously supplied with plentiful alternative prey
throughout the experiment (mainly various dipterans and plant hoppers).
Surviving spiders were counted after three weeks, and individuals not found
(any spiders found dead but intact were assumed dead from other causes
than predation) were considered eaten by its cohabitants.
To examine fitness related to distance from the waterline in wolf spiders,
female Pardosa wolf spiders carrying egg sacs were collected along inland
transects at 0 m, 20 m and 160 m from the waterline in five shores on the
island of Gräsö. The number of eggs (or spiderlings) in egg sacs were
counted, and egg number divided by cephalothorax width was used as a
measure of fecundity.
To examine stable isotope turnover rates and to get realistic fractionation
rates, Pardosa wolf spiders (N=62) were fed on a diet of bred dipterans
(Drosophila melanogaster) of known stable isotope composition for 13
weeks. Once a week four spiders were sampled for stable isotope analysis.
To examine the scale of wolf spider movement on shores, a markrecapture study was performed in a 60*20 m area (60 m along the waterline,
20 m inland) on two shores, one long shore (open shore meadow with short
vegetation) and one short shore (forest about five meters from the waterline).
On each shore, Pardosa wolf spiders were captured, marked and released in
three 5 m2 squares 30 m apart along the waterline during two subsequent
days. Spiders were captured using pitfall traps and hand capture. Marking
was made using water based acrylic dye, with one colour being used for each
release point and each day. The third day spiders were captured using pitfall
traps set 2 m apart in the 60*20 m grid.
17
Stable isotope analysis
Stable isotope analysis has become a widely used tool for the study of energy and nutrient flows in food webs and for the study of long-term differences in feeding behaviour between consumers. While stable isotope analysis, as all methods, has its drawbacks, one of its many advantages is the ability to distinguish between local production and inflows from other systems,
as long as there is a detectable difference in isotopic ratios between sources.
Stable isotope analysis has been used for this purpose in a large number of
studies concerned with aquatic subsidies (for example Collier et al. 2002;
Barrett et al. 2005; Briers et al. 2005; Ballinger & Lake 2006; Catennazzi &
Donnelly 2007; Paetzold et al. 2008), and is used in four out of the five papers in this thesis in order to understand variation among organisms in their
usage of marine subsidies. Separating marine and terrestrial carbon sources
is possible since basal resources in marine and terrestrial environments typically differ in carbon stable isotope composition. Due to differences in photosynthetic apparatus and carbon sources (CO2 vs CO3-), terrestrial plants are
typically much more 13C-depleted than marine plants and algae (Fry 2006),
though different algal species and marine plants also differ in carbon isotope
composition (Fredrikssen 2003, Edward et al. 2008; Paper I, V).
Stable isotope composition of plant and animal tissues is a combination of
stable isotope ratios within sources and of fractionation processes. Fractionation occurs during chemical reactions since different stable isotopes of the
same element behave somewhat differently in chemical reactions (though the
magnitude of the fractionation differs a lot between elements): The lighter
isotope of an element typically reacts faster than the heavy isotope, which
requires more energy for the formation or breaking of chemical bonds (Fry
2006). This causes heavy isotopes to have slower incorporation rates in tissues than lighter isotopes of the same element. As a result stable isotope
ratios of carbon and nitrogen (δ13C=13C/12C and δ15N=15N/14N) differ between carbon dioxide and nitrogen in the air on one hand and the carbon and
nitrogen composition in plant tissues on the other hand. Also, plants using
different modes of photosynthesis, as well as different carbon sources, may
differ in stable isotope ratios (Fry 2006). Fractionation will occur for every
chemical reaction as elements travel up the food chain, resulting in the heavier isotope being retained longer in an open system, and enriched at higher
levels of the food chain. For carbon, only minor fractionation of carbon isotopes typically occur for each trophic step (typically <0.1 %), but this is
much smaller than the known difference in carbon isotope ratios between the
primary carbon sources in this thesis (typically >0.8%, Fry 2006;
McCutchan et al. 2003). While fractionation rates may vary not only between groups, species or populations, but also between tissues and individu-
18
als, the large differences between aquatic and terrestrial carbon sources in
this thesis enable tracing of primary carbon sources up the food chain even
without detailed knowledge of carbon fractionation rates of consumers. Still,
several studies have shown that intrapopulation variation in δ13C (δ13C
=13C/12C) may lead to under- or overestimations of dietary variation (Bolnick
et al. 2003; Matthews & Mazumder 2004; Araújo et al. 2007) which is a
reason for caution in the use of stable isotope analysis. One aspect of this
topic is addressed in Paper IV.
Nitrogen stable isotope ratios similarly show different signals in marine
and terrestrial environments, but with a much larger spatial variability and a
larger and more variable nitrogen isotopic fractionation in relation to differences both among sites and among organisms (Vanderklift & Ponsard 2003;
Jennings & Warr 2003, Caut et al. 2009). Nitrogen stable isotope ratios are
therefore less useful than carbon stable isotope ratios for delineating trophic
links. Variability in 15N between primary producers and consumers may
nevertheless provide information on the relative length of trophic chains
(Post 2002), but due to the problem with larger and more variable fractionation rates, particularly for generalist predators, few conclusions in this thesis
are based on nitrogen stable analysis. In generalist predators with a high
prevalence of behaviours such as intraguild predation and cannibalism and a
large number of prey species in their diet (such as many spiders) (Foelix
1996; Wise 2006), fractionation rates are often difficult or impossible to
determine since the feeding on multiple trophic levels cause large variations
in stable isotope composition.
19
Papers
Paper I
Utilization of marine nutrients by coastal arthropod predators in the Baltic
Sea area: a stable isotope study.
The aim of paper I was to examine which arthropods on shores, in particular
which predators, utilize carbon of marine origin and to what extent, and to
use stable isotope analysis to construct a bottom-up food web (Fig. 5) based
on the three main types of available and distinguishable carbon sources on
these shores: marine algal detritus, marine prey (chironomid midges) and
terrestrial material (plants, prey, detritus).
Spiders
Insect predators
Small spiders
(<3mm)
Chironomidae
Herbivores
Terrestrial
detritivores
Collembola
Marine algae
Terrestrial plants
Algal detritus
Terrestrial detritus
Nutrients
Figure 5. Suggested coastal food web of Baltic Sea shores.
20
Diets of arthropods on shores were examined using stable isotope analysis.
The results show that spiders are the terrestrial predators in my study system
mainly utilizing nutrients, while most insect predators (Fig. 6) and all herbivores mainly utilize carbon of terrestrial origin. This indicates that marine
subsidies in the area may be more important when arriving as an inflow of
prey rather than as an inflow of detritus. However, since terrestrial plants
cannot utilize aquatic carbon sources but can still benefit much from algal
detritus as a nutrient source, our methods were insufficient to determine the
extent to which plants, and thereby also herbivores feeding on shore vegetation, utilize marine subsidies.
Predator groups
Theridiidae 3
Tetragnathidae 13
Salticidae 12
Pisauridae 4
P hilo dro midae 5
Lyco sidae 57
Linyphiidae
Gnapho sidae 9
Dictynidae 1
Clubio nidae 7
A raneidae 6
Staphylinidae 7
Silphidae 1
Saldidae 4
Nabidae 18
Chryso pidae 7
Carabidae 5
Opilio nes 13
A carina 6
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
Marine carbon proportion
Figure 6 Carbon mixing model with fractionation of either -0.5 (A) or 0.5 (B) at a
family level. Grey bars show proportions of marine carbon in terrestrial arthropod
diets (± confidence intervals).
It is clear from this study that the predators that benefit most from an inflow
of marine prey are spiders, indicating that eventual effects of marine subsidies for the coastal ecosystem are likely to be mediated by spiders.
21
1
Paper II
Edge or dispersal effects – Their relative importance on arthropod densities
on small islands.
In Paper II the relative importance of edge effects and dispersal effects was
examined on small islands of different sizes and isolation. Carbon stable
isotope analysis of wolf spiders was also made to examine utilization rates of
carbon of marine origin both within and between islands.
Different responses of density to island size were observed for different
taxa. There was a negative relationship between density and island size for
leaf hoppers, and a positive relationship for ants, while dipteran and web
spider densities did not show any response to either island size or isolation.
An interactive effect of island size and isolation was found for wolf spiders
and parasitoids, whith a positive relationship between densities and island
size on distant islands and a negative relationship on close islands. The results suggested that the relative importance of dispersal and edge effects
differ between taxa, with dispersal behaviour being more important than
edge effects in wolf spiders, parasitoid wasps and possibly leaf hoppers
(Homoptera). Edge effects are more important in ants and collembolans, and
results were inconclusive for web spiders and dipterans. The stable isotope
analysis showed higher δ13C-values on islands closer to the mainland, but all
spiders had a carbon stable isotope signal close to that of the two common
subsidy types in the area, marine algae and emerging chironomid midges.
The implications of this study are that both edge effects and dispersal should
be considered when examining effects of patch size and isolation for densityarea relationships.
22
Paper III
Effects of subsidized predators on coastal food webs in the Baltic Sea area.
To examine top-down effects of spiders in coastal food webs in relation to
marine subsidies, we performed a large scale removal experiment where
spiders were removed from small islands and effects on densities of other
arthropods and plants were measured.
The results show that spider removal caused insect predator densities to
increase (Fig 7), suggesting that insect predators are negatively affected by
the high density of spiders on shores. Further studies are needed to understand the underlying mechanisms for this, but direct mortality from spider
predation may have been a decisive factor. No treatment effects were found
on either herbivore or detritivore densities (Fig 7) or on plant biomass, and
we suggest that eventual negative effects of the high spider densities on control islands may be at least partly compensated by an increased effect of
insect predators utilizing mainly terrestrial prey on treatment (removal) islands.
This observation does not exclude the possibility of top-down effects on
lower levels of the food web from spiders, but if they do exist, they are likely
behaviour mediated rather than a result of predation. Behaviours such as
cannibalism and intraguild predation are common in spiders and may
dampen cascading effects, making top-down effects by spiders difficult to
observe (Polis et al. 2000; Finke & Denno 2005). Behavioural effects may
include decreased activity and switches in habitat and diet selection (Denno
et al. 2003; Cronin et al. 2004; Moran et al. 1996; Schmitz 2004; Beckerman
et al. 1997), and to find such effects necessitates a different experimental
design. Without additional behavioural studies, the only conclusion that can
be drawn is that spiders tend to affect other arthropod predators negatively,
while top-down effects of spiders on lower trophic levels of the food web are
either nonexistent or may be compensated by effects of insect predators on
treatment islands, resulting in little or no measurable effect of spider removal
on plants, herbivores or detritivores. Another possibility is that top-down
effects are only apparent with a more complete spider removal treatment: as
50% of spiders remained on treatment islands.
23
100
a. web-spiders
10
c. staphylinid beetles
10
b. carabid beetles
10
1
Density
10
1
1
1
0.1
0.1
10
e. parasitoid hymenoptera
10
1
f. herbivorous heteroptera
1
0.1
100
d. predatory heteroptera
0.1
g. homoptera
1000
h. collembola
100
10
10
1
1
2004
2005
2006
2007
2004
2005
2006
2007
Year
Figure 7. Density responses by broad arthropod groups to removal of lycosid spiders (solid line = control islands, hatched line = spider removal islands).
24
Paper IV
Individual variation between spiders on shores in the utilization of aquatic
subsidies
The aim of Paper IV was to examine variability in the utilization of marine
subsidies in shore-dwelling spiders along an eutrophication gradient, using
stable isotope analysis. The study was carried out inside and on islands outside Norrtäljeviken, a highly nutrient enriched eutrophicated bay in the Baltic Sea.
The results showed that spiders inside the bay all utilized mainly terrestrial prey, while spiders outside the bay, in particular wolf spiders, were
separated into individuals utilizing either terrestrial or aquatic prey (Fig. 8).
The total population niche width was therefore larger outside than inside the
bay. This individual specialization may be related to the differences in nutrient enrichment in the aquatic ecosystem and/or salinity between sites in- and
outside the bay, but we could not fully explain the processes behind the narrowing niche width. We suggest that eutrophication in combination with a
more variable habitat structure in the less nutrient enriched sites may affect
trade offs in spiders, causing a decrease total niche width by affecting prey
availability and prey choice of individual predators.
Another important implication from this study is that large intrapopulation variation in isotopic values may lead to over- or underestimations of
dietary variation between populations or species. Such large intraguild variation is common in spiders, which often are generalist predators, and our
study underlines that while stable isotope analysis remains a useful tool for
examining flows across ecosystem boundaries, caution is needed in the interpretation of data with large intrapopulation variation.
25
Aquatic algae and terrestrial plants
0.6
Brown algae
Green algae
0.4
Terrestrial plants
Frequency
0.2
Phragmites
australis
0
Dipterans
0.6
0.4
Chironomid midges
Brachycerid flies
0.2
0
Wolf spiders
0.6
0.4
Outside
Inside
0.2
0
Web weaving spiders
0.6
Outside
0.4
Inside
0.2
0
-30
-26
-22
-18
-14
-10
-6
δ13C
Figure 8. δ13C of primary producers and dipterans across all sites,and δ13C of wolf
spiders and web weaving spiders separated between sites inside and outside the bay.
26
Paper V
Linking land and sea –terrestrial arthropod vectors for marine subsidies
In Paper V we examined the inland reach of marine subsidies and the role of
arthropod vectors in this inland transport. The study was carried out on
Swedish and Australian shores (Fig.2), making it possible for us to use the
large differences in shore type, vegetation, productivity and topography between the two study systems to study processes behind inland transport of
subsidies.
The results showed that spiders and dipterans are important vectors for
the inland transport of marine subsidies. The large differences between our
Swedish and Australian study systems (very different types of ecosystems
both in the sea, on shores and further inland) allow us to draw general conclusions about the underlying processes of inland transport of marine subsidies. Emerging dipterans from the sea in Sweden and from wrack on beaches
in Australia seem to fill a very similar role, with flying dipterans in both
cases reaching further inland than wrack and subsidize spiders further inland
as well as on the shores. While the effect decreases along inland transects,
we still find a marine carbon stable isotope signal as far as 160 m inland in
Sweden and 80-100 m in Australia (Fig. 9). This may not solely be related to
the movement of dipteran vectors, but also to the movement of the spiders
hunting them. In particular wolf spiders are cursorial and fast moving predators, and are both among the most common and abundant predators on
shores and among the ones utilizing marine subsidies to the largest extent.
While other spider groups on the shores may utilize marine subsidies to a
similar extent, these spiders are often less mobile and less abundant than the
wolf spiders, making the latter group better suited to function as a vector for
the inland transport of subsidies.
The study also indicate that landscape characteristics such as vegetation
and topography can play an important role for the inland transport, and that
these characteristics will affect different vectors differently. Both topography
and higher vegetation will decrease the reach of wrack, but the vegetation
may also contain the wrack longer on shores. The inland reach of dipterans
will be less affected than wrack, but will still reach further inland in a flatter,
more open landscape. The same is likely true for cursorial spiders that live in
open habitats, such as many wolf spiders.
27
Sweden
Flies
Australia
dipterans
Wolf spiders
Pardosa
Web spiders
Web spiders
Jumping spiders
0m
(waterline)
10
Distance 0
(tideline)
20
8
15
6
10
4
5
2
0
0
-30
-25
-20
-15
-30
20 m
10
20
8
-25
-20
-15
-10
Distance 2 (upper beach, approx.
20 m from tideline)
15
6
10
4
5
2
δ15N
0
0
-30
-25
-20
-15
-30
20
10
15
6
-20
-15
-10
-15
-10
-15
-10
Distance 5 (dunes, approx.
80 m from tideline)
80 m
8
-25
10
4
5
2
0
0
-30
10
-25
-20
-30
-15
8
-20
Distance 6 (dunes, approx.
180-200 m from tideline)
20
160 m
-25
15
6
10
4
5
2
0
0
-30
-25
-20
-15
-30
-25
-20
δ13C
Figure 9. δ13C and δ15N of dipterans and spiders along inland transects in Sweden
and Australia.
28
Studies not included in papers: Results
The intraguild predation experiment
The intraguild predation experiment showed that larger wolf spiders prey on
smaller wolf spiders to a high degree even when alternative prey is available.
In 20 of the 30 containers, one or several of the four spiders were eaten by
another spider. Intraguild predation was more common where the size difference between spiders were large; however, Pardosa was seemingly more
aggressive and large individuals also killed small Trochosa spiders, despite
the similarity in size.
Wolf spider fitness along inland transects
There was a significant difference in Pardosa wolf spider egg production
along inland transects, with lower production in spiders caught very close to
the waterline (F=5.7; p<0.05) (Fig. 10).
12
Fecundity
10
Figure 10. Fecundity
of female Pardosa
wolf spiders along
inland transects.
8
6
4
2
0
0
20
160
Distance (m)
Stabile isotope turnover time
The experiment where spiders were fed a Drosophila diet showed that δ13C
ratios of wolf spiders became more similar to prey ratios over time (Fig. 11),
and at the end of the study (after 12 weeks) Dδ13C = 1.2 and Dδ15N = 1.6
between Drosophila flies and spiders. This remaining difference in δ13C
ratios between spiders and prey after 12 weeks may be explained by the
turnover time of spiders being longer than 12 weeks, but may also be related
to a high carbon fractionation.
29
10.00
δ15N
8.00
Drosophila
Pardosa week 1
6.00
Pardosa week 13
4.00
2.00
0.00
-25.00
-20.00
-15.00
δ13C
Figure 11. δ13C and δ15N of Drosophila flies and of Pardosa wolf spiders sampled
week 1 (not fed on Drosophila flies) and week 13 (fed Drosophila flies for 12
weeks).
Mark-recapture of wolf spiders
In the mark-recapture study, 410 Pardosa wolf spiders were marked and
released on the long shore and 171 on the short shore; of these 39 were recaptured on the long and 21 on the short shore. 69% of the recaptured spiders on the long shore and 95% of the spiders on the short shore were captured within 5 m from the waterline, most in the vicinity of the original release, but a few individuals were recaptured on the opposite side of the
60*20 m grid, 60 m away. The results indicate that wolf spiders are able to
move over large areas, but mainly move along the waterline rather than
inland, even on the long shore.
12
Figure 12. Average numbers of caught Pardosa wolf
spiders per trap on one long
and one short Baltic Sea
shore.
Spiders/trap
10
8
long shore
6
short shore
4
2
0
0-5
5-10
10-20
Distance (m)
The total number of spiders caught in the 60*20 m grid was, including recaptures, 1562 on the long shore and 398 on the short shore. This study
thereby also show that wolf spiders aggregate near the waterline on Baltic
Sea shores and that densities decrease rapidly inland (Fig.12). Wolf spider
densities were also much larger on the long shore (open shore meadow) than
on the shore with forest close to the waterline.
30
Discussion
Stable isotope ecology of shore ecosystems
The results from paper I provide a framework for closer examinations of
feeding relationships in the coastal food web, some of which are addressed
in Paper III, IV and V. We used information from carbon and nitrogen stable
isotope analyses in Paper I to construct a bottom-up coastal food web (Fig.
6) based on terrestrial and marine carbon sources. Still, the picture remained
obscure in some aspects, mainly because of difficulties in differentiating
between the effects of the two inflow types: algae and the emerging chironomid midges. In paper I, we concluded that spiders are the main beneficiaries of the marine inflow, but we could not examine possible bottom up
effects by algal inflow on herbivores and insect predators since we were
unable to quantify this inflow. The algal inflow is likely important as a carbon and nutrient source for detritivores and as a nutrient source for terrestrial
plants, but the importance of the algal subsidy to shore ecosystems in the
Baltic Sea area is probably underestimated in this study due to the difficulties in differentiating between marine sources (Paper I). In this thesis, with
the exception of Paper V, I therefore mainly discuss inflows consisting of
emerging insects. On the predator level of the food web, a high inflow of
alternative prey bypassing lower levels of the food chain and subsidizing
predators directly could theoretically still be more important than the algal
subsidy, and such direct subsidies to predators could also enhance top-down
effects in the ecosystem (Polis et al. 1997; Henschel 2004). Another aspect
not discussed in the papers in this thesis is temporal variation in inflow rates.
One of the advantages of stable isotope analysis as a method is however that
it provides long term dietary information rather than a "snapshot picture",
and since the turnover rate for Pardosa wolf spiders is longer than 12 weeks,
the marine carbon stable isotope signal found in wolf spiders on shores indicate that carbon of marine origin is a regular part of spider diets in this area.
Results from the spider stable isotope analyses (Paper IV) show a large
individual variation in spider diets (Fig. 9) beside the interspecific variation
in stable isotope composition, and that this individual variation decreases
along a gradient of decreasing salinity and increasing eutrophication. Intrapopulation variation is common in nature, and many generalist species
31
consists of individual specialists; such variation is however often (but not
always) associated with differences in resource use between age classes,
sexes and morphs (Bolnick et al. 2002; Bolnick et al. 2003; Svanbäck &
Persson 2004). In our case, the increased individual variation in sites outside
compared to inside the Norrtäljeviken bay is consistent with the mechanism
for individual specialization suggested by Bolnick (2003), where the total
niche width is expanded but individual niches are constrained by trade offs
making it difficult for any single individual to utilize all available resources.
We were unable in this study to separate between effects of salinity, eutrophication and habitat characteristics, but suggest that the effect of eutrophication on ecosystem dynamics may be influenced not only by species richness per se, but by the narrowing of total niche width in consumers otherwise utilizing a wider range of resources. Individual variation in wolf spiders on shores may also explain some of the large variation in δ13C ratios of
spiders on Baltic Sea shores found in Paper I and Paper V.
Most studies of marine subsidies to shores have been made in areas with
large differences in productivity between donor and receptor systems, where
marine nutrient inflows greatly surpass local primary production (Polis and
Hurd 1996; Polis et al. 2004; Orr et al. 2005; Catenazzi & Donnelly 2007;
Ince et al. 2007; Marczak et al. 2007). This is true for the Australian shores
(Paper V) as well, though the difference in this system is not as large (Ince et
al. 2007) as in the studies by Polis & Hurd (1996), Polis et al. (2004) and
Catenazzi & Donnelly (2007) of very arid, low productivity terrestrial systems and highly productive marine systems. In the Baltic study system (Paper V) there may be a difference in productivity, but it is likely much
smaller, making effects of marine subsidies harder to find. This smaller productivity difference between systems is another factor setting the Baltic Sea
area as a study system apart from previous studies of marine subsidies. In the
only study previously made between systems of similar productivity, Paetzold et al. (2008) found very little effect of subsidies on predator δ13C ratios
above the intertidal zone on a temperate, productive, forested island subsidized by an inflow of algal detritus. Our results from Swedish study sites
(Paper V) differ in this respect, with a larger inland reach of the subsidy, but
also a larger variation in δ13C ratios.
In the study by Paetzold et al. (2008), wrack was the only type of marine
inflow, while in the Baltic Sea there is also a substantial inflow of emerging
chironomid midges. The larger importance of marine subsidies in Baltic
shore ecosystems may be due to a combination of a more efficient inland
transport of subsidies (flying insects) and an inflow directly subsidizing
predator levels of the food web rather than indirectly, through an increased
primary production. Inflows of emerging insect have previously been studied
mainly in stream ecosystems, and have been shown to be important for food
webs in stream corridors (Murakami & Nakano 2002; Nakano & Murakami
2001; Power et al. 2004). Both Paetzold et al. (2008) and Marczak et al.
32
(2007) also stress that the productivity gradient alone is insufficient for explaining variation in responses to spatial subsidies. This conclusion is supported by our results that densities are affected by other factors such as landscape characteristics (Paper V), edge effects and dispersal (Paper II). In Paper V we also discuss effects of landscape characteristics of the receptor
ecosystem. We found that both higher shore vegetation (forest) and topography may decrease the inland reach of marine subsidies, factors that are not
often discussed in literature (but see Marczak et al. 2007 and Stempniewicz
et al. 2007). Stempniewicz et al (2007) found a much larger eutrophication
effect of guano deposition of little auk (Alle alle) chickens compared to that
of guillemots (Uria sp.), related to differences in shore topography of nesting
sites.
Top-down effects in shore ecosystems
The traditional approach for studying effects of arthropod predators on lower
trophic levels is through small-scale enclosure studies (e.g., Schmitz 2003;
Beckerman et al. 1997), or by selectively removing predators on focal plant
individuals (e.g., Marquis & Whelan 1994). While such studies are well
suited for examining underlying mechanisms, they may be less informative
about large scale effects since small scale studies may impede natural
movements and exclude population level feedbacks (Inouye et al. 2005). In
our mark-recapture study of Pardosa wolf spiders on Baltic Sea shores, results indicate that Pardosa wolf spiders can move up to 60 m in a 48 h time
period (Fig. 13). One of the aims in Paper III was to examine these processes
on a larger scale, both temporally and spatially, than in traditional enclosure
studies.
Using islands as natural enclosures made differences due to treatment
(spider removal) less apparent, perhaps due to the large variability both between islands and between years. No evidence of top-down effects on levels
below predator levels of the ecosystem could be found (Paper III), in contrast to previous small-scale studies (Polis et al. 1997; Schmitz et al. 2000;
Halaj & Wise 2001; Denno et al. 2003; Anderson & Polis 2004; Schmitz
2004). Factors such as isolation, patch size and edge effects were also shown
to affect arthropod densities on islands, and the importance of edge effects
and dispersal differ between taxa (Paper II). While the investigated islands
were all located close to the mainland (Paper III), the importance of edge
effects may still vary between both islands and years and our observation of
limited top-down effects may be partly related to the treatment not being
effective enough considering the large natural variability in our study system. Still, we were able to show that insect predator densities are negatively
affected by spiders on shores (Paper III), which may appear contradictory
since nearly all common and abundant spiders on shores utilize mainly car-
33
bon of marine origin (Paper I), while most of the utilize mainly carbon of
terrestrial origin (Paper I). The very high spider densities, however, may
cause even a small inclusion of terrestrial prey in spider diets to affect densities of the less abundant insect predators. Competition from spiders for terrestrial prey may further reinforce the negative effect of spider intraguild
predation on insect predator densities, even with terrestrial prey making up a
very small part of the carbon signal of individual spiders. From this follows
that the lack of top-down effects on herbivore and detritivore populations in
this study may be explained by spider predation on control islands being
compensated for by the increases in densities of more specialized insect
predators on removal islands. If this is the case, removal of all arthropod
predators would be necessary to discover top-down effects on herbivore and
detritivore levels of the food web.
Density effects of marine subsidies
Marine subsidies are often considered one of the main explanations for aggregation of predators on shores (Polis et al. 1997; Nakano & Murakami
2000; Murakami & Nakano 2002; Henschel 2004; Power et al. 2004). We
found little evidence for aggregation of spiders on Baltic Sea shores (Paper
V), but this result may be obscured by large variation, and densities may also
be underestimated by the suction sampling method used for the density estimates. Suction sampling is considered a highly effective method for sampling terrestrial arthropods in short vegetation (Henderson 2003), but may be
less effective for large, fast cursorial animals such as for example wolf spiders. In the Australian study sites, spider densities were higher on beaches
than further inland, but here the density estimates were made using pitfall
traps (Paper V). To examine the mobility of wolf spiders on Baltic Sea
shores we did a mark-recapture study on two shores on the island of Gräsö
(two of the five shores in paper V), one long and one short shore. The results
suggest that wolf spider densities do decrease further inland also on Baltic
Sea shores, even on a small spatial scale (20 m). The mark-recapture also
indicated that densities are higher on long shores than short shores, though
the difference in densities may be related to differences between sites. Wolf
spider fitness is not included in Paper V, but fecundity of female Pardosa
wolf spiders was measured along inland transects on the Swedish sites and
the results indicate that fecundity is lower closer to the waterline. This may
be related to higher densities, and thereby more competition and larger risk
of intraguild predation and cannibalism.
In this thesis I mainly discuss the role of spiders as predators in shore
food webs, but in many shore ecosystem larger vertebrate predators such as
birds, lizards and bats are important predators and often utilize marine subsidies to a high degree (Polis 1994; Power et al. 2004; Murakami & Nakano
34
2002; Barrett et al. 2005; Catenazzi & Donnelly 2007). The higher spider
densities on shores both in the Baltic and Australia (Paper V) is likely related
to higher prey availability closer to the waterline, but an additional factor
affecting differences in spider density may be increased predation risk from
larger insectivorous predators such as lizards further inland. In Australia
(Paper V), lizards were often encountered in the dunes above beaches and
often caught in our pitfall traps. We have not included any vertebrates in this
study, but other studies found evidence of lizards utilizing inflows of marine
prey both directly and by feeding on smaller terrestrial predators such as
arachnids (Barrett et al. 2005; Catenazzi & Donnelly 2007). Lizards thereby
also function as vectors in our Australian study systems. Other studies have
found birds and bats to be important vectors for marine inflows (Murakami
& Nakano 2001; Murakami & Nakano 2002; Power et al. 2004), and these
groups are often seen hunting over water surfaces in Baltic shallow bays.
The failure to include all levels of the food chain may lead to underestimations of the inland reach of marine subsidies, and in future studies it would
be interesting to also include vertebrate predators.
35
Concluding remarks
This thesis has a dual theme: the importance of marine subsidies and
the role of spiders in shore ecosystems. The results in Papers I,
supported by results in paper II, IV and V, indicate that spiders on
shores often utilize marine resources to a high degree, and different
aspects of this relationship are examined in several of the papers.
Paper I also showed that spiders utilize marine subsidies to a larger
extent than other arthropod predators on shores. In Paper II, responses
of density to island size differed between different arthropod groups
on small islands, implicating that the relative importance of dispersal
and edge effects differ between taxa. In Paper III, top-down effects
were examined in a spider removal study using small islands as
enclosures. The results showed that spiders have a negative effect on
insect predators, but treatment had no measurable effects on lower
levels of the food web. The results of Paper IV showed that there is an
individual variation in spiders, in particular wolf spiders, on shores,
and that total niche width of spiders decrease along an eutrophication
gradient. In paper V the inland reach of marine subsidies was
examined in a comparative study of Swedish shore meadows and
Australian sandy beaches. The results indicate that spiders and
dipterans are important vectors for the inland transport of marine
subsidies in both these systems, and that the inland reach of subsidies
is also affected by landscape characteristics of the shores.
36
Acknowledgements
Although this summary was written by me, it is based on the work of
all authors and coauthors of the papers and manuscripts that it is based
on (Paper I-V), and for that reason I would like to thank Peter Hambäck, Örjan Östman, Lena Kautsky, Paul Lavery, Glenn Hyndes and
Maria Enskog. I also want to thank my supervisor Peter Hambäck and
my assistant supervisor Lena Kautsky for valuable comments on this
summary.
37
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Svensk sammanfattning
Denna avhandling syftar till att bättre förstå hur marina subsidier påverkar
strandekosystem, med fokus på spindlars roll i strandfödoväven.
Avhandlingen innehåller fem studier, som förutom en fältstudie på
sandstränder i sydvästra Australien är utförda i Östersjöområdet, främst i
Upplands skärgård. Den metod som främst använts för att studera betydelsen
av marina näringsflöden är stabil isotopanalys av kol och kväve. I Paper I
används isotopanalyser för att kvantifiera i vilken utsträckning leddjur på
stränder utnyttjar marina näringsflöden och för att konstruera en födoväv för
strandekosystem på Östersjöstränder. I Paper II undersöks hur kanteffekter
och spridning påverkar tätheten av insekter och spindlar på små öar, i Paper
III studeras top-down effekter i strandfödoväven. I Paper IV undersöks
variationer i spindlars utnyttjande av marina susidier i relation till
övergödning i Norrtäljeviken. I Paper V undersöks hur långt inåt land marina
subsidier når, och hur olika typer av subsidiers räckvidd påverkas av
egenskaper hos strandekosystemet. De marina subsidier som förekommer i
avhandlingen är marina alger och växter som spolas upp på stränderna och
insekter, främst fjädermygg (Chironomidae), med larvstadier i vattnet och
vuxenstadier på land. Resultaten visar att spindlar utnyttjar marina
näringsflöden till stor del, och att de gör det i högre grad än andra rovdjur
(rovinsekter) på stränder. Ett storskaligt försök där spindlar togs bort från
små öar visade att spindlar har en negativ effekt på tätheter av rovinsekter på
stränder. Inga effekter av behandlingen kunde dock påvisas på lägre nivåer i
födoväven (herbivorer, nedbrytare och växtbiomassa). Tätheterna av olika
leddjur på stränder påverkas inte bara av subsidier, och en undersökning av
tätheter på öar av olika storlek och på olika avstånd från fastlandet visade att
kanteffekter och spridning har olika effekt på olika djurgrupper.
Vargspindlars nischbredd minskar längs en gradient av ökande övergödning
i den övergödda Norrtäljeviken, och denna undersökning visar också att det
finns en stor individuell variation i spindlars utnyttjande av marina subsidier.
Den jämförande studien mellan svenska och australiska strandekosystem
visar att spindlar och tvåvingar är viktiga vektorer för transporten inåt land
av marina subsidier i båda studiesystemen. Subsidiernas räckvidd påverkas
också av landskapskaraktärer hos stränderna, som topografi och
vegetationstyp.
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Tack!
Det finns så många som på ett eller annat sätt, direkt eller indirekt, är
delaktiga i den här avhandlingens tillblivelse, hoppas att jag inte glömmer
någon! Först ett tack till min handledare Peter, för att du gav mig möjlighet
att få jobba med spindelekologi och för att du är en kreativ forskningsmiljö i
dig själv (jag hamnar ständigt ohjälpligt på efterkälken, såväl vetenskapligt
som i fält)! Tack även till min biträdande handledare Lena, som alltid bidrar
med ett välbehövligt marint perspektiv.
Tack till alla andra forskare och doktorander på Botan som förgyllt
doktorandtiden här, jag tänker inte nämna er allihop vid namn men ingen är
glömd! Tack till Sonja för att du varit en så trevlig rumskamrat och för tips
och råd under den första förvirrande tiden som doktorand. Tack till alla
assistenter, kolleger och andra som varit med i fält de här åren, inte bara för
arbetsinsatserna utan även för trevligt sällskap. Tack Örjan och Anna för alla
trevliga fikastunder och middagar i Öregrund. Tack Torbjörn, för goda råd
och trevliga samtal om spindlar i allmänhet och vargspindlar i synnerhet.
Tack till Lars som på allvar väckte doktorandtankarna, även om det blev
spindlar i stället för fiskar.
Tack Marina, för alla trevliga luncher, quizpubs och diskussioner om
doktorandlivet. Och naturligtvis insatsen som djur- och blomvakt medan jag
var i Australien, något inte vem som helst tagit på sig! I synnerhet som jag
vet att vissa av mina lägenhetsinvånare skrämde dig lite – du är nog den
enda som kommit hem till mig, sett dig omkring bland
fågelspindelterrarierna och sagt ”Åh nej, du har blommor!”.
Thanks to all the nice people I met and worked with in Australia. Paul,
Kathryn, Glenn, Peter, I had a wonderful time! Thank you Kathryn for all
your hospitality, and thanks Paul for giving me the opportunity to do some
field work “down under”.
Tack mamma och pappa, ni har alltid stöttat och uppmuntrat mig att läsa
och lära och undersöka, oavsett hur konstiga mina intressen än tett sig och
hur illa ni än tyckt om alla spindlar och andra kryp jag dragit in. Och tack till
mina systrar, som alltid ger mig nya och annorlunda perspektiv på vad jag
sysslar med. Tack hela familjen för såväl uppmuntran som praktisk hjälp
med fältjobb, bildbehandling med mera.
Tack alla vänner nere i Skåne, ni vet vilka ni är och det är alltid lika trevligt
att träffa er även om det inte blir ofta nog...
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Sist men inte minst ett tack till alla trevliga människor i Sjövärnskårens
Musikkår och Solna Brass. Ni har varit en viktig del av min doktorandtid –
man blir så lätt så insnöad på sitt lilla lilla forskningsfält, och då behövs
något som ger en lite perspektiv på tillvaron och påminner om att det faktiskt
finns en värld utanför marina subsidier, strandfödovävar och spindlar, och att
den världen är full av vacker blåsmusik!
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