ARE SALT MARSH INVASIONS BY THE GRASS ELYMUS ATHERICUS A THREAT FOR TWO DOMINANT HALOPHILIC WOLF SPIDERS?

2005·7.6 MB·English

by  Julien Pétillon

2005. The Journal of Arachnology 33:236-242
ARE SALT MARSH INVASIONS BY THE GRASS
ELYMUS ATHERICUS A THREAT FOR TWO DOMINANT
HALOPHILIC WOLF SPIDERS?
Julien Petillon^’^, Frederic Ysnel\ Jean-Claude Lefeuvre^ and Alain Canard^: ^ E.R.T.
“Biodiversite fonctionnelle et Gestion des territoires”, Universite de Rennes 1, 263 Av,
du General Leclerc, CS 74205, 35042 Rennes Cedex, France. E-mail: julien.
[email protected]; ^ U.M.R. C.N.R.S. “Fonctionnement des Ecosystemes et
Biologie de la Conservation”, Universite de Rennes 1, 263 Av. du General Leclerc,
CS 74205, 35042 Rennes Cedex, France; ^ Laboratoire d’Evolution des Systemes
Naturels et Modifies, Museum National d’Histoire Naturelle, 36 rue Geoffroy Saint
Hilaire 75005 Paris, France
ABSTRACT. As a result of the Elymus athericus (Poaceae) invasion in the last ten yeai's, a major
change in vegetation cover has occurred in salt marshes of the Mont Saint-Michel bay (France). In this
study, we investigated if the high conservation value of invaded salt marshes is preserved. Abundances,
densities and flood resistance abilities of the dominant halophilic species Arctosafulvolineata (nocturnal
lycosid) and Pardosa purbeckensis (diurnal lycosid) were compared in both natural and invaded habitats.
Elymus invasion involved both positive and negative aspects with respect to the conservation value ofthe
salt marshes invaded: the P. purbeckensis population was clearly reduced in invaded habitats, whereasA.
fulvolineata seemed to derive high benefits from the invasion. We supposed that abiotic parameters ofthe
new habitat (mainly vegetation and litter characteristics) affected the two species differently with respect
to their aut-ecology and their flood resistance abilities. Furthermore, food resources (estimatedby different
macrofauna density measurements) were likely to be reduced for P. purbeckensis in invaded habitats and
unchanged for A. fulvolineata. Lastly, we hypothesize that individuals of P. purbeckensis are subject to
increased interspecific competition (measured as intra-guild densities), whereas spiders from the same
guild as A. fulvolineata have not increased in invaded habitats, resulting in an unchanged competition
level.
Keywords: Conservation value, halophilic species, habitat change, invasive species, food resources
Salt marshes are one of the rarest ecosys- nities dominated by some “halophilic arthro-
tems in the world (Lefeuvre et al. 2000), with pods” (Foster & Treherne 1976; Irmler et al.
a linear but fragmented distribution along 2002; Petillon et al. 2004).
coasts, therefore representing a high interest In the Mont Saint-Michel bay, France, salt
in terms of nature conservation (e.g., Gibbs marshes have been invaded by a native spe-
2000; Bakker et al. 2002). In fact, these eco- cies Elymus athericus (Link) Kerguelen (Va-
systems host a poorly diversified flora and lery et al. 2004). This high marsh-living grass
fauna, which possess a low number of species (Poaceae) started to invade the salt marsh 10
that are threatened directly orindirectly by hu- years ago (Bouchard et al. 1995) and now
man activities such as habitat destruction, dif- covers some marshes to the mean sea tide lev-
fuse soil pollution from adjacent cultivated el. This progression is characteristic of an in-
fields, and overgrazing (Desender & Maelfait vasive species that is common in European
1999; Adam 2002). The high conservation salt marshes (Bockelmann & Neuhaus 1999).
value of these habitats is also due to the high Nevertheless, the effects of E. athericus in-
specificity of its fauna, adapted to two main vasions on the high conservation value of salt
abiotic factors, high soil salinity and regular marshes remain unknown. Spiders constitute
submergence by seawater. Due to tidal events, an abundant, diversified and well-known tax-
natural salt marshes present a specific plant onomic group in salt marshes (Fouillet 1986;
cover (spatial succession from the high to the Desender & Maelfait 1999), including spe-
low marsh) and specific invertebrate commu- cialist (stenotopic) species, the so-called “hal-
236
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PETILLON ET AL.—GRASS INVASION ON SALT MARSH SPIDERS 237
—
Table L Synthesis offield experiments carried out in the Mont Saint-Michel bay salt marshes (‘Ferme
Foucault' and ‘Vivier-sur-mer’ sites).
Number of
Parameter Methodology Period replicates
Target species abundances Pitfall trap (r = 5 cm) April-November 2002 8
Target species densities Depletion quadrat (1 m^) June-July 2003 32
Target species abundances be- Pitfall trap (r = 5 cm) April 2004 12
fore and after the tide
Abiotic parameters WET sensor and manual mea- July-August 2002 16
surements
Macrofauna densities Soil cores (r = 5 cm) October 2003 6
Amphipod densities Depletion quadrats (1 m^) June 2003 4
Total spider densities Depletion quadrats (0.25 m^) June 2002 16
ophilic species” (Hanggi et al. 1995) or “salt the two species ArctosafulvoUneata and Par-
marsh resident species” (Petillon et al. 2004). dosa purbeckensis.
In Europe, the numerically dominant hunting
METHODS
spiders in salt marsh ecosystems are the rare
lycosid species Pardosa purbeckensis EO. Study sites and field surveys. The Mont
Pickard-Cambridge 1895 and Arctosafulvolb Saint-Michel bay, located between Brittany
neata (Lucas 1846) (Fouillet 1986; Baert & and Normandy (North West France), is unique
Maelfait 1999; Elkaim & Rybarczyck 2000). in Europe for its tidal amplitude, which reach-
Pardosa purbeckensis will be considered in es 15 meters (the second largest in the world).
this study as different from P. agrestis, dom- This exceptional phenomenon results in the
inant in Central Europe agricultural land- extension ofsalt marshes and mud flats, which
scapes (e.g., Samu & Szinetar 2002) and in- together cover 250 km^. Salt marshes are only
land salt marshes (e.g., Zulka et al. 1997), submerged during spring tides (i.e. monthly
mainly because of its morphological charac- strong tides) and are then inundated during
teristics (as described in Locket & Millidge two hours per tide (Lefeuvre et al. 2000). The
1951) and its osmoregulatory abilities (Hey- uppermost parts ofthe salt-marshes are delim-
demann 1970). ArctosafulvoUneata is classi- ited by dikes and are not submerged during
fied as a nationally endangered species in the high tides.
United Kingdom (Harvey et al. 2002) and be- Natural stations (dominated by Atriplex
longs to the “rare species” category in the portulacoides, Chenopodiaceae) and invaded
Ramsar Convention, stations (dominated by Elymus athericus, Po-
The aim of the study was to investigate if aceae) were located at the same distance from
the newly created vegetation coverinducedby the dike. Habitat characterization and mea-
the invasion of Elymus athericus still consti- surements of spider activity and density were
tutes a suitable habitat for these two species carried out at four stations (two natural and
ofhigh conservation value. To explain the po- two invaded) located at the ‘Ferme Foucault'
tential impact of the invasive plant on spider site’ (Normandy, 48°55'N, 1°52'W) whereas
species habitat preferences, natural and invad- experiments on species reactions to flooding
ed habitats were characterized by abiotic and were carried out at six stations (three natural
biotic components. We hypothesized that Ely- and three invaded) located at the ‘Vivier-sur-
mus athericus changed the general microhab- mer’ site (Brittany, 48—°60^N, 1°78'W).
itat characteristics and induced 1) an effect on Spider sampling. Spider abundances:
species density, activity density and flooding Spiders were sampled with pitfall traps, con-
resistance due to less appropriate abiotic pa- sisting of polypropylene cups (10 cm diame-
rameters (e.g., salinity or litter depth), 2) a ter, 17 cm deep) set into the ground so that
change in the quality and quantity offood re- the lips were flush with the soil surface. Eth-
sources, and 3) a variation in the predation ylene glycol was used as preservative, be-
rate (measured as intra-guild densities) upon cause of its lack of effects on spider catches
238 THE JOURNAL OF ARACHNOLOGY
—
Table 2. Mean spider activity abundances (number ofindividuals/day/meter XlO) and mean densities
(number of individuals/m^) in invaded and natural vegetations, and statistical comparisons using ANOVA
(Code: n.s. = non significant, * = F < 0.05, ** = p < 0.01).
Natural plots Invaded plots F-ratio Code
Spider abundances
Arctosafulvolineata 9.31 ± 3.09 9.83 ± 3.15 0.01 n.s.
Pardosa purbeckensis 98.5 ± 23.80 41.72 ± 7.70 5.13 *
Spider densities
Arctosafulvolineata 0.03 ± 0.03 0.43 ± 0.12 9.71
Pardosa purbeckensis 3.93 ± 1.24 1.37 ± 0.27 4.09 *
(Topping & Luff 1995). Traps were covered Characterization of natural and invaded
—
with a raised wooden roof to keep out rain. habitats. Abiotic characteristics: To char-
Catches in pitfall traps were related to trap- acterize plant communities, vegetation was
m
ping duration and pitfall perimeter, which cal- described four times within a radius of 1
culates an ‘activity trapability density’ (num- around each pitfall trap (4 replicates per sta-
ber of individuals per day and per meter: tion) at the ‘Ferme Foucault’ site: litter depth
Sunderland et al. 1995). Four pitfall traps (to the nearest mm) and vegetation height (to
were installed at each station and spaced 10 the nearest cm). Soil salinity (estimated by
meters apart, considered as the minimum dis- pore water electrical conductivity), soil water
tance for avoiding interference between traps content and temperature were measured using
&
for spider catches (Topping Sunderland a W.E.T. Sensorconnected to a moisture meter
1992). Activity trapability density of spiders HH2 (Delta-T Devices Ltd., Cambridge, UK)
was followed during the entire period of ac- and made with a specific clay soil calibration.
tivity of the two species from April-Novem- All abiotic variables were assessed in summer
ber 2002 except during high tides (Table 1). 2002 (Table 1). Soil temperatures were mea-
Spider densities: To compare absolute den- sured at llh a.m. to compare this micro-cli-
sities of the two lycosids, Im X Im plastic mate parameter between habitats, but not to
m
quadrats (1 height) were used. Quadrats characterize it along time.
were sampled by regular hand catches and pit- Biotic characteristics: Food resources were
fall traps (one placed in the middle of the estimated by vertical sampling using a soil
quadrat) until there were no more individuals. core (12 cm depth and 10 cm for diameter),
Eight spatial replicates were sampled in each except for the very mobile amphipod (see be-
vegetation type (i.e. invasive and natural), and low). Six cores were collected in each habitat
this technique was used four times in June and (natural or invaded) during October 2003 (Ta-
July 2003 (i.e. 32 replicates per vegetation: ble 1). Cores were then sieved through a 250
Table 1). jjim mesh screen. Because cannibalism and in-
Ejfects offlooding: To determine the role of traguild predation are a general rule in spiders,
vegetation cover in modifying species ability especially in structurally simple ecosystems
to resist flooding, spiders were sampled before (e.g., Schaefer 1974), spider densities were in-
and after a spring tide (tidal range: 13.35 m) cluded in total arthropod densities when com-
in April 2004 (Table 1). Three natural andthree paring the food resources between invaded
invaded stations were studied at three salt and natural areas. The results do not include
marsh levels ranging from the high to the low an important macrofauna component, the gas-
marsh (i.e. 50, 150 and 200 meters from the tropod Phytia bidentata because this species
dike). Four pitfall traps per station were acti- is not likely to be consumed by the two ly-
vated during three consecutive days before and cosid species.
after the high tide. Catches were completed by Because the amphipod Orchestia gammar-
hand collecting during activation andcollection ella represents one of the more abundant ar-
of pitfall traps for a total of 1.5 hours per sta- thropods in west European salt marshes (e.g.,
tion before and after the high tide. Meijer 1980), special attention was paid to
——
PETILLON ET AL.—GRASS INVASION ON SALT MARSH SPIDERS 239
—
Table 3. Comparison of mean activity abundances measured by pitfall traps (number of individuals/
day/meter; in parentheses: total numberofindividuals) before and afterthehightidein natural andinvaded
stations, and statistical comparisons using ANOVA (Code: n.s. = non significant, * = p < 0.05, ** = p
< 001
. ).
Species abundances
Before the high tide After the high tide F-ratio Code
Natural stations
ArctosafulvoUneata 1.77 ± 0.49 (40) 0.44 ± 0.21 (18) 6.14 *
Pardosa purbeckensis 3.36 ± 1.14 (364) 2.12 ± 0.47 (274) 1.00 n.s.
Invaded stations
ArctosafulvoUneata 0.97 ± 0.28 (31) 0.80 ± 0.27 (23) 0.21 n.s.
Pardosa purbeckensis 4.69 ± 1.06 (214) 2.03 ± 0.51 (163) 5.04 *
this species. Densities of O. gammarella were RESULTS
calculated in natural and invaded habitats us- Comparison of habitat preference.
ing a depletion method. Within each plant
Catches by pitfall traps revealed that Pardosa
community, four Im^ quadrats were randomly
purbeckensis activity abundances were signif-
sampled in June 2003 by hand catches and
icantly higher in natural than in invaded plots
pitfall traps until there were no more individ-
(Table 2). In accordance with trends found in
uals.
activity, P. purbeckensis had a significantly
To measure nocturnal and diurnal wanderer
higher density in natural plots. However, Arc-
densities, spider densities were estimated by
tosafulvoUneata had much higher densities in
the quadrat-flotation technique. Homogeneous
0.25 m^ areas were isolated by iron quadrats invaded than in natural plots, whereas its
abundance did not differ significantly between
(0.5 meter width and 1 meter height) set into
the two vegetation types (Table 2
the ground to a depth of 0.20 m. All vegeta- ).
tion was removed, stored in sealed bags and Comparison of flooding effec—ts between
natural and invaded vegetation. Based on
analyzed in the laboratory. The spiders re-
maining within the quadrat were then hand their distribution change after the high tide,
ArctosafulvoUneata and Pardosa purbecken-
collected on the bare soil until none were vis-
ible. A pitfall trap was placed in the middle sis presented similar reactions to flooding,
of the quadrat and left in place until all re- with an unmodified distribution (in terms of
maining moving individuals were caught, A presence/absence) in natural and invaded sta-
last hand collection was also carried out dur- tions. OnlyA.fulvoUneata almost disappeared
ing the time of pitfall trapping. The spider from one natural station (level 2), where less
density was calculated by summing individu- than 5 individuals were caught after the high
als of the initial and final hand collections tide. In fact, this species presentedcomparable
with the catches of both pitfall trap and veg- abundances before and after the high tide in
etation samplings. Four replicates were per- invaded stations (Table 3), whereas its mean
formed at each station and repeated fourtimes abundance significantly decreased in natural
during June 2002 (Table 1). stations. Pardosapurbeckensis showed signif-
Identification and data analyses.—All the icantly decreased abundances after the high
spiders collected were preserved in 70% eth- tide in invaded stations and constant abun-
anol, transported to the laboratory for species dances in natural stations (Table 3).
identification and kept in the University collec- Comparison of habitat characteristics,
tion (Rennes, France). In the tables, all means Invaded stations {Elymus athericus) differed
are presented with standarderror(mean ± s.e.). significantly from natural stations (Atriplex
Mean environmental and species variables portulacoides), i.e. they had deeper litter and
were compared using ANOVA tests after ver- taller plant cover (Table 4). No significant dif-
ification of normal distribution according to ferences were found regarding soil salinity,
Kolmogorov-Smimov tests (MINITAB). soil water content and temperature. Regarding
240 THE JOURNAL OF ARACHNOLOGY
potential prey, Acari and Amphipoda {Or- salt marsh species to the invasion can be in-
chestia gammarella) densities were signifi- fluenced by many other factors than abiotic
cantly lower in invaded areas, leading to a de- components of habitats. In particular for sim-
crease in total pedofauna at these stations ple ecosystems such as salt marshes, interspe-
(Table 4). No significant difference was found cific competition may reduce spider popula-
for Collembola. tions (Schaefer 1972 according to Wise 1993;
Species from the guild of Pardosa pur- Marshall & Rypstra 1999). We propose that
beckensis (diurnal wanderers) were found in invaded habitats, by hosting other species
significantly higher densities in invaded than from the same guild as P. purbeckensis (es-
in natural stations (the more dominant species pecially Pardosa prativaga, P. proxima and
were the non-coastal Pardosa prativaga, P. P. pullata), increased inter-specific competi-
proxima and P. pullata), whereas no differ- tion. Contrary to P. purbeckensis, A. fulvoli-
ence between vegetation types was found for neata seemed not to be subjected to higher
species from the Arctosa fulvolineata guild levels of competition in invaded habitats, as
(nocturnal wanderers, mainly including the nocturnal wanderer densities were the same in
non-coastal Agroeca lusatica, Clubiona stag- natural and invaded habitats. Little is known
natilis and Zelotes latreillei). about salt marsh spider diets, except predation
upon Collembola for P. purbeckensis (Schaef-
DISCUSSION
fer 1974) and for the linyphiid Erigone arctica
Changes in hab—itat characteristics after (White 1852) (Legel & Wingerden 1980). So,
Elymus invasion. Lycosids in general and at the moment there is no evidence for food
the genus Pardosa in particular are known to limitation, even if some potential prey de-
prefer open habitats (e.g., Aart 1973), and creased in invaded habitats (e.g., the amphi-
Harvey et al. (2002) suggested that adults of pod Orchestia gammarella).
P. purbeckensis are favored by low vegeta- Change—s in flood resistance in invaded
tion. Kessler & Slings (1980) demonstrated habitats. In this study, the two salt marsh
the tendency among juveniles of P. purbeck- species responded differently to flooding in
ensis to aggregate and to select grass with invaded habitats. Pardosa purbeckensis abun-
high shoot densities, probably to avoid can- dances decreased only in invaded stations af-
nibalism. Because more diurnal wandering ter the high tide. If this species uses under-
spiders are hunting in invaded plots, we sug- ground refuges during flooding, as do several
gest that young specimens of P. purbeckensis intertidal invertebrates (Foster & Treherne
are exposed to a higher predation risk than in 1976; Kneib 1984), then it would be disfa-
natural areas, contributing to the lower adult vored in invaded areas (deep but thin roots of
density in the invaded areas. Thus we propose Elymus) compared to natural areas (short and
that the habitat structure of Elymus (mainly large roots of Atriplex portulacoides). Con-
tall vegetation and deep litter) was not suitable trary to P. purbeckensis, A. fulvolineata seem
for this species and contributed to its lower to derive benefits from the invasive plant be-
occurrence in invaded habitats. In contrast, cause its abundance remained the same after
Arctosa fulvolineata might be favored by the the high tide in the invaded stations and de-
structure ofthe invaded habitats: a deeper and creased in the natural stations. This litter-liv-
more complex litter due to a lower rate ofEly- ing species might be favored by Elymus ath-
mus litter decomposition (Valery et al. 2004). ericus that seems to improve its habitat by
As a general rule, deep litter, by providing increasing the food resources and the amount
new microhabitats and microclimate condi- of air in the litter.
tions (Wise 1993), tends to favor nocturnal Conflicting aspe—cts of the grass invasion:
wanderers (case ofArctosafulvolineata), am- how to manage it. Habitat suitability ofnat-
bush hunters (thomisids) and “litter-sensi- ural and invaded areas clearly differed be-
tive” sheet-weavers (Bell et al. 2001). Thus tween the two wolf spiders. Activity trappa-
we suggest that A. fulvolineata prefers more bility density of P. purbeckensis was strongly
heterogeneous litter of invaded areas, where it reduced in invaded areas indicating a decrease
is often found during the day, at 3-4 cm depth in the density and/or the mobility of individ-
(Petillon pers. obs.). uals, whereas activity trappability density of
Differences in the responses of dominant A. fulvolineata was enhanced in invaded sta-
PETILLON ET AL.—GRASS INVASION ON SALT MARSH SPIDERS 241
—
Table 4. Habitat characteristics (mean ± s.e.) of natural and invaded stations, and statistical compar-
isons using ANOVA (Code: n.s. = non significant. * = P < 0.05, ** == P < 0.01).
:
Units Natural plots Invaded plots F-ratio Code
Structural components
Vegetation height cm 29 -1- 0.63 78 -i- 1.64 331.43 **
Litter depth cm 0 4 -H 0.23 754.60 **
%
Soil water content 60.13 -+- 4.21 57.72 -1- 1.12 0.31 n.s.
Soil salinity mS/m 1009.7 -f- 50.1 1013.8 -+- 45.1 <0.01 n.s.
Soil diurnal tempera-
ture °C 10.86 0.62 1 1.00 -H 0.38 0.03 n.s.
Biotic components
Amphipoda Number/m^ 979 311 205 -t- 51 6.02 *
Acari Number/m^ 55302 -1- 8688 20202 -1- 3085 14.49 **
Collembola Number/m^ 5008 + 2391 5645 -t- 2574 0.03 n.s.
Total pedofauna Number/m^ 60352 -h 7053 26293 -1- 3707 18.27
Diurnal wanderers Number/m^ 6.50 -+- 2.10 29.25 -h 6.97 9.77 *
Nocturnal wanderers Number/m^ 0.75 0.48 5.25 -1- 2.78 2.54 n.s.
ACKNOWLEDGMENTS
tions. Here, quadrat results confirmed that
changes in activity trappability densities also We would like to thank R Amaya Garcia,
reflected changes in the species’ population Q. Le Mouland, S. Michaud, G. Perrin, E. Pe-
densities. tillon and E. Villani for field assistance, Gobi
More than ten years after the grass inva- Camberlein and Julien Cucherousset for En-
sion, the two dominant halophilic species are glish improvement. S. Toft and two anony-
still present in invaded salt marshes (Fouillet mous reviewers greatly improved the manu-
1986; present study). But our results indicate script by their remarks and suggestions. This
that in the near future microhabitat changes study was supported by the French Ministry
and (perhaps) competition for space and food of Environment (Programme ‘Especes inva-
between native and non-coastal (immigrated sives’) and the CNRS Zone Atelier ‘La Baie
after the Elymus invasion) species could lead du Mont Saint-Michel et ses bassins versants’.
to a decline in the absolute density of the
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