Are You suprised ? - Zoology, Department of

advertisement
1
1
2
3
Dispersal limitation and post-settlement survival of an introduced ascidian (Botrylloides violaceus) in San Juan
Islands, WA
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Abstract
Distributions of invasive species are often patchy; however, the mechanisms regulating these patchy
distributions are poorly understood. Organisms with short-lived larvae provide an opportunity to test whether these
distributions are dispersal-limited or mediated through post-metamorphic processes. Here we used the invasive
colonial tunicate Botrylloides violaceus to examine the roles of dispersal and predation in determining its
distribution through field outplants of recently settled juveniles to locations with and without adult B. violaceus
colonies. Survival and growth were not different between caged and uncaged treatments, suggesting that predation
is not controlling the distribution of this species. However, survival and growth were different among sites. One site
without established colonies had significantly lower growth and survival than all others, indicating the importance of
post-settlement factors at this site. The other site where adults were absent had similar growth and survival to sites
with established colonies, indicating dispersal limitation at this site. Our study suggests that the distribution of B.
violaceus is limited both by dispersal and environmental conditions that affect juveniles after settlement.
Keywords: colonial tunicate, Botrylloides violaceus, San Juan Islands, dispersal, juveniles
2
20
Introduction
21
Colonization success of invasive species and interactions between native and non-native species can vary
22
spatially and lead to different patterns of species richness and diversity (Fridley et al. 2007; Sax and Gaines 2008).
23
Although the known range of a species may encompass a wide span of latitudes, its actual distribution can be patchy
24
(MacArthur 1972; Pennington 1996). Understanding the factors that create these mosaic distributions is critical to
25
predicting the population dynamics of invasive species.
26
A number of explanations have been proposed to explain the success of invasive species, such as the
27
evolution of increased competitive ability (Blossey and Nötzold 1995), the empty niche model (Elton 1958) and the
28
enemy release hypothesis (Elton 1958). However, for a species to colonize a new area, propagules must first
29
disperse and subsequently survive ambient biotic and abiotic stresses. The idea that dispersal to new locations is
30
fundamental to the spread of invasive species has been widely recognized (Williamson and Fitter 1996; Blackburn
31
and Duncan 2001; Drake and Lodge 2004; Lockwood et al. 2005, Lockwood et al. 2009); however, few studies have
32
directly measured whether dispersal limitation or environmental factors are more important in limiting invasive
33
species distributions (but see Pierson and Mack 1990; Von Holle and Simberloff 2005).
34
In many marine species, propagules are pelagic larvae that are capable of greater dispersal than the adults.
35
These larvae not only have to survive their planktonic larval stage, but then make the transition to a benthic juvenile.
36
Furthermore, their small size at metamorphosis increases their susceptibility to predation. For a non-native species,
37
establishment in an area may be inhibited by both native and introduced predators. A number of studies have
38
examined the preferences of native predators for invaders (Shinen et al. 2009; Simoncini and Miller 2007), and
39
others have demonstrated the role native predators may play in setting the range limits of invasive species (deRivera
40
et al. 2005).
41
The invasive colonial tunicate Botrylloides violaceus is native to the coasts of Asia in the NW Pacific
42
Ocean (Cohen 2005) and was first documented in the San Juan Island Archipelago (Washington, USA) in 1998 (A.
43
Cohen et al. 1998). Since that time B. violaceus has spread to a number of other locations around the San Juan
44
Islands, while remaining absent from others. The factors that limit the spread of this non-native species are
45
currently unknown. In an attempt to predict the distributions of B. violaceus based on abiotic factors, Epelbaum and
46
others (2009a) found very few sites unsuitable for its survival in the waters of British Columbia, yet the species
47
remains patchily distributed. Native predators are known to consume B. violaceus, but have not been demonstrated
3
48
to control B. violaceus and prefer native prey (Epelbaum et al. 2009b; Simoncini and Miller 2007; Whitlatch and
49
Osman 2009). At sites in the San Juan Islands where the species is already a resident member of the fouling
50
community, caged field experiments with B. violaceus also suggest little influence of predation on recruitment
51
(Jacobs 2006). However, there is some evidence that predation can be important in the first week after settlement
52
(Osman & Whitlatch 2004).
53
Here, we examine whether the distribution of B. violaceus is determined by dispersal limitation or post-
54
settlement factors in the San Juan Islands. We artificially increased dispersal by outplanting newly settled juveniles
55
and measured their survival and growth at sites with and without established adult colonies. We hypothesize that if
56
the distribution of B. violaceus is limited by dispersal, then juveniles will grow and survive equally well at all sites
57
in both caged and uncaged treatments. Alternatively, if colonies are limited by post-settlement factors we predict
58
that juveniles will experience reduced growth or survival at the sites where adults are absent. Post-settlement
59
limitation may occur through biotic or abiotic factors. If higher mortality occurs in uncaged treatments at sites
60
where adults are absent, then predation is likely restricting its distribution. Equal mortality across caging treatments
61
would suggest abiotic factors play an important role.
62
63
Methods
64
Study organism and study sites
65
Botrylloides violaceus (Botryllidae) is a colonial ascidian that forms large sheets over many hard substrata
66
and commonly overgrows other native species. It is native to the northwest Pacific along the coasts of Siberia,
67
China and Japan (Cohen 2005). B. violaceus has been introduced to both coasts of the United States and a number
68
of other locations worldwide including the Netherlands, Italy and Australia. Along the west coast of the United
69
States it was first reported from San Francisco Bay in 1973. However, due to some confusion with identification it
70
may have been present elsewhere on the west coast of the United States, including Willapa Bay and Puget Sound, in
71
the 1970s. Subsequently, B. violaceus was reported in a variety of bays along the coasts of California, Oregon and
72
Washington in the 1990s and 2000s (Cohen 2005). B. violaceus was first seen in the San Juan Islands, Washington,
73
USA in 1998 (A. Cohen et al. 1998).
74
75
In the San Juan Islands, B. violaceus is abundant at both Fisherman’s Bay (FB) and Roche Harbor (RH),
while it is absent at Friday Harbor Laboratories (FHL) and Jensen’s Shipyard (JS) in Friday Harbor (Fig. 1). All
4
76
sites except FHL are public or private marinas, with heavy boat traffic; the FHL dock has a lower level of research
77
vessel traffic.
78
79
80
Larval collection and settlement
Adult colonies of B. violaceus were collected on 24 July 2009 from the floating docks at RH and placed in
81
a dark flow-through seawater tank at FHL for 36 h prior to spawning. To induce spawning, all colonies were placed
82
in a common tank, exposed to bright light, and gently torn. All actively swimming tadpole larvae were collected
83
and placed on settlement plates (roughened Petri dishes; method from Marshall et al. 2006). Within 4 h,
84
approximately 95% settlement had occurred. All larvae that were not firmly attached to settlement plates at this
85
time were removed from the experiment. In the case of pairs of larvae that were in close proximity with each other,
86
one was randomly selected for removal to eliminate the effects of space competition and colony fusion among
87
settlers; both phenomena have been observed in B. violaceus (C. Cohen et al. 1998; Marshall et al. 2006). After
88
thinning, each plate had 2-7 settled larvae, with a mean of 3.825 (± 1.18 SD).
89
Settlers were marked and photographed using a Micropublisher 3.3 RTV camera with QCapture 3.1.1
90
(Leeds Precision Instruments, Inc.). Plates were then attached to a Plexiglas array (approx. 10 cm x 1m) and
91
deployed in the field. Each array contained ten settling plates with two small holes so that the plates could be
92
attached to the arrays using plastic cables. Half of the plates were randomly selected to be covered with Vexar®
93
mesh serving as a cage treatment to exclude predators larger than the 4 mm mesh size. Plates were randomly
94
assigned to an array within treatments, ensuring equal numbers of caged and uncaged plates on each array, and then
95
randomly assigned to a position on the array. Arrays were placed in the field on 27 July 2009, suspended on ropes
96
approximately 1 m below the underside of floating docks with two arrays at each of four sites: FB, RH, FHL and JS.
97
98
Growth, survival, and physical data
99
For the first week of the experiment, the sites were surveyed for temperature and salinity using a YSI probe
100
every two days. After six days the arrays were returned to the lab, where each colony was photographed for analysis
101
of survival, growth, and number of zooids per colony. They were subsequently returned to the field for fifteen
102
additional days, and final survival, growth, and zooid number were assessed as described above on 16 August 2009
103
after 21 days in the field.
5
104
105
Feeding experiment
106
To estimate the potential amount of predation on newly settled larvae, a predation experiment was
107
conducted with B. violaceus and Pandalus danae (dock shrimp or coon shrimp), a mobile predator that is present at
108
the FHL and JS docks in large numbers and in much smaller numbers or absent at RH and FB (authors’ pers. obs.).
109
Fifteen individuals of P. danae were collected at the FHL dock using a net and starved for approximately 72 h in
110
individual cages. B. violaceus individuals were collected from RH, kept in the dark overnight, and spawned
111
according to the above procedure. 12 to 15 larvae were settled in each of 15 pre-roughened Petri dishes as described
112
above. After settling for approximately 24 h, settlers were thinned so that each plate contained 10 individuals.
113
Fifteen separate feeding trials were run by placing one starved shrimp in a Tupperware® container with one
114
plate containing 10 B. violaceus settlers in a sea table. The plates were glued to the lids of the container so that the
115
settlers were presented to the shrimp in the same orientation as in the field experiment. After 24 h the number of
116
settlers remaining was counted. The cephalothorax length of each shrimp was measured with digital calipers as an
117
estimate of size.
118
119
Data analysis
120
Physical data (temperature and salinity) were analyzed graphically for differences among sites and dates.
121
Photographs of the settlers, six-day-old colonies, and 21-day-old colonies were analyzed with ImageJ. The zooid
122
area of each individual was measured three times and replicates were averaged to minimize measurement error. To
123
measure the effect of initial settler size on final size, we ran an ANCOVA on final zooid area by site, with initial
124
zooid area as a covariate at both six and 21 days.
125
We calculated size-specific growth rates using the following formula:
126
127
G = [(Af) – (Ai)] / [Ai*(tf-ti)]
128
129
where Af is area (mm2) at time f (final), Ai is area (mm2) at time i (initial), and t is time. Data were arcsine square-
130
root transformed, followed by a log transformation for normality. These transformed data were averaged per plate
6
131
and then analyzed using a two-way factorial ANOVA with site and caging as fixed factors.
132
was analyzed using a two-way factorial ANOVA with site and caging as fixed factors as above.
The number of zooids
133
Differences in survivorship after 6 and 21 days were compared within sites between caged and uncaged
134
treatments using Fisher’s exact tests. Differences in survivorship among sites were compared using contingency
135
analysis via permutation testing (an extension of Fisher’s exact test to a 2x4 table). The permutation test was run
136
with 100,000 iterations using a program developed by W.R. Rice, University of California, Santa Barbara. Pairwise
137
comparisons among sites were made using Fisher’s exact tests. Logistic regressions were used to relate final
138
survivorship to initial size for data pooled from all sites.
139
We calculated the mean number and standard error of settlers eaten per individual shrimp and correlated
140
cephalothorax length of each P. danae with the number of settlers eaten (Spearman’s rho, analysis done in R ver.
141
2.8.1). All analyses were run in JMP ver. 7, unless otherwise indicated.
142
143
Results
144
Growth
145
After 6 days, growth was similar among all sites (Fig 2A) and between both caged and uncaged treatments.
146
The transformed relative growth data were both normal (Shapiro-Wilk test, W = 0.964, p = 0.09) and homoscedastic
147
(Levene’s test, p = 0.90). The two-way factorial ANOVA showed no significant effect of site, cage, or site*cage
148
interaction (Fig. 2A, Table 1). There was a significant effect of initial zooid size on final zooid size as determined
149
by ANCOVA, but no effect of site on final zooid size (Table 2, Fig. 3A).
150
The number of zooids per colony showed a significant effect of site (Table 3). The number of zooids per
151
colony showed neither a site*cage interaction nor a cage effect. A one-way ANOVA among sites (pooling
152
caged/uncaged data) followed by Tukey tests showed that there were significantly more zooids per colony at FB
153
than at any of the other sites (Fig. 4A). The temperature at FB was consistently higher (15.4-19.0°C) than at the
154
other three sites, which clustered together around 13ºC, while salinity measurements were consistent across sites.
155
After 21 days, growth was significantly different among sites (Fig. 2B). Low survival at JS excluded this
156
site from all growth analyses due to lack of power. The transformed relative growth data were both normal (Shapiro-
157
Wilk test, W = 0.981, p = 0.58) and homoscedastic (Levene’s test, p = 0.75). The two-way factorial ANOVA
7
158
showed a significant effect of site, but no significant effect of cage or site*cage interaction (Fig. 2B, Table 1). A
159
one-way ANOVA by site on pooled caged/uncaged data followed by a post-hoc Tukey test showed that FB had the
160
largest growth rate (Fig. 2B). There was a significant effect of site on final zooid size as determined by ANCOVA.
161
However, there was no effect of initial settler size or site*size interaction (Table 2, Fig. 3B).
162
The number of zooids per colony showed a significant effect of site (Table 3) with neither a site*cage
163
interaction nor a cage effect. A one-way ANOVA among sites (pooling caged/uncaged data) followed by Tukey
164
tests showed that there were significantly more zooids per colony at FB than at any of the other sites; RH and FHL
165
were not different (Fig. 4B).
166
167
168
Survival
Early survival (Fig. 5A) was similar at all sites and between both caged and uncaged treatments. Survival
169
was not significantly different between caged and uncaged treatments at all sites (p > 0.25 in all comparisons).
170
Caged and uncaged treatments were subsequently pooled and survivorship was not significantly different among
171
sites (p = 0.08). Low mortality limited sample sizes, preventing the use of logisitic regression to examine the effect
172
of initial settler size on early survival.
173
After 21 days, survivorship continued to be similar between caged and uncaged treatments (all p > 0.09).
174
Caged and uncaged treatments were again pooled. However, survival was significantly different among sites (p <
175
0.001, Fig. 5B) at this later time point. Pairwise comparisons revealed that JS had the lowest survivorship (all p <
176
0.001). FB and FHL did not have significantly different survivorship (p = 0.63) while RH had the highest
177
survivorship (all p < 0.03). Logistic regression showed no significant effect of initial settler size on final survival
178
among sites or when all data were pooled.
179
180
181
Feeding experiment
P. danae were found to consume recently settled B. violaceus. Each shrimp consumed an average of 3.7
182
colonies (± 1.17 SE) out of the 10 colonies offered. However, variability was high with some individuals
183
consuming all colonies and others consuming no colonies. There was no correlation between shrimp size and the
184
number of colonies consumed (Spearman’s rho = -0.235, p = 0.40). The mean cephalothorax length of the P. danae
185
individuals was 29.28 mm (± 0.80 mm SE).
8
186
187
188
Discussion
Our results suggest that the factors determining the distribution of B. violaceus in the San Juan Islands vary
189
spatially. Initially, post-metamorphic growth and survival were similar at all sites (Figs. 2A & 5A), with no
190
difference between caged and uncaged treatments in the first six days of the experiment. After 21 days growth and
191
survival was still similar between caged and uncaged treatments, indicating predation is not an important factor in
192
determining the distribution of juvenile colonies of B. violaceus. However, we observed site-specific differences in
193
growth (Fig. 2B) and mortality (Fig. 5B). JS had significantly lower growth and higher mortality than the other
194
three sites, suggesting post-settlement factors prevent B. violaceus from colonizing this marina. Dispersal limitation
195
may also explain why adult colonies are not present here, but propagules that do disperse to JS will not be able to
196
persist The second site where adults were absent (FHL) had the same survivorship as FB and the same growth rates
197
as RH, both sites where adults are abundant. This suggests that the limited dispersal ability of B. violaceus prevents
198
establishment at FHL. The highest growth rates were found at FB and the highest survivorship was found at RH,
199
indicating site-specific differences in the mechanisms regulating the establishment of new B. violaceus populations.
200
At RH persistence of B. violaceus may rely on high survival, while at FB B. violaceus may be successful through
201
increased growth.
202
Offspring size can strongly influence the success of subsequent life-history stages (Pechenik 2006). The
203
relationship between offspring size and fitness can vary considerably with environmental quality where there is an
204
advantage to size at intermediate levels of environmental quality, while this advantage is lost in both benign and
205
extremely harsh environments (Moran and Emlet 2001; Allen et al. 2008). At six days overall initial settler size was
206
correlated with colony size at all sites with no differences between sites (Fig. 3A). However, this relationship
207
disappeared by 21 days (Fig. 3B). Additionally, no relationship was found between initial settler size and survival.
208
This suggests larger initial sizes do not confer an advantage in growth or survival of B. violaceus at our study sites.
209
Elevated temperatures likely caused the high growth rates in colony size and zooid number observed at FB.
210
FB is a relatively shallow, artificially enclosed bay with limited connectivity to other water sources causing water
211
temperatures to be elevated. Other studies have also found elevated growth rates of B. violacues at higher
212
temperatures (Yamaguchi 1975; McCarthy et al. 2006). Epelbaum et al. (2009a) found B. violaceus zooid number
213
increases the fastest in temperatures ranging from 20 to 25ºC, while only some growth was observed between 5 and
9
214
15ºC. Temperatures in FB remained near optimal temperatures for growth during this experiment, which likely
215
contributed to the increased growth rates of B. violaceus at this site. The degree to which new colonies allocated
216
energy to growth of individual colonies or zooid production also varied by site. Zooid number was significantly
217
greater at FB compared to all other sites at six days and at 21 days (Fig. 4). At six days, zooid number was greater
218
at FB even though colonies were of similar size at all sites. This differential resource allocation to generating
219
greater numbers of zooids could increase both feeding ability and future fecundity at FB.
220
Although there was substantial mortality at several of our sites after 21 days in the field, there were no
221
significant differences in survival between caged and uncaged treatments after six or 21 days (Fig. 5), indicating that
222
predation did not limit early post-metamorphic survival in B. violaceus. Whitlach and Osman (2009) also found low
223
predation on B. violaceus by a gastropod predator in Long Island Sound. However, our arrays were hung 1m below
224
the surface of the water and may not have been exposed to the full suite of predators that are present in a natural
225
fouling community that could limit recruitment, settlement, and subsequent survival of B. violaceus. Osman and
226
Whitlatch (2004) tested for this effect and did find higher mortality in one week old colonies for plates closer to the
227
floating docks compared to suspended plates. Despite this potential limitation, significant differences in mortality
228
were not observed until colonies were older than one week and Osman and Whitlach (2004) did not find differences
229
in predation on two week or three week old colonies deploying plates in different ways.
230
Our feeding trials with P. danae showed that, when starved, some shrimp ate all available settlers of B.
231
violaceus, while others did not eat any settlers within the 24 h trial. This indicates that although shrimp are able to
232
consume B. violaceus, they are unlikely to control a newly introduced population. Given that P. danae is not the
233
only species present in the area that could eat juvenile ascidians, further studies could test the feeding rates of
234
different predators on B. violaceus (as in Epelbaum et al. 2009b), as well as the preference of these predators for B.
235
violaceus or other prey.
236
Abiotic factors are likely more important than predation in determining the growth and survival of juvenile
237
colonies of B. violaceus. A number of environmental factors are known to influence reproduction and development
238
in ascidians including temperature, salinity, food, light, UV exposure, and anthropogenic disturbance (Bates 2005;
239
Lambert 2005). Environmental characteristics of the water column may have played a role in the decreased growth
240
and survival at JS.
Future field measurements of food availability and water quality, and additional experimental
10
241
tests of the effects of compounds present in the water on B. violaceus could help distinguish among abiotic factors
242
influencing post-settlement survival of B. violaceus, especially the reduced survival at JS.
243
Several lines of evidence suggest that the spread of B. violaceus is dispersal-limited at FHL and potentially
244
JS as well. First, the larval behavior and rapidity of settlement of this species are consistent with short-distance
245
dispersal. Botrylloides spp. have lecithotrophic tadpole larvae that settle within minutes to hours once they begin
246
swimming, reaching distances as short as 0.6m from the parent colony (Worcester 1994), but the larvae can also
247
disperse longer distances (Grosberg 1987; Jacobs 2006). Furthermore, evidence of small-scale variation in genetic
248
population structure and population dynamics of colonial ascidians over scales as small as 20 m suggests
249
populations are dispersal-limited and effectively closed to immigration (Ayre et al. 1997; Yund and O’Neil 2000;
250
Grosholz 2001; Yund and Stires 2002). Within the San Juan Islands, sites without B. violaceus colonies are
251
separated by straight-line distances of more than 5 km, suggesting that populations within the islands may also be
252
dispersal-limited. A study modeling dispersal in the San Juan Islands found shorter dispersal distances within the
253
archipelago relative to the more open areas of the Strait of Juan de Fuca south and east of the islands (Engie and
254
Klinger 2007). Thus, B. violaceus populations within the San Juan Islands may be particularly dispersal-limited
255
relative to other sites within Puget Sound. As a result, populations of B. violaceus may slowly spread within areas
256
of suitable habitat until reaching a habitat barrier across which the planktonic duration is too short to cross (Forrest
257
et al. 2009). This could lead to the patchy distribution observed within the San Juan Islands (Fig. 1).
258
Due to the limited larval dispersal of B. violaceus, its range expansion may depend more on long-distance
259
dispersal events caused by transport of adults (Kinlan et al. 2005) via shipping (Lambert and Lambert 2003),
260
aquaculture (Dijikstra et al. 2007) or rafting on macrophytes (Worcester 1994). It has been suggested that B.
261
violaceus was originally transported from Asia to North America in the early 20 th century during a boom in trans-
262
Pacific shipping (Lambert and Lambert 2003). The tunicate is thought to have been subsequently introduced into
263
local sites in the Puget Sound primarily as a hitchhiker on aquaculture oysters (Fuller 2009). Man-made structures
264
at aquaculture facilities and marinas provide extensive substrate for local expansion, and many invasive ascidians
265
are found exclusively on these substrates (Lambert & Lambert 2003). Moreover, the mobility of many of these
266
structures provides the means to transport colonies to new areas. Once at the new location, adult colonies can
267
reattach to new substrates (Edland and Koehl 1998; Bullard et al. 2007) or larvae can establish new colonies nearby.
268
While the short larval duration of B. violaceus limits larval dispersal to new locations, this life history characteristic
11
269
is favorable for larval retention and may promote self-recruitment and rapid expansion in a newly established site
270
(Dunstan and Bax 2007). Site-specific oceanographic features that increase the residence time within an area can
271
increase larval retention and allow faster population growth rates (Dunstan and Bax 2007), which should cause the
272
rate of expansion within an area to vary with location.
273
274
275
276
Acknowledgements
We would like to thank R. Strathmann, R. Emlet, M. Jacobs, and K. Chan for invaluable advice and
277
guidance as well as G. Lambert and C. Lambert for sharing their ascidian expertise. We would also like to thank the
278
staff at Roche Harbor Marina, Lopez Islander Resort and Marina, Albert Jensen and Sons Boatyard, and Friday
279
Harbor Labs for dock access.
12
280
Literature Cited
281
Allen RM, Buckley YM, Marshall DJ (2008) Offspring size plasticity in response to intraspecific competition: an
282
adaptive maternal effect across life-history stages. Amer Nat 171:225-237
283
284
Ayre DJ, Davis AR, Billingham M, Llorens T, Styan C (1997) Genetic evidence for contrasting patterns of dispersal
285
in solitary and colonial ascidians. Mar Biol130:51-61
286
287
Bak RPM, Joenje M, de Jong I, Lambrechts DYM, Nieuwland G (1998) Bacterial suspension feeding by coral reef
288
benthic organisms. Mar Ecol Prog Ser 175:285-288
289
290
Bates WR (2005) Environmental factors affecting reproduction and development in ascidians and other
291
protochordates. Can J Zool 83:51-61
292
293
Blackburn TM, Duncan RP (2001) Determinants of establishment success in introduced birds. Nature 414:195-197
294
295
Blossey B, Nötzold R (1995) Evolution of increased competitive ability in invasive nonindigenous plants: a
296
hypothesis. J Ecol 83:887-889
297
298
Bullard SG, Sedlack B, Reinhardt JF, Litty C, Gareau K, Whitlatch RB (2007) Fragmentation of colonial ascidians:
299
differences in reattachment capability among species. J Exp Mar Biol Ecol 342:166-168
300
301
Cohen AN (2005) Guide to the exotic species of San Francisco Bay. San Francisco Estuary Institute, Oakland, CA
302
303
Cohen AN, Mills CE, Berry H, Wonham M, Bingham B, Bookheim B, Carlton JT, Chapman JW, Cordell JR, Harris
304
LH, Klinger T, Kohn A, Lambert CC, Lambert G, Li K, Secord D, Toft J. 1998. The 1998 Puget Sound Expedition:
305
a rapid assessment survey of nonindigenous species in shallow waters of Puget Sound. Washington State
306
Department of Natural Resources, Olympia, WA
307
13
308
Cohen, CS, Saito Y, Weissman IL (1998) Evolution of allorecognition in botryllid ascidians inferred from a
309
molecular phylogeny. Evolution 52: 746-756
310
311
deRivera CE, Ruiz GM, Hines AH, Jivoff P (2005) Biotic resistance to invasion: native predator limits abundance
312
and distributions of an introduced crab. Ecology 86:3364-3376
313
314
Dijkstra J, Harris LG, Westerman E (2007) Distribution and long-term temporal patterns of four invasive colonial
315
ascidians in the Gulf of Maine. J Exp Mar Biol Ecol 342:61-68
316
317
Drake JM, Lodge DM, Lewis M (2005) Theory and preliminary analysis of species invasions from ballast water:
318
controlling discharge volume and location. Amer Midland Nat 154:459-470
319
320
Drake JM, Lodge DM (2004) Global hotspots of biological invasions: evaluating options for ballast-water
321
management. Proc R Soc Lond [Biol] 271:575-580
322
323
Dunstan, PK, Bax NJ (2007) How far can marine species go? Influence of population biology and larval movement
324
on future range limits.Mar Ecol Prog Ser 344:15-28
325
326
Edlund AF, Koehl MA (1998) Adhesion and reattachment of compound ascidians to various substrata: weak glue
327
can prevent tissue damage. J ExpBiol201:2397-2402
328
329
Elton C (1958) The ecology of invasions by animals and plants. Forward by Daniel Simberloff. University of
330
Chicago Press, 2000, Chicago
331
332
Engie K, Klinger T(2007) Modeling passive dispersal through a large estuarine system to evaluate marine reserve
333
network connections. Estuaries Coasts 30:201-213
334
14
335
Epelbaum A, Herborg LM, Therriault TW, Pearce CM (2009a) Temperature and salinity effects on growth, survival,
336
reproduction, and potential distribution of two non-indigenous botryllid ascidians in British Columbia. J Exp Mar
337
Biol Ecol 369:43-52
338
339
Epebaum A, Pearce CM, Barker DJ, Paulson A, Therriault TW (2009b) Susceptibility of non-indigenous ascidian
340
species in British Columbia (Canada) to invertebrate predation. Mar Biol 156:1311-1320
341
342
Forrest BM, Gardner JPA, Taylor MD (2006) Internal borders for managing invasive marine species. Journal of
343
Applied Ecology 46:46-54
344
345
Fridley JD, Stachowicz JJ, Naeem, S, Sax DF, Seabloom EW, Smith MD, Stohlgren TJ, Tilman D, Von Holle B
346
(2007) The invasion paradox: reconciling pattern and process in species invasions. Ecology 88:3-17
347
348
Fuller P (2009) Botrylloides violaceus. In: USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
349
http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=2418. Accessed 9 September 2009
350
351
Goodbody I (1993) The ascidian fauna of a Jamaican lagoon – 30 years of change. Rev Biol Trop 41:35-38
352
353
Grosberg RK (1987) Limited dispersal and proximity-dependent mating success in the colonial ascidian Botryllus
354
schlosseri. Evolution 41:372-384
355
356
Grosholz E (2001) Small spatial-scale differentiation among populations of an introduced colonial
357
invertebrate.Oecol 129:58-64
358
359
Jacobs MW (2006) Developmental and ecological consequences of variation in larval size, planktonic period, and
360
timing of metamorphosis: testing hypotheses with ascidians. Dissertation, University of Washington
361
15
362
Kinlan, BP, Gaines SD, Lester SE (2005) Propagule dispersal and the scales of marine community process.
363
Diversity Distrib11:139-148
364
365
Lambert G (2005) Ecology and natural history of the protochordates. Can J Zool 83:34-50
366
367
Lambert CC, Lambert G (2003) Persistence and differential distribution of nonindigenous ascidians in harbors of the
368
Southern California Bight. Mar Ecol Prog Ser 259:145-161
369
370
Lockwood JL, Cassey P, Blackburn T (2005) The role of propagule pressure in explaining species invasions. Trends
371
Ecol Evolut 20:223-228
372
373
Lockwood, JL, Cassey P, Blackburn TM. (2009) The more you introduce the more you get: the role of colonization
374
pressure and propagule pressure in invasion ecology. Diversity Distrib 15: 904-910
375
376
MacArthur RH (1972) Geographical ecology. Princeton University Press, Princeton
377
378
Marshall DJ, Cook CN, Emlet RB. 2006. Offspring size effects mediate competitive interactions in a colonial
379
marine invertebrate. Ecology 87:214-225
380
381
McCarthy A, Osman RW, Whitlatch RB (2007) Effects of temperature on growth rates of colonial ascidians: a
382
comparison of Didemnum sp. to Botryllusschlosseri and Botrylloidesviolaceus. J Exp Mar Biol Ecol 342:172-174
383
384
Memmott, J, Craze PG, Harman MH, Syrett P, Fowler SV (2005) The effect of propagule size on the invasion of an
385
alien insect. J Anim Ecol 74:50-62
386
387
Moran AL, Emlet RB (2001) Offspring size and performance in variable environments: field studies on a marine
388
snail. Ecology 82:1597-1612
16
389
390
Naranjo SA, Carball JL, Garcia-Gomez JC (1996) Effects of environmental stress on ascidian populations in
391
Algeciras Bay (southern Spain). Possible marine bioindicators? Mar Ecol Prog Ser 144:119-131
392
393
Osman RW, Whitlatch RB(1998) Local control of recruitment in an epifaunal community and the consequences to
394
colonization processes. Hydrobiologia 375/376:113-123
395
396
Osman RW, Whitlatch RB (2004) The control of the development of a marine benthic community by predation on
397
recruits. J Exp Mar Biol Ecol 311:117-145
398
399
Pechenik JA (2006) Larval experience and latent effects--metamorphosis is not a new beginning. Integ Comp Biol
400
46:323-333
401
402
Pennington M (1996) Estimating the mean and variance from highly skewed marine data. Fish Bull 94:498-505
403
404
Pierson EA, Mack RN (1990) The population biology of Bromus tectorum in forests: distinguishing the opputunity
405
for dispersal from environmental restriction. Oceol 84:519-525
406
407
Sax DF, Gaines SD (2008) Species invasions and extinctions; the future of native biodiversity on islands. PNAS
408
105: 11490-11497
409
410
Simoncini M, Miller RJ (2007) Feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for a
411
dominant native ascidian, Aplididum glabrum, relative to the invasive ascidian Botrylloides violaceus. J Exp Mar
412
Biol Ecol 342:93-98
413
414
Shinen JS, Morgan SG, Chan AL (2009) Invasion resistance on rocky shores: indirect effects of three native
415
predators on an exotic and a native prey species. Mar Ecol Prog Ser 378:47-54
416
17
417
Von Holle B., Simberloff D (2005) Ecological resistance to biological invasion overwhelmed by propagule pressure.
418
Ecology 86:3213-3218
419
420
Whitlatch RB, Osman RW (2009) Post-settlement predation on ascidian recruits: predator responses to changing
421
prey density. Aquat Invasions 4:121-131
422
423
Williamson MH, Fitter A (1996) The characters of successful invaders. Biol Conserv 78: 163-170
424
425
Worcester SE (1994) Adult rafting versus larval swimming: dispersal and recruitment of a botryllid ascidian on
426
eelgrass. Mar Biol 121:309-317
427
428
Yamaguchi M (1975) Growth and reproductive cycles of marine fouling ascidians Ciona intestinalis, Styela plicata,
429
Botrylloides violaceus, and Leptoclinum mitsukurii at Aburatsubo-Moroiso Inlet (Central Japan). Mar Biol 29:253-
430
259
431
432
Yund PO, Stires A (2002) Spatial variation in population dynamics in a colonial ascidian. Mar Biol 141:955-963
433
434
Yund PO, O’Neil PG (2000) Microgeographic genetic differentiation in a colonial ascidian (Botryllus schlosseri)
435
population. Mar Biol 137:583-588
436
18
437
438
439
Table legend:
440
Botrylloides violaceus at six and 21 days using caging and site as fixed effects.
Table 1: Two-way Model I ANOVA for arcsine square root transformed data of relative zooid growth of
441
442
Table 2. ANCOVA examining the effect of site on final zooid size of Botrylloides violaceus with initial size as a
443
covariate at day six and day 21.
444
445
Table 3. Two-way Model I ANOVAon the number of zooids of Botrylloides violaceus at six and 21 days using
446
caging and site as fixed effects.
447
448
Table 4. Temperature (°C) and salinity (psu) measured at all four sites included in this study; FB: Fisherman’s Bay,
449
FHL: Friday Harbor Labs, RH: Roche Harbor, JS: Jensen’s Shipyard.
450
451
19
452
453
Table 1
Source
Day 6
Cage
Site
Cage*Site
Error
Total
Day 21
Cage
Site
Cage*Site
Error
Total
454
455
DF
SS
F
p
1
3
3
49
56
0.0422
0.010
0.046
1.444
1.561
1.432
0.112
0.521
0.237
0.953
0.6698
1
3
3
49
56
0.0001
0.2886
0.0228
0.3521
0.6856
0.0122
11.476
0.9067
0.91
< 0.001*
0.45
20
456
457
Table 2
Source
Day 6
Initial size
Site
Initial size*Site
Error
Total
Day 21
Initial size
Site
Initial size*Site
Error
Total
458
459
DF
SS
F
p
1
3
3
68
75
0.0596
40.671
1.2050
141.36
188.83
0.0287
6.5213
0.1932
0.87
< 0.01*
0.90
1
3
3
93
100
183.122
9.079
17.033
252.402
493.618
67.473
1.115
2.092
< 0.001*
0.347
0.107
21
460
461
Table 3
Source
Day 6
Cage
Site
Cage*Site
Error
Total
Day 21
Cage
Site
Cage*Site
Error
Total
462
463
464
DF
SS
F
p
1
2
2
40
45
0.1975
4.8775
0.1327
19.216
24.709
0.411
5.076
0.138
0.53
0.01*
0.87
1
3
3
152
159
2.778
67.387
3.452
156.145
231.944
2.704
21.866
1.120
0.102
< 0.001*
0.343
22
465
466
Table 4
Temp. (°C)
29-Jul
31-Jul
2-Aug
29-Jul
31-Jul
2-Aug
FB
19
15.4
17
30.15
30.35
30.4
RH
12.8
13
13
29.6
29.8
30
FHL
12.25
13.5
12.3
29.75
29.45
30.55
14
12
12.1
30
29.8
30.3
JS
467
468
469
470
Salinity (psu)
23
471
472
473
Figure captions:
474
Figure 1. A map of the study sites in the San Juan Islands, Washington, USA. Filled symbols indicate sites where
475
Botrylloides violaceus colonies are present; open symbols indicate sites with no known adult colonies (data from A.
476
Cohen et al. 1998; G. Lambert, pers. comm). Stars represent study sites.
477
Figure 2. Mean relative growth (± SE) of zooids at each site after 6 d (A) and 21 d (B) where adults are currently
478
present (FB and RH) and absent (FHL and RH). A two-way factorial Model I ANOVA of relative growth rates
479
showed no significant effect of site, cage, or site*cage after 6 d. After 21 d FB showed significantly higher growth
480
rates than RH and FHL. JS was excluded form the analysis in (B) due to lack of data. Letters represent significant
481
differences among sites based on post-hoc Tukey tests.
482
483
Figure 3. Linear regression of final zooid size by initial zooid size after 6 d (A) and 21 d (B) where adults are
484
currently present (FB and RH) and absent (FHL and JS)). ANCOVA results of final zooid size (mm2) by site, with
485
initial zooid size as a covariate showed a significant effect of initial zooid size, but no effect of site on final zooid
486
size at day 6. A significant effect of site, but not of initial size, was observed after 21 days. JS was excluded from
487
(B) due to lack of data.
488
489
Figure 4. Mean number of zooids per colony (±SE) at each site after 6 d (A) and 21 d (B) where adults are currently
490
present (FB and RH) and absent (FHL and RH). A two-way factorial Model I ANOVA of number of zooids by site
491
and cage showed no effect of cage or site*cage; letters represent significant differences among sites based on post-
492
hoc Tukey tests. JS was excluded from the analysis in (B) due to lack of data.
493
494
Figure 5. Final percent survivorship of B. violaceus juveniles after 6 d (A) and 21 d (B) where adults are currently
495
present . A Fisher’s exact test showed significant differences between sites but no differences between caged and
496
uncaged plates. Letters based on significance of pairwise comparisons of pooled caged and uncaged data.
497
24
498
499
500
501
502
503
Figure 1
25
Relative zooid growth (day)-1
A
Relative zooid growth (day)-1
B
0.5
Caged
Uncaged
0.4
0.3
0.2
0.1
0.0
0.18
0.16
a
0.14
0.12
0.10
0.08
b
0.06
b
0.04
0.02
n.d.
0.00
FB
505
506
507
509
FHL
Location
504
508
RH
Figure 2
JS
26
A
14
FB
RH
FHL
JS
FB
RH
FHL
JS
12
Day 6 size (mm2)
10
8
6
4
2
0
B
10
Day 21 size (mm2)
8
6
4
2
0
0.4
0.8
1.0
1.2
Initial size (mm2)
510
511
512
513
0.6
Figure 3
1.4
1.6
1.8
27
514
515
516
Figure 4
28
517
518
Figure 5
Download