Journal of Experimental Marine Biology and Ecology

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1 Journal of Experimental Marine Biology and Ecology 399 (2011) Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: Positive and negative effects of a dominant competitor on the settlement, growth, and survival of competing species in an epibenthic community Danielle C. Claar a,b,, Kyle F. Edwards a, John J. Stachowicz a a Bodega Marine Laboratory, University of California at Davis, P.O Box 247, Bodega Bay, CA 94923, United States b University of Hawaii at Hilo, 200 W. Kawili St. Hilo, HI 96720, United States article info abstract Article history: Received 6 July 2010 Received in revised form 29 January 2011 Accepted 3 February 2011 Keywords: Coexistence Diversity Facilitation Fouling community Larval recruitment Secondary substrate Because dominant competitors can monopolize resources, any positive effects they have on other species can have large community impacts. The solitary tunicate Ascidia ceratodes is a dominant competitor in the fouling community in Bodega Harbor, CA. This tunicate preempts primary substratum from competitors, but its thick tunic also allows other species to grow on its surface. The net effect of Ascidia on the community as a whole therefore depends on the balance between competitive and facilitative effects. In this study we evaluate the facilitative effects of Ascidia on different life stages of common competing species. We quantified larval settlement onto Ascidia compared to unoccupied space; we quantified the growth rate on Ascidia of small colonies of two common species; and we measured whether established colonies could escape overgrowth by Ascidia by growing onto its tunic. We found evidence for high rates of settlement on Ascidia, with some species showing higher and others lower settlement relative to that observed on free space. The growth rate of settlers was generally lower on Ascidia compared to primary substratum. We also found that colonial species established on primary space commonly escape overgrowth from Ascidia by growing onto Ascidia, as this occurs in about half of all encounters. This study indicates that the total effect of Ascidia on the community will depend on species-specific and life stage-specific effects, with likely long-term consequences for the diversity and composition of the community Elsevier B.V. All rights reserved. 1. Introduction A fundamental question in ecology is how different species coexist while requiring the same resources. A majority of the research on this topic has focused on resource competition and the different kinds of resource partitioning that can maintain diversity (e.g. Titman, 1976; Nandakumar et al., 1993; Silvertown, 2004; Yoshiyama et al., 2009). However, competition has not provided a complete understanding of species diversity patterns in nature, and more recent work has emphasized the effects of positive interactions, such as facilitation, on species diversity (Hacker and Gaines, 1997; Rowan, 1998; Bertness et al., 1999; Stachowicz, 2001; Bruno et al., 2003, Sellheim et al., 2010). Facilitative interactions can maintain diversity via several mechanisms. Dominant species can create better abiotic conditions for interacting species by modifying the physical environment (Hacker and Gaines, 1997); dominant species can provide additional space for epiphytic and epizootic organisms to settle and grow (Kimbro and Grosholz, 2006); and species that are resistant to Corresponding author at: C/O John J. Stachowicz, Department of Evolution and Ecology & Director, Center for Population Biology, University of California, Davis, CA 95616, United States. Tel.: ; fax: address: claar@hawaii.edu (D.C. Claar). predation can provide an associational refuge for other species that are susceptible to predators (Hay, 1986). In this study, we examine how the facilitative provisioning of additional substrate may help maintain species diversity in an assemblage of marine invertebrates that compete strongly for space. The sessile marine invertebrates that live in fouling communities often compete strongly for space (e.g. Nandakumar and Tanaka, 1997; Stachowicz et al., 2002), but nonetheless these communities are typically diverse. Predation and disturbance have been shown to increase diversity by reducing the ability of dominant competitors to monopolize space and exclude competitively subordinate species (Osman, 1977; Sutherland and Karlson, 1977; Karlson, 1978). Furthermore, we have observed strong tradeoffs between colonization ability and longevity, and between colonization ability and competitive ability, that will act to maintain diversity (Edwards and Stachowicz, 2010). Positive interactions have been less well studied, but observation suggests they are common and likely provide additional mechanisms by which diversity can be maintained. For example, some species can colonize directly on the tough surface of other species, instead of on free primary space (Sellheim et al., 2010). Settling on secondary substrate could provide a refuge from competition with other species. Additionally, the increased complexity of secondary substrate can provide sessile recruits with a refuge from predation (Wahl and Hay, 1995) /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.jembe

2 D.C. Claar et al. / Journal of Experimental Marine Biology and Ecology 399 (2011) When present in a fouling community, tunicates are often the dominant competitors (Gutt et al., 1999; Mazouni et al., 2001; Johnston et al., 2002). Ascidia ceratodes, a solitary tunicate, is a dominant competitor in the fouling community at Spud Point Marina in Bodega Bay, CA. Ascidia recruits in low numbers during most of the year, but once established it is capable of overgrowing nearly all other species and is relatively invulnerable to displacement (Edwards and Stachowicz, 2010; Edwards and Stachowicz, in press). Preempting and overtaking space through competition is a strong negative effect on the other species in the community, but Ascidia could also benefit some species. The thick tunic of Ascidia provides habitat onto which other organisms may recruit, or onto which nearby colonial species may grow. Provided the solitary ascidian's siphons are not overgrown, such overgrowth by other species appears to have minimal effect on the Ascidia (K. F. Edwards & J. J. Stachowicz, pers. obs.). In addition to providing secondary substrate for larvae to settle on, Ascidia could also increase biodiversity by discouraging predation by crawling predators, like limpets and chitons, that are attached to primary substrate (Nydam and Stachowicz, 2007). This study addresses potential facilitative effects of Ascidia and how these might contribute to the diversity of the fouling community. Specifically, we asked: (1) Do subordinate competitors settle on Ascidia, and how does the rate of settlement compare to settlement on primary space? (2) For species that regularly settle on Ascidia, how do survival and growth compare between colonies on Ascidia vs. primary space? (3) For colonial species established on primary space, is overgrowth by Ascidia mitigated by growth onto the surface of Ascidia? We use our data to discuss the effects of Ascidia on the persistence of subordinate species; ultimately, this data will be useful for synthetic models that simultaneously incorporate negative and positive interactions to understand community dynamics. 2. Materials and methods This study was conducted in Spud Point Marina, Bodega Bay, California (38 19 N, W). The fouling community is a diverse system resulting from the colonization of organisms on docks, boats, piers and other manmade and natural surfaces (Nydam and Stachowicz, 2007; Stachowicz and Byrnes, 2006). The most common sessile organisms found in the fouling community include tunicates, bryozoans, mussels, barnacles, polychaetes, sponges, and anemones. The primary larval recruits recorded in this study were from the colonial tunicates Diplosoma listerianum, Didemnum vexillum, Botryllus schlosseri, Botrylloides diegensis, Botrylloides violaceus, Distaplia occidentalis; the bryozoans Bugula neritina, Bugula californica, and Watersipora subtorquata; and the sponge Haliclona sp Effects of Ascidia ceratodes on the settlement of subordinate competitors We quantified the settlement of inferior competitors on Ascidia ceratodes (hereafter Ascidia) and on bare space. We deployed settling plates with and without Ascidia and measured the number and species of recruits that had settled on each after one week. Ascidia were obtained by removing mature individuals from settlement plates at Spud Point Marina. The settlement plates were placed in a tub of seawater after they were pulled onto the docks. Ascidia were carefully detached from the plates by hand, immediately transported to the laboratory in a cooler, and stored in a flow-through seawater tank. All extant recruits and colonies were removed from each Ascidia using tweezers and verified under a dissecting microscope. Ascidia were kept in water that was changed frequently to avoid drastic temperature changes. After the Ascidia were cleaned, they were attached with superglue to 10 cm by 10 cm sanded PVC plates (n=16) at approximately 80% cover. Additional bare plates (10 cm by 10 cm, n=16) served as a control. All panels were left in the holding tank overnight to allow development of a film on the plates to encourage larval settlement. The control plates and the Ascidiacovered plates were attached to racks made of PVC pipes, which were suspended from docks at Spud Point Marina at 1 m depth; each rack held four plates of each treatment, with haphazard spatial positions. After one week, we removed the plates from the water, placed them in plastic containers, and transported them to the lab for evaluation. Any Ascidia that had died during the experiment were removed from the panels to avoid causing mortality to other individuals during transport. Settlers on dead Ascidia were not counted, because dead Ascidia quickly began to decompose, and did not provide a secure substrate for recruits. A dissecting microscope was used to identify and count the organisms that were present on the plates. In order to count the recruits on the surface of Ascidia, each Ascidia was carefully removed from its plate with a small metal spatula and observed directly under the microscope. For the Ascidia plates, percentage cover of Ascidia was measured, and the number of recruits on Ascidia and the recruits that settled on the control plate were counted separately. Because the species composition and abundance of settlers can vary from week to week, we conducted this experiment 2 times, with plates collected and counted on July 3 and July 10, The effect of Ascidia on growth rates of recruits In order to quantify the effect of Ascidia on the growth rate of recruits, we measured the increase in number of zooids for two species of recruits over a three week period. We used PVC plates, and used superglue to attach Ascidia to one side of the plate with a cover of approximately 50%. Sixteen plates were deployed at Spud Point Marina on June 23, After one week, plates were brought back to the lab, and photographs were taken of each plate. These pictures were printed, and colonies of Watersipora (n=70) and Botrylloides violaceus (n=136) were marked on the photographs of both Ascidia and control plates. Initial number of zooids in each colony was recorded. We then returned the plates to the marina. After two more weeks (3 weeks after initial deployment), the plates were returned to the lab for analysis. The photographs taken at Week 1 were used to locate recruits on the plates and Ascidia; we then counted the number of zooids at Week 3 for each recruit documented in Week 1. The growth of each colony was measured by the change in number of zooids between Weeks 1 and 3. In our analysis we compared growth and size of colonies on Ascidia to colonies of the same species on adjacent substrate on the same plate Measurement of Ascidia surface area To understand whether the increased number of settlers of some species on Ascidia was due to the increased surface area per substratum area provided by Ascidia, we determined the relationship between the primary space covered by a single Ascidia (n=28) and the amount of surface area Ascidia presents for settlement. In order to do this, we took pictures of Ascidia on our settlement plates, then cut the tunic off of each Ascidia so that it would lay flat on a surface, and photographed it again. We used ImageJ (Rasband, ) to measure the primary substrate area occupied by each individual, and its surface area. We used this data in a linear regression to relate space occupied to surface area for Ascidia. This measurement method may underestimate the full surface area of Ascidia, as bumps and crevices on the tunic may create additional surface area that is not accounted for. However, we consider this measurement to be an acceptable approximation of the degree to which Ascidia creates more settlement space Effect of Ascidia on the survival of neighboring colonial species In order to evaluate the effect that Ascidia has on neighboring colonies as it grows, we recorded the outcomes of neighbor neighbor

3 132 D.C. Claar et al. / Journal of Experimental Marine Biology and Ecology 399 (2011) interactions using photographs taken from August 2008 to March Plates were deployed at Spud Point Marina, face-down at 1 m depth, and were photographed biweekly at the marina. As Ascidia individuals grew, they invariably captured the primary space occupied by neighboring colonies (see also Edwards and Stachowicz, in press). Each interaction between Ascidia and another species (n=52) was evaluated as either death, overgrowth onto Ascidia, or persistence of interacting colony onto primary substrate. In no cases did an Ascidia individual die as a result of the interaction Statistical analyses In order to test the effect of Ascidia on settlement rates, we performed a series of ANCOVAs. Each model had three explanatory factors: temporal set (July 3 vs. July 10), treatment (Ascidia as substrate vs. control), and surface area available for settlement. By including both treatment and surface area in the model, we can ask whether the effect of Ascidia on settlement is due solely to increased surface area, or if there is an additional effect of substrate type beyond surface area. We also included a treatment-by-set interaction in the initial model, to ask whether the effect of treatment varied over time, but this interaction was nonsignificant for all species except Diplosoma, and was therefore dropped from those models. We used this model with a series of response variables: total settler abundance, settler abundance for each individual species, and settler species richness. To test the effect of Ascidia on the growth rate of newly settled colonies, we performed a two-way ANOVA with an effect for treatment (Ascidia vs. bare substrate), and with a blocking effect for the plate on which the colony occurred (16 total plates). We treat each measured colony as a replicate measure of the effect of substrate type on growth. 3. Results During this experiment, four species recruited in high abundance, and comprised 89% of total recruits counted. These species were the colonial ascidians Botrylloides violaceus, Distaplia occidentalis, and Diplosoma listerianum, and the bryozoan Watersipora subtorquata. Considering settlers of all species together, there was a higher total settler density per unit substrate on Ascidia (mean±se, Ascidia: 0.7± 0.1 cm 2, Control: 0.3±0.1 cm 2 ), but this can be accounted for by the increased surface area provided (Table 1, significant effect of surface area but not treatment on total settler abundance). The species-specific responses to Ascidia were heterogeneous. The two bryozoans (Watersipora and Bugula) both had higher settlement density per unit substrate in the Ascidia treatment (Fig. 1). This was driven by a significant effect of surface area for both species, but for Watersipora there was an additional, significantly positive effect of Fig. 1. Comparison of mean (±SE) number of individuals settling directly on Ascidia tunics vs. on bare panels per cm 2 of primary substrate. For Ascidia, settlement density is calculated per unit area of primary space covered by Ascidia (not the surface area of Ascidia). Ascidia not accounted for by surface area (Table 1). For one of the colonial ascidians (Distaplia), settlement density was significantly lower on Ascidia, and for another colonial ascidian (Diplosoma), there was a significant set-by-treatment interaction, indicating a strong reduction of settlement on Ascidia plates during one of the sets (Table 1, Fig. 1). There was no detectable effect of surface area for any of the colonial ascidians or the sponge Haliclona sp. Species richness was greater in the second time period than the first (respective means 7 vs. 5.65, F 1,28 =3.3, p =0.044). However, species richness was not significantly affected by surface area (F 1,28 =2.4, p=0.13) or treatment (F 1,28 =3.3, p=0.08). Both treatments also had a similar total number of species found on all plates. Out of 13 identified species, a total of 12 species were identified on Ascidia, while 10 species were identified on the control plates. When we measured the area of primary substrate used by Ascidia compared to the surface area it provides, we found that the surface area provided by Ascidia was approximately 150% of the primary substrate occupied (regression equation y=1.53*x+1.11, F 1,26 =190,pb0.001). An average Ascidia individual uses up 19.8 cm 2 of primary substrate, and provides 31.4 cm 2 of surface area. We measured the effect of Ascidia on the growth rates of Botrylloides and Watersipora colonies that settled on Ascidia. Our results showed decreased growth on Ascidia compared to the primary Table 1 Theeffect of Ascidia on densityof settlersforeach species, andforthe totalnumberof settlers. For each species we performed an ANCOVA, including effects for treatment, set (time period), surface area, and a set-by-treatment interaction. The interaction between treatment and set was significant only for Diplosoma,(F 1,28 =4.3, p=0.047), and the interaction term was dropped for the other species. For each test, df=(1, 28). * pb0.05, ** pb0.01. Species Treatment F-value Set F-value Surface area F-value Botrylloides violaceus ** 2.5 Watersipora subtorquata 22** 13** 12** Distaplia occidentalis 8.4** Botryllus schlosseri ** 0.23 Didemnum vexillum Bugula neritina * Haliclona sp * 0.24 Diplosoma listerianum Total ** 7.0* Fig. 2. Growth rates of two common colonial species growing on Ascidia,comparedto neighboring primary space. Proportional growth rate was measured as (Final # zooids Initial # zooids)/(initial # zooids). Error bars are±1 SE.

4 D.C. Claar et al. / Journal of Experimental Marine Biology and Ecology 399 (2011) substrate for both of the species studied (Fig. 2). In the case of Watersipora, the difference between the two treatments was significant (2-way ANOVA; factors were Treatment and Plate; Treatment: F 1,51 =4.4, p=0.04), while the difference in Botrylloides growth between the two treatments was not (2-way ANOVA; Treatment: F 1,112 =1.7, p=0.20). The interaction between plate and treatment was not significant for the analyses of either species. When we examined the outcome of interactions between colonial ascidians on primary substrate and Ascidia, we found that subordinate species commonly evaded mortality by growing onto a neighboring Ascidia tunic. For the three most commonly observed species, successful growth onto the tunic occurred in about half of the encounters (B. violaceus, 13 of 24 observations; B. schlosseri, 3 of 6 observations; W. subtorquata, 4 of 11 observations). Of the remaining interactions, most resulted in mortality of the subordinate competitor (B. violaceus, 9 of 11 observations; B. schlosseri, 3 of 3 observations, W. subtorquata, 5 of 7 observations). The colonies in the remaining interactions survived, but their growth was restricted by the growth of the nearby Ascidia individual. 4. Discussion For several species, benefits gained by settlement or growth onto Ascidia tunics may counteract the strong competitive ability of Ascidia, helping maintain diversity in this community. We found that the total number of recruits per cm 2 of substrate was greater on Ascidia than on bare plates, indicating that Ascidia positively affects the total magnitude of recruitment in the fouling community. This effect appears to be driven by the greater surface area provided by Ascidia, because surface area but not treatment was a significant predictor of total recruitment. However, individual species showed both increases and decreases in recruitment on Ascidia (Fig. 1). This suggests that species-specific mechanisms generate recruitment differences between surfaces. The two bryozoans studied, Watersipora and Bugula, showed greater recruitment on Ascidia. Both species were positively affected by greater surface area, but Watersipora showed an additional positive effect of Ascidia beyond the effect of surface area (Table 1). This result is consistent with prior work showing that bryozoans settle more often on structurally complex surfaces (Walters and Wethey, 1996). The various species of colonial ascidians did not significantly benefit from the greater surface area of Ascidia, and two of the species had significantly lower recruitment on Ascidia (Table 1). One possible mechanism for this decline is that these species are avoiding settlement near Ascidia because it is a strong competitor (Grosberg, 1981). The bryozoan Watersipora subtorquata recruits at a five times greater rate to Ascidia, but this difference does not appear to translate into more adult colonies on Ascidia in the field (Edwards, pers. obs.). Our growth experiment showed that growth of Watersipora was reduced by ~30% when it grew on Ascidia compared to the control. This effect on growth may counteract increased recruitment on Ascidia, although competition from other species growing on Ascidia may affect Watersipora as well. For the colonial ascidian Botrylloides violaceus, there is a marginally significant trend towards lower growth rates on Ascidia compared to adjacent bare space. Because both Watersipora and Botrylloides tended to have lower growth rates on Ascidia, Ascidia may in general reduce the growth rates of epibionts, although the mechanism behind this difference is unknown. Although Ascidia is a dominant competitor for space, our results indicate that the net effect of Ascidia on the persistence of competing species will be determined by a complex interaction of positive and negative, direct and indirect effects. The direct effect of Ascidia on recruitment can be positive or negative (Fig. 1); the direct effect of Ascidia on epibiotic growth tends to be negative, but with varying strength (Fig. 2); and the direct effect of growing Ascidia on neighboring colonies is negative, but mitigated by epibiotic growth. The indirect effects of Ascidia on a particular species will depend upon the effects of Ascidia on that species' competitors. A key difference between Ascidia and primary substrate is that Ascidia experiences mortality (typical lifespan is 6 12 months, Edwards and Stachowicz, in press), which will lead to incidental mortality of epibionts. Dominance of Ascidia should therefore provide a relative advantage for species that can colonize, grow rapidly, and reproduce on the substrate provided by Ascidia, before it dies. For example, Didemnum is one of the longest-lived colonial ascidians in the fouling community, but it has low fecundity and growth rate (Edwards and Stachowicz, 2010). It is also a persistent invader worldwide (Bullard et al., 2007) and can overgrow nearly all other species in this community (Edwards and Stachowicz, 2010). The turnover of substrate created by Ascidia may prevent species such as Didemnum from completely excluding inferior competitors. An interesting question is whether species with a longer coevolutionary history are more or less likely to experience facilitative effects from Ascidia. However, in this experiment while Ascidia is native, nearly all the other species are non-native; therefore, it is difficult to assess the importance of evolutionary history from our results. Our results are comparable to other studies showing that dominant sessile species can alter the success of species that are epibionts. The epibiotic community on intertidal mussels can be similar to the community that attaches to the rock, but abundances can differ between substrates (Lohse, 1993). Algae that are epibiotic on mussels can have increased survival and growth compared to primary substrate (Aquilino et al., 2009). Algae that are epibiotic on other algae can obtain a refuge from predators by associating with competitors that are resistant to those predators (Wahl and Hay, 1995). An important challenge for future research is to understand how the direct and indirect interactions between epibionts and their hosts combine to affect the distribution of organisms and the maintenance of species diversity. As described above, epibiotic growth may favor species with fast life histories over their competitors. 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