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OIKOS 106: 217 /224, 2004 Competition, predation and species responses to environmental change Lin Jiang and Alexander Kulczycki Jiang, L. and Kulczycki, A. 2004. Competition, predation and species responses to environmental change. / Oikos 106: 217 /224. Despite much effort over the past decade on the ecological consequences of global warming, ecologists still have little understanding of the importance of interspecific interactions in species responses to environmental change. Models predict that predation should mitigate species responses to environmental change, and that interspecific competition should aggravate species responses to environmental change. To test this prediction, we studied how predation and competition affected the responses of two ciliates, Colpidium striatum and Paramecium tetraurelia , to temperature change in laboratory microcosms. We found that neither predation nor competition altered the responses of Colpidium striatum to temperature change, and that competition but not predation altered the responses of Paramecium tetraurelia to temperature change. Asymmetric interactions and temperature-dependent interactions may have contributed to the disparity between model predictions and experimental results. Our results suggest that models ignoring inherent complexities in ecological communities may be inadequate in forecasting species responses to environmental change. L. Jiang and A. Kulczycki, Dept of Ecology, Evolution and Natural Resources, Cook College, Rutgers Univ., 14 College Farm Road, New Brunswick, NJ 08901-8551, USA. Present address for LJ: Inst. of Marine and Coastal Sciences, Rutgers Univ., 71 Dudley Road, New Brunswick, NJ 08901, USA ([email protected]). Over the 20th century, global surface temperature has increased by an average of 0.68C, and is projected to increase by 1.4 to 5.88C over the period 1990 to 2100 (IPCC 2001). Such rapid climate change, unparalleled at least in the past 1000 years, is likely to have profound ecological consequences. Ecologists have taken on the challenge of uncovering the potential impacts of climate change on ecological systems (Lubchenco et al. 1991). The past decade has seen a large number of climaterelated ecological studies that significantly improved our understanding of the ecological impacts of climate change (reviewed by Hughes 2000, McCarty 2001, Walther et al. 2001). Many studies have documented ecological changes in natural communities associated with climate change. Species have responded by changes in their demographic rates (Myneni et al. 1997, Post and Stenseth 1999, Barber et al. 2000), altered timings in lifehistory events (Beebee 1995, Crick et al. 1997, Bradley et al. 1999, Brown et al. 1999, Dunn and Winkler 1999, Post and Stenseth 1999, Inouye et al. 2000, Menzel and Estrella 2001, Post et al. 2001), and changes in geographic range and abundance (Graham and Grimm 1990, Grabherr et al. 1994, Barry et al. 1995, Forchhammer et al. 1998, Reid et al. 1998, Thomas and Lennon 1999, Kiesecker et al. 2001, Parmesan et al. 1999, Sæther et al. 2000, Sturm et al. 2001). Population extinctions (Pounds et al. 1999, Harrison 2000, Kiesecker et al. 2001) and changes in community structure (Grabherr et al. 1994, Brown et al. 1997, Alward et al. 1999, Post et al. 1999) have also occurred. Despite these considerable efforts, ecologists still do not have a good understanding of the role of species Accepted 21 November 2003 Copyright # OIKOS 2004 ISSN 0030-1299 OIKOS 106:2 (2004) 217 interactions in responses to environmental change, partly because past studies have primarily focused on responses of single-species populations or whole ecosystems, largely ignoring the involvement of species interactions. This approach has often been justified by assuming that the observed species responses essentially reflected their direct physiological responses to environmental change. However, species interactions, particularly competition and predation, have been known to play dominant roles in determining species abundance and distribution (Connell 1983, Schoener 1983, Sih et al. 1985, Begon et al. 1998, Morin 1999), and they may potentially affect how species respond to environment change (Ives and Gilchrist 1993, Ives 1995, Lawton 1995, Abrams 2002). Indeed, several empirical studies have indicated the important effect of community structure on species responses to environment change (Brown et al. 1997, Davis et al. 1998a,b Post et al. 1999, Fox and Morin 2001). In this study, we examined the effects of interspecific competition and predation on species responses to temperature change. Species interactions may either alleviate or aggravate a species’ response to environmental change. Models, based on the assumption that populations follow stationary population distributions (analogous to stable equilibria, Ives and Gilchrist 1993) and that environmental change affects population growth rates, predict that species occupying distinct ecological roles are buffered against environmental change and that species sharing similar ecological positions are susceptible to environment change (Ives and Gilchrist 1993, Ives 1995). A corollary that follows is that predator /prey interactions should buffer species against environmental change, and that competitive interactions should exacerbate species response to environmental change (Ives and Gilchrist 1993, Ives 1995). Empirical studies on lake ecosystems also led to similar speculations (Carpenter et al. 1993). These predictions make biological sense. Considering a linear food chain, if the abundance of the prey increases (decreases) in response to environmental change, such positive (negative) response will lead to the increase (decrease) in predator abundance, which should put a limit on the prey response. The same logic applies to the predator. In contrast, for species involved in competitive interactions, an increase (decrease) in one species’ abundance, in response to environmental change, will lead to the decrease (increase) in the abundance of other species. Such among-species inverse density dependence should make competing species relatively sensitive to environmental change. Only one study has tested these predictions (Fox and Morin 2001). In their experiments, Fox and Morin raised temperature gradually to simulate global warming. This approach, although a good representation of ongoing climate change, makes it difficult for populations to 218 reach stationary population distributions. Because Ives and Gilchrist’s models were based on the stationary distribution assumption, the study of Fox and Morin (2001) did not constitute a strictly valid test of the models. In this study, we instead used different constant temperatures to simulate environmental change, as populations are more likely to approach stationary distributions in constant environments. We assembled predator /prey communities and communities of competitors in laboratory microcosms, subjected them to different temperature regimes, and examined how species in different communities responded to warming. The short generation times (/3 to 24 hours) of protozoan species used in the experiments allowed us to collect multi-generational population dynamics data over a period of several weeks. We expected that the average abundance of prey species engaged in predator /prey interactions would be more buffered against temperature change compared to without predators, and that the average abundance of species engaged in competitive interactions would be more affected by temperature change compared to without competitors. Material and methods Three ciliate species were used: Colpidium striatum (hereafter Colpidium ), Paramecium tetraurelia (hereafter Paramecium ), and Didinium nasutum (hereafter Didinium ). Colpidium and Didinium were obtained from Carolina Biological Supply Company (Burlington, NC, USA), and Paramecium from the American Type Culture Collection (Rockville, MD, USA). Colpidium is reniform-shaped, about 50 mm in size; Paramecium is slipper-shaped, about 120 /180 mm in size. Both are freeswimming bacterivores. Didinium is an oval-shaped predator ranging in size from 100 to 200 mm, and it actively searches and attacks suitable prey, such as Colpidium and Paramecium , in the water column. The experimental microcosms were 240 ml capped glass bottles each containing 100 ml of nutrient medium plus 2 wheat seeds. The medium was made from 0.55 g of protozoan pellets (Carolina Biological Supply, Burlington, NC, USA) dissolved in 1 liter of well water. Bottles, medium and wheat seeds were autoclaved before use. Medium was first inoculated with three bacterial species: Serratia marcescens, Bacillus cereus and Bacillus subtilis, all obtained from Carolina Biological Supply Company (Burlington, NC, USA). These bacteria served as food resources for Colpidium and Paramecium . Bacterivores were introduced into the microcosms 24 hours after bacteria inoculation. The predatory species Didinium was introduced into the microcosms one week after the introduction of bacterivores. OIKOS 106:2 (2004) Our main experiments employed a factorial design in which temperature and presence/absence of interspecific interactions were the two main factors. The temperature factor had three constant levels: 22, 26, and 308C. The species interaction factor had three treatments: control treatment with the bacterivore (Colpidium or Paramecium ) raised in monocultures, competition treatment with the two bacterivores competing for resources (Colpidium /Paramecium ), and predation treatment in which the bacterivore was preyed upon by predators (Colpidium /Didinium or Paramecium /Didinium ). We set up three, four, and six replicates for the control, competition, and predation treatment, respectively. Higher replication was used in the predation treatment, because among-replicate variation in the predation treatment is generally of larger magnitude than in the control and competition treatment (L. Jiang, pers. obs.). Both control and competition experiments ran for 44 days. Colpidium /Didinium and Paramecium /Didinium predation experiments ran for 37 and 18 days, respectively, reflecting the longer persistence time of Colpidium than Paramecium in the predation treatment. For each bacterivore (Colpidium and Paramecium ), we also measured its intrinsic growth rate at 22, 26, and 308C, according to the procedures described in Fox and Morin (2001). We monitored population dynamics of each protist in the microcosms every two to three days. To sample, we swirled each microcosm to mix its content and used a Pasteur pipette to withdraw approximately 0.35 ml of the medium. We weighed each sample on an electronic scale to determine its exact volume, and counted the number of each protist in the sample under a Nikon SMZ-U dissecting microscope. We replaced 10% of the medium in each microcosm with fresh medium every week to replenish nutrients and prevent metabolic waste buildup. We recorded density for each protist species as the number of individuals per ml and log transformed the data (log10(individuals/ml/1)) to reduce heteroscedasticity in preparation for statistical analysis. We calculated geometric mean density over time for each species, excluding the post-extinction zero counts if species went extinct before the end of the experiment. Our analyses did not involve the predatory species Didinium , because we were only interested in how predation and competition affect the responses of the two bacterivores, Colpidium and Paramecium , to environmental warming. We used geometric mean densities of bacterivores as the main response variables in our analyses. We performed separate analyses for the two bacterivores, Colpidium and Paramecium . For each of them, we first ran a one-way ANOVA to test whether its growth rate and mean density in the control treatment (monocultures) was affected by ambient temperature. We then performed two-way ANOVA on the data from control and competition treatments for each bacterivore, with OIKOS 106:2 (2004) temperature and presence/absence of interspecific competition as the two main factors. Significant interaction terms in the two-way ANOVA would indicate that interspecific competition influences response to environmental change. We performed two-way ANOVA on the data from control and predation treatments for each bacterivore, with temperature and presence/absence of predation as the two main factors. Significant interaction terms in the two-way ANOVA would indicate that predation influences responses to environmental change. Results The effects of temperature on Colpidium Increasing temperature significantly reduced Colpidium growth rate (one-way ANOVA, temperature: F2,6 / 47.18, P/0.002; Fig. 1). At all three temperatures in monocultures (the control treatment), Colpidium grew very quickly to high density, and then declined to relatively steady density (Fig. 2, left panels). The steady density was lower at higher temperatures (Fig. 2, left panels). Increasing temperature significantly reduced Colpidium average density in monocultures (one-way ANOVA, temperature: F2,6 /179.41, P B/0.0001; Fig. 3). In the Colpidium /Didinium predation experiment, Didinium increased in abundance and reduced Colpidium abundance at all three temperatures (Fig. 2, right panels). Sometimes population fluctuation of both species was observed, but extinction of Colpidium never occurred during the experimental period. Didinium persisted for a shorter period of time at higher temperatures, perhaps due to lower prey abundance and higher metabolic costs at higher temperatures. Both increasing Fig. 1. The effects of temperature on the intrinsic growth rates of Colpidium and Paramecium . Error bars represent one standard error. 219 Fig. 3. Effects of temperature on mean population densities of Colpidium and Paramecium . Upper and lower panels represent Colpidium and Paramecium , respectively. Errors bars indicate one standard error. Fig. 2. Colpidium population dynamics in the control and predation treatments at different temperatures. Left panels represent the control treatment in which Colpidium was in monoculture, and right panels represent the predation treatment in which Colpidium was a prey of Didinium . Each panel corresponds to one representative microcosm. temperature and Didinium predation significantly reduced Colpidium average abundance (two-way ANOVA, temperature: F2,21 /28.99, P B/0.0001, predation: F1,21 /41.10, P B/0.0001; Fig. 3). However, the nonsignificant interaction terms in the two-way ANOVA (F2,21 /0.89, P/0.4256) indicated that predation from Didinium did not change the responses of Colpidium to temperature change. In the Colpidium /Paramecium competition experiment, Colpidium was not negatively affected by the presence of Paramecium . In fact, Colpidium attained higher abundance in the competition treatment than in the controls (two-way ANOVA, competition: F1,15 / 12.22, P /0.0033; Fig. 3, 4). The non-significant interaction terms (F2,15 /0.21, P/0.8116) in the two-way ANOVA indicated that competition from Paramecium did not change the response of Colpidium to temperature change. 220 The effects of temperature on Paramecium Increasing temperature significantly increased Paramecium growth rate (one-way ANOVA, temperature: F2,6 /6.29, P/0.0336; Fig. 1). In monocultures, Paramecium maintained a relatively steady density after initial rapid population growth (Fig. 5, left panels). Temperature change did not affect Paramecium mean density in monocultures (one-way ANOVA, temperature: F2,6 /3.75, P /0.0879; Fig. 3). In the Paramecium /Didinium predation experiment, predation from Didinium reduced Paramecium abundance, and eventually caused Paramecium extinction in all replicates at all three temperatures (Fig. 5, right panels). In all microcosms, Paramecium uniformly went extinct four days after the introduction of Didinium . Predation from Didinium significantly reduced Paramecium average abundance before extinction (two-way ANOVA, predation: F1,21 /808.21, P B/0.0001; Fig. 3). However, the non-significant interaction terms in the two-way ANOVA (F2,21 /1.49, P/0.2477) indicated that predation from Didinium did not change the responses of Paramecium to temperature change. In the Colpidium /Paramecium competition experiment, Paramecium coexisted with Colpidium as the OIKOS 106:2 (2004) Fig. 4. Colpidium and Paramecium population dynamics in the competition treatment at different temperatures. Each panel corresponds to one representative microcosm. sub-dominant species at 228C, went extinct in all replicates before the end of the experiment at 268C, and coexisted with Colpidium as the co-dominant species at 308C (Fig. 4). Competition from Colpidium significantly reduced Paramecium average abundance (two-way ANOVA, competition: F1,15 /673.72, PB/0.0001). Significant interaction terms between temperature and competition in the two-way ANOVA (F2,15 /103.16, P B/0.0001) revealed that competition from Colpidium altered the response of Paramecium to temperature change. Discussion Our experimental results, to a large extent, did not support the hypothesis that a species engaged in predator /prey interactions should be more buffered against environmental change, and that species engaged in competitive interactions should be more susceptible to environmental change, compared to responses in the absence of interspecific interactions. Increasing temperaOIKOS 106:2 (2004) ture reduced both the growth rate and average density of Colpidium in monocultures, but neither competition from Paramecium nor predation from Didinium changed its response to elevated temperature, although both changed its average abundance. Increasing temperature increased the growth rate but did not affect the average density of Paramecium in monocultures. Predation from Didinium also did not alter Paramecium’s response to temperature change. However, competition from Colpidium did change the way Paramecium responded to elevated temperatures. As the only case consistent with the hypothesis, Paramecium was more affected by temperature change in the presence of interspecific competition: it went extinct at 268C, but coexisted with Colpidium at 22 and 308C. The extinction of Paramecium at 268C in the competition treatment is unexpected, because increasing temperature raised its growth rate and did not affect its abundance in the absence of interspecific interactions, and because increasing temperature reduced both the growth rate and abundance of its competitor, Colpidium . This contrasts with more general findings that species coexist at intermediate temperatures and competitive 221 Fig. 5. Paramecium population dynamics in the control and predation treatments at different temperatures. Left panels represent the control treatment in which Paramecium was in monoculture, and right panels represent the predation treatment in which Paramecium was a prey of Didinium . Each panel corresponds to one representative microcosm. extinction or dominance occurs at extreme temperatures (Park 1954, Hairston and Kellermann 1965, Wilson et al. 1984, Phillips et al. 1995, Davis et al. 1998a,b, Taniguchi and Nakano 2000). A probable explanation is that at 268C the higher feeding rate of Colpidium outweighed its lower density (compared with 228C), translating into larger overall competitive effects on Paramecium populations. The surprising responses of Paramecium in the competition treatment suggest that, at least for some species, it may be impossible to use their direct physiological responses to predict how they would respond to environmental change, if they are affected by interspecific interactions. So why is there such a discrepancy between the hypothesis and our data? First, asymmetric interactions between species may have complicated responses to temperature change. In particular, our data showed that interspecific competition negatively affected Paramecium but not Colpidium . This competitive asymmetry 222 was probably responsible for the differential responses of Colpidium and Paramecium to temperature change in the competition treatment. Interspecific competition may be more likely to affect species responses to environmental change in communities characterized by diffuse competition, i.e. competitive interactions in which species are affected more or less equally (MacArthur 1972). Such communities may go through significant changes in species composition and community structure, in the event of environmental change. A second plausible explanation for the discrepancy lies in the assumption of the models. In formulating their models, Ives and Gilchrist (1993) assumed that environmental change only affects species growth rates. This assumption fails to hold if environment change also alters species interaction strength. For instance, temperature generally affects attack rates and handling times of predators on their prey (Murdoch et al. 1984, MacRae and Croft 1993, Elliott and Leggett 1996, Sanford 1999). In our experiments, we did not estimate the functional responses of the predatory species Didinium on its prey, but it is possible that temperaturedependent functional responses have contributed to the finding that predation did not affect species responses to elevated temperatures. Temperature may also alter strength of competitive interactions. For species involved in resource competition, such as Colpidium and Paramecium in this study, higher temperatures means individuals of each species must take up more resource to meet their higher metabolic needs, exerting higher percapita competitive intensity upon each other. The fact that Paramecium was competitively excluded by Colpidium at 268C suggest that stronger per-capita competitive strength of Colpidium on Paramecium more than compensated the lower abundance of Colpidium (compared with 228C). As variation in interaction strength may often occur in association with environmental change, it is important that future studies take it into consideration when predicting or explaining the responses of communities to environmental change. In another study, Fox and Morin (2001) were also unsuccessful in finding evidence supporting Ives and Gilchrist’s predictions. They argued that strong density dependence within species made it impossible to detect any effects of interspecific interactions on species responses to environmental change. This is not the case in our experiments. Weak density dependence governed Colpidium because both its growth rate and abundance declined with temperature, and strong density dependence governed Paramecium because its growth rate but not abundance increased with temperature. Yet it was Paramecium not Colpidium whose response to temperature change was altered by competition. Different temperature regimes between our and their experiments (see Introduction) may have contributed to the differential results. OIKOS 106:2 (2004) It is important to note that the ciliates used in this experiment do not possess complex life-history characteristics. All three ciliates reproduce asexually, and populations are not structured. This simplicity allowed us to avoid potential complications caused by complex life-history traits, and focus on the actual effects of species interactions. In nature, many ecosystems are dominated by more complex organisms. These organisms often possess complex life-history traits that may affect their response to environmental change, and these species-level responses may further translate into responses at the community level. For example, Beisner et al. (1996, 1997) found that temperature affected the reproductive strategy of an invertebrate carnivore on prey Daphnia , and increasing temperature destabilized the predator /prey interaction and switched the system from bottom-up to top-down trophic control. Post et al. (1999) found that climate variation caused changes in wolf hunting behavior, leading to community-wide responses including changes in herbivore abundance and primary production. Davis et al. (1998a,b) study on Drosophila demonstrated the importance of dispersal in species responses to environmental change. Environmental change may also alter plant resource allocation among growth, reproduction, storage, and defense against herbivory, affecting the outcome of herbivoreplant interactions (Ayres 1993). Conclusions Despite the simple life history characteristics of the experimental organisms, our data failed in large part to support the hypothesis that predation should lessen and competition should magnify species’ direct response to environmental change. Asymmetric interactions between species and temperature-dependent interactions may have contributed to the discrepancy between the hypothesis and the data. Communities containing organisms with complex lifehistory characteristics may have more complex and unexpected responses to environmental change, which cannot be readily predicted by general models such as those of Ives and Gilchrist (1993) and Ives (1995). Given the complexities of ecological communities, if one were to accurately predict the response of the species to environmental change, more mechanistic approaches such as individual-based models may be more appropriate (Dunham 1993, Murdoch 1993). However, individual-based models are notoriously known to be structurally complex, data hungry, and only pertain to the particular system being modeled. Predicting ecological consequences of environmental change remains a serious challenge. Acknowledgements / Tim Casey, Christina Kaunzinger, Zac Long, Timon McPhearson, Peter Morin, Jennifer Price and OIKOS 106:2 (2004) Christina Steiner provided comments that improved this manuscript. Funding was provided by US NSF grant DEB9806427 to Peter Morin and Tim Casey. Lin Jiang was supported by Rutgers University Bevier Fellowship. Alex Kulczycki was supported by REU supplement to US NSF grant DEB-9806427. References Abrams, P. A. 2002. 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