A competitive hierarchy model integrating roles of physiological competence and competitive ability does not provide a mechanistic explanation for the zonation of three intertidal Fucus species in Europe
TL;DR: Keddy's competitive hierarchy model, which has not been tested in marine systems, provides an explanation for the zonation of plant species on environmental gradients but requires empirical testing in each species assemblage, to ensure that the negative outcome reported here does not invalidate the concept in general.
Abstract: It has been clear for the last 20 yr that both competitive ability and physiological competence on the intertidal gradient of rocky shores determine the zoned distribution of fucoid seaweeds in the North Atlantic. However to this point, there has been no concept integrating these functions for seaweed vegetation in a single mechanistic explanation. Keddy's competitive hierarchy model, which has not been tested in marine systems (i.e. not on seaweed vegetation) provides an explanation for the zonation of plant species on environmental gradients. The model proposes that competitive abilities of species on a stress or resource gradient are inversely related to fundamental niche breadths. We tested 2 assumptions of the model to determine whether it provided a comprehensive explanation of zonation of 3 Fucus species on the island of Helgoland in the North Sea. The 2 assumptions translate into specific predictions for the Fucus assemblage (where F. serratus occurs on the low shore, F. spiralis on the high shore, and F. vesiculosus in between): I. competitive ranking is F. serratus > F. vesiculosus > F. spiralis; 2. fundamental niche breadth rankings are the reverse of competitive ability rankings. Pairwise competition experiments were done in the field. A highly modified substitutive design was used, in order to take into account the shortcomings of this approach. The empirically derived competitive hierarchy did not fit prediction I. Transplant experiments with adults and juveniles provided results contrary to prediction 2. Since none of the assumptions of the model fit, it cannot be used to provide a mechanistic explanation for the zonation of Fucus species on Helgoland. Unlike other important models of competition, Keddy's approach does not claim universal validity in all communities. This means that it requires empirical testing in each species assemblage, and more importantly, that the negative outcome reported here does not invalidate the concept in general.
Summary (5 min read)
\ competitive hierarchy
- The surveyed gradient was parallel to the shoreline of Canadian lakes.
- Only partial supporting evidence could be found (Reader et al. 1994) .
Description of the study site
- Thus, the mean angle of gradient is less than 0.5".
- In the uppermost zone of the main NE-Intertidal.
- The area surrounding these boulders is populated from ca March to September by dense stands of green ephemeral algae.
- More comprehensive descriptions of the distribution of Helgoland's intertidal species are given by Markham and Munda (1980) , Janke (1986) and some of the studies summarized by Harms (1993) .
- Crnsztses of tlutural Fucus gernzling dmsit), in the Helgolarzd intrrtidc~l zone Censuses in the Helgoland intertidal zone of subjectively "dense" stands of Fucus spp. were done in order to estimate appropriate experimental densities.
- Numbers were assigned to 20 patches of Fzrcus germlings distributed across the fucoid belt.
- Seven of them were chosen randomly and algae from 5 small areas (5 x 5 cm. coordinates with random number tables) of each were scraped from the rock and brought to the laboratory.
- Densities of samples and size class frequency distributions from subsamples were estimated.
Replacet7zent series esprrinzent
- A suspension hierarchy hypothesis, it was not necessary to test comof zygotes was poured into each tray and agitated by petitive interactions between F. spiralis and F. serrutus filling the tray with additional sterile seawater immedidirectly.
- Later in spring, extensive deterioration of receptacles occurred.
- Before installation, the lengths of 25 unbranched randomly chosen individal Fucus juveniles (from each species in mixtures) were measured from each plot.
- The lengths and dry weights were measured for 50 juveniles of each species not used in the replacement series.
- With polynomial regression equations the lengths (L. in cm) of the juveniles on the experimental plots give an estimate of the average mass (in mg) of individual shoots and, with knowledge of the density, of the total starting biomasses of each Fucus species in each plot.
- At the end of the experiment, the remaining shoots were separated to species, censused and their dry weights measured individually.
- Dry weight was obtained by drying (at 60°C for 3 d) thalli that had been rinsed in tap water.
- Replacement series graphs were drawn by plotting final yield of each component species of a replacement series experiment against its proportion in mixtures, as explained by Khan et al. (1975) .
- Individual and relative crowding coefficients were calculated from initial dry weight of germlings and final biomass.
- Initial weight was calculated with polynomial regression equations as indicated above.
- The individual crowding coefficient K, (Khan et al. 1975) is calculated for species i as: where o, is the per plant yield in mixed culture, o, is the initial per plant mass in mixed culture, mi,, the per plant yield in the corresponding monoculture and o,, the inital per plant mass in monoculture.
- Input-output diagrams were plotted for mixtures of F. serrutlis with F. cesiculosus and for mixtures of F. cesicillosus with F. spircllis for each total density separately.
Conzparison of intruspecific and iriterspecjfic interactions in additice designs.
- In substitutive designs of competition experiments, such as the replacement series, individuals of one species are "replaced" by individuals of a second species in the different plots.
- The density of one species is held constant while that of the other is varied (Firbank and Watkinson 1990) .
- The competition experiment performed here had a replacement series configuration, but some plots can also be analysed according to the additive design.
- Prior to ANOVA, homoscedasticity of data was tested with Cochran's test.
- To obtain the ranking predicted by the competitive hierarchy hypothesis, competition should be asymmetric.
Fundamental niche breadths
- Transplants oj' laboratory gernzlirigs In.stullation and rnairztenunce of the esperiment.
- 3 tiles bearing germlings of one of the Fucus species were removed and distributed among the 3 zones on the seawall (one per zone = total of 3: repeated for each of 3 F~rcusspecies).
- Remaining algae were scraped from the tiles, censused and dry weights were determined.
- Tr-ansplants uf' ad~tlt ,field plunts I11,srallution atld nzait~tenanceof the e.~perimerzt.
- 'A' compares the interspecific effect of adding 500 F. srrrutus, 'B' the intraspecific effect of adding 500 F. crsiculo.rus. 'C' and 'D' depict substitutive comparisons between intra-and interspecific effects at medium density = 500 and high density = 1000 shoots, 10 x 10 cm, respectively.
Replacement series graplzs
- At none of the 3 densities was there a linear relationship between yield and mixture proportion.
- Yields of F. ~~esiculosus were elevated in all mixed cultures, while yields of F. serratus were depressed.
- Hence, for F. cesiculosus, the curves are totally (medium density) or partially (low and high density) convex, while those of F. serratus have a concave shape.
- F. spirulis performed poorly in most plots.
- This means that growth of F. serratu.7 was depressed in mixtures (compared with its monocultures).
- Conversely, all individual crowding coefficients except one for F. cesiculosus.
- K,. from the same mixtures were > 1, indicating that growth of F. cesiculo.s~iswas elevated in these mixtures.
- All values of K, were bigger than the corresponding K,. hence all relative crowding coefficients K,, were > 1. all K,, < 1. in concordance with analysis by replacement series graphs.
Input-output ratlo diagrunzs
- All output ratios are less than input ratios.
- Three tiles, each bearing a different Fucus species (upper right), were fixed to each Fucus zone on the seawall .
- All data points for F. cesiculosus-F. spiralis mixtures lie above the line of unit slope; hence, in all mixtures, more biomass of F. resiculo.sus was gained per initial mass at the expense of F. ,spiralis and.
- Thus, the analysis through input-output diagrams is in accord with the graphical analysis and with crowding coefficients.
- Although unrelated to the main experimental objectives.
Cornparison of intraspecific and interspecific interactions in additire designs
- Information on intra-and interspecific effect was obtained from a few plots of the replacement series experiments that fit to the design suggested by Underwood (1986) (EAD: extended additive design).
- Field-grown fucoid thalli originating from concrete blocks in the Helgoland intertidal and transplanted to the three Fuclrs zones.
- When grown together with another 250 vs another 750 F. serrcrtus.
- At the medium density, the yield of 250 F. serratus was more depressed by inter-than by intraspecific competition.
- Comparisons of intra-with interspecific effects revealed a stronger reduction (assumed. since there is no control) in yield through Intraspecific competition.
- According to Shipley and Keddy's (1994) defin~tion.
- The competitive relationship between F. t.eaiculosu~and F. serratus is asymmetric, the former being the dominant species.
- The findings of data analysis in an additive design are in agreement with those of substitutive replacement series.
F.serratus:F. vesiculosus F.vesiculosus: F. spiralis
- Neither of the higher density treatments differed significantly from control mean yield (EAD, Tukey-Kramer .* > 0.05, Fig. 8c ).
- Intra-and interspecific effects were significantly different at medium density, in that yield of 250 F. cesiculosus was less when grown with 250 additional F. cesiculostrs (8.95 g) as opposed to 250 additional F. mixture proportion spiralis (21.47 g).
- Since differences in I-way ANOVA were nearly significant ( p = 0.066) and differences in TK test were close to critical differences for interspecific effects (diff. = 4.4, crit. diff. = 4.9).
- The only significant pairwise difference was from an irrelevant comparison (i.e. between treatments with different species and different total density).
- Yield of F. spiralis was reduced both by intra-and interspecific competition, the latter having the stronger effect.
Sunimary of results for competition experirnerzt
- Replacement series graphs, crowding coefficients, ratio diagrams and EAD and MASD comparisons unequivocally gave this result.
- F. .spiralis performed poorly in replacement series experiments at all three initial densities.
- The findings are not concordant with the prediction from the competitive hierarchy model (F. serratus > F. cesiculo.sus > F. spirrilis).
Transplants of laboratory gerrnlirzgs
- And total dry weight per experimental tile for each Fucus species-zone combination are shown in Table 5 .
- For all parameters considered here, F. spiralis and F. r:esiculo.szr.s clearly performed worst in the lowerlnost zone, contrary to the predictions of the competitive hierarchy model.
Tran.splants of adult jell-gron.n thalli
- Initial and final numbers of transplants of all three Fucus species are shown in Table 2 .
- Differences in percentage survival were tested with ANOVA after arcsine transformation.
- All three Fucus species showed the greatest percentage survival in the zone of F. srrra-tus and successively fewer surviving plants in the zones above this tidal level.
- The most complete data for F. serratu.r parallel those from the summer experiment.
- This may also have led to the high survival, compared with summer transplants. of F. rc~siculosus and F. .spiralis in the highest zone.
- The authors tested whether the Fucus species on Helgoland conform to the predictions of the "competitive hierarchy hypothesis" (Keddy 1989a ).
- The authors determined the competitive ranks of Helgoland Fucus species, and their fundamental niche breadths in the section of the intertidal zone naturally populated by members of the genus.
- In replacement series experiments set up in the Helgoland intertidal zone.
- The competitive exclusion of F. spiralis from the mid-intertidal zone occupied by F. cesiculosus was also shown by Schonbeck and Norton (1980) .
- The competitive dominance of F. cesiculosus over F. serratus was less pronounced than that over F. spimlis.
- Hence, the hypothesis that competitive ranks of Helgoland Fucus species can be explained with the predictions from the competitive hierarchy model must be rejected.
Assessment of replacement series as experimental design
- Initial and final numbers of shoots from concrete fragments that withstood the storm.
- Few went as far as Connolly (1986) who stated "that it [the replacement series] is usually a misleading tool for research on mixtures", and most concluded that it may be a valuable method.
- Replacement series experiments have their difficulties, as have other designs, in detecting competition.
- For this to be true, both species must achieve constant final yield, when grown alone at the density of their mixture proportion (Taylor and Aarssen 1989.
- In the present study, no tests of this kind were done explicitly.
- For each Funrs species, final densities of monospecific treatments were not significantly different, even though starting densities ranged between 100 and 1000 plants per tile.
- The assumption of similar sizes was probably met with congeneric Fucus species in this study.
Transplant experiments and fundamental niche breadths in 3 Fucus species
- A fundamental tenet of the competitive hierarchy model is that competitive abilities are negatively correlated with fundamental niche breadth, perhaps because of an inherent trade-off between ability for interference competition and ability to tolerate low resource levels (Keddy 1989a ).
- The model predictions for fundamental niche breadths are: Patterns of transplants of laboratory germlings were similar in that F. ~r~iciilo.s~is was able clearly to survive in the uppermost zone and F. srrrarus performed poorly when transplanted above its natural zone.
- Effects of high temperature and desiccation should be ameliorated on this NE-facing side of the seawall and.
- It is not appropriate in this context to compare yields among species among zones simultaneously.
Application of a "relaxed" version of the competitive hiearchy model
- The survival of F. z~esiculosusabove the zone of its natural occurrence has been observed repeatedly before (Schonbeck and Norton 1978 .
- This variant seems more realistic compared with the originally strict assumptions of sharp borders of occurrence inevitably linked with invariable competitive ability and permits dominant subordinate pairs to change rank under different environmental conditions and, hence.
- Gradient" would have been a term more appropriate, not suggesting a commonly used definition of "resource" (emphasizing the possibility of consun~ption).
- Only controlled experiments, as performed in this study, will reveal the validity of Keddy's model.
- It can be argued that the mid-intertidal zone occupied by F. cesiculosus is the central benign habitat.
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Cites background or methods from "A competitive hierarchy model integ..."
...(ii) The second model was described by Keddy (1989) as a ‘Competitive hierarchy model’ and was further developed for seaweeds by Chapman (e.g. Chapman, 1990; Karez & Chapman, 1998)....
...Fundamental niches of inferior species include those of competitively superior species (see Karez & Chapman, 1998, for details)....
...In contrast, F. serratus was clearly able to invade the lower zone, if L. digitata was removed (Hawkins & Harkin, 1985; Hawkins & Hartnoll, 1985) showing that the competitive strength depends on the environment and supporting the relaxed model of Keddy proposed by Karez & Chapman (1998)....
Cites background or result from "A competitive hierarchy model integ..."
...spiralis a z with results reported by other authors (Karez and Chapman, 1998)....
...According to Karez and Chapman (1998), the competitive capacity of the species is inversely related to the occupied area in the rocky substratum....
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