Separation anxiety: mussels self-organize into similar power-law clusters regardless of predation threat cues (original) (raw)
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A fractal approach for detecting spatial hierarchy and structure on mussel beds
Marine Biology, 2001
Within beds of blue mussel (Mytilus edulis L.), individuals are aggregated into small patches, which in turn are incorporated into bigger patches, revealing a complex hierarchy of spatial structure. The present study was done to find the different scales of variation in the distribution of mussel biomass, and to describe the spatial heterogeneity on these scales. The three approaches compared for this purpose were fractal analysis, spatial autocorrelation and hierarchical (or nested) analysis of variances (ANOVA). The complexity (i.e. patchiness) of mussel aggregations was described with fractal dimension, calculated with the semivariogram method. Three intertidal mussel beds were studied on the west coast of Sweden. The distribution of wet biomass was studied along transects up to 128 m. The average biomasses of blue mussels on the three mussel beds were 1825±210, 179±21 and 576±66 g per0.1 m2, respectively, and the fractal dimensions of the mussel distribution were 1.726±0.010, 1.842±0.014 and 1.939±0.029 on transects 1–3, respectively. Distributions of mussels revealed multiscaling behaviour. The fractal dimension significantly changed twice on different scales on the first bed (thus showing three scaling regions), the second and third beds revealed two and three scaling regions, respectively. High fractal dimension was followed by significant spatial autocorrelation on smaller scales. The fractal analysis detects the multiple scaling regions of spatial variance even when the spatial structure may not be distinguished significantly by conventional statistical inference. The study shows that the fractal analysis, the spatial autocorrelation analysis and the hierarchical ANOVA give complementary information about the spatial variability in mussel populations.
Ecological theory predicts that two species with similar requirements will fail to show long-term coexistence in situations where shared resources are limiting, especially at spatial scales that are small relative to the size of the organisms. Two species of intertidal mussels, the indigenous Perna perna and the invasive Mytilus galloprovincialis, form mixed beds on the south coast of South Africa in a situation that has been stable for several generations of these species, even though these populations are often limited by the availability of space. We examined the spatial structure of these species where they co-exist at small spatial scales in the absence of apparent environmental heterogeneity at two sites, testing: whether conspecific aggregation of mussels can occur (using spatial Monte-Carlo tests); the degree of patchiness (using Korcak B patchiness exponent), and whether there was a relationship between percent cover and patchiness. We found that under certain circumstances there is non-random conspecific aggregation, but that in other circumstances there may be random distribution (i.e. the two species are mixed), so that spatial patterns are context-dependent. The relative cover of the species differed between sites, and within each site, the species with higher cover showed low Korcak B values (indicating low patchiness, i.e. the existence of fewer, larger patches), while the less abundant species showed the reverse, i.e. high patchiness. This relationship did not hold for either species within sites. We conclude that coexistence between these mussels is possible, even at small spatial scales because each species is an ecological engineer and, while they have been shown to compete for space, this is preceded by initial facilitation. We suggest that a patchy pattern of coexistence is possible because of a balance between direct (competitive) and indirect (facilitative) interactions.
The interaction of prey distribution patterns and predator behavior can mediate predator-prey dynamics. Inter-patch distance (lag) may be especially important in the interacting effects of aggregation and interference among predators on their search and prey-handling ability. Interactions of blue crabs Callinectes sapidus preying upon thin-shelled clams Macoma balthica in Chesapeake Bay provide a test of how the opposing forces of aggregation and interference interact with the spatial distribution of prey patches to influence rates of prey consumption. Blue crabs can detect clam patches from tip to 15 m away using chemosensory cues, and they aggregate on them, thus facilitating predation, but exhibit agonistic behaviors when closer than 5 in to another crab, thus reducing feeding efficiency. We used these patterns of aggregation and interference to modify a generalized functional response model to describe individual crab foraging efficiency as a function of distance between patches, The model predicted highest predation rates at an intermediate lag of 6.6 m. We tested this a priori hypothesis with a set of field experiments wherein prey patches were established with lags of 1, 7, 10, and 50 m. Predation rates were highest. at intermediate lags, as predicted. This work highlights the importance of the interaction between spatial scales and ecological processes, demonstrating that spatial heterogeneity is not. noise that obscures processes, but an active component of the predator-prey dynamic.
ABSTRACT: To test the assumption that there is no spatial structure in small-scale recruitment variability of rocky shore mussels, we examined spatial dependence in the distribution of density of recruits (late plantigrades: 0.5 to 3.5 mm; larger recruits: 3.5 to 10 mm) and adults of the brown mussel Perna perna within local scales (30 lags ranging between 0.35 and 10.5 m) in mid- and upper mussel beds. Spatial heterogeneity was estimated by analyzing scaling properties of semivariograms using a fractal approach. Relationships between density of mussel recruits and adults and biomass of the red alga Gelidium pristoides at the different scales were examined by cross-semivariograms. We found that the distribution of adults showed spatial dependence at all transects, often with higher spatial heterogeneity (higher fractal dimension, D) at smaller scales (1st scaling region). The distribution of larger recruits exhibited spatial dependence at all transects, revealing a spatial structure, which was related to that of adults. In contrast, the distribution of late plantigrades showed mainly spatial independence (random pattern; 1.97 < D ≤ 2). Densities of both size classes of recruits were positively related to those of adults at all transects and scales, but the relationship was stronger for larger recruits than late plantigrades, explaining why there was clearer spatial structure of larger recruits. The relationship with algae was mainly negative for larger recruits, while it tended to be positive at many scales for late plantigrades. Thus, both adult mussels and G. pristoides are suitable habitats for plantigrades, while mussels are the main habitat for larger recruits. This may mean that recruits on algae either die or migrate to mussel clumps at a certain size. This study highlights the importance of recruit size when analyzing recruitment patchiness of mussels, and has implications for sustainable management of P. perna.
Morphology of the foolish mussel(Mytilus trossulus) : variation and defense
2004
Mussels (Mytilus spp.) are dominant members of the rocky intertidal ecosystem, where interactions with predators and competitors are well documented. However, little is known about how variation in defensive morphological traits of mussels affects the outcome of interactions, or how predation affects the variation observed. In this thesis, I show thaim~rpholo~ical diversity in foolish mussels (M trossulus) in Howe Sound, British Columbia, is potentially important to interactions with mussel predators of the rocky intertidal community. Byssal attachment strength is positively related to the presence of crab and seaduck predators, but variation in several other morphological features is not easily attributable to predators. In addition, I experimentally demonstrate that seastars preferentially consume mussels with uneven shell margins and small crabs preferentially consume thinner-shelled mussels. However, large crabs either have no preference, or consume more thick-shelled mussels. F...