Distribution of hydroids along fronds of the kelp Ecklonia radiata

A full understanding of life history characteristics of invasive species is a fundamental prerequisite for the development of management strategies. Two introduced goby species (Rhinogobius cliffordpopei and Rhinogobius giurinus) have established highly abundant populations in Lake Erhai (China). In the present study, we examined the reproductive biology of these two species with the aim of improving the efficiency of management strategy. The results indicated that R. cliffordpopei spawned from February to June, whereas R. giurinus spawned from April to August. Rhinogobius cliffordpopei showed higher gonado-somatic indices and had larger eggs than R. giurinus. The adult sex ratio of R. cliffordpopei was female skewed, but that of R. giurinus was equal. Rhinogobius cliffordpopei showed a male-skewed sexual size dimorphism, whereas the body size of R. giurinus varied only slightly between males and females. The different reproductive traits appear as a crucial biologic aspect for developing control programs. Specifically, control measures should be implemented and/or intensified from September to February for R. cliffordpopei and from January to April for R. giurinus. The body size of the smaller R. cliffordpopei females is the determinant for minimal mesh size of the nets used in physical removals of R. cliffordpopei.     

Phlorotannins versus other factors affecting epiphyte abundance on the kelp Ecklonia radiata

We examined factors affecting the abundance and distribution of epiphytes (fouling) on the sublittoral kelp Ecklonia radiata. We first assessed the importance of phlorotannins as chemical defences against epiphytes by (a) correlating epiphyte loads on different parts of the thallus with the phlorotannin content of those tissues, and (b) experimentally testing the effects of variation in phlorotannin concentration against the settlement and growth of gametes of Ulva lactuca, a common epiphyte in the system. Tissue phlorotannin content was, at best, only weakly related to epiphyte loads, with r ² values typically <0.10. Inhibition of Ulva gametes only occurred at concentrations >10 mg l⁻¹, which is 5 orders of magnitude greater than levels of phlorotannins in the water column around beds of E. radiata, and 1–3 orders of magnitude greater than estimated levels in the boundary layer at the surface of the plant. We concluded that phlorotannins have a negligible impact on patterns of epiphytism on E. radiata, and proceeded to investigate other factors influencing the distribution and abundance of epiphytes. In our samples the relative age of different parts of the thallus was strongly correlated with epiphyte abundance, with epiphyte densities greatest on the oldest tissue and least on the youngest. Distal parts of the thalli also had greater epiphyte loads than basal parts. Field experiments in which kelp tissue was suspended at two heights in an E. radiata forest for varying lengths of time confirmed the importance of the length of time that the tissue was in the water, and its height in the water column, to the development of an epiphyte community. Comparison of epiphyte loads on tissue from primary (smooth) and secondary (rough) laminae in these experiments indicated that surface rugosity also affected fouling. Macroherbivores were rare on E. radiata, and abundances of mesofauna and epiphytes were positively related, suggesting that grazers were not important determinants of patterns of epiphyte abundance. Although phlorotannins have been previously suggested to play an important role as defences against epiphytes, we suggest that water-soluble compounds such as phlorotannins are less likely to be effective defences against epiphytes than non-polar metabolites, which can adhere to the surface of the producing organism.     

Seasonal and spatial variation in photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient

Photosynthesis Research - Understanding the photoacclimation response of macroalgae across broad spatial and temporal scales is necessary for predicting their vulnerability to environmental changes...     

Seasonal variation in the relationship between growth rate and phlorotannin production in the kelp Ecklonia radiata

Most theories for the evolution of plant chemical defences assume that defences are costly. In order to investigate the potential cost of phlorotannin production in the brown alga (kelp) Ecklonia radiata, I measured the correlation between changes in phlorotannin levels and growth rates for E. radiata in two seasons, spring and autumn. In spring, when both phlorotannin levels and growth rates in E. radiata were highest, there was a significant negative relationship between changes in phlorotannin levels and growth, consistent with a cost of phlorotannin production. No relationship was evident in autumn, indicating that moderate levels of phlorotannins and growth could be maintained simultaneously. These and related results suggest that the cost of defence will vary as a function of (1) endogenous patterns of growth or metabolite production and (2) variation in available resources. Evidence for a cost of phlorotannins in E. radiata adds to the paradox of the maintenance of high levels of phlorotannins in Australasian brown algae.     

Characterisation of microsatellite loci in the habitat-forming kelp, Ecklonia radiata (Phaeophyceae, Laminariales)

Habitat forming algae play an important role in the ecology of temperate reefs worldwide. Despite this, our understanding of levels of gene flow within and among populations of algae is largely limited to studies on intertidal species; we know comparatively little about important habitat-forming subtidal algae. Here, we develop eight polymorphic microsatellite markers for the characterisation of population genetic diversity and structure in the subtidal kelp, Ecklonia radiata. This large macroalga is the most abundant habitat-forming kelp on the subtidal rocky reefs of temperate Australia and New Zealand where it forms extensive forests that support an astounding diversity of associated taxa.     

Holdfast aggregation in relation to morphology,age, attachment and drag for the kelp Ecklonia radiata

This study quantified the prevalence of holdfast aggregation (fusion of holdfasts) for the kelp Ecklonia radiata on subtidal reefs in southwestern Australia, and tested whether morphology, age, attachment or drag were different between kelps growing alone (solitary) or in aggregates. Wave-sheltered in-shore reefs consistently had fewer aggregates than wave-exposed off-shore reefs (15–20% versus 20–30%). On average, individual thalli from aggregates were longer (97.8 cm ± 2.2 S.E. versus 88.0 cm ± 2.0 S.E.) and had smaller holdfasts (32.9 g fresh wt ± 1.7 S.E. versus 45.8 g fresh wt ± 2.9 S.E.) than solitary thalli, whereas there were no significant differences in other morphological characters, including total biomass (805.1 g fresh wt ± 38.7 S.E. versus 831.5 g fresh wt ± 38.5 S.E.), stipe length (7.93 cm ± 0.47 S.E. versus 7.65 cm ± 0.40 S.E.) and stipe diameter (12.6 mm ± 0.23 S.E. versus 13.0 mm ± 0.25 S.E.). There was no difference in age between solitary (2.7–3.0 years) and aggregated (2.4–2.8 years) individuals. While the attachment force of whole aggregates (256.5 N ± 21.6 S.E.) was found to be significantly larger than attachment force for solitary individuals (162.5 N ± 12.9 S.E.), attachment areas were also larger for aggregates (90.7 cm² ± 5.40 S.E. versus 64.3 cm² ± 5.54 S.E.) and consequently there were no differences in attachment strength between aggregates (2.92 N cm⁻² ± 0.26 S.E.) and solitary thalli (2.71 N cm⁻² ± 0.22 S.E.). Aggregates had significantly smaller (17%) roughness factors (equivalent to drag coefficients) than solitary individuals and a negative relationship (r = −0.68) between roughness factors and biomass suggested that this was related to the scope for compaction and rearrangement of the thalli. Further, there was no relationship between roughness factors of solitary individuals and the aggregates they produced when combined, suggesting that roughness factors are not additive or multiplicative. The spatial distribution of holdfast aggregates, the morphological differences between solitary and aggregated individuals as well as their attachment and drag characteristics were all consistent with aggregation reducing the rate of fatal kelp dislodgment.     

Detrital carbon production and export in high latitude kelp forests

Oecologia - The production and fate of seaweed detritus is a major unknown in the global C-budget. Knowing the quantity of detritus produced, the form it takes (size) and its timing of delivery are...     

Biological interactions maintain the boundaries between kelp forests and urchin barrens in the Aleutian Archipelago

This study used benthic surveys and manipulative experiments to examine (1) if boundaries between kelp forests and urchin barrens exist at multiple locations spanning the Aleutian Archipelago, (2) if these boundaries are spatially stable, and (3) how changes in algal density within the kelp forests influence the ability of urchins to invade them. Our results demonstrate that sharply punctuated kelp forest-urchin barren boundaries occur throughout the Archipelago, and they are spatially stable for at least 2 years. Further, when all macroalgae were experimentally removed from the kelp forest side of the boundaries, urchins rapidly moved into these clearings and excluded macroalgae for up to 2 years. However, these movements were not observed where 75% or less of the macroalgae was removed (leaving 25% or more in place), suggesting that even low macroalgal abundances can prevent urchins from invading the kelp forests. Further, urchin densities were negatively related to kelp density, again indicating that kelp can reduce urchin densities. While the ability of urchins to overgraze kelp forests is widely known, our results indicate that kelp can inhibit urchins, that these inhibitory influences are a widely recurrent phenomenon, and that this interaction is important to maintaining kelp forests across the Aleutians.     

Predator diversity strengthens trophic cascades in kelp forests by modifying herbivore behaviour

Although human-mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat-providing species.     

Habitat characteristics can influence fish assemblages in high latitude kelp forests

Kelp forests are patchy fish-associated habitats, which can vary greatly in their size, foundation species, and several physical habitat attributes. The structure of fish assemblages can vary with these characteristics and with the location of the assemblage within the forest, i.e. edges versus interiors. This study quantified the biological and physical heterogeneity within different sized kelp forests and identified which factors are important in structuring the associated fish assemblages. Fish and habitat surveys were conducted at the edge and interiors of ten kelp forests of varying sizes in Kachemak Bay, south-central Alaska. Fish assemblage structure was not correlated with the species composition of surface canopy forming foundation kelps (Nereocystis leutkeana and Eualaria fistulosa) or with kelp forest size. Instead, it correlated with the abundances of two understory foundation kelps (Agarum clathratum and Saccharina latissima), substratum rugosity, and water depth. Together these benthic attributes explained 53.6 % of the fish assemblage variability. Additionally, significantly different fish assemblages were found at edge compared to interior locations with the relative abundance of seven species (Artedius fenestralis, Ammodytes hexapterus, Blepsias cirrhosis, Gadus macrocephalus, Hexagrammos stelleri, Pholis laeta, and Sebastes melanops) explaining 91.4 % of the variability. This study highlights the importance of habitat characteristics such as understory foundation species, substratum rugosity, water depth and location within a patch on the variability of fish assemblages in high latitude kelp forests.     

Stability of strong species interactions resist the synergistic effects of local and global pollution in kelp forests

Foundation species, such as kelp, exert disproportionately strong community effects and persist, in part, by dominating taxa that inhibit their regeneration. Human activities which benefit their competitors, however, may reduce stability of communities, increasing the probability of phase-shifts. We tested whether a foundation species (kelp) would continue to inhibit a key competitor (turf-forming algae) under moderately increased local (nutrient) and near-future forecasted global pollution (CO(2)). Our results reveal that in the absence of kelp, local and global pollutants combined to cause the greatest cover and mass of turfs, a synergistic response whereby turfs increased more than would be predicted by adding the independent effects of treatments (kelp absence, elevated nutrients, forecasted CO(2)). The positive effects of nutrient and CO(2) enrichment on turfs were, however, inhibited by the presence of kelp, indicating the competitive effect of kelp was stronger than synergistic effects of moderate enrichment of local and global pollutants. Quantification of physicochemical parameters within experimental mesocosms suggests turf inhibition was likely due to an effect of kelp on physical (i.e. shading) rather than chemical conditions. Such results indicate that while forecasted climates may increase the probability of phase-shifts, maintenance of intact populations of foundation species could enable the continued strength of interactions and persistence of communities.     

Variation in the degree of specialization can maintain local diversity in model communities

We hypothesize that the continuum between generalist and specialist adaptations is an important general trade-off axis in the maintenance of local diversity, and we explore this idea with a simple model in which there are patch types to which species arrive as propagules and compete. Each patch type is defined by a competitive ranking of all species. A highly specialist species is the top competitor in one patch type but has a relatively low average ranking across different patch types, while a generalist species has a high average rank across patch types but is not the top competitor in any patch type. We use random dispersal and vary the fecundity of all species together to vary total propagule density and therefore recruitment limitation and density-dependent mortality. When fecundity is very high, each patch becomes occupied by its specialist species and generalists go extinct, so the number of species at equilibrium is equal to the number of patch types. If fecundity is very low, generalists dominate and specialists go extinct. There is a range of fecundity levels in which specialists, generalists, and intermediates coexist, and the number of species is substantially greater than the number of patch types. While coexistence of specialists and generalists has been considered a problem in evolutionary ecology, our results suggest to the contrary that this trade-off contributes to the maintenance of local diversity.     

Thoughts on the processes that maintain local species diversity of ectomycorrhizal fungi

Ectomycorrhizal fungi exhibit high diversity even in small monoculture forests. Roughly 20 to 35 species typically occupy such sites. Explanations for this diversity can be based on resource partitioning, disturbance, competition, or interaction with other organisms. Mycorrhizal fungi compete for two general classes of resources: host-derived carbon and soil or detritus derived mineral nutrients. Both types of resources are arrayed in space (e.g., soil depth, distance from tree) and time (e.g., season, host successional series). Some species seem to be partitioned in space and time at these scales, but the question of how widespread these patterns are remains largely unanswered. Mineral resources are distributed in discrete substrates in soil, litter, and within other soil microorganisms; the biochemical diversity exhibited by fungi may translate into differences in access to these resources among species. Small-scale natural disturbances that sever roots, mix soil horizons and litter layers, or change local pH and nutrient availability, are likely to create additional habitats for ectomycorrhizal fungi. Evidence from fruiting patterns and differences in colonization strategies suggest that such disturbances may be important for establishment of some species. Competitive replacement networks among species have the theoretical potential to increase diversity. The frequency of species replacements, observed co-infections of ectomycorrhizal fungi on single host roots, and high rates of rootlet turn-over all suggest that competition is important, but whether it plays a creative role in maintaining diversity remains to be demonstrated. Other organisms could be important in the maintenance of diversity, if they effect competition among mycorrhizal fungi. Bacteria and soil invertebrates are the most likely groups for such interactions. Technological advances in root observation and PCR methods for indentification of mycorrhizae make many of these theories testable.     

Post-glacial redistribution and shifts in productivity of giant kelp forests

Quaternary glacial–interglacial cycles create lasting biogeographic, demographic and genetic effects on ecosystems, yet the ecological effects of ice ages on benthic marine communities are unknown. We analysed long-term datasets to develop a niche-based model of southern Californian giant kelp (Macrocystis pyrifera) forest distribution as a function of oceanography and geomorphology, and synthesized palaeo-oceanographic records to show that late Quaternary climate change probably drove high millennial variability in the distribution and productivity of this foundation species. Our predictions suggest that kelp forest biomass increased up to threefold from the glacial maximum to the mid-Holocene, then rapidly declined by 40–70 per cent to present levels. The peak in kelp forest productivity would have coincided with the earliest coastal archaeological sites in the New World. Similar late Quaternary changes in kelp forest distribution and productivity probably occurred in coastal upwelling systems along active continental margins worldwide, which would have resulted in complex shifts in the relative productivity of terrestrial and marine components of coastal ecosystems.     

The effects of harvesting of the South African kelp (Ecklonia maxima) on kelp population structure, growth rate and recruitment

Ecklonia maxima is an economically important kelp in South Africa. The harvested kelp is used mainly as feed for cultured Haliotis midae (abalone) on farms all along the South African South and West Coast. The effects that different harvesting methods have on the growth of sub-canopy kelps, kelp population structure and kelp recruitment were tested in a kelp bed at Bordjies Rif near Cape Town. Two 30 × 10 m sites were set up, about 100 m apart, in near monoculture stands of E. maxima. Each 30×10 m area was subdivided into three treatments. In treatment 1 (T1) the whole ‘head’ of each kelp sporophyte that reached the surface was cut off between the bulb and the primary blade (‘lethal’ method). In treatment 2 (T2) (‘non-lethal’ method), the secondary fronds of all sporophytes that reached the surface were cut 20–30 cm from their junction with the primary blade, and removed. In the control plot, the kelp plants were not treated. Harvesting treatments were done approximately every four months, at low spring tide, from 3 March 2003 to 3 November 2003 (three treatments). The effects of harvesting on the kelps depend largely on the size of plant and the time the fronds were removed; however, no seasonal pattern could be observed. The different treatments had no effect on the growth rate, population structure or recruitment of the kelp. This means that factors other than light play an important role in the growth, structure and recruitment of the kelp beds in False Bay. Results are discussed in relation to current commercial harvesting practices.     

Composition and growth of subtidal parvosilvosa from Californian kelp forests

Summary 1. Ecological studies of the kelp forests of Pacific North America have understandably stressed the larger vegetational components. An important vegetational component, previously neglected, is a taxonomically complex turf or parvosilvosa (Gislén 1930), composed of small algae and microscopic reproductive or developmental forms of larger algae.2. This report considers the ecological importance of this smaller plant component within the framework of the overall vegetational structure. Both diving studies and the cultivation of experimental material in the laboratory under controlled conditions have been utilized to probe the questions involved.3. Rocks and shells covered with representative algal turf have been cultured in the laboratory in naturally or artificially-illuminated tanks provided with a continuous supply of filtered seawater, and the resulting species composition has been compared with that occurring in nature.4. Both luxuriant forms of previously collected material and new forms have been observed in these cultures. Certain of these algae grow extremely well in culture and in some cases continue to reproduce, generating successive crops of plants. Growth rates under laboratory conditions for a few species have been measured and approach what might be expected in the field.5. Elective culture results suggest that many benthic algae may well exist for periods of time as resting or resistant stages, awaiting favorable environmental conditions. Stages such as those postulated have been produced in the laboratory.6. Complete taxonomic or ecological surveys of subtidal areas, and particularly the deeper benthic regions near the lower limits of plant distribution would benefit from culture studies of the type described. Subtidal benthic algae also lend themselves readily to experimental manipulation, and the results obtained to date, although preliminary, suggest possible experimental approache to subtidal ecology.

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Komposition und Wachstum von Parvosilvosa aus dem untergetauchten Gezeitenbereich kalifornischer Tangwälder

Kurzfassung Als Parvosilvosa werden taxonomisch komplexe, rasenartige Pflanzenbestände bezeichnet, die aus kleinen Algen und mikroskopischen Entwicklungsstadien größerer Algen bestehen. Die vorliegende Untersuchung erörtert die ökologische Bedeutung dieser Pflanzenkomponente als Teil der örtlichen Gesamtflora; sie basiert auf Schwimmtauch-Studien und Laboratoriumsexperimenten. Mit Parvosilvosa bedeckte Felsstücke und Molluskenschalen wurden unter Laboratoriumsbedingungen natürlichem und künstlichem Licht ausgesetzt. In den Kulturen konnten sowohl luxurierende Formen von bereits aus dem Biotop bekanntem Material als auch neue Formen festgestellt werden. Einige dieser Algen wuchsen sehr gut in Kultur und schritten zur sexuellen Fortpflanzung. Offenbar existieren viele benthonische Algen zeitweise in Form von Ruhestadien. Unter günstigen Bedingungen — wie sie beispielsweise in unseren Kulturen geboten werden — wachsen sie dann zu adulten Pflanzen heran. Die Erfassung des gesamten Algenbestandes eines Biotops dürfte durch die hier angewandten Methoden erleichtert werden. Die durchgeführten Untersuchungen ergeben neue Ansatzpunkte für experimentelle Arbeiten im Litoral.

     

Disturbance and the structure of coral reef fish communities on the reef slope

Three levels of physical disturbance were applied to corals in permanent 10x10 m quadrats along a section of fringing reef at Lizard Island on the Great Barrier Reef to investigate the response of fish assemblages. Tabular and corymbose corals were overturned and left in situ, reducing total hard coral cover from ˜55% to ˜47%, ˜43%, and ˜34%. Despite pre-existing associations with benthic cover, all fish groups examined (pomacentrids, labrids, chaetodontids, and acanthurids) were resistent to benthic disturbances at the level and scale at which they were applied. Partial Mantel's tests, in combination with partial Canonical Correspondence Analysis enabled spatial and temporal variation to be factored out from experimental effects. Most of the variation in the fish community could be assigned to spatio-temporal variables, indicating that spatial structure over the reef landscape may moderate localised disturbance effects. This study indicates that coral reef fish assemblages may be more resistant to disturbance than many correlative studies would suggest, and highlights a need for further information on levels and scales of natural habitat disturbance in order to apply a structured approach to the experimental investigation of the importance of habitat in structuring coral-reef fish assemblages.     

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity--and thus information on the relevance of experimental findings--for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25-69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis--derived from studies in less diverse ecosystems--of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.