Per Åberg

A continental scale evaluation of the role of limpet grazing on rocky shores

It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17° of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.

Spatial and temporal variation in settlement and recruitment of the intertidal barnacle Semibalanus balanoides (L.) (Crustacea : Cirripedia) over a European scale

Variation in the level of settlement and recruitment in the intertidal barnacle Semibalanus balanoides was studied using a hierarchical sampling programme. The effect of three spatial scales, 10s of metres (sites), 1000s of metres (shores) and 100s of kilometres (locations), was determined. The largest spatial scale represented the distance between four widely separated locations, Sweden, the Isle of Man, SW Ireland and SW England, covering a large part of the range of S. balanoides in Europe. Temporal variation was determined by comparison between two years, 1997 and 1998. The settlement period of S. balanoides varied in length and timing, being earlier and shorter at the most northerly location, Sweden. The duration of settlement showed little difference among shores within locations, but the pattern of settlement did vary. Estimates of total settlement throughout the settlement period and of recruitment at the end of this period both showed substantial variation among locations which was dependent on the year of study. There was little consistency in the ranking of locations between the two years. Recruitment showed significant variation at the lower spatial scales of shore and site. In addition, examination of variance components showed a high degree of variation between replicates within sites in 1997. There was a significant relationship between settlement and recruitment at three of the four locations. Across all locations variation in settlement explained between 29 and 99% of variation in recruitment. However, locations showed distinct differences in the level of post-settlement survival.

Habitat and feeding preferences of crustacean mesoherbivores inhabiting the brown seaweed Ascophyllum nodosum (L.) Le Jol. and its epiphytic macroalgae

Abstract Habitat and feeding preferences of crustacean mesoherbivores inhabiting the brown alga Ascophyllum nodosum (L.) Le Jol. and its macroepiphytes were examined on the Swedish west coast. After an initial survey of the epifauna, three species of mesoherbivores, the isopods Idotea granulosa Rahtke and Jaera albifrons Leach, and the amphipod Gammarus locusta (L.), were selected for further studies. The role of macroepiphytes in the distribution, abundance and recolonization rates of the mesoherbivores were investigated in observational and manipulative field experiments. The results from these experiments were compared with results from multiple-choice feeding experiments in order to see if the distribution pattern of the mesoherbivores was related to, or independent of, feeding preferences. The epifauna was dominated in numbers by gammarid amphipods, but the most abundant crustacean species in terms of biomass was the isopod I. granulosa. The field experiments showed that macroepiphytes had large effects on the distribution and abundance of the amphipod G. locusta, with 5–8 times higher mean densities on A. nodosum plants with macroepiphytes. The effect of macroepiphytes on the abundance of I. granulosa varied strongly among sites, and for the smaller isopod J. albifrons no effects could be observed. The manipulative field experiments showed that complete recolonization by the mesoherbivores on defaunated A. nodosum plants was rapid, occurring on a time-scale of hours or days. Natural densities on plants with and without macroepiphytes were already found after the first 24 h for G. locusta, implying that the distribution pattern was the result of habitat selection by the amphipod, rather than differential predation. Feeding preferences differed among, as well as within, mesoherbivore species. Large individuals of G. locusta clearly preferred to feed on macroepiphytes, while large individuals of I. granulosa mostly grazed on meristematic apices of A. nodosum. Small individuals of both species grazed on both A. nodosum and its macroepiphytes, with a preference for the epiphytes. For J. albifrons no significant feeding preferences could be detected. The general conclusion of this study was that macroepiphytes are important as habitat and food for some of the crustacean mesoherbivores inhabiting A. nodosum, but that large variations in mesoherbivore–epiphyte interactions occur among and within species. Furthermore, the results supported the conception that many crustacean mesoherbivores display large mobility.

A DEMOGRAPHIC STUDY OF TWO POPULATIONS OF THE SEAWEED ASCOPHYLLUM NODOSUM

Very little is known about the demography in size-structured seaweed pop- ulations, and this is especially true for populations in variable environments. Thus, the demography of the brown alga Ascophyllum nodosum was analyzed with a matrix popu- lation model. This was built on a 3-yr study in two populations on the Swedish west coast, where > 1 100 individuals were marked and followed twice each year. The environmental variability is due to the presence of ice during some of the winters. The frequency of ice years in the study area is known, and is higher at one site compared to the other. During the study there were 2 yr with ice and one without at both sites, and these temporal changes in the environment resulted in large variations in the vital rates of A. nodosum. The individuals were divided into five size classes and the population dynamics at years without ice were characterized by low mortality rates and high transition probabilities for growth to larger sizes, while years with ice had high mortality rates and high transition probabilities for breakage to smaller sizes. Of the 25 possible transitions in the life cycle graph all except 2 had nonzero entries, which means that the adult life of A. nodosum individuals can be described as plastic growth between all five size classes. A crude estimate of the recruitment showed that both populations will increase in numbers in ice-free years, but will decrease in years with ice. The size-dependent fertility rate is probably subject to errors, and thus the survival matrices were scaled with a fertility function built on total reproductive biomass per individual, allowing analyses of the demography at different levels of the asymptotic growth rate XA. One of the main differences between the populations was that the stable size distribution for survival matrices in ice-free years was dominated by size class 4 at one population but class 5 at the other. Thus, to achieve the same value of XA higher fertility rates were needed at the population dominated by class 4. Elasticity analysis of the transition matrices showed the same trend, although the main result from that analysis was that growth to larger sizes or remaining in the same class contributed more to X, than repro- duction. This was valid for all levels of XA investigated.

Interactions between wave action and grazing control the distribution of intertidal macroalgae

Canopy-forming macroalgae are key species on temperate rocky shores. However, there is a lack of understanding of how the relative balance of physical and biological factors controls the establishment and persistence of intertidal macroalgae. Here we present an integrated study of the relative importance of wave-induced forces and grazing for the recruitment and survival of the canopy-forming intertidal macroalgae Fucus vesiculosus and F. spiralis. A set of overtopped breakwaters provided a nearly unconfounded gradient in wave exposure between seaward and landward sides. A biomechanical analysis was performed based on empirical measurements of maximum drag forces in breaking waves, a model of long-term maximum wave height, and the breaking stress of Fucus spp. The estimated maximum flow speed (7-8 m/s) on the seaward side of the breakwaters was predicted to completely dislodge or prune Fucus spp. larger than approximately 10 cm, while dislodgment was highly unlikely on the landward side for all sizes. Experimental transplantation of Fucus spp. supported the biomechanical analysis but also suggested that mechanical abrasion may further limit survival in wave-exposed locations. Experimental removal of the limpet Patella vulgata, which was the principal grazer at this site, resulted in recruitment of Fucus spp. on the seaward side. We present a model of limpet grazing that indicates that limpet densities >5-20 individuals/m2 provide a proximate mechanism preventing establishment of Fucus spp., whereas wave action >2 m/s reduces persistence through dislodgment and battering. In a conceptual model we further propose that recruitment and survival of juvenile Fucus spp. are controlled indirectly by wave exposure through higher limpet densities at exposed locations. This model predicts that climate change, and in particular an increased frequency of storm events in the northeast Atlantic, will restrict fucoids to more sheltered locations.

OPTIMAL DEFENSE THEORY: ELASTICITY ANALYSIS AS A TOOL TO PREDICT INTRAPLANT VARIATION IN DEFENSES

Optimal defense theory (ODT) predicts that plants should have the highest defense levels in parts that have the highest value in terms of fitness. Assumptions about differences in fitness value among plant parts have previously been based on general reasoning, e.g., that reproductive tissue is more valuable than vegetative tissue since fitness ultimately depends on reproduction. Here we present a first example of how demographic elasticity (proportional sensitivity) analysis can be used as a tool to generate more objective and specific estimates of the fitness value of different plant parts, based on the entire life cycle. These estimates were further used to generate testable predictions about intraplant variation in defenses of the brown seaweed Ascophyllum nodosum (knotted wrack). The predictions were clearly supported by the results from both analyses of chemical defense levels (phlorotannins) and bioassays (feeding preference experiments), which showed that reproductive tissues had the lowest defense level of the three examined tissue types, in contrast to previous assumptions. We suggest that a stronger focus on variation in life history among species and populations, with demographic perturbation analysis as one important tool, could lead to a better understanding of intraplant variation in defense levels, in seaweeds as well as vascular plants and other modular organisms.

Spatial variation in polyphenolic content of Ascophyllum nodosum (Fucales, Phaeophyta)

Spatial variation in polyphenolic content in annual shoots of the brown alga Ascophyllum nodosum was quantified using a hierarchical sampling design. Three sampling levels, covering distances of 100-106 m, were used. Comparisons were made between two areas, Tjarno on the Swedish west-coast and the Isle of Man in the Irish Sea, with very different types of environmental conditions. No significant differences in mean polyphenolic levels were found between the two study areas (6.6% of dry mass at Tjarno and 9.2% at the Isle of Man), whereas significant and substantial differences were found among sites within areas (range 5.7%—11.4%) and among quadrats within sites (range 3.7%—13.1%). The extensive variation at the smaller spatial scales points out the importance of using thorough sampling procedures at all levels in large-scale studies on algal polyphenolics, e.g. biogeographical comparisons, which have been neglected in several previous studies. Moreover, the results imply that experiments on causal factors of polyphenolic variation should be designed to explain the spatial scales on which the factors are important. This study also investigated the relationship between polyphenolic concentration and both plant size and mean area of annual shoots. The mean area was used as an estimate of the mean growth rate of the annual shoots within an individual. No significant relationships were found between shoot growth rate, or plant size, and polyphenolic levels in annual shoots at any of the three spatial scales that were investigated.

Population resistance to climate change: modelling the effects of low recruitment in open populations

Isolated populations or those at the edge of their distribution are usually more sensitive to changes in the environment, such as climate change. For the barnacle Semibalanus balanoides (L.), one possible effect of climate change is that unpredictable spring weather could lead to the mismatching of larval release with spring phytoplankton bloom, hence reducing the recruitment. In this paper, model simulations of a variable open population with space limited recruitment were used to investigate the effects of low and zero recruitment on population abundance in S. balanoides. Data for model parameters was taken from an isolated population in the Isle of Man, British Isles. Model simulations with observed frequencies of years with low recruitment showed only small changes in population dynamics. Increased frequencies of low recruitment had large effects on the variation in population growth rate and free space and on population structure. Furthermore, populations with intermediate to high frequencies of low recruitment appeared more sensitive to additional changes in recruitment. Exchanging low recruitment with zero recruitment severely increased the risk of local extinctions. Simulations with consecutive years of low recruitment showed a substantial increase in free space and an increase in the time taken to recover to normal densities. In conclusion, model simulations indicate that variable populations can be well buffered to changes in the demography caused by introduced environmental noise, but also, that intermediate to high frequencies of disturbance can lead to a swift change in population dynamics, which in turn, may affect the dynamics of whole communities.

Effects of wave exposure and depth on biomass, density and fertility of the fucoid seaweed Sargassum polyceratium (Phaeophyta, Sargassaceae)

Sargassum polyceratium is widely distributed around the island of Curaçao (Netherlands Antilles) where it inhabits strongly contrasting habitats. Changes in population structure have been followed in three bays with increasing levels of wave exposure at two depths: shallow (0 m) and deep (18 m). The effects of increasing wave exposure were investigated by studying three deep-water populations; and the effects of depth by studying shallow- and deep-water populations in the two calmer bays. Total density and stage density (reproductive thalli, juvenile thalli) were determined and total and individual thallus biomass was estimated non-destructively. In the most wave-exposed deep-water population thalli were twice as long with more than twice the biomass than in the calmest deep-water population. Total density and juvenile density were highest in the bay with intermediate wave exposure. Depth was an important factor at both the individual and population level. Shallow-water thalli had basal holdfast areas that were four times larger than those from deep water, and thallus biomass was positively correlated with holdfast area. Shallow-water juveniles invested more in the development of a holdfast and lateral growth than deep-water juveniles. Total biomass per quadrat was up to 10-fold higher in shallow- than in deep-water populations. In shallow-water populations reproductive thalli were present throughout the year whereas in deep-water populations they were present only during autumn and winter. We conclude that both wave exposure and depth affect population structure. Thalli were generally bigger and total biomass higher in the more exposed bay(s) and in shallower water, contradicting the general pattern in macroalgae of reduced size and biomass with increasing wave exposure.

PHENOTYPIC DIFFERENTIATION AT SOUTHERN LIMIT BORDERS: THE CASE STUDY OF TWO FUCOID MACROALGAL SPECIES WITH DIFFERENT LIFE-HISTORY TRAITS1

Marginal populations are often geographically isolated, smaller, and more fragmented than central populations and may frequently have to face suboptimal local environmental conditions. Persistence of these populations frequently involves the development of adaptive traits at phenotypic and genetic levels. We compared population structure and demographic variables in two fucoid macroalgal species contrasting in patterns of genetic diversity and phenotypic plasticity at their southern distribution limit with a more central location. Models were Ascophyllum nodosum (L.) Le Jol. (whose extreme longevity and generation overlap may buffer genetic loss by drift) and Fucus serratus L. (with low genetic diversity at southern margins). At edge locations, both species exhibited trends in life‐history traits compatible with population persistence but by using different mechanisms. Marginal populations of A. nodosum had higher reproductive output in spite of similar mortality rates at all life stages, making edge populations denser and with smaller individuals. In F. serratus, rather than demographic changes, marginal populations differed in habitat, occurring restricted to a narrower vertical habitat range. We conclude that persistence of both A. nodosum and F. serratus at the southern‐edge locations depends on different strategies. Marginal population persistence in A. nodosum relies on a differentiation in life‐history traits, whereas F. serratus, putatively poorer in evolvability potential, is restricted to a narrower vertical range at border locations. These results contribute to the general understanding of mechanisms that lead to population persistence at distributional limits and to predict population resilience under a scenario of environmental change.