Keiichi Fukaya

Seasonality in the strength and spatial scale of processes determining intertidal barnacle population growth

1. Population growth rate is determined by both density-dependent and density-independent processes. In the temperate zone, the strength and spatial scale of these processes are likely to differ seasonally, but such differences have rarely been quantitatively examined. 2. Coverage, the area occupied by organisms, is a measure of resource use in sessile marine populations. Population models used for density-based studies should be able to characterize effectively fluctuations in coverage, but few have tried to apply such models to sessile populations. 3. We observed coverage of the intertidal barnacle Chthamalus challengeri at 20 plots on four shores along the Pacific coast of Japan over 8 years. We then fitted a population model that incorporated both a density-dependent process (strength of density dependence) and density-independent processes (intrinsic growth rate and stochastic fluctuation at different spatial scales) to these data to analyse the seasonal variation of these processes and answer the following two questions: (i) How do the effects of density-dependent and density-independent processes on population growth vary seasonally? (ii) At what spatial scale, regional (tens of kilometres), shore (hundreds of metres), or rock (tens of centimetres), does density-independent stochastic fluctuation most strongly affect population size changes? 4. Barnacle population size tended to decrease in summer, when population dynamics were characterized by a relatively lower intrinsic growth rate, weaker density dependence and stronger stochastic fluctuation. In contrast, population size tended to increase in winter, reflecting a higher intrinsic growth rate, strong density dependence and weak stochastic fluctuation. 5. In summer, population growth rate was strongly affected by regional-scale stochastic fluctuation, whereas in winter it was affected more by rock-scale stochastic fluctuation, suggesting that populations were strongly affected by regional-scale processes in summer but not in winter. 6. These results indicate that seasonally variable density-dependent and density-independent processes determine the population dynamics of C. challengeri. Therefore, to understand fluctuation patterns of populations of this species, seasonality should be taken into account. Moreover, this study demonstrates that population models commonly used for density-based studies are also applicable to coverage-based population studies.

Variable processes that determine population growth and an invariant mean‐variance relationship of intertidal barnacles

Although researchers recognize that population dynamics can vary in space and time as a result of differences in biotic and abiotic conditions, spatial and temporal variability in the patterns and processes of population dynamics have not been well documented on a seasonal time frame. We quantified seasonal changes in the coverage of intertidal barnacles, Chthamalus spp., with data collected for as many as 9 years at 88 plots in five regions located along more than 1800 km of the Pacific coastline of Japan from 31° N to 43° N. To examine how seasonal changes and the spatial heterogeneity of environments can interact to influence patterns and processes of population dynamics, we analyzed the data with two models of population variability: a population dynamics model, which provides knowledge about processes that determine population growth rates; and Taylors power law, which summarizes the relationship between the temporal mean and variance of the size of a population (temporal mean-variance relationship). We found that seasonal differences were prevalent in population growth rates, as well as in the strength and spatial scales of processes that determine population growth rates. In addition, the seasonality of these rates and processes varied between habitats at different spatial scales ranging from the scale of among-rocks within a shore to that of among-regions located in different latitudes, suggesting that the effects of seasonal environmental fluctuations on population growth can depend on the spatial heterogeneity of biotic and abiotic conditions that vary at multiple spatial scales. In contrast, the evidence for spatiotemporal differences in temporal mean-variance relationships was weak. Unlike theoretical expectations, spatiotemporal differences in the variability of population size were best explained by a unique power law, despite remarkable regional and seasonal differences in the processes that determine population growth rates. These results suggest that spatiotemporal environmental variability can affect population dynamics at multiple spatial scales but do not necessarily alter the scaling law of population size variability.

Heritability of male mandible length in the stag beetle Cyclommatus metallifer

Numerous coleopteran species express male‐specific “weapon traits” that often show size variations among males, even within a single population. Many empirical studies have demonstrated that environmental conditions during development affect absolute weapon size. However, relatively few studies in horned beetles support the hypothesis that the relationship between weapon size and body size, also referred to as a “scaling relationship” or “static allometry”, is largely determined by genetic factors. In this study, the heritability of absolute mandible length and static allometry between mandible length and body size were estimated in the stag beetle Cyclommatus metallifer. While no significant heritable variation was observed in absolute mandible length, high heritability (h2 = 0.57 ± 0.25) was detected in the static allometry between mandible length and body size. This is the first report on the genetic effect on male mandible size in Lucanidae, suggesting that absolute mandible size is largely determined by environmental conditions while the static allometry between weapon size and body size is primarily determined by genetic factors.

Effects of spatial structure of population size on the population dynamics of barnacles across their elevational range

Explanations for why population dynamics vary across the range of a species reflect two contrasting hypotheses: (i) temporal variability of populations is larger in the centre of the range compared to the margins because overcompensatory density dependence destabilizes population dynamics and (ii) population variability is larger near the margins, where populations are more susceptible to environmental fluctuations. In both of these hypotheses, positions within the range are assumed to affect population variability. In contrast, the fact that population variability is often related to mean population size implies that the spatial structure of the population size within the range of a species may also be a useful predictor of the spatial variation in temporal variability of population size over the range of the species. To explore how population temporal variability varies spatially and the underlying processes responsible for the spatial variation, we focused on the intertidal barnacle Chthamalus dalli and examined differences in its population dynamics along the tidal levels it inhabits. Changes in coverage of barnacle populations were monitored for 10.5 years at 25 plots spanning the elevational range of this species. Data were analysed by fitting a population dynamics model to estimate the effects of density-dependent and density-independent processes on population growth. We also examined the temporal mean-variance relationship of population size with parameters estimated from the population dynamics model. We found that the relative variability of populations tended to increase from the centre of the elevational range towards the margins because of an increase in the magnitude of stochastic fluctuations of growth rates. Thus, our results supported hypothesis (2). We also found that spatial variations in temporal population variability were well characterized by Taylors power law, the relative population variability being inversely related to the mean population size. Results suggest that understanding the population dynamics of a species over its range may be facilitated by taking the spatial structure of population size into account as well as by considering changes in population processes as a function of position within the range of the species.

Recovery of rocky intertidal zonation: two years after the 2011 Great East Japan Earthquake

To assess the course and status of recovery of rocky intertidal zonation after massive subsidence caused by the 2011 Great East Japan Earthquake, from 2011 to 2013 we censused the vertical distribution of 10 dominant macrobenthic species (six sessile and four mobile species) in the mid-shore zone of 23 sites along the Sanriku coastline, 150–160 km north-northwest of the earthquake epicentre, and compared the vertical distributions of each species with their vertical distributions in the pre-earthquake period. The dynamics of rocky intertidal zonation varied substantially among species. Among sessile species, one barnacle dramatically increased in abundance and expanded its vertical range in 2011, but then decreased and completely disappeared from all plots by 2013. Zonations of other sessile species shifted downward following the subsidence in 2011. With some species, there was no clear change in abundance immediately after the earthquake, but they then began to increase and move upward after a few years; with other species, abundance continuously decreased. There was no clear change in the vertical distribution of any of the mobile species immediately after the earthquake. Abundance of two mobile species was unchanged, but abundance of the others decreased from 2012 and had not recovered as of 2013.

Rocky Intertidal Zonation: Impacts and Recovery from the Great East Japan Earthquake

We assessed the course and status of the recovery of rocky intertidal zonation after the Great East Japan Earthquake by conducting a census of the vertical distribution of 11 dominant macrobenthos (7 sessile and 4 mobile species) around the mid-tidal elevation at 23 sites along the Sanriku Coast, 150–160 km north-northwest of the epicenter of the earthquake. Sites were observed from 2011 to 2013, and the vertical distributions of each species were compared with those from the pre-earthquake period. Our results show that the earthquake considerably altered rocky intertidal zonation, mainly through coseismic subsidence rather than by the subsequent tsunami. By 28 months after the earthquake, the zonation of late-successional sessile taxa had not recovered, suggesting that the rocky intertidal community will experience a long delay before recovering from the influence of the earthquake. The dynamics of rocky intertidal zonation after the earthquake and accompanying subsidence includes two unique features: a delayed negative impact and an occasional increase in population sizes of several taxa. Neither of which has been reported following earthquakes of similar magnitude with accompanying uplift, in which there were mass mortalities of zone-forming species within 1 year after the event, preceding downward shifts in their zonations.

Rocky Intertidal Barnacle Population Dynamics: Impacts and Recovery from the Great East Japan Earthquake

The abundances and distributions of rocky intertidal sessile organisms were considered to be changed immediately as a direct result of either the tsunami or the subsidence and then further altered by changes in population processes such as mortality and recruitment. We investigated the influence of the Great East Japan Earthquake on the population dynamics of a rocky intertidal barnacle, Chthamalus challengeri, especially focusing on changes in vertical distribution, as well as alteration of recruitment and its causes and consequences. Light after the earthquake, C. challengeri zonation shifted vertically downward 50 cm, but the abundance did not clearly changed, suggesting that the barnacle did not suffer immediate mortality from either the tsunami or the subsidence. After the earthquake, the zone of C. challengeri expanded upward, and the species’ abundance increased over the entire study area until 2012. Then the abundance below and around the lower limit of the species’ normal vertical range decreased, and consequently, the zonation returned almost to the state before the earthquake by July 2013. During the study period, the population dynamics of this barnacle were driven in complex ways by interactions among larval supply, density-dependent intraspecific interactions, and interspecific competition, which were affected by factors including free-space availability for larval setting and the densities of conspecifics and competing species, of all which varied with elevation and time after the earthquake. One important finding is that the earthquake altered larval supply at the metapopulation level by changing the benthic population size.

Quantitative Evaluation of the Impact of the Great East Japan Earthquake and Tsunami on the Rocky Intertidal Community

It has been widely assumed that tsunamis associated with mega-earthquakes severely damage coastal ecosystems. In no previous studies, however, have the community-level impacts of tsunamis been credibly quantified. We evaluated the impacts of the tsunami following the Great East Japan Earthquake (the 2011 Tohoku earthquake tsunami) on species abundances in a rocky intertidal metacommunity encompassing 30 km of shoreline located 150–160 km north-northwest of the epicenter of the earthquake. For this purpose, we generated an indicator of the tsunami impact—the change in mean abundances between before and after the tsunami, standardized to population variability before the earthquake—for each of five sessile and six mobile species. We then used the averaged values of this indicator as a quantitative measure of tsunami impacts for the entire community and for the sessile and mobile subgroups. The tsunami had an overall significant negative impact on the rocky intertidal community at the regional scale, although the negative impact seemed smaller than those of similar large-scale disturbances on rocky intertidal communities. There was a significant negative impact of the tsunami on mobile species, but not on sessiles. The results suggest that mobiles on rocky shores are more vulnerable than sessiles to tsunamis. At a regional scale, the rocky intertidal community suffered minor damage from the 2011 Tohoku earthquake tsunami.

Seasonal changes in community structure along a vertical gradient: patterns and processes in rocky intertidal sessile assemblages

Here we considered two fundamental questions in community ecology regarding the relationship between seasonal changes in community structure and environmental gradients: (i) How does the magnitude of seasonal changes in community structure vary along an environmental gradient? (ii) How do the processes driving seasonal changes in community structure vary along an environmental gradient? To examine these questions, we investigated intertidal sessile assemblages inhabiting a notable vertical environmental gradient and fitted a transition probability matrix model to decadal time series data gathered at 25 plots along the Pacific coast of eastern Hokkaido, Japan. We found that the magnitude of seasonal changes in community structure was the largest at mid shore. The major processes driving seasonal changes in community structure changed vertically, reflecting the indirect influence of vertical changes in the physical environment on the vertical distributions of species. An unexpected finding was that the magnitude of seasonal changes in community structure did not reflect the strength of seasonal variation in the physical environment. One explanation may be that sessile organisms living on the high shore have a broad tolerance to environmental stress and are thus less sensitive to the large seasonal variation in physical stress.

A multistate dynamic site occupancy model for spatially aggregated sessile communities

Markov community models have been applied to sessile organisms because such models facilitate estimation of transition probabilities by tracking species occupancy at many fixed observation points over multiple periods of time. Estimation of transition probabilities of sessile communities seems easy in principle but may still be difficult in practice because resampling error (i.e., a failure to resample exactly the same location at fixed points) may cause significant estimation bias. Previous studies have developed novel analytical methods to correct for this estimation bias. However, they did not consider the local structure of community composition induced by the aggregated distribution of organisms that is typically observed in sessile assemblages and is very likely to affect observations. In this study, we developed a multistate dynamic site occupancy model to estimate transition probabilities that accounts for resampling errors associated with local community structure. The model applies a nonparametric multivariate kernel smoothing methodology to the latent occupancy component to estimate the local state composition near each observation point, which is assumed to determine the probability distribution of data conditional on the occurrence of resampling error. By using computer simulations, we confirmed that an observation process that depends on local community structure may bias inferences about transition probabilities. By applying the proposed model to a real dataset of intertidal sessile communities, we also showed that estimates of transition probabilities and of the properties of community dynamics may differ considerably when spatial dependence is taken into account. Our approach can even accommodate an anisotropic spatial correlation of species composition, and may serve as a basis for inferring complex nonlinear ecological dynamics.