Soyoka Muko

SPACE PREEMPTION, SIZE-DEPENDENT COMPETITION, AND THE COEXISTENCE OF CLONAL GROWTH FORMS

Interspecific interactions that produce nontransitive competitive networks have been proposed to promote diversity in a broad range of systems, including coral reefs. In this paper, we model the effect of size-dependent shifts in competitive ability on the coexistence of canopy-forming and understory coral species, and we predict how these shifts influence patterns of community structure along large-scale gradients in disturbance and recruitment limitation. We consider three models, representing a gradient from purely hierarchical competition in which the canopy-former is dominant, to competition involving standoffs and reversals between the understory species and juvenile canopy-formers. Analysis of these models leads to two key conclusions. First, as competition becomes less transitive, coexistence may be promoted or inhibited, depending upon the extent to which the canopy-former can sustain itself by clonal propagation. Specifically, when clonal growth alone is adequate to sustain the canopy-former, increasing nontransitivity promotes coexistence. When it is not, nontransitivity inhibits coexistence. Secondly, size-dependent nontransitivity dramatically changes how gradients in disturbance and recruitment affect species coexistence. In contrast to hierarchical interactions, standoffs and reversals do not show an “intermediate recruitment” phenomenon, in which coexistence is facilitated at intermediate levels of recruitment. Moreover, under hierarchical competition, the dominant always benefits more than the subordinate as recruitment is increasingly facilitated. Under standoffs and reversals, however, increasing recruitment often favors the canopy-former at some levels of disturbance, but the understory species at other levels of disturbance. These results differ markedly from previous models of stage-dependent competition, suggesting that promotion of coexistence by ontogenetic shifts in competitive ability depends upon the mechanisms by which competition occurs in particular ecological contexts. The results also indicate that the effects of gradients in disturbance and recruitment on community structure depend fundamentally on how species compete for space.

Incomplete mixing promotes species coexistence in a lottery model with permanent spatial heterogeneity

For many marine organisms, the population dynamics in multiple habitats are affected by migration of planktonic larvae. We herein examine the effect of incomplete larval mixing on the condition for species coexistence. The system consists of two heterogeneous habitats, each composed of a number of sites occupied by adults of two species. Larvae produced in a habitat form a pool and migrate to the pool of the other habitat. When an adult dies, the vacant site becomes occupied by an individual randomly chosen from the larval pool. We study (1). the invasibility of a inferior species which has no advantage in either habitats, (2). the dynamics when larval migration and competition among adults are symmetric between habitats, and (3). the case with unidirectional migration. The coexistence of competitors is more likely to occur when larval migration is weak.

Long-term effect of coral transplantation: Restoration goals and the choice of species

The transplantation is an important method for the restoration of degraded ecosystem. However, it is unclear how the choice of species and transplantation mode affects the community dynamics during recovery from a disaster, particularly for long-lived organisms such as corals. To address this issue, we study a population dynamic model of multiple species in multiple habitats connected by larval dispersal. We first consider two species showing the trade-off relationship between growth rate and mortality and examine three restoration goals to evaluate the effectiveness of transplantation: (1) total coverage; (2) species diversity; (3) spatial heterogeneity of species composition. To promote the rapid development of total coverage, the transplantation of fast-growing species should be adopted. To maintain a high level of regional species diversity, the transplantation of slow-growing species or short-dispersal species is effective. Next, we suppose four genera of corals - Acropora, Pocillopora, Porites, and Favites - as an example of coral community in Okinawa where Pocillopora is facing to local extinction. In addition to three indexes; (4) recovery of locally endangered species is evaluated as a restoration goal. Results show that to promote the recovery of Pocillopora, the transplantation of the same species is clearly the most effective choice. In contrast, the transplantations of Acropora and Porites led to undesirable results. In summary, these results indicate that both the restoration goal and the transplanted species must be carefully selected before conducting transplantation operations.

Spatial heterogeneity of mortality and temporal fluctuation in fertility promote coexistence but not vice versa: a random-community approach.

Both spatial heterogeneity and temporal fluctuation of the environment are important mechanisms promoting species coexistence, but they work in different manners. We consider many pairs of species with randomly generated survivorship and fertility in the lottery model, and examine how the variability in demographic processes affects the outcome of competition. The results are: [corrected] (1) Coexistence is easier if habitat difference in mortality is greater, or if year-to-year variation in reproductive rate is larger. But neither habitat [corrected] difference in fertility nor temporal variation in mortality promotes coexistence. [corrected] (2) Mean fertility does not affect the outcome if the coefficient of variation [corrected] remains constant. In contrast, enhanced mean mortality decreases the fraction of coexisting pairs if the environment fluctuates temporally. [corrected] (3) We also investigate the effect of limited dispersal of propagules between habitats. Compared with the complete mixing case, the fraction of coexisting pairs is clearly enhanced if the spatial heterogeneity is the major source of environmental variation, but shows slight increase if the temporal fluctuation is dominant. We conclude that spatial heterogeneity is likely to work more effectively in promoting species coexistence than temporal fluctuation, especially when the species suffer relatively high mortality, and disperse their propagules in a limited spatial scale.

Critical Information Gaps Impeding Understanding of the Role of Larval Connectivity Among Coral Reef Islands in an Era of Global Change

Populations of marine organisms on coral reef islands (CRI) are connected in space and time by seawater that transports propagules of plants, animals, and algae. Yet, despite this reality, it is often assumed that routine replenishment of populations of marine organisms on CRI is supported by locally-sourced propagules (hereafter, larvae). Following large disturbances, however, distantly-sourced larvae from less disturbed CRI within a regional meta-population are likely to be important for local population recovery, but evaluating the roles of locally- versus distantly- sourced larvae remains difficult. While larval sources are relatively well known for many fishes, they remain virtually unknown for most taxa, particularly those associated with the benthos, including hermatypic corals. We make the case that CRI provide natural laboratories in which studies of connectivity can enhance understanding of community dynamics under future disturbance regimes, especially where ongoing changes have created novel systems that are functioning in ways differing from the recent past. However, this potential cannot be realized due to the limited breadth, detail, and spatio-temporal concordance of exiting research. Targeted research on the role of connectivity in mediating ecosystem resilience of CRI is required to understand how populations of marine organisms will change in a future affected by large-scale disturbances of anthropogenic origin. Using the coral reefs of Mo’orea (French Polynesia), Okinawa (Japan), and St. John (US Virgin Islands) as examples, we describe the data required to achieve this objective, and discuss why provision of these data will require new modes of multidisciplinary and collaborative research.