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Beta diversity, species replacement and nestedness are often examined through pairwise comparisons of sites based on presence-absence data, and the relative importance of these ecological phenomena is evaluated by operations with dissimilarity coefficients. An example is the nestedness resultant dissimilarity (NRD) procedure recently proposed by Baselga (2010, Global Ecology andBiogeography 19: 134–143) to disentangle the nestedness fraction of beta diversity from species replacement. In our view, the component terms in this measure are not scaled uniformly and the nestedness fraction cannot be quantified properly without giving clear definitions for its measurement. We suggest to distinguish among three additive fractions of the species set of two sites: number of species shared (overlap), species replacement (=spatial turnover) and richness difference. Then, absolute beta diversity is obtained as a composite of the second two fractions (known as βWB), while nestedness is derived from the first and the third. To express beta diversity and nestedness in a relativized form, the respective sums are divided by the total number of species. These allow defining a new index to measure the fraction of beta diversity which is shared by nestedness as well, and is calculated as relativized richness difference with the condition that the two sites being compared have at least one species in common. It is called diversity-nestedness intersection coefficient (F). Baselga’s nestedness resultant dissimilarity and the diversity-nestedness intersection coefficient are compared graphically using artificial and actual examples. These functions follow a mathematical relationship for perfectly nested data, otherwise their results are divergent. Discrepancy increases when beta diversity is large, especially if richness differences override species replacement effects in shaping presence-absence data structures. An advantage of F is its compatibility with a general theoretical and methodological framework for revealing pattern in presence-absence data matrices.
There is increasing evidence that regime shifts occur at several scales in ecosystems (from the spatiotemporal alternation of two species to large-scale, ecosystem-level rearrangements). Yet, the theoretical background for understanding these changes is far from clear. Since fishing down in marine ecosystems is well-documented trend, and its top-down cascading effects in food webs have been richly documented, it is a current question whether overfishing, in general, can also influence regime shifts at lower levels. We model simple marine ecosystems by dynamical food webs and investigate the probability of regime shifts emerging among primary consumers. We considered cases where only one of the primary consumers is persistent in the stationary state. By perturbing the death rates in the food web, we studied the circumstances when the previously persistent primary producer is indirectly changed by the previously non-persistent one. Whether and how regime shifts (e.g., change in primary consumers) can occur depends on (1) food web topology (presence of top-predator and alternative producer), (2) the relative strength of perturbation of primary consumers’ death rates, and (3) the dynamical parameters of the recovering consumer. We found that overfishing, food web topology and dynamical parameters together determine the probability of regime shifts. Thus, integrative and complex models are needed in multispecies fisheries.
Vegetation dynamics is a stochastic process of species replacement after disturbance. It occurs because individual species are limited by general constraints and trade-offs. As these constraints and trade-offs are becoming better known, we understand more about the relationships between disturbance dynamics, species pools, and vegetation dynamics. This paper provides a summary of recent work on plant scaling and ecological trade-offs, and explores its implications for vegetation dynamics. Those aspects of succession that are predictable . given the local species complement . can be understood as consequences of these general patterns and constraints. Several are explored in this paper. The inherently stochastic nature of the process derives from the disturbance dynamics that forces it, from the sampling processes that are responsible for selecting potential invaders, and from the chance processes involved in species interactions. The dynamics of species that invade established communities is the least understood but potentially the most crucial aspect of vegetation dynamics. The relation of community invasion to gap creation and to scaling constraints is briefly discussed.
The present study uses transition matrices to compare successional processes (colonization, disturbance, persistence and replacement) of fouling communities submitted to different light effects on Cabo Frio Island, a seasonal upwelling region. Twelve functional groups were identified, and differences in the transition probabilities shown by the matrices suggest a preference for the replacement property of functional groups, which indicates the facilitation successional mechanism. The probability of colonization of these groups differed according to the direction of the substrate, which caused a negative effect of light reduction on algae with a greater probability of disturbance (sensu species replacement), which is typical of a more stressful environment. Species of the same functional group replace each other through competition and herbivory, which promotes the distinction between earlier and later groups on the successional process. Successional trajectories evaluated through global transition matrices change at each time step because they depend on the species turnover rate, and therefore, they are informative of the changing processes of the community. The probabilistic rate of changes related to successional processes may be used to evaluate future conditions of fouling communities, and the deterministic components and stochastic elements will render these communities self-organizable.
How are bryophyte alpha and beta diversities distributed across spatial scales along an elevational gradient in an oceanic island? Which mechanisms and drivers operate to shape them? Starting from a multiscale hierarchical sampling approach along an 1000 m elevational transect, we used additive diversity partitioning and null modeling to evaluate the contributions of the alpha and beta diversity components to overall bryophyte diversity in Terceira Island, Azores. Substrate-level diversity patterns were explored by means of the Sørensen Similarity Index and the Lloyd Index of Patchiness. Elevation-level beta diversity was decomposed into its replacement and richness differences components, with several environmental variables being evaluated as diversity predictors. Bryophyte diversity proved to be primarily due to beta diversity between elevation sites, followed by diversity among substrates. Compositional differences between neighboring sites decreased with elevation, being mainly caused by species replacement and correlating with differences in relative humidity and disturbance. At the substrate level, we found a great homogeneity in terms of species composition, coupled with a low substrate specialization rate. We conclude that, in Terceira’s native vegetation patches, regional processes, such as environmental gradients associated with elevation, play a greater role in shaping bryophyte diversity than local processes. Moister and less disturbed areas at mid-high elevation harbor a richer bryoflora, consistently more similar and stable between neighbouring sites. Simultaneously, the different substrates available are somewhat ecologically redundant, supporting few specialized species, pointing to these areas providing optimal habitat conditions for bryophytes. Our findings provide a better understanding of how bryophyte diversity is generated in Terceira Island, indicating that management and conservation measures should focus on island-level approaches, aiming to protect and rehabilitate additional natural vegetation patches at different elevations, especially in the severely disturbed lowlands.
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