Jurek Kolasa

Notes on ecotone attributes and functions

We attempt to identify general properties of ecotones. Earlier attempts to do so encountered difficulties resulting from contradictory conceptions of ecotones. Thus, we begin with and center our discussion on a definition of ecotone. The definition is complex. It includes scaling, structural, and functional aspects. Based on this complex definition, we offer a brief review of what is an ecotone, what attributes it has, and how it influences other habitats of interest. We identify feedback as a possibly important but ignored function of ecotones. This discussion is presented in general terms which apply to a variety of ecological situations. We point out that results of an evaluation of ecotone attributes largely depends on the spatial and temporal scale at which ecotone is conceptualized and data are collected. We suggest that some of ecotone determinants scale naturally in a repeatable fashion among various aquatic systems. Finally, we point to the concentration of dynamic properties of ecotones as applied to land/water interface.

Environmental variability alters the relationship between richness and variability of community abundances in aquatic rock pool microcosms

Abstract The effect of species richness on the temporal variability of communities and populations continues to inspire investigations and debates; however, few empirical studies have addressed the crucial question of how the relationship between richness and variability changes along a gradient of environmental variability. We determined the relationship between species richness (S) and variability (coefficient of variation, CV) for both community and population abundances of aquatic invertebrates inhabiting 49 tropical coastal rock pools that differ in environmental variability. When all pools are considered, results support the hypothesis that variability in community abundance decreases with increases in species richness. In contrast, abundances of individual populations in more speciose communities vary no more than in species-poor communities. Richness-community variability relationships were detected in rock pools with low environmental variability (as measured by a multivariate index of environmental variability) and in rock pools with low variability in specific physicochemical variables, i.e., temperature, salinity, dissolved oxygen, and pH. The presence of richness-variability relationships in the less environmentally variable rock pools and not in the more variable rock pools suggests that environmental variability may play an important role in modulating richness-variability relationships.

Resource limitation, biodiversity, and competitive effects interact to determine the invasibility of rock pool microcosms

The success of species invasions depends on both the characteristics of the invaded habitat and the traits of the invasive species. At local scales biodiversity may act as a barrier to invasion; however, the mechanism by which biodiversity confers invasion resistance to a community has been the subject of considerable debate. The purpose of this study was to test the hypothesis that productivity and diversity affected the ability of a regionally available species to colonize communities from which it is absent. We hypothesized that the invasibility of rock pool invertebrate communities would increase with increasing nutrients and decrease with increasing diversity. We tested this possibility using naturally invaded outdoor aquatic microcosms. We demonstrated that the invasibility of an experimental multi-trophic aquatic community by a competitive native midge species (Ceratopogonidae: Dasyhelea sp.) was determined by an interaction between resource availability, diversity, and the densities of two competitive ostracods species. Nutrient enrichment increased invasion success; however, within nutrient-enriched microcosms, invasion success was highest in the low-diversity treatments. Our results suggest that resource availability may in fact be the principal mechanism determining invasibility at local scales in multi-trophic rock pool communities; however resource availability can be determined by both nutrient input as well as by the diversity of the biotic community.

DESICCATION FREQUENCY REDUCES SPECIES DIVERSITY AND PREDICTABILITY OF COMMUNITY STRUCTURE IN COASTAL ROCK POOLS

Success of aquatic organisms in temporary ponds depends on both survival through the dry period (i.e., resting eggs) and population dynamics through the wet period (i.e., recolonization, dispersal). We hypothesized that pools with high desiccation frequency would contain simpler communities (fewer species), and these communities would have greater compositional constancy (persistence) due to a restricted set of species. To test these hypotheses we examined diversity, species richness, abundance, evenness, community persistence, and community structure (species ranking) over time using 49 erosional rock pools located on the north coast of Jamaica. Diversity, species richness, and abundance decreased with increased frequency of pool desiccation. Pools were classified as temporary or permanent. In temporary pools, despite substantial inter-pool differences, diversity, species richness, evenness, and abundance did not change significantly over time. Contrary to our hypothesis, community persistence (compositi...

Physical determinants of richness, diversity, evenness and abundance in natural aquatic microcosms

Histories, biotic factors and abiotic factors interact to determine biodiversity. A comparison of systems formed over a range of physical conditions may help to evaluate the role of physical factors in determining community structure. We investigated abiotic determinants of species richness, abundance, biomass, diversity and evenness using a series of erosional rock pools on the northern coast of Jamaica. Aquatic invertebrate community data were collected from 1989 to 1997, along with repeated measurements of physical variables, including descriptors of pool conditions, morphometric characteristics and other derived measures to a total of 17 variables. These variables were used to evaluate a range of regression models, from simple to complex, which explained the observed biodiversity. Simple regression models were occasionally significant but explained little variance. Multiple regression models (using a forward stepwise approach) significantly increased the explained variance of these biodiversity models. Diversity models that emphasized species richness (rather than dominance or evenness) of the community, were preferred. These results suggest that much of the observed species richness/diversity is determined by abiotic pool conditions and represents an accumulation of tolerant species, either directly or indirectly (i.e. modulated by biotic interactions). However, regression models attempting to predict abundance/biomass based on abiotic variables explained less variance than did those predicting richness/diversity. This may indicate that biotic factors within pools at population or community levels are controlling species densities.

Rapoport's rule: an explanation or a byproduct of the latitudinal gradient in species richness?

A recent explanation of the declining species richness gradient with increasing latitude away from the tropics implicated broad scale habitat variability, an associated range expansion, and a resulting increase in niche breadth. The niche breadth in turn was thought to affect richness by competition and rescue effect. While all three factors appear to be correlated, neither the postulated nor alternative causal mechanisms have been tested. We conduct such a test using a system which has all the attributes of the large scale pattern but which, in contrast to continental scale observations, allows for estimation or control of crucial variables such as taxonomic composition, habitat heterogeneity, habitat variability, exact species distribution, and local richness. Specifically, we test the alternative that the correlation between the geographical range of species and local diversity is a function of differential species survival and link this phenomenon to habitat variability. We use 40 species of aquatic invertebrates inhabiting a landscape of 49 miniature rock pools on the coast of Jamaica. The system we examined exhibits a gradient of increasing richness with decreasing habitat variability, analogous to the broad scale latitudinal pattern. Furthermore, species with broader ecological ranges are also broadly distributed. Superficially, this appears to be in agreement with the older explanations but two facts suggest different causes. First, there is no evidence of a ‘rescue effect’ maintaining high richness in many habitats despite their proximity to species sources. Second, ecologically broad species coexist with habitat specialists without reducing richness in jointly occupied habitats.

Hierarchy underlies Patterns of Variability in Species Inhabiting Natural Microcosms

Relative variability of species has been shown to increase significantly with a decrease in their ecological range. Similarly, the distribution of collapse (e.g., extinctions, disturbances, population declines) magnitudes has also been shown to follow an inverse power-law form described by the 1/f ω curve. We hypothesized that the two, possibly general, patterns associated with ecological systems share a common underlying cause: the hierarchical structure of the system itself. To test the hypothesis we used a model system of 49 natural rock pools inhabited by 40 species of invertebrates. Three measures of species variability based on changes in abundance, distribution, and persistence in individual pools conform with the postulated negative exponential curves. Correspondingly, frequency distributions of changes of various magnitudes conform to the 1/f ω pattern. Examination of the contributions of species to the 1/f ω pattern revealed that species low in the system hierarchy (habitat specialists in this case) are responsible for the majority of small variation events (correlations between the ecological range and position on the 1/f ω curve range from 0.625 to 0.807 on the three measures of variability). This permits the conclusion that the two patterns are linked and constitute different expressions of the same hierarchical system structure.

Simplifying the complexity of temporal diversity dynamics: A differentiation approach

Abstract A simple, yet highly promising method of quantifying temporal diversity dynamics is to use the ratio of mean species richness, S, to potential species richness, P (cumulative S at a locality). In a rock pool meta-community of aquatic micro-invertebrates, this ratio, which we call the dynamics index (I), proved reliable as a predictor of assemblage type. We calculated within-habitat diversity dynamics for each of 49 pools that differed in environmental variability. The resulting dynamics indices provide an easily quantified measure of the diversity dynamics at various scales and, specifically, provide a measure of within-habitat temporal turnover in habitats where P can be reliably evaluated.

Ecosystem stress and health: an expansion of the conceptual basis

The assessment of the ecosystem health and departures from it requires clarity of what the system, its structure, dynamics, and healthy conditions are. Available definitions provide inadequate tools to acquire this clarity and may lead to arbitrary diagnoses of ecosystem health but such diagnoses can be overturned on a variety of scientific, philosophical, or political grounds.Nested hierarchy of ecosystem structure compounds the difficulty in the assessment of stress and health because both states may occur simultaneously at different hierarchical levels: with stress at one level being a necessary condition of health at another.An approach based on a formal definition of system change is advanced. First, a conceptual model identifies a self-maintaining minimum interactive structure (MIS) at each level of ecosystem organization. Components of MIS are complementary, coordinated, and exchanging information — they are integrated. Function is defined as a contribution of a component to the maintenance of the whole. In this context health is viewed as persistence of the system at a given temporal and spatial scale. Impairment of the function is stress and is contrasted with change of system structure (loss, addition, or replacement of components of MIS) which is disturbance. Stress can be measured directly by changes of function or indirectly by changes in integration. Even though undesirable from the human point of view, a changed system may again be considered healthy.

Integration and Synthesis

“Science is a spiders web. Confidence in any one strand of the web is maintained by the tension and resiliency of the entire web.“