Important Determinant Of Abundance Research Articles

Spatial semiparametric models improve estimates of species abundance and distribution

Accurate estimates of abundance are imperative for successful conservation and management. Classical, stratified abundance estimators provide unbiased estimates of abundance, but such estimators may be imprecise and impede assessment of population status and trend when the distribution of individuals is highly variable in space. Model-based procedures that account for important environmental covariates can improve overall precision, but frequently there is uncertainty about the contribution of particular environmental variables and a lack of information about variables that are important determinants of abundance. We develop a general semiparametric mixture model that incorporates measured habitat variables and a nonparametric smoothing term to account for unmeasured variables. We contrast this spatial habitat approach with two stratified abundance estimators and compare the three models using an intensively managed marine fish, darkblotched rockfish (Sebastes crameri). We show that the spatial habitat model yields more precise, biologically reasonable, and interpretable estimates of abundance than the classical methods. Our results suggest that while design-based estimators are unbiased, they may exaggerate temporal variability of populations and strongly influence inference about population trend. Furthermore, when such estimates are used in broader meta-analyses, such imprecision may affect the broader biological inference (e.g., the causes and consequences of the variability of populations).

Light availability indirectly limits herbivore growth and abundance in a high rocky intertidal community during the winter

Light availability and consumer pressure are fundamental factors that structure aquatic communities, but their integrated effects are rarely studied in marine benthic systems. Using a factorial experiment, I investigated the effects of light availability and grazing by limpets (Lottia digitalis) to determine the relative influence of each on the growth and abundance of producers and consumers in a rocky intertidal community during the winter. Light reduction via shading reduced the abundance of filamentous algae and reduced the abundance of the herbivorous periwinkle Littorina sitkana. The effects of limpet grazing reduced the abundance of filamentous algae, Porphyra spp., and diatom mats. Higher limpet densities were also associated with significantly lower densities of adult, but not juvenile, Littorina spp. Light and limpet density interacted to determine Lottia growth, which was high in unshaded, single‐limpet enclosures but was negligible in plots with two limpets, shades, or both. Variation in bottom‐up (resource‐driven) and top‐down (consumer‐driven) forces are important determinants of abundance at both trophic levels, but the effects are complex, taxon‐specific, and, for littorine snails, size‐specific. Because the study species accomplish most of their growth and reproduction during the winter, the interplay of light and herbivory during this season will likely influence long‐term community dynamics.

Factors Affecting Long-term Trends in the Estuarine Abundance of Pogge (Agonus cataphractus )

Long-term data assembled from a power station on the Thames estuary were used to model fluctuations in the abundance of pogge (Agonus cataphractus) as a function of estuarine environmental, seasonal and prey availability variables using multiple regression. Temperature was the most important determinant of abundance, with seasonal variables and changes in the relative abundance of age-0 dab, Limanda limanda, possibly as prey, also holding important consequences for pogge abundance. Pogge abundance was also moderately influenced by: salinity, flow, salinity-flow interactions, changes in the relative abundance of Dover sole, Solea solea, and a trend variable. Regular patterns of seasonal (spring/winter) occurrences were associated with the use of warmer estuarine waters to optimize growth and reproductive potential and/or correlated with inshore spawning migrations and the availability of prey species. Both mechanisms suggest opportunistic use of estuarine habitats by pogge. Available temperature-abundance relationships further suggest a well-defined estuarine thermal niche, with an optimum at 9·5°C. There remains a requirement for improved information on the biology of pogge and factors governing the population dynamics of this important estuarine species.