Reliable indicators of species richness (e.g., particular species), if they can be found, offer potentially significant benefits for management planning. Few efficient and statistically valid methods for identifying potential indicators of species richness currently exist. We used Bayesian-based Poisson modeling to explore whether species richness of butterflies in the Great Basin could be modeled as a function of the occurrence (presence or absence) of certain species of butterflies. We used an extensive data set on the occurrence of butterflies of the Toquima Range (Nevada, USA) to build the models. Poisson models based on the occurrence of five and four indicator species explained 88% and 77% of the deviance of observed species richness of resident and montane-resident butterfly assemblages, respectively. We then developed a test framework, including formally defined "rejection criteria," for validating and refining the models. The sensitivity of the models to inventory intensity (number of years of data) and knowledge about the potential indicators was incorporated into this evaluation phase. We conducted a test of our models by using an existing set of data on butterflies in the neighboring Toiyabe Range. Predicted values of species richness were significantly rank correlated with the observed values. Thus, the models appear to have promise for predicting species richness based on the occurrence of certain taxa.
We explored how a woody plant invader affected riparian bird assemblages. We surveyed 15 200-m-long transects in riparian zones in a much-changed landscape of eastern Victoria, Australia. Abundance, species-richness, foraging-guild richness and composition of birds were compared in transects in three habitat types: (i) riparian zones dominated by the invasive willow Salix × rubens; (ii) riparian zones lined with native woody species; and (iii) riparian zones cleared of almost all woody vegetation. We also measured abundance and richness of arthropods and habitat structure to explore further the effects of food resources and habitat on the avifauna. We observed 67 bird species from 14 foraging guilds. Native riparian transects had more birds, bird species and foraging guilds than willow-invaded or cleared transects. Habitat complexity increased from cleared to willow-invaded to native riparian transects, as did abundance of native and woodland-dependent birds. Native shrub and trees species had more foliage and branch-associated arthropods than did willows, consistent with a greater abundance and variety of foraging guilds of birds dependent on this resource. Willow spread into cleared areas is unlikely to facilitate greatly native bird abundance and diversity even though habitat complexity is increased. Willow invasion into the native riparian zone, by decreasing food resources and altering habitat, is likely to reduce native bird biodiversity and further disrupt connectivity of the riparian zone.
Abstract Resolving whether area per se or habitat heterogeneity has the greater influence in controlling species richness remains a controversial yet important question. Here we show that avian species richness of same‐sized transects (1 ha) is independent of the remnant area (of buloke woodland) within which a transect is positioned. We also show that avi‐faunal similarity of pairs of transects randomly placed within the largest remnants (≥ 48 ha) is not consistently related to either proximity (i. e. being within the same remnant) nor to physiognomic characteristics of the transects. We believe that much of the controversy over area/habitat heterogeneity effects is probably related to scalar issues and propose a protocol by which some resolution of the question might be reached. The protocol involves ‘zoom’ sampling in which successively larger transect sizes are used, and measures of faunal richness and habitat heterogeneity are made at these different grains of resolution. One of our intentions is to stimulate discussion on how heterogeneity might be measured when grains increase from typical transect sizes ( ca 1 ha) up to much larger grains ( ca 128 ha).
Abstract: Ecologists often seek to predict species distributions as functions of abiotic environmental variables. Statistical models are useful for making predictions about the occurrence of species based on variables derived from remote sensing or geographic information systems. We previously used 14 topographically based environmental variables from 49 locations in the Toquima Range ( Nevada, U.S.A. ) and species inventories conducted over 4 years ( 1996–1999 ) to model logistically the occurrence of resident butterfly species. To test the models, we collected new validation data in 39 locations in the nearby Shoshone Mountains in 2000–2001. We used a series of “classification rules” based on conventional logistic and Bayesian criteria to assess the success rates of predictions. The classification rules represented a gradient of stringency in the “certainty” with which predictions were made. More stringent rules reduced the number of predictions made but greatly increased the success rate of predictions. For comparisons of classification rules making similar numbers of predictions, conventional logistic and Bayesian criteria produced similar outcomes. Success rates for predicted absences were uniformly higher than for predicted presences. Increasing the temporal extent of data from 1 to 2 years elevated success rates for predicted presences but decreased success rates for predicted absences, leaving the overall success rates essentially the same. Although species occurrence rates ( the proportion of locations in which each species was found ) were correlated between the modeling and validation data sets, occurrence rates for many species increased or decreased substantially; erroneous predictions were more likely for those taxa. Model fit ( measured by the explained deviance ) was an indicator of the probable success rate of predicted presences but not of predicted absences or overall success rates. We suggest that classification rules for predicting likely presences and absences may be decoupled to improve overall predictive success. Our general framework for modeling species occurrence is applicable to virtually any taxonomic group or ecosystem.
Abstract. While the eddy covariance technique has become an important technique for estimating long-term ecosystem carbon balance, under certain conditions the measured turbulent flux of CO2 at a given height above an ecosystem does not represent the true surface flux. Profile systems have been deployed to measure periodic storage of CO2 below the measurement height, but have not been widely adopted. This is most likely due to the additional expense and complexity and possibly also the perception, given that net storage over intervals exceeding 24 h is generally negligible, that these measurements are not particularly important. In this study, we used a 3-year record of net ecosystem exchange of CO2 and simultaneous measurements of CO2 storage to ascertain the relative contributions of turbulent CO2 flux, storage, and advection (calculated as a residual quantity) to the nocturnal CO2 balance and to quantify the effect of neglecting storage. The conditions at the site are in relative terms highly favourable for eddy covariance measurements, yet we found a substantial contribution (∼ 40 %) of advection to nocturnal turbulent flux underestimation. The most likely mechanism for advection is cooling-induced drainage flows, the effects of which were observed in the storage measurements. The remaining ∼ 60 % of flux underestimation was due to storage of CO2. We also showed that substantial underestimation of carbon uptake (approximately 80 gC m−2 a−1, or 25 % of annual carbon uptake) arose when standard methods (u∗ filtering) of nocturnal flux correction were implemented in the absence of storage estimates. These biases were reduced to approximately 40–45 gC m−2 a−1 when the filter was applied over the entire diel period, but they were nonetheless large relative to quantifiable uncertainties in the data. Neglect of storage also distorted the relationships between the CO2 exchange processes (respiration and photosynthesis) and their key controls (light and temperature respectively). We conclude that the addition of storage measurements to eddy covariance sites with all but the lowest measurement heights should be a high priority for the flux measurement community.
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