Predictors Of Species Distributions Research Articles

Metacommunity organisation of Amazonian stream fish assemblages: The importance of spatial and environmental factors

AbstractMetacommunity theory seeks to explain how local and regional processes contribute to the organisation of biological communities. Recent conceptual frameworks of this theory indicate that the dynamics of populations and communities are jointly established by dispersal, species responses to environmental conditions and biotic interactions dependent on species density. Here, we use hierarchical joint species distribution modelling to verify the effects of environmental factors (at multiple spatial scales), drainage network configuration, spatial autocorrelation and interspecific interactions on the structure of fish assemblages in terra‐firme streams in the Eastern Brazilian Amazon. Thirty streams were sampled in two field surveys, and 89 species were collected in total. Our results confirm the effects of limited dispersal, environmental conditions (at multiple scales) and the configuration of the drainage network on the dynamics of these assemblages. Furthermore, our results suggest that interspecific interactions are less relevant for the structuring of fish metacommunities in small Amazonian headwater streams. Abiotic factors like drainage networks and environmental heterogeneity were better predictors of species distributions than co‐occurrence patterns with potential competitors or predators.

Leaf turgor loss point shapes local and regional distributions of evergreen but not deciduous tropical trees.

Summary The effects of climate change on tropical forests will depend on how diverse tropical tree species respond to drought. Current distributions of evergreen and deciduous tree species across local and regional moisture gradients reflect their ability to tolerate drought stress, and might be explained by functional traits.We measured leaf water potential at turgor loss (i.e. ‘wilting point’; πtlp), wood density (WD) and leaf mass per area (LMA) on 50 of the most abundant tree species in central Panama. We then tested their ability to explain distributions of evergreen and deciduous species within a 50 ha plot on Barro Colorado Island and across a 70 km rainfall gradient spanning the Isthmus of Panama.Among evergreen trees, species with lower πtlp were associated with drier habitats, with πtlp explaining 28% and 32% of habitat association on local and regional scales, respectively, greatly exceeding the predictive power of WD and LMA. In contrast, πtlp did not predict habitat associations among deciduous species.Across spatial scales, πtlp is a useful indicator of habitat preference for tropical tree species that retain their leaves during periods of water stress, and holds the potential to predict vegetation responses to climate change.

Sapling performance along resource gradients drives tree species distributions within and across tropical forests

Niche differentiation is a major hypothesized determinant of species distributions, but its practical importance is heavily debated and its underlying mechanisms are poorly understood. Trait‐based approaches have been used to infer niche differentiation and predict species distributions. For understanding underlying mechanisms, individual traits should be scaled up to whole‐plant performance, which has rarely been done.We measured seven key traits that are important for carbon and water balance for 37 tropical tree species. We used a process‐based plant physiological model to simulate the carbon budget of saplings along gradients of light and water availability, and quantified the performance of the species in terms of their light compensation points (a proxy for shade tolerance), water compensation points (proxy for drought tolerance), and maximum carbon gain rates (proxy for potential growth rate). We linked species performances to their observed distributions (the realized niches) at two spatial scales in Bolivian lowland forests: along a canopy openness gradient at local scale (∼1 km2) and along a rainfall gradient (1100–2200 mm/yr) at regional (∼1000 km) scale.We show that the water compensation point was the best predictor of species distributions along water and light resource gradients within and across tropical forests. A sensitivity analysis suggests that the stomatal regulation of minimum leaf water potentials, rather than stem hydraulic traits (sapwood area and specific conductivity), contributed to the species differences in the water compensation point of saplings. The light compensation point and maximum carbon gain, both driven by leaf area index and leaf nitrogen concentration, also contributed to differential species distributions at the local scale, but not or only marginally at the regional scale. Trait‐and‐physiology‐based simulations of whole‐plant performance thus help to evaluate the possible roles of individual traits in physiological processes underlying species performance along environmental gradients. The development of such whole‐plant concepts will improve our ability to understand responses of plant communities to shifts in resource availability and stress under global change.

Representing calcification in distribution models for aquatic invasive species: surrogates perform as well as CaCO3 saturation state

Modeling of invasive species’ potential distributions is critical for preventing new invasions. Distribution models use environmental characteristics of observed distributions to determine the suite of conditions that allows invasion. Some variables are directly related to species’ biological needs, while others are surrogates that do not directly influence distributions. For aquatic invasive species (AIS) with calcified shells and exoskeletons, one common limiting factor is the chemical restriction on forming calcium carbonate (CaCO3), which is usually represented by conductivity or dissolved calcium. We predicted that conductivity and calcium would not serve as accurate surrogates for CaCO3 saturation state, which is more directly linked to calcification potential. To test this prediction, we fit classification trees to field surveys of northern Wisconsin lakes for three calcifying AIS—rusty crayfish (Orconectes rusticus), banded mystery snail (Viviparus georgianus), and Chinese mystery snail (Cipanguladina chinensis). Along with lake area, variables associated with calcification were the best predictors of species distributions. Even though saturation state is the most biologically mechanistic variable, it did not improve model predictions or explain more variability in species presence than its surrogates. Overall, surrogates are sufficient for species distribution modeling in these lakes that exhibit a wide range of chemical states.

Environmental correlates of tree species distributions vary among age classes in a northern temperate forest

Processes involved in the structuring of forest communities include: (1) ecological sorting, where species poorly suited to local conditions are subject to environmental filtering and competitive displacement; (2) disturbance, resulting in stochastic removal of individuals and reinitiating successional regimes and (3) dispersal limitation, inhibiting the infiltration of species into preferred sites. Temporal dynamics in these processes lead to difficulty inferring causal landscape–biota correlations. Complicating factors include potential for ontogenetic variation in habitat preferences among age classes, and inherent ambiguity regarding severity and coverage of historical disturbance events. Sorting species into age groups can provide relevant temporal information. Fundy National Park is a northern, mixed-temperate forest in Atlantic Canada (Acadian forest type), which was pervasively altered upon European settlement. Species frequency data for three tree age classes (saplings, juveniles and adults) in permanent sample plots (400 m2, n = 33) were compared to environmental data, including soil chemistry, understory light conditions, physiography and disturbance history using ordination and randomization techniques. Abiotic and disturbance-related predictors of species distributions differed among life stages. Specifically, in the adult stage, stand age was a critical predictor of distribution, whereas in younger age classes environmental variables such as nutrient availability and soil moisture and drainage were key drivers of distribution. It is concluded that older populations were increasingly less constrained by environmental conditions, suggesting that adult populations bear the legacy of stochastic landscape alteration, thus appearing randomly distributed along environmental gradients. As younger populations gradually expand in distribution, they are filtered into preferred conditions by ecological sorting. These findings indicate the importance of considering age-class effects and site history in further assessments of interactions between landscapes and flora.

Identifying transit corridors for elephant using a long time-series

The role of corridors in mitigating the effects of landscape fragmentation on biodiversity is controversial. Recent studies have highlighted the need for new approaches in corridor design using long-term datasets. We present a method to identify transit corridors for elephant at a population scale over a large area and an extended period of time using long-term aerial surveys. We investigated environmental and anthropogenic factors directly and indirectly related to the wet versus dry season distribution of elephant and its transit corridors. Four environmental variables predicted the presence of elephant at the landscape scale in both seasons: distance from permanent water, protected areas and settlements and vegetation structure. Path analysis revealed that altitude and monthly average NDVI, and distance from temporary water had a significant indirect effect on elephant distribution at local scale in dry and wet seasons respectively. Five transit corridors connecting Tarangire National Park and the northern as well as south-eastern wet season dispersal areas were identified and matched the wildlife migration routes described in the 1960s. The corridors are stable over the decades, providing landscape connectivity for elephant. Our approach yielded insights how advanced spatial analysis can be integrated with biological data available from long-term datasets to identify actual transit corridors and predictors of species distribution.

The distributions of one invasive and two native crayfishes in relation to coarse-scale natural and anthropogenic factors

Summary 1. Native crayfishes are often extirpated from portions of their range because of interactions with invasive species, anthropogenic alterations to environmental conditions or a combination of these factors. Our goal was to identify coarse-scale natural and anthropogenic factors related to the current distributions of the invasive crayfish, Orconectes hylas, and two endemic crayfishes, Orconectes peruncus and Orconectes quadruncus in the St. Francis River drainage, Missouri, U.S.A. and to provide wider insights into the potential role of anthropogenic factors in facilitating species displacement. 2. We used classification trees to model coarse-scale natural and anthropogenic environmental factors and their relation to the presence or absence of each species. Model results were then used to predict probability of presence for each species within each stream segment throughout the entire St. Francis River drainage. 3. Factors related to geology and soils were the best predictors of species distributions. A dichotomy of these factors explained much of the discrete distributions of the two native species. Agricultural-related factors were identified as the most influential anthropogenic activity related to species distributions. All associations between the invasive species and anthropogenic factors were negative which suggested the invader was not likely to establish in heavily impacted areas. Overall, our models had high correct classification rates, and we were able to reliably predict the presence of the invader in the invaded drainage. 4. Given the negative associations of the invader with anthropogenic alterations at a coarse spatial scale, we believe other mechanisms are likely to be responsible for the widespread displacement of the two native species. These findings can be used to assist in conservation activities such as creation of refugia for native species and may direct future research to identify the mechanism(s) of species displacement.

Distribution patterns of flora and fauna in southern Chilean Coastal rain forests: Integrating Natural History and GIS

Knowledge of species richness centers is necessary for the design of conservation areas. In this study, we present a GIS analysis of two years of field data on animal and plant diversity distributions in evergreen, coastal rain forests of southern Chile (39°30′–41°25′ S). Despite their high endemism, these forests have remained largely unprotected. Field records were complemented with data from museum collections and scientific literature. We used selected environmental variables (evapotranspiration, altitude) and, in some cases, forest types as predictors of species distributions. Our study focused on the distribution of forest bryophytes, vascular plants, soil invertebrates, amphibians and birds. We generated distributional maps for each taxa based on their field records in the study area, complemented by natural history information, except in the case of bryophytes and soil invertebrates. In general, species richness was lower at 600 m elevation or above for all the taxa studied. Species richness tends to increase in the northern sector of the study area. We observed a greater richness of vascular plants near rivers and streams, and noted important floristic differences between west and east-facing slopes of the Coastal Range, with more species in the oriental side. Because species in high altitude forests are not a subset of those found at lower elevations, we propose that conservation strategies should prioritize the protection of the entire altitudinal gradient of the southern Coastal Range, especially in the more diverse oriental and northern sectors.

Effect of adjacent agricultural habitat on the distribution of passerines in natural grasslands

Habitat heterogeneity has often been acknowledged as a key factor determining which species occur in a landscape, but we still know little on the nature and relative importance of the processes controlling species distribution and abundance within complex landscape mosaics. We used a framework defined by the compensation, supplementation and complementation models to study the distribution of four species of passerines typical of steppe habitats and identify the degree to which landscape context accounts for their distribution in the steppe-agriculture mosaic of the Crau, southern France. Steppe habitat was the best predictor of species distribution in all cases and that densities were highest in steppe. One of these species, the Calandra lark, is strictly tied to the natural steppe habitats independently of its context. For another species, the Tawny pipit, we document a significant positive effect of the presence, near steppe habitat, of extensive pastoral habitats, such as fallows and grazed crops. Finally, the abundance of the two remaining species, Skylark and Short-toed lark, was negatively affected by the presence of non-herbaceous habitats in the vicinity of steppe habitat. This suggests that the quality of the steppe habitat, as perceived by these species is conditional to the nature of adjacent habitats. Our results suggest that the management of the context in which steppe habitat is imbedded will significantly affect the distribution and abundance of this group of passerines and determine both habitat quality and conservation value at a landscape scale.

Factors Influencing the Distribution of Species of Californian Coastal Sage Scrub

The distributions of species and species groups of southern coastal sage scrub in Alta and Baja California are examined in relation to structural and environmental variables. Variables examined include vegetative structure, geographic and topographic position, soil physical characteristics and macronutrient levels, substrate, temperature, precipitation, and factors of disturbance (fire, light grazing, air pollution). Evapotranspirative stress during summer months seems to be a major factor influencing species distributions. Mean temperature of the warmest month appears to be a better predictor of this stress than seasonal or annual precipitation levels. Many of the dominant species also respond sensitively to differences in soil fertility and parent material, with 18 of the 21 dominant species examined showing significant preference for a single substrate among seven classes. Among soil nutrients, levels of organic and nitrate nitrogen appear as particularly useful predictors of species distributions. Of disturbance variables, regional levels of air pollutants are correlated most strongly with floristic differences. Multiple linear regression equations are presented permitting prediction of species' foliar cover levels from levels of environmental variables. Geographic segregation of floristic associations within the sage scrub flora is most marked along a northwest—southeast axis paralleling the Pacific coastline, although southern inland (Riversidian) associations show affinities with the northern coastal flora. Medium—scale patchiness (° 0.1 ha) in floristic composition occurs within the sage scrub due to local dominance by one or a small number of shrub species. The possible ecological significance of this is briefly discussed.