Abundance Of Potential Competitors Research Articles

The relative importance of biotic and abiotic determinants of temporal occupancy for avian species in North America

AbstractAimWe examined the relative importance of competitor abundance and environmental variables in determining the species distributions of 175 bird species across North America. Unlike previous studies, which tend to model distributions in terms of presence and absence, we take advantage of a geographically extensive dataset of community time series to model the temporal occupancy of species at sites throughout their expected range.LocationNorth America.Time period2001–2015.Major taxa studiedOne hundred and seventy‐five bird species.MethodsWe calculated variation in temporal occupancy across species’ geographic ranges and used variance partitioning and Bayesian hierarchical models to evaluate the relative importance of (a) the abundance of potential competitors and (b) the environment (elevation, temperature, precipitation, vegetation index) for determining temporal occupancy. We also created a null model to test whether designated competitor species predicted variation in temporal occupancy better than non‐competitor species.ResultsOn average, the environment explained more variance in temporal occupancy than competitor abundance, but this varied by species. For certain species, competitor abundance explained more variance than the environment. Migrant species with smaller range sizes and greater range overlap with competitors had a higher proportion of variance explained by competitor abundance than the environment. The abundance of competitor species had a stronger effect on focal species temporal occupancy than non‐competitor species in the null model.Main conclusionsTemporal occupancy represents an underutilized method for describing species distributions that is complementary to presence/absence or abundance. Geographic variation in temporal occupancy was explained by both biotic and abiotic drivers, and abiotic drivers explained more variation in temporal occupancy than abundance on average. Species traits also play a role in determining whether variation in temporal occupancy is best explained by biotic or abiotic drivers. The results of our study can improve species distribution models, particularly by accounting for competitive interactions.

Distribution patterns of the introduced encrusting bryozoan Conopeum chesapeakensis (Osburn 1944; Banta et al. 1995) in an estuarine environment in upper San Francisco Bay

The factors shaping the distributions of nonindigenous species (NIS) are of particular interest for understanding their success and potential impacts within their invaded ranges. In the San Francisco Bay estuary, the encrusting bryozoan Conopeum chesapeakensis (Osburn, 1944; Banta et al., 1995) occurs in peak abundances in lower salinity hard substrate habitats, with lower abundances upstream and downstream; however, little is known about the factors that control its distribution. To investigate several hypotheses about what allows this broadly tolerant invader to be numerically dominant in this region, a field transplant experiment was conducted across three sites in upper San Francisco Bay estuary. Colonies settled on PVC plates in the peak abundance zone and transplanted to upstream and downstream treatment sites, or returned to the settlement site, which served as a control. Salinity, temperature, chlorophyll a levels, and the abundance of interspecific competitors varied at each site and were measured throughout the experiment. Mixed effects models incorporating these measurements compared net growth rate and zooid size observed across treatment sites. Colonies transplanted upstream experienced high barnacle settlement, a potential competitive threat, and decreased salinity, and exhibited an average net growth rate of 6.60 zooids/day. Colonies at the control site faced almost no potential interspecific competition, intermediate salinity, and had an average net growth rate of 4.96 zooids/day. At the downstream site, colonies grew an average of 4.62 zooids/day and experienced high potential competition from serpulid polychaete settlement and the highest salinity of all sites. The best-fit models indicated that overall abundance of potential competitors, especially the serpulid Ficopomatus enigmaticus (Fauvel, 1923), was negatively correlated with the net growth rate of C. chesapeakensis colonies. Zooid size was also negatively correlated with F. enigmaticus abundance and temperature, though the relationship weakened over time. Many colonies at both the upstream and downstream transplant sites experienced fast initial growth following transplantation, but then experienced partial colony loss corresponding with an increase in the abundance of potential competitors. In contrast, colonies at the control site showed slow but continuous growth throughout the study in absence of interspecific competitors. These results suggest that the numerical dominance of C. chesapeakensis in upper-estuary habitat may be partly explained by a lack of potential interspecific competitors. As San Francisco Bay and other estuaries face high invasion pressure in brackish upper-estuarine regions, understanding which factors influence the distribution of NIS can help predict impacts to resident communities.

Association of Ant Predators and Edaphic Conditions with Termite Diversity in an Amazonian Rain Forest

AbstractPredation is a key determinant of prey community structure, but few studies have measured the effect of multiple predators on a highly diverse prey community. In this study, we asked whether the abundance, species richness, and species composition of a species‐rich assemblage of termites in an Amazonian rain forest is more strongly associated with the density of predatory ants or with measures of vegetation, and soil texture and chemistry. We sampled termite assemblages with standardized hand‐collecting in 30 transects arranged in a 5 km × 6 km grid in a terra firme Amazonian rain forest. For each transect, we also measured vegetation structure, soil texture, and soil phosphorus, and estimated the density of predatory ants from baits, pitfall traps, and Winkler samples. Seventy‐nine termite species were recorded, and the total density of predatory ants was the strongest single predictor of local termite abundance (r = −0.66) and termite species richness (r = −0.44). In contrast, termite abundance and species richness were not strongly correlated with edaphic conditions (¦r¦ < 0.01), or with the density of non‐predatory ants (rabund = −0.27; rs = −0.06). Termite species composition was correlated with soil phosphorus content (r = 0.79), clay content (r = −0.75), and tree density (r = −0.42). Assemblage patterns were consistent with the hypothesis that ants collectively behaved as generalist predators, reducing total termite abundance, and species richness. There was no evidence that ants behaved as keystone predators, or that any single termite species benefited from the reduction in the abundance of potential competitors.

Ecological release in White Sands lizards.

Ecological opportunity is any change that allows populations to escape selection from competition and predation. After encountering ecological opportunity, populations may experience ecological release: enlarged population size, broadened resource use, and/or increased morphological variation. We identified ecological opportunity and tested for ecological release in three lizard colonists of White Sands, New Mexico (Sceloporus undulatus, Holbrookia maculata, and Aspidoscelis inornata). First, we provide evidence for ecological opportunity by demonstrating reduced species richness and abundance of potential competitors and predators at White Sands relative to nearby dark soils habitats. Second, we characterize ecological release at White Sands by demonstrating density compensation in the three White Sands lizard species and expanded resource use in White Sands S. undulatus. Contrary to predictions from ecological release models, we observed directional trait change but not increased trait variation in S. undulatus. Our results suggest that ecological opportunity and ecological release can be identified in natural populations, especially those that have recently colonized isolated ecosystems.

Spatial ecology of multiple parasitoids of a patchily-distributed host: implications for species coexistence

1. The coexistence of multiple species sharing similar but spatially fragmented resources (e.g. parasitoids sharing a host species) may depend on their relative competitive and dispersal abilities, or on fine-scale resource partitioning. Four generalist and one specialist parasitoid species associated with the holly leaf miner, Phytomyza ilicis, in a woodland network of 127 holly trees were investigated. 2. To understand coexistence and persistence of these potential competitors, patterns of occurrence in relation to patch size and isolation, vertical stratum within patches, and incidence and abundance of potential competitors were documented. Field experiments creating empty habitat patches suggested that dispersal rather than local demographic processes determines abundance and incidence. 3. Parasitoids showed species-specific responses to patch properties, with the incidence of species determined mostly by patch size. Parasitism rates were less clearly related to patch characteristics, but parasitism rates for most species were lower in patches where the numerically dominant parasitoid species, Chrysocharis gemma, was present. No evidence of vertical stratification was found in species composition or abundance within patches, making it unlikely that coexistence is enhanced by fine-scale resource division. 4. Overall, the patterns detected may be attributed to the distribution of C. gemma and differences in species' ecology other than dispersal ability. The life history of C. gemma may allow it to pre-emptively exploit a large fraction of the available hosts, avoiding direct competition with other parasitoids. In contrast, direct competition is more likely among the pupal parasitoids Cyrtogaster vulgaris, Chrysocharis pubicornis, and Sphegigaster flavicornis which have a similar biology and phenology. For these species, coexistence may be facilitated by contrasting incidence in relation to patch size and isolation.

New records of the Endangered Andean mountain cat in northern Argentina

Few published records exist for the Endangered Andean mountain cat Oreailurus jacobita , a rare and little known felid restricted to high altitudes of the Andes of South America. We present 20 new records for the species, and analyse its altitudinal overlap with the sympatric pampas cat Oncifelis colocolo , a widespread habitat generalist, in north-west Argentina. Our data confirm the recent presence of the Andean mountain cat at several locations in Jujuy, Salta, and Catamarca provinces. Nevertheless, densities appear to be much lower than those of the pampas cat. This Lower Risk, near threatened species represented 68% of the records for small cats at high altitudes and its altitudinal range overlapped with the lower 1,000 m of the range of the Andean mountain cat. Protected areas for the Andean mountain cat in Argentina should be above 3,500 m and have a high proportion of habitat above 4,000 m, where the abundance of potential competitors is lower.

Effects of Recreational Fishing on the Population Dynamics of Lake-Dwelling Brook Trout

Data collected over 7 consecutive years were used to examine the population dynamics of the exploited brook trout Salvelinus fontinalis of Meach Lake in central Ontario. Numbers of age-1 and older trout averaged 975 among years. The largest trout were 490 mm long and weighed 1,807 g. Few trout lived beyond age 4. Annual mortality rates varied among age-classes and between sexes; average total instantaneous mortality was 0.95 and average total and fishing mortalities were 78% and 44%, respectively. Exploitation was greatest for age-2 and age-3 brook trout of both sexes. Catchability was uncorrelated with abundance. There was evidence of density-dependent growth at age-2 or age-3 but no indication of density-dependent mortality. A “hockey stick” model appeared to best describe the stock–recruitment relationship. Abundance of potential competitors was not correlated with brook trout size, although abundance of white suckers Catostomus commersoni was correlated with numbers of age-2 male brook trout. We conclude that total mortality rates are naturally very high and existing levels of harvests in this lake may not be increasing total mortality. To ensure the sustainability of brook trout fisheries, lake-dwelling populations should be managed to maintain total mortality rates below those observed in this study.

An overview of the LEAR (Lobster Ecology and Recruitment) project: results of field and experimental studies on the juvenile ecology of Homarus gammarus in cobble

Field and experimental studies were undertaken in four European countries on clawed lobster (Homarus gammarus) ecology and recruitment. The aims were to assess: (a) abundance of early benthic phase (EBP) lobsters and other benthic species at cobble sites, (b) the effect of these species on EBP growth and survival, and (c) influence of these factors on recruitment and/or viability of restocking. Suction sampling of cobble sites revealed a wide diversity and abundance of potential competitors but no EBP lobsters. Hatchery-produced EBPs were therefore used for small-scale mesocosm experiments. In nature, predation on EBP was rapid without shelter, whereas in-vitro mortality, growth, and behaviour experiments showed that cover potentially increases survival. Scarcity per se may not completely explain the apparently absence of EBP, given that not a single EBP lobster was located either during this study or during any other of the extensive benthic investigations. The present paper discusses the implications of their apparent absence and whether it arises because EBPs are very scarce, cannot be captured by suction sampling, are not within lobster habitat, or occupy a specialized niche within adult habitat outside the range of current sampling methods.