Variation In Species Relative Abundance Research Articles

Factors shaping the abundance of two butterflies sharing resources and enemies across a biogeographic region

AbstractAimIntraspecific variation in species relative abundance is shaped by a complex interplay of abiotic and biotic factors, making it both necessary and challenging to assess their combined relative importance in explaining variations across space and time. We used two congeneric butterfly species for which extensive count data and a deep understanding of their natural history is available to test three hypotheses explaining intraspecific variation in their abundance: (H1) seasonal dispersal behaviour driven by climate, (H2) resource availability and (H3) apparent competition mediated via shared parasitoids.TaxonGonepteryx rhamni (Brimstone) and G. cleopatra (Cleopatra).LocationNE Iberian Peninsula, where both species coexist, and a nearby archipelago (Balearic Islands), where only Cleopatra occurs.MethodsWe analysed spatial abundance variations for both species in the mainland and island–mainland differences in the abundance of Cleopatra. Abiotic and biotic factors, including temperature, host plant and overwintering habitat availability, larval parasitism and density dependence, were tested to explain the observed variations.ResultsH1 can explain variation in butterfly abundance between mainland regions since in warmer summers populations increased in cooler areas but decreased in warmer areas. H2 explains the variation within mainland climate regions with a strong positive relationship between resource availability and abundance but is unlikely to explain the island–mainland variation in the abundance of Cleopatra. H3 could neither explain biogeographical variation in abundance because although richer parasitoid communities were found on the mainland, larval mortality rates were similar or lower on the mainland than in the islands.Main ConclusionsClimate and resource availability jointly account for variation in butterfly abundance across the mainland, but neither these factors nor parasitism can explain island–mainland differences. Both coexisting butterfly species and their larval parasitoids may have undergone evolutionary processes, resulting in spatial segregation that promotes the coexistence of the two butterfly species on the mainland.

Estimating parasite host range.

Estimating the number of host species that a parasite can infect (i.e. host range) provides key insights into the evolution of host specialism and is a central concept in disease ecology. Host range is rarely estimated in real systems, however, because variation in species relative abundance and the detection of rare species makes it challenging to confidently estimate host range. We applied a non-parametric richness indicator to estimate host range in simulated and empirical data, allowing us to assess the influence of sampling heterogeneity and data completeness. After validating our method on simulated data, we estimated parasite host range for a sparsely sampled global parasite occurrence database (Global Mammal Parasite Database) and a repeatedly sampled set of parasites of small mammals from New Mexico (Sevilleta Long Term Ecological Research Program). Estimation accuracy varied strongly with parasite taxonomy, number of parasite occurrence records, and the shape of host species-abundance distribution (i.e. the dominance and rareness of species in the host community). Our findings suggest that between 20% and 40% of parasite host ranges are currently unknown, highlighting a major gap in our understanding of parasite specificity, host-parasite network structure, and parasite burdens.

Foliar phosphorus content predicts species relative abundance in P-limited Tibetan alpine meadows

Traits that favor rapid plant growth are expected to have an important influence on species relative abundance (SRA) in semi-natural grasslands where competitive exclusion is reduced by grazing. However, which traits are more strongly associated with variation in SRA remains largely unknown, especially in P-limited but species-rich grasslands. We assessed the relative importance of traits predicting SRA in P-limited Tibetan alpine meadows subject to long term grazing. We assessed abundance in terms of both aboveground biomass and ramet numbers in relation to seven traits (specific leaf area, LDMC: leaf dry matter content, leaf carbon concentration, leaf nitrogen concentration, LPC: leaf phosphorus concentration, mature height, and seed size) for all component species in grazed and ungrazed plots in each of three contrasting sites on the Tibetan Plateau. We used CATS regression (i.e. regression for Community Assembly through Trait Selection) to quantify the relative importance of different traits in predicting SRA, and tested dependence of trait importance values on both environmental context and alternative measures of abundance. Species were primarily differentiated along a trade-off axis involving traits promoting nutrient acquisition for fast growth vs resource conservation and competitive ability. A higher LPC consistently was the most important trait predicting species abundance in grazed plots, while species with low LPC but higher LDMC held an advantage in ungrazed plots. This suggests that in competition-released communities, species with a high ability to uptake soil available P tend to become abundant, while in competition-dominated communities, species using fixed P efficiently do better. The relative importance of traits in ungrazed plots did not change across sites, but in grazed plots species that have traits associated with nutrient conservation were favored in colder and drier conditions. The results highlight the importance of traits affecting acquisition of soil available P in these Tibetan alpine meadow communities. Species tradeoffs in nutrient acquisition versus conservation are a primary determinant of SRA, but the relative importance of traits depended on both environmental context and whether SRA was estimated as above-ground biomass or as number of ramets.

Seasonal and Spatial Variations in Fish and Macroinvertebrate Communities of Oyster and Adjacent Habitats in a Mississippi Estuary

In Grand Bay National Estuarine Research Reserve (Grand Bay NERR), Mississippi, we used quantitative drop sampling in three common shallow estuarine habitats—low profile oyster reef (oyster), vegetated marsh edge (VME), and nonvegetated bottom (NVB)—to address the dearth in research comparing nekton utilization of oyster relative to adjacent habitats. The three habitats were sampled at two distinct marsh complexes within Grand Bay NERR. We collected a total of 633 individual fishes representing 41 taxa in 22 families. The most diverse fish family was Gobiidae (seven species) followed by Blennidae and Poeciliidae (three species each). We collected a total of 2,734 invertebrates representing 24 taxa in 11 families. The most diverse invertebrate family was Xanthidae (six species) followed by Palaemonidae (five species). We used ordination techniques to examine variation in species relative abundance among habitats, seasons, and sampling areas, and to identify environmental gradients correlated with species relative abundances. Our resulted indicated that oyster provided a similarly complex and important function as the adjacent VME. We documented three basic trends related to the importance of oyster and VME habitats: 1) Oyster and VME provide habitat for significantly more species relative to NVB, 2) Oyster and VME provide habitat for rare species, and 3) Several species collected across multiple habitats occurred at higher abundances in oyster or VME habitat. We also found that salinity, temperature, and depth were associated with seasonal and spatial shifts in nekton communities. Lastly, we found that the relative location of the two marsh complexes we studied within the context of the whole estuary may also explain some of the temporal and spatial differences in communities. We conclude that oyster habitat supported a temporally diverse and spatially distinct nekton community and deserves further attention in research and estuarine conservation efforts.

Effects of predation and variation in species relative abundance on the parameters of neutral models

Hubbell (2001) proposes that random demographic processes (i.e., neutral dynamics) can explain observed levels of variation in the richness and abundance of species within and among communities. Hubbell’s neutral models have drawn attention because they reproduce several characteristic features of natural communities. But neutral models are criticized for ignoring nonrandom processes known to cause species densities to fluctuate. We parameterized neutral models using the population counts of 64 species of aquatic invertebrates collected from 49 discrete rock pools over a 13 year period. We used Hubbell ‘s numerical modeling approach to evaluate the effect of natural population fluctuations on the parameter settings. We also analyzed the effect of observed variation on the species proportional abundance predicted by neutral models. We find that observed levels of variation in abundance are much higher than predicted by neutral models, forcing estimates of the migration probability, m, and fundamental biodiversity parameter, θ, to fluctuate over time. Much of the observed variation is mediated by predator-prey interactions. Low predator densities are associated with fewer species and less even relative abundances of species, resulting in lower estimates of m and θ compared to periods of high predator densities. Our results show that by assuming an identical survival probability for all species, neutral models misrepresent substantial aspects of community dynamics.