Mean Number Of Taxa Research Articles

Colonization of New Experimental Ponds by Benthic Macroinvertebrates

Benthic macroinvertebrate colonization of 12 0.04-ha experimental ponds was studied for 1 yr. The ponds were filled in late January 1988 and artificial substrate samples were retrieved at 4-wk intervals from 25 February 1988 to 25 February 1989. The first organisms ( Chironomus and Ceratopogonidae) were collected in the first set of samples 4 wk after filling. The colonization sequence was Diptera (primarily several genera of chironomids), followed by Coleoptera, Ephemeroptera, and Odonata. The arrival of taxa corresponded to adult flight periods. Noninsect taxa (Oligochaeta, Nematoda, Amphipoda) were collected in relatively low numbers late in the year. Twenty-nine taxa were collected, 13 of which were present in all 12 ponds. Mean number of taxa, diversity, and evenness increased rapidly during the first 7 mo, but never became very high. Mean density increased slowly throughout the spring and summer to about 200 individuals per sampler and then increased rapidly during the autumn to about 600 individuals per sampler. Community structure was dominated by Chironomidae (≈85%) and Ephemeroptera (≈9%). Trophic function was dominated by detritus-feeding collectors (≈79%). Factors that influenced the structure and function of the new experimental ponds appeared to be: lack of connection to colonized waters, small size, and simple, homogeneous habitat and food resources.

Spatiotemporal Variation in Insect Mutualists of a Neotropical Herb

We observed striking spatiotemporal variation in the antguard and pollinator mutualist assemblages of a neotropical herb. Fifteen different ant taxa (in five subfamalies) and five pollinator taxa (four euglossine bees and one anthrophorid bee) were associated with the plants during censuses at four sites. Ants and pollinators were censused for 4 and 3 yr, respectively. The mean number of taxa per site per year was 6.8 for ants and 3.6 for pollinators. The ant assemblage varied more than the pollinator assemblage, paticularly spatially. The ant assemblage varied more through space than through time (means of the proportional similarities for pairwise comparisons of assemblages were 0.37 and 0.51, respectively), while the pollinator assemblage varied equally through space and time (means of the proportional similarities were 0.53 and 0.50, respectively). For individual taxa of both assemblages, the coefficient of variation in relative abundance was large (>75%) for the majority of spatial and temporal comparisons. In each assemblage there was a single taxon (in the antguard, a rare taxon, but in the pollinators, an abundant taxon) that was the least variable, surprisingly in both time and space, in its relative abundance. These results, coupled with our previous findings of significant variation among mutualist taxa in the magnitude of their beneficial effects, indicate that plants may be subject to highly variable selection by their mutualist assemblages. The most beneficial antguard was consistently rare; the most beneficial pollinator was abundant in one site—year but rare or absent in most site—years. The taxon that was consistently abundant was not an efficient pollinator. Evolutionary specialization of plants on particular animals may be constrained by lack of constancy in the relative abundance of animals and the opportunity for specialization may differ greatly between interactions (e.g., antguards and pollinators) due to divergent patterns of constancy, rarity, and quality.

EFFECTS OF SPATIAL ORIENTATION OF MULTIPLE PLATE ARTIFICIAL SUBSTRATES ON INVERTEBRATE COLONIZATION1

ABSTRACT: Jumbo multiple plate samplers were suspended in a river at 0.3 and 1 m depth in one of three orientations: interplate spaces closed to downwelling light and open to flow, open to light and flow, or open to light and closed to flow. Mean numbers of colonizing taxa and individuals were not significantly different among orientations after eight weeks of submergence. All variables tested decreased significantly with depth. Mean number of taxa, number of individuals (1 m only), and insect diversity decreased between samplings at four and eight weeks for samplers that were closed to light and open to flow. All orientations, depths, and durations had one‐half to two‐thirds of the total taxa. Colonization was affected by location in the river and position on sampler suspension equipment. The results indicate lack of orientation effects on colonization or high variability that obscured such effects. The sampler suspension equipment possibly increased among‐sampler variability by forming artificial snag habitats, and interplate light and flow conditions at different orientations may not have been sufficiently distinct to elicit different biological responses. Individual samplers provided diverse microhabitats regardless of orientation, but it would be prudent to include orientation among the variables considered in use of multiple plate samplers.

Biology and chemistry of three Pennsylvania lakes: Responses to acid precipitation

The biology and chemistry of three northeastern Pennsylvania lakes was studied from summer 1981 through summer 1983 to evaluate lakes with different sensitivities to acidification. At the acidified lake (total alkalinity ≤ 0.0 μeq L−1) there were fewer phytoplankton and zooplankton species than at the moderately sensitive lakes. The most numerous plankton species in all three lakes are reportedly acid tolerant. Among the benthic macro- invertebrates (BMI) there were more acid tolerant Chironomidae at the acidified lake, but more acid intolerant Ephemeroptera and Mollusca and a higher wet weight at the least sensitive lake. There were no differences among the lakes' BMI mean total numbers or mean number of taxa. The fish community at the acidified lake was dominated by stunted Lepomis gibbosus, but L. machrochirous were most abundant in the other lakes. Principal component analysis suggested a shift in all three lakes over the sampling period toward combined lower pH, alkalinity, specific conductance, Ca and Mg and higher Al and Mn. Such chemical changes have been associated with acidification. The rate and extent of acidification appeared to be controlled by geological and hydrological characteristics of the drainage basins.

A comparative study of seven grabs used for sampling benthic macroinvertebrates in rivers

SUMMARY. After considering the large number of grabs described in the literature, seven grabs of weight < 25 kg were chosen for manual operation from a small boat: Van‐Veen grab, weighted and unweighted Ponar grabs, Friedinger version of the Petersen grab, Dietz‐La Fond mud‐snapper, pole‐operated Birge‐Ekman grab and pole‐operated Allan grab.Random samples (number of sampling units n= 10) were taken in a large tank with a known number of 2‐mm cylindrical plastic pellets amongst stones of uniform size. Separate experiments were performed with four sizes of stones (model ranges: 2–4 mm, 8–16 mm, 16–32 mm, 32–64 mm). Stratified random samples (n= 10) were taken in rivers and the modal particle sizes at four sites were 0.004–0.06 mm, 0.5–2 mm, 16–64 mm and 64–128 mm. All grabs usually took a representative sample of the substratum at each site with no strong bias towards a particular particle size. The general performance of the Friedinger, Dietz‐La Fond and Allan grabs was poor, except on a muddy bottom, with frequent failure to operate, small samples of substratum and a mean depth of penetration < 3 cm in all substrata except mud for the Dietz‐La Fond and Allan grabs. The Van‐Veen and Birge‐Ekman grabs sampled to a mean depth < 3 cm in mud and fine gravel (2–4 mm), but the Birge‐Ekman jammed frequently in fine gravel. Both Ponar grabs operated well and sampled to a mean depth ≥ 5 cm in mud and fine gravel, > 3 cm when small stones (8–16 mm) were present and 2 cm (weighted Ponar only) when larger stones (> 16 mm) were present in a gravel bottom. The mean depth was <0.8 cm for all grabs when larger stones (>16 mm) were predominant on the bottom.In the tank experiments with pellets, the efficiencies for the total catches of the Friedinger, Dietz‐La Fond and Allan grabs were low with values <45% for fine gravel (2–4 mm), < 22% for small stones (8–16 mm) and <5% for a substratum of larger stones (>16 mm). If 50% is the minimum acceptable efficiency, then the Ponar, Van‐Veen and Birge‐Ekman grabs were adequate for fine gravel, only the two Ponar grabs were adequate for small stones and no grabs were adequate for sampling a substratum of larger stones (>16 mm).In field trials, the relative abundances of major taxa were similar for most grabs at each site; Friedinger and Dietz‐La Fond grabs were the major exceptions. In terms of both mean number of taxa and mean number of invertebrates m−2 the Ponar, Birge‐Ekman and Allan grabs performed well on the predominantly muddy substratum at site 1, but only the weighted Ponar grab performed adequately on the predominantly gravel bottom with some large stones (>16 mm) at site 2. All grabs performed badly when larger stones (>16 mm) were predominant on the bottom (sites 3, 4).The relationship between the variances and means of the samples taken with each grab followed a power law for the catches of pellets in tank experiments, and for major taxa and total numbers at each site in field trials. Values of exponents in the power law lay within the range 1.14–2.34. The coefficient of variation was also frequently related to the sample mean and was an unreliable statistic for comparing the precision of grabs.