The invasibility of marine algal assemblages: role of functional diversity and identity

The emergence of the biodiversity-ecosystem functioning debate in the last decade has renewed interest in understanding why some communities are more easily invaded than others and how the impact of invasion on recipient communities and ecosystems varies. To date most of the research on invasibility has focused on taxonomic diversity, i.e., species richness. However, functional diversity of the communities should be more relevant for the resistance of the community to invasions, as the extent of functional differences among the species in an assemblage is a major determinant of ecosystem processes. Although coastal marine habitats are among the most heavily invaded ecosystems, studies on community invasibility and vulnerability in these habitats are scarce. We carried out a manipulative field experiment in tide pools of the rocky intertidal to test the hypothesis that increasing functional richness reduces the susceptibility of macroalgal communities to invasion. We selected a priori four functional groups on the basis of previous knowledge of local species characteristics: encrusting, turf, subcanopy, and canopy species. Synthetic assemblages containing one, two, three, or four different functional groups of seaweeds were created, and invasion by native species was monitored over an eight-month period. Cover and resource availability in the assemblages with only one functional group showed different patterns in the use of space and light, confirming true functional differences among our groups. Experimental results showed that the identity of functional groups was more important than functional richness in determining the ability of macroalgal communities to resist invasion and that resistance to invasion was resource-mediated.     

In search of quorum effects in metacommunity structure: species co-occurrence analyses

Alternative models of community assembly emphasize regional, stochastic, dispersal-based processes or local, deterministic, niche-based processes. Community ecology's historical focus on local processes implicitly assumes that local processes surpass regional processes over time or across space to derive nonrandom metacommunity structure (i.e., a quorum effect). Quorum effects are expected late in succession among nearby sites, whereas quorum effects are not expected early in succession among distant sites. I conducted a meta-analysis of zooplankton data sets encompassing time scales of one to thousands of years and spatial scales of <1 m to thousands of kilometers. Species co-occurrence analyses statistically evaluated presence/absence patterns relative to random patterns obtained with Monte Carlo null models. A series of weighted analyses was conducted and alternative randomization algorithms and null models were evaluated. Most zooplankton metacommunities were randomly structured in unweighted analyses, and the distribution of significant structure did not follow quorum effect predictions. Weighted analyses (e.g., by habitat area) revealed significant, nonrandom structure in most zooplankton metacommunities, but the distribution of significant structure still did not adhere well to quorum effect predictions. Finally, additional weighting for study scale (number of sites) nullified most significant area-weighted structure, and again, the distribution of significant structure did not follow quorum effect predictions. Overall, a quorum effect was not supported, perhaps related to zooplankton life histories and energetics and/or the quorum effect itself. Results at the presence/absence level of resolution indicated that local processes did not generally override regional processes over time or across space to drive community structure. A full integration of dispersal- and niche-based concepts in metacommunity dynamics will be most fruitful for unraveling community assembly. Species co-occurrence analyses were scale dependent (habitat area and study size). Future analyses should use weights for important factors (e.g., habitat area), and meta-analyses should include study scale as an additional factor contributing to apparent patterns.     

Developing multiple hypotheses in behavioral ecology

Researchers in behavioral ecology are increasingly turning to research methods that allow the simultaneous evaluation of hypotheses. This approach has great potential to increase our scientific understanding, but researchers interested in the approach should be aware of its long and somewhat contentious history. Also, prior to implementing multiple hypothesis evaluation, researchers should be aware of the importance of clearly specifying a priori hypotheses. This is one of the more difficult aspects of research based on multiple hypothesis evaluation, and we outline and provide examples of three approaches for doing so. Finally, multiple hypothesis evaluation has some limitations important to behavioral ecologists; we discuss two practical issues behavioral ecologists are likely to face.     

The impact of a rice based diet on urinary arsenic

Rice is elevated in arsenic (As) compared to other staple grains. The Bangladeshi community living in the United Kingdom (UK) has a ca. 30-fold higher consumption of rice than white Caucasians. In order to assess the impact of this difference in rice consumption, urinary arsenicals of 49 volunteers in the UK (Bangladeshi n = 37; white Caucasians n = 12) were monitored along with dietary habits. Total urinary arsenic (As(t)) and speciation analysis for dimethylarsinic acid (DMA), monomethylarsonic acid (MA) and inorganic arsenic (iAs) was conducted. Although no significant difference was found for As(t) (median: Bangladeshis 28.4 μg L(-1)) and white Caucasians (20.6 μg L(-1)), the sum of medians of DMA, MA and iAs for the Bangladeshi group was found to be over 3-fold higher (17.9 μg L(-1)) than for the Caucasians (3.50 μg L(-1)). Urinary DMA was significantly higher (p < 0.001) in the UK Bangladeshis (median: 16.9 μg DMA L(-1)) than in the white Caucasians (3.16 μg DMA L(-1)) as well as iAs (p < 0.001) with a median of 0.630 μg iAs L(-1) for Bangladeshi and 0.250 μg iAs L(-1) for Caucasians. Cationic compounds were significantly lower in the Bangladeshis (2.93 μg L(-1)) than in Caucasians (14.9 μg L(-1)). The higher DMA and iAs levels in the Bangladeshis are mainly the result of higher rice consumption: arsenic is speciated in rice as both iAs and DMA, and iAs can be metabolized, through MA, to DMA by humans. This study shows that a higher dietary intake of DMA alters the DMA/MA ratio in urine. Consequently, DMA/MA ratio as an indication of methylation capacity in populations consuming large quantities of rice should be applied with caution since variation in the quantity and type of rice eaten may alter this ratio.     

Swimming behavior of juvenile anchovies (<Emphasis Type="Italic">Anchoa</Emphasis> spp.) in an episodically hypoxic estuary: implications for individual energetics and trophic dynamics

Diel swimming behaviors of juvenile anchovies (Anchoa spp.) were observed using stationary hydroacoustics and synoptic physicochemical and zooplankton profiles during four unique water quality scenarios in the Neuse River Estuary, NC, USA. Vertical distribution of fish was restricted to waters with DO greater than 2.5 mg O₂ l⁻¹, except when greater than 70% of the water column was hypoxic and a subset of fish were occupying water with 1 mg O₂ l⁻¹. We made the prediction that an individual fish would select a swim speed that would maximize net energy gain given the abundance and availability of prey in the normoxic waters. During the day, fish adopted swim speeds between 7 and 8.8 bl s⁻¹ that were near the theoretical optimum speeds between 7.0 and 8.0 bl s⁻¹. An exception was found during severe hypoxia, when fish were swimming at 60% above the optimum speed (observed speed = 10.6 bl s⁻¹, expected = 6.4 bl s⁻¹). The anchovy is a visual planktivore; therefore, we expected a diel activity pattern characteristic of a diurnal species, with quiescence at night to minimize energetic costs. Under stratified and hypoxic conditions with high fish density coupled with limited prey availability, anchovies sustained high swimming speeds at night. The sustained nighttime activity resulted in estimated daily energy expenditure over 20% greater than fish that adopted a diurnal activity pattern. We provide evidence that the sustained nighttime activity patterns are a result of foraging at night due to a lower ration achieved during the day. During severe hypoxic events, we also observed individual fish making brief forays into the hypoxic hypolimnion. These bottom waters generally contained higher prey (copepod) concentrations than the surface waters. The bay anchovy, a facultative particle forager, adopts a range of behaviors to compensate for the effects of increased conspecific density and reduced prey availability in the presence of stratification-induced hypoxia.     

Simulating secondary succession of elk forage values in a managed forest landscape,western Washington

Modern timber management practices often influence forage production for elk (Cervus elaphus) on broad temporal and spatial scales in forested landscapes. We incorporated site-specific information on postharvesting forest succession and forage characteristics in a simulation model to evaluate past and future influences of forest management practices on forage values for elk in a commercially managed Douglas fir (Pseudotsuga menziesii, PSME)-western hemlock (Tsuga heterophylla, TSHE) forest in western Washington. We evaluated future effects of: (1) clear-cut logging 0, 20, and 40% of harvestable stands every five years; (2) thinning 20-year-old Douglas fir forests; and (3) reducing the harvesting cycle from 60 to 45 years. Reconstruction of historical patterns of vegetation succession indicated that forage values peaked in the 1960s and declined from the 1970s to the present, but recent values still were higher than may have existed in the unmanaged landscape in 1945. Increased forest harvesting rates had little short-term influence on forage trends because harvestable stands were scarce. Simulations of forest thinning also produced negligible benefits because thinning did not improve forage productivity appreciably at the stand level. Simulations of reduced harvesting cycles shortened the duration of declining forage values from approximately 30 to 15 years. We concluded that simulation models are useful tools for examining landscape responses of forage production to forest management strategies, but the options examined provided little potential for improving elk forages in the immediate future.     

Environmental effects of soil property changes with off-road vehicle use

The effects of off-road vehicles (ORVs) on the physical and chemical properties of 6 soil series were measured at Hollister Hills State Vehicular Recreation Area in central California. Accelerated soil erosion and the alteration of surface strength, bulk density, soil moisture, temperature, and soil nutrients were quantified to gain an insight into the difficulty of revegetating altered, or modified, areas.Erosion is severe at Hollister Hills, particularly in coarse grained soils on steep slopes. Erosion displaced 0.5 and 3.0 metric tons per square meter on 2 trails on gravelly sandy loam, and 0.3 metric tons/m² from a trail on sandy loam. The surface strength and bulk density increased while the soil moisture decreased in gravelly sandy loam, coarse sandy loam, sandy loam, and clay. Clay loam had an increased surface strength with variably increased bulk density and no decrease in soil moisture. Diurnal temperature fluctuations increased and organic material and soil nutrients decreased in soil modified by vehicles.These property changes increase the erosion potential of the soil, impede germination of seedlings, and slow natural revegetation. Management methods in ORV-use areas should include planning trails by prior application of the universal soil loss equation and soil surveys, trail closure before complete loss of the soil mantle, and revegetation of closed areas.     

THE INFLUENCE OF ENGINEERING DESIGN AND OPERATION AND OTHER ENVIRONMENTAL FACTORS ON RESERVOIR FISHERY RESOURCES1

The apparent effect of selected reservoir environmental variables-including surface area, mean depth, outlet depth, thermocline depth, water level fluctuation, storage ratio, shore development, total dissolved solids, growing season and age of reservoir–on fish standing crop in 140 large impoundments has been explored through partial correlation and multiple regression analyses. The sample was partitioned into 25 subsamples based on reservoir use type, water exchange rate, thermocline formation and water chemistry. Fish standing crops were estimated by summer rotenone sampling of coves or open water areas enclosed by blockoff net. Logarithmic partial correlation revealed highly significant (0.01 confidence shore development and dissolved solids on At the 0.20 confidence level, the crop of storage ratio and shore level) positive effects of outlet depth, total standing crop in the entire sample. all sport fishes is positively influenced by outlet depth, development and negatively by mean depth. In 54 hydropower reservoirs with a stable thermocline, positive effects of increased storage ratio and dissolved solids on t o t a l crop are evident at the 0.05 confidence interva. Increase in thermocline depth has a negative effect. In 25 hydropower reservoirs without a stable thermocline, clupeid (shad) crop is negatively correlated with surface area, mean depth and fluctuation. Reservoirs with a thermocline have higher standing crops than those without. At the species or species group level, partial correlation of nine environmental variables a t the 0.05 confidence interval reveals: Positive effect of surface area on pike and pickerel; buffalo-fishes, white crappie and total sport fish crop; positive effect of outlet depth on largemouth bass, catfishes, total sport fish crop and buffalofishes; a negative effect of water level fluctuation on pike and pickerel, redear sunfish and gizzard shad; a positive relationship between storage ratio and channel catfish and bull- heads and a negative one with flathead catfish and suckers; a positive effect of total dissolved solids on black and whit basses, catfishes, gizzard shad, carpsuckers and carp. A morphoedaphic expression, total dissolved solids divided by mean depth, provides a useful index t o reservoir fish production. The relationship is curvilinear, with maximum crops expected at index values of 5 t o 30.The index accounts for 62 percent of the variability in hydropower storage reservoir crops. Several multivariable regressions have been derived f o r predictive purposes. Examples are included, with R values of 35 t o 60.