The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). II. Abundance, biomass and production

 The abundance and biomass of Corophium multisetosum Stock, 1952 were determined from benthic corer samples collected monthly over 1 yr in the upper reaches of Canal de Mira (Ria de Aveiro, Portugal). Both density and biomass over the sampling period were negatively correlated with water temperature and positively correlated with chlorophyll a concentration in the sediment. C. multisetosum density was significantly negatively correlated with plant biomass and positively correlated with salinity. The nature of the sediment, favourable environmental conditions, high availability of food and low interspecific competition allowed the population to reach a maximal density of 200 × 10³ individuals m⁻² and a maximal biomass (ash-free dry wt, AFDW) of 62 g_AFDW m⁻². The population was highly productive, especially during the autumn/winter period. Production, estimated by two different methods (Hynes method: 251 g_AFDW m⁻² yr⁻¹; Morin–Bourassa method: 308 g_DW m⁻² yr⁻¹), was much higher than the values reported for other Corophium species. The annual P:Bˉ ratio (10) was high, but similar to values reported for Swedish populations of C. volutator and lower than the values estimated from Mediterranean populations of C. insidiosum. Received: 8 October 1999 / Accepted: 22 June 2000     

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). I. Life history and aspects of reproductive biology

The population of Corophium multisetosum Stock, 1952 in Areão displayed a semiannual, iteroparous life history. Mean longevity was ～6?mo, with the estimated life span longer for overwintering individuals born in autumn than for individuals born in spring. Length-frequency data indicated that the length increment per moult is probably higher in males than females; however females moulted more frequently and achieved a larger body size. Preliminary growth rates were 100?μm?d^?1 for juveniles and 19 to 29?μm?d^?1 for mature females, with the lower values occurring during the winter. It was estimated that under favourable conditions females may attain reproductive size and mature within 1?mo. Although incubating females were present all year round, recruitment occurred in spring, almost ceased during the summer, peaked in autumn, and decreased again during the winter. Extreme temperatures and very low salinities during winter and summer may have deterred breeding, while moderate temperatures (15 to 20?°C) and salinities > 1?psu in spring and autumn were apparently favourable for reproduction. The unfavourable summer conditions constrained breeding and synchronised the timing of reproduction. In late-autumn and during the winter, as temperature decreased and brooding time increased, synchrony was progressively lost. Brood size varied as a function of embryonic developmental stage, size of incubating females, and season. The life-history pattern and reproductive features of C. multisetosum in Areão are closely related to temperature and salinity; other environmental conditions such as oxygen content of the water and food availability may also be relevant.     

Growth and development of nauplii and copepodites of the estuarine copepod Acartia tonsa from southern Europe (Ria de Aveiro, Portugal) under saturating food conditions

A temperature-dependent growth model is presented for nauplii and copepodites of the estuarine calanoid copepod Acartia tonsa from southern Europe (Portugal). Development was followed from egg to adult in the laboratory at four temperatures (10, 15, 18 and 22°C) and under saturating food conditions (>1,000 μg C l⁻¹). Development times versus incubation temperature were fitted to a Belehradek’s function, showing that development times decreased with increasing incubation temperature: at 10°C, A. tonsa need 40.3 days to reach adult stage, decreasing to 8.9 days when reared at 22°C. ANCOVA (homogeneity of slopes) showed that temperature (P<0.001) and growth phase (P<0.01) had a significant effect on the growth rate. Over the range of temperatures tested in this study, highest weight-specific growth rates were found during naupliar development (NI–NVI) and varied from 0.185 day⁻¹ (10°C) to 0.880 day⁻¹ (22°C) with a Q ₁₀ equal to 3.66. During copepodite growth (CI–CV), the weight-specific growth rates ranged from 0.125 day⁻¹ (10°C) to 0.488 day⁻¹ (22°C) with a Q ₁₀ equal to 3.12. The weight-specific growth rates (g) followed temperature (T) by a linear relationship and described as ln g=−2.962+0.130 T (r ²=0.99, P<0.001) for naupliar stages and ln g=−3.134+0.114T (r ²=0.97, P<0.001) for copepodite stages. By comparing in situ growth rates (juvenile growth and fecundity) for A. tonsa taken from the literature with the temperature-dependent growth model defined here we suggest that the adult females of A. tonsa are more frequently food limited than juveniles.     

Trace Organic Matter in the Ria Formosa,Portugal

The Ria Formosa lagoon, Portugal, receives organic matter from many natural and anthropogenic sources. This paper presents an overview of the contribution these sources made through the use of lipid biomarkers. Sixty-one surface sediment samples were collected and analysed for a range of lipids. Sewage materials were confined to regions within 2 km of discharge points; phytoplankton biomarkers (sterols and fatty acids) suggest production was greatest in the inner parts of the lagoon that also have greatest inorganic nutrient enrichment; terrestrial organic matter was present in relatively low concentrations as shown by both sterol and fatty alcohols; bacteria were widespread although the Sulphate Reducing Bacteria (SRBs, shown by the presence of 3-OH fatty acids) were located near but not adjacent to sewage discharge points.     

Microbiological monitoring of Ruditapes decussatus from Ria Formosa Lagoon (South of Portugal)

Ria Formosa Lagoon is the most important bivalve production region of the South Portuguese coast. However, the microbial contamination has been increasing in the northwest area of the lagoon. This study assesses the microbiological contamination of clams Ruditapes decussatus from the most contaminated area for 1 year period. Escherichia coli levels were weekly determined and related with variations in several environmental parameters: precipitation, atmospheric temperature, solar radiation, wind velocity, humidity and tide height. Highest E. coli levels in clams were associated with periods of heaviest rainfall and lowest temperature, mainly due to diffuse sources of faecal discharges. A seasonal trend was observed: microbial contamination in clams was lowest in summer, with highest temperatures, intense solar radiation and dry weather and higher in the remaining seasons, especially in winter due to the runoff of faecal contaminated waters. Several measurements not only to protect the quality of the lagoon, but also to protect public health were recommended. The most imperative will be to detect and extinguish the diffuse contamination sources. Otherwise it might be necessary to implement an interdiction period of bivalve harvesting after heavy rainfalls.     

Trace Organic Matter in the Ria Formosa, Portugal

The Ria Formosa lagoon, Portugal, receives organic matter from many natural and anthropogenic sources. This paper presents an overview of the contribution these sources made through the use of lipid biomarkers. Sixty-one surface sediment samples were collected and analysed for a range of lipids. Sewage materials were confined to regions within 2 km of discharge points; phytoplankton biomarkers (sterols and fatty acids) suggest production was greatest in the inner parts of the lagoon that also have greatest inorganic nutrient enrichment; terrestrial organic matter was present in relatively low concentrations as shown by both sterol and fatty alcohols; bacteria were widespread although the Sulphate Reducing Bacteria (SRBs, shown by the presence of 3-OH fatty acids) were located near but not adjacent to sewage discharge points.     

Spatial and temporal changes in the demersal fish community of the Ria de Arosa (NW Spain)

The demersal fish community was sampled seasonally in three habitats (beach, raft and middle) of the inner, middle, and outer Ria de Arosa (NW Spain). A total of 73 species of demersal fish, dominated by the family Gobiidae, were found. There were no important seasonal changes in abundance, with similarity values always higher than 60% between samples from two different sampling periods. Raft stations in the ria show the highest values of abundance all year, containing almost 1 individual m^-2 in winter. The highest diversity indices were those from beach areas (2.11 to 2.27) and the lowest ones those from middle areas (1.13 to 1.86). Similarity indices and factorial analysis revealed three major groupings of species and samples, related to water depth and nature of the bottom.     

Population age structure and rhizome growth of<Emphasis Type="Italic"> Cymodocea nodosa</Emphasis> in the Ria Formosa (southern Portugal)

The population age structure and derived population dynamics (recruitment, growth and mortality), rhizome growth and flowering effort of Cymodocea nodosa in the Ria Formosa (south of Portugal) were examined using reconstruction techniques. The horizontal rhizome elongation rates were low (13.8–30.7 cm year^–1), which is consistent with the low population growth rate (–2.35 to 0.29 year^–1). The vertical elongation rate was proportional to the horizontal growth except for one stand that was subjected to sand movements and had higher elongation rates. The mean leaf production varied from 11.8 to 14 leaves year^–1. The average shoot age varied from 0.70 to 1.04 years, and the oldest shoot found was 7.6 years. The mortality rates were the highest as yet reported for this species (0.99–3.70 year^–1). Observed low growth and high mortality are possibly associated with the development of eutrophic conditions in the Ria Formosa. The flowering probability found in Ria Formosa stands (<0.009–0.90 shoots^–1 year^–1) was comparable to that found in the Mediterranean and off the Canary Islands. Only two stands with flowering events were found inthe Ria Formosa population, which indicates that the stands are highly dependent on clonal growth to maintain themselves inthe Ria Formosa.Communicated by O. Kinne, Oldendorf/Luhe     

Feeding ecology of Liocarcinus depurator (Decapoda: Portunidae) in the Ria de Arousa (Galicia,north-west Spain): effects of habitat,season and life history

The diet of the portunid crab Liocarcinus depurator (L.) in soft-bottom areas of the Ría de Arousa (Galicia, north-west Spain) was studied by analysis of stomach contents, and by comparison of habitats of contrasting abundance, composition and diversity of prey communities. Monthly samples were taken from July 1989 to June 1990 (N=3747) in mussel (Mytilus galloprovincialis) raft-culture areas and central channel zones. Ninety-two food items were identified: crustaceans, molluscs, polychaetes, ophiuroids and fishes were the dominant prey. The raft epifauna and the mussels constituted the dominant group of prey in the raft areas and inner channel station (the epifaunal crab Pisidia longicornis comprised 30 to 52% of the diet), whereas the benthic macroinfauna was the dominant food component in the mid-outer channel (polychaetes constituted 32% of the diet). In the epifaunal community, diet selection for P. longicornis was positive and for amphipods was negative, whereas within the infauna diet selection for bivalves and ophiuroids was positive and for polychaetes negative or neutral. The seasonal importance of P. longicornis and M. galloprovincialis varied in the diet of L. depurator parallel to seasonal activities connected with mussel culture (seeding, thinning, harvesting) that determine the availability of prey from the rafts. Major changes in the composition of the diet related to the life history of L. depurator occurred during ontogeny. In the raft zones, consumption of mussels and fishes increased with increasing body size, while the consumption of Natantia, non-decapod crustaceans, gastropods, polychaetes Pectinariidae and plants decreased with increasing crab size. Maximum consumption of P. longicornis was by L. depurator individuals of intermediate size (15 to 44 mm carapace width). Stomach fullness (measured as dry weight of food) increased with increasing body size, but with negative allometry. During the late pre-moult and early post-moult periods L. depurator did not consume any food; maximum stomach fullness was recorded for the late post-moult and intermoult stages. Female food consumption decreased during egg incubation. Food consumption was greater in autumn and winter than at other seasons; this appears to be related to seasonal changes in reproduction and moulting.     

The abundance and life histories of terrestrial isopods in a salt marsh of the Ria Formosa lagoon system,southern Portugal

Four species of isopod characteristic of salt marsh habitats, Tylos ponticus, Porcellio lamellatus, Halophiloscia couchii and Armadillidium album coexist in the upper reaches of the Ria Formosa lagoon salt marsh system in southern Portugal. In this locality, T. ponticus is the most abundant of the four species with mean annual densities of 2,950 m⁻² and a peak density of 10,387 m⁻² in July 1998 which is very much higher than what has previously been recorded for any isopod in any habitat. The mean annual densities for the other species were lower: P. lamellatus 36 m⁻², A. album 19 m⁻² and H. couchii 3 m⁻², indicating a less significant role in this ecosystem. Tylos ponticus and A. album started to breed on May, 24 and 12 months after release from the marsupium, respectively, where as other species start to breed in March, 12 months after their release from the marsupium. Tylos ponticus has a relative growth rate (RGR) of 0.23 between release from the marsupium and time of first breeding in July of its second year and breeds at a mature mass of 3.6 mg AFDM whereas the other three species mature after 10–12 months, have more than double this RGR but because of the shorter pre-reproductive period breed at masses of 1.8 mg AFDM for P. lamellatus, 1.0 mg AFDM for H. couchii, and 1.1 mg AFDM for A. album, respectively. The mass specific fecundity of all three of the less abundant species was higher than that of T. ponticus but the offspring of T. ponticus were ten times heavier than those of the next largest species, P. lammellatus. The difference in abundances between the species is interpreted as being due to the larger mass of the offspring of the most successful species. This larger mass confers an adaptive advantage due to larger size being associated with reduced juvenile mortality for isopods under abiotically stressful conditions. M. Sprung died in an accident on June 18, 2003     

Corophium multisetosum (Amphipoda: Corophiidae) in Canal de Mira,Portugal: Some factors that affect its distribution

The population ofCorophium multisetosum (L.) from Canal de Mira, Ria de Aveiro, Portugal, was sampled seasonally from December 1985 to September 1986, as part of a larger survey of the benthic invertebrate macrofauna. Its distribution along the channel exposes the species to a range of salinities from freshwater to above 30. A principal components analysis, using the physical and chemical parameters of the sediment as variables and sampling stations as operational taxonomic units, indicated that abundance is negatively correlated with salinity, depth, and the occurrence of sediments rich in particles below 125µm and rich in organic matter. Abundance is positively correlated with temperature. The distribution of the species does not seem to be affected by the occurrence of sediment grades between 125 and 1 000µm. Paired-choice salinity experiments indicated thatC. multisetosum prefers salinities within the range 2.5 to 10. In multi-choice experiments concerning sediment grade, amphipods did not show any significant preference within the 125 to 500µm range, although the 125µm grade was chosen less frequently. The influence of temperature on the overall distribution ofC. multisetosum is discussed.     

Three-dimensional hydrodynamic modeling of the Chicago River, Illinois

We present and describe results from a three-dimensional model applied to the Chicago River under a wet weather condition. Chicago River is an integral part of Chicago, the third largest city in United States, for recreational, commercial and industrial purposes. Although the flow in the Chicago River is supposed to be from north to south, the flow acquires a three-dimensional nature at various points in the river. The reasons for the three dimensionality of the flow can be attributed to the dynamic boundary conditions generated by combined sewer overflow (CSO) events during wet weather conditions, as well as the presence of confluences of various branches of the river with creeks and slips. This work presents the hydrodynamic application and validation of a three dimensional numerical model, which was used to simulate the flow in the Chicago River for a period of 8?days in September of 2008, during which the torrential rains in the city of Chicago led to intense CSO events. The numerical model used for the aforementioned exercise is the Environmental Fluid Dynamics Code (EFDC) which solves the three dimensional vertically hydrostatic, free surface, turbulence averaged equations of motions for a variable density fluid. The results obtained by the numerical model were validated with the help of stage values obtained from the USGS gauging station [Station No. 05536123] present inside the domain of interest. It was estimated that during the storm September 2008, several billion gallons of water were discharged into Lake Michigan from the Chicago River Controlling Works to prevent city flooding. The 3D model also provides a tool for the management of the waterways, in particular for future analysis of different alternatives being considered to reverse the Chicago River once again with the goal of preventing the passage of invasive species (e.g. Asian carp) towards the Great Lakes.     

Carbon partitioning among the first trophic levels in the North Western Adriatic Basin

In the frame of PRISMA II Project samples for plankton analyses were carried out during four cruises (June, 1996; February and June, 1997; February, 1998) in order to assess the relative importance in term of biomasses of the three main size fractions (pico- <2 r µm, nano- 2-20 r µm and micro-plankton >20 r µm). Spatial and temporal distribution of the three plankton fractions were described as abundance and contribution to the total carbon content in an area between Po River mouth and Rimini. The relative contribution of picoplankton resulted higher in the offshore zone, while that of nanoplankton in the inshore waters. In February 1998 microphytoplankton, mainly constituted by diatoms, was very abundant in the inshore waters. Micro-zooplankton was always very scarce. Cluster analyses performed on these data grouped the stations on the basis of their community structure, and agreed with the hydrological features. Small size classes contributed more significantly to the total plankton carbon content in most of the situations. Microplankton fraction contribution was relevant only during spring diatom bloom of February 1998 and with a less extent in the confined coastal summer blooms.     

Biogeochemical characterization of surface waters in the Aljustrel mining area (South Portugal)

Aljustrel mining area (South Portugal) is a part of the Iberian Pyrite Belt and encloses six sulfide mineral masses. This mine is classified of high environmental risk due to the large tailings’ volume and acid mine drainage (AMD)-affected waters generated by sulfides’ oxidation. The use of biological indicators (e.g., diatoms) revealed to be an important tool to address the degree of AMD contamination in waters. Multivariate analysis has been used as a relevant approach for the characterization of AMD processes. Cluster analysis was used to integrate the significant amount and diversity of variables (physicochemical and biological), discriminating the different types of waters, characterized by the high complexity occurring in this region. The distinction of two main marked phenomena was achieved: (1) the circumneutral-Na-Cl water type (sites DA, PF, BX, BF, RO, CB), expressing the geological contributions of the Cenozoic sediments of Sado river basin, with high diatom diversity (predominating brackish diatoms as Entomoneis alata); and (2) the acid–metal-sulfated water type (sites BM, JU, RJ, AA, MR, BE, PC, AF), reflecting both the AMD contamination and the dissolution of minerals (e.g., silicates) from the hosting rocks, potentiated by the extremely low pH. This last group of sites showed lower diatom diversity but with typical diatoms from acid- and metal-contaminated waters (e.g., Pinnularia aljustrelica). In addition to these two water types, this hierarchical classification method also allowed to distinguish individual cases in subclusters, for example, treated dams (DC, DD), with alkaline substances (lime/limestone), that changed the physicochemical dynamics of the contaminated waters.

     

Trophic levels and trophic tangles: the prevalence of omnivory in real food webs

The concept of trophic levels is one of the oldest in ecology and informs our understanding of energy flow and top-down control within food webs, but it has been criticized for ignoring omnivory. We tested whether trophic levels were apparent in 58 real food webs in four habitat types by examining patterns of trophic position. A large proportion of taxa (64.4%) occupied integer trophic positions, suggesting that discrete trophic levels do exist. Importantly however, the majority of those trophic positions were aggregated around integer values of 0 and 1, representing plants and herbivores. For the majority of the real food webs considered here, secondary consumers were no more likely to occupy an integer trophic position than in randomized food webs. This means that, above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores. Omnivory was most common in marine systems, rarest in streams, and intermediate in lakes and terrestrial food webs. Trophic-level-based concepts such as trophic cascades may apply to systems with short food chains, but they become less valid as food chains lengthen.     

The cladoceran trophic status in the nitrogen limited ecosystem of Lake Kinneret (Israel)

The impactof cladocerans metabolic activities on the carbon (C), nitrogen (N) and phosphorus (P) dynamics in Lake Kinneret (Israel) is presented. The study, is based on the incorporation of field data and experimental measurements. Grazing, respiration and production rates of Diaphanosoma spp., Ceriodaphnia spp. and Bosmina spp. were experimentally measured at three temperatures, and the results were extrapolated to the field biomass distribution atthese respective temperatures, and the total lake capacity was calculated using the following equation: consumption = respiration + production + excretion. The field capacity of consumption, respiration and production were found to be mostly correlated with biomass density, but the temporal fluctuation of the percent of excretion from consumed energy differed. The increase in P, decline in N and decrease in the TN/TP mass ratio in the epilimnion of Lake Kinneret during 1969-2004 created N limitation. An increase in C and dedine in TN, with a consequential increase in the C/TN ratio were documented. TP was augmented but the C/TP ratio was only slightly increased. During 1975-2004, P was probably, a minor limitation for cladoceran growth. The positive impact of recycled P by cladocerans underthe N limitation in Lake Kinneret is discussed. There is a current threat on the water quality, derived from N limitation (mostly in summer-fall) and consequent Cyanophyta blooms. Thus, the role of recycled P bygrazers may be significant.     

Effects of ultraviolet-B enhancement on marine trophic levels in a stratified coastal system

We examined the effects of enhanced UV-B radiation (relative to ambient) on marine trophic levels inhabiting a stratified coastal ecosystem, using living models (13 000 liter marine enclosures) of a temperate estuarine water column. The experiment was carried out in June and July 1994 on a plankton community drawn from lower Narragansett Bay, Rhode Island, USA. The effects of altered UV-B radiation (elevated 50% over ambient, tenfold DNA-weighted) on three trophic strata: the primary producers (photosynthetic algae), primary herbivores (copepods), and fish eggs and larvae (Anchoa mitchilli Cuvier and Valenciennes) were examined. The goal was to determine if UV-B–induced alterations at the base of the food chain had impacts on other elements of the trophic web. Phytoplankton abundance (P=0.02) and biomass (P=0.007) were significantly reduced in UV-B–enhanced treatments, above but not below the thermocline (2.25 m), during the second week of the study. Copepod nauplii were significantly less abundant in UV-B–enhanced mesocosms than in control treatments during the third and fourth weeks of the study (P=0.01). A portion of the impact on nauplii may be a result of alterations at the base of the food web. The greater mortality of nauplii in UV-B–enhanced systems did not translate to reduced abundance of copepodite (P=0.83) or adult (P=0.29) copepods. No significant effects were observed for microzooplankton (P=0.15). Neither the mortality rates nor the growth rates of larval anchovy were significantly affected by the experimental increase in UV-B (P>0.05). Despite the tenfold increase in biologically damaging UV-B, effects were not seen at higher trophic levels, most likely because of the rapid extinction of UV-B in the highly colored coastal water. Received: 13 November 1996 / Accepted: 18 July 1997     

Dynamic fate modeling of γ-hexachlorocyclohexane in the lower reaches of the Liao River

A QWASI model dependent on temperature is parameterized to describe the long-term fate of persistent organic pollutants (POPs) in the Liao River. The model parameters, namely fugacity capacity, degradation rate, and transfer coefficient, are profoundly affected by temperature. This model is used to simulate the fate of γ-hexachlorocyclohexane (γ-HCH) in the lower reaches of the Liao River from 1998 to 2008. Modeling results show that γ-HCH fugacity capacities in air, water, and sediment increase as temperature decreases, and the transfer and transformation rate coefficients increase as temperature increases. The variations of transfer and transformation parameter D values depend on fugacity capacities, and transfer and transformation coefficients simultaneously. The performance of the model is evaluated by comparing the predicted and observed concentrations in the water and sediment of the Liao River. The predicted values agree well with the observed value in the order of magnitude, in most cases within the factor of 3. It is believed that the model is appropriate for simulating the long term fate of POPs in the Liao River.     

Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems

Summary. Chemical, physiological and behavioral components of pheromone communication have been described for a number of bark beetle species, yet our understanding of how these signals function under natural conditions remains relatively limited. Development of ecologically based models is complicated by the multiple functions and sources of variability inherent in bark beetle semiochemistry. This discussion addresses four ecological issues of chemical signaling in bark beetles: the effects of aggregation on individual fitness, the possibility of cheating, how plants can defend themselves against herbivores that employ aggregation pheromones, and the implications of variability in chemical communication systems to predator avoidance. An analysis of published data from thirteen scolytid – conifer systems indicates that the net benefit and optimal colonization density vary with host condition and beetle species. When beetles attack live trees, the benefit of cooperative host procurement exceeds losses due to competition for the limited substrate, at least up to moderate densities. When beetles colonize dead tissue, however, the effect of subsequently arriving beetles on initial colonizers is almost entirely negative. This suggests that aggregation originated as exploitation of senders, but evolved into manipulation of receivers. It is also proposed that the optimal colonization density which typifies each species or population may offer a more objective and less value–laden index of behavior than current labels such as “aggressiveness”. Beetles can maximize the relative benefits of group attack by incorporating instantaneous measures of host resistance into their colonization behavior, and by adjusting oviposition with colonization density. This system may provide opportunities for cheating. However a number of factors may select against a fixed strategy of cheating, including the linkage between tree allelochemistry and beetle semiochemistry, the reduced quality of substrate available to late arrivers, the short adult lifespans of most bark beetles, differential exposure to some predators, the difficulty of locating signalers during extensive endemic periods, and the low costs incurred during host assessment. However, the possibility that beetles employ flexible, density – dependent strategies deserves heightened attention. The ability of bark beetles to collectively exhaust host defenses poses a particular problem for plant defense. It is argued here that the ideal defense should include both direct resistance mechanisms against invading beetles, and indirect mechanisms that inhibit chemical communication. Evidence for the latter mechanism is explored. The ability of predators to efficiently exploit aggregation pheromones as kairomones in prey finding poses significant risk to bark beetles. It is proposed that minor alterations in pheromone components may provide colonizers with partial escape from such natural enemies while maintaining intraspecific functionality. Traditional interpretations emphasized the fidelity and consistency of pheromones, but under natural conditions chemical signals are modified by unpredictable features of the biotic and abiotic environment. Although we typically view variation in pheromonal signals as experimental noise or simple deviations from a population norm, such variation may reflect evolutionary dynamics. Complex ecological interactions may impose trade-offs between the clarity versus diversity of their signals. Received 3 July 2000; accepted 8 January 2001