Oceanographic coupling across three trophic levels shapes source–sink dynamics in marine metacommunities

A central goal of metapopulation ecology is to determine which subpopulations have the greatest value to the larger metapopulation. That is, where are the ‘sources’ that are most essential to persistence? This question is especially relevant to benthic marine systems, where dispersal and recruitment are greatly affected by oceanographic processes. In a single‐species context, theoretical models typically identify ‘hotspots’ with high recruitment, especially high self‐recruitment, as having the highest value. However, the oceanographic forces affecting larval delivery of a given species may also influence the recruitment of that species’ predators, prey, and competitors.We present evidence from the Virgin Islands and Bahamas that oceanographic forces produce spatial coupling between the recruitment of planktivorous fishes, the recruitment of their predators, and the productivity of their zooplankton prey. We examined the consequences of this type of multi‐trophic coupling using a simple analytical population model and a multispecies numerical simulation model with parameter values based on the Virgin Islands system. In both analyses, strong coupling caused planktivores at the highest recruitment sites to experience higher mortality (a consequence of higher predator densities) but faster growth and higher fecundity (a consequence of higher zooplankton densities) than planktivores at low recruitment sites. As such, the relative strength of oceanographic coupling between the three trophic levels strongly determined whether a particular reef acted as a source or sink. In the simulation model, density‐dependent competition for zooplankton limited overall metapopulation biomass more severely than predation, so oceanographic coupling between planktivore larval supply and zooplankton productivity had a stronger effect on the metapopulation value of a patch. We argue that the potential for such tri‐trophic coupling should be incorporated into future metacommunity models and has considerable implications for the design and evaluation of marine reserves.     

Time-patterns of macrobenthic species in Moreton Bay

Data were obtained at 2-month intervals for ea 31/2 years by quintuplicate sampling with an 0.1 m² Smith-Mclntyre grab at six selected sites in Moreton Bay. A total of 312 species-in-sites was obtained; when considering species-in-all-sites this was reduced to 158. Both sets of data were analysed with and without data (species) reduction. After trials of several analytical methods single cyclical regressions involving Fourier transforms were selected as the main method. By prescribing a range of wavelengths, those giving the best and most significant fit to the data were obtained. Two ranges of wavelengths were used, near-annual and long-term. The former showed a wide range of optimal periodicities from ca 38 to 60 weeks for one set of data and ca 38 to ca 65 weeks for the other. Species with the more rapid cycles are believed to be under particularly severe control by predator/disturbers. ‘Imposing’ a 52-week cycle on the data and determining the estimated times of attainment of the first maximum shows a modal distribution with most species attaining maximum populations in September. This agrees with previous, but less intensive work, in Moreton Bay. More species show significant long-term cycles than show annual cycles, with a spectrum of cycles of from 2 to 7 years (the range scanned), with concentrations at 3, 5 and 6 years. It is impossible to ‘explain’ these varying cycles by any conceivable combination of abiotic factors. Considering cycles of 5, 6 and 7 years, there is convincing evidence in each case that an earlier group of species is replaced, in a synchronous fashion, by a later group. Quite often species disappear from one site and are synchronously replaced at another. It was suspected that the overall situation would be complex, and although this complexity has been partially partitioned and partly clarified, much remains unexplained.     

The recent population expansion of boarfish,Capros aper (Linnaeus, 1758): interactions of climate,growth and recruitment

The objectives of this study were to evaluate whether temperature changes in the Northeast Atlantic influence the growth and recruitment dynamics of boarfish, Capros aper. Two geographically separate areas were examined, ‘north’ at the northern distribution range west of Ireland and ‘south’ on the main fishing grounds south of Ireland. No significant differences in length‐at‐age were observed between the two areas. Interannual otolith growth patterns were similar between the two areas with distinct years of faster and slower growth. In the ‘north’, no significant relationship between adult growth and temperature was observed, while growth in the ‘south’ was positively related to temperature up to approximately 16°C growth rates were suppressed in the years with temperatures above that. Recruitment showed a positive correlation with adult growth the previous year for the Spanish recruitment index only, suggesting spatial connectivity between the Celtic Sea and the Bay of Biscay. The age distributions were similar in both areas and despite the boarfish's longevity of >30 years, are dominated by the age classes corresponding to the years with high recruitment, suggesting that increased recruitment is responsible for the observed stock expansion.     

Regional patterns of recruitment success and failure in two endemic California oaks

Oak woodlands and savannas are key defining landscapes in the California Floristic Province, making up almost a quarter of the region's forests and woodlands. Two endemic Californian oak species, valley oak (Quercus lobata Neé) and blue oak (Quercus douglasii Hook. & Arn.), are widely considered at risk of decline from persistent recruitment failures in the last century. However, decades of research have produced no definitive conclusion about the existence, extent, or causes of this ‘regeneration problem’. Underlying causes of perceived recruitment failure are unclear and could include drivers at distribution‐wide to local scales including climate and atmospheric changes, habitat fragmentation, altered herbivore populations, changing fire regimes, exotic plant and animal invasions, livestock grazing, and soil conditions altered by past land uses. We performed meta‐analyses of existing stand‐scale data from the published and grey literatures on seedling and sapling recruitment in blue and valley oaks throughout each of their distributions. We sought to evaluate whether distribution‐wide regeneration ‘problems’ exist for either species and to assess what factors correlate with distribution‐wide recruitment patterns. Nearly 80% of sites surveyed for blue oaks but fewer than 50% of sites surveyed for valley oaks contained some evidence of seedling or sapling recruitment. A majority of sites surveyed for both species appear to have insufficient recruitment to replace adult populations, though further demographic work would be required to quantify minimum replacement recruitment rates. Reserve sites were seven times more likely than non‐reserve sites to contain valley oak populations with evidence of recruitment. Blue oak recruitment patterns were weakly related to climate and geographical factors and strongly variable at subregional scales. We suggest several lines of additional research that could fill gaps in the existing literature and clarify the patterns emerging from this analysis.     

Habitat effects on second-order predation of the seed predator Harpalus rufipes and implications for weed seedbank management

Seed predators provide a valuable ecosystem service to farmers by reducing densities of weed seeds, and, in turn, densities of weed seedlings they must manage. The predominant invertebrate weed seed predator in Maine, USA, agroecosystems is the carabid beetle Harpalus rufipes DeGeer. Pitfall trapping has shown that H. rufipes prefers sites with vegetative cover to fallow sites, preference speculated to be driven by predator avoidance behavior. To test this hypothesis, ‘second-order predation assays’ were developed, in which live H. rufipes prey were presented to second-order predators. Field experiments were conducted to determine foremost if H. rufipes was subject to second-order predation, and secondly, whether (a) vegetative cover affords H. rufipes protection from second-order predators, and (b) high rates of second-order predation correspond with decreased invertebrate seed predation rates. Two 72-h experiments were conducted (mid August and September 2012) at crop and non-crop sites across a 28 ha diversified farm in Stillwater, ME, USA.Second-order predation was 2.8% per day. Based on images from motion-sensing cameras, H. rufipes’ predators included birds and small mammals. Neither a relationship between second-order predation and vegetative treatment, nor an empirical relationship between second-order predation and invertebrate seed predation were detected. However, a simulation model predicted that 2.8% per day second-order predation could increase the number of seeds entering the seedbank by more than 17% annually. Additionally, complex habitats supported higher rates of second-order predation than did simple habitats.     

Herbivore damage increases avian and ant predation of caterpillars on trees along a complete elevational forest gradient in Papua New Guinea

Signals given off by plants to alert predators to herbivore attack may provide exciting examples of coevolution among organisms from multiple trophic levels. We examined whether signals from mechanically damaged trees (simulating damage by herbivores) attract predators of insects along a complete elevational rainforest gradient in tropical region, where various predators are expected to occur at particular elevational belts. We studied predation of artificial caterpillars on trees with and without ‘herbivorous’ damage; as well as diversity and abundances of potential predators at eight study sites along the elevational gradient (200–3700 m a.s.l.). We focused on attacks by ants and birds, as the main predators of herbivorous insect. The predation rate decreased with elevation from 10% d^?1 at 200 m a.s.l. to 1.8% d^?1 at 3700 m a.s.l. Ants were relatively more important predators in the lowlands, while birds became dominant predators above 1700 m a.s.l. Caterpillars exposed on trees with herbivorous damage were attacked significantly more than caterpillars exposed on trees without damage. Results suggest that relative importance of predators varies along elevational gradient, and that observed predation rates correspond with abundances of predators. Results further show that herbivorous damage attracts both ants and birds, but its effect is stronger for ants.     

Spatially correlated recruitment of a marine predator and its prey shapes the large-scale pattern of density-dependent prey mortality

Patterns of predator dispersal can be critical to the dynamics of prey metapopulations. In marine systems, oceanic currents may shape the dispersal of planktonic larvae of both predators and prey, producing spatial correlations in the recruitment of both species and distinctive geographic patterns of prey mortality. I examined the potential for this phenomenon in two fishes, a wrasse and its grouper predator, at a Caribbean island where the near-shore oceanographic regime produces a temporally consistent spatial pattern of fish recruitment. I found that recruitment and adult abundance of groupers were spatially correlated with recruitment of wrasse prey. Furthermore, the local abundance of predators strongly affected the nature of density-dependent prey mortality. At sites with few predators, wrasse mortality was inversely density-dependent, while mortality was positively density-dependent at sites with higher predator densities. This phenomenon could be important to the dynamics of any metacommunity in which physical forces produce correlated dispersal.     

Indirect interactions among tropical tree species through shared rodent seed predators: a novel mechanism of tree species coexistence

The coexistence of numerous tree species in tropical forests is commonly explained by negative dependence of recruitment on the conspecific seed and tree density due to specialist natural enemies that attack seeds and seedlings (‘Janzen–Connell’ effects). Less known is whether guilds of shared seed predators can induce a negative dependence of recruitment on the density of different species of the same plant functional group. We studied 54 plots in tropical forest on Barro Colorado Island, Panama, with contrasting mature tree densities of three coexisting large seeded tree species with shared seed predators. Levels of seed predation were far better explained by incorporating seed densities of all three focal species than by conspecific seed density alone. Both positive and negative density dependencies were observed for different species combinations. Thus, indirect interactions via shared seed predators can either promote or reduce the coexistence of different plant functional groups in tropical forest.     

Predator–prey naïveté, antipredator behavior,and the ecology of predator invasions

We present a framework for explaining variation in predator invasion success and predator impacts on native prey that integrates information about predator–prey naïveté, predator and prey behavioral responses to each other, consumptive and non‐consumptive effects of predators on prey, and interacting effects of multiple species interactions. We begin with the ‘naïve prey’ hypothesis that posits that naïve, native prey that lack evolutionary history with non‐native predators suffer heavy predation because they exhibit ineffective antipredator responses to novel predators. Not all naïve prey, however, show ineffective antipredator responses to novel predators. To explain variation in prey response to novel predators, we focus on the interaction between prey use of general versus specific cues and responses, and the functional similarity of non‐native and native predators. Effective antipredator responses reduce predation rates (reduce consumptive effects of predators, CEs), but often also carry costs that result in non‐consumptive effects (NCEs) of predators. We contrast expected CEs versus NCEs for non‐native versus native predators, and discuss how differences in the relative magnitudes of CEs and NCEs might influence invasion dynamics. Going beyond the effects of naïve prey, we discuss how the ‘naïve prey’, ‘enemy release’ and ‘evolution of increased competitive ability’ (EICA) hypotheses are inter‐related, and how the importance of all three might be mediated by prey and predator naïveté. These ideas hinge on the notion that non‐native predators enjoy a ‘novelty advantage’ associated with the naïveté of native prey and top predators. However, non‐native predators could instead suffer from a novelty disadvantage because they are also naïve to their new prey and potential predators. We hypothesize that patterns of community similarity and evolution might explain the variation in novelty advantage that can underlie variation in invasion outcomes. Finally, we discuss management implications of our framework, including suggestions for managing invasive predators, predator reintroductions and biological control.     

Cadmium and copper levels in seals,penguins and skuas from the Weddell Sea in 1982/1983

Summary The presence of empty shells of Laternula elliptica that provide refuge sites for juvenile fishes on the soft-bottom surface of South Bay, Antarctica, has been difficult to explain because this bivalve normally lives deeply buried (>50 cm). L. elliptica individuals unburied and aggregated at the surface by the action of icebergs could be consumed by mobile predators. This paper examines experimentally both burying rates and predation upon unburied L. elliptica in the field at South Bay. Live and intact L. elliptica were hand-collected by diving, and separated in 16 groups of 10 each, and placed on mud-sand bottoms protected from the ice disturbance at a depth of 17 m in South Bay. Eight groups were confined by open wooden frames so as to exclude predators, mainly star-fish and gastropods, the other eight remained as control. Species and number of predators and burying rates were estimated during 23 days. Within the first 24 h, 30% of the bivalves buried themselves, after which, rates fell abruptly. By the end of the day 23, only 60% had successfully buried; of the 40% which remained unburied, 20% were consumed by the following sequence of predators: Odontaster validus, Cryptasterias turqueti, Parborlasia corrugatus, Neobuccinum eatoni and two species of amphipods; all completely consumed one bivalve in 5–7 days. The 20% that remained unburied constituted potential prey though they showed no signs of being attacked by O. validus during the observation period. In addition to serving as prey, L. elliptica provides empty and intact shells used as refuge sites by juvenile fishes.     

Small mammal controls on the climate‐driven range shift of woody plant species

Climate change is resulting in shifts in species’ ranges as species inhabit new climatically suitable areas. A key factor affecting range‐shifts is the interaction with predators. Small mammals, being primary seed predators and dispersers in forest ecosystems, may play a major role in determining which plant species will successfully expand and the rate at which range‐shifts will occur. Plants dispersing seeds beyond the species’ current range limits will encounter seed predators to which these seeds are novel; however, empirical studies of seed predator–novel seed interactions are lacking. The aims of our study were to: 1) quantify seed selection by small mammals presented with ‘novel’ seeds; 2) quantify the post‐selection fate of ‘novel’ seeds; and 3) identify seed traits that affect seed selection and post‐selection seed fate. We designed a field experiment exposing small mammal communities to novel seeds produced by plants expected to shift their ranges in response to climate change. We matched novel seeds with reference ‘familiar’ seeds and studied key steps defining interactions between small mammals and novel seeds. We found that the probability of selection of a novel seed varied among species and was, at times, higher than the selection probability of familiar seeds. Key traits that affected seed selection and the distance a seed was dispersed for caching were shell hardness and seed mass. We also found that 33% of dispersed seeds were cached in optimal germination sites (e.g. within fallen logs and buried under the leaf litter mat). Through seed emergence trials we found that emergence was higher for larger seeds, suggesting that the role of small mammals may be modulated by emergence rates. Our results suggest that the interaction between small mammals and novel seeds may have cascading effects on climate‐induced plant range shifts and community composition.     

Effects of nest features and surrounding landscape on predation rates of artificial nests

Artificial nest predation experiments were carried out in northern Italy in woods which varied in size, isolation and surrounding landscape structure. Multivariate analyses, including logistic regression, showed that: (1) size and isolation of woods did not significantly affect predation rates; (2) nests on the edge of woods did not suffer higher predation rates than nests inside the wood; (3) nest camouflage greatly influenced predation rates, suggesting that predators were mainly using visual clues to identify nests; (4) the type of habitat that surrounded the woods emerged as a crucial factor in nest survival and the amount of human settlements in the vicinity of the wood was inversely correlated with nest survival, probably due to predators associated with humans; (5) other habitat variables, which were apparently individually unimportant, were found to have an effect on nesting success, if combined in a single ‘suitability index’. It is impossible to generalize about the influence of landscape fragmentation on nest predation because local landscape history and predator guilds, together with the scale of fragmentation, probably interact to determine the suitability of nest sites and their vulnerability to predators.     

A surplus of seeds: High rates of post-dispersal seed predation in a flooded grassland in monsoonal Australia

Abstract Oryza meridionalis is an annual emergent wetland grass which produces between 26 and 260 kg seeds ha^-1 annually. Seed shed occurs at the end of the wet season, when the plains are usually still partially flooded. The juvenile recruitment of key native vertebrate species, such as the Magpie Goose (Anseranus semipalmata) and the Dusky Plains Rat (Rattus colletti), coincides with seed shed. This study investigated predation of O. meridionalis seeds at two sites on the South Alligator River floodplain in monsoonal Australia. The effects of inundation and the presence of a background density of seeds on seed removal were investigated by stratified sampling with respect to position down the topographic slope, to include the ‘dry’ floodplain margin, ‘damp’Oryza zone, and ‘flooded’Oryza zone. The effect of seed lot size on the proportion of seed removed was also investigated, and exclosures were used to identify the principal predator group. The proportion of seeds removed was not affected by the presence/absence of a background of seeds nor the number of seeds placed in experimental ‘lots’. The majority of seeds (75%) was consumed by vertebrate predators (most likely the abundant Dusky Plains Rat). Inundation afforded some protection from predation. Despite high losses of seeds exposed to predators, O. meridionalis is an abundant and widespread species on these floodplains, possibly because of the protection from predation afforded by inundation to those seeds which are shed into the water column. It is likely that there is a complex interaction between topography, rainfall and predator and prey relationships, which ultimately determines the importance of seed removal for the maintenance of populations of O. meridionalis. These high losses of seeds to predation have implications for wetland rehabilitation where seed broadcasting is proposed.     

The composite trophic status index (TRIX) as a potential tool for the regulation of Turkish marine aquaculture as applied to the eastern Aegean coast (Izmir Bay)

This research examines the current water quality status of Izmir Bay, using the trophic index (TRIX) as a tool for the regulation of Turkish marine finfish aquaculture. In 2007, new legislation was enacted in Turkey related to the protection of coastal waters, especially those of enclosed bays and gulfs, from pollution by fish farming. However, the legislation does not apply to any other coastal zone stakeholders; for this reason the entire Izmir Bay was examined. Use of the composite trophic status index (TRIX) produced mean TRIX values of 3.6 for the aquaculture area (AA) and 2.5 for areas of the outer bay where no aquaculture takes place; this indicates ‘no risk of eutrophication’ as defined by Turkish law. In the inner Izmir Bay, there is a mean TRIX value of 4.3, which is above the threshold of four and typical for ‘high eutrophication risk’ areas, as expected because of heavy urbanization. The study then applied the UNTRIX indices adjusted to local conditions, revealing that both the inner bay and the aquaculture area (AA) can be classified as of ‘poor’ status whereas the outer bay can be defined as ‘good’. The UNTRIX‐based trophic classification is in good agreement with TRIX for both the outer and inner parts of the bay; however, there is no agreement regarding the classification of the aquaculture area.     

Recruitment,mortality and growth of mangrove (<Emphasis Type="Italic">Rhizophora</Emphasis> sp.) seedlings in Ulugan Bay,Palawan, Philippines

Propagule dispersal, establishment and recruitment to the sapling stage are critical steps in the life cycle of mangroves. Specific (i.e., per capita) rates of recruitment and mortality, and the growth rates of Rhizophora seedlings in three mangrove stands in Ulugan Bay (Palawan, Philippines) were estimated between March 1999 and February 2001. Recruitment and mortality were variable in space and time, with mortality exceeding rates of recruitment at all sites. The specific rates of seedling recruitment and mortality were higher in Buenavista (0.66 year^–1 and –1.67 year^–1) than in Umalagan (0.05 year^–1 and –0.33 year^–1) and Oyster Bay (0.13 year^–1 and –0.24 year^–1). The annual rate of production of internodes by the main stem was similar at the three sites (5.4–5.5 internodes year^–1), but the annual rate of elongation of the main stem was higher in Buenavista (10.6 cm year^–1) than in Oyster Bay (7.6 cm year^–1) and Umalagan (5.6 cm year^–1).     

The Adaptive Significance of Avian Mobbing V. An Experimental Test of the ‘Move On’ Hypothesis

This paper presents the first experimental study of the effects of mobbing on predators. Two captive little owls Athene noctua and a tawny owl Strix aluco, having stabilized activity patterns and roosting sites, were singly confronted with one or two mobbing blackbirds Turdus merula. The little owls showed marked stress (gular fluttering, panting and panic flights), abandoned chosen roosts and remained unsettled for several days. A permanent alternative roost was chosen after one mobbing encounter, other encounters resulted in several days of ‘floating’ before returning to the original sites. One little owl used ‘camouflage’ by ‘freezing’ during repeated harassments, the other tended to retaliate if mobbing was close and weak. The tawny owl seemed to ignore the rather weak blackbird mobbing, but abandoned its roost for up to 36 h when mobbed by larger numbers of woodland birds at close range. It is concluded that mobbing causes profound distress, especially in small, vulnerable predators like the little owl and forces it to change its daytime roost. If abandoning the area is not possible, time and energy are expended in increased vigilance as a protection against further mobbing attacks. Thus there is strong support for the ‘Move on’ hypothesis. Results also support the ‘Perception advertisement’ hypothesis, implying that the detected hunter deliberately forfeits the surprise element. Which effect prevails depends on predator evaluation of the signals involved.     

Competition between Quercus petraea and Carpinus betulus in an ancient wood in England: seedling survivorship

Abstract. Age structures of populations of canopy trees in Wormley Wood are consistent with reports that successional change is occurring, with Carpinus betulus replacing Quercus petraea as the dominant species. TWINSPAN analysis of data from a vegetation survey identifies three communities (described in earlier work), the ‘Bracken’, ‘Bramble’, and‘Bareground’ societies, characterized by increasing prominence of C. betulus and loss of species diversity. An experiment was set up in which C. betulus, Q. petraea and Betula pendula seedlings were explanted into each of the communities. Survival of seedlings was monitored over 860 days and differences investigated. Cohort survivorship differed between species and sites. C. betulus seedlings survived longer than Q. petraea in Bracken and Bramble communities. Species- and site-specific variation in the types and effects of herbivory were found. Herbivory did not appear to be a critical factor in the survival of Q. petraea seedlings. The photosynthetic light response of the seedling species was measured in the field. The light compensation point for Q. petraea seedlings (77 μmol photon m^-2 s^-1) is higher than the maximum available light under the main tree canopies in the wood. In contrast, seedlings of C. betulus have a lower light compensation point (15 μmol photon m² s^-1) and the mature tree casts a deeper shade than Q. petraea. It is suggested that the invasion of the canopy by C. betulus, following the cessation of coppicing, is creating light levels too low for Q. petraea seedling banks to persist.     

Small-scale plant dynamics in temperate Themeda triandra grasslands of southeastern Australia

Abstract. Vegetation subjected to two long-term burning regimes (annual or biennial burning) was studied in permanent plots, at two spatial scales: 0.01 m² and 1 m², to determine the small-scale dynamics of plants in temperate Themeda triandra grasslands of southeastern Australia. Species turnover rates were estimated by presence/absence data while species mobility was assessed using cumulative frequency data. While mean species richness did not fluctuate greatly between years, the vegetation was internally dynamic rather than static. Cumulative species richness increased by 50% at both spatial scales and sites over the 4-yr study period. However, few species became cumulatively frequent (i.e. occurred in 80% of plots in the first and/or subsequent years), suggesting that cumulative species richness increases were due to small- or local-scale movements of plants, rather than ‘shifting clouds’ of species moving across the entire site. The vegetation's dynamics did not differ greatly at sites subject to different (frequent) fire intervals. Species turnover and mobility were individualistic, but the dynamics of many species was greater at the smaller spatial scale: 31–48% of the species present at both spatial scales at the two sites had higher turnover rates at the 0.01 m² scale. Similarly, some ‘non-mobile’ species at the 1-m² scale (i.e. ‘constant’ or ‘local’ mobility types), were more mobile at the smaller-scale. Turnover rate and mobility type were strongly associated with life form in some cases, particularly at the annually-burnt site. In general, therophytes (and to a lesser degree, geophytes) were positively associated with high turnover and mobility in most years, while hemicryptophytes were negatively associated with high turnover in many instances. Hemicryptophytes included many species with a range of mobility types and hence, few significant associations between mobility and this life form were found. The previously unrecognized internal dynamics of this community under ‘stable’ management regimes contributes to species coexistence by allowing plants with different dynamics properties to persist in a spatially and temporally unpredictable manner. Frequent burning is presumably the important component driving much of the non-directional, small-scale dynamics because it regularly destroys individual plants and aerial plant parts and creates opportunities for seedling regeneration, whilst permitting the vegetative persistence and spread of established plants in non-light-limited microsites.     

Evolution of prolonged development: a life table analysis for periodical cicadas

According to conventional wisdom, natural selection should favor early reproduction. Prolonged development of 13 and 17 yr has been difficult to explain for periodical cicadas. Earlier, I hypothesized that development may be long for periodical cicadas because fecundity increases as a function of longer development with little increased risk of mortality. In this article, I tested whether the fecundity was greater for 17-yr cicadas than for 13-yr cicadas and estimated the shape of the survivorship curve. A cohort of 17-yr cicadas was followed from 1979 through 1996. Most mortality occurred during the first 2 yr; thereafter mortality was uncommon. Across 17 yr, adult densities increased at five out of seven sites, and in no case did the change exceed three times. Seventeen-year adults had heavier ovaries than did 13-yr adults; this effect was greater for Magicicada cassini (1.80 times) than for Magicicada decim (1.16 times). For M. cassini, the extra fecundity associated with 17-yr development swamped the potential advantages of more frequent reproduction of 13-yr forms under most conditions. For M. decim, realistically low rates of mortality roughly matched the small effect of development on fecundity. Prolonged development in periodical cicadas may be explained adequately by the demographic hypothesis, although it may also reduce predation risk to adults or result from strong selection for large body size.     

Factors affecting the distribution of juvenile estuarine and inshore fish

The differential distributions of juveniles and adults of 25 species of teleost were investigated and compared from four habitat types in sub-tropical Moreton Bay, Queensland. The aim of the study was to identify factors influencing the distribution of juveniles, particularly the species which enter estuaries. The following habitats were sampled: a shallow, sheltered tidal estuary (Caboolture); a shallow, exposed bay with muddy substrates (Deception Bay); an exposed area of sandy substrates and seagrass (Toorbol Point) and a sheltered oceanic site with sandy substrates and seagrass (Kooringal). Data on diet, spawning seasons and recruitment periods of fry are presented together with measurements of salinity, temperature and turbidity. Species entering estuaries recruited mainly in summer (rainy season). The possible preference of juveniles for calm water, the roles of food and predation pressure, the effects of salinity, temperature and turbidity are discussed in relation to the biology and distribution of the fish. Salinity and temperature were probably not important to most juvenile fish. The effects of calm water, suitable food and predators vary according to species. Although all juveniles studied preferred shallow water, in the case of those entering estuaries, turbidity was the single most important factor. Juveniles of the same species occurred in both the estuary and Deception Bay where abiotic and biotic factors other than turbidity were different. During summer, turbidity gradients extended from east to west in Moreton Bay with highest turbidities in Caboolture estuary and Deception Bay. In winter, turbidities throughout Moreton Bay were low and relatively uniform. At this time many of the ‘clear water’ species occurred in Deception Bay. The influence of high turbidity on fish may be linked to reduced predation pressure and perhaps food supply in shallow water. Turbidity gradients in summer may aid fry in locating estuarine nursery grounds. It is apparent however, that juveniles of many species are probably not attracted to estuaries per se but to shallow turbid areas.