Population regulation by post-settlement mortality in two temperate reef fishes

 Input of individuals dispersing into open populations can be highly variable, yet the consequences of such variation for subsequent population densities are not well understood. I explored the influence of variable input (”supply”) on subsequent densities of juveniles and adults in open local populations of two temperate reef fishes, the bluebanded goby (Lythrypnus dalli) and the blackeye goby (Coryphopterus nicholsii). Variable recruitment was simulated by stocking a natural range of densities of young fishes on replicate patch reefs. Density and mortality of the stocked cohorts were followed over time, until the fishes reached maturity. Over the first day of the experiments, mortality of both species was significantly density-dependent; however, there was still a very strong relationship between density on day 1 and density on day 0 (i.e., simulated recruitment was still an excellent predictor of population density). At this point in the study, the main effects of density-dependent mortality were to reduce mean densities and variation about the mean. Over the period from the start of the experiments until the time when maturity was reached by each species (about 1 and 3 months for Lythrypnus and Coryphopterus, respectively), mortality was strongly density-dependent. Such strong density-dependent mortality virtually eliminated any linear relationship between adult density and ”recruit” density. However, for both species, the relationship between these two variables was well fit by an asymptotic curve, with the asymptotic density of adults equal to c. 3/m² for Coryphopterus, and c. 10/m² for Lythrypnus. Natural recruitment (via settlement of larvae) to the reefs over the period of the study (9 months) was above the asymptotic densities of adults for the two species, even though the study did not encompass the periods of peak annual recruitment of either species. This suggests that adult populations of these two gobies may often be limited, and regulated, by post-settlement processes, rather than by input of settlers. Other studies have shown that mortality of the two species is density-independent, or only weakly density-dependent, on reefs from which predators have been excluded. Hence, it appears that predators cause density-dependent mortality in these fishes. Received: 26 November 1996 / Accepted: 5 April 1997     

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish

 Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly. Received: 10 October 1996 / Accepted: 5 April 1997     

Tree invasion into a mountain-top meadow in the Oregon Coast Range,USA

Abstract. The grassy meadow on the top of Marys Peak, in the Oregon Coast Range, is being invaded, primarily along the margins, by Abies procera. To examine vegetation and environmental changes across the forest-meadow transition and to evaluate factors affecting tree invasion, belt transects were established at 10 sites. Correlations of environmental variables with ordination axes from Detrended Correspondence Analysis suggest that the sites were distributed along a moisture gradient. Although the sites varied considerably in species composition and environment, tree invasion of the meadow was occurring at all sites. Reduction in abundance of the dense herbaceous vegetation of the meadow is required before the small seedlings of Abies procera can become established. Almost all tree invasion of the meadow occurs in the narrow forest-meadow ecocline because trees at the edge of the forest reduce the cover of herbaceous plants in the adjacent meadow. On some sites, above average snow accumulations further reduce vigor of meadow vegetation and increase tree establishment. On the driest sites wet summers facilitate tree establishment. Infrequent fires, which remove the fire sensitive Abies procera, are probably required to reverse the slow but persistent trend of forest expansion and ultimately maintain the meadow.     

Direct and indirect effects of predation,herbivory and surface rugosity on mussel recruitment

Summary The predatory gastropod Nucella lapillus, commonly preys upon the mussel, Mytilus edulis, and is thought to control the distribution and abundance of mussels on the rocky shores of New England, USA. In this study, done in Maine, USA, not only the presence of Nucella lapillus but also the roughness of the experimental surface and the presence of the herbivorous gastropod, Littorina littorea, were manipulated. Four types of surfaces were used as recruitment substrata for mussels: smooth bare granite, aggregations of the barnacle, Semibalanus balanoides, fiberglass resin castings of smooth bare granite and resin castings of aggregations of S. balanoides. To ensure that caged N. lapillus were not starving, barnacles were provided as alternative prey. Experiments showed no detectable effect of N. lapillus on the recruitment of M. edulis. Mussel recruitment was enhanced by surface rugosity and depressed by the activities of L. littorea. Analysis of covariance, using the number of algal species as the covariate, suggested that L. littorea reduced the number of newlyrecruited mussels by removing algae that provided recruitment sites, but no manipulations were done to test this conjecture. It is likely that previous reports of N. lapillus controlling mussel abundance are attributable to N. lapillus preying upon barnacles, which increase surface rugosity and enhance mussel recruitment. Review of literature on feeding preferences of N. lapillus supports this view. When handling times and prey availability are taken into account, Nucella shows a clear preference for barnacles over mussels.     

Is the small clutch size of a Corsican blue tit population optimal?

In an attempt to test predictions of the optimisation hypothesis of life history traits in birds, we estimated fitness consequences of brood size manipulations. Experiments were carried out over a period of 4 years in a Mediterranean population of blue tits Parus caeruleus which is confronted with a particular set of environmental constraints. Effects of brood size manipulation were investigated in relation to year-to-year variation in environmental conditions, especially caterpillar abundance. There was a strong variation in the effects of brood size manipulation depending on year. Most effects were on offspring quality (fledging mass, tarsus length). The absolute number of recruits did not significantly differ among categories (reduced, control, enlarged broods) but varied considerably among years. Females recruited from enlarged broods were of lower quality, started to breed later and laid fewer eggs than those recruited from control and reduced broods. Neither parental survival nor reproductive performances of adults in year n + 1 was affected by brood size manipulation in year n. Thus there was no evidence for a cost of reproduction in this population. Since the number of recruits did not depend on brood size manipulation (recruitment rates were higher in reduced broods), but recruits from reduced broods were of better quality compared with other groups, we conclude that adults lay a clutch that is larger than that which is predicted by the optimisation hypothesis. Producing more young could incur some penalties because offspring from large broods are of lower quality and less likely to recruit in the population. Two possible reasons why decision rules in this population seem to be suboptimal are discussed. Received: 10 March 1998 / Accepted: 1 July 1998     

Reduced immunocompetence of nestlings in replacement clutches of the European magpie (Pica pica)

Laying date is one of the most important determinants of reproductive success and recruitment probability in birds. Late breeders usually fledge fewer chicks than individuals with earlier breeding dates, and fledglings produced late in the season have high mortality rates. Food availability and nestling mass have been evoked as the principal mechanistic links between laying date and offspring survival. Here we suggest that another factor may actually account for the difference in survival rate between early and late offspring: immunocompetence. We predicted that nestlings produced later in the season or in replacement clutches should have lower immune responses when challenged with an antigen, than early nestlings or nestlings produced in first clutches. This hypothesis was tested in a population of magpies (Pica pica), in which we experimentally induced breeding failure in a group of nests and compared the immune response of nestlings in replacement clutches with the immune response of first clutch nestlings. Cellular immune response, as measured by wing web swelling (a correlate of T-lymphocyte production after injection of phytohaemagglutinin-P), significantly decreased with hatching date and was significantly lower in nestlings of replacement clutches. Furthermore, coefficients of intraclutch variation in immune response were higher in nestlings of replacement clutches. This experiment demonstrates an inverse relationship between immune responsiveness and breeding date, and reduced recruitment probability of late nestlings may be a direct consequence of their inability to cope with parasites.     

Use of fish behavior in assessing the effects of Hurricane Iniki on the Hawaiian island of Kaua'i

Synopsis Declared the most compact and powerful storm known to have reached the Hawaiian Archipelago, Hurricane Iniki destroyed terrestrial and freshwater habitats island-wide when it struck Kaua'i late on the afternoon of Friday, 11 September 1992. Five weeks after the storm, a research group began underwater studies on the north shore of Kaua'i to assess the physical and biological effects of the hurricane and to track at four-month intervals the recovery of three high quality streams that are home to Hawaii's native and mostly endemic freshwater fishes, mollusks, and crustaceans. Early in the investigation, information on the social behavior of resident fishes showed particular promise for assessing the impact of the hurricane and for tracing the recovery of stream ecosystems. In Hanakapi'ai Stream, the stream damaged most by the hurricane and a subsequent flash flood, two downstream species,Eleotris sandwicensis andStenogobius hawaiiensis, were missing, and members of a third one,Sicyopterus stimpsoni, were sufficiently crowded into the lower section of the stream that aggressive behavior, occasionally resulting in injury, sharply curtailed feeding and prevented all but the earliest phases of courtship. Although the study began in the middle of the spawning season forAwaous guamensis, the few males found were in poor condition and showed neither territorial behavior in response to other males nor courtship when gravid females were encountered. Farther upstream, males ofLentipes concolor directed an inordinate amount of effort toward maintaining territories. Similar, but less dramatic alterations in distribution and behavior were seen in fishes from the less damaged Wainiha River. In Nu'alolo Stream, protected from hurricane winds by nearly vertical valley walls, the behavior was typical for fishes throughout its length except forE. sandwicensis in the damaged section at the stream mouth. Most progress in the return to social behaviors characteristic of pre-hurricane conditions for fishes in Hanakapi'ai Stream and Wainiha River occurred during the first four months of the study, but it was not until October 1993, more than a year after Iniki, that the behavior of the fishes signaled a return to conditions that existed before the storm. The study demonstrated that Hawaii's stream fishes are remarkably capable of rebounding from the effects of hurricanes because of the hardiness of adults and the recruitment of young animals from the ocean as a natural part of each species' amphidromous life cycle.     

Spatio-temporal patterns in the mortality of bay scallop recruits in North Carolina: investigation of a life history anomaly

By timing reproduction to occur when predatory mortality on progeny is minimal, organisms may maximize recruitment to adult populations. Accordingly, an hypothesis to explain the greater importance of fall than spring spawning to North Carolina populations of bay scallops (Argopecten irradians) is that predatory mortality of bay scallop recruits is lower in fall and winter than spring and summer. To test this hypothesis, we measured predatory mortality of scallop recruits monthly. To infer the identities of predators that are most important in determining patterns of mortality of bay scallop recruits in spring and in fall, predatory mortality of bay scallop recruits was compared between the edge and interior of sheltered and exposed seagrass patches during the day and at night in May and November. Consideration of predatory mortality throughout the year indicated that mortality of scallop recruits over late spring and summer approaches 100% but is negligible over late fall and winter. In May, predatory mortality of scallop recruits was similar during day and night but greater at exposed than sheltered sites. In November, predatory mortality was greater during night than day and slightly greater at sheltered than exposed sites. In neither month did position within patch influence mortality, and at all times and places, missing and crushed scallops contributed a higher proportion than drilled scallops to the total dead. These spatio-temporal patterns of mortality of scallop recruits suggest that mud crabs, Dyspanopeus sayi, which are more abundant in exposed than sheltered seagrass beds during spring and can feed by day and night, are a likely major contributor to spatio-temporal pattern in mortality of scallop recruits in North Carolina. Blue crabs, Callinectes sapidus, which are many times more abundant in summer than winter, may also contribute to observed seasonal patterns in mortality. The dramatically lower rates of predation on bay scallops over the winter months appear to provide fall settlers with a temporal window of opportunity to recruit to the adult population. Although spring spawning contributes little to adult populations in most years because of high rates of predatory mortality during summer, we hypothesize that spring spawning persists because infrequent devastating perturbations, such as hurricanes and red tides, can result in complete failure of fall recruitment.     

Sapling dynamics in a southeastern Texas floodplain forest

Abstract. How much temporal variation in recruitment, mortality and change in size class occurs in the sapling layer of mature temperate forests in the absence of large‐scale exogenous disturbance? Using 15 years of data from a flood‐plain forest in Big Thicket National Preserve, we found that year‐to‐year variation in demographic parameters was greater than we originally expected. Death rates were generally more variable than recruitment rates, and were much more variable for large saplings than for small ones. Small saplings of the 10 most common species had at least one year when they experienced two to eight times their long‐term mean recruitment and death rates. Large saplings had at least one year when they experienced three to 10 times their long‐term mean death rates and at least one year with two to seven times the long‐term mean recruitment rate. Temporal patterns in sapling death rates were related to flooding patterns, while temporal patterns in recruitment were related to the Palmer Drought Severity Index, an indicator of drought severity and soil moisture availability. We also identified apparently synchronous patterns of demographic response among less flood‐tolerant species which differed from the responses of more flood‐tolerant species. We demonstrated the effects of both climatic variation and light variation in affecting stand‐wide sapling demographics in a forest where canopy gaps are important for regeneration, and where chronic understorey disturbance favours growth over survivorship as a sapling strategy.     

Spatial patterns in abundance of a damselfish reflect availability of suitable habitat

For species with metapopulation structures, variation in abundance among patches can arise from variation in the input rate of colonists. For reef fishes, variability in larval supply frequently is invoked as a major determinant of spatial patterns. We examined the extent to which spatial variation in the amount of suitable habitat predicted variation in the abundance of the damselfish Dascyllus aruanus, an abundant planktivore that occupies live, branched coral throughout the Indo-Pacific. Reef surveys established that size, branching structure and location (proximity to sand) of the coral colonies together determined the ”suitability” of microhabitats for different ontogenetic stages of D. aruanus. Once these criteria were known, patterns of habitat use were quantified within lagoons of five Pacific islands. Availability of suitable habitat generally was an excellent predictor of density, and patterns were qualitatively consistent at several spatial scales, including among different lagoons on the same island, among different islands and between the central (French Polynesia and Rarotonga) and western (Great Barrier Reef, Australia) South Pacific. A field experiment that varied the amount of suitable coral among local plots indicated that habitat for settlers accounted for almost all of the spatial variation in the number of D. aruanus that settled at that location, suggesting that spatial patterns of abundance can be established at settlement without spatial variation in larval supply. Surveys of four other species of reef-associated fish revealed that a substantial fraction of their spatial variation in density also was explained by availability of suitable reef habitat, suggesting that habitat may be a prevalent determinant of spatial patterns. The results underscore the critical need to identify accurately the resource requirements of different species and life stages when evaluating causes of spatial variation in abundance of reef fishes. Received: 18 May 1999 / Accepted: 9 January 1999