Habitat Units Research Articles

The Comparison of Remotely Sensed and Ground-Based Habitat Area Data Using Species-Area Models

As long as high resolution landscape data are more expensive than low resolution data, there needs to be some form of cost-benefit analysis to inform the choice of data for a particular landscape ecological application. We suggest an approach whereby models describing a common ecological data set are compared using high and low resolution landscape data. First, the detection of habitat units was compared on the two maps and, secondly, an ecological model with a common set of species data was fitted to both sets of landscape data. The control was to fit the model using the higher resolution data but including only those habitat patches that could be detected on the low resolution map. The approach is demonstrated using data from a study relating the number of species of resident birds to habitat area. Woodland areas were calculated using Ordnance Survey (OS) maps supplemented with field survey for comparison with areas estimated from the remotely-sensed ITE Land Cover Map, which was derived from Landsat TM data with a pixel size of 25 x 25 m. The discrepancies between the two sets of area values were greatest at the smallest woodland size. Only 18% of woodlands smaller than I ha (as measured from the ground-based methods), but almost all woodlands greater than 2 ha, could be detected on the Land Cover Map. Estimates of woodland area tended to be smaller according to the remotely sensed map, especially at the smaller sizes. Species-area relationships using ground-based data described the bird species number better than those from remotely sensed data, even when comparing only those woodlands found on both maps. At this scale, ground-based maps gave a better quality of information than the Land Cover Map, but at greater cost. However, the remotely sensed data are of sufficient resolution for coarse estimates of species-area relationships.

IN‐STREAM HABITAT UNIT CLASSIFICATION: INADEQUACIES FOR MONITORING AND SOME CONSEQUENCES FOR MANAGEMENT1

ABSTRACT: Habitat unit classification can be a useful descriptive tool in hierarchical stream classification. However, a critical evaluation reveals that it is applied inappropriately when used to quantify aquatic habitat or channel morphology in an attempt to monitor the response of individual streams to human activities. First, due to the subjectivity of the measure, observer bias seriously compromises repeatability, precision, and transferability of the method. Second, important geomorphic and ecological changes in stream habitats are not always manifested as changes in habitat‐unit frequency or characteristics. Third, classification data are nominal, which can intrinsically limit their amenability to statistical analysis. Finally, using the frequency of specific habitat unit types (e.g., pool/riffle ratio or percent pool) as a response variable for stream monitoring commonly leads to the establishment of management thresholds or targets for habitat‐unit types. This, in turn, encourages managers to focus on direct manipulation or replacement of habitat structures while neglecting long‐term maintenance or re‐establishment of habitat‐forming biophysical processes. Stream habitat managers and scientists should only use habitat unit classification to descriptively stratify in‐stream conditions. They should not use habitat unit classification as a means of quantifying and monitoring aquatic habitat and channel morphology. Monitoring must instead focus on direct, repeatable, cost‐efficient, and quantitative measures of selected physical, chemical, and biological components and processes spanning several scales of resolution.

Reproductive traits, habitat use and templet theory: a synthesis of world‐wide data on aquatic insects

1. Using world‐wide data on the reproductive biology of 131 species (in eight orders) of aquatic insects, we used multivariate analyses to examine: (i) relationships among reproductive traits determining life cycle, fecundity, morphology, behaviour and physiology; (ii) relationships among traits determining spatial and temporal habitat characteristics at different scales; and (iii) the relationship between reproductive and habitat‐use traits. This provided a test of predictions of the habitat templet concept on trends of species traits along gradients of habitat heterogeneity.2. The major trends observed in the relationships among reproductive traits were that larger females had larger eggs, which were more vulnerable to perturbations such as droughts and often laid in cocoons. In addition, they laid the eggs in larger numbers of smaller clutches than smaller females. Other traits (e.g. egg number or incubation time) did not show clear trends.3. Females that deposited eggs at sites of low local temporal heterogeneity (within plants) used, at the same time, gross habitats of high temporal heterogeneity (temporary waters). In contrast, traits in habitat use hardly differed on well‐known gradients of temporal heterogeneity along running waters (from source to estuary). The number of habitat units used by ovipositing females generally increased with the spatial scale considered, most species oviposited in a single small habitat unit but in several gross habitats.4. A significant (P < 0.01) relationship between traits in reproduction and habitat use demonstrated that habitat acted as a templet for reproductive strategies. This relationship was dominated by larger females having larger, unattached eggs which were more vulnerable to droughts and were oviposited in temporally more stable small‐scale habitats (within wood or macrophytes, or within cocoons spun by the female) but more unstable large‐scale habitats (primarily temporary waters). Thus, only on the small habitat scale did some of our observations correspond to the predictions of the habitat templet concept (e.g. larger size or higher vulnerability in more stable habitats). However, many species had traits in reproduction that did not show trends as predicted by the concept.5. This and other recent studies of the relationships between traits of freshwater organisms and the heterogeneity of their habitats have shown that habitat acts as a templet for species life history traits. However, many of the details observed in these studies did not correspond to predictions of the templet concept because of trade‐offs among the traits and scale problems in the description of habitat heterogeneity. Therefore, future studies should focus on groups of organisms which are as similar as possible in the trade‐offs among their species traits and on the potential relationships of habitat heterogeneity across multiple scales.

A Comparison of Basinwide and Representative Reach Habitat Survey Techniques in Three Southern Appalachian Watersheds

We compared estimates of stream habitat at the watershed scale using the basinwide visual estimation technique (BVET) and the representative reach extrapolation technique (RRET) in three small watersheds in the Appalachian Mountains. Within each watershed, all habitat units were sampled by the BVET; in contrast, three or four 100-m reaches were sampled with the RRET. The number of pools was higher and the number of cascades was lower when estimated by the RRET than they were when estimated by the BVET, whereas the average areas of all habitat types estimated by the RRET were smaller. At the watershed scale, eight out of nine estimates of total habitat area by habitat type derived from the RRET were outside the 95% confidence intervals derived from the BVET. Depth estimates were consistently smaller with the RRET than with the BVET Large woody debris estimates with the RRET were less than with the BVET in two of three watersheds and were greater in one watershed. We observed that the degree to which habitat in a RRET assessment reflects conditions at a larger scale depends on the selection of representative reaches. Habitat estimates based on the BVET were a more accurate reflection of conditions existing in the three small southern Appalachian watersheds than estimates derived from the RRET. The BVET permitted greater amounts of habitat to be surveyed with known accuracy and precision.

Assessing Risks to Biodiversity from Future Landscape Change

We examined the impacts of possible future land development patterns on the biodiversity of a landscape. Our landscape data included a remote sensing derived map of the current habitat of the study area and six maps of future habitat distributions resulting from different land development scenarios. Our species data included lists of all bird, mammal, reptile, and amphibian species in the study area, their habitat associations, and area requirements for each. We estimated the area requirements using home ranges, sampled population densities, or genetic area requirements that incorporate dispersal distances. Our measures of biodiversity were species richness and habitat abundance. We calculated habitat abundance in two ways. First, we computed the total habitat area for each species in each landscape. Second, we calculated the number of habitat units for each species in each landscape by dividing the size of each habitat patch in the landscape by the area requirement and summing over all patches. Species richness was based on presence of habitat. Species became extinct in the landscape if they had no habitat area or no habitat units, respectively. We then computed ratios of habitat abundance in each future landscape to habitat abundance in the present for each species. We also computed the ratio of future to present species richness. We then calculated summary statistics across all species. Species richness changed little from present to future. There were distinctly greater risks to habitat abundance in landscapes that extrapolated from present trends or zoning patterns, however, as opposed to landscapes in which land development activities followed more constrained patterns. These results were stable when tested using Monte Carlo simulations and sensitivity tests on the area requirements. We conclude that this methodology can begin to discriminate the effects of potential changes in land development on vertebrate biodiversity.

Factors Affecting Competitive Dominance of Rainbow Trout over Brook Trout in Southern Appalachian Streams: Implications of an Individual-Based Model

Abstract We used an individual-based model to examine possible explanations for the dominance of rainbow trout Oncorhynchus mykiss over brook trout Salvelinus fontinalis in southern Appalachian streams. Model simulations were used to quantify the effects on interspecific competition of (1) competitive advantage for feeding sites by rainbow trout, (2) latitudinal differences in stream temperatures, flows, and daylight, (3) year-class failures, (4) lower fecundity of brook trout, and (5) reductions in spawning habitat. The model tracks the daily spawning, growth, and survival of individuals of both species throughout their lifetime in a series of connected stream habitat units (pools, runs, or riffles). Average densities of each species based on 100-year simulations were compared for several levels of each of the five factors and for sympatric and allopatric conditions. Based on model results and empirical information, we conclude that more frequent year-class failures and the lower fecundity of brook trout...

A Habitat-Based Design for Sampling and Monitoring Stream Amphibians with an Illustration from Redwood National Park

We present a sampling method for use in small to medium-sized streams that is rigorous and repeatable, and contains the necessary flexibility to be speciesor habitat-specific. Our habitat-based method generates large sample sizes for reliable statistical analyses yet remains cost effective. Streams are mapped, typed, and cross-stream sampling units (belts) are placed to systematically sample each stream based on habitat composition. Biotic and abiotic attributes of the habitat associated with each belt and animal capture are measured or estimated. We employed this sampling design to compare amphibian densities within streams in an old-growth coast redwood (Sequoia sempervirens) ecosystem in northwestern California. Our method targets stream-dwelling larval amphibians, but is also effective for adults and neotenes that share lotic habitats with larvae. This method provides baseline data on amphibian presence and abundance, as well as habitat-specific density estimates. It can also yield highly accurate population estimates for streams where sample sizes are sufficiently large. first signs of environmental stress usually occur at the population level, affecting especially sensitive species (Odum 1992, p 542). Amphibians are potentially excellent bioindicators of both qualitative and quantitative changes in environmental conditions because of their high densities, long life, site fidelity, and marked physiological sensitivity (Baringa 1990; Blaustein and Wake 1990; Vitt et al. 1990; Wake and Morowitz 1990; Wyman 1990; Wake 1991; Blaustein 1994) (but see Pechmann and Wilbur 1994). However, rigorous, repeatable sampling methods are needed to permit investigators to test and exploit the intrinsic advan-tages of amphibians as bioindicators. Stream amphibians are challenging to sample because they are unevenly distributed, cryptically colored, and often hidden within the substrate. Corn and Bury (1989) used an area-constrained search technique for sampling stream amphibians that involved intensive searching of all substrates in a single, 10-m long plot located in a typical stretch of stream. They first walked a few hundred meters of the stream and then selected 1 site that appeared representative of the area they had viewed (purposive selection). However, a single purposively selected sample unit per stream does not allow assessment of variability within streams, and does not allow valid statistical analyses. Welsh (1987) modified Corn and Bury's (1989) method to include 3 systematically spaced, 5-m long sample units per stream, which provided a minimum estimate of variation and allowed a greater choice of analytic approaches. Bury and Corn (1991) later concurred in recommending a minimum of 3 samples per stream. None of these approaches, however, adequately addressed the problem of differential use of stream habitats by amphibian species, and the resulting potential for poor sample estimates. The 2-stage designs proposed by Hankin (1984, 1986) for surveys of stream-dwelling fishes supposed that some unbiased method of estimation could be used to estimate numbers of fish present in natural stream habitat units. In his example applications, Hankin assumed

Trout Use of Woody Debris and Habitat in Appalachian Wilderness Streams of North Carolina

Abstract Wilderness areas in the Appalachian Mountains of North Carolina are set aside to preserve characteristics of both old-growth and second-growth forests and associated streams. Woody debris loadings, trout habitat, and trout were inventoried in three southern Appalachian wilderness streams in North Carolina by the basin-wide visual estimation technique. Two streams in old-growth wilderness areas contained more large woody debris (LWD, diameter >10 cm) and more and smaller pools and riffles than did a stream in a second-growth area managed as wilderness. Furthermore, the size distribution of woody debris in the second-growth stream was skewed to smaller size-classes than that in the old-growth streams. Brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta in the three streams were always found in habitat units that had large amounts of LWD but were present in only 70–90% of the large number of units with little or no LWD. In the absence of high fishing pr...

Protecting Oregon old-growth forests from fires: How much is it worth?

Current fire management policies in the USDA Forest Service includes traditional multiple uses, but these policies do not adequately incorporate non.traditional uses such as preservation of biodiversity and related nongame and endangered animals. A contingent valuation methodology was used for valuing the general public's desire to know that rare and unique ecosystems exist and will be protected from fire for current and future generations. The methodology was applied to old-growth forests and critical habitat units for the northern spotted owl in Oregon. A mail survey describing a simulated voter referendum on an Oregon old-growth fire prevention and control fund that reduces by half the number of acres of old-growth burned each year was sent to a random sample of 1,000 Oregon households. Each household was randomly assigned one of 20 alternative program cost levels ranging from $2 to $300. The mean dichotomus choice willingness to pay estimate was of $90. By expanding the sample to Oregon's populations yields, estimates ranged from $45 to $99 million for the whole State (a low of $45 to $90 per household). The resulting value per acre saved from fire under the proposed scenario is more than $24,000; and the cost per acre of old growth protected is $28.

Estimating number of species and relative abundances in stream-fish communities: effects of sampling effort and discontinuous spatial distributions

We sampled fishes and measured microhabitat in series of contiguous habitat units (riffles, runs, pools) in three Virginia streams. We used Monte Carlo simulations to construct hypothetical series of habitat units, then examined how number of species, similarity in relative abundances, and number of microhabitats accumulated with increasing number of habitat units (i.e., sampling effort). Proportions of all species and microhabitats represented were relatively low and variable at low sampling effort, but increased asymptotically and became less variable with greater sampling effort. To facilitate comparisons among streams, we fitted simulation results to negative exponential curves. The curves indicated that 90% of the species present were usually found by sampling 5 to 14 habitat units (stream length of 22–67 stream widths). Estimates of species relative abundances required less sampling effort for a given accuracy than estimates of number of species. Rates of species accumulation (with effort) varied among streams and reflected discontinuity in species distributions among habitat units. Most discontinuity seemed to be due to low population density rather than to habitat selectivity. Results from an Illinois stream corroborated our findings from Virginia, and suggested that greater sampling effort is needed to characterize fish community structure in more homogeneous stream reaches.

Planning and operational guidelines for mitigation banking for wetland impacts

Mitigation of wetland losses due to development activities is an important public policy issue in the United States. Such mitigation could include avoidance, minimization, rectification, reduction, or compensation. If the first four measures are not feasible, then compensation is required. Wetland mitigation banking is one type of compensation; it refers to a process where a developer purchases a land area and restores, enhances, or creates plant or animal wetland habitat prior to any development activities. These improvements are quantified and credit is placed in a bank; at a later date, when unavoidable impacts due to the development occur, the credit can be withdrawn. Due to the relative infancy of mitigation banking, a questionnaire survey of 19 existing and 20 proposed banks in the United States has been conducted to determine the state-of-practice relative to eight identified planning and/or operational issues. The issues included bank goals, site selection, bank operator, policies for credit (habitat units) establishment and usage, preferred wetland development options, criteria for bank usage, long-term management plans, and construction and maintenance requirements. The survey results were then used to formulate 10 guidelines for planning and operating banks; they are: (1) the goal of resource agencies and developers should be to provide an efficient and effective means of providing mitigation while at the same time decreasing the time involved in the permitting process; (2) site selection and construction should take place prior to development project impacts; (3) the bank operator should be the developer and/or a combination of resource/regulatory agencies whose responsibilities are to carry out administrative and technical duties and develop long-term management plans; (4) establishment of bank credits/debits should be based on recognized habitat methods; (5) the preferred order of wetland development option usage are restoration, enhancement, construction, and preservation; (6) development projects eligible for bank use must have proven to the applicable regulatory agencies that the sequential mitigation hierarchy is not possible and the project has no alternatives; (7) earmarked accounts, trust funds, or long-term interest bearing accounts should be used to guarantee funds for long-term management; (8) during bank construction, best management practices for erosion and fugitive dust control should be implemented to protect adjacent ecosystems; (9) postmonitoring should be conducted annually for at least 10 years to determine the success or failure of a bank site and to provide information for future maintenance efforts; and (10) bank account statements should be sent semiannually to bank users and after each transaction to signess of any Memorandum of Understanding/Agreement for their response and approval.

Phloem-Feeding Specialists Sharing a Host Tree: Resource Partitioning Minimizes Interference Competition among Galling Aphid Species

This study deals with a guild of specialist, phloem feeding aphids (Homoptera; Pemphigidae; Fordinae) which form galls on Pistachia trees (Anacardiaceae). In Israel, each of two Pistacia species carries five common species of galling aphids, which may occur in large numbers on the same individual tree, shoot, leaf or even leaflet. These Fordinae have very similar life cycles, they all feed on phloem sap, and all of them need very young, unfolding leaves for gall formation. Our data show, however, that most pairs of species are unlikely to compete for galling sites because their niches are separated either temporally (fundatrices arrive at different times and, therefore, occupy different leaves) or spatially, by attacking different sites on the tree, shoot, or leaf. In 1991-1993, we calculated niche breadth (B) of each species, and proportional similarity (PS) between each species pair on shoots colonized by more than one species. PS between different species pairs on leaves within shoots ranged between 0 and 0.48. This means that, on the niche dimension represented by the shoot, the two species may share some of the habitat units. If the two species compete, we expected that niche breadth of at least one of them would be reduced on the cohabited shoots. This was not the case. B was not negatively affected when pairs of species occupied the same shoot, compared with shoots where only one species was present. B was positively correlated with density: the chance of niche overlap increased when more galls occupied the same shoot. In species sharing leaves within shoots, B showed similar trends. On P. atlantica, Smynthurodes betae and Forda riccobonii had the highest PS. Both the species make their galls on leaflet margins and occur frequently on the same leaves. Only at this level did we detect negative interactions: the frequency of co-occurrence of galls of both species on the same leaflet (within galled leaves) was significantly less than expected by chance. Taking into account our deliberate non-random selection of trees, shoots, and leaves, where competition was likely to occur, we conclude that interspecific interference competition for galling sites is not a major determinant of the resource partitioning of these closely-related herbivores

Observer Variability in Classifying Habitat Types in Stream Surveys

Abstract We report on the ability of trained observers to independently classify habitat units within stream reaches into primary (pools, riffles, and glides) and secondary (types of pools and types of riffles) habitat types. Differences among observers in classifying habitat types increased with the number of habitat types and decreased with level of observer training. Observer variability also seemed to he affected by reach-specific physical attributes, such as gradient and the amount of wood in the stream channel. Attempts to classify stream habitats will be more consistent and useful if observers receive sufficient uniform training and are required to distinguish between fewer habitat types.

A field study of fish predation on juvenile crown-of-thorns starfish

To test whether commercially exploited fishes could regulate populations of crown-of-thorns starfish, laboratory reared juvenile Acanthaster planci were planced on small habitat units in an area of a lagoon where a number of species of fish that feed on benthic invertebrates occurred. Predators were excluded from half the units using wire mesh. In 35 days, losses were low and there was no statistically significant difference between caged and uncaged units. A difference in mortality rate of 1% of individuals per day would have been detected with >85% probability.However, the observed mean difference, the maximum estimate of predatory mortality, was 0.13% of starfish per day. It thus seems unlikely that predation by any large fishes was important in the population dynamics of juvenile A. planci at that site at the time of this experiment. Juvenile starfish were presented to lethrinids in the field at two reefs. Thirteen percent of juvenile A. planci presented at one reef were eaten, but in no presentation did lethrinids eat all the available starfish and those that were eaten were often mouthed and rejected by several fish before being swallowed. No juveniles were taken in a smaller number of trials at the second reef. These results do not favour the hypothesis that predation on juveniles by large fish is important in the population dynamics of A. planci but experiments at more sites will be required before this conclusion can be generalized.

Spatial distribution, survival, and growth of sibling groups of juvenile coho salmon (Oncorhynchus kisutch) in an experimental stream channel

The spatial distribution, survival, and growth of two full-sibling families of coho salmon (Oncorhynchus kisutch) in an experimental outdoor stream channel were compared between the families and with the performance of one of the families in an allopatric control channel after 75 d of rearing. No difference in survival was observed between families (81.2% overall), but their spatial distributions in the channel differed markedly. One family or the other numerically dominated 5 of 7 riffle-pool habitat units within the sympatric treatment channel. The family that was larger at the beginning of the experiment (0.52 vs. 0.37 g mean mass) was also larger at the end (2.03 vs. 1.58 g). While there was little evidence of density-dependent growth overall, the growth of each family was correlated with the density of siblings in the unit. However, the larger bodied family showed negative density-dependent growth, whereas the smaller bodied family showed positive density-dependent growth. These patterns of distribution and growth may have arisen from sibling recognition or some other proximate mechanism such as assortative distribution based on size or interfamily variation in aggression. Regardless of the mechanism, intrapopulation variation in distribution and growth may lead to considerable variation in fitness among families because these factors affect success in subsequent freshwater and marine life history stages.

The CORINE biotopes project: a database for conservation of nature and wildlife in the European community

The CORINE Programme was established by the European Commission to create a harmonized geographical information system on the state of the environment in the European Community. The ‘biotopes’ inventory of sites of major importance for nature conservation was a priority topic. At the time of writing, 6144 sites have been described, covering 12.2 per cent of the EC land surface area. The database records the presence of over 30000 habitat units and contains species data for 83 per cent of the sites. It has been used to guide environmental policy in the EC including the planning of the Habitats Directive, the most comprehensive piece of environmental legislation yet adopted by the Community, and is serving as an important tool in its implementation. Future development will follow the transformation of CORINE into an operational system within the European Environment Agency. The scope of the Biotopes database is currently being extended to include countries of Central and Eastern Europe, and members of EFTA.

Fish Habitat and Fish Populations in a Southern Appalachian Watershed before and after Hurricane Hugo

Habitat features and relative abundance of all fish species were estimated in 8.4 km of a small mountain stream system before and 11 months after Hurricane Hugo crossed the southern Appalachians in September 1989. There was no change in the total amount (area) of each habitat type but the total number of habitat units decreased and average size and depth of habitat units increased. Transport and sorting of streambed sediments was evident from the increased proportion of habitat units in which cobbles and small boulders were the most common constituents. Large woody debris more than doubled from 228 to 559 pieces/km of stream channel. At the watershed scale, there were only minor changes in the fish community 11 months after the hurricane, Eleven species were found both before and after the storm, and most species were uncommon. Among common species, densities increased in riffles for darters Etheostoma spp., increased in pools for blacknose dace Rhinichthys atratulus, and were largely unaffected for rainbow trout Oncorhynchus mykiss. The results of this case study suggest that the effects of catastrophic disturbances on fish habitat and populations depend on the predisturbance condition of instream and riparian habitat, timing of the disturbance, and life histories of individual species.

Large-scale Analysis of Atlantic Nova Scotia American Lobster (Homarus americanus) Landings with Respect to Habitat, Temperature, and Wind Conditions

American lobster (Homarus americanus) landings per unit of habitat, calculated for 25 statistical districts (SD) of Atlantic Nova Scotia (N.S.), were consistently higher in northern Cape Breton (SD 1–7) and southwestern N.S. (SD 32–34) than from the eastern (SD 15–18) and southern (SD 22–31) shores. Analyses suggest that changes in lobster populations of Atlantic N.S. take place over distances in the order of 100 km and over periods of about 5 yr and that lobster abundance is not limited by habitat availability. Five large homogeneous regions subject to similar historical landing trends could be identified on the basis of their thermal characteristics. Regions of lowest lobster landings (SD 8–31) are oriented in the axis of dominant southwest winds, which generate coastal upwelling, resulting in the lowest cumulative number of degree-days and a high year-to-year variability. Conversely, the high landings recorded in southwestern N.S. can be related to warm and stable temperature conditions, allowing lobsters to enhance yearly thermal gains via winter migrations into deep (> 100 m), warm Slope water. The definition of homogeneous regions characterized by common historical landing trends and thermal regime suggests that lobster abundance in Atlantic N.S. is ultimately controlled by large-scale oceanographic processes.

Spatial and temporal patterns in the fish assemblages of individual pools in a midwestern stream (U.S.A.)

The composition and consistency of fish assemblages in 14 adjacent pools (6–120 m long) of a clear-water, limestone and gravel creek in midwestern U.S.A. were quantified in eight snorkeling surveys over 19 months, to establish a baseline of natural variation in the system at this scale. The fauna of the stream was dominated numerically by minnows (Cyprinidae), sunfish and black bass (Centrarchidae), and topminnows (Fundulidae). The pool fish fauna of the total 1 km reach (including all 14 pools) was highly consistent throughout the study, despite two major floods. Assemblages in individual pools generally were consistent, but there was more variation within pools than at the scale of the entire reach. Throughout the study, most individual pools remained within discrete subsets of the total occupied multivariate space in a principal components analysis based on fish species abundances. Sunfishes (Lepomis spp.) and bass (Micropterus spp.) were more consistent in their distribution among pools than were minnows (Cyprinidae) or a topminnow (Fundulus). There were 25 significant correlations in occurrence of species pairs among stream pools, out of 91 possible comparisons of the 14 most abundant taxa in the reach. Many pools contained assemblages either dominated by large centrarchids or by abundant cyprinids and juvenile centrarchids, but intermediate assemblages also were observed. The dynamics of distribution of fish species and fish assemblages among individual stream pools are likely influenced by a combination of species-specific behaviors and habitat selection, predator constraints on use of individual pools by small fishes, riffles as size-selective barriers to fish movements between pools, dispersal of young-of-the-year fishes, and abiotic phenomena like floods. Individual stream pools appear to be discrete habitat units for fishes, and do represent an appropriate scale for biologically meaningful studies of fish assemblages or their effects on streams.

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss.

In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1-2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. "Settlement" is considered to be the arrival of new individuals from the pelagic habitat, and "recruitment" is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1-2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.