Habitat Complexity Index Research Articles

Status of health and conservation classification of tropical coral reefs in Lakshadweep archipelago

Coral reefs of Lakshadweep perform a range of vital ecosystem functions and sustain the livelihoods of island communities. Coral reefs provide ecosystem services that are vital to human societies and industries through fisheries, coastal protection and tourism. The present study was undertaken during September and October 2015 to assess the status of reef health in 10 inhabited Islands of Lakshadweep following reef classification to imply conservation measures and also for the sustainable management by the dependent communities. Live coral cover, habitat complexity index, coral species richness, reef condition index, mortality index, morphological dominance, morphological diversity and coral conservation class were used to get an aggregate value of ‘composite condition’ index for determining the overall conservation values of coral reefs in these Islands. Kadmat reef had the highest live coral cover of 64.50% among the 10 inhabited islands. Bitra and Kiltan reef had good live coral cover of 62.75 and 52.25% respectively. The major benthic component in all the studied islands is live coral except in Amini and Andrott Islands. The recovery of corals has increased significantly in the last 5 years in Kadmat, Bitra, Kiltan and Bangarm after the 2010 mass bleaching event. Among the eight attributes, significant positive correlations were found between coral reef condition and live coral cover (r = 0.93, p 60% of stress tolerators. CC3 was assigned to Bangaram and Kadmat due to the presence of > 75% of ruderals. Mixed coral morphology was recorded in Agatti, Andrott, Kalpeni, Kavaratti, and Kiltan with approximately equal proportions of ruderals, competitors and stress tolerators, and these coral reefs were designated as CC4. Protection and conservation management measures, as well as sustainable resource use, have been suggested based on the live coral cover, conservation class, and coral reef health status.

Relative influence of environmental factors and fishing on coral reef fish assemblages.

Understanding whether assemblages of species respond more strongly to bottom-up (availability of trophic resources or habitats) or top-down (predation pressure) processes is important for effective management of resources and ecosystems. We determined the relative influence of environmental factors and predation by humans in shaping the density, biomass, and species richness of 4 medium-bodied (10-40 cm total length [TL]) coral reef fish groups targeted by fishers (mesopredators, planktivores, grazer and detritivores, and scrapers) and the density of 2 groups not targeted by fishers (invertivores, small fish ≤10 cm TL) in the central Philippines. Boosted regression trees were used to model the response of each fish group to 21 predictor variables: 13 habitat variables, 5 island variables, and 3 fishing variables (no-take marine reserve [NTMR] presence or absence, NTMR size, and NTMR age). Targeted and nontargeted fish groups responded most strongly to habitat variables, then island variables. Fishing (NTMR) variables generally had less influence on fish groups. Of the habitat variables, live hard coral cover, structural complexity or habitat complexity index, and depth had the greatest effects on density, biomass, and species richness of targeted fish groups and on the density of nontargeted fishes. Of the island variables, proximity to the nearest river and island elevation had the most influence on fish groups. The NTMRs affected only fishes targeted by fishers; NTMR size positively correlated with density, biomass, and species richness of targeted fishes, particularly mesopredatory, and grazing and detritivorous fishes. Importantly, NTMRs as small as 15 ha positively affected medium-bodied fishes. This finding provides reassurance for regions that have invested in small-scale community-managed NTMRs. However, management strategies that integrate sound coastal land-use practices to conserve adjacent reef fish habitat, strategic NTMR placement, and establishment of larger NTMRs will be crucial for maintaining biodiversity and fisheries.

Algal complexity positively affects the abundance, richness and diversity of molluscan assemblages of a semiarid hypersaline mangrove

Marine macroalgae serve as habitats that provide ideal shelter for a rich and diverse community of associated molluscs, but the true effects of the complexity of these habitats on fauna are still poorly understood. Replicates of four macroalgal species: Gracilaria domingensis (Kutzing) Sonder ex Dickie 1874, Solieria filiformis (Kutzing) Gabrielson 1985, Gracilaria cuneata Sonder ex Kutzing 1859 and Dictyota ciliolata Areschoug 1854 of a hypersaline estuary were collected to investigate the effects that macroalgal habitat complexity index has on the abundance, richness and diversity of associated molluscs. Habitat complexity index was determined by measuring several morphological attributes of each replicate of the four macroalgae species as well as weight and the fractal dimension. The studied macroalgae differed in habitat complexity index, with Gracilaria domingensis being the most complex. Microgastropods were the most species rich and abundant group of molluscs with 59 of 90 species and 876 of the total 1006 individuals. Mollusc community abundance and diversity differed significantly among the studied species of macroalgae. Habitat complexity had a positive effect on associated mollusc abundance, richness and diversity. This study revealed a rich biodiversity of micromolluscs associated with the studied macroalgae and reinforces the positive effects that increased habitat complexity has on the coexistence of this rich and diverse fauna in phytal ecosystems. Conservation efforts to protect hypersaline mangroves should include the protection of macroalgae banks as an important tool for habitat management and the maintenance of the associated biota.

Fine-scale benthic biodiversity patterns inferred from image processing

Despite potentially considerable advantages over traditional sampling techniques, image-derived indices of habitat complexity have rarely been used to predict patterns in marine biodiversity. Advantages include increased speed and coverage of sampling, avoidance of destructive sampling, and substantially reduced processing time compared to traditional taxonomic approaches, thus providing a starting point for more detailed analysis if warranted. In this study, we test the idea that the mean information gain (MIG) and mean mutual information (MMI), two indices of image heterogeneity that we derived from photographs of marine benthic assemblages, represent good preliminary predictors of biodiversity patterns for 133 benthic invertebrate and algal taxa on jetty pylons in Gulf St Vincent, South Australia. Both MIG and MMI were spatially structured, with evidence of among-site differences that were also evident in the benthic data. When combined with information on the spatial structure within the dataset (site and depth), MIG and MMI explained ∼35% of deviance in invertebrate species richness, ∼43% in Shannon's evenness and up to 50% of dissimilarity in species composition. This explanatory power is of a similar magnitude to many other, less readily available, surrogate measures of biodiversity. These results corroborate the idea that indices of image heterogeneity can provide useful and cost-effective complements to traditional methods used for describing (or predicting) marine epibiota biodiversity patterns. This approach can be applied to many case studies for which photographic data are available, and has the potential to result in substantial time and cost savings.

Recolonizing sea otters spatially segregate from pinnipeds on the Canadian Pacific coastline: The implications of segregation for species conservation

The reintroduction and recolonization of species extirpated from former ranges are key components of species conservation. Resource availability affects recolonization success and resulting distribution patterns, but top-down processes may also play a role through density-mediated or trait-mediated indirect effects that may exclude a species from otherwise suitable habitat. We predicted that the spatial distribution of recolonizing sea otters, Enhydra lutris, on the Canadian Pacific coastline was explained by resources as well as interspecific interactions – spatial segregation from pinnipeds, the preferred prey of killer whales Orcinus orca. We surveyed the summer occurrence of sea otters and pinnipeds on Vancouver Island, Canada. We quantified coastline density and bathymetry at multiple spatial scales as indices of habitat complexity and foraging habitat availability. We used generalized linear model selection to test hypotheses about sea otters’ spatial relationship to resources and heterospecifics. Pinniped presence negatively predicted sea otter presence, even after accounting for complexity and foraging habitat. Sea otters may segregate from pinnipeds due to trait-mediated indirect effects of predation, leading us to hypothesize apparent competition between sea otters and pinnipeds. Research is needed to test this hypothesis; if true, refuge from apparent competitors may be a key component of recolonization habitat for sea otters. Species distribution models should quantify resource landscapes but also species-scapes: the spatial plane of species interactions that combines with resources to drive species distributions. Conservation plans based on recolonization models that include only resources may overestimate available habitat, carrying capacity, and recolonization success.

Green crab (Carcinus maenas (Linnaeus, 1758)) foraging on soft-shell clams (Mya arenaria Linnaeus, 1758) across seagrass complexity: Behavioural mechanisms and a new habitat complexity index

Although many studies have examined foraging success across seagrass complexity, few have identified underlying behavioural mechanisms or examined effects of belowground complexity. Here, I used a new habitat complexity index (IBG), behavioural data, and laboratory manipulations of artificial above- and belowground structures and predator sizes to understand foraging of invasive green crabs (Carcinus maenas) on soft-shell clams (Mya arenaria) in seagrass (Zostera marina) beds. IBG was calculated as interstitial area between rhizomes (i.e., belowground complexity) divided by claw area; belowground complexity was varied while claw area was held constant. This meant that as IBG increased, belowground complexity itself decreased. Belowground complexity strongly affected crab foraging, and a sigmoid function described predation rate across increasing IBG. This relationship was not evident when aboveground complexity was high (i.e., interstitial area between shoots was small), and no patterns in predation across a gradient of aboveground complexity were observed. Important behavioural mechanisms included encounter rate with prey and the probability of capture upon encounter, both of which increased hyperbolically as IBG increased, and handling time per prey, which decreased exponentially as IBG increased. Most handling time was spent excavating prey from the sediments. Predator size did not change these relationships, although larger crabs had more difficulty capturing prey at low IBG than smaller crabs. Clearly, success of crabs feeding on infauna in seagrass beds is limited most by claw size relative to opening size from which prey are extracted. IBG incorporates this limitation, and can be used to predict effects of habitat complexity on foraging success of epibenthic predators in various habitat types.

A comparison of two techniques for the rapid assessment of marine habitat complexity

Summary Monitoring and assessment of the status and distribution of marine seabed habitats is needed to support existing and emerging environmental policy commitments. Traditional monitoring of habitats and associated species using grabs and trawls is costly and labour intensive and might usefully be complemented by cheaper and more readily automated methods that can be used at higher frequencies and/or on larger spatial scales. We develop and apply two methods to measure seabed habitat complexity and demonstrate how they can be used to describe impacts (e.g. fishing gear impacts) and monitor recovery. The first method relies on the analysis of deviations in a laser line projected on the seabed. The second method is based on the pixel value distribution in seabed photographs. We use both methods to quantify the complexity created by different substrates and habitat‐forming species and to establish links between habitat complexity and faunal diversity (richness) and abundance. The habitat complexity index calculated with the laser line method provided a reliable index of complexity across a range of habitat types, showing a monotonic increase with coarseness of the substratum and the abundance of sessile epifauna. Pixel value distributions in the photographs did not reflect the increase in complexity due to sessile epifauna but only reflected substratum differences. Results suggested that the laser line method would be suitable for monitoring the effect of disturbance on habitats ranging from gravelly sands to rock, and their subsequent recovery. The photographic method would be better suited to assessing complexity and heterogeneity of the substratum. Both methods complement conventional biological sampling and can be used at higher frequencies and/or on larger spatial scales per unit cost. The laser line method has considerable potential to support demands for frequent monitoring of seabed habitats and human impacts at a range of spatial scales. It is less costly and labour intensive than existing approaches and can be deployed from vessels of many sizes.

Bot fly parasitism of the red-backed vole: host survival, infection risk, and population growth

Parasites can play an important role in the dynamics of host populations, but empirical evidence remains sparse. We investigated the role of bot fly (Cuterebra spp.) parasitism in red-backed voles (Myodes gapperi) by first assessing the impacts of the parasite on the probability of vole survival under stressful conditions as well as on the reproductive activity of females. We then identified the main factors driving both the individual risk of infection and the abundance of bot flies inside red-backed voles. Finally, we evaluated the impacts of bot fly prevalence on the growth rate of vole populations between mid-July and mid-August. Thirty-six populations of red-backed voles were sampled in the boreal forest of Québec, Canada. The presence and the abundance of parasites in voles, two host life history traits (sex and body condition), three indices of habitat complexity (tree basal area, sapling basal area, coarse woody debris volume), and vole abundance were considered in models evaluating the effects of bot flies on host populations. We found that the probability of survival of red-backed voles in live traps decreased with bot fly infection. Both the individual risk of infection and the abundance of bot flies in red-backed voles were driven mainly by vole abundance rather than by the two host life history traits or the three variables of habitat complexity. Parasitism had population consequences: bot fly prevalence was linked to a decrease in short-term growth rate of vole populations over the summer. We found that bot flies have the potential to reduce survival of red-backed voles, an effect that may apply to large portions of populations.

Space size relative to prey width (Sp/Py) influences macrofaunal colonization of artificial structures

We tested an index of habitat complexity, Sp/Py (inter-structural space size/prey body width), by deploying artificial structures on a coral reef near Dibba, United Arab Emirates. The struc- tures were polyethylene tube bundles with tube diameters of 3, 12 and 24 mm and a mixed treatment composed of all 3 tube sizes. We counted and identified mobile macrofauna that colonized the inte- rior of the tubes and divided them into 3 body width categories: small ( 12 mm). Medium and large fauna were excluded from 3 mm tubes, and large fauna were excluded from 12 mm tubes. Small fauna were significantly more abundant in 24 mm tubes compared with 3 mm tubes, with intermediate abundance in 12 mm tubes. A second experiment determined that small fauna probably responded positively to tube interior volume, or responded to differences in hydrodynamic properties between the treatments. Medium fauna were more likely to be found in 12 mm tubes that matched their own body width than in 24 mm tubes. Species richness was signifi- cantly lower in the 3 mm tubes compared with the other treatments, in part because errant poly- chaetes, gastropods, caprellid amphipods and crabs were less likely to be found in the 3 mm tubes. These results indicate that habitats with very narrow inter-structural space sizes may be 'too com- plex' and may exclude certain fauna, resulting in decreased species richness. Also, some fauna may prefer habitats with intermediate space sizes that match their own body width.

Distribution of sympatric seahorse species along a gradient of habitat complexity in a seagrass-dominated community

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 291:81-91 (2005) - doi:10.3354/meps291081 Distribution of sympatric seahorse species along a gradient of habitat complexity in a seagrass-dominated community Janelle M. R. Curtis1,2,*, Amanda C. J. Vincent1,2 1Department of Biology, McGill University, 1205 Dr. Penfield Avenue, Montréal, Québec H3A 1B1, Canada2Project Seahorse, Fisheries Centre, University of British Columbia, 2204 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada *Email: janelle.curtis@elf.mcgill.ca ABSTRACT: We present estimates of local population abundance, distribution and habitat preference for 2 European seahorse species, Hippocampus guttulatus and H. hippocampus. We predicted that these sympatric species would partition their habitat into 2 broadly defined habitat types: complex vegetated habitats and sparsely vegetated sand flats. We sampled populations using underwater visual census techniques over landscape (100s to 1000s m) and microhabitat scales (<1 m). Over landscape scales, we estimated abundance and quantified habitat associations using generalized linear models. Over microhabitat scales, we tested for holdfast (attachment site) preferences using selection indices. Both species were patchy in distribution, but H. guttulatus mean density (0.073 ind. m–2) was one order of magnitude greater than that of H. hippocampus (0.007 ind. m–2). At a landscape scale, H. guttulatus abundance was positively correlated with an index of habitat complexity, the percentage of substrate covered by flora and sessile fauna. Conversely, H. hippocampus used more open and less speciose habitats that were subjected to greater oceanic influences. At microhabitat scales, both species significantly preferred grasping holdfasts over barren surfaces, but the species differed in holdfast preferences: H. guttulatus grasped all prospective holdfast types with equal probability while H. hippocampus significantly avoided both fauna and flora that formed large colonies or tracts of dense vegetation. Patterns in habitat use were consistent with differences in morphology and foraging strategy. Despite similar life histories, these sympatric species may respond differently to disturbances that modify habitat structure and complexity over landscape or microhabitat scales. KEY WORDS: Habitat selection · Habitat complexity · Hippocampus · Syngnathidae · Habitat suitability models · Underwater visual census · Foraging strategy Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 291. Online publication date: April 28, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Inferring versus measuring rates of recovery in no-take marine reserves

Can rates of biomass recovery of fished species be inferred reliably from once-only spatial comparisons of no-take marine reserves of different ages and fished areas? We used underwater visual census at 15 no-take marine reserves in the Philippines to both infer and measure such rates. We made a single estimate of the biomass of large predatory fishes (Serranidae, Lutjanidae, Lethrinidae) targeted heavily by fisheries in each of 13 well protected no-take reserves (age range 0.5 to 13 yr), and in nearby nonreserve (fished) sites. We also measured rates of biomass buildup of these fish regularly for 18 yr (1983 to 2001) in 2 no-take reserves (Sumilon, Apo) and nonreserve sites. The duration of protection required to detect significantly higher reserve biomass was similar, but lower for temporal monitoring (3 to 4 yr) than for spatial comparisons (6 yr). The reserve:nonreserve biomass ratios at maximum duration of reserve protection were similar for inferred (9.0) and measured (6.3 to 9.8) estimates. Thus, results of long-term monitoring of 2 reserves may have regional generality. The inferred rate of change of a reserve effect index (log 10 [Reserve biomass + 1 / Nonreserve biomass + 1]) with duration of protection did not differ significantly from the measured rate at Sumilon, but was higher than that measured at Apo. A habitat complexity index did not affect estimates of ‘reserve effects’ significantly in this study, and reserve protection was generally effective. Thus, using similar methods of reserve protection and census on the same target group in similar areas, one can make useful inferences about rates of recovery in no-take marine reserves.

EXAMINING PATTERNS OF BAT ACTIVITY IN BANDELIER NATIONAL MONUMENT, NEW MEXICO, BY USING WALKING POINT TRANSECTS

Abstract We conducted a preliminary study using small field crews, a single Anabat II detector coupled with a laptop computer, and point transects to examine patterns of bat activity at a scale of interest to local resource managers. The study was conducted during summers of 1996–1998 in Bandelier National Monument in the Jemez Mountains of northern New Mexico, a landscape with distinct vegetation zones and high species richness of bats. We developed simple models that described general patterns of acoustic activity within 4 vegetation zones based primarily on nightly variation and a qualitative index of habitat complexity. Bat acoustic activity (number of bat passes&sol point) did not vary dramatically among a limited sample of transects within a vegetation zone during 1996. In 1997 and 1998, single transects within each vegetation zone were established, and bat activity did not vary annually within these zones. Acoustic activity differed among the 4 vegetation zones of interest, with the greatest activi...

Indicators for monitoring minesite rehabilitation: trends on waste-rock dumps, northern Australia

Minesite rehabilitation needs to be monitored with easily measured indicators so that trends can be plotted and assessed over time to address closure criteria. Our aim was to evaluate two traditional vegetation indicators used in woodlands and forests (tree composition and size), and two new landscape surface indicators (integrity of rip-lines and a nutrient cycling index), as well as a new habitat complexity index. We measured these five indicators on rehabilitated waste-rock dumps of differing ages on two mines in northern Australia, and compared these measures to those for nearby natural savannas. Our results confirm that tree composition and size were useful traditional indicators of vegetation development on rehabilitation because trends were towards that expected for nearby savannas. Surface roughness, as indicated by rip-lines, was also a useful indicator of the potential for a landscape to retain resources because these rip-lines persisted until vegetation was well established to assume this role. As this vegetation developed, soil surface condition, as indicated by a nutrient cycling index, also progressed towards values found on nearby natural savannas. A habitat complexity index suggested that older rehabilitation sites were developing the structural features needed by fauna. Although our findings need to be confirmed for other mines and for older sites, they do suggest that these five ecological indicators are useful for monitoring minesite rehabilitation.

HABITAT COMPLEXITY, SPATIAL INTERFERENCE, AND “MINIMUM RISK DISTRIBUTION”: A FRAMEWORK FOR POPULATION STABILITY

In the past century, the debate over whether or not density-dependent factors regulate populations has generally focused on changes in mean population density, ignoring the spatial variance around the mean as unimportant noise. In an attempt to provide a different framework for understanding population dynamics based on individual fitness, this paper discusses the crucial role of spatial variability itself on the stability of insect populations. The advantages of this method are the following: (1) it is founded on evolutionary principles rather than post hoc assumptions; (2) it erects hypotheses that can be tested; and (3) it links disparate ecological schools, including spatial dynamics, behavioral ecology, preference-performance, and plant apparency into an overall framework. At the core of this framework, habitat complexity governs insect spatial variance. which in turn determines population stability. First, the minimum risk distribution (MRD) is defined as the spatial distribution of individuals that results in the minimum number of premature deaths in a population given the distribution of mortality risk in the habitat (and, therefore, leading to maximized population growth). The greater the divergence of actual spatial patterns of individuals from the MRD, the greater the reduction of population growth and size from high, unstable levels. Then, based on extensive data from 29 populations of the processionary caterpillar, Ochrogaster lunifer, four steps are used to test the effect of habitat interference on population growth rates. (1) The costs (increasing the risk of scramble competition) and benefits (decreasing the risk of inverse density-dependent predation) of egg and larval aggregation are quantified. (2) These costs and benefits, along with the distribution of resources, are used to construct the MRD for each habitat. (3) The MRD is used as a benchmark against which the actual spatial pattern of individuals is compared. The degree of divergence of the actual spatial pattern from the MRD is quantified for each of the 29 habitats. (4) Finally, indices of habitat complexity are used to provide highly accurate predictions of spatial divergence from the MRD, showing that habitat interference reduces population growth rates from high, unstable levels. The reason for the divergence appears to be that high levels of background vegetation (vegetation other than host plants) interfere with female host-searching behavior. This leads to a spatial distribution of egg batches with high mortality risk, and therefore lower population growth. Knowledge of the MRD in other species should be a highly effective means of predicting trends in population dynamics. Species with high divergence between their actual spatial distribution and their MRD may display relatively stable dynamics at low population levels. In contrast, species with low divergence should experience high levels of intragenerational population growth leading to frequent habitat-wide outbreaks and unstable dynamics in the long term. Six hypotheses, erected under the framework of spatial interference, are discussed, and future tests are suggested.

Effects of Habitat Area and Complexity on Colorado River Cutthroat Trout Density in Uinta Mountain Streams

Habitat degradation has reduced the complexity and connectivity of streams on the north slope of the Uinta Mountains in northeastern Utah. These changes have diminished the historical range of Colorado River cutthroat trout Oncorhynchus clarki pleuriticus, isolated the populations of this subspecies, and perhaps increased its risk of extinction. We assessed the effects of fragment area and habitat complexity on Colorado River cutthroat trout density. We studied 88 reaches in 4 isolated stream fragments. At the fragment scale, both the density of adults and habitat complexity increased significantly as fragment size increased. In the smaller fragments, the density of adults was lower while that of juveniles was higher. Habitat differed substantially among fragments. At the reach scale, the density of adults was positively related to elevation, the percentage of undercut banks, and mean substrate particle size and negatively related to residual pool depth and the extent of large woody debris. The density of juveniles was positively related to the extent of large woody debris and negatively related to residual pool depth and stream width. The habitat complexity index was weakly related to adult density at the reach scale. We were not able to distinguish the influence of habitat area or complexity on the density of adults, but a population living in an isolated stream fragment with low habitat complexity probably requires more area to persist than a population of the same size living in a highly complex habitat.

The reproductive success of Blackbirds Turdus merula in relation to habitat structure and choice of nest site

The reproductive success of a population of Blackbirds Turdus merula occupying farmland and woodland was studied over 3 years to investigate the effects of habitat on breeding success. Territory distribution was patchy in both farmland and woodland; some areas were unoccupied, while other areas were occupied at variable densities. Habitat structure appeared to influence occupation: the index of habitat complexity (“cover score”) was higher in occupied areas than in unoccupied areas and high‐density territories had higher cover scores than low‐density territories. However, habitat structure had no significant effect on reproductive success because the cover scores of territories where pairs were successful did not differ significantly from those of territories where there were no successful breeding attempts. There was no evidence of differential mortality rates in adults according to habitat. The height, bulk and exposure of c. 430 nests were measured to determine the effect of nest and nest‐site characteristics on reproductive success. Nest exposure was the only feature that differed between successful and failed nests, successful nests being less exposed than failed nests. The major cause of breeding failure was nest predation, but the effect of nest exposure operated only during the laying and incubation period and not during the nestling period. The significance of habitat structure for variation in population densities between habitats is discussed.

Species‐Area Relationship for Stream Fishes

We sampled riffles and pool habitats of small streams in Minnesota, Illinois, and Panama to examine variation in species—area relationships within and between the respective fish faunas. For six of the seven steams studies, habitat lume was a better predictor of species richness than was habitat area, and number of individuals was a better predictor of species richness than habitat volume. Slopes of species–volume relationships were similar among regions, but the number of species per unit volume was greater in Panama. Multiple regression analyses indicated that knowledge of habitat complexity and volume did not enhance appreciably the capability of linear models to predict species richness from number of individuals in the sample. These results support the hypothesis that species—area relationships may often be epiphenomena stemming from the more comprehensive community "samples" intercepted by larger habitat patches. Although number of individuals was the best single predictor of species richness, habitat structure and type clearly influenced species' distributions in some streams, thereby indicating that species—area relationships were not strictly sampling phenomena. An index of habitat complexity based on depth, current, and bottom type was correlated with species richness in two Panama streams. Also, the abundance of individual species was more likely to be correlated with habitat volume in Panama than in Illinois or Minnesota, and species relative abundances were more similar between years in Panama than in Illinois or Minnesota streams, especially in pools. These patterns suggest that in streams subject to strong seasonal and annual environmental variation, habitat features are poorer predictors of fish distribution and abundance than in streams subject to less environmental variability. We speculate that annual variability in reproductive success and harsh winters interact to maintain imbalance between the fish assemblages and their habitat in Minnesota. Weak relationships between species richness and habitat volume or complexity may be indicative of population variability and the predominance of extinction/recolonization processes in community organization.