Spatial Patterns Emphasize the Importance of Coastal Zones as Nursery Areas for Larval Walleye in Western Lake Erie
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Ichthyoplankton
Spatial heterogeneity
Summary The spatial pattern of tree species retains signatures of factors and processes such as dispersal, available resource patches for establishment, competition and demographics. Comparison of the spatial pattern of different size classes can thus help to reveal the importance and characteristics of the underlying processes. However, tree dynamics may be masked by large‐scale heterogeneous site conditions, e.g. when the restricting size of regeneration sites superimposes emergent patterns. Here we ask how environmental heterogeneity may influence the spatial dynamics of plant communities. We compared the spatial patterns and demographics of western hemlock in a homogeneous and a heterogeneous site of old‐growth Douglas‐fir forests on Vancouver Island using recent techniques of point pattern analysis. We used homogeneous and inhomogeneous K ‐ and pair‐correlation functions, and case‐control studies to quantify the change in spatial distribution for different size classes of western hemlock. Our comparative analyses show that biological processes interacted with spatial heterogeneity, leading to qualitatively different population dynamics at the two sites. Population structure, survival and size structure of western hemlock were different in the heterogeneous stand in such a way that, compared to the homogeneous stand, seedlings were more clustered, seedling densities higher, seedling mortality lower, adult growth faster and adult mortality higher. Under homogeneous site conditions, seedling survival was mainly abiotically determined by random arrival in small gaps with limiting light. At the heterogeneous site, seedling densities and initial survival were much higher, leading to strong density‐dependent mortality and selection for faster growing individuals in larger size classes. We hypothesise that the dynamics of the heterogeneous stand were faster due to asymmetric competition with disproportionate benefit to taller plants. Synthesis . Our study supports the hypothesis that successional dynamics are intensified in heterogeneous forest stands with strong spatial structures and outlines the importance of spatial heterogeneity as a determinant of plant population dynamics and pattern formation.
Spatial heterogeneity
Demographics
Forest dynamics
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Point pattern analysis
Spatial heterogeneity
Temperate forest
Temperate rainforest
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Organisms usually benefit from heterogeneous conditions, but, by doing so, may reduce the degree of heterogeneity. The question therefore arises how heterogeneity is maintained. We investigated within-year spatiotemporal patterns in a monospecific stand of a submerged plant (fennel pondweed, Potamogeton pectinatus), with the novelty that we distinguished between different forms of heterogeneity: spatial variance (the frequency distribution of densities) and spatial pattern (the spatial distribution of densities). We repeatedly measured plant biomass that was affected by swan predation, winter mortality, and summer regrowth. Spatial variance was enhanced mostly by swan foraging, despite the fact that swans appear to exploit patches to the same threshold level. Spatial pattern, which had vanished after swan foraging, reestablished due to spatial pattern in winter mortality and was further enhanced by plant regrowth. We found that variance and pattern each have their own temporal dynamics and are maintained by different biological processes. We therefore advocate that it is pivotal to distinguish between variance and pattern in the study of spatial heterogeneity.
Persistence (discontinuity)
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Abstract We conducted larval and adult fish surveys on the southern flank of Georges Bank during the spring of two years (1990 and 1995) with contrasting physical conditions. We employed canonical correspondence analysis (CCA) to examine the relationships between physical variables and the spatial distribution of pelagic fish and ichthyoplankton. Surface temperature bottom temperature, and vertical stratification were significant factors affecting larval fish distributions, and there were groups of species with similar responses to these variables. There were also consistent relationships between physical variables and pelagic fish and squid abundances and spatial distributions. Pelagic fish and ichthyoplankton with similar responses to hydrographic variables had high spatial overlap, and variation in hydrographic regimes modulated the strength of this interaction. Pelagic fish and squids are potentially important predators of both larval and juvenile fish on Georges Bank. Hydrographic structure modulates the degree of spatial overlap and therefore likely influences the strength of predator–prey interactions.
Ichthyoplankton
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Ichthyoplankton
Spawn (biology)
Mesopelagic zone
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Ichthyoplankton
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Ichthyoplankton
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Spatial heterogeneity
Steppe
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Patchiness is an important feature in grassland ecosystems, but the distribution pattern of plant species in small patches on the Qinghai-Tibetan Plateau (QTP) is largely unknown. We questioned: (1) whether the power law can be applied to determine the spatial pattern of plant species in small patches; and (2) how species frequency (dominant, companion and rare) affects the spatial heterogeneity of small patches on the QTP? The power law was used to examine the spatial pattern of plant species and examine the relationship between the plant species presence frequency and spatial pattern in three types of small patches (Cyperaceae, Polygonaceae and Compositae), each with three spatial extents according to the size of the area (scale 1: 0.6–0.9 m2, scale 2: 3.0–3.8 m2 and scale 3: 6.5–8.8 m2) on the QTP. Plant species in patches showed an aggregated distribution and the spatial heterogeneity index differed among species. The spatial heterogeneities of Cyperaceae and Polygonaceae patches were influenced strongly by dominant and companion plant species, while the Compositae patch was influenced by dominant and rare species. The Compositae patches exhibited a lower species diversity and higher spatial heterogeneity index than the other patches, and the extensive distribution of Compositae community can be linked to degradation processes in the alpine meadow. We concluded that the spatial pattern of plant species and the heterogeneity index for small patches could be determined by the species presence frequency on the QTP.
Polygonaceae
Cyperaceae
Spatial heterogeneity
Rare species
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Abstract Understanding sampling effectiveness is critical to gear selection and the determination of larval fish dynamics. We evaluated bow‐mounted push nets for collecting larval fish across reservoirs and habitats and compared them with traditional tow nets. By means of a pushed 0.5‐m‐diameter conical net and towed 0.5‐m‐diameter and 0.75‐m‐diameter conical nets, ichthyoplankton samples were taken during daylight hours from May to July on 21 reservoirs that varied in morphological and environmental characteristics. The push net had higher catches than the same‐diameter tow net. However, the push net was not as efficient as the larger‐sized tow net in July, when larvae are larger. For pelagic habitats, bow‐mounted push nets or large tow nets will sample the larval fish community more efficiently than traditional 0.5‐m‐diameter conical tow nets. We also assessed push nets for sampling nearshore littoral habitats (<1.0 m in depth). Across reservoirs the littoral areas had much higher catch rates than did the offshore pelagic zone; estimates of peak larval fish densities were four times as high in the littoral zone. Deriving estimates of larval fish abundance from pelagic habitats only will probably lead to underestimation of total larval fish densities. The versatility of the push net in sampling littoral habitats is an important consideration when designing surveys to estimate larval fish communities. Bow‐mounted push nets can be used to effectively sample both pelagic and littoral larval fish communities, whereas traditional tow nets are only suited for pelagic habitats.
Ichthyoplankton
Diel vertical migration
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