Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open‐canopy stream (high ambient light) and a closed‐canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast‐decomposing (alder, Alnus glutinosa) and slow‐decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open‐canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within‐stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.     

Leaf litter and irrigation can increase seed germination and early seedling survival of the recalcitrant‐seeded tree Beilschmiedia miersii

Although regeneration of recalcitrant‐seeded tree species can be affected by prolonged drought, especially in Mediterranean regions, little is known about the response of such species to varying site conditions. A field experiment was performed to determine the effect of irrigation and leaf litter cover on seed germination and early seedling survival of the vulnerable recalcitrant‐seeded tree Beilschmiedia miersii (Lauraceae). Two levels of irrigation (non‐irrigated and irrigated units) and three levels of leaf litter depth (0, 5 and 12 cm) were applied to 72 groups of 30 seeds along a wet ravine of the Coastal Range of Central Chile, equally distributed across sites with different levels of canopy cover. Seed germination was significantly increased by irrigation only under closed‐canopy cover, and by leaf litter cover (>5 cm) under all canopy conditions. The effect of leaf litter on germination increased with canopy openness, while the effect of irrigation did not show any tendency. Meanwhile, early seedling survival was significantly increased by irrigation under intermediate canopy cover, and by leaf litter (>5 cm) under closed‐canopy cover. As a result of its overall positive effect on germination, leaf litter should be maintained within B. miersii communities, particularly under intermediate to closed‐canopy conditions, where it can also increase early seedling survival, and both seed germination and early seedling survival might be increased through additional water inputs. The presence of leaf litter might help retaining such inputs, prolonging their effect on regeneration of B. miersii communities. We see this as a baseline assessment of regeneration and persistence that needs further testing on species with similar traits, given the expected increase in the frequency and length of dry periods into the future.     

Relationships between intra‐specific variation in seed size and recruitment in four species in two contrasting habitats

Large seeds contain more stored resources, and seedlings germinating from large seeds generally cope better with environmental stresses such as shading, competition and thick litter layers, than seedlings germinating from small seeds. A pattern with small‐seeded species being associated with open habitats and large‐seeded species being associated with closed (shaded) habitats has been suggested and supported by comparative studies. However, few studies have assessed the intra‐specific relationship between seed size and recruitment, comparing plant communities differing in canopy cover. Here, seeds from four plant species commonly occurring in ecotones between open and closed habitats (Convallaria majalis, Frangula alnus, Prunus padus and Prunus spinosa) were weighed and sown individually (3200 seeds per species) in open and closed‐canopy sites, and seedling emergence and survival recorded over 3 years. Our results show a generally positive, albeit weak, relationship between seed size and recruitment. In only one of the species, C. majalis, was there an association between closed canopy habitat and a positive seed size effect on recruitment. We conclude that there is a weak selection gradient favouring larger seeds, but that this selection gradient is not clearly related to habitat.     

Pond canopy cover: a resource gradient for anuran larvae

1. The gradient in pond canopy cover strongly influences freshwater species distributions. This study tested the effects of canopy cover on the performance of two species of larval anurans, a canopy cover generalist (Rana sylvatica, the wood frog) and an open‐canopy specialist (R. pipiens, the leopard frog), and tested which factors co‐varying with canopy cover mediate these effects. 2. A field transplant experiment demonstrated that canopy cover had negative performance effects on both species. However, leopard frogs, which grow faster than wood frogs in open‐canopy ponds, were more strongly affected by closed‐canopy pond conditions. 3. Closed‐canopy ponds had lower temperature, dissolved oxygen (DO), and food nutritional quality as indicated by carbon‐to‐nitrogen ratio (C : N) analysis of field‐sampled food types, and of gut contents of transplanted larvae. 4. Laboratory experiments demonstrated that higher temperature and food quality but not DO substantially increased larval growth. However, only food quality increased growth rates of leopard frogs more than wood frogs. 5. The strong correlation of growth rates to gut content C : N in the field, and the similarity of growth curves as a function of resource quality in the field and laboratory, strongly suggest that resources are of primary importance in mediating intraspecific, and especially interspecific differences in performance across the canopy cover gradient.     

Leaf litter input mediates tadpole performance across forest canopy treatments

Understanding the mechanisms limiting the distributions of organisms is necessary for predicting changes in community composition along habitat gradients. In many areas of the USA, land originally cleared for agriculture has been undergoing a process of reforestation, creating a gradient of canopy cover. For small temporary wetlands, this gradient can alter abiotic conditions and influence the resource base of wetland food webs by affecting litter inputs. As distributions of amphibians and many other temporary wetland taxa correlate with canopy cover, we experimentally manipulated shade levels and litter types in pond mesocosms to explore mechanisms limiting species performance in wetlands with canopy cover. Most differences between ponds were mediated by litter type rather than direct effects of shading. Although all three amphibian species tested are open-canopy specialists, spring peepers were the only species to show decreased survival in shaded ponds. Pond litter type generally had strong effects on growth and development rates, with tadpoles of two species in grass litter ponds growing to twice the size of, and metamorphosing 7 days earlier than, those in leaf litter ponds. Contrary to our initial hypothesis, shade level and litter type showed very few significant interactions. Our results indicate that the effects of shading cannot be considered in isolation of vegetation changes in pond basins when evaluating the effects of forest succession on temporary pond communities.     

Subsidies of long‐chain polyunsaturated fatty acids from aquatic to terrestrial environments via amphibian emergence

相似文献   

Canopy Closure,Competition, and the Endangered Dusky Gopher Frog

Abstract: A major challenge facing wildlife biologists is understanding why some species go extinct while others persist in the same habitat. To address this question, we investigated whether tree canopy closure over ponds affects growth and survival of rare and common tadpoles within ponds and mediates competitive interactions among species. We conducted 2 experiments to test whether canopy closure and competition may have contributed to the decline of the endangered dusky gopher frog (Rana sevosa), but allowed the persistence of the southern leopard frog (R. sphenocephala). We explored the response of both species to canopy closure in single-species and mixed- (1:1) species treatments of identical total tadpole density. An experiment using aquatic enclosures in temporary ponds showed that canopy closure reduced tadpole growth approximately 20% for both species. Survival of dusky gopher frog tadpoles was higher in mixed-species enclosures than in single-species enclosures. In a complementary experiment using artificial ponds, dusky gopher frogs had lower survival to metamorphosis, reduced size at metamorphosis, and produced a lower total biomass of metamorphosed juveniles in shaded ponds. Southern leopard frogs exhibited reduced body size at metamorphosis only when shaded. These studies suggest that pond canopy closure, not larval competition, may be contributing to the decline of the dusky gopher frog. The different responses to canopy closure suggest a potential mechanism for the loss of dusky gopher frogs and the persistence of southern leopard frogs. Removal of trees from historically open-canopy ponds may help facilitate the recovery of dusky gopher frogs and benefit similar species.     

Reintroduction of Wild Lupine (Lupinus perennis L.) Depends on Variation in Canopy,Vegetation, and Litter Cover

We experimentally examined the effects of canopy, vegetation, and leaf litter cover on the demography of Wild lupines (Lupinus perennis) in a central North American oak savanna spanning 9 years. We also compared the distribution of Wild lupine across the landscape to results predicted by the demographic experiments. With less canopy cover, soil temperatures were warmer and seedlings emerged earlier. Seedling survival increased 14% with each additional leaf grown. Seedling survival was four times greater in openings and partial shade than in dense shade. Seedling survival was also influenced by interactions between canopy cover and vegetation cover, between canopy cover and leaf litter, and among canopy cover, vegetation cover, and litter cover. In openings, seedlings had higher survival when vegetative cover was present, suggesting a positive shading effect on survival, but with greater canopy cover vegetative cover reduced survival. Seedling survival was greater for plants that experienced herbivory, a result that was probably related to plant size and quality rather than having been eaten. Survival of lupines to 9 years after seed planting was greatest in the partial shade, moderate in openings, and least in dense shade. Wild lupine cover across the landscape was greatest when litter cover was low and canopy cover and ground layer cover were moderate. Reduction of canopy cover by burning or cutting, and reduction of leaf litter by prescribed burning will benefit the reintroduction of Wild lupine by increasing light, reducing litter cover, and creating disturbances; however, the reduction of vegetation cover in openings may hinder lupine reintroduction.     

不同厚度雪被对高山森林6种凋落物分解过程中酸溶性和酸不溶性组分的影响

高山森林冬季不同厚度雪被格局可能通过影响凋落物的分解过程中酸溶性和酸不溶性组分特征,改变凋落物分解过程,但缺乏必要关注。采用凋落物分解袋法,研究了高山森林林窗中央至林下形成的天然雪被厚度梯度(厚型雪被、中型雪被、薄型雪被和无雪被)覆盖下,6种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albo-sinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)、康定柳(Salix paraplesia)和高山杜鹃(Rhododendron lapponicum)凋落物在不同关键时期(雪被形成期、雪被覆盖期和雪被融化期)的酸溶性组分和酸不溶性组分变化特征。经历一个冬季的分解后,6种凋落物酸溶性组分绝对含量呈降低趋势,除红桦外5种凋落物酸不溶性组分绝对含量呈增加趋势。不同厚度雪被显著影响雪被覆盖期和融化期凋落物酸不溶性和酸溶性组分绝对变化量;其中方枝柏、红桦和康定柳凋落物酸不溶性组分增加量在厚型雪被下显著高于其它雪被覆盖;而相对于阔叶凋落物酸溶性组分变化量在薄型雪被和无雪被梯度达到最大值,针叶凋落物酸溶性组分在厚型雪被下具有最大的变化量。一个冬季分解结束后,表征6种凋落物酸溶性和酸不溶性组分含量相对比例的LCI指数(Lignocellulose index)总体升高,雪被对LCI指数的影响主要表现在雪被覆盖期和融化期,且方枝柏、岷江冷杉和康定柳凋落物LCI在冬季分解后均在厚型雪被达到最高值。同时统计分析结果表明,物种极显著影响冬季不同阶段凋落物酸溶性和酸不溶性组分的变化。这些结果意味着气候变暖情景下,高山森林冬季雪被和冻融格局的改变将显著影响凋落物分解过程中酸溶性、酸不溶性组分以及LCI指数代表的抵抗性组分结构的变化,且影响趋势受到凋落物质量的调控。     

Microgeographic morphological variation across larval wood frog populations associated with environment despite gene flow

Gene flow has historically been thought to constrain local adaptation; yet, recent research suggests that populations can diverge despite exchanging genes. Here I use a common garden experiment to assess the combined effects of gene flow and natural selection on morphological variation of 16 wood frog (Rana sylvatica) populations, a species known to experience divergent selection pressures in open‐ and closed‐canopy ponds across relatively small geographic scales. Wood frog tadpoles from different ponds showed significant morphological variation associated with canopy type with a trade‐off between tail length and body depth consistent with previous research. In contrast, neutral genetic differentiation of nine microsatellite loci as measured by Jost's D was not associated with canopy type, indicating no pattern of isolation by environment. Genetic structure analyses indicated some substructure across the 16 ponds (K = 4); however, three out of four assigned clusters included both open‐ and closed‐canopy ponds. Together, these results suggest that morphological divergence among these wood frog populations is occurring despite gene flow and that selection within these environments is strong. Furthermore, morphological variation among ponds differed across two sampling periods during larval development, demonstrating the importance of evaluating phenotypic divergence over multiple time periods and at a time relevant to the processes being studied.     

Woody plant encroachment reduces species richness of herb‐rich woodlands in southern Australia

Abstract Woody plants have been increasing in many woodland and savanna ecosystems owing to land use changes in recent decades. We examined the effects of encroachment by the indigenous shrub Leptospermum scoparium (Myrtaceae) on herb‐rich Eucalyptus camaldulensis woodlands in southern Australia. Species richness and compositional patterns were examined under the canopy of L. scoparium and in surrounding open areas to determine the species most susceptible to structural changes. Richness was significantly lower in areas of moderate to high L. scoparium cover (>15%), suggesting that a threshold shrub cover caused major change in this ecosystem. Shrubs were associated with a significant reduction in above‐ground biomass of the ground‐layer flora and a significant shift in community composition. The few species that were positively associated with high L. scoparium cover were also common in the woodland flora; no new species were recorded under the shrub canopy. Important environmental changes associated with L. scoparium cover were decreased light availability and increased litter cover, which were likely a consequence of encroachment. Leptospermum scoparium cover was also associated with greater surface soil moisture, which may be a consequence of increased shading under the shrub canopy or indicate favourable soil conditions for L. scoparium establishment. Reductions in species richness and abundance of the germinable seed bank were found in soil samples taken from under L. scoparium. With ongoing recruitment of L. scoparium and consequent increases in shrub cover, ground‐layer diversity in these species‐rich woodlands should continue to decline over time.     

Amphibian species richness across environmental gradients

Large‐scale field patterns are a fundamental source of inferences on processes responsible for variation in species richness among habitats. We examined species richness of larval amphibian communities in 37 ponds over seven years on the Univ. of Michigan's E. S. George Reserve. Ordination of the community incidence matrix indicated a strong major axis of variation in species associations that was correlated with pond hydroperiod, surface area and forest canopy cover. Communities were significantly nested with those species found in ponds with high canopy cover, small area and short hydroperiod being nested subsets of those found in ponds with contrasting characteristics. Presence of fish had strong negative effects on species richness; relaxation of this effect also was apparent when fish were extirpated from ponds by drought. We employed a model selection analysis to identify the most appropriate statistical model for predicting the long‐term average species richness of these ponds from local abiotic and biotic (predator and competitor density) factors. A model including only the abiotic factors was overwhelmingly superior for the anurans; hierarchical partitioning indicated that area and canopy cover alone accounted for over 70% of the independent effects of predictor variables. The global model including both abiotic and biotic factors was the best supported model for the caudates, and correspondingly hierarchical partitioning suggested that area, hydroperiod, invertebrate predators and caudate biomass all accounted for 9–16% of the independent effects. Overall, biotic factors accounted for much less of the variation in species richness than abiotic factors. The patterns in larger, open‐canopy ponds provided little evidence of competitive effects on species richness, though there were patterns consistent with competitive effects in small, closed‐canopy ponds. The unusual temporal and spatial extent of these data enabled us to critically evaluate ideas regarding patterns in larval amphibian communities, and the effects of area, disturbance (hydroperiod) and productivity (canopy cover) on species richness of these communities. These results have important implications to the conservation of amphibian species richness in freshwater wetlands, which are among the most threatened ecosystems worldwide.     

Anthropogenic changes to leaf litter input affect the fitness of a larval amphibian

相似文献   

Linking the evolution of habitat choice to ecosystem functioning: direct and indirect effects of pond-reproducing fire salamanders on aquatic-terrestrial subsidies

Shifts in life history traits and in the behaviour of species can potentially alter ecosystem functioning. The reproduction of the central European fire salamander (Salamandra salamandra), which usually deposits its larvae in first-order streams, in small pool and pond-like habitats, is an example of a recent local adaptation in this species. Here we aimed to quantify the direct and indirect effects of the predatory larvae on the aquatic food webs in the ponds and on the flux of matter between the ponds and adjacent terrestrial habitats. Our estimates are based on biomass data of the present pond fauna as well as on the analysis of stomach content data, growth rates and population dynamics of the salamander larvae in pond habitats. By their deposition of larvae in early spring, female fire salamanders import between 0.07 and 2.86 g dry mass m^?2 larval biomass into the ponds. Due to high mortality rates in the larval phase and the relatively small size at metamorphosis of the pond-adapted salamanders compared to stream-adapted ones, the biomass export of the metamorphosed salamanders clearly falls below the initial biomass import. Catastrophic events such as high water temperatures and low oxygen levels may even occasionally result in mass mortalities of salamander larvae and thus in a net 100 % import of the salamander biomass into the pond food webs. Indirect effects further accelerate this net import of matter into the aquatic habitat, e.g. the feeding of salamanders on aquatic insect larvae with the emergence of terrestrial adults—thus preventing export—and on terrestrial organisms that fall on the water surface (supporting import). This study demonstrates that the adaptation of salamanders to pond reproduction can alter food web linkages across ecosystem boundaries by enhancing the flux of materials and energy from terrestrial (i.e. forest) to the aquatic (i.e. pond) habitat.     

Effects on soil,microclimate and vegetation of the native-invasive <Emphasis Type="Italic">Retama monosperma</Emphasis> (L.) in coastal dunes

The aim of this study was to analyse and quantify the effects of the canopy of the native-invasive N-fixer woody shrub Retama monosperma in the dune ecosystem, affecting the structure and function of the dune environment as well as plant community, in the context of the facilitation mechanism. Air temperature and relative humidity; soil pH, electric conductivity, organic matter (OM) and nutrient content; above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were determined and compared from sampling plots below the R. monosperma canopy and in canopy gaps within a coastal dune system in SW Spain. The relationships between soil OM and nutrient contents and above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were also assessed. A predominance of positive interactions was confirmed. The canopy of R. monosperma ameliorated temperature extremes beneath, and soil OM and nutrient concentrations were increased by 188–466%, compared to those found in gaps. Plant biomass increased by 442% beneath the canopy and was composed almost exclusively of herbaceous annuals. Plant diversity was not affected. Plant communities were clearly structured as fertility islands, distributed in an environmentally stressful dune matrix characterized by scarce vegetation cover and low biomass.     

Experimental analysis of the evolutionary potential of hybridization in leopard frogs (Anura: Ranidae)

Artificial crossing using Rana blairi and R. sphenocephala frogs produced conspecific, interspecific and F₁ backcross hybrid genotypes. Although hybrid males used in the crosses were sterile, crosses using hybrid females produced viable larvae. The larval performance of resultant parental and hybrid genotypes was measured in experimental ponds at two densities. Density significantly affected survival, body mass at metamorphosis, larval period length and metamorphosis for all genotypes. Survival was the same among genotypes, but decreased with increasing density. Body mass at metamorphosis was the same among genotypes, but decreased with increasing density. Larval period increased with increasing density. Among genotypes, larvae from the conspecific R. sphenocephala cross had the shortest larval period while larvae from the conspecific R. blairi cross had the longest larval period. All hybrid genotypes had larval periods longer than R. sphenocephala, but shorter than R. blairi. The percentage of individuals metamorphosing was highest for R. sphenocephala ponds and lowest for R. blairi ponds across densities. Ponds with hybrid larvae produced a greater proportion of metamorphs than those with R. blairi larvae, but a smaller proportion than R. sphenocephala ponds. Equivalent or increased relative larval performance of hybrid genotypes under the conditions of our experiment suggests that hybrid genotypes may possess similar or higher fitnesses relative to their progenitors in some environments. Reduced fertility of adult hybrid males is a powerful selective force against natural hybridization. Nevertheless, because of the successful reproduction by female hybrids, natural hybridization has the potential to serve as a mechanism for the introgression of novel genetic variation that can benefit both R. blairi and R. sphenocephala in fluctuating and unpredictable larval environments. Experimental determination of the fitness of parental and hybrid genotypes is crucial for a comprehensive understanding of the effects of hybridization on organismal evolution.     

Amphibian phenotypic variation along a gradient in canopy cover: species differences and plasticity

The distributions of many freshwater organisms correlate with a gradient in canopy cover, ranging from sunny wetlands to closed woodland ponds. Little is known about mechanisms that exclude species from some sections of the gradient while allowing persistence in others. I addressed this question by manipulating shading in 740‐l outdoor mesocosms and measuring several ecologically‐relevant traits in three species of amphibian larva (Rana temporaria and Triturus alpestris, generalists occupying the entire gradient; and Hyla arborea, a specialist in open habitats). Shading caused delayed development, but had no effect on survival and increased the growth rate of R. temporaria. Body and tail color were darker in the shade. Plasticity in morphological shape, consisting of reduced gut width and increased tail size under shaded conditions, may reflect poor food availability and low dissolved oxygen. The canopy generalist R. temporaria increased activity in the shade, spent more time basking in shallow water, and maintained high larval performance. Unexpectedly, the specialist H. arborea was also highly plastic. These results describe extensive phenotypic plasticity induced by shade, and highlight traits that may influence performance along the canopy gradient.     

Energy flow and subsidies associated with the complex life cycle of ambystomatid salamanders in ponds and adjacent forest in southern Illinois

Breeding adults and metamorphosing larval amphibians transfer energy between freshwater and terrestrial ecosystems during seasonal migrations and emergences, although rarely has this been quantified. We intensively sampled ambystomatid salamander assemblages (Ambystoma opacum,A. maculatum, and A. tigrinum) in five forested ponds in southern Illinois to quantify energy flow associated with egg deposition, larval production, and emergence of metamorphosed larvae. Oviposition by female salamanders added 7.0–761.4 g ash-free dry mass (AFDM) year⁻¹ to ponds (up to 5.5 g AFDM m⁻² year⁻¹). Larval production ranged from 0.4 to 7.4 g AFDM m⁻² year⁻¹ among populations in three ponds that did not dry during larval development, with as much as 7.9 g AFDM m⁻² year⁻¹ produced by an entire assemblage. Mean larval biomass during cohort production intervals in these three ponds ranged from 0.1 to 2.3 g AFDM m⁻² and annual P/B (production/biomass) ranged from 4 to 21 for individual taxa. Emergent biomass averaged 10% (range=2–35%) of larval production; larval mortality within ponds accounted for the difference. Hydroperiod and intraguild predation limited larval production in some ponds, but emerging metamorphs exported an average of 70.0±33.9 g AFDM year⁻¹ (range=21.0–135.2 g AFDM year⁻¹) from ponds to surrounding forest. For the three ponds where larvae survived to metamorphosis, salamander assemblages provided an average net flux of 349.5±140.8 g AFDM year⁻¹ into pond habitats. Among all ponds, net flux into ponds was highest for the largest pond and decreased for smaller ponds with higher perimeter to surface area ratios (r ² =0.94, P<0.05, n=5). These results are important in understanding the multiple functional roles of salamanders and the impact of amphibian population declines on ecosystems. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

The importance of allochthonous litter input on the biomass of an alien crayfish in farm ponds

To evaluate the effects of allochthonous litter input on the population density of invasive red swamp crayfish (Procambarus clarkii) in Japanese farm ponds, we analyzed gut contents, stable isotope ratios, and the correlation between crayfish biomass and environmental factors in the ponds. For our correlation analysis, we used Akaike’s information criterion (AIC) corrected for small sample size (AIC_C) to select appropriate models within the generalized linear model. Allochthonous litter input was the most influential variable affecting crayfish biomass, followed by pond area. Gut content analysis demonstrated a positive correlation between the percentage of litter in the crayfish gut and the amount of litter input into the pond from which animals were collected. Crayfish δ¹³C became increasingly similar to litter δ¹³C as litter input into ponds increased. Nitrogen isotope signature analysis suggested that microorganisms attached to litter may contribute to crayfish diet. The above results obtained by three complementary approaches demonstrated an important influence of allochthonous litter input on crayfish biomass in farm ponds. We propose that the appropriate management of surrounding forests may be effective in controlling the abundance of exotic crayfish with minimized impacts on native communities.