Eric D. Dibble

Measurement of Plant Architecture in Seven Aquatic Plants

ABSTRACT We quantified architectural characteristics of seven aquatic macrophytes by measuring spatial complexity and shading properties. The plants were: Egeria densa, Hydrilla verticillata, Myriophyllum spicatum Potamogeton nodosus, Potamogeton pectinatus, Vallisineria americana, and Zosterella dubia). Two replicates of each plant were cultured in aquaria in concrete raceways. Light transparency and vertical and horizontal interstices measurements were taken at three strata level in each plant species. Spatial complexity was calculated by adding the ratio between interstices frequency and size from both vertical and horizontal axes. Mean density and size of plant interstices differed significantly among plant species (x2=123, df=6, P<0.01) and were greatest along horizontal axes. Plant complexity was lowest in M. spicatum and highest in P. pectinatus. Shading properties were significantly different among plants (x2=40, df=5, P<0.01), and significant inter- and intra-plant strata differences were noted.

Differential influence of a monotypic and diverse native aquatic plant bed on a macroinvertebrate assemblage; an experimental implication of exotic plant induced habitat.

Aquatic plants mediate ecological processes in aquatic habitats, specifically predator–prey (bluegill sunfish (Lepomis macrochirus Rafinesque)-macroinvertebrate) interactions. Macroinvertebrate colonization is directly and indirectly influenced by substrate heterogeneity, interstitial space, and surface complexity. Exotic invasive plant species, such as Hydrilla verticillata L.F. Royle, may alter the available structure in aquatic habitat by creating a shift to a homogeneous habitat, thus affecting the macroinvertebrate community. Since macroinvertebrates provide a food base for young phytophilic fishes, changes in their density and abundance may alter food webs. We investigated the hypothesis that macroinvertebrate community structure is influenced by differences in habitat heterogeneity by measuring difference between a heterogeneous native aquatic plant bed, homogenous hydrilla plant bed, and habitat with no plants. Studies were conducted in the field (pond) and the experimental treatments were: (1) no plants, (2) monotypic bed of hydrilla, and (3) diverse native plants. Aquatic plants, regardless of species, supported greater macroinvertebrate abundance, richness, and biomass. Macroinvertebrate abundance, richness, and biomass in a hydrilla-dominated habitat did not differ significantly from a diverse plant habitat, except for richness in October. Indicator taxa did differ significantly between respective treatments, suggesting a change in species composition. However, no significant effect of fish predation on macroinvertebrate populations and/or community structure was documented. The data suggest that a shift from a natural mosaic of vegetated habitat to a highly complex monotypic habitat (e.g., exotic hydrilla) may reduce spatial heterogeneity important to structuring a macroinvertebrate assemblage.

Foraging efficiency of juvenile bluegill, Lepomis macrochirus, among different vegetated habitats

Optimal foraging theory is devoted to understanding how organisms maximize net energy gain. However, both the theory and empirical studies lack critical components, such as effects of environmental variables across habitats. We addressed the hypothesis that energetic returns of juvenile bluegill are affected by environmental variables characteristic of the vegetated habitats. Predicted optimal diet breadths were calculated and compared to prey items eaten by juvenile bluegill to determine if bluegill were foraging to maximize energetic gain. Differences in habitat profitability among vegetated sites were determined by comparing predictions of maximized energetic return rates (cal s-1) with prey contents of bluegill stomachs. Sizes of most prey items eaten by juvenile bluegill throughout the vegetated sites were smaller than the predicted optimal diet breadths. However, inclusion of smaller prey items in the diet did not seem to affect rate of energetic gain. Energetic return rates were maximized at the 1.5 and 2 mm prey size classes and declined only slightly with inclusion of smaller prey sizes. Predicted energetic return rates and average mass in bluegill stomachs were related negatively. Average mass in bluegill stomachs also was associated negatively with Elodea canadensis stem densities and percent of light transfer, suggesting that foraging efficiency of bluegill decreased as plant density and percent of light increased. Results of our research indicate that maximization of energetic return rates is dependent upon availability of prey sizes that contribute to optimal foraging. Thus, determination of those habitats that provide the highest availability of benthic invertebrate prey with the least interference by stems is critical. Enhanced foraging capabilities can promote recruitment, faster growth, better body condition and survival.

Spatial Complexity Measured at a Multi-Scale in Three Aquatic Plant Species

ABSTRACT We investigated the hypothesis that available habitat heterogeneity within an aquatic plant bed is scale dependent. A multi-scale approach was used to quantify spatial-complexity at five different scales of resolution in three different aquatic plants species (Cabomba furcata, Najas microcarpa, and Utricularia foliosa) collected from lagoons in the Upper Paraná River floodplain basin, Brazil. The effect of scale upon spatial-complexity was highly significant and interactions between species and scale were also significant, suggesting that habitat heterogeneity was scale dependent and plants contained a species-specific threshold where complexity was highest. Our data represent scale dependent heterogeneity potentially important at mediating biological interactions in aquatic plants and provide a starting point for investigating multi-scale based hypotheses explaining physical and biological interactions among aquatic organisms and vegetated habitat.

Use of Fractal Dimension to Assess Habitat Complexity and Its Influence on Dominant Invertebrates Inhabiting Tropical and Temperate Macrophytes

ABSTRACT We evaluated the feasibility of using fractal geometry to measure the structural complexity innate to 11 species of temperate and tropical macrophytes. The efficacy of fractal dimension (D) as a surrogate of plant complexity was tested by using D values to predict the density of two dominant invertebrate taxa (Annelida and Odonata). Plants and invertebrates were collected from lagoons in the upper Paraná River, Brazil, and from a lake in central Minnesota, USA. Fractal dimensions varied from 1.16 (SD=0.03) in Potamogeton illinoiensis to 1.68 in Najas conferta (SD=0.07) and Myriophyllum spicatum (SD=0.02). Spatial scale did not affect D values, since the results obtained for pictures taken at 25 cm2, 100 cm2 and 600 cm2 did not differ for five tropical species. Using the results of D recorded at 100 cm2, a positive and significant relationship between plant complexity and Annelida and Odonata densities was observed. The biological significance of the positive correlations between D and invertebrate densities and the feasibility in calculating D make this method a potential candidate for measuring plant complexities at small scales.

Implications of a Hierarchical Relationship Among Channel Form, Instream Habitat, and Stream Communities for Restoration of Channelized Streams

Hierarchy theory provides a conceptual framework for understanding the influence of differently scaled processes on the structure of stream communities. Channel form, instream habitat, and stream communities appear to be hierarchically related, but the strength of the relationships among all components of this hypothesized hierarchy have not been examined. We sampled channel form, instream habitat, fishes, and macroinvertebrates in a channelized stream in Mississippi and Alabama to examine the hypothesis that a hierarchical relationship exists among channel form, instream habitat, and stream communities. Instream habitat, fishes, and macroinvertebrates were sampled in May, July, and September 2000. Measurements of channel form were obtained in July 2000. Mantel tests, multiple regressions, and correlation analyses were used to assess strength of the relationships among channel form, instream habitat, and stream communities. Positive correlations were observed between channel form and instream habitat, and correlations observed between these factors were the greatest observed in our study. Overall, fish and macroinvertebrate communities exhibited stronger relationships with instream habitat than with channel form. Species richness, evenness, and abundance tended to exhibit greater correlations with instream habitat, while species composition had greater correlations with channel form. We concluded that channel form, instream habitat, and stream communities were hierarchically related.

Ecological mechanisms of invasion success in aquatic macrophytes

Aquatic plants (macrophytes) are important components of freshwater ecosystems and serve numerous purposes that structure aquatic communities. Although macrophytes represent an essential component of stable aquatic communities, invasive macrophytes negatively alter ecosystem properties. Non-native, invasive species have been identified as a major cause of biodiversity loss and the increasing prevalence of invasive species has prompted studies to help understand their impacts and to conserve biodiversity. Studying mechanisms of invasion also give insight into how communities are structured and assembled. This paper examined mechanisms that contribute to macrophyte invasion through a literature review. Mechanisms identified with this review included competition, enemy release, evolution of increased competitive ability, mutualisms, invasional meltdown, novel weapons, allelopathy, phenotypic plasticity, naturalization of related species, empty niche, fluctuating resources, opportunity windows, and propagule pressure; and were then placed within the context of the invasion process. Results of this review indicated that many invasion mechanisms have been tested with fully aquatic macrophytes with varied levels of support (i.e., some mechanisms are not supported by evidence in the context of macrophyte invasions). Future research should continue the search for evidence of invasion mechanisms that allow introduced species to establish. It is likely that general principles governing these invasions do not exist, at least among comparisons across ecosystem types. However, ecologists should continue to search for general patterns within definable ecosystem units to increase understanding about factors contributing to invasibility.

An experimental simulation of an exotic aquatic macrophyte invasion and its influence on foraging behavior of bluegill.

ABSTRACT We investigated the hypothesis that a shift from a native aquatic plant bed to an exotic invasive Hydrilla verticillata (hydrilla) plant bed would alter spatial complexity and light transmittance and therefore alter Lepomis macrochirus (bluegill) foraging success. Experimental treatments included: (i) intermediate densities of native-mixed plants and (ii) 50% native—50% hydrilla, (iii) hydrilla, (iv) high density of hydrilla, and (v) no plants. Spatial complexity and light transmittance were influenced by increasing the homogeneity of hydrilla in vegetated aquatic habitat. In addition, bluegill foraging efficiency was affected negatively by increasing spatial complexity of a hydrilla dominated habitat. As a result bluegill searched faster, exhibited 60% more mean foraging bouts, and recognized 38% more food items in a native diverse habitat versus a hydrilla dominated habitat.

A simple method to estimate spatial complexity in aquatic plants

We used a computerized approach to measure spatial complexity for the structural habitat provided by eight aquatic plant species collected from backwater lagoons located in the Upper Parana River floodplain, Brazil. The plant species were: Cabomba furcata Schult. and Schult.f., Eichhornia azurea (Sw.) Kunth (stems and roots), Eichhornia crassipes (Mart.) Solms, Egeria najas Planchon, Heteranthera cf. zosterifolia, Potamogeton cf pusillus, Utricularia foliosa L., and Nymphaea amazonum Mart. and Zucc. The upper 0.5 m length of the terminal stems was quantified for complexity. Mean frequency and length of the interstices were significantly different among plant species. Spatial complexity varied (F = 17.30; p < 0.0001) among the different plant species with E. azurea roots and U. foliosa exhibiting the highest levels and E. azurea and N. amazonum stems the lowest. These unique spatial complexities suggested that aquatic plants possessed a differential contribution to the habitat heterogeneity in Upper Parana River lagoons.

Effects of invasive macrophyte on trophic diversity and position of secondary consumers

Invasive species are one of the widespread stressors of aquatic ecosystems. Several studies document food web effects of invasive fish, but little information is available on the effects of invasive macrophytes. We studied differences in food chain length as well as trophic position and trophic diversity of fish and odonates in lakes dominated by native plants or invasive Eurasian watermilfoil. Trophic position and food chain length were determined using baseline-adjusted δ15N isotope signatures. Trophic diversity, or isotope niche width, was estimated from convex hull area analysis. Results show that trophic position of secondary consumers was not affected by the invasive macrophyte, whereas trophic diversity was greater in watermilfoil-dominated lakes. The direction of isotopic niche expansion was different in fish and odonates, suggesting potential decoupling in predator–prey interactions. This study shows that dominant non-native macrophytes may cause significant changes in food web structure of invaded ecosystems. Trophic diversity may be a more sensitive indicator of environmental stress than trophic position and has the potential to be used for assessment of invasive species impacts and restoration success.