Jorge L. Gutiérrez

Non-indigenous invasive bivalves as ecosystem engineers

Several non-indigenous bivalve species have been colonising aquatic ecosystems worldwide, in some cases with great ecological and economic impacts. In this paper, we focus on the ecosystem engineering attributes of non-indigenous invasive bivalves (i.e., the capacities of these organisms to directly or indirectly affect the availability of resources to other species by physically modifying the environment). By reviewing the ecology of several invasive bivalves we identify a variety of mechanisms via which they modify, maintain and/or create habitats. Given the usually high densities and broad spatial distributions of such bivalves, their engineering activities can significantly alter ecosystem structure and functioning (e.g., changes in sediment chemistry, grain size, and organic matter content via bioturbation, increased light penetration into the water column due to filter feeding, changes in near bed flows and shear stress due to the presence of shells, provision of colonisable substrate and refuges by shells). In addition, changes in ecosystem structure and functioning due to engineering by invasive bivalves often have very large economic impacts. Given the worldwide spread of non-indigenous bivalves and the varied ways in which they physically modify habitats, their engineering effects should receive more serious consideration in restoration and management initiatives.

Physical Ecosystem Engineers as Agents of Biogeochemical Heterogeneity

Abstract Physical ecosystem engineers are organisms that physically modify the abiotic environment. They can affect biogeochemical processing by changing the availability of resources for microbes (e.g., carbon, nutrients) or by changing abiotic conditions affecting microbial process rates (e.g., soil moisture or temperature). Physical ecosystem engineers can therefore create biogeochemical heterogeneity in soils and sediments. They do so via general mechanisms influencing the flows of materials (i.e., modification of fluid dynamic properties, fluid pumping, and material transport) or the transfer of heat (i.e., modification of heat transfer properties, direct heat transfer, and convective forcing). The consequences of physical ecosystem engineering for biogeochemical processes can be predicted by considering the resources or abiotic conditions that limit or promote a reaction, and the effect of physical ecosystem engineering on these resources or abiotic conditions via the control they exert on material flows and heat transfer.

Physical Ecosystem Engineers and the Functioning of Estuaries and Coasts

A great diversity of organisms modify the physical structure of estuarine and coastal environments. These physical ecosystem engineers – particularly, dune and marsh plants, mangroves, seagrasses, kelps, reef-forming corals and bivalves, burrowing crustaceans, and infauna – often have substantive functional impacts over large areas and across distinct geographic regions. Here, we use a general framework for physical ecosystem engineering to illustrate how these organisms can exert control on sedimentary processes, coastal protection, and habitat availability to other organisms. We then discuss the management implications of coastal and estuarine engineering, concluding with a brief prospectus on research and management challenges.

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m−2) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m−2 d−1 and 4.80 mg m−2 d−1 of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.

1 - On the Purpose, Meaning, and Usage of the Physical Ecosystem Engineering Concept

This chapter presents a perspective on selected aspects of the purpose, meaning, and usage of the physical ecosystem engineering concept, including some new thoughts, some clarification, and some reification. The chapter briefly describes the domain, general purpose, and components of the concept. The chapter also defines two coupled, direct interactions comprising ecosystem engineering—the physical ecosystem engineering process responsible for abiotic change, and physical ecosystem engineering consequence that addresses biotic effects of abiotic change. The chapter also clarifies the meaning of “ecosystem” in ecosystem engineer . The chapter addresses causes of process ubiquity and how they lead to general expectations of consequence. It also examines sources of context-dependent variation in engineer effect magnitude and significance and what needs to be known to predict effects. The chapter also defines conditions for detectable engineering effects and the condition for large effects, all other factors being equal (i.e., ceteris paribus). The chapter argues against unspecified conflation of process and consequence.

Conditional responses of organisms to habitat structure: an example from intertidal mudflats

Habitat structure is often assumed to be a predictor of habitat function. However, habitat structure may be insufficient to predict the functional significance of a habitat if the level of resources in the habitat is a consequence of the interaction between the habitat structure and physical or biological factors. In this study, we investigated whether depressions in tidal flat sediments generated by stout razor clams, Tagelus plebeius, affect the spatial patterns of pit digging by deposit-feeding burrowing crabs, Chasmagnathus granulata. The pits dug by crabs while feeding overlapped with clam siphon holes at a frequency higher than expected at random, and measurements of pit-digging by crabs integrated over several days indicated a higher frequency of feeding in the sediment of depressions. The daily frequency of pit-digging by crabs in depressions was positively related to the organic matter content of their sediments, but was significantly higher than the frequency of pit-digging away from clam siphon holes only after events of high bedload sediment transport, when the organic matter in the sediments of these depressions peaked. This example demonstrates the conditional nature of the relationship between habitat structure and function by illustrating how a physical process—bedload sediment transport—may introduce variation in the function that depressions play as feeding sites for burrowing crabs. Published information suggests that such conditional responses of organisms to habitat structure: (1) occur in a variety of habitats; (2) involve a variety of structures either of biotic or abiotic origin; and (3) are the consequence of either physical or biological controls that vary in importance according to the general mechanism through which habitat structure affect resources. This broad experimental evidence suggests that the accuracy of predictive models linking habitat structure and function can be improved by incorporating a mechanistic perspective that allows recognition of the potential for conditional responses of organisms to habitat structure.

Hormonal regulation of the basic peroxidase isoenzyme from Zinnia elegans

Xylem differentiation in plants is under strict hormonal regulation. Auxins and cytokinins, together with brassinosteroids (BRs), appear to be the main hormones controlling vascular differentiation. In this report, we study the effect of these hormones on the basic peroxidase isoenzyme from Zinnia elegans (ZePrx), an enzyme involved in lignin biosynthesis. Results showed that auxins and cytokinins induce ZePrx, similarly to the way in which they induce seedling secondary growth (in particular, metaxylem differentiation). Likewise, the exogenous application of BR reduces the levels of ZePrx, in a similar way to their capacity to inhibit seedling secondary growth. Consistent with this notion, the exogenous application of BR reverses the auxin/cytokinin-induced ZePrx expression, but has no effect on the auxin/cytokinin-induced secondary growth. This differential hormonal response is supported by the analysis of the ZePrx promoter, which contains (a) cis-elements directly responsive to these hormones and (b) cis-elements targets of the plethora of transcription factors, such as NAC, MYB, AP2, MADS and class III HD Zip, which are up-regulated during the auxin- and cytokinin-induced secondary growth. Taken together, these results suggest that ZePrx is directly and indirectly regulated by the plethora of hormones that control xylem differentiation, supporting the role of ZePrx in xylem lignification.

Wounding induces local resistance but systemic susceptibility to Botrytis cinerea in pepper plants

Cotyledon wounding in pepper caused the early generation of hydrogen peroxide both locally (cotyledons) and systemically (upper true leaves). However, 72 h later there is a different wound response between local and systemic organs, as shown by resistance to the pathogenic fungus Botrytis cinerea, that increased locally and decreased systemically. Signaling by ethylene and jasmonic acid was assessed by using two inhibitors: 1-methylcyclopropene (MCP, inhibitor of ethylene receptors) and ibuprofen (inhibitor of jasmonate biosynthesis). MCP did not affect the modulation of resistance levels to Botrytis by wounding, ruling out the involvement of ethylene signaling. Ibuprofen did not inhibit wound-induced resistance at the local level, but inhibited wound-induced systemic susceptibility. Moreover, changes of biochemical and structural defenses in response to wounding were studied. Peroxidase activity and the expression of a peroxidase gene (CAPO1) increased locally as a response to wounding, but no changes were observed systemically. Lignin deposition was induced in wounded cotyledons, but was repressed in systemic leaves of wounded plants, whereas soluble phenolics did not change locally and decreased systemically. The expression of two other genes involved in plant defense (CABPR1 and CASC1) was also differentially regulated locally and systemically, pointing to a generalized increase in plant defenses at the local level and a systemic decrease as a response to wounding. Wound-induced defenses at the local level coincided with resistance to the necrotroph fungus B. cinerea, whereas depleted defenses in systemic leaves of wounded plants correlated to induced susceptibility against this pathogen. It may be that the local response acts as a sink of energy resources to mount a defense against pathogens, whereas in systemic organs the resources for defense are lower.

Downregulation of the basic peroxidase isoenzyme from Zinnia elegans by gibberellic acid.

Hypocotyl formation during the epigeal germination of seedlings is under strict hormonal regulation. In a 3 d old Zinnia elegans seedling system, gibberellic acid (GA(3)) exerts an opposite effect to that exerted by light on hypocotyl photomorphogenesis because GA(3) promotes an etiolated-like growth with an inhibition of radial (secondary) growth. For this reason, the effect of GA(3) on the basic peroxidase isoenzyme from Z. elegans (ZePrx), an enzyme involved in hypocotyl lignin biosynthesis, was studied. The results showed that GA(3) reduces ZePrx activity, similarly to the way in which it reduces seedling secondary growth. This hormonal response is supported by the analysis of the ZePrx promoter, which contains four types of GA(3)-responsive cis-elements: the W Box/O2S; the Pyr Box; the GARE; and the Amy Box. Taken together, these results suggest that ZePrx is directly regulated by GA(3), with this effect matching the inhibitory effect of GA on the hypocotyl secondary growth.

Modification of Habitat Quality by Non-native Species

Non-native species can affect the quality of habitats available to other organisms and, in turn, the ecosystem services they provide or regulate. Although much research to date has focused on the impacts of non-native species on habitats, the links between habitat impacts and the provision or modulation of ecosystem services have remained elusive. This review illustrates two general kinds of non-native species impact on the abiotic conditions and resources available in a habitat: (1) assimilatory-dissimilatory impacts from the uptake and release of energy and materials and (2) physical ecosystem engineering impacts that arise from structural modification of environments caused by species presence and/or activities. Additionally, it distinguishes between physical ecosystem engineering impacts that result from the creation or modification of physical structures per se (e.g., effects on living space) and those that occur because of the interactions of physical structures and different forms of kinetic energy, such as heat or fluid flows (e.g., wind attenuation by trees). Examples are given to illustrate the co-occurrence of multiple impact pathways and their often compound impacts on single habitat attributes. Finally, the habitat-mediated impacts of non-native species on food and raw materials, climate, and tourism and recreation are discussed as examples of cascading impacts on provisioning, regulating, and cultural services, respectively.