Is there a distinct tropical ecotoxicology?

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Introduction

To discuss the existence of a tropical ecotoxicology, we must recognize first that ecology plays an important role. The basic principles of toxicology, developed in the 70s remain: experimental testing, analysis of concentration and/or dose–effect relationships, and estimation of effect concentrations, such as the exposure concentration at which an x% effect is observed within a certain period (ECx). However, they have been enhanced by the addition of ecological considerations (i.e., ecotoxicology). Robinson (1978) first asked the question of whether or not there is a distinct tropical ecotoxicology. A no answer would indicate that ecological phenomena and mechanisms change as a continuum from temperate to tropical areas. A yes answer would indicate an abrupt change from temperate to tropical areas. However, verifying a yes or no answer is not a simple task. Below, we consider this question in terms of ecology, stressors, and toxicology respectively and then address environmental management issues.

Ecology

Although inter- and intraspecific relationships between species and ecological mechanisms do not show major differences along latitudinal gradients, there is an increase in biodiversity from the poles to the tropics, with an increase in structural complexity. Whether increased species diversity may affect ecosystem function or lead to greater functional redundancy is still unclear (Johnson et al. 1996); however, it is even more unclear how this is affected by the presence of environmental stressors. Biological complexity is greater in the tropics than in temperate regions, but it remains an open question how this species diversity can be influenced further by contaminant and noncontaminant stressors in the tropics. To study megadiverse communities, one needs an ecology associated with an appropriate theoretical approach; however, it is still very complex to measure spatial and temporal variation in such communities, let alone biological and evolutionary traits, and establish resilience patterns.

Stressors

Sources of chemical contamination and other stressors in tropical environments are similar to those in temperate ecosystems. However, their manifestation differs between tropic and temperate areas. For example, the final effect of eutrophication is the same for tropical and temperate waters: massive changes in biological diversity and ecosystem services (Smith et al. 1999). However, differences are encountered in the trajectory towards these massive changes, as tropical waters tend to have constant high temperatures with high light input. Biological processes play a determinant role regarding eutrophication, bacterial activity on the recycling of organic matter, zooplankton grazing and excretion, macrophyte interaction, and fish traits, to cite some. In temperate waters, where temperature and light vary seasonally, physical processes dominate over biological processes (Kilham and Kilham 1990).

Ecotoxicology

Most standard ecotoxicology test species are from temperate regions and, as a rule, they have been accepted as an adequate tool to assess pollution in the tropics. However, as noted by Baird et al. (1995), tropical species may differ markedly from these standard test organisms in their response to numerous types of contaminants, particularly given that tropical environmental conditions do not match standard test conditions for temperate species.

In tropical ecosystems, biological variables play an important role in determining whether contamination becomes pollution. For example, due to the higher temperatures in the tropics than in temperate regions, decomposition rates, organic matter recycling, and nutrient remobilization all proceed at accelerated rates year-round, affecting the bioavailability and thus the toxicity of chemical contaminants. For example, the production of MeHg in surface sediments is approximately 30 times lower than in macrophyte roots, and its bioavailability is probably limited, as well as the sediment water flux of MeHg (Guimarães et al. 2000). Clearly, this again demonstrates a higher level of biological complexity in the tropics.

Environmental management

Environmental management of tropical ecosystems, in comparison to temperate ones, has a myriad of complex issues resultant from natural complexities: habitat and species diversity; species interactions; population dynamics; and community relationships. To start with, environmental risk assessments (ERA) are not a mandatory protocol in tropical countries; instead, environmental impact assessments are typically conducted. The number of scientists and environmental managers is still proportionally lower than in countries from the Northern Hemisphere. Moreover, politicians legislate for the present, typically without concern for the future. Stakeholders involved in the management of tropical ecosystems are also diverse in terms of cultures, language, scale, nature, and impact of organization, ability to access a growing knowledge base, financial resources, and technical skills.

Environmental managers largely continue the practice of transferring technology developed in the Northern Hemisphere to tropical ecosystems, without taking into account the fact that the relative sensitivities of tropical and temperate species are noticeably different to different stressors. Additionally, ecological differences of the tropics are often overlooked: higher temperatures, higher organic matter turnover with faster oxidation-reduction activities, and also a possible multi-stressor scenario with anthropogenic modifying factors.

Concluding remarks

Robinson (1978), basing his reasoning on conditions in coral reefs and tropical forests, concluded that there is a tropical ecology; however, his focus was on the higher level of complexity in the tropics. We are certain that this does not constitute an argument for the existence of a tropical ecology. One of the tools for achieving an ecological understanding is generalization; this is critical in ecological research, because without pattern, we cannot determine the significance of a prospective explanation (Pickett et al. 2007), and there is no specific generalization for a tropical ecotoxicology that does not hold for ecology. Moreover, inter- and intraspecific relationships between species and ecological mechanisms do not show major differences along latitudinal gradients as hypothesized by Robinson (1978).

Ecotoxicology is a subdiscipline of ecology, integrated with toxicology, and its development in the tropics is dependent upon the use of appropriate tools that recognize the biological complexities and develop local management models to prevent the loss of valuable ecological services. In fact, most of the ecotoxicology that is being carried out in the tropics is still based on the toxicology testing-based approach that can serve to derive maximum acceptable chemical concentrations, but has not been effective as far as ecological understanding is concerned.

Although we cannot agree with the arguments of Robinson (1978), we cannot deny that biological processes play a more important role in the tropics, and the task of the scientist is to convince environmental managers and policymakers that adequate methodologies should be generated to recognize this, adapting the ERA protocols to the specific conditions of different sites.

The uniqueness of tropical ecotoxicology represents an enormous challenge, because the vast majority of the world's most threatened biodiversity hotspots are found in the tropics. Therefore, appropriate tools need to be determined or developed to prevent the loss of valuable ecological services.

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