Effect of Agricultural Pesticides and Land Use Intensification on Amphibian Larval Development

Abstract

The increase in human population had to increase the demand for vital resources, including food, generating intensive and extractive exploitation, and impacting natural ecosystems and biodiversity. Land degradation of ecosystems is a serious and widespread problem in the world. The expansion of the agricultural frontier is by direct or indirect human-induced processes, expressed as long-term reduction or loss of biodiversity. The expansion and industrialization of agriculture had been negatively affected by soil fertility, the climate, biogeochemical cycles, bodies of water, and loss of biodiversity on different spatiotemporal scales. Intensive agriculture, in the form of monocultures, is subjected to strict pest controls for the use of highly toxic agrochemicals. Pesticides are used in monocultures by spraying aqueous dilutions. Knowing the toxic effect of pesticides and agrochemicals on amphibians is very important. These animals have special ecophysiological conditions because they have biphasic life cycles composed of an embryonic and larval aquatic development stage and the adult stage in humid terrestrial environments. For these reasons, the amphibians have been observed with increased mortality rates, reduced prey availability, and affected growth rates.

1. Introduction

Ecosystem transformation may be an inevitable outcome of the combined impacts of multiple drivers [1, 2, 3], including the increase in the human population, their activities, and the demand for resources [4, 5]. The conversion of natural habitat to other land covers through changes in human land use is a principal cause of deforestation, the loss of biodiversity, and local/global extinction of species in natural terrestrial ecosystems [1, 3, 6]. In the world, ecosystem transformation is associated with the rise of agricultural systems, these new ecosystems increase human appropriation of the Earth’s net primary production, reducing the amount of energy available for all other species, and influencing a range of ecosystem processes and services [3, 4, 7].

To provide for human needs, over 50% of the global usable land is already for pastoral or intensive agricultural uses [3, 5]. The increase in the agricultural frontier and overexploitation lead to the use of a high number of chemical products that affect both the communities present in bodies of water and the soil [8, 9, 10]. Among the communities most affected by these agents are amphibians, which have limited spatial mobility [11, 12, 13], physiological and ecological specificities, that restrict their distribution and habitat use [14, 15, 16].

Amphibians present a biphasic life cycle in their development, an aquatic larval stage, adapted for rapid growth, and a terrestrial adapted for dispersal and reproduction [17, 18, 19]. The aquatic environment is the first habitat faced by anurans with complex life cycles [16, 18], because different restrictions imposed by the environmental gradient may be present during larval development [16, 17].

In the terrestrial environment, adult anurans select and colonize different habitats for oviposition and development of their young [20, 21, 22]. Tadpole uses different microhabitats, such as semi-permanent pools, permanent pools, temporary pools, phytotelmata seasonal and permanent streams or rivers, and under leaf litter [16, 18]. These bodies of water can be mesotrophic or oligotrophic, and present different biotic and abiotic characteristics, which could affect their adaptation [20, 23].

The larval stages of anurans in disturbed environments and recovery processes are crucial for the persistence of the species in a specific locality [20, 24, 25]. Their survival depends directly or indirectly on the requirements of these organisms for conditioning and the presence of suitable microhabitats for the species throughout their life cycle [24, 26, 27].

Environmental and spatial processes present in anuran assemblages with complex life cycles respond differently within water bodies [22]. These organisms play an important role in aquatic ecosystems, especially in the absence of higher trophic levels [28]. These organisms present different traits in the oral disc and diets that allow them to occupy more functional space. The tadpole's filter-feeding can change the composition and abundance of algal species, thus affecting the amount of chlorophyll and primary productivity [25, 28]. Scraper larvae can affect other primary consumers as epiphytes, present on the bottoms of water bodies. The changes in the structure and function of freshwater ecosystems provide insight into the mechanisms of interaction between anuran assemblages and their relationship with the ecosystem [25, 28]. However, numerous environmental variables can directly or indirectly affect anuran larval growth, survival, and mortality rates [20, 29, 30]. Tadpoles can inhabit water bodies that present a gradient of permanence over time [20]. For the tadpoles, the opportunity to remain longer in a water body would imply increasing size and decreasing the probability of mortality during metamorphosis [19, 31]. The larger size would be related to an earlier onset of reproductive maturity, which is advantageous in terms of egg production and sexual attraction, among other variables [21, 31, 32].

Agrosystems use fertilizers and pesticides, these affect the amphibians populations of the terrestrial and aquatic ecosystem. Pesticides affect amphibian populations with lethal, as well as sublethal effects. The sublethal effects occur in the medium and long, decreasing the availability of prey for amphibians, which together with ecological and physiological stressors could restrict the transmission of matter and energy and growth rates [33, 34]. We reviewed the literature on dermal pesticide absorption and toxicity studies for aquatic life stages of amphibians.

2. Transformation of the territory and its impact on biodiversity

The transformation of ecosystems and ecological processes in the world is associated with the establishment and needs of human populations and their socio-cultural processes [5, 35]. The demographic increase has been progressive in the world. Humans have transformed between 40–50% of the natural ecosystems of savannas, forests, and wetlands into agricultural and urban systems [5, 36]. Over 50% of the global usable land is already in pastoral or intensive agricultural uses and urban systems, reducing the potential for the sustainable provision of many goods and services from natural ecosystems. These new systems have been causing negative effects on climate, soil fertility, biogeochemical cycles, land use, and diversity at different spatial scales [5, 37, 38].

Currently, more than half of the world's population lives in cities, occupying 3% of the earth's surface [3, 35, 39]. Humans appropriate one-third of the net primary productivity of the land and 8% of the ocean to meet their food, energy, and production and consumption needs for goods and services [39, 40]. Natural terrestrial ecosystems are a major source of timber, fuels, fuelwood, resins, and fibers, which provide provisioning services for humans [2, 3]. However, their extension and coverage have been replaced by new crop areas (12%) and pastures for livestock (25%) to produce the necessary food for a constantly increasing population; being the main drivers of the transformation of the structure and functionality of the landscape at different spatial and temporal scales [1, 5, 37]. The intensification of agricultural systems transforms natural terrestrial ecosystems by changing land cover and land use, resulting in deforestation, defaunation, and land-use change [1, 5, 9, 36].

Forest clearing processes have been permanent and at different intensities around the world, configuring different patterns of land use and, therefore, different deforestation dynamics [5, 41]. In the case of tropical rainforests, deforestation has increased, giving way to agricultural, mining, and silvopastoral uses [5, 36]. These changes in the landscape present differences in cover types, being heterogeneous matrices where the radiation balance and temperature have extreme fluctuations day and night [42, 43]. The matrices are characterized by having greater exposure and change in abiotic variables, such as wind intensity and frequency, temperature, solar radiation, relative humidity, greater water flow, and high water or saline erosion of soils [5, 36, 43]. In this sense, depending on the use, anthropogenic systems affect the quality and quantity of habitat for both vegetation and fauna found on the edges of the remaining forest fragments [1, 5, 44].

These matrices are surrounded by small fragmented patches of forest, which have different soil types and vegetation, and vary in size, shape, and isolation [45, 46]. The biological and physical interaction of these two transition zones generates an effect of changing environmental and biotic conditions at the edges [45, 47]. Thus, the edge effect is mostly associated with fragmentation and habitat loss [6, 46]. As habitat fragments, the geographic extent of the anthropogenic matrix increases, and the edge effect on remnant native forests increases, which increases their isolation as habitat loss is accentuated [20, 48]. As landscapes are transformed, the original continuity of native cover is broken, decreasing the reproductive success of native species and the genetic exchange between populations [1, 7, 49].

Fragmentation in tropical ecosystems directly affects population densities and the number of species, especially endemic species [50, 51]. Likewise, the edge effect and fragmentation generate an alteration of the habitat of native species [1, 7, 49] and favor the appearance of new exotic/invasive or disturbance-adapted species that compete with the rest of the species for resources [50, 52]. Habitat degradation and loss caused by ecosystem transformation processes [44, 52] result in the extinction of species of the taxonomic groups most sensitive to disturbance, such as amphibians and some reptiles [49, 52].

In 2000, it was considered that approximately 60% of the world's tropical forests were degraded due to anthropogenic disturbances [2]. Worldwide 70% of native forests are located less than 1000 m from a productive system [53]. In the productive system the activities, such as burning, spraying, logging, unmanaged soil fertilization, use of pesticides, and herbicides, are traditionally carried out [5]. These activities generate strong impacts not only locally, but also lead to ecological footprints on a regional scale. Elements, such as the edge effect, alterations in the composition of biota, presence of water bodies, and soil permeability, negatively affect biodiversity and ecosystem services [37, 54].

3. Response of amphibian assemblages to ecosystem transformation

Amphibians are essential components of many natural ecosystems. They are indicators of ecosystem health [55] and have important roles in natural food webs [56]. These animals are sensitive to environmental changes and require moist habitats with relatively low and constant temperatures [55]. In addition, the distribution of amphibians in forests may be determined by the heterogeneity of ground cover [57, 58] and the availability of microhabitats [56, 59]. The stages of larval of anurans (eggs, embryology, and tadpoles) are subject to different biotic and abiotic conditions that affect survival, development, and size during development [31, 60, 61]. Many anuran larvae exhibit phenotypic, physiological, and ethological adaptations in response to changes in the environment that will directly influence premetamorphic growth and development, developmental speed, growth rates, body size, weight at the end of metamorphosis and the length of the larval cycle [17, 62, 63].

The choice of the aquatic environment, by adult amphibians for oviposition, influences the rates of fertilization, embryonic development, larval growth, survival, and mortality of these organisms (Figure 1) [64, 65, 66]. Likewise, the structural and physiognomic characteristics of the aquatic habitat are determined by: 1) the temperature of the medium, 2) the amount of food available, 3) the amount of radiant energy through sunlight, 4) the amount of oxygen available, 5) the amount of accumulated excreted waste, 6) bacterial growth, 7) predation, 8) the space available per organism (density), 9) presence of growth-inhibiting substances and 10) variation in habitat size [16, 67]. In anurans, changes in environmental selective pressures accompany and determine development, body size, weight, and the timing of metamorphosis [18, 68], affecting the distribution of species and their use of different habitats [66].

Figure 1.

Summary of factors affecting the biology and ecology of the anurans (adults, eggs, hatchlings, and tadpoles). These processes are influenced by primary factors in or of the biotic environment. The secondary factors are reflected in the reproductive ecology and behavior of adults; they can affect the biotic environment of a larva (microhabitat selection). The arrows indicate interactions and their directions, which need not be directly causal. Data from [18].

The amphibian's different life-history stages may occur in disjunct habitats and function at different spatiotemporal scales (Figure 1) [22, 23, 69, 70]. Changes in the physical and biotic environment primarily affect physiological, ecological, and biological processes [18].

The interactions of these factors in anuran larvae (Figure 1) depend on the characteristics of the habitats selected. The parents select the site to lay eggs, as well as the characteristics of the microhabitats they use as a mechanism to mitigate competition and predation, due to different relationships over time [18, 70]. Tadpole survival, growth, and development are secondarily affected by the behavior of each species in predator-prey relationships and their phenotypic plasticity (Figure 1) [18, 70].

Biotic factors include but are not limited to food availability and quantity, population density (crowding), and predation [62]. Abiotic factors, such as environment, ambient temperature, photoperiod, and water body (water level, quality) directly influence amphibian larvae and physiological mechanisms related to growth and differentiation [62, 71, 72, 73]. The anthropogenic transformation of the landscape tends to be heterogeneous, thus imposing new adaptation challenges to amphibians, presenting in some cases high biodiversity and only those species that have greater phenotypic plasticity to abiotic variations and ecosystem disturbance can adapt [16, 17]. However, the patchy distribution of aquatic habitats together with the reduced dispersal capacity of some amphibian species could impede the colonization of new habitats by adults, so the selection of breeding sites could determine the occurrence of tadpoles in each habitat [22]. In the water body, tadpoles may select occurrence sites according to microhabitat characteristics based on food availability and predation risk [16, 22].

Intensification of agricultural practices is observed on a global scale, generating habitat loss, reduced landscape heterogeneity, and connectivity [10, 12, 74]. Amphibians depend on the quality of aquatic habitats for reproduction and development, as well as the surrounding landscape for the terrestrial phase [34]. In agrosystems the composition and abundance of amphibian species are low. Presenting generalist species that have different traits that allow these organisms to survive. Amphibians that lay their eggs in lentic water bodies and whose larvae develop there (without parental care) tend to be more tolerant to the anthropogenic transformation of the landscape [20, 75]. An example of this is foam nests which can be a successful strategy to colonize highly dynamic and ephemeral anthropogenic bodies [20].

4. Effect of pesticides on the development and growth of amphibian larvae

In the world, with the expansion of the agricultural frontier, the use of fertilizers and pesticides has become widespread, with negative effects on amphibian populations [76], and the use of herbicides and pesticides contaminate the air, soil, surface water, and subsoil, generating serious problems for community dynamics and biodiversity [76, 77, 78]. Chemical pesticides have been one of the main resources used in intensive and conventional agrosystems for the control of some pathogenic fungi, insect pests, and weeds [7, 77]. The chemical composition of pesticides varies according to the degree of toxicity and persistence associated with their origin (natural and synthetic). Natural pesticides originate from pyrethrin, nicotine, and rotenone [7, 79].

Synthetic pesticides have hydrocarbons, chlorinated pesticides, organophosphates, and carbamates as their groups of origin. However, cyclodienes, carbamates, and organophosphates were eliminated from 50% of the world commercial market [77, 79], due to their high toxicity and collateral effects on other nontarget species [77]. In wild populations (amphibians, reptiles, birds, and mammals) [79], some effects are caused by residues of organic insecticides and organophosphates, which are abundant in the environment, are known [77, 79].

Pesticides in amphibians, not only reduce prey availability, but also reduce the transmission of matter and energy and, ultimately, growth rates [20, 34]. Within agroecosystems malformations in adult anurans and tadpoles are very common, being one of the main causes of high amphibian mortality rates [34, 80].

Among the different adverse effects of organophosphates on anurans are changes in survival and growth rates, morphological malformations, and some behavioral problems [33]. Organophosphate pesticides, such as endosulfan, are highly toxic in the environment [77, 79] and degrade slowly, leading to accumulations in food chains [79]. The impact of endosulfan on wildlife is associated with lethal effects on some fish and on the larval stage of amphibians present in water bodies near the areas where the pesticide is applied. Among the nonlethal effects, there are delays in the growth and development rates of amphibian larvae [31, 81].

Chlorpyrifos remains in the water for only a few days or weeks [33]. The tadpoles to high concentrations of chlorpyrifos have significant negative effects on growth and metamorphosis development time. On the other hand, the use of pyrethroids are pest controllers due to their low toxicity in birds and mammals, nevertheless showing that they are highly toxic to aquatic organisms [82]. The tadpoles both lethal and sublethal effects have been recorded, which are associated with behavioral changes, affecting larval cohorts and gregarious behavior patterns that favor the search for food and cause greater predation in intoxicated larvae [78, 82].

Among the pyrethroids is cypermethrin (Cy), which is highly active and effective against a wide range of pests that affect agriculture, public health, and domestic animals [33, 78], but it also reduces the biodiversity of aquatic organisms, such as crustaceans, aquatic insects, fish, and anuran larvae, which are not part of the target species for control [78, 82]. The cypermethrin in amphibian larvae causes the death of nerve cells in anurans, and have also been determined that, when exposed to this pesticide during the early stages, developmental inhibitions are observed [78]. However, the different concentrations of cipermectrin that produce mortality and those required to produce malformations are different.

Pesticides are a group of substances with varying degrees of toxicity and very diverse characteristics, among which two large groups can be distinguished: (1) elements that are defined by the type of use of the pesticide, according to the organism on which they act, such as insecticides, herbicides, acaricides, fungicides, and rodenticides. (2) according to the chemical structure of the substances with pesticidal activity, which are divided into organophosphates, organochlorines, carbamates, carboxylic acids, pyrethroids, amides, anilines, alkyl derivatives of urea, heterocyclic compounds with nitrogen, phenols, imides, inorganic compounds [77].

Sediment entrainment by drainage and irrigation systems could generate eutrophication processes in aquatic systems and greenhouse gas emanations [83]. Contamination of water sources has negative effects on populations, for example, the amphibians do not have shelled eggs and their skin is a permeable organ. These animals are more sensitive to pollution and deterioration of the environment [33, 78, 82]. Tadpoles could be directly or indirectly in agricultural areas, being exposed to contaminants present in both their aquatic and terrestrial habitats [20, 84, 85]. The sublethal effects of cypermethrin are abnormalities, changes in behavior, the acceleration or delay of metamorphosis due to chemical stress, loss of appetite, mutations, death of amphibian nerve cells, low rates of the embryo, and tadpole development [78, 82] and lethal effects (high mortality rates).

The incidence of agrochemical ecotoxicity in anuran larvae has shown different responses about the species, the concentration of the contaminant, its degradation rate, exposure times, and to predator pressures [86]. Some studies have shown that in crops, the use of agricultural inputs at different concentrations is often not high enough to cause immediate mortality but generates sublethal consequences, such as depressions in the immune system of amphibians, which makes them more susceptible to parasites and malformations in their morphology [33, 78, 82] or the decline of the different species and populations of the impacted habitat, where alterations in the food chain may have greater consequences than direct chemical effects [86].

5. Case study

Under experimental conditions, we evaluated the effects of the biocide cypermethrin (Cy: the substance of the pyrethroid chemical group), the active compound of the commercial product ®Fuminate in 80 larvae of Mannophryne vulcano, at larval development stages 25 and 26 according to Gosner’s table [87]. Three treatments with ®Fuminate and control were designed, each with 20 replicates.

In the experimental subjects exposed to different concentrations of cypermethrin and the control, the following variables were studied: (1) stage according to Gosner [87]; (2) body weight (g), and (3) total length (TL). Follow-up of the test for each treatment and larva was carried out until the completion of the metamorphosis process, observing the sublethal effects on its development.

In each of the experiments, the following records were obtained: with treatment level 1) in concentrations of one (1) ml of cypermethrin, diluted in one (1) liter of water (concentration suggested by the manufacturer for the bathing of cattle, horses, and domestic dogs) obtained 100% mortality in less than 12 h. The high degree of toxicity of this pyrethroid to aquatic life organisms is demonstrated, especially to anuran larvae, which cause lethal and sublethal effects [78, 82]. In the treatments of cypermethrin, level 2) at 0.4125 ml/l, level 3) at 0.206 ml/l, and the control treatment (no insecticide), no deaths were recorded during the first 24 h.

The variables studied indicate that Mannophryne volcano tadpoles exposed to sublethal concentrations of cypermethrin were affected in the rate of weight gain (Figure 2), weight (Figure 3), and growth (Figure 4), until the end of the metamorphosis. However, no increase in length during the larval period or malformations were observed as suggested by Carr et al. [88] for similar cases. This could be related to the time of exposure to the xenobiotic substance and the embryonic stage in which the organisms were found, being more likely to observe these effects in organisms exposed since the laying of eggs by the female.

Figure 2.

Weight gain of Mannophryne vulcano larvae from the average values, during the evaluation of the three treatments.

Figure 3.

Average size at the end of the metamorphosis of Mannophryne vulcano, during the experiment with three types of pesticides.

Figure 4.

Average weight at the end of the metamorphosis of Mannophryne vulcano, during the experiment with three types of pesticides.

Tadpole growth and weight gain showed similar behavior in the three levels evaluated. In the larvae of M. vulcano there were no significant differences between the different levels in both development and growth, which indicates that the first and final stages are the most critical in larval development [33].

Survival in the two treatments with cypermethrin concentrations registered the lowest values. The highest pesticide concentration had the lowest survival of metamorphs compared to those recorded for the other levels. It is important to keep in mind that tadpoles are more susceptible to contaminants when they are in the transition from an aquatic to a terrestrial phase, where sublethal effects could be greater during this critical phase.

It is also important to highlight that the growth, development, and timing of the larval period depend on environmental factors, such as temperature, quality, quantity of food, and density, which were previously evaluated. According to Izaguirre et al. [78] larvae from temporary water bodies tend to be more sensitive to cypermethrin concentrations compared to larvae from lotic habitats.

Acknowledgments

We acknowledged the anonymous reviewers and the associate editor Dr. Mohammad Manjur Shah who provided insightful comments on an earlier version of the manuscripts. Our case study was financially supported by the Chicago Zoological Society.

Competing interests

We declare that we have no competing interests.