Methodological approach to assess biodiversity in agro ecosystems – A case study

Abordagem metodológica para a avaliação da biodiversidade em ecossistemas agrários - estudo de caso

Catarina Tavares¹, Ana F. Gouveia², Luís Crespo³, Célia Mateus⁴ e M. Teresa Rebelo⁵

 

1 Departamento de Recursos Naturais, Ambiente e Território. Tapada da Ajuda, 1349-017 Lisboa, Portugal. Email: ctavares@isa.utl.pt,author for correspondence

2 AmBioDiv Valor Natural, Lda. Rua Filipe da Mata, 10, 1º Frente, 1600-071 Lisboa, Portugal

3 IMAR –Universidade dos Açores, Grupo da Biodiversidade dos Açores (CITA-A). 9700-042 Angra do Heroísmo Terceira, Açores, Portugal

4 Instituto Nacional de Investigação Agrária e Veterinária, IP. Av. República, Quinta do Marquês, 2784-505 Oeiras, Portugal

5 Departamento de Biologia Animal/ Centro de Estudos do Ambiente e do Mar (CESAM), Faculdade de Ciências da Universidade de Lisboa. Rua Ernesto Vasconcelos, Ed. C2, Campo Grande, 1749-016 Lisboa, Portugal

 

ABSTRACT

In agroecosystems, the evaluation of biodiversity is extremely important given its relation with their sustainability, however it is a difficult and expensive task. The use of the higher taxa surrogacy has been proposed as an effective and less expensive way to assess biodiversity, but is still being used mainly in natural, more stable, ecosystems. In this study, we tested the use of the higher taxa surrogacy approach for spiders in pear orchards, under different production systems, in the “Oeste” region of Portugal. Spiders were chosen for this case study, since they are important predators, acting as biological control agents in agroecosystems. We concluded that genus can be used as a higher taxa surrogate of species richness with a good accuracy.

Key-Words: Higher taxa surrogacy, pear orchards, spiders.

 

RESUMO

Em ecossistemas agrícolas, a avaliação da biodiversidade é extremamente importante dada a sua relação com a sustentabilidade, sendo contudo uma tarefa difícil e dispendiosa. O uso da relação entre o número de espécies e o número de táxones de alto nível, tem sido proposto como uma alternativa eficaz e económica para a avaliação da biodiversidade, contudo a sua utilização tem estado praticamente restringida aos ecossistemas naturais, mais estáveis. Neste estudo, testou-se o uso da relação entre o número de espécies e o número de táxones de alto nível em aranhas, em pomares de pêra sob diferentes modos de produção, na região Oeste de Portugal. As aranhas foram seleccionadas para este estudo de caso por serem importantes predadores, actuando como agentes de luta biológica contra pragas. Concluiu-se que o género pode ser usado na avaliação da riqueza de espécies, com um elevado grau de precisão.

Palavras-chave: Aranhas, pomares de pêra, relação entre táxones

 

Introduction

In the last few years, the interest in associating nature conservation to agriculture has increased enormously in Europe and several measures have been taken for the implementation of a sustainable agriculture (Amaro, 2003).

One way of measuring the sustainability of an agroecosystem is through the evaluation of its biodiversity (Moonen and Bàrberi, 2008; Paoletti, 1999). Biodiversity is often measured by the number of species sampled in an ecosystem (Primack, 2002), which is a rather difficult and expensive task, especially when considering invertebrate taxa (Duelli et al., 1999; Martín-Piera, 2000). However, since invertebrates account for more than 90% of the genetic variability in agroecosystems, it seems necessary to use them as the closest correlates to the overall biodiversity (Duelli, 1997).

The use of the higher taxa surrogacy approach (order, family, genus) has been proposed by several authors as a way of measuring diversity, based on the existence of a strong relationship between higher taxa richness and species richness (Andersen, 1995; Gaston and Williams, 1993; Wilkie et al., 2003; Groc et al., 2010). Therefore with a lower effort and using fewer resources, a large amount of information from several taxa can be obtained without identification to the species level (Gaston and Williams, 1993). In fact, the higher taxa approach has already been used for several different organisms (e.g. Gaston and Williams, 1993; Andersen, 1995; Gaston and Blackburn, 1995; Martín-Piera, 2000; Ricotta et al., 2002; Biaggini et al., 2007; Cotes et al., 2010; Groc et al., 2010), including spiders (Borges et al., 2002; Cardoso et al., 2004; Gouveia, 2004).

However, some limitations must be considered when using this approach, because several factors can influence the relationship between species richness and higher taxa richness, namely, genealogy and status of higher taxa, choice of higher taxa rank, spatial scale at which the assessment occurs, areas with unusual patterns of speciation, and sampling period (Gaston and Williams, 1993; Andersen, 1995). Spiders, as predators, are beneficial arthropods in agroecosystems, helping in pest control, and so their monitoring is important (Duelli, 1997; Marc et al., 1999; Tavares et al., 2011).

Very few studies using higher taxa surrogacy have been conducted in agroecosystems (Duelli, 1997; Cotes et al., 2010); and most of them deal with natural ecosystems conservation. The higher taxa approach has already been used on spider’s priority conservation areas in Portugal (Borges et al., 2002; Cardoso et al., 2004; Gouveia, 2004), but not yet in agroecosystems.

In this work, the use of the higher taxa surrogacy approach was tested for spiders in pear orchards in the “Oeste” region of Portugal.

 

Material and Methods

The study took place in six pear orchards, in the Oeste region of Portugal (Figure 1), from June to November 2006 and from April to June 2007, every two weeks. Two orchards of each type of production system were sampled: Organic Agriculture (OA1 and OA2), Integrated Pest Management (IPM1 and IPM2) and Conventional Agriculture (CA1 and CA2). Conventional Agriculture (CA) uses broadspectrum pesticides applied rigidly at certain established periods; Integrated Pest Management(IPM), uses chemical control (selective pesticides) together with other control measures when considered necessary after previous monitoring procedures; and Organic Agriculture (OA), does not use synthetic pesticides. OA1 orchard (0.6 ha), in Vilar village, was situated at a house’s backyard, surrounded by a non-cultivated field, pear orchards, a vineyard and vegetable plots. It was bordered by wildflower strips and by a stone wall covered with weeds. Vegetation covered the orchard’s inter-rows. OA2 (1 ha), in Freiria village, was also situated at a house’s backyard. It was bordered by a road and shrub hedges, surrounded by a vineyard, pear orchards and a non-cultivated field. It had wildflower strips along the field edge, inter-rows covered with vegetation and botanically rich alleyways. IPM1(3.5 ha), near Caldas da Rainha city, was at a valley, surrounded by pine trees, eucalyptus and riparian vegetation, and bordered by other orchards (pear and apple). It had wildflower strips along the orchard edges, botanically rich alleyways and vegetation covering the inter-rows. IPM2 (3 ha), in Peral village was bordered by a road, a small brook with riparian vegetation, wildflowers strips, hedges with trees and shrubs, and was surrounded by orchards and non-cultivated fields. Vegetation covered the inter-rows. CA1 (0.1 ha), also in Peral village, was above a valley, bordered by vegetable crops, apple trees, weeds and piles of dried plants (reeds). On one side was a wooden wall with nearby grassland. Inter-rows were covered with vegetation. CA2 (4 ha), in Vermelha village, was at a valley, bordered by other pear orchards, riparian vegetation near a small brook, wildflowers strips, shrubs and tree hedges and surrounded by pine trees and eucalyptus. Inter-rows were covered with vegetation. Spiders were collected by ten pit-fall traps (10 cm diameter opening) in each orchard, placed along the intertree strips, separated by 5 meters from each other. Plywood square was placed as a roof over the traps, allowing spiders to crawl under it, but avoiding the accumulation of debris or rain. Pitfall trapping was chosen, as it is a method directed at epigeous spiders that requires the less experience, effort and cost to employ (Cardoso, 2004; Cardoso et al., 2007). Additionally, it can capture almost half of the spider species living in a typical Mediterranean habitat and can capture a great number of individuals (Cardoso et al., 2009), which is important for the statistical robustness of the data.

 

 

Pit-falls had 30% ethylene glycol inside. The liquid content was filtered with a filter paper and the solid content was stored in 75% alcohol. Spiders were later separated from the remaining material in laboratory and stored in 70% alcohol and glycerine. Adult specimens were identified to species level or morph species.

For the evaluation of the potential use of family and genus as higher taxa surrogates, we used the regression analysis for each orchard and for all of them together. Three regression models were tested: a linear model (y=a+bx), an exponential model (y =a·ebx) and a log-log model (logy=a+b·logx). Species were tested as the dependent variable and the higher taxonomic categories as the independent variable. Statistical analysis was performed with STATISTICA 7.0 and SPSS softwares.

 

Results

In this study, 1288 adult’s spiders were collected with the pit-fall traps. Fifteen families, 49 genera and 64 species and morphospecies were identified (Table 1).

 

 

All 42 regressions (Table 2) performed were highly significant (p<0.001), except the exponential model for the family-species relationship in CA2 (p=0.004) (Table 2 right). Both higher taxa tested (family and genus) presented good relationships with species, independently of the regression models applied, being the genus-species relationships stronger than the family-species ones. When all orchards were analysed together, r² was strong (r²>0.80) for both higher taxa-species relationships. When orchards were analysed separately, no model predominated over the others, in terms of fitness. When analysing all orchards together, the linear model was the strongest one (highest r²) for the relationship genus-species and the exponential model for the relationship family-species.

 

 

 

Discussion

The relationship between family-species and genus-species was statistically significant. According to Martín-Piera (2000), an increase in higher taxa implies an increase in the number of species.

Three regression models were tested, as proposed by Borges et al. (2002). According to those authors, the log-log model is usually the most liable model, because it normalizes residuals distribution and consequently has a more powerful statistical prediction. However, in the present study, when orchards were analysed separately, no model revealed itself stronger than the other, and when all orchards were analysed together, the linear model was the most adequate for the relationship genus-species and the exponential one for the relationship family-species.

The same conclusion was reached by Cardoso et al. (2004) and Gouveia (2004). In the case of the CA2 orchard the exponential model for the family-species relationship was not adequate. This is due to the fact that although the number of families is relatively high in this plot and close to the number of families found for all the orchards, the number of species is much lower than the one found for the other orchards (Table 1). Thus, the exponential model will not be adequate, since in this particular case, as long as we increase the number of families, the number of species will not increase exponentially. CA2 revealed a very low diversity of species, being overall dominated by a single species Zelotes egregioides. Abundance of a single species is characteristic of very disturbed habitats (Samu, Sunderland and Szinetár, 1999).

The taxon that best predicted species richness was the genus, as also concluded in other studies, namely Borges et al. (2002), Cardoso et al. (2004), Gouveia (2004) and Groc et al. (2010), conducted in different ecosystems. According to Martín-Piera (2000), the genus explains a higher percentage of variance than the family. Additionally, when analysing the different orchards separately, the relationship family-species presented r² values of a wider range, while those of the relationship genus-species were less variable. Attention must be made to the fact that Andersen (1995), applying the higher taxa surrogacy approach for species of ants in different regions of Australia, found that the relationship genus-species was very strong within regions, but varied substantially between regions. However, in Portugal this has not been observed: studies conducted in different regions presented similar results (Borges et al, 2002; Cardoso et al., 2004; Gouveia, 2004). According to Cardoso et al. (2004), neither geographical location nor habitat type appears to have a strong influence in the higher taxa approach of spiders in Portugal.

 

Conclusions

This study presents additional evidence that higher taxa can be used to predict species richness in a given area with reasonable accuracy, being the genus the best surrogate for species as proposed by Cardoso et al. 2004. Identification to genus level and posterior extrapolation through a linear model revealed to be a good methodology to be used for spider’s species. The relationship family-species revealed a lower correlation, and is not adequate for all situations such as the orchard CA2. It may be possible that disturbed habitats present a different family-species relationship.

The higher taxa surrogacy approach for spiders seems to be an interesting method for a less expensive and time consuming biodiversity assessment in agro ecosystems.

 

Acknowledgements

To Elisabete Figueiredo (ISA/UTL) and Filomena Magalhães (FCUL) for helping on the statistical analysis and to Fernanda Amaro (INRB) for the incentive. This study was financed by the Agro Programme – Portugal (Project Agro 545).

 

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Recebido/Received: 2012.07.12

Aceitação/Accepted: 2013.05.10