Ecosystem service evaluation and development of synthetic indicators
Within each field, three equally-spaced sampling points (200 m apart) were located along a 400 m transect, starting at least 50 m from the field edge. Ant communities and soil-based ecosystem services were sampled at each of these points between June and August 2011. Methodologies and results for the assessment of soil-based ecosystem services have already been detailed by Lavelle et al. (2014), and will thus not be described in full. Briefly, a suite of soil physical, chemical and biological parameters were measured to evaluate key ecosystems services including: nutrient provision, water regulation, maintenance of soil structure, biodiversity, and climate regulation. Nutrient provision services were assessed by measuring soil chemical fertility parameters (pH, SOM, total N, available P, Al saturation, cation exchange capacity, and macronutrient concentrations) at the 0-10 cm and 10-20 cm depths. Soil hydrological services (water regulation) were assessed by soil physical characteristics including volumetric and gravimetric moisture content, porosity, plant available water (based on water retention curves), aggregate stability, bulk density, penetration resistance and shear strength resistance.
The maintenance of soil structure (particularly relevant for these easily compacted soils) was assessed by visual inspection of soil macroaggregates (> 4 mm) to determine the origins of soil structure (i.e., fauna vs. root vs. microbial or physical aggregation processes vs. nonmacroaggregated soils). This method, outlined by Velasquez et al. (2007), provides an integrative measure of recent soil biological activity and offers an important proxy measurement for C stabilization, crop rooting potential, water infiltration and aeration. Soil biodiversity and biological activity was assessed via the collection and sorting of soil macrofauna (including ants) into 16 taxonomic groups (e.g., Oligochaeta, Isoptera, Coleoptera) largely separated by order (see details below for sampling of ant communities). Finally, climate regulation was evaluated by measuring aboveground C in living biomass, soil C in the surface layer (0-20 cm), and greenhouse gas (GHG) emissions of CO2, CH4, and N2O at monthly intervals between June and November of 2011.
The data sets generated for nutrient provision, soil hydrological services, maintenance of soil structure, and biodiversity were summarized into a set of four synthetic indicators of ecosystem services (as well as a more direct indicator for climate regulation mentioned below) according to methods adapted from Velasquez et al. (2007). Principal components analysis (PCA) was used to determine the parameters that best capture the variance within each dataset across the five land uses sampled. This was done by selecting those with a significant contribution (>50% of the variance explained by the most influential variable) to either of the first two principal axes. The selected variables were then combined into a single value and scaled to a number ranging from 0.1 to 1.0 using a homothetic transformation. Since widely accepted standard conventions already exist for climate regulation services, GHG fluxes were converted into global warming potential based on CO2 equivalents, while ecosystem C storage was calculated as the sum of aboveground biomass and soil C in the surface (0-20 cm) layer. The resulting numbers for all services were then scaled to between 0.1 and 1.0 (using the inverse value for GHG emissions, since high emissions imply a negative service and low emissions a positive service) and then averaged to generate a single indicator of climate regulation.
Ant diversity sampling and calculations
At the sampling points described above, ants were collected along with other groups of soil macrofauna by employing a modified TSBF collection method (Anderson and Ingram, 1993). Sampling involved the excavation and hand-sorting of litter and soil from a central monolith (25 x 25cm x 20 cm deep) and two adjacent monoliths (25cm x 25cm x 10 cm deep) located 10 m to the North and South of each central monolith. Standing plant biomass was cut 2-3 cm above the soil surface and removed prior to sampling. With three sampling points per transect, a total of nine soil monoliths were collected for each field. In the laboratory, ants were separated from other macrofauna and the samples were cleaned and preserved in 96% alcohol. Identification of ants to the genus level was performed following keys of Palacio and Fernandez (2003) and Bolton (1994); updated keys that are specific for each gender were used for finer level identifications according to AntWeb (http://www.antweb.org) and Longino (2003). We performed a descriptive analysis of the percentage of collection by the different subfamilies, genera and species, based on morphological assessment of the number of ant species for each sampling point and land use. Effectiveness of sampling was estimated with species accumulation curves using the program EstimatesS v.8.2.0 (Colwell 2013).
Two indices were calculated to compare the five soil uses: a) species richness (density) per sampling point, and b) diversity using the exponential of the Shannon index (eH’) calculated as: eH’ = exp ( −Σ 𝐩𝒊 𝐋𝐨𝐠 𝐩𝒊 𝒔𝒊 =𝟏 ).
Where pi is the proportion of workers of the species i and s the total number of species. The value eH’ can be interpreted as the ‘effective number of species’, and provides a more intuitive comparison of species diversity than the traditional Shannon index (Jost 2006).
Identification of ant indicators using the IndVal method
Indicator ant species for ecosystem services were identified using the IndVal method developed by Dufrêne and Legendre (1997), based on the degree of specificity (occurrence of a species in a particular category, but not in others) and fidelity (frequency of a species in samples from a category) of a species to a particular habitat or soil condition (i.e., level of ecosystem service provision). The association of species with ecosystem service provision was accomplished by defining three categories of performance (irrespective of land use) based on the synthetic indicators described above. Each of the sampled fields was considered to have low (0.1-0.4), medium (0.4-0.7), or high performance (0.7-1.0) for each of the five consolidated ecosystem services, based on a similar methodology applied by Rousseau et al. (2013). An ant species (or morphospecies) is thus considered a bioindicator of ecosystem services if it is commonly found in samples of a particular category (but not in other categories) and is present in most samples from this category. The IndVal value for each species is calculated as follows: Aij = N.ind (ij) / N.ind (i).
Comparison of ant communities between land use systems
When considering the total number of species encountered across the fifteen replicates of each treatment, the land use with the highest species richness (gamma diversity) was improved pasture (52 species), followed by semi-natural savanna (45), rubber plantations (39), oil palm (38) and annual crops (20) (Table 3). Management systems with the highest number of unique species (i.e., those not found in other soil use systems) encountered were improved pasture (11), rubber (10) and semi-natural savanna (9) (Table S1).
According to the exponential of Shannon, еH’, improved pasture continued to be the most diverse system and suggested an effective community diversity of 33 species (Table 3). In comparing the effective number of species across systems, rubber and savanna were found to possess 94.4% and 92.9%, respectively, of the effective species diversity observed in improved pasture. At the same time oil palm contains only 72.2%, and transitory crops have less than 50% of the species occurring in the three most diverse management systems.
Developing indicators of soil-based ecosystem services
The principal objective of this research was to identify ant species that could serve as bioindicators for the provision of ecosystem services in the Llanos region. In total, we identified fourteen species that were deemed significant indicators of soil-based ecosystem services. Twelve were associated with the high provision of soil-based ecosystem services, while two were associated with the poor provision of ecosystem services (Table 2). For example, species (or morphospecies) such as Acromyrmex sp. 1, Pheidole inversa, Solenopsis sp. 1, were associated with the maintenance of soil structure (i.e., the presence of biogenic aggregates and overall soil aggregation). These species are likely to be indicators, since they are actively involved in relevant ecosystem processes related to soil aggregation. For example, large colonies typically associated with the genus Acromyrmex can greatly modify the structure of the soil though nest building activities, which alter the chemical and physical properties of soil, accelerate nutrient turnover and promote plant growth (Moutinho et al., 2003). More specifically, enhanced soil porosity associated with ant mounds has been suggested to enhance organic matter turnover by creating more favorable conditions for microbial growth (Petal and Kusinska, 1994). Similarly, species of the genera Pheidole and Solenopsis, often build their mounds from excavated soil and incorporate different organic materials, such as seeds and leaf fragments (pers. comm.). In the case of climate regulation, the indicator species C. curvispinosa is known to nest in woody stems and occupy habitats with perennial vegetation (www.AntWeb.org). Such systems are generally associated with reduced disturbance and lower fertility inputs (at least relative to annual crops), both attributes that favor C storage and lower GHG emissions (Lavelle et al., 2014). For many of the bioindicators of ecosystem services identified in this study, their life strategies and impacts on soil functioning are still not well understood and it is difficult to know if these species contribute to the regulation of the ecosystem service to which they are associated or if their presence is simply governed by the same soil properties that drive the ecosystem services they indicate.
Table of contents :
CHAPTER 1: GENERAL INTRODUCTION
1.1 GENERAL CONTEXT
1.1.1 Importance of Soils and the ecosystem services they provide
1.1.2. Importance of soil macrofauna for soil functioning
1.1.3. The role of termite communities
1.1.4. The role of ant communities
1.1.4. Impacts of agriculture practices on soil fauna
1.2. THE CASE OF COLOMBIAN LLANOS
1.3. STRUCTURE OF THE THESIS AND GOALS
CHAPTER 2: ANTS AS INDICATORS OF SOIL-BASED ECOSYSTEM SERVICES48
2.2 MATERIALS AND METHODS
2.2.1 Study region and design
2.2.2 Ecosystem service evaluation and development of synthetic indicators
2.2.3 Ant diversity sampling and calculations
2.2.4 Comparison of land uses
2.3.1 Composition of ant communities
2.3.2 Indicators for ecosystem services
2.3.3 Comparison of ant communities between land use systems
2.4.1 Developing indicators of soil-based ecosystem services
2.4.2 Response of ant communities to manangement
CHAPTER 3 : INFLUENCE OF LANDSCAPE AND SOIL PROPERTIES ON SOIL ENGINEERS COMMUNITIES
3.2 MATERIALS AND METHODS
3.2.1 Study region and sampling design
3.2.2 Physical and Chemical Soil analyses
3.2.3 Ants and termites biodiversity
3.2.4 Statistical analysis
3.3.1 Soil engineers diversity
3.3.2 Effect of region or land use on soil properties
3.3.3 Effect of region or land use on community structure
3.3.4 Effect of environmental factors on soil engineer species occurrence
3.4.1 Effect of the regions or land uses on soil properties
3.4.2 Effect of the regions or land uses and ant or termite communities.
3.4.3 Effect of soil properties, regions and land uses on ant or termite species
CHAPTER 4: RESPONSE OF ANT TRAITS TO AGRICULTURAL CHANGES
4.2 MATERIALS AND METHODS
4.2.1 Study area
4.2.2 Sampling scheme
4.2.3 Ant identification.
4.2.4 Soil characteristics
4.2.5 Trait description
4.2.6 Data analysis
4.3.1 Joint analisys of ecological traits, environmental variables and ant community .
4.3.2 Classification of species based on environmental variables and species ecological traits
4.3.3 Co-inertia analysis between morphologic and ecological traits
4.4.1 Joint structure between ant species distribution, environment and species traits.
4.4.2 What explains the ant species distribution?
4.4.3 Relation between morphological and ecological traits.
CHAPTER 5: DISCUSSION, PERSPECTIVES AND CONCLUSIONS
5.1.1 Overview and main results of the thesis
5.1.2 Species richness and composition
5.1.3 Effect of the agricultural landscape on soil communities
5.1.4 Ecosystem services and soil engineers
5.1.5 An approach to landscape ecology
5.2. CONCLUSION AND PERSPECTIVES