Earthworm population ecology; Guyana, S A

Earthworms are regarded as the bio-indicators of soil quality and are perhaps the most significant regulators of soil structure and organic matter content in a variety of terrestrial soil ecosystems, paving the way for sustainable green agriculture and land rehabilitation. Due to the steady increase in industrialization and shifts in global climate, their population is now more susceptible to change/decline as a result of the strains placed on soil ecosystems by agriculture, mining and deforestation. This research aimed to and successfully established the composition of earthworm populations present in Guyana while exploring their relationship with the biogeographical regions and pedobiological components of their respective ecosystem. Earthworms and soil samples were collected from 15 sites per natural region after which they were taxonomically identified following methodological dissections which yielded 68 distinct species. Of the four natural regions, the earthworm population of Highland Region was found to be the most diverse, rich, even and dense. Earthworm abundance, epigeic abundance, endogeic abundance, anecic abundance and species richness among the four natural regions of Guyana, were all of statistical significant difference, likewise, earthworm abundance in the various climate and soil types along with disturbance were of statistical significant difference. It was found that epigeic earthworms were significantly affected by phosphorus (0.01), moisture (0.01) and calcium (0.02) while anecic earthworms were significantly affected by magnesium (0.04), and the degree at which these affect the various ecotype is different among natural regions. This study has proven with conviction that earthworm population structure varies depending on the biogeographical and pedobiological factors present within any respective terrestrial ecosystem.


INTRODUCTION
Guyana is a tropical country located on the north-eastern part of South America. It comprises of four distinct biogeographical zones: Coastal alluvial plain (LCP) which supports mangrove and agricultural systems; Hilly sand and clay region (HSCR) which supports wallaba and dacama forests, Highland region (HR) which supports montane forests; Interior savanna (IS) which is predominantly a savanna habitat.
Earthworms' role in an ecosystem is almost always defined by their feeding and burrowing habits and the soil 5 10 15 20 horizons they occupy, ergo earthworms occupy different niches within the same ecosystem. This was first noted by Lee in 1959 and then by Bouche in 1971, who determined that earthworms belong to three main ecological categories. Epigeic are small earthworms (10-30mm) which are considered as 'litter dwellers' which form no burrows and are highly pigmented dorsally and ventrally, Edwards and Bohlen, 1996. They impart significantly on the decomposition of organic matter which accounts for the humus layer above soils. Because of where they occupy, they are more susceptible to predation and are highly impacted by both environmental and anthropogenic variables, Bouche, 1977. Anecic species, (110-110cm), are relatively pigmented dorsally and unpigmented ventrally and they form permanent vertical burrows within the soil, extending well into the C horizon, Lal, 2007. They feed on surface litter/organic debris, which they pull into their burrows. They contribute to the movement of nutrients down through the soil profile, making it more accessible to plant roots, Lavelle and Spain, 2001. Endogeic are unpigmented earthworms that form continuous horizontal burrows where they feed on the soil, hence they are termed as The aim of this study was to establish the population structure of earthworms in Guyana through the exploration of three hypotheses: (1) Variations in earthworm population structure among Guyana's geographical zones will be significantly different. (2) Earthworm abundance will be significantly affected by Guyana's biogeographical factors.
(3) Earthworm population structure will be significantly correlated with soil physico-chemical factors.

Description of Study Area
The 60 sites that were studied were classified based on their climate, habitat and soil type. The

Experimental Design
Fifteen sampling sites were randomly established within each of the 4 biogeographical regions of Guyana and their respective GPS coordinates were recorded. In each of the 60 sites, 15 sampling points of 0.5m³ were established along a linear transect at 6m intervals. The soil was removed from each sample point by manually digging after which the removed soil was hand sorted for earthworms which were categorized and counted. Special note was taken to colouration, to determine ecotype, and the presence of a clitellum, to discern adults from juveniles. 2 adults from each morphospecie were collected and placed in separate labelled ziplock bags containing 95% ethanol to preserve genetic integrity for identification. The circumference and length of each morphospecie was recorded along with the type and position of key external features such as segmentation, prostomium, genital opening and setal data.

RESULTS
68 taxonomically distinct species were identified of which Pontoscolex corethrurus was found to be the most abundant, accounting for 11.8% of the population, while a species from the Kynotidae family was found to be the least abundant, accounting for 0.01% of the population.
Geography was found to have a statistical significant impact on earthworm abundance (p=0.0078). Of the 4 biogeographical regions of Guyana, The Highland Region (HR) was found to have the highest abundance while the Interior Savanna (IS) was found to have the least, (Table 1.0).
Density among ecotypes were found to be of statistical significant difference (p=4.58E-08) with epigeics being the 70 most abundant (accounting for 52.9% of the population), followed by endogeics (accounting for 41.5% of the population) and anecics (accounting for 5.6% of the population), Table 2.0. The highest percentage of epigeics was within the Low Coastal Plain (LCP), 25.2%, while the least was found in the Interior Savanna (IS), 3.1%. The highest abundance of endogeics was in the Highland Region, 16.1%, while the least was in the LCP, 4%.
The density among the soil types was found to be of statistical significant difference with a p value of 0.025, with anthrosols being associated with the highest average density per m², Table 3.0 . Of the anthrosols, hortic, also known as black earth, was seen to have the highest density of earthworms. Hydragic anthrol was found to be associated with the lowest density of the anthrosols. On the opposite end of the spectrum were the leptosols, which displayed the lowest average density. Hortic anthrosols were found to be associated with higher species richness (7) while hydragic anthrosols were associated with lower species richness (1).
Of the 7 identified habitat types, farms were found to be associated with the highest average density (78). This was then followed by montane forests and grasslands while the lowest density was associated with savannas, Table 4.0.
The density among the habitat types however, was not of statistical significant difference (p=0.061).
Climate was found to have a significant impact on earthworm density as evidenced by the p value, 6.77E-05.
Monsoon climate was found to be associated with the highest average density by a landslide, followed by equatorial and tropical savanna, Table 5.0.
For Guyana, there were a total of thirty-five disturbed sites with an average abundance of 321, and twenty-five conserved sites with an average abundance of 432. The average richness for the conserved sites was four while the average richness for the disturbed sites was three. It was found that richness was more heavily impacted by disturbance than abundance as seen in Table 6.0.
A regression analysis was conducted on the variables to determine the relationship among the dependent and independent variables. The analysis on the total density was low, with an r 2 value of 0.3. To determine if ecotypes are affected differently, earthworms were separated into their various ecotype after which the same analysis was carried out again. It was found that epigeic density and the independent variables had a stronger connection based on the r 2 value, 0.6. Anecic density and endogeic density was found to have lower r 2 values.
Additionally it was found that phosphorous, calcium and moisture significantly affected epigeic density while 85 90 95 100 105 110 magnesium was found to significantly affect anecic density, Table 7.0.

DISCUSSION
The observed abundance of P. corethrurus is attributed to the ecological plasticity of the organism in that it can outcompete other native earthworm species and successfully exploit new niches that have developed as a result of anthropogenic activities, thriving in a multitude of ecosystems. Unlike P. corethrurus, the species with the least abundance is found to be associated with only mangrove ecosystems, only two of which were sampled, which explains its low overall abundance.
The Highland Region is a montane environment covered in tropical forests and grasslands which creates a unique environment in which detritus is in abundance. Due to the warm temperatures, however, the rate of decomposition increases and coupled with the high precipitation rates, leaching of nutrients occur, Lavelle, 1983. Despite the above stated conundrum, the density in this region is the highest of the pedogeographical regions which is most logically due to: epigeic species being R-strategists, being smaller with high fecundity rates, while most anecic species are Kstrategists, being larger with low fecundity rates, (Cosin, et al., 2011). Endogeic species on the other hand feeds on the soil, and together with their selective feeding habits and gut microflora, this ecotype flourishes in this environment, Lavelle and Spain, 2001.
Additionally, the difference in the ecotypes' reproductive strategies explains the major gaps in their abundance. The lacking of anecic species in the LCP and HSCR is due to the high levels of pedological anthropogenic activities, such as logging, and agriculture, which brings them to the surface exposing them to predators.
The Interior Savanna unlike the Highlands is a tropical savanna which experiences dry spells with less precipitation and high temperatures. All of the above are known to have negative consequences on earthworm abundance, Bohlen, 2002, which was reflected in the data collected.
Black earth/ hortic anthrosol (accounting for the highest density), are highly fertile, loose soils due to century's worth of practice of adding organic waste and charcoal to the soil, Sombroek, 1966. This creates a pedobiological environment which can sustain high densities of earthworms which can occupy the same niche as interspecific and intraspecific competition is unlikely to occur. Hydragic anthrols are water saturated soils which creates an anaerobic environment, (Bohlen, et al., 2004), making the occupation of the deeper layers of the soil and the niches it provides almost impossible and as such it is only possible for the surface dwelling epigeic species to thrive at best, hence why only one specie was observed. CONCLUSION 68 taxonomically distinct species of earthworms were found in Guyana. Of the four natural regions, the earthworm population of Highland Region was found to be the most diverse, rich, even and dense, due to the abundance of organic matter, high precipitation, low temperatures and in some cases, no human disturbance. Based on the data collected, it was determined that earthworm abundance, epigeic abundance, endogeic abundance, anecic abundance and species richness among the four natural regions of Guyana, were all of statistical significant difference.

Earthworm abundance among Natural Regions of Guyana
Each data point represents the total number of earthworms found at each respective site      p values for disturbed vs conserved sites Each data point represents the p-value obtained when comparing the abundance and richness of earthworms among conserved and disturbed sites, per Natural Region.  p values for density per soil parameter-Regression analysis Each data point represents the p-value obtained for the regression analysis of the soil parameters against earthworm density and ecotype density.