Abstract
BackgroundFragmentation of tsetse habitat in eastern Zambia is largely due to encroachments by subsistence farmers into new areas in search of new agricultural land. The impact of habitat fragmentation on tsetse populations is not clearly understood. This study was aimed at establishing the impact of habitat fragmentation on physiological and demographic parameters of tsetse flies in order to enhance the understanding of the relationship between fragmentation and African animal trypanosomosis (AAT) risk.MethodsA longitudinal study was conducted to establish the age structure, abundance, proportion of females and trypanosome infection rate of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) in areas of varying degrees of habitat fragmentation in Eastern Zambia. Black screen fly rounds were used to sample tsetse populations monthly for 1 year. Logistic regression was used to analyse age, proportion of females and infection rate data.ResultsFlies got significantly older as fragmentation increased (p < 0.004). The proportion of old flies, i.e. above ovarian category four, increased significantly (P < 0.001) from 25.9 % (CI 21.4–31.1) at the least fragmented site (Lusandwa) to 74.2 % (CI 56.8–86.3) at the highly fragmented site (Chisulo). In the most fragmented area (Kasamanda), tsetse flies had almost disappeared. In the highly fragmented area a significantly higher trypanosome infection rate in tsetse (P < 0.001) than in areas with lower fragmentation was observed. Consequently a comparatively high trypanosomosis incidence rate in livestock was observed there despite lower tsetse density (p < 0.001). The overall proportion of captured female flies increased significantly (P < 0.005) as fragmentation reduced. The proportion increased from 0.135 (CI 0.10–0.18) to 0.285 (CI 0.26–0.31) at the highly and least fragmented sites, respectively.ConclusionsHabitat fragmentation creates conditions to which tsetse populations respond physiologically and demographically thereby affecting tsetse-trypanosome interactions and hence influencing trypanosomosis risk. Temperature rise due to fragmentation coupled with dominance of old flies in populations increases infection rate in tsetse and hence creates high risk of trypanosomosis in fragmented areas. Possibilities of how correlations between biological characteristics of populations and the degree of fragmentation can be used to structure populations based on their well-being, using integrated GIS and remote sensing techniques are discussed.
Highlights
Fragmentation of tsetse habitat in eastern Zambia is largely due to encroachments by subsistence farmers into new areas in search of new agricultural land
From the fragmentation indices obtained and the index of apparent abundance (IAA) established at study sites, it appeared that wherever host animals are available, an area with large natural patches is more favourable for G. m. morsitans than one with small natural patches as was seen at Chisulo (c) with 0.0357 IAA compared to Kasamanda (d) with 0.001 IAA
The following were apparent assuming that the bias of the fly round sampling method in favour of males, young flies and hungry females [25] remained constant irrespective of fragmentation (i) despite the differences in the dynamics of the IAA of G. m. morsitans at study sites, [increase at one site and reduction at another over the same period], it reduces as fragmentation increases (ii) the age distribution of captured G. m. morsitans is distorted by fragmentation in this area (iii) the proportion of female flies captured increase as fragmentation reduces (Fig. 6), (iv) the proportion of old female flies increases significantly as fragmentation increases (Fig. 5d), (v) the trypanosomal infection rate in G. m. morsitans increases significantly with fragmentation (Fig. 7a)
Summary
Fragmentation of tsetse habitat in eastern Zambia is largely due to encroachments by subsistence farmers into new areas in search of new agricultural land. Ecological and environmental factors such as host availability, predation, shelter, temperature and humidity are critical in determining the dynamics of a tsetse population [1, 2]. These factors are affected by the degree of habitat fragmentation [3, 4]. Other notable responses of tsetse to ecological conditions include (i) increase in the proportion of female flies captured by fly round methods in a population as a result of nutritional stress [8]; (ii) life expectancy being longer in cool than in hot seasons [9]; (iii) body size of the offspring (as measured by wing vein length) that may change depending on the conditions experienced by the parent flies during the period preceding the capture [10, 11]
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