Fluctuating asymmetry and habitat use of aquatic predators in an altered landscape. Highlights from the arid region monegros

Zusammenfassung

Agriculture-induced habitat degradation and loss together with changes in climate patterns are among the greatest global threats to species, communities and ecosystem functioning. Biota inhabiting agricultural wetlands in many arid and semiarid regions have been subject to increasing isolation and decreased quality of their habitats. As well as having to withstand severe climatic variations they are often exposed to sudden changes in land-use, flooding-drying regime and agrichemicals, all these affecting major life-history traits such as growth, reproduction and survival. Advancing our understanding on how organisms cope and adapt to environmental variations lies at the core of ecology and evolutionary biology. This has become especially relevant in the light of increased human-induced environmental change as a major threat to global biodiversity. When predicting species' responses to environmental degradation it is crucial to consider spatial patterns of local adaptation and population dynamics at different regional and geographical scales. This thesis stemmed out of the need to shed light on major biological and ecological processes of agricultural waterbodies in arid landscapes, as well as to quantify biota capacity to reflect the magnitude of environmental stress in these ecosystems. For this I assessed the degree of influence a number of environmental factors have on morphological development and species-habitat relationships in aquatic invertebrate and vertebrate predators at several spatial and temporal scales. The study was carried out in central Monegros, an arid area in NE Spain. During the last six decades this region has experienced radical landscape transformation and degradation due to intensive agriculture, including increased soil salinity, erosion and agrichemical pollution. The work is divided into two major sections. In the first section I analyzed the population dynamics of three ubiquitous predatory aquatic beetles (Hydroglyphus pusillus, Laccophilus minutus and Rhantus suturalis) in relationship with surrounding environmental factors. Furthermore I assessed their developmental instability as reflected in traits fluctuating asymmetry (FA, i.e. small random deviations from symmetry of bilateral traits) and their trace element uptake and bioindication capacity. The second section regarded the developmental instability of amphibian (Pelophylax perezi) populations at different developmental stages, together with an overview of the species feeding ecology. The results indicated that drought, an important factor controlling temporary ponds, local climate variations, habitat characteristics and species life cycles were the major drivers of aquatic beetles' community dynamics. The response to climatic factors (i.e. ultraviolet-B radiation and precipitations) was species-specific, likely due to differences in their life histories. Hydroperiod and habitat permanence, as well as vegetation cover and type were the main habitat determinants of beetles' distribution. Dytiscid beetles showed a fairly good capacity to bioindicate trace elements in the environment. While this ability varied with each species, R. suturalis was the most sensitive to reflect agricultural pollution in Monegros. In terms of body developmental stability the results revealed that it changed among beetle species, traits, and habitats. Paddy populations generally experienced a higher FA than those from reservoirs. A number of natural (e.g. aquatic and emergent vegetation) and anthropogenic (e.g. land-use and road proximity) factors were linked to high asymmetry levels. However landscape heterogeneity, such as the proximity of irrigated land, was associated with a low FA suggesting that it may contribute to an increased persistence of predatory aquatic beetles in agricultural landscapes. On the other hand, reservoir populations reflected better background climatic factors such as mean precipitation and frequencies of rain, dew, and freeze days. Correlation patterns occurred between multiple trait FA in distinct populations of H. pusillus according to trait functionality and structural complexity. Highly functional and complex traits such as elytra and legs exhibited comparatively lower levels of FA than other morphological traits and their FAs were significantly correlated. Moreover, close related segments such as femur and tibia were more correlated in terms of asymmetry than distanced ones. An analysis of traits concordance revealed both, an individual and a population asymmetry patterns. It indicates that individuals have a common genetic background and a general ability to buffer stress incurred during development by mechanisms which are to be further investigated. Among vertebrates, amphibians are one of the most endangered groups due to agriculture intensification. As key players of ecosystem function, amphibians are important sentinels of its integrity. The second part of the thesis revealed that the Iberian frog Pelophylax perezi is a generalist predator that uses the food resources available in the environment regardless of prey size or type. This foraging behavior may have helped it to thrive in the irrigated areas. Some basic phenotypic indicators such as body size and mass failed to detect stress in its populations. Despite of an increased body condition in paddy environment, the frogs experienced a high level of FA as reflected by their skeletal traits. This indicates that a catch-up growth of these populations may actually enhance developmental errors. These findings imply that fluctuating asymmetry is a valuable tool to identify amphibians under stress in landscapes converted to agriculture. One major conclusion of this thesis is that multiple trait FA of aquatic invertebrate and vertebrate predators can reliably predict environmental stressed populations in habitats with different level of management. Several climatic factors also appeared to play a significant role in the developmental stability of these organisms. These could be important determinants of species survival in constantly changing environments. Despite of species vulnerability to many habitat constraints, heterogeneous irrigated landscapes can however lower the risk of regional population declines under weather perturbations. Populations of invertebrate and vertebrate predators can apparently adapt well to these environments by dispersal and appropriate feeding strategies, but their persistence in these landscapes over time is uncertain.