دانلود رایگان مقاله انگلیسی تاثیرات استفاده از زمین و زیستگاه توسط سوبسید بالقوه بی مهره های رودخانه برای شبکه مواد غذایی زمینی - اشپرینگر 2018

Abstract

Adult aquatic insects with a terrestrial life-stage are important vectors transferring resources assimilated in freshwater environments to terrestrial consumers. Research on this linkage has focused particularly on how terrestrial environmental features affect dispersal of adult aquatic insects, and on the responses of terrestrial consumers. However, both the timing and extent of dispersal by adult aquatic insects are further regulated by their species-specific life history traits. We sampled aquatic invertebrates from nine streams in central Sweden, and assessed how the composition of key traits related to dispersal and life history varied between in-stream habitats (riffles, pools), seasons (autumn, spring), and among streams differing in catchment land use (forested, agriculture). Traits indicative of more limited adult dispersal (e.g. small adult size and weak flying strength), along with traits indicative of strongly pulsed peaks in emergence (e.g. univoltinism and well-synchronised emergence) were all more abundant in the agricultural than forested streams in the autumn. However, these differences had disappeared by late spring, possibly reflecting early emergence by the univoltine taxa that dominated the agricultural stream communities and/or elevated mortality in the agricultural streams. Riffles supported higher abundances of insects with strongly flying adults, whereas traits associated with more limited dispersal were characteristic of insect assemblages in pools, which also supported the highest proportion of invertebrates completely lacking an adult flying stage. This result is likely to have implications at larger scales, given the dominance of soft-bottomed pool habitats and scarcity of riffles in many agricultural landscapes. Overall, our analysis indicates that while overall production of aquatic invertebrates with a winged adult was greater in agricultural streams, availability of this productivity for terrestrial consumers is more likely to be spatially restricted closer to the stream channel, and potentially also more temporally pulsed .

Discussion

Abundances of all invertebrates, including those both with and without a terrestrial flying adult stage, were higher overall in agricultural than forested streams. This likely reflects higher autotrophic productivity associated with reduced shading and increased nutrient inputs in agricultural streams (Delong and Brusven 1998; Harding et al. 1999), particularly given there were no differences in the overall size of the two stream types (Carlson et al. 2016). Abundances of most of our focal species traits were similarly higher in the agricultural streams, and in riffle habitats. However, we also observed multiple interactions between land use, season and habitat, both in our analyses of all traits combined, and when analysing individual traits separately. These interactions reflect differences in the predominant life history strategies characterising the macroinvertebrate communities of forested and agricultural streams, and of riffle and pool habitats, and have implications for potential spatio-temporal variability in aquatic subsidy production and availability for terrestrial consumers. Most of the traits characterising aquatic insect assemblages from the agricultural streams are indicative of more limited dispersal by the adult stages, including small adult size, short adult life span and weak adult flying strength. These traits were associated especially with small-sized Diptera having short-lived adults, particularly Chironomidae, which dominated our agricultural assemblages. Individuals characterized by these traits will generally stay close to the stream edge following emergence, and their short adult life span will both limit the time available both for extensive dispersal, and for consumption by terrestrial consumers (Kovats et al. 1996; Malmqvist 2002; Petersen et al. 2004; Krosch et al. 2009). Greenwood and Booker (2016) similarly found that small body size was a characteristic trait of invertebrate assemblages from streams with more pasture upstream, though this was associated with strong rather than poorly dispersing taxa. In our study, only one trait indicative of greater dispersal, long female flight distances, was characteristic of the agricultural streams, associated with blood-feeding Diptera (Ceratopogonidae and Simuliidae) that have females which disperse farther in search of meals (Lassen et al. 2012; McCreadie and Adler 2012). The influence that this might have on connectivity between freshwater and terrestrial habitats is demonstrated by Carlson et al.’s (2016) finding that the proportion of blood feeding Ceratopogonidae was associated with greater dispersal distance by adult Diptera overall.