Land‐use effects on terrestrial consumers through changed size structure of aquatic insects

Abstract

Summary We assessed the influence of agricultural land use on aquatic–terrestrial linkages along streams arising from changes in the emergence of aquatic insects. We expected that terrestrial predators would respond to a change in the abundance and/or the size structure of the emerging aquatic insects by an increase or decrease in population size. We measured the flux of emergent aquatic insects and the abundance of terrestrial invertebrate predators and birds along 10 streams across a forest‐to‐agriculture land‐use gradient. We also performed stable isotope analyses (hydrogen, carbon and nitrogen) of terrestrial invertebrate predators. Small aquatic insects (Nematocera) were most abundant under agricultural land use, whereas larger bodied aquatic insects (Plecoptera and Trichoptera) were more associated with forest land use. Carabid beetles and linyphiid spiders were associated with agricultural streams (where there was a high abundance of small aquatic insects), whereas lycosid spiders and birds were associated with forest streams and a high abundance of large‐sized aquatic insects. The contribution of aquatic insects to the diets of riparian Lycosidae, Linyphiidae and Carabidae was estimated to be 44%, 60% and 43%, respectively, indicating the importance of aquatic subsidies to the terrestrial system. Our results show that agricultural land use in an overall forested landscape can have significant effects on the abundance and diet of terrestrial consumers through its impact on the size structure of the assemblage of emerging insects, rather than the overall magnitude (numbers) of the aquatic subsidy. Hence, our results suggest that the composition, not just quantity, of a cross‐habitat resource may influence the recipient system.

Species traits reveal effects of land use, season and habitat on the potential subsidy of stream invertebrates to terrestrial food webs

“…Large body size is an indicator of stronger flying strength, and the availability of both larger sized and smaller sized subsidy might allow for optimal feeding across a broader size range of terrestrial consumers (Brose et al 2008). Our finding that longer-lived, larger sized and stronger flying taxa did not differ between forested and agricultural streams emphasises the need to quantify as biomass how much aquatic subsidy these taxa export, compared with much more abundant but weaker flying smaller taxa (Stenroth et al 2015). Significantly, most of the taxa associated with these traits were almost entirely absent in the survey of adult flying insects conducted by Carlson et al (2016), possibly because these taxa are either too large and strong, or fly too high, to be routinely trapped by their sticky trap sampling method.…”

“…Significantly, most of the taxa associated with these traits were almost entirely absent in the survey of adult flying insects conducted by Carlson et al (2016), possibly because these taxa are either too large and strong, or fly too high, to be routinely trapped by their sticky trap sampling method. It is not known whether dispersal of these taxa is inhibited to the same extent in agricultural landscapes as observed for smaller sized and weak flying Diptera (Carlson et al 2016), though Stenroth et al (2015) notably found that adults of larger sized aquatic taxa were more associated with forested than agricultural streams.…”

“…Accordingly, much research attention has focused on how terrestrial environmental features regulate subsidy dispersal and the responses of terrestrial consumers (e.g. forest cover, complexity of ground habitats, availability of terrestrial resources), and on how human activities alter these linkages (Petersen et al 2004;Greenwood and McIntosh 2008;Marczak and Richardson 2008;Stenroth et al 2015). However, both the timing and extent of dispersal by adult aquatic insects are further regulated by their species-specific life history traits (Greenwood and Booker 2016).…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Land use -Almost all ecosystems suffer from some degradation due to impacts of land use, such as by agriculture, urbanization and deforestation or forest harvesting [108,118,119], which were the dominant changes in the past 100 years and seriously impacted biodiversity and ecosystem functioning at local, regional and global scales [120]. During the past 50 years, agricultural land use was and will continue to be the main reason of ecological changes in both aquatic and terrestrial ecosystems [121].…”

“…Trichoptera and Plecoptera) were more related to forest land use, and this size change of prey subsidy is associated with the distribution of different types of terrestrial predators, causing a different terrestrial predator community structure [119]. -Land use can change the magnitude of subsidies, Francis and Schindler (2009) found that at all geographical scales, shoreline development negatively influenced terrestrial invertebrate subsidies, with 100% of the diet of fish were terrestrial insects in undeveloped lakes, whereas it was only 2% in developed lakes [108].…”