Persistence in the longitudinal distribution of lotic insects in a changing climate: a tale of two rivers

The longitudinal distribution of many taxa in rivers is influenced by temperature. Here we took advantage of two older datasets on net-spinning caddisflies (Hydropsychidae) from contrasting European rivers to assess changes in species occurrence and relative abundance along the river by resampling the same sites, postulating that an increase in river temperature over the intervening period should have resulted in cool-adapted species retreating into the headwaters and warm adapted species expanding upstream. Distributional changes in the Welsh Usk were slight between 1968/69 and 2010, one rare species appearing at a single headwater site and one warm-adapted species disappearing from the main river. Distributional changes in the French Loire, between 1989–93 and 2005, were similarly modest, with no consistent movement of species up- or downstream. We estimate that the decadal rate of increase in the mean summer daily maximum in the Usk was only 0.1 °C at one ‘summer cool’ headwater site, while a neighbouring ‘summer warm’ tributary increased by 0.16 °C per decade, and the main river by 0.22 °C. The Loire is warmer than the Usk and the mean decadal rates of increase, over the period 1989–2005, at three sites along the lower reaches were 0.39, 0.48 and 0.77 °C. Increases in stream and river temperature, therefore, were spatially variable and were not associated with consistent upstream movement of species in either of these (very different) rivers. We conclude that either the temperature increases have hitherto been insufficient to affect species distribution or, more speculatively, that it may not be possible for river organisms (that do not respond only to temperature) to move upstream because of a developing spatial mismatch between key habitat characteristics, some of them changing with the climate but others not

‘House and garden’:larval galleries enhance resource availability for a sedentary caddisfly

1. Sedentary grazers can be numerous in fresh waters, despite the constraints on resource availability and the increased predation risk inherent in this lifestyle. The retreats of sedentary grazers have been assumed to provide protection to the resident (a ‘house’), but also may provide additional fertilised food for the grazer (i.e. a ‘garden’). If retreats function as a garden, then they should (i) contain a higher quality and/or quantity of food than the alternative food source. Furthermore, the proportion of retreat-derived carbon and nitrogen assimilated by the resident should be (ii) related to overall resource availability (more when resources are limited) or (iii) perhaps also to larval density. Alternatively, if retreats provide a less risky food source, then (iv) assimilation of material from the retreat is likely to be greater under conditions in which the risk of emerging from the retreat is high. 2. We tested these four hypotheses for the common and widespread gallery-building grazing caddisfly Tinodes waeneri. Resource availability, larval density and biomass, and exposure were measured for populations from six lakes of differing productivity in August, October and January. 3. Galleries always contained more algal food than the surrounding epilithon, suggesting that gardening is effective. Furthermore, gallery chlorophyll a content in August, and the disparity in food quality (assessed from the C : N ratio) between gallery and epilithon (quality higher in the former) in October were positively related to the proportion of larval biomass that was derived from the gallery. Larval density and wave exposure parameters were not related to larval assimilation of gallery material. 4. Galleries that are fertilised by the occupant provide more, and sometimes also better quality, food (in terms of the C : N ratio) than is otherwise available. Thus, the gallery plays a substantial role in larval nutrition, and this role is greater at key times of food shortage

Gardening by the psychomyiid caddisfly Tinodes waeneri:evidence from stable isotopes

Sedentary species face a trade-off between the benefits of exploiting food close to their homes and the cost of defending it. In aquatic systems, it has been suggested that some sedentary grazers can increase the range of circumstances under which they are at an advantage over mobile grazers by enhancing food resources within their feeding territories through ‘gardening’. We examined this for the retreat-building sedentary larvae of the caddis Tinodes waeneri, which are often dominant in the littoral of lakes. We hypothesised that T. waeneri gardens by fertilising its retreat (a fixed ‘gallery’ on which algae and other microorganisms grow), and that gardening would be more important in lower productivity lakes. We tested this by analysing the carbon and nitrogen stable isotope ratios of larvae, their galleries and the general background epilithon, collected from rocks in the littoral zones of six lakes spread across a natural nutrient gradient. We found evidence of nutrient recycling within the Tinodes gallery community in all lakes. Galleries were 15N-depleted compared to the epilithon, suggesting that algae on galleries preferentially assimilated 14N from larval excretions that were themselves 15N-depleted relative to the larval food source. Mixing model results indicate that galleries formed an important larval carbon and nitrogen source, with mean gallery dietary contributions of over 40% in at least one sample period in all lakes studied. Gallery contributions were greater between April and October than in January and, contrary to our initial hypothesis, greater in the more productive lakes of those surveyed. Nevertheless, T. waeneri galleries do act as a fertilised garden. ‘Gardening’ appears to be widespread in this species, and may affect productivity and patterns of nitrogen retention within the stony littoral of lakes

Insect dispersal does not limit the biological recovery of streams from acidification

1.The ecological recovery of streams from large-scale perturbations, such as acidification, requires aquatic insects to disperse between catchments. While adults can usually fly, dispersal is seldom observed directly. Catches of insects in transects of traps perpendicular from streams suggest that lateral adult dispersal is limited. This paper evaluates whether this could explain limited biological recovery in streams recovering chemically from acidification.2.At the replicate Llyn Brianne experimental catchments (Wales), Malaise traps (2000) and benthic sampling (1985–2005) were used to appraise inter-catchment dispersal in acid-sensitive Ephemeroptera, Plecoptera and Trichoptera. The results provide direct evidence for inter-catchment dispersal: eight species from three Orders were caught as adults alongside acid streams where larvae never occurred in 21 years' benthic sampling.3.These data refute the hypothesis that limited dispersal per se explains delayed biological recovery from acidification in Welsh streams. Other factors affecting colonization (e.g. ‘propagule pressure’, mating or oviposition behaviour) and persistence (e.g. continued acid episodes) must be involved, with the first of these possibilities still poorly understood. These data add to a growing body of literature illustrating insect dispersal between catchments, and they have wider relevance to the recovery and restoration of river ecosystems following basin-scale impacts

Consequences of inferring diet from feeding guilds when estimating and interpreting consumer-resource stoichiometry

1. Imbalances between the supply of elements from resources and their demand from consumers may constrain key ecological processes, such as growth and production. Most previous studies have estimated such stoichiometric imbalances between consumers and resources by inferring the diet of the former from functional classifications rather than by direct assessments of the diet. However, this does not allow for potentially plastic responses of consumers to a restricted supply of elemental resources. 2. Here, for three streams of very contrasting nutrient availability, we calculated elemental imbalances between consumers and resources using diets derived from empirical gut contents analysis and compared them with those inferred for the functional feeding guilds of the species concerned. 3. In almost every case, elemental imbalances (C:P and N:P) based on the realised diet differed significantly from those expected from the inferred diet, the former revealing greater alignment between the elemental composition of consumers and their resources, particularly for P. 4. Simply inferring the diet, as is commonly done, results in erroneous estimates of elemental imbalances and misleading conclusions about stoichiometric constraints on consumer