Hydrological controls on oviposition habitat are associated with egg‐laying phenology of some caddisflies

Abstract

Seasonal variation in resource availability can have strong effects on life histories and population densities. Emergent rocks (ERs) are an essential oviposition resource for multiple species of stream insects. The availability of ERs depends upon water depth and clast size, which vary with discharge and river geomorphology, respectively. Recruitment success for populations may depend on whether peak egg‐laying periods occur at times when ERs are also abundant. For multiple species that oviposit on ERs, we tested whether seasonal fluctuations in ER abundance were concurrent with oviposition phenology. We also tested whether high discharge drowned ERs for sufficiently long periods to preclude egg laying, and whether this problem varied between rivers differing in channel morphology and particle size distribution. We obtained a continuous timeseries of water level (WL) measured every 30 min for 2 years at sites on three rivers in south‐eastern Australia with similar hydrology but different geomorphology. A relationship between WL and ER numbers was determined empirically at each site and these relationships were used to predict ER availability over the 2 years. Egg masses of 10 species of caddisflies were enumerated each month for a year in one river to establish oviposition phenology. Abundance of ERs was inversely related to discharge in all three rivers. ERs were most abundant during autumn and scarce during spring. Site‐specific geomorphology resulted in skewed or multimodal distributions of ER abundance each year. Between years, catchment‐scale hydrometeorology mediated patterns of ER availability, despite the close proximity of sites. Temporal variance in ER availability was not consistently correlated with mean WL or WL variance. ER variance increased with WL variance, when WL was below a threshold equivalent to mean annual WL. Above this threshold, most ERs were likely to be submerged. Oviposition phenology varied strongly among the 10 species of caddisflies, with egg‐laying ranging from 1–2 months to year‐round. Temporal variations in ER and egg mass abundance were not correlated for most species. Below a threshold minimum number of ERs, egg masses were highly crowded onto the few available ERs, which is evidence that ERs were in short supply. For five species, high egg mass abundance was positively associated with periods of the year when the time above the threshold number of ERs was high. Unusually, two species laid most egg masses during winter and when the time above this threshold was short. Three species showed no association between egg mass abundance and time above this threshold; two of these species laid eggs year‐round. Regional hydrometeorology controlled the availability of ERs, but between‐river differences were sufficient to deliver different outcomes in the availability of oviposition sites between years and seasons. Caddisflies were rarely prevented from laying eggs but periods when ERs were in short supply created crowding, which may be associated with negative fitness effects...

Facilitating the recovery of insect communities in restored streams by increasing oviposition habitat

“…Furthermore, a third metric developed by Lang et al (2017) of egg mass aggregation on individual rocks indicated that, despite there being more rocks and more rock surface area in restored‐treated and reference streams, females still selected a few specific protruding rocks. Previous studies reported higher aggregations of egg masses with lower abundance of protruding rocks (Lancaster et al, 2021), but it is common for stream insects to aggregate their eggs on a small proportion of the available egg‐laying habitat (e.g., Encalada, 2005; Lancaster & Downes, 2010; Peckarsky et al, 2000). The 106% higher aggregation of egg masses in restored‐treated versus restored‐untreated streams is evidence that the addition of protruding rocks increased the availability of egg‐laying habitat preferred by females.…”

“…It is unlikely these eggs all hatched, and there were no residual egg chorions visible on the rocks from hatched eggs; hence, this movement likely resulted in egg mortality. We posited that increasing egg‐laying habitat would cause egg masses to be distributed over more rocks (Lancaster et al, 2021), which might reduce egg mortality by decreasing the probability that if any one protruding rock was disturbed, it would not contain a large proportion of egg masses. The prevalence and aggregation results discussed above suggest that the extent to which this could be realized is low for highly selective females that aggregate their eggs on specific habitats, and it would depend strongly on the added egg‐laying habitat being preferred by females.…”

“…Egg masses per stream area censused was used rather than total stream area of each 50‐m study section because on some occasions (eight of 222), the entire 50‐m study section could not be censused due to lightning storms. We also calculated the mean number of egg masses per individual rock area, as previous research revealed that when rocks were scarce, the density of egg masses on individual rocks is high (Jordt & Taylor, 2021b) and more aggregated (Lancaster et al, 2021); therefore, we tested whether the density of egg masses on individual rocks in the restored‐treated streams changed from before to after treatment. Mean egg masses per individual rock area was calculated as the mean of the sum of masses on an individual rock divided by the underside two‐dimensional area of the rock for each stream, census date, and year.…”

“…For instance, we need to improve our understanding of egg‐laying habitat suitability and especially selection by females and develop ways to enhance habitat stability and maintain protruding rock habitat via natural processes to increase the ecological and economic effectiveness and longevity of restoration efforts. Additionally, climate change is predicted to cause increases in rainfall intensity (Madsen & Figdor, 2007) as well as droughts (Leeper et al, 2022), and these aspects of water availability are known to have direct and indirect negative effects on egg‐laying habitat quality and quantity (Kennedy et al, 2016; Lancaster et al, 2021; Peckarsky et al, 2000). These and similar habitats are vital to a multitude of other aquatic organisms that use specific rocks, or sediment sizes, for their reproductive habitat (salmon: Kondolf & Wolman, 1993, bluehead chub: Sabaj et al, 2000, salamanders: Unger et al, 2020) that, in some cases, have been the focus of restoration and conservation efforts (salmon: Barlaup et al, 2008, Madtom catfish: Cope et al, 2019, various fishes: Taylor et al, 2019, salamanders: Button et al, 2020).…”

Seasonal differences in amounts of oviposition habitat and egg‐laying by caddisflies in rivers with regulated versus unregulated flows

“…While average water levels vary predictably with discharge, relationships between water level and ER number may not be straightforward across a full range of discharges (Jordt & Taylor, 2021;Miller et al, 2020). Lancaster et al (2021) compared water levels and ER densities across 2 years in three unregulated rivers. Emergent rock densities were consistently low at the highest water levels but were variable at low water levels, due to differences in local bed topography among sites.…”