1.To understand species losses from disturbed landscapes, it is important to distinguish the effects of degraded environmental conditions from those caused by barriers to dispersal between habitat patches. To assess the relative importance of these effects, we developed a new approach using permutation and association tests applied to rank abundance data, using the invertebrate fauna of two rivers in two seasons. 2. Our study streams were Hughes Creek and Seven Creeks, in south-eastern Australia, which have both been degraded by agriculture in downstream sections. We collected benthic invertebrates and also dispersing individuals (drift, terrestrial adults) during two seasons in [2007][2008]. Study sites spanned strong environmental gradients as well as the main dispersal route (up-and down-channel). Environmental data were analysed to set up permutation tests on rank abundances. Survey and disperser data were contrasted using contingency table analyses. 3. The results suggest dispersal plays a strong role in community structure. Environmental effects were evident and strongest upstream, but evidence of environmental effects was weak over much of the gradient. Many species had different distributions in different data sets or dispersers that were abundant at locations distant from centres of benthic distribution. 4. Our results differ from many studies, but few have been able to evaluate dispersal effects directly. Our method provides a practical approach for evaluating the role dispersal plays in driving species abundance patterns across landscapes, thus bridging a gap between theory and practice. 5. Synthesis and applications. Managers typically use indices of ecosystem health that assume environmental conditions largely determine species diversity and abundance. Dispersal between habitat patches is known to be important, but there are no reliable methods to assess the role dispersal may play. We provide an approach that allows both dispersal and environmental effects on species distributions to be evaluated from survey data. This may open the way for dispersal information to be incorporated into management actions. Additionally, the approach should allow improved siting of restoration projects that depend greatly on successful dispersal of individuals for successful outcomes.

Summary Under the preference–performance hypothesis (PPH), oviparous females select oviposition sites that optimise the fitness of their offspring (eggs or larvae). The resulting distribution and fitness of offspring may have knock‐on effects for population distribution patterns and dynamics during larval and adult stages. We tested the PPH for Australian caddisflies from two genera (family: Hydrobiosidae) that oviposit in different flow conditions. Apsilochorema spp. oviposit in slow flowing water, whereas Ulmerochorema sp. favour fast flows. We expected hatching success to be higher in velocities favoured by ovipositing females. In a field experiment, newly laid egg masses of each species were exposed to experimental ‘fast’ and ‘slow’ flow treatments throughout development and monitored until they hatched or died. In a second field experiment, we placed egg masses in a range of velocities (0.0–1.5 ms−1) to determine the threshold beyond which eggs were damaged by shear forces. The results supported the PPH for one species. Apsilochorema egg masses were sheared from the substratum in fast flows, but hatched with 100% success in favoured slow flows. The threshold velocity for Apsilochorema was 0.6–0.7 ms−1, well beyond the natural oviposition range of up to 0.3 ms−1. Ulmerochorema eggs hatched in all flows, suggesting that flow‐related mortality of the egg stage is unimportant for this species. Oviposition in fast flows might enhance the fitness of Ulmerochorema larvae or ovipositing females instead. Vulnerability to shear forces appears to explain why Apsilochorema lay eggs exclusively in slow flows. Shear may be a common cause of mortality for lotic insect eggs, and unseasonal spates or regulated flows may significantly affect recruitment of larvae. Selective oviposition affects the spatial distribution and survival of eggs and thus affects larval supply, and these supply dynamics are under‐studied in stream ecology.

Summary Egg predation is seldom considered in life‐history studies of freshwater insects, but could be an important source of mortality with potential to limit population numbers. Costs of egg predation to prey can be considered at two levels: (i) fitness costs to individuals via reduced reproductive output; and (ii) population costs via reduced recruitment of benthic larvae. Larvae of Orthotrichia armata (Trichoptera: Hydroptilidae) feed on the egg masses of caddisflies in the Little River, central Victoria, Australia, but the small size of O. armata suggests that they may be unable to penetrate egg masses that are covered with a thick layer of spumaline jelly. We predicted that predation by O. armata would be more severe for egg masses with thinner spumaline. With field surveys throughout the peak caddisfly oviposition period (summer), we compared predation by O. armata on the egg masses of nine caddisfly taxa. Egg masses were categorised, according to the thicknesses of spumaline, into bulbous (thickly jellied), thinly jellied and jelly‐free morphotypes, with three species per morphotype. Bulbous egg masses were rarely consumed. Thinly jellied egg masses comprised just 4–13% of all egg masses in each survey, but attracted 52–93% of all egg predators. Jelly‐free egg masses were readily consumed, but O. armata were disproportionately scarce on them. In a second survey, we compared the proportions of eggs that were consumed from egg masses with small and large clutch sizes, by monitoring the thinly jellied egg masses of Taschorema sp. (598 ± 95 eggs per mass) and Ethochorema turbidum (1195 ± 192 eggs per mass) to eclosion. More than 25% of Taschorema sp. egg masses were entirely consumed, causing complete reproductive failure of parent females (fitness costs to individuals). Consumption rates were < 25% for most egg masses of E. turbidum. Orthotrichia armata consumed 48% of all Taschorema sp. eggs and 20% of E. turbidum eggs, suggesting that egg predation may substantially curb larval recruitment (costs to populations), particularly for species with small clutch sizes. Our results demonstrate that egg mortality varies for aquatic insect species that package their eggs in different ways. Combined with recent data from the same system, our results suggest egg mass morphology–mortality trade‐offs, with bulbous egg masses particularly vulnerable to flow forces (shear) and thinly jellied/jelly‐free egg masses particularly vulnerable to predation. Given the wide array of aquatic invertebrate species that lay single egg masses on hard substrata, it is feasible that this is a common trade‐off that warrants more research on the impacts of egg predation on aquatic species.

Detrital inputs to ecosystems provide potential food sources and can produce trophic cascades, but this effect is influenced by whether species specialise in consuming or inhabiting accumulations of detritus. To test whether species are differentially associated with leaves or sand, we compared densities of stream invertebrate species in patches of leaves and bare sand in two sandy-bed creeks in south-eastern Australia, in summer and spring. We also assessed the quality of information on diet and substrate association in the literature. Most species showed no density differences between leaf and sand patches (‘microhabitat generalists’), but categorisation as generalists, leaf or sand species differed between datasets. We developed a method for identifying important effect sizes; power analyses showed that many species were true generalists, but many non-significant results were potentially Type II errors. The literature provided information that was broadly consistent with our data, but few studies publish reliable information about either diet or patch use. Our results support a contention that few Australian stream invertebrates are obligate shredders, and this may also be true for streams elsewhere. Predicting and detecting the responses of such generalist taxa to detrital inputs will be very challenging.

Understanding what regulates population sizes of organisms with complex life cycles is challenging because limits on population sizes can occur at any stage or transition. We extend a conceptual framework to explore whether numbers of successfully laid eggs determine densities of later stages in insects, fish, amphibians, and snails inhabiting marine, freshwater, or terrestrial habitats. Our review suggests novel hypotheses, which propose characteristics of species or environments that create spatial variation in egg densities and predict when such patterns are maintained throughout subsequent life-cycle stages. Existing data, although limited, suggest that persistent, strong associations between egg and subsequent juvenile densities are likely for species where suitable egg-laying habitat is in short supply. Those associations are weakened in some environments and for some species by density-dependent losses of eggs or hatchlings. Such cross-ecosystem comparisons are fundamental to generality in ecology but demand place-based understandings of species’ biology and natural history. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 52 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.