Rising temperatures are amplifying drought-induced stress and mortality in forests globally. It remains uncertain, however, whether tree mortality across drought-stricken landscapes will be concentrated in particular climatic and competitive environments. We investigated the effects of long-term average climate [i.e. 35-year mean annual climatic water deficit (CWD)] and competition (i.e. tree basal area) on tree mortality patterns, using extensive aerial mortality surveys conducted throughout the forests of California during a 4-year statewide extreme drought lasting from 2012 to 2015. During this period, tree mortality increased by an order of magnitude, typically from tens to hundreds of dead trees per km , rising dramatically during the fourth year of drought. Mortality rates increased independently with average CWD and with basal area, and they increased disproportionately in areas that were both dry and dense. These results can assist forest managers and policy-makers in identifying the most drought-vulnerable forests across broad geographic areas.

2. Two fully-factorial mesocosm experiments (one for each crayfish pair) were used to investigate crayfish diet, and their impact on benthic invertebrate community structure, benthic algal standing stock and leaf litter decomposition rates, in allopatric and sympatric populations, compared to a crayfish free control. We used stable isotope analysis to examine crayfish diet in the mesocosms and in allopatric populations of each species in the Thames catchment.3. Isotopic niche width did not vary significantly between allopatric and sympatric populations of crayfish pairs in the mesocosms and isotopic niche partitioning in all the wild populations suggests the invaders can coexist.4. All four species altered benthic invertebrate community structure but with differing functional effects, often mediated via trophic cascades. Red swamp crayfish predation upon snails evidently promoted benthic algal standing stock via reduction in grazing pressure. However, a trophic cascade whereby the crayfish consumed native invertebrate shredders, causing a reduction in net leaf litter decomposition, was decoupled by red swamp and signal crayfish since they consumed leaf litter directly and thus moderated the cascade to a trickle when in sympatry with Turkish or virile crayfish, respectively. 6. Our results suggest that the combined effect of multiple invasions on the ecosystem can reflect either an additive effect of their independent impacts, or an amplified effect, which is greater than the sum of their independent impacts. A lack of general pattern in their effects makes any potential management strategy more complex.

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1. Monitoring is crucial for the conservation of endangered crayfish species as well as for managing and controlling invasive species. Yet, relatively few studies have assessed relationships between native white-clawed crayfish Austropotamobius pallipes and non-native signal Pacifastacus leniusculus, narrow-clawed Astacus leptodactylus, virile Orconectes virilis and red-swamp Procambarus clarkii crayfishes in freshwater environments at ecologically relevant spatial and temporal scales.2. Thus, the present study examines the distribution dynamics of native and non-native crayfishes in the River Thames Basin District (16 133 km 2 , south-eastern England) over four decades .3. To reveal geographical bioinvasion and decline patterns, data on occurrences (sampling site, date) of native white-clawed and four non-native crayfishes were used to create sequences of decade maps for each species. Cumulative numbers of occupied grid squares were used to construct 'invasion/decline' curves and rates. To quantify dynamic features, the frequencies of survey occurrence for each species were subjected to time-series analysis, followed by cross-correlation to determine relationships between crayfishes.4. White-clawed crayfish was initially widespread, but gradually disappeared after the mid-1970s, coinciding with the introduction and initial dispersal of signal crayfish. During 2001-2010 white-clawed crayfish numbers declined steeply. Introduced in the mid-1980s, narrow-clawed crayfish spread within a few years, but is now restricted to the east of the basin, where virile and red-swamp crayfishes were reported with low presence since their introductions in 2006. Dynamics of white-clawed and signal crayfishes showed the opposing trends with a highly significant negative association.5. Geographical patterns indicate that non-native crayfishes were introduced in multiple locations in the study area. Recovery of native crayfish seems unlikely, given the impacts of signal crayfish and the continuing arrival of new crayfish species. This paper demonstrates the utility of using broad temporal and spatial scales for revealing accurate insights on invasion and decline dynamics.

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