Effects of seed size and emergence time on tree seedling establishment: importance of developmental constraints

Arafura filesnakes (Acrochordus arafurae) are large (up to 2.5 m, 5 kg) aquatic nonvenomous snakes that feed entirely on fishes. A 10-year field study in the Australian wet-dry tropics revealed strong correlations between rainfall patterns, fish abundance, and snake population dynamics. All of these characteristics showed considerable annual variation. High rainfall late in the wet season (February–March) caused prolonged inundation of the floodplain. Following such years, dry-season sampling revealed that fishes were abundant, filesnakes were in good body condition, and a high proportion of adult female filesnakes were reproductive. Annual variation in recruitment to the population (as judged by the relative abundance of yearling snakes) was also correlated with fish abundance and thus, with rainfall patterns in the late-wet season. Our results fit well with those from other studies on a diverse array of aquatic and terrestrial species within the wet-dry tropics. Annual variation in rainfall patterns, via its effects on prey abundance, may drive the population dynamics of many tropical predators. Received: 28 June 1999 / Accepted: 29 February 2000     

Rainfall-Driven Amplification of Seasonal Acidification in Poorly Buffered Tropical Streams

Acidification in freshwater ecosystems has important ecological and biogeochemical effects. Temperate streams affected by anthropogenic acidification have been extensively studied, but our understanding of natural acidification in tropical streams has been constrained by the lack of long-term datasets. Here, we analyze 14?years of monthly observations from 13 sampling stations in eight tropical streams in lowland Costa Rica. Stream pH increased during the 4-month dry season and declined throughout the wet season. The magnitude of the seasonal pH decline was greatest following the driest dry seasons, including the historically large El Ni?o Southern Oscillation event in 1998 when pH values dropped below 4.0 in some streams. Dissolved CO₂ accounts for the low baseline pH in the poorly buffered study streams, and we hypothesize that an influx of soil-derived CO₂ via subsurface flow paths contributes to the observed seasonal pH declines. Our results show tight coupling between rainfall, terrestrial, and aquatic ecosystems in the tropics. Predicted decreases in dry season rainfall for the tropics may lead to an increased magnitude of seasonal acidification.     

Fire weather in the wet-dry tropics of the World Heritage Kakadu National Park,Australia

Abstract Seasonal changes of weather and fuels in the wet-dry tropics are dramatic; fires follow suit. In this paper, we examine quantitatively rainfall, evaporation, wind, temperature and humidity information, and indices derived from them, for Kapalga Research Station and nearby Jabiru in World Heritage Kakadu National Park, Northern Australia. At Kapalga, the average annual rainfall of about 1200mm mostly falls during a 6 month wet season. Grasses, green in the wet, begin to desiccate during the early dry season. Perennial grasses cure more slowly than the annuals, and grasses in drainages cure later than those on ridges. Fire weather is usually most severe in September-October (late dry season) and least severe in January-February (late wet season). As the dry season progresses to its peak, daily wind patterns change, daily maximum temperatures increase to an average of 36°C, dew points drop to a minimum, and soil moisture is severely depleted. In the early dry season (cf. later), fires have a greater tendency to go out at night compared with later perhaps because winds then are calmer, fuels are more discontinuous, and relights from burning logs are less likely to occur. Fire weather in the north of Australia appears less severe than that in the southeast of the continent where socially disastrous fires occur periodically.     

Adapting to the unpredictable: reproductive biology of vertebrates in the Australian wet-dry tropics

In the wet-dry tropics of northern Australia, temperatures are high and stable year-round but monsoonal rainfall is highly seasonal and variable both annually and spatially. Many features of reproduction in vertebrates of this region may be adaptations to dealing with this unpredictable variation in precipitation, notably by (i) using direct proximate (rainfall-affected) cues to synchronize the timing and extent of breeding with rainfall events, (ii) placing the eggs or offspring in conditions where they will be buffered from rainfall extremes, and (iii) evolving developmental plasticity, such that the timing and trajectory of embryonic differentiation flexibly respond to local conditions. For example, organisms as diverse as snakes (Liasis fuscus, Acrochordus arafurae), crocodiles (Crocodylus porosus), birds (Anseranas semipalmata) and wallabies (Macropus agilis) show extreme annual variation in reproductive rates, linked to stochastic variation in wet season rainfall. The seasonal timing of initiation and cessation of breeding in snakes (Tropidonophis mairii) and rats (Rattus colletti) also varies among years, depending upon precipitation. An alternative adaptive route is to buffer the effects of rainfall variability on offspring by parental care (including viviparity) or by judicious selection of nest sites in oviparous taxa without parental care. A third type of adaptive response involves flexible embryonic responses (including embryonic diapause, facultative hatching and temperature-dependent sex determination) to incubation conditions, as seen in squamates, crocodilians and turtles. Such flexibility fine-tunes developmental rates and trajectories to conditions--especially, rainfall patterns--that are not predictable at the time of oviposition.     

The Effects of Interannual Rainfall Variability on Tree–Grass Composition Along Kalahari Rainfall Gradient

Precipitation variability has been predicted to increase in a global warmer climate, and is expected to greatly affect plant growth, interspecies interactions, plant community composition, and other ecosystem processes. Although previous studies have investigated the effect of intra-annual rainfall variability on plant growth and ecosystem dynamics, the impacts of interannual rainfall variability remain understudied. This paper uses satellite data and develops a new mechanistic model to investigate the response of tree–grass composition to increasing interannual rainfall variability in arid to sub-humid ecosystems along the Kalahari Transect in Southern Africa. Both satellite data and model results show that increasing interannual rainfall fluctuations favor deep-rooted trees over shallow-rooted grasses in drier environments (that is, mean annual rainfall, MAP < 900–1000 mm) but favor grasses over trees in wetter environments (that is, MAP > 900–1000 mm). Trees have a competitive advantage over grasses in dry environments because their generally deeper root systems allow them to have exclusive access to the increased deep soil water resources expected to occur in wet years as a result of the stronger interannual rainfall fluctuations. In relatively wet environments, grasses are favored because of their high growth rate that allows them to take advantage of the window of opportunity existing in years with above average precipitation and thus increase fire-induced tree mortality. Thus, under increasing interannual rainfall fluctuations both direct effects on soil water availability and indirect effects mediated by tree–grass interactions and fire dynamics are expected to play an important role in determining changes in plant community composition.     

Patterns of tree growth in relation to environmental variability in the tropical dry deciduous forest at Mudumalai, southern India

Tree diameter growth is sensitive to environmental fluctuations and tropical dry forests experience high seasonal and inter-annual environmental variation. Tree growth rates in a large permanent plot at Mudumalai, southern India, were examined for the influences of rainfall and three intrinsic factors (size, species and growth form) during three 4-year intervals over the period 1988–2000. Most trees had lowest growth during the second interval when rainfall was lowest, and skewness and kurtosis of growth distributions were reduced during this interval. Tree diameter generally explained <10% of growth variation and had less influence on growth than species identity or time interval. Intraspecific variation was high, yet species identity accounted for up to 16% of growth variation in the community. There were no consistent differences between canopy and understory tree growth rates; however, a few subgroups of species may potentially represent canopy and understory growth guilds. Environmentally-induced temporal variations in growth generally did not reduce the odds of subsequent survival. Growth rates appear to be strongly influenced by species identity and environmental variability in the Mudumalai dry forest. Understanding and predicting vegetation dynamics in the dry tropics thus also requires information on temporal variability in local climate.     

Breeding cues in a wetland‐dependent Australian passerine of the seasonally wet‐dry tropics

The Capricorn yellow chat Epthianura crocea macgregori (Aves: Meliphagidae) occurs in the seasonal wet‐dry tropics. This region, although coastal, is typified by highly variable annual rainfall. The Capricorn yellow chat breeds in wetlands, predominantly in the summer–autumn period, but has the capacity to breed in response to out‐of‐season rainfall events, consistent with an opportunistically breeding species. Most studies of breeding cues in passerines have been on species centred on temperate climates with predictable rainfall season, arid biomes with a highly variable rainfall quantity and season, and the relatively non‐seasonal wet tropics. This study was focused on a species that occurs in an intermediate situation with a highly variable but summer dominant rainfall season. It aimed to identify which proximal cues are used by birds in such environments to prepare for breeding. Monthly observations at a breeding ground over a 45‐month period were regressed against environmental and climatic variables. There was a significant positive relationship of chat abundance with average minimum monthly air temperature and the extent of inundation. Invertebrate food availability was also sampled. Cross‐correlation with prior monthly rainfall showed that abundance of insects (Diptera, Hemiptera and Lepidoptera) and semi‐aquatic invertebrates peaked 1–2 months following large rainfall events, coinciding with peaks in presence of dependent young of Capricorn yellow chats. Thus, the Capricorn yellow chat matches the model for arid‐adapted birds in which seasonal cues (e.g. increasing day‐length or temperature) in spring lead to breeding preparedness, but breeding only occurs in response to proximal factors such as rainfall. However, the Capricorn yellow chat differs in that breeding is delayed until rainfall is sufficient to inundate its wetland habitat and stimulate the production of food resources associated with the low vegetation and muddy margins of the temporarily flooded pools and channels; suggesting that inundation may be the most important breeding cue.     

Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long‐standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above‐ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32‐year precipitation–ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.     

Variation in the composition and structure of tropical savannas as a function of rainfall and soil texture along a large-scale climatic gradient in the Northern Territory, Australia

Abstract. Variation in structural and compositional attributes of tropical savannas are described in relation to variation in annual rainfall and soil texture along a subcontinental-scale gradient of rainfall in the wet-dry tropics of the Northern Territory, Australia. Rainfall varies along the gradient from over 1500 mm p.a. in the Darwin region ( c . 12° S) to less than 500 mm in the Tennant Creek region ( c . 18° S). Soils are patchy, and sands, loams and clays may occur in all major districts within the region. We utilized a large data set (1657 quadrats ° 291 woody species; with numerous measured and derived sample variables) covering an area of 0.5 million km². Correlations between floristic composition of woody species and environmental variables were assessed using DCA ordination and vector fitting of environmental variables. Vectors of annual rainfall and soil texture were highly correlated with variation in species composition. Multiple regression analyses incorporating linear and quadratic components of mean annual rainfall and topsoil clay content were performed on three structural attributes (tree height, tree cover, tree basal area) and two compositional attributes (woody species richness, deciduous tree species richness). Tree height declined with decreasing rainfall; cover, basal area, woody species richness and deciduous species richness all declined with decreasing rainfall and increasing soil clay content. Regression models accounted for between 17% and 45% of the variation in the data sets. Variation in other factors such as soil depth, landscape position and recent land-use practices (for which there were no data on an individual quadrat basis) are likely to have contributed to the large residual variation in the data set.     

Long-term trends in fruit production in a tropical forest at Ngogo,Kibale National Park,Uganda

Fruit production in tropical forests varies considerably in space and time, with important implications for frugivorous consumers. Characterizing temporal variation in forest productivity is thus critical for understanding adaptations of tropical forest frugivores, yet long-term phenology data from the tropics, in particular from African forests, are still scarce. Similarly, as the abiotic factors driving phenology in the tropics are predicted to change with a warming climate, studies documenting the relationship between climatic variables and fruit production are increasingly important. Here, we present data from 19 years of monitoring the phenology of 20 tree species at Ngogo in Kibale National Park, Uganda. Our aims were to characterize short- and long-term trends in productivity and to understand the abiotic factors driving temporal variability in fruit production. Short-term (month-to-month) variability in fruiting was relatively low at Ngogo, and overall fruit production increased significantly through the first half of the study. Among the abiotic variables, we expected to influence phenology patterns (including rainfall, solar irradiance, and average temperature), only average temperature was a significant predictor of monthly fruit production. We discuss these findings as they relate to the resource base of the frugivorous vertebrate community inhabiting Ngogo.     

Two climate‐sensitive tree‐ring chronologies from Arnhem Land,monsoonal Australia

The ecology of the Australian monsoon tropics is fundamentally shaped by dry conditions between May and October followed by highly variable rainfall over the months of November to April. Due to its crucial ecological importance, a better understanding of past hydroclimate variability in the region is of great interest to land managers and custodians in this region. Short instrumental records also make highly resolved terrestrial palaeoclimate records for northern Australia prior to 1900 CE of considerable scientific importance. Here, we present two new well‐replicated Callitris intratropica ring‐width chronologies from Arnhem Land in northern Australia, one of which extends the tree‐ring record in the region by another 86 years, back to 1761. Both chronologies have clearly defined regional patterns of correlations with temperature, precipitation, potential evapotranspiration and two drought indices (the self‐calibrating Palmer Drought Severity Index (PDSI) and the Standardised Precipitation Evapotranspiration Index (SPEI)) across the lower latitudes of the Northern Territory. Results indicate considerable scope for hydroclimatic reconstructions based on C. intratropica for transitional periods into and out of the wettest time of the year. This suggests that such reconstructions would reflect variability in the duration of the wet period. While precipitation or streamflow reconstructions may be possible for both these transitional periods, drought reconstructions will be best focused on the months of March–May at the end of the wet period. Hydroclimate reconstructions would provide important baseline information for understanding the rate and magnitude of current regional climate change for these ecologically and culturally important transitional periods.     

Convergence in growth responses of tropical trees to climate driven by water stress

Widely documented for temperate and cold forests in both hemispheres, variations in tree growth responses to climate along environmental gradients have rarely been investigated in the tropics. Seven tree‐ring chronologies of Centrolobium microchaete (Fabaceae) in the Cerrado tropical forests of Bolivia are used to determine the growth responses to climate along a precipitation gradient. Chronologies are distributed from the humid Guarayos forests (annual precipitation > 1600 mm) in the transition to the Amazonia to the dry‐mesic Chiquitos forests (annual precipitation < 1200 mm) in the proximity to the dry Chaco. On a large spatial scale, radial growth is positively influenced by rainfall and negatively by temperature at the end of the dry season. However, this regional pattern in climate‐tree growth relationship shows differences along the precipitation gradient. Relationships with climate are highly significant and extend over longer periods of the year in sites with low rainfall and extremely severe dry seasons. At wet sites, larger water soil capacity and endogenous forest dynamics partially mask the direct influence of climate on tree growth. Stronger similarities in tree‐growth responses to climate occur between sites in the dry Central Chiquitos and in the transition to the Guarayos forests. In contrast, the relationships show fewer similarities between sites in the humid Guarayos. We conclude that growth responses to climate in the tropics are more similar between sites with limited rainfall and severe and prolonged dry seasons. Our study points to a convergence in the patterns of growth responses of tropical trees to climate, modulated by scarce rainfall and marked seasonality. The negative impact of water deficits on tree physiological processes induces not only the documented reduction in forest species richness, but also a convergence in tree‐growth responses to climate in dry tropical forests.     

The effect of soil on the growth performance of tropical species with contrasting distributions

Within the tropics, a marked gradient in rainfall between dry and wet forests correlates with a well documented turnover of plant species. While water availability along these gradients is an important determinant of species distributions, other abiotic and biotic factors correlate with rainfall and may also contribute to limit species distribution. One of these is soil fertility, which is often lower in the wetter forests. To test its possible role in species distribution along a rainfall gradient, we performed a screen‐house experiment where we measured the growth performance of seedlings of 23 species with contrasting distributions across the Isthmus of Panama. We grew seedlings in soils collected from the drier Pacific side and the wetter Atlantic side. Differences in soil fertility across the Isthmus were large enough to significantly influence the growth performance of the seedlings. However, we found no evidence of home‐soil advantage among species with contrasting distributions. Dry‐distribution species grew on average slower than wet‐distribution species suggesting a cost to drought adaptations. The response to soil differences correlated with the growth rate of the species, such that fast‐growing species responded more to changes in soil quality. We hypothesize that inherently slow growth rates of some dry‐distribution tropical species may be a more important factor limiting their colonization of wetter sites along the rainfall gradient.     

Climate‐driven fluctuations in surface‐water availability and the buffering role of artificial pumping in an African savanna: Potential implication for herbivore dynamics

Abstract In arid and semiarid environments surface‐water strongly constrains the distribution and abundance of large herbivores during the dry season. Surprisingly, we know very little about its variability in natural ecosystems. Here we used long‐term data on the dry‐season occurrence of water at individual waterholes to model the surface‐water availability across Hwange National Park, Zimbabwe, under contrasted climatic and management scenarios. Without artificial pumping only 19.6% of the park occurred within 5 km of water under average climatic conditions. However surface‐water availability was strongly influenced by annual rainfall, and over 20 years the variability of the surface area of the park occurring within 5 km of water was slightly larger than the variability of rainfall. This contrasts with the usual buffered response of vegetation production to rainfall fluctuations, and suggests that the variability in dry‐season foraging range determined by surface‐water availability could be the main mechanism regulating the population dynamics of large herbivores in this environment. Artificial pumping increased surface‐water availability and reduced its variability over time. Because changes in surface‐water availability could cause the greatest changes in forage availability for large herbivores, we urge ecologists to investigate and report on the variability of surface‐water in natural ecosystems, particularly where rapid climate changes are expected.     

Population demography of frillneck lizards (Chlamydosaurus kingii,Gray 1825) in the wet‐dry tropics of Australia

We explore the effects of biotic and abiotic factors on the population demography of frillneck lizards (Chlamydosaurus kingii) in the Australian wet‐dry tropics. Annual growth rates of males were significantly higher across all body sizes compared to females, resulting in a significant larger maximum body size in males. Both male and female lizards were highly philopatric and 81% of the among‐year recapture distances were less than 200 m. Juvenile and adult frillnecks were subjected to low but highly variable annual survival rates. Both proportion of juveniles and relative proportion of reproductive females showed extensive among‐year variations. No relationship was, however, observed between proportion of gravid females and that of juveniles captured during the subsequent year. High rainfall in January was negatively correlated with recruitment most likely caused by increased egg/embryo mortality due to flooding of nest sites. We therefore suggest that the lack of association between female reproduction and juvenile recruitment was due to the effects of stochastic variation in January rainfall. Lizard numbers increased during the first five years of the study followed by a decline during the subsequent four years. Our analyses show that annual variation in survival constituted the main determinant in driving the annual change in frillneck numbers. Surprisingly, no relationship was observed between fillneck population dynamics and annual variation in juvenile recruitment. We suggest that the 7‐years over which these analyses were conducted were insufficient to detect any significant effects of recruitment on lizard numbers, demonstrating the need for long‐term studies to accurately document vertebrate population demographic processes in areas experiencing stochastic variations rainfall such as the Australian wet‐dry tropics.     

Dynamics of habitats and macroinvertebrate assemblages in rivers of the Australian dry tropics

1. The dry tropics are characterised by episodic summer rainfall such that the majority of annual river flow occurs in a short period of time. This dryland hydrological cycle leads to variably connected channels and waterholes along the length of a river bed. 2. We investigated the seasonal changes in biophysical characteristics and macroinvertebrate assemblage composition in dry‐tropics rivers at 15 sites on four rivers, each sampled five times (representing one annual hydrological cycle), in the Burdekin catchment, north Queensland, Australia. 3. Assemblages and their temporal trajectories differed among seasons, sites and habitats, even within the same habitat and/or river. Wet season flooding did not appear to ‘reset’ assemblages, with post‐wet season assemblages differing between years. 4. We found no consistent pattern in taxonomic richness over time, and sites within rivers showed no consistent convergence or divergence (i.e. turnover) in macroinvertebrate assemblage composition. However, biophysical variables associated with the rigours of the late dry season had significant effects on macroinvertebrate assemblages, highlighting the variable and often harsh conditions of dry‐tropics rivers. Underlying these patterns were different resistance and resilience traits of invertebrates (such as colonisation and establishment abilities), as well as the local‐scale effects of biophysical variables. 5. The dynamic nature of dryland rivers presents major challenges to monitoring programmes, and our results suggest a more complex scenario for monitoring and management than previously described.     

Condensation onto the skin as a means for water gain by tree frogs in tropical Australia

Green tree frogs, Litoria caerulea, in the wet-dry tropics of northern Australia remain active during the dry season with apparently no available water and temperatures that approach their lower critical temperature. We hypothesized that this surprising activity might be because frogs that are cooled during nighttime activity gain water from condensation by returning to a warm, humid tree hollow. We measured the mass gained when a cool frog moved into either a natural or an artificial hollow. In both hollows, water condensed on cool L. caerulea, resulting in water gains of up to 0.93% of body mass. We estimated that the water gained was more than the water that would be lost to evaporation during activity. The use of condensation as a means for water gain may be a significant source of water uptake for species like L. caerulea that occur in areas where free water is unavailable over extended periods.     

Effects of Drought,Pest Pressure and Light Availability on Seedling Establishment and Growth: Their Role for Distribution of Tree Species across a Tropical Rainfall Gradient

Tree species distributions associated with rainfall are among the most prominent patterns in tropical forests. Understanding the mechanisms shaping these patterns is important to project impacts of global climate change on tree distributions and diversity in the tropics. Beside direct effects of water availability, additional factors co-varying with rainfall have been hypothesized to play an important role, including pest pressure and light availability. While low water availability is expected to exclude drought-intolerant wet forest species from drier forests (physiological tolerance hypothesis), high pest pressure or low light availability are hypothesized to exclude dry forest species from wetter forests (pest pressure gradient and light availability hypothesis, respectively). To test these hypotheses at the seed-to-seedling transition, the potentially most critical stage for species discrimination, we conducted a reciprocal transplant experiment combined with a pest exclosure treatment at a wet and a dry forest site in Panama with seeds of 26 species with contrasting origin. Establishment success after one year did not reflect species distribution patterns. However, in the wet forest, wet origin species had a home advantage over dry forest species through higher growth rates. At the same time, drought limited survival of wet origin species in the dry forest, supporting the physiological tolerance hypothesis. Together these processes sort species over longer time frames, and exclude species outside their respective home range. Although we found pronounced effects of pests and some effects of light availability on the seedlings, they did not corroborate the pest pressure nor light availability hypotheses at the seed-to-seedling transition. Our results underline that changes in water availability due to climate change will have direct consequences on tree regeneration and distributions along tropical rainfall gradients, while indirect effects of light and pests are less important.     

Rainfall, ambient temperature, and Clethrionomys californicus capture frequency

Sixty Western Red-backed Voles Clethrionomys californicus inhabiting an old-growth mixed conifer forest in north-western California (USA) were studied over 2 years to assess relationships among vole capture frequency, rainfall, and ambient temperature. Red-backed Voles were encountered more often and recapture rates per vole doubled with the tripling of yearly rainfall. In addition, more voles were captured during wet months than dry months. Analysed over weekly intervals, captures were positively correlated with the amount of weekly rainfall, a greater proportion of the known vole population was encountered during wet weeks than dry weeks, and increased weekly rainfall resulted in an increased proportion of the vole population captured. Significant relationships were not found between daily rainfall and vole captures, but both minimum and maximum daily ambient temperatures were lower when voles were trapped. Vole captures increased with decreases in daily and weekly minimum and maximum ambient temperatures. Weekly rainfall in combination with minimum weekly temperatures accounted for the greatest explanation of variability in vole capture frequencies. These results document that rainfall and ambient temperature have an impact on Western Red-backed Vole capture frequency; thus these weather variables should be addressed as covariates in analyses focused on capture data for this species.     

Anthropogenic subsidies mitigate environmental variability for insular rodents

The exogenous input of nutrients and energy into island systems fuels a large array of consumers and drives bottom-up trophic cascades in island communities. The input of anthropogenic resources has increased on islands and particularly supplemented non-native consumers with extra resources. We test the hypothesis that the anthropogenic establishments of super-abundant gulls and invasive iceplants Carpobrotus spp. have both altered the dynamics of an introduced black rat Rattus rattus population. On Bagaud Island, two habitats have been substantially modified by the anthropogenic subsidies of gulls and iceplants, in contrast to the native Mediterranean scrubland with no anthropogenic inputs. Rats were trapped in all three habitats over two contrasting years of rainfall patterns to investigate: (1) the effect of anthropogenic subsidies on rat density, age-ratio and growth rates, and (2) the role of rainfall variability in modulating the effects of subsidies between years. We found that the growth rates of rats dwelling in the non-subsidized habitat varied with environmental fluctuation, whereas rats dwelling in the gull colony maintained high growth rates during both dry and rainy years. The presence of anthropogenic subsidies apparently mitigated environmental stress. Age ratio and rat density varied significantly and predictably among years, seasons, and habitats. While rat densities always peaked higher in the gull colony, especially after rat breeding in spring, higher captures of immature rats were recorded during the second year in all habitats, associated with higher rainfall. The potential for non-native rats to benefit from anthropogenic resources has important implications for the management of similar species on islands.