Pockets of resistance: Response of arid‐land fish communities to climate,hydrology, and wildfire

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Disturbance regime and limits on benefits of refuge use for fishes in a fluctuating hydroscape

Refuge habitats increase survival rate and recovery time of populations experiencing environmental disturbance, but limits on the ability of refuges to buffer communities are poorly understood. We hypothesized that importance of refuges in preventing population declines and alteration in community structure has a non‐linear relationship with severity of disturbance. In the Florida Everglades, alligator ponds are used as refuge habitat by fishes during seasonal drying of marsh habitats. Using an 11‐year record of hydrological conditions and fish abundance in 10 marshes and 34 alligator ponds from two regions of the Everglades, we sought to characterize patterns of refuge use and temporal dynamics of fish abundance and community structure across changing intensity, duration, and frequency of drought disturbance. Abundance in alligator ponds was positively related to refuge size, distance from alternative refugia (e.g. canals), and abundance in surrounding marsh prior to hydrologic disturbance. Variables negatively related to abundance in alligator ponds included water level in surrounding marsh and abundance of disturbance‐tolerant species. Refuge community structure did not differ between regions because the same subset of species in both regions used alligator ponds during droughts. When time between disturbances was short, fish abundance declined in marshes, and in the region with the most spatially extensive pattern of disturbance, community structure was altered in both marshes and alligator ponds because of an increased proportion of species more resistant to disturbance. These changes in community structure were associated with increases in both duration and frequency of hydrologic disturbance. Use of refuge habitat had a modal relationship with severity of disturbance regime. Spatial patterns of response suggest that decline in refuge use was because of decreased effectiveness of refuge habitat in reducing mortality and providing sufficient time for recovery for fish communities experiencing reduced time between disturbance events.     

Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

1. Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.
2. Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in-stream predators such as fishes.
3. We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.
4. Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.
5. There was an increase over time in algal biomass (chlorophyll-a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.
6. Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

     

Stream community structure in relation to spatial and temporal variation: a habitat templet study of two contrasting New Zealand streams

• 1 The physical characteristics of two contrasting streams, and habitat types within these streams, are described in terms of a two-dimensional physical habitat templet in which disturbance frequency and the availability of spatial refugia are the temporal and spatial axes.
• 2 It is predicted that habitats experiencing a high disturbance frequency and low refuge availability will be characterized by a low invertebrate species diversity, a low biomass of epilithic algae and particulate organic matter and a community made up of mobile, weedy species. Habitats having a low disturbance frequency and high refuge availability will be characterized by a diverse community containing sedentary and specialist species, with high algal and particulate organic matter levels.
• 3 A lower median substrate particle size and higher shear stress regime in Timber Creek were indicative of a higher disturbance frequency than in the Kyeburn. Substrate diversity was lower in Timber Creek than in the Kyeburn and indicated that the availability of refugia was lower in Timber Creek. In both streams, pools were found to have a higher disturbance frequency and lower availability of refugia than riffles.
• 4 Invertebrate species diversity, the biomass of epilithic algae and particulate organic matter and the representation of sedentary species, filter feeders and shredders were higher in the more temporally stable and spatially heterogeneous Kyeburn. The community of Timber Creek, frequently disturbed and having low refuge availability, had a high proportion of mobile and weedy species, with the highly mobile, generalist-feeding Deleatidium spp. (Ephemeroptera; Leptophlebiidae) being the most dominant organisms.
• 5 The predictions made about stream community structure and species characteristics in relation to disturbance frequency and the availability of spatial refugia are generally supported. Now a larger scale investigation is required to test the generality of the predictions. We conclude that the habitat templet approach offers a sound framework within which to pose questions in stream ecology.

     

Fish metacommunity responses to experimental drought are determined by habitat heterogeneity and connectivity

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Changes in the Spatial Distribution of Fishes in Lakes Along a Residential Development Gradient

As the human demand for freshwater natural resources such as fish and drinking water increases, we may rely more heavily on models to predict the response of aquatic ecosystems to natural and anthropogenic disturbance. Theses models in turn implicitly depend on the underlying spatial distribution of organisms. In terrestrial ecosystems, increased natural resource utilization has transformed habitat and changed the spatial distribution of organisms, with subsequent negative effects on biota. Recent studies in lakes demonstrate that human development of lakeshores alters the physical habitat and nutrient cycles. The impact of such disturbance by humans on the spatial distribution of aquatic organisms, however, remains unknown. Here we quantify the effect of lakeshore development on the spatial distribution of fishes in 23 lakes in the US Pacific Northwest. We found a significant decrease in the spatial aggregation of fishes with increased shoreline development by humans, reflecting a loss of refugia and resource heterogeneity that favors aggregation among fishes. We also found that lakes with a high perimeter–surface-area ratio and a relatively shallow littoral zone had much higher levels of fish aggregation, suggesting the importance of terrestrial inputs to lakes. Finally, we found a marginally significant decrease in fish spatial aggregation with increased total phosphorus concentration, but no effect of chlorophyll concentration, water transparency, the predator–prey ratio, or number of species on fish spatial distributions. These results suggest that anthropogenic modification of shorelines is significantly altering the spatial distribution of important aquatic organisms, and that these changes may have important implications for predictive modeling of ecosystem dynamics.     

Ecological effects of perturbation by drought in flowing waters

• 1 Knowledge of the ecology of droughts in flowing waters is scattered and fragmentary, with much of the available information being gathered opportunistically. Studies on intermittent and arid‐zone streams have provided most of the information.
• 2 Drought in streams may be viewed as a disturbance in which water inflow, river flow and water availability fall to extremely low levels for extended periods of time. As an ecological perturbation, there is the disturbance of drought and the responses of the biota to the drought.
• 3 Droughts can either be periodic, seasonal or supra‐seasonal events. The types of disturbance for seasonal droughts are presses and for supra‐seasonal droughts, ramps.
• 4 In droughts, hydrological connectivity is disrupted. Such disruption range from flow reduction to complete loss of surface water and connectivity. The longitudinal patterns along streams as to where flow ceases and drying up occurs differs between streams. Three patterns are outlined: ‘downstream drying’, ‘headwater drying’ and ‘mid‐reach drying’.
• 5 There are both direct and indirect effects of drought on stream ecosystems. Marked direct effects include loss of water, loss of habitat for aquatic organisms and loss of stream connectivity. Indirect effects include the deterioration of water quality, alteration of food resources, and changes in the strength and structure of interspecific interactions.
• 6 Droughts have marked effects on the densities and size‐ or age‐structure of populations, on community composition and diversity, and on ecosystem processes.
• 7 Organisms can resist the effects of drought by the use of refugia. Survival in refugia may strongly influence the capacity of the biota to recover from droughts once they break.
• 8 Recovery by biota varies markedly between seasonal and supra‐seasonal droughts. Faunal recovery from seasonal droughts follows predictable sequences, whilst recovery from supra‐seasonal droughts varies from one case to another and may be marked by dense populations of transient species and the depletion of biota that normally occur in the streams.
• 9 The restoration of streams must include the provision of drought refugia and the inclusion of drought in the long‐term flow regime.

     

Drought impacts and recovery are driven by local variation in species turnover

In forest systems, drought acts as a disturbance that can generate impacts on community structure and composition at multiple scales. This study focused on a 2-month drought event within an early successional forest system to determine the mechanism(s) of community response to, and recovery from, drought. Drought induced a 28% decline in neighborhood species richness and cover as a result of decreased colonization and increased extinction rates. Following drought, neighborhood richness quickly recovered via increased colonization rates while extinction rates were unaltered. Drought had little long-term effect on neighborhood structure (species richness and cover) and generated only subtle changes in neighborhood composition. Ruderal (annual and biennial) species were more likely to change (increase or decrease) in cover and frequency than the more stress tolerant perennial and woody species. However, population dynamics appeared to be generally driven by stochastic species turnover among fields and not by uniform shifts in species performance across the site. Although drought impacts and recovery appeared predictable at the neighborhood scale, population responses to drought within the site were rather unpredictable. Our findings suggest that stochastic fine-scale processes can generate predictable course-scale dynamics within a disturbed system. The scale-specific mechanisms of community change presented here should be explored in other systems to determine the extent of their generality in driving disturbance impacts on communities.     

Disturbance frequency and community structure in a twenty-five year intervention study

Models of community regulation commonly incorporate gradients of disturbance inversely related to the role of biotic interactions in regulating intermediate trophic levels. Higher trophic-level organisms are predicted to be more strongly limited by intermediate levels of disturbance than are the organisms they consume. We used a manipulation of the frequency of hydrological disturbance in an intervention analysis to examine its effects on small-fish communities in the Everglades, USA. From 1978 to 2002, we monitored fishes at one long-hydroperiod (average 350 days) and at one short-hydroperiod (average 259 days; monitoring started here in 1985) site. At a third site, managers intervened in 1985 to diminish the frequency and duration of marsh drying. By the late 1990s, the successional dynamics of density and relative abundance at the intervention site converged on those of the long-hydroperiod site. Community change was manifested over 3 to 5 years following a dry-down if a site remained inundated; the number of days since the most recent drying event and length of the preceding dry period were useful for predicting population dynamics. Community dissimilarity was positively correlated with the time since last dry. Community dynamics resulted from change in the relative abundance of three groups of species linked by life-history responses to drought. Drought frequency and intensity covaried in response to hydrological manipulation at the landscape scale; community-level successional dynamics converged on a relatively small range of species compositions when drought return-time extended beyond 4 years. The density of small fishes increased with diminution of drought frequency, consistent with disturbance-limited community structure; less-frequent drying than experienced in this study (i.e., longer return times) yields predator-dominated regulation of small-fish communities in some parts of the Everglades. Electronic Supplementary Material Supplementary material is available for this article at     

Drought and aquatic ecosystems: an introduction

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Assessing the net effect of anthropogenic disturbance on aquatic communities in wetlands: community structure relative to distance from canals

Anthropogenic alterations of natural hydrology are common in wetlands and often increase water permanence, converting ephemeral habitats into permanent ones. Since aquatic organisms segregate strongly along hydroperiod gradients, added water permanence caused by canals can dramatically change the structure of aquatic communities. We examined the impact of canals on the abundance and structure of wetland communities in South Florida, USA. We sampled fishes and macroinvertebrates from marsh transects originating at canals in the central and southern Everglades. Density of all aquatic organisms sampled increased in the immediate proximity of canals, but was accompanied by few compositional changes based on analysis of relative abundance. Large fish ( >8 cm), small fish ( <8 cm) and macroinvertebrates ( >5 mm) increased in density within 5 m of canals. This pattern was most pronounced in the dry season, suggesting that canals may serve as dry-down refugia. Increases in aquatic animal density closely matched gradients of phosphorus enrichment that decreased with distance from canals. Thus, the most apparent impact of canals on adjacent marsh communities was as conduits for nutrients that stimulated local productivity; any impact of their role as sources of increased sources of predators was not apparent. The effect of predation close to canals was overcompensated by increased secondary productivity and/or immigration toward areas adjacent to canals in the dry season. Alternatively, the consumptive effect of predatory fishes using canals as dry-season refuges is very small or spread over the expanse of marshes with open access to canals. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Extreme habitats are not refuges: poeciliids suffer from increased aerial predation risk in sulphidic southern Mexican habitats

Extreme environments are often considered a predation refuge for organisms living in them. In southern Mexico several species of poeciliid fishes are undergoing incipient speciation in a variety of extreme (i.e. permanently dark and/or sulphidic) freshwater systems, and previous research has demonstrated reproductive isolation between populations from sulphidic and adjacent benign habitats. In the present study, we investigated bird predation rates (measured as successful captures per minute) in two sulphidic surface and several benign surface habitats, to test the hypothesis that extreme habitats are predation refuges. We found capture rates to be approximately 20 times higher in sulphidic environments: probably facilitated by extremophile poeciliids spending most of their time at the water surface, where they engage in aquatic surface respiration as a direct response to hypoxia. Even birds that are usually not considered major fish predators regularly engage in fish predation in the toxic habitats of southern Mexico. Our results demonstrate that extreme environments do not necessarily represent a refuge from predation, and we discuss the general importance of predation in driving incipient speciation in these systems. Finally, we hypothesize that natural selection via avian predation may play an important role in maintaining reproductive isolation between divergent poeciliid populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 417–426.     

Strong zonation of benthic communities across a tidal freshwater height gradient

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Fish community responses to drying disturbances in an intermittent stream: a landscape perspective

1. Refugia are critical to the persistence of individuals, populations and communities in disturbed environments, yet few studies have considered how the position of refugia within the landscape interacts with the behavioural responses of component species to determine the influence of disturbance events on mobile animals. 2. An 18‐month quantitative electrofishing survey was undertaken on the Selwyn River, a stream that is intermittent in its middle reaches, to determine how the direction and distance to refugia affect the response of fish populations to drying, and how landscape context interacts with flow permanence to produce spatial patterns in communities. 3. Overall, the propensity of fish to take refuge in perennial reaches during drying episodes, and the rate and extent of recolonization from these refugia upon rewetting, depended upon the direction and distance to refugia and the behaviour of component species. 4. In the upper river, Canterbury galaxias (Galaxias vulgaris), upland bullies (Gobiomorphus breviceps) and brown trout (Salmo trutta) migrated upstream to permanent water as the stream dried from the bottom up, but frequent drying and slow recolonization by most species combined to produce a fish community in intermittent reaches that was quantitatively and qualitatively different to that in neighbouring perennial reaches. 5. In the lower river, fish did not appear to migrate downstream to permanent water as the stream dried from the top down, but a lower frequency of drying episodes and faster recolonization by upland bullies and eels (Anguilla spp.) from downstream refugia allowed the fish community in intermittent reaches to converge with that in neighbouring perennial reaches during prolonged wetted periods. 6. Longitudinal patterns of increasing fish density and species richness with flow permanence are interpreted as the product of species‐specific responses to drying events and the spatial position of refugia within the riverscape.     

Habitat enhancement and native fish conservation: can enhancement of channel complexity promote the coexistence of native and introduced fishes?

Native fishes worldwide have declined as a consequence of habitat loss and degradation and introduction of non-native species. In response to these declines, river restoration projects have been initiated to enhance habitat and remove introduced fishes; however, non-native fish removal is not always logistically feasible or socially acceptable. Consequently, managers often seek to enhance degraded habitat in such a way that native fishes can coexist with introduced species. We quantified dynamics of fish communities to three newly constructed side channels in the Provo River, Utah, USA, to determine if and how they promoted coexistence between native fishes (nine species) and non-native brown trout (Salmo trutta L.). Native and introduced fishes responded differently in each side channel as a function of the unique characteristics and histories of side channels. Beaver activity in two of the three side channels caused habitat differentiation or channel isolation that facilitated the establishment of native species. The third side channel had greater connectivity to and similar habitat as the main channel of the Provo River, resulting in a similar fish community to main channel habitats (i.e. dominated by brown trout with only a few native fish species). These results demonstrate the importance of understanding habitat preferences for each species in a community to guide habitat enhancement projects and the need to create refuge habitats for native fishes.     

Homogenization of fish assemblages in different lake depth strata at local and regional scales

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Direct and indirect impacts of wildfire on faunal communities of Mediterranean temporary ponds

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Life history plasticity does not confer resilience to environmental change in the mole salamander (<Emphasis Type="Italic">Ambystoma talpoideum)</Emphasis>

Plasticity in life history strategies can be advantageous for species that occupy spatially or temporally variable environments. We examined how phenotypic plasticity influences responses of the mole salamander, Ambystoma talpoideum, to disturbance events at the St. Marks National Wildlife Refuge (SMNWR), FL, USA from 2009 to 2014. We observed periods of extensive drought early in the study, in contrast to high rainfall and expansive flooding events in later years. Flooding facilitated colonization of predatory fishes to isolated wetlands across the refuge. We employed multistate occupancy models to determine how this natural experiment influenced the occurrence of aquatic larvae and paedomorphic adults and what implications this may have for the population. We found that, in terms of occurrence, responses to environmental variation differed between larvae and paedomorphs, but plasticity (i.e. the ability to metamorphose rather than remain in aquatic environment) was not sufficient to buffer populations from declining as a result of environmental perturbations. Drought and fish presence negatively influenced occurrence dynamics of larval and paedomorphic mole salamanders and, consequently, contributed to observed short-term declines of this species. Overall occurrence of larval salamanders decreased from 0.611 in 2009 to 0.075 in 2014 and paedomorph occurrence decreased from 0.311 in 2009 to 0.121 in 2014. Although variation in selection pressures has likely maintained this polyphenism previously, our results suggest that continued changes in environmental variability and the persistence of fish in isolated wetlands could lead to a loss of paedomorphosis in the SMNWR population and, ultimately, impact regional persistence in the future.     

Resistance to drought affects persistence of alternative regimes in shallow lakes of the Boreal Plains (Alberta,Canada)

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Zooplankton response to flooding of a drought refuge and implications for the endangered fish species Craterocephalus fluviatilis cohabiting with alien Gambusia holbrooki

Disruption to a river’s natural flow regime changes its ecological character, which becomes unfavourable for previously adapted biota. The zooplankton particularly are affected, and survival of larval and juvenile fish is largely determined by their availability. Alien fishes can also impact on recruitment in native fishes, sometimes through competition. In this regard, the invasive eastern Gambusia Gambusia holbrooki is linked to the decline of several fish species. It can have a substantial influence in shaping plankton communities, which implies that it competes with native fish that rely on the microfauna. The effects of river regulation and over abstraction of water in the Murray–Darling Basin, south-eastern Australia, were exacerbated by drought from 1997 to 2010. Consequently, the endangered Murray hardyhead Craterocephalus fluviatilis underwent substantial population decline and extirpations. The purpose of this study is to determine if a link exists between zooplankton response to flooding of a drought refuge and the recruitment success of C. fluviatilis in the presence of G. holbrooki. Flooding triggered sharp and substantial increases in the zooplankton and their eggs, which was the sole food of C. fluviatilis. This apparently benefitted the recruitment of C. fluviatilis, and sometimes alleviated diet overlap with G. holbrooki. Conversely, the zooplankton in a nearby non-flooded refuge was low in abundance and diversity, and all fish species were extirpated. The findings indicate that the flooding of drought refugia with relatively small volumes of water can be timed with ecological cues that would otherwise be desynchronized in highly regulated rivers, particularly during drought.