Alexander S. Flecker

Biodiversity conservation in running waters

In the concerns about biodiversity conservation, fresh waters have received less attention than tropical forests and oceans. However, running waters harbor a diverse panoply of species, habitats, and ecosystems, including some of the most threatened and many having great value to human society. An overview of the biological diversity of running waters and the state of imperilment is presented. Six major factors that threaten destruction of running water species and ecosystems are discussed: habitat loss and degradation; species invasions; overharvesting; secondary extinctions; chemical and organic pollution; global climate change. General measures for recovery and restoration of running waters conclude the article.

An ecosystem engineer, the beaver, increases species richness at the landscape scale

Abstract. Ecosystem engineering – the physical modification of habitats by organisms – has been proposed as an important mechanism for maintaining high species richness at the landscape scale by increasing habitat heterogeneity. Dams built by beaver (Castor canadensis) dramatically alter riparian landscapes throughout much of North America. In the central Adirondacks, New York, USA, ecosystem engineering by beaver leads to the formation of extensive wetland habitat capable of supporting herbaceous plant species not found elsewhere in the riparian zone. We show that by increasing habitat heterogeneity, beaver increase the number of species of herbaceous plants in the riparian zone by over 33% at a scale that encompasses both beaver-modified patches and patches with no history of beaver occupation. We suggest that ecosystem engineers will increase species richness at the landscape scale whenever there are species present in a landscape that are restricted to engineered habitats during at least some stages of their life cycle.

Ecosystem Engineering by a Dominant Detritivore in a Diverse Tropical Stream

Prochilodus mariae (Characiformes: Prochilodontidae) is a detritivorous fish distributed throughout the Orinoco river basin of South America. Spectacular migrations of these fishes occur at the end of the rainy season into the Andean foothills. Prochilodus ingest large quantities of sediments and may thereby modify habitats in neotropical streams. The major objectives of this study were (1) to explore experimentally the importance of Prochilodus in structuring a tropical stream in the Venezuelan Andean piedmont, and (2) to determine whether there was sufficient ecological redundancy in a diverse and abundant assemblage of epibenthic fishes to compensate for the removal of Prochilodus. Community structure was compared among three experimental treatments: (1) Prochilodus exclusion, (2) Prochilodus enclosure, and (3) the natural fish assemblage. Selective exclusion of Prochilodus resulted in striking changes in community structure as measured by patterns of sediment accrual and the composition of algal and invertebrate assemblages. Highly significant increases in total dry mass and in ash-free dry mass of sediments accruing on stream-bottom substrates were observed almost immediately following the exclusion of Prochilodus. Moreover, the composition of algal and invertebrate assemblages was signif- icantly modified by Prochilodus. Taxa such as diatoms were reduced in number when Prochilodus was present; in contrast, Prochilodus appeared to facilitate nitrogen-fixing cyanobacteria. Total invertebrate densities were greatest in the Prochilodus removal treat- ment; however, a variety of responses to the experimental treatments was observed among different taxa analyzed individually, including density reductions, increases, and no mea- surable effects. This study suggests that the detritivore Prochilodus is a functionally dom- inant species in Andean foothill streams via sediment-processing activities. Moreover, it provides little evidence to support the notion that strongly interacting species are limited to simple systems with few food web components.

Fish extinctions alter nutrient recycling in tropical freshwaters

There is increasing evidence that species extinctions jeopardize the functioning of ecosystems. Overfishing and other human influences are reducing the diversity and abundance of fish worldwide, but the ecosystem-level consequences of these changes have not been assessed quantitatively. Recycling of nutrients is one important ecosystem process that is directly influenced by fish. Fish species vary widely in the rates at which they excrete nitrogen and phosphorus; thus, altering fish communities could affect nutrient recycling. Here, we use extensive field data on nutrient recycling rates and population sizes of fish species in a Neotropical river and Lake Tanganyika, Africa, to evaluate the effects of simulated extinctions on nutrient recycling. In both of these species-rich ecosystems, recycling was dominated by relatively few species, but contributions of individual species differed between nitrogen and phosphorus. Alternative extinction scenarios produced widely divergent patterns. Loss of the species targeted by fishermen led to faster declines in nutrient recycling than extinctions in order of rarity, body size, or trophic position. However, when surviving species were allowed to increase after extinctions, these compensatory responses had strong moderating effects even after losing many species. Our results underscore the complexity of predicting the consequences of extinctions from species-rich animal communities. Nevertheless, the importance of exploited species in nutrient recycling suggests that overfishing could have particularly detrimental effects on ecosystem functioning.

Fish Predation and The Evolution of Invertebrate Drift Periodicity: Evidence from Neotropical Streams

Drift activity of stream invertebrates typically is greatest during the nighttime hours in running waters throughout the world. Such diel periodicity may be an adaptive response that minimizes exposure to visually hunting, drift-feeding fishes. I tested this risk- of-predation hypothesis by examining drift behavior of mayflies in a series of Andean mountain and piedmont streams that vary in the abundance of drift-feeding fishes. Drift was primarily nocturnal in piedmont streams with natural populations of visually hunting predators. In contrast, mayfly drift activity did not differ between day and night in mountain streams that historically lack drift-feeding fishes. However, in naturally fishless Andean streams containing introduced rainbow trout, nocturnal peaks in drift were observed for the mayfly Baetis, suggesting a rapid evolutionary change in behavior in response to an exotic predator. When drift periodicity was examined along a gradient of predation regimes, activity was found to be increasingly restricted to the nighttime hours as predation risk became more intense. Diel periodicity was observed even when fish were experimentally excluded, suggesting that nocturnal activity has evolved as a fixed behavioral response to predation, and is not a direct ecological consequence of diurnal feeding by fishes. These observations support the hypothesis that predation risk is important in determining the timing of prey drift behavior.

Local adaptation in Trinidadian guppies alters ecosystem processes

Theory suggests evolutionary change can significantly influence and act in tandem with ecological forces via ecological-evolutionary feedbacks. This theory assumes that significant evolutionary change occurs over ecologically relevant timescales and that phenotypes have differential effects on the environment. Here we test the hypothesis that local adaptation causes ecosystem structure and function to diverge. We demonstrate that populations of Trinidadian guppies (Poecilia reticulata), characterized by differences in phenotypic and population-level traits, differ in their impact on ecosystem properties. We report results from a replicated, common garden mesocosm experiment and show that differences between guppy phenotypes result in the divergence of ecosystem structure (algal, invertebrate, and detrital standing stocks) and function (gross primary productivity, leaf decomposition rates, and nutrient flux). These phenotypic effects are further modified by effects of guppy density. We evaluated the generality of these effects by replicating the experiment using guppies derived from two independent origins of the phenotype. Finally, we tested the ability of multiple guppy traits to explain observed differences in the mesocosms. Our findings demonstrate that evolution can significantly affect both ecosystem structure and function. The ecosystem differences reported here are consistent with patterns observed across natural streams and argue that guppies play a significant role in shaping these ecosystems.

Community-Wide Consequences of Trout Introduction in New Zealand Streams

Twenty-five years ago not one of these rivers had the least interest for the angler … the rod of the fisherman never cast a shadow on their waters; every one of these mighty rivers, every one of the thousand creeks and streams that flow into them … were tenantless and profitless to the sportsman (Spackman 1892, as cited in McDowall 1990a) … brown trout have since been stocked in almost every conceivable lake, river, or stream, such that the present naturalized population encompasses every suitable ecological niche within the confines of New Zealand (MacCrimmon and Marshall 1968).

Fish distributions and nutrient cycling in streams: can fish create biogeochemical hotspots?

Rates of biogeochemical processes often vary widely in space and time, and characterizing this variation is critical for understanding ecosystem functioning. In streams, spatial hotspots of nutrient transformations are generally attributed to physical and microbial processes. Here we examine the potential for heterogeneous distributions of fish to generate hotspots of nutrient recycling. We measured nitrogen (N) and phosphorus (P) excretion rates of 47 species of fish in an N-limited Neotropical stream, and we combined these data with population densities in each of 49 stream channel units to estimate unit- and reach-scale nutrient recycling. Species varied widely in rates of N and P excretion as well as excreted N:P ratios (6-176 molar). At the reach scale, fish excretion could meet >75% of ecosystem demand for dissolved inorganic N and turn over the ambient NH4 pool in <0.3 km. Areal N excretion estimates varied 47-fold among channel units, suggesting that fish distributions could influence local N availability. P excretion rates varied 14-fold among units but were low relative to ambient concentrations. Spatial variation in aggregate nutrient excretion by fish reflected the effects of habitat characteristics (depth, water velocity) on community structure (body size, density, species composition), and the preference of large-bodied species for deep runs was particularly important. We conclude that the spatial distribution of fish could indeed create hotspots of nutrient recycling during the dry season in this species-rich tropical stream. The prevalence of patchy distributions of stream fish and invertebrates suggests that hotspots of consumer nutrient recycling may often occur in stream ecosystems.

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

Fish Trophic Guilds and the Stucture of a Tropical Stream: Weak Direct vs. Strong Indirect Effects

Freshwater fishes exhibit spectacular biodiversity in the tropics. In contrast to temperate streams, where insectivorous fishes generally predominate, detrital— and algalfeeding fishes are widespread components of tropical ichthyofaunas. I compared the direct and indirect effects of a variety of fishes in structuring a neotropical stream insect assemblage. An exclosure/enclosure experiment was preformed to examine the role of different fish trophic guilds and densities on patterns of patch colonization by invertebrates. Insectivorous characid fishes had relatively weak impacts on overall invertebrate abundance, although significant reductions were observed for selected taxa (the mayflies Baetis sp. 2 and Leptohyphes, and the caddisflies Smicridea and Alisotrichia). This contrasted sharply with the strong effects of the grazing armored catfish Chaetostoma milesi and the detritivore Prochilodus mariae, which caused significant reductions in abundance of common taxa throughout the invertebrate assemblage. In addition, grazers and detritivores substantially reduced the amount of sediments and associated detritus accumulated on stream bottom substrata. A second experiment was preformed to evaluate the mechanism by which grazing and detritivorous fishes influence insect abundance. Insect abundance was compared between a treatment where resource depression and bioturbation was simulated by removing sediments manually to fish exclosure/enclosure treatments. As in the previous experiment, grazers and detritivores significantly reduced the abundance of many common invertebrate taxa, as well as detrital and algal resources. However, taxon—specific differences in colonization of the sediment—removal treatment suggest that several factors are important in explaining responses to fishes by insects. Many common taxa displayed decreases in abundance both in sediment—removal and fish treatments, indicating that sediment—processing activities by epibenthic fishes are important in explaining patch colonization for many members of the invertebrate assemblage. However, sediment depletion is not sufficient to explain the response of the abundant invertebrate, Baetis sp. 1, which showed substantial reductions in cages containing fishes (open and grazer enclosures), but high densities in both the fish—exclusion and the sediment—removal treatments. These data suggest that intimidation by fish, as measured by the presence of grazers, results in low rates of patch use by this numerically dominant mayfly. These findings contrast with studies reporting relatively weak effects of fishes in temperate zone streams. Fishes may be strong interactors in some neotropical steams, not through direct lethal consumption of prey, but rather by modifying the distribution and abundance of resources important to stream insects. In some instances the simple intimidation associate with the presence of a diverse and abundant fauna of bottom—feeding fishes also may influence insect abundance and distribution.