Evolution of dispersal in explicitly spatial metacommunities

We apply an evolutionary game theoretic approach to the evolution of dispersal in explicitly spatial metacommunities, using a flexible parametric class of dispersal kernels, namely 2Dt kernels, and study the resulting evolutionary dynamics and outcomes. We observe strong selective pressure on mean dispersal distance (i.e., the first moment), and weaker, but significant, one on the shape of dispersal kernel (i.e., higher moments). We investigate the effects of landscape topology and spatial heterogeneity on the resulting ‘optimal’ dispersal kernels. The shape—importantly the tail structure—and stability of evolutionarily optimal dispersal strategies are strongly affected by landscape topology or connectivity. Specifically, the results suggest that the optimal dispersal kernels in the river network topology have heavier tails and are stable, while those in the direct topology, where organisms are allowed to travel directly from one location to another, have relatively thin tails and may be unstable. We also find that habitat spatial heterogeneity enables coexistence and controls spatial distribution of distinct groups of dispersal strategies and that alteration in topology alone may not be sufficient to change such coexistence. This work provides a tool to translate environmental changes such as global climate change and human intervention into changes in dispersal behavior, which in turn may lead to important alterations of biodiversity and biological invasion patterns.     

Scale-dependent effects of natural environmental gradients,industrial emissions and dispersal processes on zooplankton metacommunity structure: Implications for the bioassessment of boreal lakes

Environmental controls were traditionally considered as sole determinants of community assembly for freshwater bioassessment studies, whereas potential importance of dispersal processes and spatial scale have received limited attention. We conducted a bioassessment of lakes across northeast Alberta, Canada using crustacean zooplankton to develop a framework for evaluating if and how atmospheric emissions from the nearby Athabasca Oil Sands Region could impact their community assemblages. We quantified the effects of environmental gradients and spatially contingent dispersal processes for determining zooplankton community composition of 97 lakes at two spatial scales (regional and sub-regional) using constrained ordination, spatial modeling and variance partitioning techniques. Our findings indicated that effects of both environmental gradients and dispersal processes on species composition were scale-dependent. Zooplankton community composition was significantly correlated to environmental parameters that are directly and indirectly sensitive to industrial deposition including nitrate, sulphate, dissolved organic carbon, base cation, chloride, trace metal concentrations and predation regime, indicating their potential to track future environmental impacts. The relative importance of these environmental predictors varied with spatial scale, yet unraveling the effects of natural environmental heterogeneity vs. industrial deposition on this scale-dependency was not possible due to lack of regional baseline information. Dispersal processes were not important in shaping zooplankton communities at the sub-regional scale, but had limited, yet significant influence on species composition at the regional scale, emphasizing the need for cautious interpretation of broad-scale community patterns. Beyond establishing crucial regional baselines, our study highlights the necessity for explicit incorporation of dispersal effects and spatial scale in bioassessment of lakes across landscapes.     

Density-dependent dispersal and relative dispersal affect the stability of predator-prey metacommunities

Although density-dependent dispersal and relative dispersal (the difference in dispersal rates between species) have been documented in natural systems, their effects on the stability of metacommunities are poorly understood. Here we investigate the effects of intra- and interspecific density-dependent dispersal on the regional stability in a predator-prey metacommunity model. We show that, when the dynamics of the populations reach equilibrium, the stability of the metacommunity is not affected by density-dependent dispersal. However, the regional stability, measured as the regional variability or the persistence, can be modified by density-dependent dispersal when local populations fluctuate over time. Moreover these effects depend on the relative dispersal of the predator and the prey. Regional stability is modified through changes in spatial synchrony. Interspecific density-dependent dispersal always desynchronizses local dynamics, whereas intraspecific density-dependent dispersal may either synchronize or desynchronize it depending on dispersal rates. Moreover, intra- and interspecific density-dependent dispersal strengthen the top-down control of the prey by the predator at intermediate dispersal rates. As a consequence the regional stability of the metacommunity is increased at intermediate dispersal rates. Our results show that density-dependent dispersal and relative dispersal of species are keys to understanding the response of ecosystems to fragmentation.     

The effects of between-habitat dispersal rate on protist communities and metacommunities in microcosms at two spatial scales

The effect of manipulation of between-habitat dispersal rates in multiple patch systems was examined experimentally using protist communities in laboratory microcosms. Replicate landscapes of eight microcosms (patches) at two spatial scales (patch sizes) were inoculated with 13 species of protists. Dispersal was carried out by transferring a small random sample of medium and protists from one randomly selected microcosm to another within a landscape. Four dispersal rates (24, 6, 2 and 0 transfers very 3 days) were used, and the microcosms were sampled after 6 and 12 weeks. Patch size had a consistent effect on within-path (community) and within-landscape (metacommunity) diversity, both being lower in small patch systems. Higher dispersal rates had a slight effect on community and metacommunity diversity after 12 weeks, with a tendency for higher dispersal to slightly offset the rate of loss of species. Both dispersal and patch size had effects on the abundance of many individual species, though in a variety of ways. The individual species results suggest that extinction is selective with respect to both patch size and dispersal rate treatments, and may be influenced by species interactions. It seems likely that in metacommunity systems of this sort, rather than mainland-island systems, the potential effect of between-patch dispersal rate in rescuing and recolonizing where local extinctions occur may be much reduced by the effect of selective extinction, relative to that expected under the assumption of random extinction.     

The effects of resource enrichment, dispersal, and predation on local and metacommunity structure

Community structure is the observable outcome of numerous processes. We conducted a laboratory experiment using a microbial model system to disentangle effects of nutrient enrichment, dispersal, and predation on prey species richness and predator abundance at local and metacommunity scales. Prey species included: Chilomonas sp., Colpidium striatum, Colpoda cucullus, Paramecium tetraurelia, P. caudatum, Philodina sp., Spirostomum sp., Tetrahymena thermophila, and Uronema sp., and Stentor coeruleus was the predator used. We hypothesized that: (1) increased basal resources should maintain greater species richness and higher predator abundance; (2) dispersal should maintain greater species richness; and (3) predation should reduce species richness, especially in the high resource treatments relative to no-predator treatments. Our results support all three hypotheses. Further, we show that dispersal affects richness at the local community scale but not at the metacommunity scale. However, predation seems to have major effects at both the local and metacommunity scale. Overall, our results show that effects of resource enrichment, dispersal, and predation were mostly additive rather than interactive, indicating that it may be sometimes easier to understand their effects than generally thought due to complex interactive effects.     

Forecasting spatially structured populations: the role of dispersal and scale

We forecasted spatially structured population models with complex dynamics, focusing on the effect of dispersal and spatial scale on the predictive capability of nonlinear forecasting (NLF). Dispersal influences NLF ability by its influence on population dynamics. For simple 2-cell models, when dispersal is small, our ability to predict abundance in subpopulations decreased and then increased with increasing dispersal. Spatial heterogeneity, dispersal manner, and environmental noise did not qualitatively change this result. But results are not clear for complex spatial configurations because of complicated dispersal interactions across subpopulations. Populations undergoing periodic fluctuations could be forecasted perfectly for all deterministic cases that we studied, but less reliably when environmental noise was incorporated. More importantly, for all models that we have examined, NLF was much worse at larger spatial scales as a consequence of the asynchronous dynamics of subpopulations when the dispersal rate was below some critical value. The only difference among models was the critical value of dispersal rate, which varied with growth rate, carrying capacity, mode of dispersal, and spatial configuration. These results were robust even when environmental noise was incorporated. Intermittency, common in the dynamics of spatially structured populations, lowered the predictive capability of NLF. Forecasting population behaviour is of obvious value in resource exploitation and conservation. We suggest that forecasting at local scales holds promise, whereas forecasting abundance at regional scales may yield poor results. Improved understanding of dispersal can enhance the management and conservation of natural resources, and may help us to understand resource-exploitation strategies employed by local indigenous humans.     

Competing populations on fragmented landscapes with spatially structured heterogeneities: improved landscape generation and mixed dispersal strategies

Interactions between two species competing for space were studied using stochastic spatially explicit lattice-based simulations as well as pair approximations. The two species differed only in their dispersal strategies, which were characterized by the proportion of reproductive effort allocated to long-distance (far) dispersal versus short-distance (near) dispersal to adjacent sites. All population dynamics took place on landscapes with spatially clustered distributions of suitable habitat, described by two parameters specifying the amount and the local spatial autocorrelation of suitable habitat. Whereas previous results indicated that coexistence between pure near and far dispersers was very rare, taking place over only a very small region of the landscape parameter space, when mixed strategies are allowed, multiple strategies can coexist over a much wider variety of landscapes. On such spatially structured landscapes, the populations can partition the habitat according to local conditions, with one species using pure near dispersal to exploit large contiguous patches of suitable habitat, and another species using mixed dispersal to colonize isolated smaller patches (via far dispersal) and then rapidly exploit those patches (via near dispersal). An improved mean-field approximation which incorporates the spatially clustered habitat distribution is developed for modeling a single species on these landscapes, along with an improved Monte Carlo algorithm for generating spatially clustered habitat distributions.      

Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants

Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species? Location: Semi‐arid savanna in the Kimberley Thorn Bushveld, South Africa. Methods: We developed a two‐step protocol for the statistical differentiation of association‐promoting mechanisms in plants based on the Acacia erioloba–Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non‐spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non‐spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter. Results: We first showed a significant positive spatial association of the two species. Thereafter, the non‐spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred. Conclusions: Our two‐step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).     

Spatial autocorrelation and dispersal in mistletoes: field and simulation results

The infections of two species of mistletoes in Baja California, Mexico were investigated for spatial patterns of abundance, and for an effect of the dispersal patterns of mistletoe seeds on these spatial patterns. Mistletoe distributions were mapped and the dispersal of mistletoe seeds was observed. Most mistletoes seeds were dispersed locally to the parent tree or to nearby trees. While mistletoe distributions were highly clumped at the level of the individual tree, no spatial pattern was found above the scale of the individual tree. Infected trees were no more clumped than the overall host population, and infected trees had no more mistletoes on nearby surrounding trees than did uninfected trees. Trees showed no spatial autocorrelation in the number of mistletoes they supported. Simulations using a spatially explicit simulation model with local dispersal and stochasticity in seed dispersal, host mortality, and mistletoe mortality were used to interpret the field results. Simulation results suggest that dispersal patterns affect the overall level of variance in the number of mistletoes per tree but do not lead to spatial patterns in abundance above the scale of the tree. Thus, both simulation and field systems give the surprising result that local dispersal does not lead to spatial autocorrelation in the numbers of mistletoes per tree.Abbreviations AI = Arroyo Inspiracion - VSR = Valle San Rafael     

Inverse link between density and dispersal distance in butterflies: field evidence from six co-occurring species

A pertinent question in animal population ecology is the relationship between population abundance, density, and mobility. Two extreme ways to reach sufficient abundance for long-term persistence are to inhabit restricted locations at high densities, or large areas in low densities. The former case predicts low individual mobility, whereas the later predicts high one. This assumption is rarely tested using across-species comparisons, due to scarcity of data on both mobility and population sizes for multiple species. We used data on dispersal and local population densities of six butterfly species gained by mark-recapture, and data on their (relative) regional abundance obtained by walking transects in a landscape surrounding the mark-recapture sites. We correlated both local density and regional abundance against slopes of the inverse power function, appropriate for describing the shape of dispersal kernel. Local densities correlated negatively with the dispersal kernel slopes both when sexes were treated as independent data points and if treated together. For regional abundance, the correlation was also negative but only marginally significant. Our results corroborate the notion that a trade-off exists between living in dense populations and having poor dispersal, and vice versa. We link this observation to resource use by individual species, and distribution of such resources as host plants in the study landscape.     

Morph-dependent fatty acid oxidation in a wing-polymorphic cricket: implications for the trade-off between dispersal and reproduction

Although a considerable amount of information is available on the ecology and physiology of wing polymorphism, much less is known about the biochemical-genetic basis of morph specialization for dispersal versus reproduction. Previous studies have shown that the dispersing morph of the wing-polymorphic cricket, Gryllus firmus, prioritizes the accumulation of triglyceride flight fuel over ovarian growth, while the opposite occurs in the flightless morph during the first week of adulthood. In this study, we compared the in vivo rate of lipid oxidation between genetic stocks of flight-capable versus flightless morphs to determine the role of lipid catabolism in morph specialization for flight versus reproduction. During the first five days of adulthood, in the absence of flight, fatty acid oxidation was substantially lower in the dispersing morph relative to the flightless morph, when either radiolabeled acetate or palmitate was used as a substrate. Differences between the morphs in fatty acid oxidation were genetically based, occurred co-incident with morph-specific differences in triglyceride accumulation and ovarian growth, and were observed on a variety of diets. A genetically based trade-off in the relative conversion of palmitate into CO(2) versus triglyceride was observed in morphs of G. firmus. Decreased oxidation of fatty acid and increased biosynthesis of triglyceride, both appear to play an important role in flight fuel accumulation, and hence morph specialization for flight. Conversely, increased oxidation of fatty acid likely fuels the enhanced ovarian growth in the flightless morph. The results of the present study on fatty acid catabolism, and previous studies on triglyceride and phospholipid biosynthesis, provide the first direct evidence that genetically based differences in in vivo flux through pathways of intermediary metabolism underlie a trade-off between flight capability and reproduction--a trade-off of central importance in insects.     

The expanding range of Undaria pinnatifida in southern New Zealand: distribution, dispersal mechanisms and the invasion of wave-exposed environments

Very few studies have addressed how the invasive kelp Undaria pinnatifida (Harvey) Suringar spreads beyond initial founding populations in harbours. Surveys of the harbours and accessible areas of open coast throughout southern New Zealand were conducted to determine how far U. pinnatifida populations had extended since initial incursions. Our findings clearly demonstrate that U. pinnatifida is capable of invading native communities and can establish reproductive populations in locations subjected to significant and consistent wave action. The extent of spread from source populations differs between harbours in which it has established. Dispersal is greatest in harbours with long established populations, those where populations have not been strategically managed, harbours with high water exchange with surrounding coastal waters, and where prevailing currents allow establishment of U. pinnatifida on suitable substrata close to harbour entrances. Dispersal along the open coast is primarily achieved by drifting adult sporophytes that are washed up in the rocky intertidal zone. Founding populations are most often found in the intertidal zone, primarily within rockpools. Subtidal transects and observations indicate that U. pinnatifida is well adapted to invade exposed coastlines and can establish within a broad range of niches in wave-exposed areas including rockpools, the low intertidal, shallow subtidal, Macrocystis pyrifera kelp forests, and in low light areas beyond the vertical extent of large native macroalgae. The current range of U. pinnatifida is much greater than expected and appears to be expanding. Due to its ability to grow in a broad range of environments and to form dense monospecific stands, U. pinnatifida has the potential to strongly modify almost all rocky subtidal and intertidal communities in temperate locations.     

Ecological relationships between phytoplankton communities and different spatial scales in European reservoirs: implications at catchment level monitoring programmes

Phytoplankton communities are structured by factors acting over temporal and spatial scales. Identifying which factors are driving spatial patterns in aquatic communities is the central aim of ecology. In this study, data sets of phytoplankton communities and environmental data of two Portuguese reservoirs types (lowland “riverine reservoirs” and higher altitude “artificial lake reservoirs”) were used to determine the importance of environmental variables at different spatial (geographical, regional and local) and time scales (seasons, years) on the community structure. In all the data sets, the multivariate ordination technique Canonical Correspondence Analysis (CCA) showed that regional and local scales explained the majority (9–18% and 13–19%, respectively) of the taxa variance. However, for “riverine reservoirs”, time variables were more important, explaining 27% of the variability in phytoplankton assemblages. Variance partitioning was used to assess the individual importance of the three spatial scales and time for the community structure of the two reservoir types. The majority of among-site variability (5.9–21.4%) was accounted for by time variables, with local, regional, and geographical scale variables accounting for 3.3–5.6%, 3.7–4.5% and 2.6–2.9%, respectively. The effects of different spatial scales on phytoplankton communities were clearly interrelated; thus, implying that phytoplankton assemblages are capable of detecting stress from catchment to site scales. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Padisak     

Gene flow and population history in high dispersal marine invertebrates: mitochondrial DNA analysis of Holothuria nobilis (Echinodermata: Holothuroidea) populations from the Indo-Pacific

The sea cucumber, Holothuria nobilis, has a long-lived planktotrophic larvae, and previous allozyme surveys have suggested that high dispersal is realized. In contrast, recent ecological studies indicate that dispersal is low. To reconcile these data, and to investigate the evolution of this Indo-Pacific species, we screened geographical variation in 559 bp of a mitochondrial gene (COI) in 360 samples from the Australasian region and La Réunion. Sequences from La Réunion differed by > 7% from others and may constitute another species. Haplotype diversity in other samples was high (0.942, SD = 0.007), but haplotypes were closely related (mean nucleotide diversity: 0.0075, SD = 0.0041). AMOVA, pairwise FST values and exact tests did not detect significant population structure. Nested clade analysis showed that one of two main clades was over-represented in west Australia, whereas the other was more common in the northern Great Barrier Reef. Isolation-by-distance was identified as the main determinant of population structure at several clade levels. Contiguous range expansion was inferred for evolutionary older clade levels and this may correspond to a late Pleistocene (88 000-193 000 years ago) population expansion inferred from haplotype mismatch distributions. Thus, the population genetic structures detected are likely to be formed prior to the last ice age, with some indications for high dispersal on shorter time scales.     

Direct and feeding‐induced interactions between two rice planthoppers, Sogatella furcifera and Nilaparvata lugens: effects on dispersal capability and performance

Abstract. 1. A series of laboratory experiments was conducted to explore the effects of inter‐specific interactions, both direct interactive effects and those induced through previous feeding, on the dispersal capability (proportion of macropterous adults) and performance (development time and survival) of two wing‐dimorphic planthoppers, the whitebacked planthopper Sogatella furcifera and the brown planthopper Nilaparvata lugens, two pests of rice throughout Asia. 2. An asymmetric effect of inter‐specific crowding on dispersal capability was detected between the two planthoppers. With density controlled, the proportion of macropterous adults in N. lugens was higher when raised in mixed populations with S. furcifera than in pure cultures comprised of conspecifics, suggesting that interspecific effects on dispersal capability are stronger than intraspecific effects. In contrast, interspecific effects on macroptery (%) were weaker than intraspecific impacts for S. furcifera. 3. This trend was parallelled by adverse competitive effects on survival, with interspecific effects stronger than intraspecific impacts for N. lugens and intraspecific effects stronger for S. furcifera. 4. Crowding also affected the incidence of macroptery indirectly through feeding‐induced changes in plant physiology. Rearing N. lugens on plants fed on previously by heterospecifics resulted in a higher incidence of macroptery in females and protracted development than for N. lugens raised on plants fed on previously by conspecifics. By contrast, the intraspecific effect of previous feeding was stronger than the interspecific effect in S. furcifera, with higher macropter production and prolonged development occurring on plants exposed previously to conspecifics. 5. The results suggest that interspecific interactions between herbivorous insects, both direct and via feeding‐induced changes in plant physiology, can have negative consequences for performance and survival and promote the production of flight‐capable adults that can disperse.     

The effect of the rate of successful dispersal of a Phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

     

The effect of the rate of successful dispersal of a phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

     

Retrospective analysis of associations between water quality and toxic blooms of golden alga (Prymnesium parvum) in Texas reservoirs: Implications for understanding dispersal mechanisms and impacts of climate change

Toxic blooms of golden alga (GA, Prymnesium parvum) in Texas typically occur in winter or early spring. In North America, they were first reported in Texas in the 1980s, and a marked range expansion occurred in 2001. Although there is concern about the influence of climate change on the future distribution of GA, factors responsible for past dispersals remain uncertain. To better understand the factors that influence toxic bloom dispersal in reservoirs, this study characterized reservoir water quality associated with toxic GA blooms since 2001, and examined trends in water quality during a 20-year period bracketing the 2001 expansion. Archived data were analyzed for six impacted and six nonimpacted reservoirs from two major Texas basins: Brazos River and Colorado River. Data were simplified for analysis by pooling spatially (across sampling stations) and temporally (winter, December–February) within reservoirs and generating depth-corrected (1 m) monthly values. Classification tree analysis [period of record (POR), 2001–2010] using salinity-associated variables (specific conductance, chloride, sulfate), dissolved oxygen (DO), pH, temperature, total hardness, potassium, nitrate+nitrite, and total phosphorus indicated that salinity best predicts the toxic bloom occurrence. Minimum estimated salinities for toxic bloom formation were 0.59 and 1.02 psu in Brazos and Colorado River reservoirs, respectively. Principal component analysis (POR, 2001–2010) indicated that GA habitat is best defined by higher salinity relative to nonimpacted reservoirs, with winter DO and pH also being slightly higher and winter temperature slightly lower in impacted reservoirs. Trend analysis, however, did not reveal monotonic changes in winter water quality of GA-impacted reservoirs during the 20-year period (1991–2010) bracketing the 2001 dispersal. Therefore, whereas minimum levels of salinity are required for GA establishment and toxic blooms in Texas reservoirs, the lack of trends in water quality suggests that conditions favorable for toxic blooms pre-date the 2001 expansion. These observations are consistent with a climate change-independent scenario of past GA dispersals in Texas reservoirs driven by novel introductions into pre-existing favorable habitat. Reports of latent GA populations in certain nonimpacted reservoirs, however, provide a plausible scenario of future dispersals characterized by prolonged periods between colonization and toxic bloom development and driven by changes in water quality, natural, or anthropogenic.