Dianna K. Padilla

Ecological neighborhoods: scaling environmental patterns

In this paper we review, develop, and differentiate among concepts associated with environmental patterning (patch, division, and heterogeneity), spatial and temporal scales of ecological processes (ecological neighborhoods), and responses of organisms to environmental patterning (relative patch size, relative patch duration, relative patch isolation, and grain response). We generalize the concept of ecological neighborhoods to represent regions of activity or influence during periods of time appropriate to particular ecological processes. Therefore, there is no single ecological neighborhood for any given organism, but rather a number of neighborhoods, each appropriate to different processes. Neighborhood sizes can be estimated by examining the cumulative distribution of activity or influence of an organism as a function of increasingly large spatial units. The spatial and temporal dimensions of neighborhoods provide the scales necessary for assessing environmental patterning relative to particular ecological processes for a given species. Consistent application of the neighborhood concept will assist in the choice of appropriate study units, comparisons among different studies, and comparisons between empirical studies and theoretical postulates.

Beyond ballast water: aquarium and ornamental trades as sources of invasive species in aquatic ecosystems

Although ballast water has received much attention as a source of aquatic invasive species, aquariums and trade in aquarium and ornamental species are emerging as another important source for species likely to invade aquatic habitats. These species are spread throughout the world in a generally unregulated industry. The recent focus on the aquarium trade as a possible mechanism for environmentally sustainable development poses an especially dangerous threat, although this has so far escaped the attention of most environmentalists, conservationists, ecologists, and policy makers.

Plastic inducible morphologies are not always adaptive: the importance of time delays in a stochastic environment

SummaryWe present a mathematical model for predicting the expected fitness of phenotypically plastic organisms experiencing a variable environment. We assume that individuals experience two discrete environments probabilistically in time (as a Markov process) and that there are two different phenotypic states, each yielding the highest fitness in one of the two environments. We compare the expected fitness of a phenotypically fixed individual to that of an individual whose phenotype is induced to produce the better phenotype in each environment with a time lag between experiencing a new environment and realization of the new phenotype. Such time lags are common in organisms where phenotypically plastic, inducible traits have been documented. We find that although plasticity is generally adaptive when time lags are short (relative to the time scale of environmental variability), plasticity can be disadvantageous for longer lag times. Asymmetries in environmental change probabilities and/or the relative fitnesses of each phenotype strongly influence whether plasticity is favoured. In contrast to other models, our model does not require costs for plasticity to be disadvantageous; costs affect the results quantitatively, not qualitatively.

ESTIMATING THE PROBABILITY OF LONG‐DISTANCE OVERLAND DISPERSAL OF INVADING AQUATIC SPECIES

Accurately predicting the pattern and rate of spread of invading species is difficult, particularly for species that disperse long distances. Though relatively rare, and often stochastic, long-distance dispersal events increase the maximum rate and geographic extent of invasion. Human activities are responsible for the spread of many exotic species, particularly aquatic species such as the zebra mussel, which are primarily transported within North America by recreational boaters. We estimated spatial and temporal patterns of boating traffic among Wisconsins inland waterbodies using results of a large, randomized survey of recreational boaters conducted by the Wisconsin Department of Natural Re- sources. Of the survey respondents, >90% of boaters traveled locally, within a county or to adjacent counties, 8.4% moved >50 km, and only 0.8% moved extreme long distances (>261 km, two standard deviations above the mean of intercounty travel). Extreme long- distance boater movements were correlated positively with greater numbers of registered boaters in source and destination counties, and with greater surface area and numbers of named lakes in destination counties. We compared the observed spatial and temporal pat- terns of the zebra mussel invasion to those estimated from recreational boater movement by simple diffusion models. Diffusion models underestimated the maximum rate and geo- graphic extent of the zebra mussel invasion and overestimated the invasion of suitable habitats within this extent. Patterns of recreational boater activity in Wisconsin were a better predictor of the observed zebra mussel invasion pattern because they provided prob- abilistic estimates of invasion at finer spatial resolution. These estimates may be used to manage the spread of boater-dispersed aquatic invaders. To slow the spread of boater- dispersed aquatic invaders such as the zebra mussel, management efforts should target high- frequency, long-distance boater movements, and regions with the greatest volume of source and/or destination boater movement.

Changes in global economies and trade: the potential spread of exotic freshwater bivalves.

The globalization of economies and trade have facilitated the spread of exotic species including the five most important freshwater suspension feeding invaders Dreissena polymorpha, D. bugensis, Corbicula fluminea, C. fluminalis, and Limnoperna fortunei. We suggest that the spread of these exotic species has not been a continuous process, but rather punctuated by periods of rapid long distance spread (jump), during which species greatly expanded their geographic ranges. Each jump has been associated with changes in the tempo of some human activity, such as the construction of shipping canals for trade, building of reservoirs for water storage and power production, political boundary changes or changes in political systems, which affected the position or permeability of national borders, human migration, changes in the mode and volume of international trade, or recent industrial practices and environmental laws. We hypothesize that the rate of spread of exotic species depends on the spatial scale of spread and may be accelerated or slowed by various human activities. In general, aquatic exotic species may quickly spread along connected waterways in a new continent they invade and soon reach their maximum range (continental scale). However, it will take much longer to colonize all isolated regions (regional scale) and longer still to spread to all isolated lakes and river systems (local scale). The difference in the rate of colonization across scales may be several orders of magnitude.

Geographic spread of exotic species : Ecological lessons and opportunities from the invasion of the zebra mussel Dreissena polymorpha

The spatial and temporal dynamics of the recent invasion of North American fresh waters by the zebra mussel Dreissena polymorpha are reviewed in terms of the mechanistic bases behind the dispersal and colonization processes. The planktonic phase of the life cycle (the veliger), the ability of the benthic stage to attach to sub- merged objects, and the prominence of human activities as vectors for dispersal has promoted rapid spread of this aquatic pest to 18 states in the USA and two provinces in Canada within the first seven years of its introduction into the Laurentian Great Lakes. So far, the majority of range expansion has occurred within commercially navi- gable waters, and thus commercial shipping appears to be the most important vector of spread within connected bodies of water, especially to areas upstream of estab- lished populations. In contrast, overland spread to iso- lated inland waters appears to occur more slowly, and by early 1994 adult mussels had only been found in eight inland lakes. Although there are many potential vectors of overland spread, transient recreational boating activity is suspected of being the primary means of overland disper- sal and several mechanisms associated with boating have been shown to be capable of transporting mussels in large numbers. Studies on waterfowl indicate that although ducks are capable of transporting zebra mussels, the rate of transport is quite small relative to boating activity. Other methods of inferring the relative importance of dis- persal vectors are outlined, and an example of predicting the spread on the basis of regional patterns of recre- ational boating traffic is given. Finally, studies on the demographic conditions necessary for the establishment of new populations are suggested as a rewarding area of further research. Copyright

Impacts of Zebra Mussels on Aquatic Communities and their Role as Ecosystem Engineers

Zebra mussels (Dreissena polymorpha) are not only an extremely aggressive invasive species, often dominating water bodies they invade, they are also very effective ecosystem engineers, altering the environments they invade. They are effective engineers, altering both ecosystem structure and function. They change existing and provide new habitat for other organisms, affect trophic interactions and the availability of foods for both pelagic species and other benthic species, and they affect the rates of other ecosystem processes including mineralization of nutrients, oxygen availability and sedimentation rates. These physical impacts on the environment feedback directly to other species that interact with or are impacted by zebra mussels, or indirectly through food chains, disturbance, succession, or other longer-term community and ecosystem processes.

THE INVASIVE BIVALVES DREISSENA POLYMORPHA AND LIMNOPERNA FORTUNEI: PARALLELS, CONTRASTS, POTENTIAL SPREAD AND INVASION IMPACTS

Abstract We contrast ecological and life history traits of the well studied freshwater invader, the zebra mussel (Dreissena polymorpha), with the lesser known invasive golden mussel (Limnoperna fortunei) to compare salient biological traits and environmental limits, and to predict the potential spread and ecosystem impacts of L. fortunei in areas where it is introduced. Both species are sessile, byssate bivalves with a planktonic larval stage and extremely high reproductive capacity. For both species adults attain much higher biomass in waterbodies they invade than all of the native invertebrates combined, and they create substrate complexity otherwise not found in freshwater systems. Both are very active suspension feeders, greatly enhance benthic-pelagic coupling, and act as effective ecosystem engineers. Although taxonomically unrelated, their ecosystem impacts are surprisingly similar and follow from the novel ecological niche they share, rather than being species specific. The golden mussel has broader environmental tolerances and therefore may be a much more successful invader than D. polymorpha in regions dominated by acidic, soft and contaminated waters. In the near future L. fortunei may colonize the southern and central parts of North America, much farther north than has been previously predicted. Although to date the zebra mussel is considered the most aggressive freshwater invader, soon many waterbodies may receive another, even more aggressive invader.

Invaders are not a random selection of species

We assembled information on 119 species of freshwater macroinvertebrate invaders in North America and Europe, and compared them to all native freshwater species in North America and Europe. We tested whether the invaders were a random or selected group among taxa (phylum or class), water quality requirements, and feeding habit. We found that freshwater macroinvertebrate invaders are not a random selection of species, and are over-represented by molluscs and crustaceans, while taxa richness of native communities are dominated by insects. Over 35% of native species of aquatic invertebrates in North America are only able to live in areas with excellent or very good water quality, and are intolerant of organic pollution. In contrast, all invaders are tolerant of at least moderate amounts of organic pollution. There was a significant difference in the distribution of feeding habits between native species and invaders: collector-filterers (including suspension feeders) were 2.5–3 times more abundant, and predators were 3–4 times less abundant among invaders than among native invertebrates. The ongoing spread of exotic species affects the biodiversity of selected taxa, shifts communities toward greater tolerance of organic pollution and increases the numbers of suspension feeders, thereby enhancing benthic pelagic coupling in waterbodies with high densities of invaders. Because these processes are very similar in Europe and North America, we suggest that the observed patterns may have a common global effect.

PREDICTING THE LIKELIHOOD OF EURASIAN WATERMILFOIL PRESENCE IN LAKES, A MACROPHYTE MONITORING TOOL

In regions with abundant and diverse freshwater resources, it is difficult and costly to survey all lakes at the level required to detect invasive plants. Effective allocation of monitoring resources requires tools that identify waterbodies where exotic species are most likely to invade. We developed and tested models that predict conditions in which Eurasian watermilfoil, Myriophyllum spicatum, is most likely to survive and successfully colonize. We used logistic regression to model the likelihood of M. spicatum presence or absence using a suite of biological, chemical, and physical lake characteristics which are easily obtainable from public databases. We evaluated model fit by the Aikake criterion and model performance by the percentage of misclassification errors as well as the costs associated with acquiring data for variables modeled. Several models fit our data well, misclassifying only 1.3-11.0% of the lakes where M. spicatum was observed, and used relatively inexpensive landscape variables (percent forest cover in a drainage basin, presence and type of public boat launch, and bedrock type) that typically exist as information layers in geographic information systems (GISs) or recreational atlases. We found that the most important factors affecting the presence or absence of M. spicatum were those that influence water quality factors known to impact M. spicatum growth, rather than factors associated with human activity and dispersal potential. In particular, the amount of forest cover in the lake watershed was consistently important and could control the level of dissolved inorganic carbon in lakes, one of the factors known to affect M. spicatum growth rates. Factors such as the number of game fish species and number and types of boat ramps or proximity to roads were generally less important lake characteristics. Our models can be useful tools for developing management strategies to prevent or slow the spread of M. spicatum and aquatic invaders, such as the zebra mussel, that can attach to it and thus be dispersed. Our models also exemplify a general approach for slowing or stopping the spread of other invading species.