Julie L. Lockwood

Progress toward understanding the ecological impacts of nonnative species.

A predictive understanding of the ecological impacts of nonnative species has been slow to develop, owing largely to an apparent dearth of clearly defined hypotheses and the lack of a broad theoretical framework. The context dependency of impact has fueled the perception that meaningful generalizations are nonexistent. Here, we identified and reviewed 19 testable hypotheses that explain temporal and spatial variation in impact. Despite poor validation of most hypotheses to date, evidence suggests that each can explain at least some impacts in some situations. Several hypotheses are broad in scope (applying to plants and animals in virtually all contexts) and some of them, intriguingly, link processes of colonization and impact. Collectively, these hypotheses highlight the importance of the functional ecology of the nonnative species and the structure, diversity, and evolutionary experience of the recipient community as general determinants of impact; thus, they could provide the foundation for a theoretical framework for understanding and predicting impact. Further substantive progress toward this goal requires explicit consideration of within-taxon and across-taxa variation in the per capita effect of invaders, and analyses of complex interactions between invaders and their biotic and abiotic environments.

Global patterns of introduction effort and establishment success in birds

Theory suggests that introduction effort (propagule size or number) should be a key determinant of establishment success for exotic species. Unfortunately, however, propagule pressure is not recorded for most introductions. Studies must therefore either use proxies whose efficacy must be largely assumed, or ignore effort altogether. The results of such studies will be flawed if effort is not distributed at random with respect to other characteristics that are predicted to influence success. We use global data for more than 600 introduction events for birds to show that introduction effort is both the strongest correlate of introduction success, and correlated with a large number of variables previously thought to influence success. Apart from effort, only habitat generalism relates to establishment success in birds.

Extinction in a field of bullets: a search for causes in the decline of the world's freshwater fishes

Abstract Because human actions alter the physical nature of aquatic ecosystems similarly worldwide, the extinction risk among many freshwater fishes that share particular life-history traits may also be similar. Determining whether taxonomic selectivity, the preferential loss (or persistence) of certain species groups, exists among the worlds freshwater fish families is then a key step in predicting future species declines and triaging future conservation efforts. We use binomial statistics to look for taxonomic patterns among the worlds freshwater fish families currently at risk of extinction. Families are identified as being at risk of extinction if at least one species within a given family is classified as either extinct or at risk of extinction by the International Union for the Conservation of Natures Redlist of Threatened Animals [Groombridge, B., Baillie, J., 1997. 1996 IUCN Red list of Threatened Animals. IUCN Gland, Switzerland and Cambridge, UK]. Eighteen freshwater families have more threatened species than expected if extinction risk was evenly distributed across all families. Next, we use a series of chi-squared analyses to determine if various family-level characteristics (e.g. geographic distribution, body length, habitat preference, etc.) produce this taxonomic pattern. We find that families that inhabit well-studied regions of the world contain more threatened species. However, we find no indication of a unifying set of extinction-promoting biological or ecological traits that contribute to extinction risk among freshwater families. A possible explanation for this discrepancy is that aquatic alterations worldwide are so severe that extinction is being driven by extrinsic rather than intrinsic factors.

Assembling ecological communities in time and space

Ecological communities do not spring into existence overnight, but develop during the process we call assembly. As in other complex systems, random assemblies may generate surprising patterns. In previous models, sequential invasion and extinction moved successive species mixes toward a persistent one. Other species present in the pool could not invade this persistent mix. Chance events early in the assembly produced persistent mixes of different compositions. Most model species survived somewhere by belonging to one or more of these different persistent mixes. We show that with more rapid invasion, communities move through complex cycles of composition, where each species gets its turn. These complementary views offer insights into the diversity of natural communities. Importantly, they have practical implications for those attempting to restore diversity to damaged ecosystems.

Pattern and process of biotic homogenization in the New Pangaea

Human activities have reorganized the earths biota resulting in spatially disparate locales becoming more or less similar in species composition over time through the processes of biotic homogenization and biotic differentiation, respectively. Despite mounting evidence suggesting that this process may be widespread in both aquatic and terrestrial systems, past studies have predominantly focused on single taxonomic groups at a single spatial scale. Furthermore, change in pairwise similarity is itself dependent on two distinct processes, spatial turnover in species composition and changes in gradients of species richness. Most past research has failed to disentangle the effect of these two mechanisms on homogenization patterns. Here, we use recent statistical advances and collate a global database of homogenization studies (20 studies, 50 datasets) to provide the first global investigation of the homogenization process across major faunal and floral groups and elucidate the relative role of changes in species richness and turnover. We found evidence of homogenization (change in similarity ranging from −0.02 to 0.09) across nearly all taxonomic groups, spatial extent and grain sizes. Partitioning of change in pairwise similarity shows that overall change in community similarity is driven by changes in species richness. Our results show that biotic homogenization is truly a global phenomenon and put into question many of the ecological mechanisms invoked in previous studies to explain patterns of homogenization.

How many, and which, plants will invade natural areas?

Of established nonindigenous plant species in California, Florida, and Tennessee, 5.8%, 9.7%, and 13.4%, respectively, invade natural areas according to designations tabulated by state Exotic Pest Plant Councils. Only Florida accords strictly with the tens rule, though California and Tennessee fall within the range loosely viewed as obeying the rule. The species that invaded natural areas in each state were likely, if they invaded either of the other states at all, to have invaded natural areas there. There was a detectable but inconsistent tendency for species that invade natural areas to come from particular families. At the genus level in California and Florida, and the family level in California, there was also a tendency for natural area invaders to come from taxa that were not represented in the native flora. All three of the above patterns deserve further studies to determine management implications. Only the first (that natural area invaders of one state are likely to invade natural areas if they invade another state) seems firm enough from our data to suggest actions on the part of managers.

A perfect storm: two ecosystem engineers interact to degrade deciduous forests of New Jersey

Ecosystem engineers play a large role in physically structuring the ecosystem in which they are embedded. The focus of much of the research surrounding these species is to document the impacts of a single engineer on community composition and ecosystem processes. However, most ecosystems harbor multiple engineering species that interact in complex ways and rarely have the dynamics of such species been fully investigated. We look at how two ecosystem engineers, the white-tailed deer (Odocoileus virginianus) and the invasive plant Japanese stilt grass (Microstegium vimineum), interact to completely alter the structure and composition of the subcanopy within northern deciduous forests. This interaction has wide-ranging repercussions on forest food webs which we explore through a case study of breeding woodland birds in the state of New Jersey, USA. We show that the guilds of birds that rely on the subcanopy have experienced greater declines from 1980 to 2005 than birds that specialize on the intact upper canopy of impacted forests. This dynamic is not restricted to immediate temporal effects and may act to derail the long-term successional pathway of northern deciduous forests. It is no longer prudent to set aside tracts of forest and expect them to retain their native biodiversity without active management.

Smoke on the water: the interplay of fire and water flow on Everglades restoration

Recent research makes clear that much of the Everglade’s flora and fauna have evolved to tolerate or require frequent fires. Nevertheless, restoration of the Everglades has thus far been conceptualized as primarily a water reallocation project. These two forces are directly linked by the influence of water flows on fire fuel moisture content, and are indirectly linked through a series of complex feedback loops. This interaction is made more complex by the alteration and compartmentalization of current water flows and fire regimes, the lack of communication between water and fire management agencies, and the already imperiled state of many local species. It is unlikely, therefore, that restoring water flows will automatically restore the appropriate fire regimes, leaving the prospect of successful restoration in some doubt. The decline of the Cape Sable seaside sparrow, and its potential for recovery, illustrates the complexity of the situation.

HAWAIIAN HONEYCREEPER HOME RANGE SIZE VARIES WITH HABITAT: IMPLICATIONS FOR NATIVE ACACIA KOA FORESTRY

Reforesting degraded landscapes with native, high-value timber trees may contribute to biodiversity protection while also increasing the economic value of the land. Ideally, reforested areas should have the habitat characteristics to support viable populations of native fauna. We investigated habitat use by Akiapolaau (Hemignathus munroi), an endangered Hawaiian honeycreeper, in three habitat types: a relatively intact old-growth forest, an old-growth forest with a long history of grazing, and a native Acacia koa plantation. We quantified habitat characteristics, calculated foraging preferences, mapped home ranges, measured territoriality, and determined reproductive success at all sites. We found that Akiapolaau were highly selective foragers, foraging primarily on A. koa despite its relative scarcity in the environment, and then secondarily on several subcanopy species. Home range size was significantly smaller at the intact old-growth site and the A. koa plantation, the two sites with the greater cover...

Invasion Science: A Horizon Scan of Emerging Challenges and Opportunities

We identified emerging scientific, technological, and sociopolitical issues likely to affect how biological invasions are studied and managed over the next two decades. Issues were ranked according to their probability of emergence, pervasiveness, potential impact, and novelty. Top-ranked issues include the application of genomic modification tools to control invasions, effects of Arctic globalization on invasion risk in the Northern Hemisphere, commercial use of microbes to facilitate crop production, the emergence of invasive microbial pathogens, and the fate of intercontinental trade agreements. These diverse issues suggest an expanding interdisciplinary role for invasion science in biosecurity and ecosystem management, burgeoning applications of biotechnology in alien species detection and control, and new frontiers in the microbial ecology of invasions.