The spread of exotic species and climate change are among the most serious global environmental threats. Each independently causes considerable ecological damage, yet few data are available to assess whether changing climate might facilitate invasions by favoring introduced over native species. Here, we compare our long-term record of weekly sessile marine invertebrate recruitment with interannual variation in water temperature to assess the likely effect of climate change on the success and spread of introduced species. For the three most abundant introduced species of ascidian (sea squirt), the timing of the initiation of recruitment was strongly negatively correlated with winter water temperature, indicating that invaders arrived earlier in the season in years with warmer winters. Total recruitment of introduced species during the following summer also was positively correlated with winter water temperature. In contrast, the magnitude of native ascidian recruitment was negatively correlated with winter temperature (more recruitment in colder years) and the timing of native recruitment was unaffected. In manipulative laboratory experiments, two introduced compound ascidians grew faster than a native species, but only at temperatures near the maximum observed in summer. These data suggest that the greatest effects of climate change on biotic communities may be due to changing maximum and minimum temperatures rather than annual means. By giving introduced species an earlier start, and increasing the magnitude of their growth and recruitment relative to natives, global warming may facilitate a shift to dominance by nonnative species, accelerating the homogenization of the global biota. P redicted increases in global temperature (1) likely will have dramatic effects on the structure and function of ecosystems worldwide (2-4). One approach to predicting the biological effects of climate change has been to examine the direct physiological consequences to resident species of changes in temperature or CO 2 concentration (5-8). Other studies have focused on the indirect consequences of altered temperature on interactions among resident species and the cascading effects on local community composition (9-12). Climatic warming on the time scale of decades also may alter the composition of the resident biota by facilitating the poleward spread of species characteristic of warmer temperature regimes (9, 13-15). However, it is also possible that climate change could facilitate quantum leaps in the range of species across ocean basins or continents. Humans already inadvertently transport countless species around the globe (16), and, although many of these inoculations presumably fail because of inhospitable climate in the recipient region (17, 18), global warming may relax this constraint. Despite considerable interest in predicting the spread and success of ''invasive'' species because of their significant effect on native communities (19), there are few data to evaluate the effects of climate change on this type of invasion...

Theory predicts that systems that are more diverse should be more resistant to exotic species, but experimental tests are needed to verify this. In experimental communities of sessile marine invertebrates, increased species richness significantly decreased invasion success, apparently because species-rich communities more completely and efficiently used available space, the limiting resource in this system. Declining biodiversity thus facilitates invasion in this system, potentially accelerating the loss of biodiversity and the homogenization of the world's biota.

AND Marine Biological Laboratoyv, Woods Hole, Massachuserts 02543 U S AAbsrruct. Because of the narure of their substratum, the sessile invertebrate species of the marine epifaunal comrnunity living on rocks occur in discrete patterns of distribution. The rocks are finite patches or habitat islands with a limited space for colonization and growth. Such a system is ideal for studying the parameters affecting the distribution of species within a comrriunity. Also, because of the small size and immobility of the adults, the system is also ideal for studying the pattern of change in species composition and diversity within a community. This study used multiple series of manipulated experimental plates, which both duplicated natural rock surfaces and could be compared with sanlples of the rocks, to investigate the developmental and distributional processes of this community.Five major factors were found to be important to both the development of the comrnunity and its distribution on the rocks: ( I ) the selecriviry of the metamorphosing larvae as to site of attachment; ( 2 ) the seawnal fluctuation in larval abundances; (3) the biological interactions within and between species; ( 4 ) the size of rock substrata; and ( 5 ) the physical disturbance of the substrata (rock turnover). Initially, tne developmental process can be uncoupled fiom the effects of substrate size and disturbance. Predation is relatively unimportant as a biological interaction within this conrrnunity, but the species can be ranked according to their ability to compere for the availablespace on a substratum. This ranking implies a type of successional sequence in the development of the community: however, the sequence is greatly affected by historical components. The colonization of a substratum is directly dependent upon the abundance of settling larvae, which in turn is a function of seasonality and selectivity. The eventual competitive outcome and development of the community will depend upon which species have immigrated onro the substratum and is thus dependent upon history. The process is, therefore, open ended: colonization will be highly variable and change seasonally and, although one species may eventually dominate the substratum, it may be one of nine different species depending upon the individual history of that area.The frequency with which a substrarum is disturbed (with the resultant extinction of its fauna) is a function of wave force and is inversely proportional to both the size of the substratum and the depth at which it occurs. Disturbance will determine when a substratum is initially exposed for colonization and how long it will have for development. In the shallow subtidal (mean low water to -2.5 m ) , the frequent disturbance of small rocks will came them to support less than their equilibrium number of species and their fauna will reflect immediate larval abundances. Large rocks will remain stable for long periods of time and will usually be dominated by a single species. Intermediate-sized rocks (1 to 10 dm") will remain ...

A venerable generalization about community resistance to invasions is that more diverse communities are more resistant to invasion. However, results of experimental and observational studies often conflict, leading to vigorous debate about the mechanistic importance of diversity in determining invasion success in the field, as well as other ecosystem properties, such as productivity and stability. In this study, we employed both field experiments and observational approaches to assess the effects of diversity on the invasion of a subtidal marine invertebrate community by three species of nonindigenous ascidians (sea squirts). In experimentally assembled communities, decreasing native diversity increased the survival and final percent cover of invaders, whereas the abundance of individual species had no effect on these measures of invasion success. Increasing native diversity also decreased the availability of open space, the limiting resource in this system, by buffering against fluctuations in the cover of individual species. This occurred because temporal patterns of abundance differed among species, so space was most consistently and completely occupied when more species were present. When we held diversity constant, but manipulated resource availability, we found that the settlement and recruitment of new invaders dramatically increased with increasing availability of open space. This suggests that the effect of diversity on invasion success is largely due to its effects on resource (space) availability. Apart from invasion resistance, the increased temporal stability found in more diverse communities may itself be considered an enhancement of ecosystem function. In field surveys, we found a strong negative correlation between native‐species richness and the number and frequency of nonnative invaders at the scale of both a single quadrat (25 × 25 cm), and an entire site (50 × 50 m). Such a pattern suggests that the means by which diversity affects invasion resistance in our experiments is important in determining the distribution of invasive species in the field. Further synthesis of mechanistic and observational approaches should be encouraged, as this will increase our understanding of the conditions under which diversity does (and does not) play an important role in determining the distribution of invaders in the field.