There
is significant interest in why and how successful invasions
occur 1) because invasions may directly
or indirectly alter local community composition, 2) because
the performance of ecosystems may be altered as a result
of changing biodiversity which may be associated with
invasions, and 3) because there may be significant economic
costs associated with these ecosystem-level changes. Although
all ecosystems do not appear to be equally invasive, the
factors determining the susceptibility of a community
to invasion are still often unclear. Invasion theory generally
holds that the more species that are present in a community,
the more competitive the environment, and the less likely
an invasion is to be successful. The most commonly cited
mechanism behind this phenomenon is that as species accumulate,
competition intensifies and fewer resources remain available
for new colonists. In this light, our interests in the
ecology of marine invasive species generally has generally
been focused on three questions:
1. What are the characteristics that make a species a successful invader?
In the field of invasion ecology it has been an exceedingly difficult task to predict which marine species will become “invasive” or “nuisance” species and which will not. It is well recognized that most non- native species do not survive where they are introduced and fewer become pests after they are transported to new habitats/regions. Explanations for this pattern generally focus on lack of tolerable environmental conditions or to the lack of an ‘open’ niche for the invader in a new habitat/region. Attempts have been made to predict life history traits of species that will be successful invaders and the focus has usually been on the number and size of propagules, age at first reproduction, organism growth rate, etc. Some studies have found correlations between life history traits and invasion success and others have suggested that attributes of the invaded community (i.e., disturbance levels, species diversity) affect its ability to resist exotic species invasion. Our approach has been undertake comparative laboratory and field experimental studies of recent marine invaders into southern New England, with particular attention to shallow subtidal epifaunal organisms.
2. What effects do invaders have on resident fauna and flora?
The focus of most biological invasions has been on competitive relationships between natives and exotics. While some of the more publicized invasions do have significant impacts on local biota, many apparently have little or no impact on native biota. At a time when marine invasions are rapidly increasing in frequency due to increased human mobility and trans-oceanic transport, there is insufficient understanding of what causes the differential susceptibility of marine communities to invasion. Our approach has been to use a combination of field and laboratory experiments to begin dissecting biotic and abiotic mechanisms which influence the interaction of invaders and resident fauna.
3. Why are some habitats apparently more vulnerable to invasion than other habitats?
Ecosystems which have reduced biodiversity or that are stressed by environmental degradation and climate change appear to be more vulnerable to invasions. For example, we have experimentally demonstrated that enhanced species diversity directly increases the resistance of subtidal fouling assemblages to invasion and that surveys in a number of coastal habitats in southern New England also revealed an inverse correlation between resident species richness and the number of non-native species in those habitats. Climate change has its greatest effects on promoting invasive species at the southern and north boundaries of ecosystem types. In addition to greater vulnerability to invasion, increasing temperature will increase the number of warmer water invaders into temperate regions. Temperate coastal regions appear to be the most vulnerable to the effects of climate change on invasion by non-native species. For example, we have correlated a doubling in the abundance of invasive species ascidians in eastern Long Island Sound to a significant increase in seawater temperatures over the past two decades in that region.
