Caroline Maier



Ph.D. Candidate



Research Statement -

I am broadly interested in biological invasion, from both species and community perspectives.  Why are some species highly successful invaders with near global distributions, while others are limited to a small native range?  Why do some communities seem exceptionally vulnerable to invasion, while others appear resistant?   Several recently-published theories describe how various environmental factors influence a species’ invasive ability and community invasibility.  I am using a lab-based aquatic protist system to empirically examine some of these invasion theories.

My first project investigates the effect of native species richness, propagule pressure, and predation on community invasibility and the establishment success of invaders.   Species richness, the number of species present in a community, strongly influences a community’s invasibility.  Its effect, however, varies depending on how and where you look.  The classic theory is that highly diverse communities are relatively resistant to invasion.  When many species are present, niches might be tightly packed in resource space, resources depleted, and indirect interactions among species exceptionally strong, blocking the entrance of potential invaders.  In contrast, depauperate communities may not completely fill available niche space, leaving resources that invaders can use and room for invaders to enter.  This negative relationship between native species richness and invasibility is observed in theoretical models and experimental work in the laboratory and field.  In contrast, large-scale uncontrolled field surveys show a positive relationship between species richness and invasibility.   Areas with high native diversity tend to have a large number of invasive species.  To reconcile these two very different patterns, several researchers have suggested that the relationship between native species richness and community invasibility varies with scale.   Across a landscape, high native diversity is linked to high invasibility because environmental factors that promote diversity also facilitate invasion.  At the smaller neighborhood scale, environmental factors are essentially homogeneous, so community invasibility is determined primarily by the relatively weaker factors of local species richness and species interactions.

One environmental factor that might be important to diversity and invasibility is propagule pressure, the number and frequency with which individuals are released into an area.  High immigration can sustain small, declining populations, enhancing both community diversity and the establishment and spread of invaders.  Multiple or large releases of invading individuals might overcome a small founding population’s naturally high extinction rate, counteract the negative influence of predation, and increase genetic variation, possibly improving the population’s ability to adapt to the new habitat.   Propagule pressure might then play a role in creating the different species richness and community invasibility patterns observed at large and small scales.  Larger geographic ranges may include several areas of high invader immigration which greatly increase invader N for the region and swamp the negative effect of native species richness.  Smaller study areas might tend to lack the pinpoint locations of high immigration, so the diversity-invasibility relationship at this level may be determined mainly by species interactions related to species richness.

In my research project, species richness and propagule pressure are independently varied so that their individual and combined contributions can be examined.  Experiments contain series of flasks that simultaneously form two different gradients.  First, there is an environmental gradient composed of identical flasks that differ only in the degree of propagule pressure, the frequency with which invaders and “native” species are introduced and the number of each individual added at a time.  Second, each propagule pressure treatment has a species richness gradient consisting of flasks with identical environments, but a range of species richness values from depauperate to diverse.  This design simulates the conditions found in both large and small scale species richness-community invasibility studies.  Various protist invaders are introduced to all the different communities, one species of invader added to each flask.  The frequency with which invaders successfully establish a population in the various communities and their mean abundance, compared to the degree of the propagule pressure across environmental gradients and species richness across richness gradients, is analyzed to determine whether any relationship exists.  If there is a positive relationship between factors that promote diversity and community invasibility, invaders should persist longer and produce larger populations in flasks that have higher propagule pressure.  If native species richness is negatively related to community invasibility, invader abundance and persistence should increase as the richness of established species decreases within a single propagule pressure treatment.

My second project examines the effect of resource specialization on species’ invasive and competitive abilities and community members’ ability to persist in the presence of an invader.  Several theoretical and empirical studies suggest that generalist invaders are exceptionally successful.  There is debate, however, whether native specialists are competitively superior to generalist invaders.  Many studies show a trade-off between a species’ competitive ability and its degree of specialization.  If native specialists are not competitively superior, they would have been eliminated by invaders long ago.  The contradictory view, that native specialists are competitively inferior to invading generalists, has also been proposed, and it has been suggested that generalist invaders on oceanic islands eliminate native specialists.  Using an aquatic protist model system, I am investigating whether degree of resource specialization affects the invasive ability of an introduced species and the vulnerability of “native” species to competitive exclusion.  Bacterivorous protist invaders that differ in their degree of resource specialization are introduced to a variety of bacterivorous protist communities.  One invading species is added to each experimental community, and each species introduced to all community types.  Control communities are identical to experimental in all ways, but lack an invader.  The abundance of the invader and all other protist species in the communities are followed for six weeks, and then plotted against time to show the outcome.  The ability of each invader to successfully establish a persistent population in the various communities, compared to its degree of resource specialization, might show whether trends exist.  If generalist protists are indeed better invaders, they would produce a detectable, stable population in a higher proportion of the communities to which they are introduced than would specialist invaders.  The effect of the invader on established species in the community is seen by the difference in the abundance of community members in control and invaded treatments.  The fate of established community members is compared to their degree of specialization to highlight whether specialists are more harmed by invasion than are generalists.  If they are, then the abundance of more specialized community members would decrease substantially and some species might be eliminated completely from the invaded community.  In contrast, generalized community members would show less change in abundance after invasion and fewer would disappear.



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