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.Invasion Ecology.Using insights gained from succession ecology, Philip Grime, Ken Thompson, and I developed a new theory to explain community invasibility. Presented in the Journal of Ecology (Davis et al. 2000), the Fluctuating Resource Availability Theory of Invasibility predicts that a plant community will become more susceptible to invasion whenever there is an increase in the amount of unused resources. This theory rests on the simple assumption that an invading species must have access to available resources, e.g., light, nutrients, and water, and that a species will enjoy greater success in invading a community if it does not encounter intense competition for these resources from resident species. This assumption is grounded in the theory that competition intensity should be inversely correlated with the amount of unused resources (Davis et al. 1998). Given this assumption, it logically follows that any factor(s) that increase the availability of limiting resources will increase the vulnerability of a community to invasion. This means that circumstances conducive to invasion may be expected to arise periodically in a wide range of habitats and vegetation types.
We suspect that it is this diversity in the range of resource-release mechanisms that partly explains the delay in formulating a general theory of invasibility, the failure to discover many consistent ecological correlates of invasibility, and the difficulty in predicting invasions. The elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but occur only intermittently and, to result in invasion, must coincide with availability of invading propagules. An increase in resource availability can occur in two basic ways: use of resources by the resident vegetation can decline, or resource supply can increase at a rate faster than the resident vegetation can sequester it. Resource use could decline due to a number of factors. A disturbance could damage or destroy some of the resident vegetation, reducing light, water, and nutrient uptake. Heavy herbivory due to grazing or a pest outbreak, or a widespread disease among the resident vegetation also would reduce resource uptake. Increases in gross resource supply could arise in a particularly wet year (increased water supply), as a consequence of eutrophication (increased nutrients), or following removal of a tree canopy (increased light for the understory vegetation). Whether resource uptake goes down for a time, or gross supply goes up, there are more resources available to invaders, and this is when the community is particularly vulnerable to invasion.
One important corollary of this theory is that a community's susceptibility to invasion is not a static or permanent attribute, but a condition that can fluctuate over time. Many of the questions that have guided thinking about invasibility of plant communities have been misleading because they tend to characterize invasibility as an inherent attribute of communities, e.g., "Why do species-rich communities repel invasions?". Rather than being an inherent property of communities, we believe that the invasibility of most communities changes from year to year and even within a given year, as the amount of unused resources fluctuates. This means that successful species invasions are likely to occur as episodic events.
The Fluctuating Resource Availability Theory of Invasibility
is the first theory of invasibility to integrate resource availability,
disturbance, and fluctuating environmental conditions. Importantly, the
theory is mechanistic, invoking a specific ecological process,
fluctuating
resource availability, to account for observed variation in
invasibility
due to a wide variety of causes. Moreover, whereas all previous
theories
of ecosystem invasibility have been qualitative in nature, the
fluctuating
resource availability theory is explicitly quantitative. Invasibility
can
be quantified in various ways, such as the probability of establishment
and survival per arriving propagule or the increase in biomass or
percent
cover of the invaders in the new environment over a specified period of
time given a defined propagule pressure. The quantitative nature of the
theory makes it especially amenable to testing by field
experiments.
A recent experimental field study (Davis and Pelsor 2001) provides the
first empirical report of the theory from a study specifically designed
to test the impact on invasibility of fluctuating resource
availability.
I believe the concept of invasibility should not just be confined to
biological invasions. Specifically, I believe invasibility
represents a fundamental condition of an ecosystem influencing
community assembly and biodiversity in general (Davis, Thompson, Grime
2005). In 2006, I reviewed the history of invasion ecology since
the publication of Elton's book in 1958 in a chapter of the edited
volume by Cadotte et al.
GLADES Project. With the help of more than 50 Macalester undergraduates, I have established a 16 ha square grid (10 x 10 m cells) covering oak savanna/ woodland and old field habitat at Cedar Creek. This project is now known as the GLADES (Grid for Landscape Analysis and DEmographic Study) Project. All trees greater than 2 cm dbh (more than 11,000) have been measured (dbh), identified, tagged, located (to the nearest m) and monitored (live or dead) since 1995. In addition, tree seedlings have been identified and counted in a circle (1.5 m radius) centered on each of the 1681 rebars (grid nodes) since 1996. A high resolution topographic data set has been constructed for the grid. Since the water table is quite flat at Cedar Creek, distance from water table can be readily calculated from topographic data. In addition, total soil N has been measured for each of the 1600 cells and percent bare soil estimated. Finally, location of current and past tree canopies has been digitized from aerial photographs. All the data is managed with a GIS (Arcinfo). The grid is divided into 3 burn units with different experimental fire histories.. In the absence of fire, this area has been transformed from a predominantly prairie matrix with scattered trees, to a predominantly woodland matrix with scattered prairie openings (Davis et al. 1997).
Analyses of the spatial patterns of the more than 11,000 trees in this species-poor temperate woodland show that all species are aggregated at small scales, that the degree of aggregation declines with distance from the trees, that the degree of aggregation of large stems is usually less than that of small stems, and that rare species are more aggregated than common species. These aggregation patterns are identical to those found in species-rich tropical forests. The introduction of fire initially increased the degree of tree aggregation in the woodland, but the level of aggregation declined with repeated fires, raising a new intermediate-disturbance hypothesis: community-wide aggregation patterns should be highest at sites with intermediate disturbance frequencies. Analysis of the spatial patterns of survival and mortality for fire-sensitive and fire-resistant trees indicates that community invasibility (the susceptibility of a community to colonization by new species) should be correlated with community-wide patterns of aggregation. The findings also show that the original intermediate-disturbance hypothesis, which holds that diversity peaks at sites with intermediate rates of disturbance, and the shifting mosaic paradigm of vegetation can be explained without having to invoke the r-K/succession paradigm that emphasizes life history differences among species Davis, Curran, et al. 2005).
I expect future data collection and analysis to help evaluate the applicability of deterministic, stochastic, and neutral models in understanding the spatial and temporal distribution of tree species at this site.
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Fire and landscape change. The impact of fire, and its absence, on the distribution of trees in a sandplain oak savanna was studied using aerial photographs taken over a 30 year period and a GIS (ArcInfo). In the absence of fire, tree canopy encroached into openings at an annual rate of 7cm/year (Davis et al. 1997). Burns that occurred less frequently than once every three years slowed but did not stop tree encroachment. Burn frequencies of once every two to three years maintained tree cover, while more frequent burns reduced tree cover (Faber-Langendoen and Davis 1995, Davis et al. 1997).
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Competition between grasses and oak seedlings. A large field experiment was established to examine how survival, growth, and photosynthesis of oak seedlings growing in old field vegetation varied along a multiple resource gradient consisting of light, nitrogen and water. Findings showed that total plant competition from herbaceous vegetation declined with increasing water availability, except in very wet shaded plots in which competition intensity increased, probably due to competition for light (Davis et al. 1998, Davis et al. 1999). These findings contradicted Grime’s theory of plant competition and were the basis for the development of the Net Resource Supply theory of competition (Davis et al. 1998). This theory states that competition intensity should be inversely correlated with Net Resource Supply, which is equal to Gross Resource Supply minus the Effective Resource Demand by the competitor. Seedling success, particularly as measured by survival, was inversely correlated with competition intensity, and survival and leaf net photosynthesis were highly correlated with soil water content. Results showed how short term changes in resource levels in this environment (soil water, soil nitrogen, light) can influence the success of oak seedling establishment via changes in competitive intensity for soil water by herbaceous vegetation.
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Fire and birds
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Savanna restoration and indigo buntings. Previous studies at Cedar Creek showed that Indigo Buntings prefer the savannas and open woodlands that are being restored through controlled burning over unburned forest habitat or old fields. We wanted to determine if male Indigo Buntings selected territories on the basis of landscape features that occur at a smaller scale than a burn compartment, and if so to characterize those features. We found that the areas used by males for purposes of singing usually had more large dead trees and fewer large live trees than other areas in the same compartment. The singing areas did not differ in the number of smaller trees (live or dead) or in the amount of shrub cover. We usually observed male buntings singing from the top of tall, dead trees throughout the day. Male buntings appear black in shade and poor light conditions, but they become a brilliant blue in full sunlight. Thus, they may choose to sing in open habitats (less live vegetation) and exposed perches (tops of dead trees) in order to be better seen by the female buntings. It is difficult to determine whether presettlement oak savannas would have been preferred habitat for Indigo Buntings. The savanna compartments at Cedar Creek have been undergoing restoration for more than 30 years. It seems that the use of fire as a restoration tool has produced landscape features that are very favorable to the buntings. The fire, combined with periodic drought, has left numerous standing dead trees, and most burn compartments contain numerous shrubs and oak saplings which have continued to resprout after fire and which have benefitted from the increase in light availability stemming from a more open tree canopy. Much of the presettlement savanna in Minnesota likely lacked both these features.
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Fire, gophers, and Penstemon grandiflorus. A combination of observational studies (Davis et al. 1991a, Davis et al. 1991b, Davis 1992), a field experiment (Davis et al. 1995), and landscape analysis using a GIS (Davis et al. 1997) showed that Penstemon grandiflorus (Scrophulariaceae) is inhibited by herbaceous vegetation and that the species is both facilitated and inhibited by pocket gophers, Geomys bursarius, and fire. Plants grew faster and reproduced sooner when growing on an old gopher mound (due to the absence of competition from herbaceous vegetation), however plants experienced higher mortality when growing in areas where gophers were active due to root herbivory by gophers and to plants being buried and killed under new mounds. Fire killed young plants, but since the distribution of the species was largely associated with the distribution of gophers (positive association) and since gophers prefer open to wooded areas, fire facilitated the presence of Penstemon in the savanna/woodland landscape by creating and maintaining openings.
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Heidi Scholtz, Susan Hofmann, Jon Dicus, Jeff Villinski, Lis Young, Kirsten Banks, Sarah McAndrew, Jodi Buckman-Fifield, Byron Ritchie, Norm Graf, Kim Gregg, Hai Tran, Abbey Duke, Tom Ibsen, Ruthanne Rhodes, Toni Shaeffer, Carolyn Muermann, Toby Query, Paul Bazakas, Michelle Crozier, Josh Huntington, Phung Gip, Nathan Dewar, Sash DePrez, Eliot Mitchell, Amy Day, Andrea Tietmeyer, Erin Donald, Christina Salazar, Seth Taapken, Claudia Curran, Andy Miller, Melissa Pelsor, Lee Dooley, Marjorie Hundtoft, Ingrid Attleson, Jessie Knoll, Ethan Bushelle, Louise Bier, Chloe Dieger, Aleta Van Brocklin, Karla Hillstrom, Dan Ungier, May Lin Kessenich, Bri Kujala, Eric Nordstrom, Ryan Burdge, Julia Eagles, Juliana Castellanos, Roxanne Clark, Anna Shamey, Said Guled, Elizabeth Goldsmith, Margaret Pettygrove, and Christa von Behren..
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