A recurring theme in my research has been the emergence of critical threshold responses to landscape structure.   Landscape connectivity becomes disrupted abruptly at a critical level of habitat loss and fragmentation, which ultimately depends upon the dispersal abilities of the species (3-5).  Different species thus possess different perceptions of landscape structure.  Habitat fragments also become isolated at a critical level of habitat, when the interpatch distances on the landscape rapidly increase (lacunarity thresholds).  This has implications for the ability of organisms to locate or colonize suitable habitat on the landscape.  These lacunarity thresholds may thus precipitate dispersal thresholds, in which dispersal success dramatically declines past some critical level of habitat abundance (6).  Experimental work with insects suggests that the movement or search behavior of some species may exhibit threshold responses to patch structure (7). Species that differ in their demographic potentials, as a function of their dispersal and reproductive capabilities, may exhibit an extinction threshold, where the population suddenly crashes at some critical level of habitat abundance and fragmentation severity (8).  Landscape structure can mitigate extinction risk for some species by lowering the critical habitat threshold at which these populations go extinct.  Landscape structure (the amount and spatial arrangement of habitat) also affects the ability of landscapes to function as overall population sources or sinks, as was demonstrated for various types of Neotropical migratory songbirds (9).  Landscape thresholds may also disrupt predator-prey interactions and affect the ability of natural enemies to control pest outbreaks.  Such thresholds in species interactions have important implications for conservation biological control, especially if native predators are influenced more than exotic species by habitat loss and fragmentation (10).  Greater reliance may be placed in the future on introducing exotic species for the purpose of biocontrol as agroecosystems become increasingly more fragmented.  This is not without ecological, as well as economic, costs.   My current research addresses how landscape dynamics affect extinction risk for migratory songbirds (11-13), a group of conservation concern, and involves a regional assessment of population viability for grassland birds in particular, which have exhibited steeper population declines than any other avian group in North America (14-15).

Publications cited above:

1. With, K. A. 1997. The application of neutral landscape models in conservation biology. Conservation Biology 11: 1069-1080.
2. With, K. A., and A. W. King. 1997. The use and misuse of neutral landscape models in ecology. Oikos 79: 219-229.
3. With, K. A., and T. O. Crist. 1995. Critical thresholds in species' responses to landscape structure. Ecology 76: 2446-2459.
4. With, K. A., and T. O. Crist. 1996. Translating across scales: simulating species distributions as the aggregate response of individuals to heterogeneity. Ecological Modelling 93: 125-137.
5. With, K. A., R. H. Gardner, and M. G. Turner. 1997. Landscape connectivity and population distributions in heterogeneous environments. Oikos 78: 151-169.
6. With, K. A., and A. W. King. 1999. Dispersal success in fractal landscapes: a consequence of lacunarity thresholds. Landscape Ecology 14: 73-82.
7. With, K. A., S. J. Cadaret, and C. Davis. 1999. Movement responses to patch structure in experimental fractal landscapes. Ecology 80: 1340-1353.  
8. With, K. A., and A. W. King. 1999. Extinction thresholds for species in fractal landscapes. Conservation Biology 13: 314-326.  
9. With, K. A., and A. W. King. 2001. Analysis of landscape sources and sinks: the effect of spatial pattern on avian demography. Biological Conservation 100: 75-88.  
10. With, K. A., D. M. Pavuk, J. L. Worchuck, R. K. Oates, and J. L. Fisher.  2002.  Threshold effects of landscape structure on biological control in agroecosystems.  Ecological Applications 12:  52-65.
11. Schrott, G. R., K. A. With, and A. W. King.  2005a.  On the importance of landscape history for assessing extinction risk.  Ecological Applications 15:  493-506.
12. Schrott, G. R., K. A. With, and A. W. King.  2005b.  Demographic limitations on the ability of habitat restoration to rescue declining populations.  Conservation Biology 19: 1181-1193.  
13. With, K. A., G. R. Schrott, and A. W. King.  2006.  The implications of metalandscape connectivity for population viability in migratory songbirds.  Landscape Ecology 21: 157-167.
14. With, K. A., A. W. King, and W. E. Jensen.  2008.  Remaining large grasslands may not be sufficient to prevent grassland bird declines.  Biological Conservation 141: 3152-3167.
15. Rahmig, C. J., W. E. Jensen, and K. A. With.  2009.  Grassland bird responses to land management in the largest remaining tallgrass prairie.  Conservation Biology 23: 420-432.

Regional Assessment of Population Viability for Grassland Birds in Agricultural Grasslands

An Experimental Model Landscape System (EMLS), inspired by neutral landscape models, was established in the field to assess how habitat  connectivity affects species interactions (predator-prey and host-parasitoid relationships) and the structure and dynamics of terrestrial arthropod communities that naturally colonized clover habitat in this agroecosystem.  This research is being done in collaboration with Dr. Daniel Pavuk of the Department of Biological Sciences at Bowling Green State University, and was supported by the National Science Foundation.  This funding and supplemental grants from the NSF Research Experience for Undergraduates Program also enabled the participation of more than a dozen students in ecological research. 

The first publication (With et al. 2002) to emerge from this project reports on how thresholds in landscape structure (lacunarity thresholds) affect the ability of natural enemies to track the distribution of insect pests (biocontrol thresholds).  Landscape thresholds precipitate similar thresholds in the distribution of aphid populations, which become isolated (fragmented) when clover habitat falls below 20%.  An exotic coccinellid (Harmonia axyridis) that was introduced specifically for the biocontrol of pea aphids was able to track this threshold in the distribution of its aphid prey, but an indigenous coccinellid (Coleomegilla maculata) was unable to do so.   An analysis of movement responses to patch structure at different scales revealed that the biocontrol agent was much more mobile, exhibiting a greater propensity to move within and between landscape plots, than the indigenous coccinellid.  The native predator was thus more sensitive to habitat fragmentation and was unable to track aphid distributions when they became scarce and patchily distributed (i.e., <20% habitat).   If native predators are generally more sensitive to habitat fragmentation than exotic species, our study suggests that we may become increasingly dependent upon the introduction of exotic biocontrol agents that are better able to track pest populations below the biocontrol threshold as agroecosystems become progressively more fragmented.   This will obviously incur great ecological, as well as economic, costs.  In addition to economic thresholds, there are also ecological thresholds that must be surmounted for successful biocontrol.