Cathy D. Collins

Habitat fragmentation and its lasting impact on Earth's ecosystems

Urgent need for conservation and restoration measures to improve landscape connectivity. We conducted an analysis of global forest cover to reveal that 70% of remaining forest is within 1 km of the forest’s edge, subject to the degrading effects of fragmentation. A synthesis of fragmentation experiments spanning multiple biomes and scales, five continents, and 35 years demonstrates that habitat fragmentation reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering nutrient cycles. Effects are greatest in the smallest and most isolated fragments, and they magnify with the passage of time. These findings indicate an urgent need for conservation and restoration measures to improve landscape connectivity, which will reduce extinction rates and help maintain ecosystem services.

Community-level consequences of mycorrhizae depend on phosphorus availability

In grasslands, arbuscular mycorrhizal fungi (AMF) mediate plant diversity; whether AMF increase or decrease diversity depends on the relative mycotrophy in dominant vs. subordinate plants. In this study we investigated whether soil nutrient levels also influence the ability of AMF to mediate plant species coexistence. First, we developed a conceptual model that predicts the influence of AMF on diversity along a soil nutrient gradient for plant communities dominated by mycotrophic and non-mycotrophic species. To test these predictions, we manipulated phosphorus to create a soil nutrient gradient for mesocosm communities composed of native prairie grasses and then compared community properties for mesocosms with and without AMF. We found that, where P was limiting, AMF increased plant diversity and productivity, and also altered community structure; however, at high P, AMF had little influence on aboveground communities. Compositional differences among treatments were due largely to a trade-off in the relative abundance of C3 vs. C4 spes. Our study emphasizes how environmental constraints on mutualisms may govern community- and ecosystem-level properties.

Land-Use History and Contemporary Management Inform an Ecological Reference Model for Longleaf Pine Woodland Understory Plant Communities

Ecological restoration is frequently guided by reference conditions describing a successfully restored ecosystem; however, the causes and magnitude of ecosystem degradation vary, making simple knowledge of reference conditions insufficient for prioritizing and guiding restoration. Ecological reference models provide further guidance by quantifying reference conditions, as well as conditions at degraded states that deviate from reference conditions. Many reference models remain qualitative, however, limiting their utility. We quantified and evaluated a reference model for southeastern U.S. longleaf pine woodland understory plant communities. We used regression trees to classify 232 longleaf pine woodland sites at three locations along the Atlantic coastal plain based on relationships between understory plant community composition, soils (which broadly structure these communities), and factors associated with understory degradation, including fire frequency, agricultural history, and tree basal area. To understand the spatial generality of this model, we classified all sites together and for each of three study locations separately. Both the regional and location-specific models produced quantifiable degradation gradients–i.e., progressive deviation from conditions at 38 reference sites, based on understory species composition, diversity and total cover, litter depth, and other attributes. Regionally, fire suppression was the most important degrading factor, followed by agricultural history, but at individual locations, agricultural history or tree basal area was most important. At one location, the influence of a degrading factor depended on soil attributes. We suggest that our regional model can help prioritize longleaf pine woodland restoration across our study region; however, due to substantial landscape-to-landscape variation, local management decisions should take into account additional factors (e.g., soil attributes). Our study demonstrates the utility of quantifying degraded states and provides a series of hypotheses for future experimental restoration work. More broadly, our work provides a framework for developing and evaluating reference models that incorporate multiple, interactive anthropogenic drivers of ecosystem degradation.

Detection and Plant Monitoring Programs: Lessons from an Intensive Survey of Asclepias meadii with Five Observers

Monitoring programs, where numbers of individuals are followed through time, are central to conservation. Although incomplete detection is expected with wildlife surveys, this topic is rarely considered with plants. However, if plants are missed in surveys, raw count data can lead to biased estimates of population abundance and vital rates. To illustrate, we had five independent observers survey patches of the rare plant Asclepias meadii at two prairie sites. We analyzed data with two mark-recapture approaches. Using the program CAPTURE, the estimated number of patches equaled the detected number for a burned site, but exceeded detected numbers by 28% for an unburned site. Analyses of detected patches using Huggins models revealed important effects of observer, patch state (flowering/nonflowering), and patch size (number of stems) on probabilities of detection. Although some results were expected (i.e. greater detection of flowering than nonflowering patches), the importance of our approach is the ability to quantify the magnitude of detection problems. We also evaluated the degree to which increased observer numbers improved detection: smaller groups (3–4 observers) generally found 90 – 99% of the patches found by all five people, but pairs of observers or single observers had high error and detection depended on which individuals were involved. We conclude that an intensive study at the start of a long-term monitoring study provides essential information about probabilities of detection and what factors cause plants to be missed. This information can guide development of monitoring programs.

The role of topography and soil characteristics in the relationship between species richness and primary productivity in a Kansas grassland

Abstract Environmental conditions influence patterns of local-scale plant productivity and species richness. We sampled soils and plant communities at a topographically-diverse, mid-successional old field site in Kansas to better understand the abiotic factors underlying a natural plant productivity gradient. We related soil texture, pH, percent nitrogen and carbon, and soil moisture to elevation, plant productivity, and species richness. Most soil qualities were significantly correlated with elevation. The four soil texture classes we identified — clay, silty clay, silty clay loam, and silt loam — were spatially clustered according to topography. The lowest elevation sites, characterized by high C and N, low pH, low light penetration, and soils containing more silt and sand which supported the most productive and least diverse plant communities. We suggest that topography drives spatial heterogeneity in soils at our site. We also suggest that although richness is influenced directly by the filtering effects of the abiotic environment on the species pool, the indirect effects environmental factors have on richness via biomass production are more important for governing plant species richness in our system.

Colonization of successional grassland by Ulmus rubra Muhl. in relation to landscape position, habitat productivity, and proximity to seed source1

Abstract Evaluating the factors that regulate woody plant invasion into grassland is important for understanding the process of succession, for predicting potential impacts of global change on grassland ecosystem structure and function and for informing the restoration and management of grasslands. In this study we investigate factors influencing seedling establishment and spatial distribution of an invading native woody plant (Ulmus rubra Muhl.) across a heterogeneous successional grassland landscape in eastern Kansas. Using data collected from a landscape-level seedling census of an abandoned hayfield undergoing succession, we relate U. rubra stem abundance to landscape gradients in topographic position, soil texture, soil fertility, plant productivity, and proximity to seed sources. We also present results from a fertilization experiment to more directly evaluate the potential effect of grassland fertility and productivity on U. rubra invasion, independent from other potential causal factors that covary with soil fertility and plant productivity across the natural topographic gradient. In the landscape census, U. rubra stem densities were greatest in low-productivity micro-sites located at higher elevations in the landscape on hill-slopes and ridges. Highly productive micro-sites at low positions in the landscape and dominated by the introduced rhizomatous hay grass, Bromus inermis, contained few if any U. rubra stems despite close proximity to a seed source. In the fertilization experiment, fertilization increased plant biomass, but reduced U. rubra stem densities, confirming the suppressive influence of high productivity on U. rubra stems in the landscape. In control plots of the fertilization experiment, U. rubra stem density was negatively correlated with distance from seed source, a pattern that was obliterated by fertilization, illustrating the interaction of habitat productivity and dispersal limitation in regulating current patterns of establishment and distribution of invading U. rubra seedlings at the site. We suggest that current patterns of woody plant invasion in this region are very different from what likely occurred historically prior to settlement, due to major anthropogenic shifts in ecological context. This altered context includes the suppression of wildfire, legacy effects of prior agricultural activity and increased availability of woody plant propagules across the region.