Outside Collaborators: Michael L. Avery, USDA APHIS, Wildlife Services; Kim J. Babbitt, University of New Hampshire; Mark Brown, University of Florida, Center for Wetlands; Joan L. Morrison, Trinity College; Robert A. Zielinski and William H. Orem, U.S. Geological Survey

Science Advisory Board. The MacArthur Agro-ecology Research Center (MAERC) formed a new Science Advisory Board (SAB) to provide outside independent review of the research program at MAERC. The SAB evaluates the research program, submits a general report to the Archbold Expeditions Trustees, and provides feedback to Archbold and MAERC scientists. The SAB held its first meeting at MAERC on 16-17 March 2000 (see photo, this page), and will meet every 18-24 months. Current SAB members are: John M. Briggs (Arizona State Univ.), David Genereaux (North Carolina State Univ.), R. Richard Lowrance (USDA-Agricultural Research Serv., Tifton, GA), and Judith L. Meyer (Univ. Georgia).

Experimental Pastures Project. The experimental pastures project continues to be the main focus of the research program at MAERC. The project consists of a 420-ha array of 16 experimental pastures, 8 each of improved summer pastures and semi-native winter pastures. The cattle stocking treatments were started in fall, 1998. The project investigates the effects of cattle stocking density on nutrient loads in surface water, soil chemistry, and various ecological characteristics of the pastures. There are two replicate treatments of four different cattle stocking densities (control, low, mid, and high) in each pasture type. Cattle are in the summer pastures during May-October and in the winter pastures during November-April. Three major projects in the experimental pastures are supported through various agencies. Funding from South Florida Water Management District, Florida Department of Environmental Protection (DEP), and U.S. Environmental Protection Agency supports collection and analysis of water samples for calculation of nutrient concentrations and loads from the experimental pastures (see photo, page 24). Funding from the U.S. Department of Agriculture’s National Research Initiative (US-NRI) supports a large collaborative project with the University of Florida (UF) that focuses on integrating the ecological and economic aspects of sustainable grazing systems, and another project that focuses on nutrient cycling processes in seasonal wetlands in the experimental pastures. The overall project also receives generous annual support from both the UF Center for Natural Resources and the Office of the Dean of Research at UF Institute of Food and Agricultural Sciences (UF-IFAS).

Preliminary results of the water quality research show that surface water concentrations and loads of total phosphorus (P) are significantly greater from the improved pastures than from the semi-native pastures. Average total P (TP) concentrations in 1998 and 1999 were 0.46 and 0.58 mg/L, respectively, in runoff from improved pastures and 0.08 and 0.12 mg/L, respectively, in runoff from semi-native pastures. The current target P concentration for beef cattle ranches is 0.35 mg/L. Total annual P loads in surface runoff were about 6 times greater, on average, from improved pastures than from semi-native pastures. The ratio of biologically active ortho-P in runoff from the improved pastures was also greater than in runoff from the semi-native pastures. The cattle stocking rate treatments had no consistent significant effect on P concentrations or loads in surface runoff but stocking rate effects are likely to be cumulative over a period of several years and may therefore develop later in the study. These results are summarized in a 2000 report by John Capece, Ken Campbell, Don Graetz, and Patrick Bohlen to the Florida DEP.

In addition to surface water monitoring, we have also been doing intensive analysis of P concentrations and forms in surface soils of the experimental pastures. This critical work, led by Don Graetz (IFAS), will help identify P sources and potential strategies for remediation of excessive P loads. Concentrations of water soluble P, the most soluble P form and thus the most susceptible to leaching, were greater in the improved pastures than in the semi-native pastures at all depths and were greatest in the upper 5 cm of soil (see Fig., this page). The degree of phosphorus saturation in surface soil was 20% in the improved pastures but only 10% in the semi-native pastures, indicating a possible saturation threshold for significant P release from surface soil into runoff water. Links between soil P pools and surface water P concentrations will be a major research focus of the project in the future. Lakeisha Hill (see page 38), UF Master’s student working under Graetz’s supervision, plans to examine soil P forms in deeper soil layers to see if these may contribute to P release to surface water.

Although P is the primary concern from a water quality standpoint, we are also measuring concentrations and loads of different forms of nitrogen (N) in surface runoff (TKN, NH₄⁺, NOx). As was the case with P, NH₄⁺ concentrations in runoff were greater from improved pastures than from semi-native pastures in both 1998 and 1999. Nitrate concentrations tended to be greater in the semi-native pastures. These patterns were consistent with soil N mineralization, which was greater in the improved pastures, and nitrification rates, which were greater in the semi-native prairies. Surprisingly, N mineralization rates were lowest in pastures with the highest stocking density. Although the reason for this pattern has not yet been determined it is likely that intensive grazing affected the soil microbial community indirectly by altering belowground allocation of resources in the grazed plants. This possibility and other possible causes for the observed influence of cattle grazing on N mineralization will be the subject of future investigations.

Numerous other data were collected in the experimental pastures and collaborating scientists are currently analyzing these data. Jeff Mullahey and his research assistants, Amy Hendrie and Jim Johnston, collected 2 years of data on grass production in relation to the cattle stocking densities. Forage utilization by the cattle varied seasonally and was closely related to stocking density. During a drought in June 1999, cattle in the high stocking density treatment utilized over 60% of available forage. Data were also collected on protein and P content and composition of the vegetation in the pastures. These data will be analyzed with respect to cattle stocking density, land use, and soil nutrient characteristics in the pastures. George Tanner analyzed bird populations in the experimental pastures and did not find many significant patterns in relation to grazing intensity but did observe some differences associated with pasture type. The eastern meadowlark is the most common species in the pastures and Tanner plans to monitor the nesting success of this species in the pastures in the next phase of the project. Robert McSorley evaluated nematode community structure in the pastures and found over 50 different genera including bacterivores, fungivores, herbivores, predators, and omnivores. Densities of most nematode genera were similar among improved and semi-native pastures. There was a suggestion that some genera may be affected by grazing although this subject will require further study and McSorley plans to have a Ph.D. student work on this question over the next 3 years. In addition to the ecological and environmental areas, the pasture project also supported development of an ecological simulation model for cow-calf operations which will eventually be used to analyze economic performance.

Wetland Research. Despite the lack of rainfall in 2000, wetland ecology continued to be an important part of the research program at MAERC. Matt Baber (see page 38) completed his doctoral research on the relative importance of landscape factors, local abiotic factors, and biotic interactions in influencing tadpole community structure in temporary wetlands at MAERC. He measured environmental characteristics in 24 isolated wetlands, and sampled tadpole communities during the 1999 wet season (June-October). Landscape characteristics, mainly wetland proximity to nearest woodland and the amount of woodland surrounding the wetland, and biotic interactions (fish predation) had the largest influence on tadpole community structure. Fish predation influenced tadpole communities more than expected due to the ubiquity of wetlands, lack of topographic relief, and dispersal abilities of several fish species. Differences in tadpole community structure among habitat types could be attributed to differences in woodland attributes and susceptibility to fish colonization. In the summer of 2000, Matt conducted a series of controlled experiments in small pools to examine fish predation on tadpoles and determine consumption rates for different species of fish on different species and sizes of tadpoles. From a conservation standpoint, his research indicated that temporary wetlands proximal to woodland areas and isolated from permanent water sources appear to be most important to amphibians.

A new project examining nutrient cycling processes in wetlands at MAERC was started in September 2000 by Patrick Bohlen, Don Graetz, and Al Steinman with funding from the US-NRI Competitive Grants Program. Despite the recognized importance of agricultural wetlands as wildlife habitat and sources of nutrient accretion, little is known about how these wetlands respond to nutrient enrichment, disturbance, or changes in land use. This new project will evaluate the effects of cattle grazing and associated land use on seasonal wetlands at MAERC, as representative of much of the grazed land in south-central Florida. The project will examine nitrogen and phosphorus cycling, biological productivity, and associated ecological characteristics in seasonal wetlands of subtropical rangeland. The 2-year project provides support for Stanley Gathumbi, MAERC Post-doctoral Research Associate (see photo, page 25), and a UF Master’s student (supervised by Graetz).

Wading Birds and Raptors. In addition to ongoing research in natural, seasonal wetlands, another project examined abundance of wading bird prey in the drainage ditches on the Ranch. These drainage ditches are created and maintained by humans but serve as important semi-natural wetlands. One of our main long-term projects examined the distribution and abundance of wading birds in the main drainage ditch network and MAERC and in 1999 Dave Anderson, Matt Baber, and Mike McMillian started a one-year study in which they examined the abundance of wading bird prey in the ditches. They sampled the ditches monthly at random locations and found that the ditches support a diverse and abundant assemblage of prey species for wading birds throughout the year. The assemblage is dominated by two species of live-bearing fishes, the mosquitofish (Gambusia holbrooki) and the least killifish (Heterandria formosa). McMillian continued his work in reproductive success of barred owls and red-shouldered hawks. We now have 6 years of data on reproductive success of these two raptors at the Ranch, including 3 dry years and 3 wet years. The results to date indicate the reproductive success of the hawk is much better in wet than in dry years.

Outside collaborators initiated new areas of research at MAERC during 1999-00. Mike Avery, and his research assistants of the USDA APHIS Wildlife Service, investigated the occurrence and extent of black vulture depredation on newborn calves. There are reports of black vultures attacking and killing young calves during or soon after birth but there is very little documentation of the frequency or extent of this problem. At the Ranch, black vultures have been observed attacking live calves and also attacking weakened cows during birthing. However, long hours in the last 2 years observing black vulture behavior at the Ranch have not provided clear evidence that black vulture predation on newborn calves is a serious concern and it likely occurs at a low frequency.

Research by Bob Zielinski and Bill Orem may help to clarify the source of excess phosphorus in runoff from pastures at the Ranch. They are evaluating phosphorus sources in water and soil by measuring uranium isotopes in a regional survey throughout central and south Florida. Uranium in phosphorus fertilizer has a unique isotopic signature, which can be distinguished from the isotopic signature of naturally distributed uranium. The source of phosphorus samples from various locations with different degrees of phosphorus enrichment can be inferred from the isotopic signature of uranium the samples. The experimental pastures at the Ranch have been included in the regional study as a site with a relatively low degree of phosphorus enrichment. Soil and water samples were obtained from the experimental pastures in 2000 and these are currently being analyzed for phosphorus content and uranium isotopes. The results may help confirm our conclusion that the excess phosphorus in runoff from our improved pastures is due to past fertilizer use.