ECOLOGICAL SUCCESS INDICATOR: REESTABLISHMENT OF HEALTHY WADING BIRD POPULATIONS
J. C. Ogden (Chair), South Florida Water Management District, G. T. Bancroft, Archbold Biological Station, and P. C. Frederick, University of Florida.
The following success indices were developed by the wading bird team as a means for assessing the recovery of more healthy populations of wading birds in the Everglades, Big Cypress and other wetland basins in south Florida. The team believes that success indices which are designed to evaluate the regional health of wading bird populations in the south Florida wetland systems should be based on annual nesting patterns. We propose the use of three categories of indices, which will use measures of, (1) the numbers of nesting birds, (2) the timing of nesting, and (3) the locations of nesting colonies. At present, the team does not consider indices based on the total number of wading birds that are foraging in south Florida wetlands to be useful for assessing the success of the restoration program, or the health of regional wading bird populations, because of current uncertainties about the factors which influence two parameters of south Florida wading bird populations, (1) the total number of birds that are foraging in south Florida wetlands at any given time, and (2) the proportion of these birds that initiate breeding. The team will reconsider the usefulness of foraging patterns trends as measures of success by the restoration program, once a regional assessment of the Systematic Reconnaissance Flights (SRF) wading bird database has been completed and reported.
In healthier, south Florida wetland ecosystems, we would expect to see increases in numbers for each of the colonially-nesting species that characteristically dominated the pre- project, mainland colonies of the region. These species are: Great Egret, Snowy Egret, Tricolored Heron, Little Blue Heron, White Ibis, and Wood Stork. We recommend that increases in the number of nesting birds be measured against a base-line established from 1986-1995 data.
This base-line is selected because it contains both a multi-year dry cycle and a multi-year wet cycle, and because total numbers of nesting birds for these species during this 10-year period were as low (Great Egrets only), or were lower (all other species) than for any known previous period (Ogden 1992,1994, Frederick 1995). The base-line will be expressed as a series of three-year running averages for each species, and for each major hydrological basin (Everglades and downstream estuaries, Big Cypress and downstream estuaries, L. Okeechobee, and Kissimmee River), for which sufficient data exist (Tables 1 & 2). Three year running averages are useful for removing much of the characteristic, between-year abruptness in the colony record, and for moderating inaccuracies in the data caused by census error. Comparisons against the base-line will use three-year running averages beginning in the 1996 nesting year (1995- 1996), for each species and each basin. Since it is known that the locations of colonies may change across large landscape scales between years, additional comparisons should be made for all basins combined.
For five species we propose a minimum population size that must be reached and maintained (based upon three year running averages), as a measure of the recovery of the south Florida wading bird populations. The proposed minimum nesting populations are three year running averages of 4,000 nesting pairs for Great Egrets, in the range of 10,000-20,000 pairs for Snowy Egrets and Tricolored Herons combined, 10,000-25,000 pairs for White Ibis, and 1,500-2,500 pairs for Wood Storks (Everglades and Big Cypress colonies combined). These recovery targets are based on the estimated size of the nesting populations for these five species during the early CSF Project period, 1940s-1960s (Robertson and Kushlan 1974, Ogden 1994). We are unable to suggest recovery objectives for the Little Blue Heron because of the lack of quantitative information on population size for this species during the pre- or early project periods, or for the base-line years.
These three species are "Great White Heron", Reddish Egret, and Roseate Spoonbill. Numbers of nesting pairs for two of these species in Florida Bay, Great White Heron and Roseate Spoonbill, appear to be as high today as at any time in this Century (W. Robertson, pers. comm., Bjork and Powell 1993, Powell et al. 1989); these numbers should be maintained or increased. Reasonable goals for these two species are 750-1,000 nesting pairs of Great White Herons, and 750-1,000 pairs of Roseate Spoonbills (Table 1). Numbers of nesting Reddish Egrets (currently estimated at 100-200 pair; R. Paul, pers. comm., Powell et al. 1989) are lower than were present in the pre-project system; these numbers should increase to the range of 250-500 nesting pairs.
The primary species to watch here is the Wood Stork, which typically formed colonies in the pre-project Everglades and Big Cypress regions between late November and early January, and since the early 1970s has formed colonies during February and March in most years (Ogden 1994). Stork colonies that form after January have much lower rates of success. Colonies that form as late as March often lose most of their nestlings to starvation once the summer rains begin. A recovery objective would be for storks to initiate nesting no later than the end of January in most years, and as early as December in some years. Ecosystem recovery also should result in earlier colony formation by other species, most notably for the White Ibis, and to a lesser extent among the other species. White Ibis in the southern Everglades once formed colonies in February-March, and in recent years in the central-northern Everglades have formed as late as April-May (Frederick 1995). In a recovered system, we would expect colonies of such species as Snowy Egret, Tricolored Heron, and White Ibis, located in the more traditional nesting areas in the southern Everglades, to form between mid-February and mid-March in most years.
The largest and most persistently used sites for mixed species colonies during the historical period, 1930s-1960s, were located in the mainland, mangrove region downstream from the southern Everglades flows (Ogden 1994). It is strongly hypothesized that this mainland estuarine subregion (Alligator and Rodgers River bays, East River and Lane River, Alligator Lake, Cuthbert and Madeira rookeries, Broad and Shark river headwaters), under pre-drainage conditions, provided higher densities of prey and a greater range of foraging site options for nesting wading birds than is possible in other regions of the system. Since the late 1960s these estuarine sites have been abandoned, or they have contained a much smaller percentage of the basin-wide nesting population of wading birds than was the case in the pre-project Everglades. Over 90% of all known nesting each year between 1932-1946 was in this estuarine subregion, compared to 6-58% (mean 26%) for the years 1986-1995 (Ogden 1992, 1994, Frederick 1995). Ecological recovery of the south Florida wetlands should result in the return of the largest, mixed-species colonies to the mainland estuarine subregion in most years. We recognize that the recovery of nesting in the mainland estuarine region may result in reduced percentages of nesting in the central and northern Everglades, and Lake Okeechobee, compared to recent years.
We suggest that a much stronger prey base was once characteristic of the broad mainland estuaries, downstream from the Everglades and Big Cypress, and that this prey base provided support for the numerous wading bird nesting colonies that were located on islands along the southwestern Gulf coast and across Florida Bay. All of the colony sites on the Gulf coast (primarily egrets and small herons), located between Chokoloskee Bay and Lostman's River (Duck Rock, Gopher, Pelican, Plover, and Pumpkin keys) have been abandoned since the 1960s (Ogden 1994). The numbers of nesting spoonbills in important colonies in northeastern Florida Bay (Tern, Porjoe keys) have become much reduced birds since the early 1970s (500 pairs down to 150 pairs; Bjork and Powell 1993). In a recovered system, we would expect to see annual nesting at one or more coastal sites between Chokoloskee Bay and the mouth of Lostman's River, and a doubling of the recent number of spoonbills nesting in northeastern Florida Bay.
For none of the five success criteria proposed above have we attempted to suggest specific, annual rates of change (rates of improvement) that would constitute acceptable levels of progress. For example, the question of how many more wading birds should nest each year, in order to show that a restoration project is meeting its objectives, is difficult to predict on short temporal scales (uncertainties regarding ecosystem lag effects) or in the absence of knowledge of the specific components of each iteration of the restoration project. Each iteration presumably will have uneven effects on the five success parameters. Never-the-less, three year running averages, calculated over 10 year periods, should reveal the level of response for numbers of nesting birds. Over these 10 year time frames, rates of improvement in numbers of nesting birds that are less than 25% of the base condition may be viewed with concern. Declines in nesting populations for several species of wading birds occurred at rates that were greater than 25% over 10 year periods of time since the 1 950s, and it may be argued that recovery should be able to occur at similar rates.
The current wading bird colony survey project needs substantial improvement in two areas, if it is to provide the quality and quantity of information that will be necessary for measuring these five indices. These areas of improvement are, (1) in the spatial extent of coverage, and (2) in the survey protocols. Currently, there is no regular colony survey conducted for the Big Cypress, Florida Bay, Gulf coast of EVER, or Lake Okeechobee. Where regular surveys are being conducted (Loxahatchee NWR, WCAs 2 & 3, and mainland EVER), differences in survey protocols have raised questions regarding levels of comparability among the results.
A potentially simple scoring process for measuring annual progress towards meeting the five recovery goals for wading birds would be to use a 0 to 5 scale. 0 would represent no improvement with any of the five indices, and 5 would represent improvement with all of the listed indices. The two indices for numbers of nesting birds would each receive an annual score of " 1 " if the most current, three year mean represented an increase above the range of means for the base-line period. The index for timing should (at least initially) be based on Wood Stork nesting, and would be scored as a " 1 " in any year when nesting by a majority of storks is initiated no later than January. The index for mainland colony locations would be scored as " 1 " if a three year running mean of the percentage of the total number of birds nesting in all Everglades colonies, which nest in the traditional mangrove ecotone colony areas, increases over the immediately preceding three year mean. The index for coastal colony locations would receive a 0.5 score for any year when one or more colonies forms on islands along the Gulf coast north of Lostman's River, and an additional 0.5 score when the number of spoonbills nesting in the northeastern Florida Bay colonies increases over the previous year, up to the target goal of 250 pair.
Bjork, R.D. and G.V.N. Powell. 1993. Relationship between hydrologic conditions and quality and quantity of foraging habitat for Roseate Spoonbills and other wading birds in the C- 111 basin. National Audubon Society, Tavernier, Florida.
Frederick, P.C. 1995. Wading bird nesting success studies in the Water Conservation Areas of the Everglades, 1992-1995. Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL.
Ogden, J.C. 1992. Wading bird colony dynamics in the central and southern Everglades. Annual report of the South Florida Research Center, Everglades National Park, Homestead, FL.
Ogden, J.C. 1994. A comparison of wading bird nesting dynamics, 1931 - 1946 and 1974-1989 as an indication of changes in ecosystem conditions in the southern Everglades. Pp. 533- 570 in: The Everglades. The ecosystem and its restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.
Powell, G.V.N., R.D. Bjork, J.C. Ogden, R.T. Paul, A.H. Powell, and W.B. Robertson, Jr. 1989. Population trends in some Florida Bay wading birds. Wilson Bulletin 101: 436-457.
Robertson, W.B.,Jr.and J.A.Kushlan. 1974. The southern Florida avifauna. Miami Geological Society Memoir 2: 414-452.
|Editor's note: the following paper was submitted to the SSG in early October, 1996, and is an application of the wading bird success criteria to the 1996 nesting season in the Everglades basin.|
The Science Sub-group to the South Florida Ecosystem Restoration Working Group is currently developing a set of ecological success measures for the restoration program. Included in this set of success measures is a recommendation for using the nesting patterns of wading birds as one indicator of change in ecological conditions during and following the implementation of the restoration projects. Ogden et al. initially proposed that three different parameters of wading bird nesting patterns in south Florida could be used to demonstrate the influence of restoration projects. These measures are (1) the number of nesting birds, (2) the timing of nesting and (3) the location of nesting colonies.
We have evaluated two of these parameters, number of nesting birds and location of colonies, to compare the 1996 nesting season with the pattern during a recent baseline period, and to compare 1996 with the nesting patterns that have been proposed as targets for restoration. WE did not evaluate the timing parameter because, at the time of this assessment, we lacked complete information on timing for many colonies during the 1996 season.
For five common species of wading birds in mainland colonies in the Everglades basin, Ogden et al. recommended that one measure of restoration success would be for the number of nesting birds to increase from the current low population to population levels comparable to those which occurred during the early CSF Project period (1940's - 1960's) the current nesting population for these five species would be determined from a 20 year base period of record (1986-1995). The five species are Great Egret, snowy Egret/Tricolored Heron combined, White Ibis, and Wood Stork. The proposed restoration targets for these five species would be for the numbers nesting in the Everglades basin, consisting of the three Water Conservation Areas and mainland Everglades National Park. Because of the substantial year-to-year variation in numbers of nesting birds in both the natural and managed systems, it was further recommended that three year running averages of numbers of nesting pairs for each species be used for comparisons between current between current and target population.
The following table lists the highest and lowest values (number of pairs) for three year running averages calculated from the base years (1986-1995), the three year running average for the period including 1996 (1994-1996), and the target three year running averages for nesting populations (pairs) based on the 1940s-1960s period, for each of the five species.
|Species||Base high/low||1994 -1996||Target|
|Great Egret||1,163 - 3,843||4,043||4,000|
|Snowy / Tricolor||903-2,939||1,508||10,000 - 20,000|
|White Ibis||2,107 - 8,020||2,172||10,000 - 20,000|
|Wood Stork||130 - 294||343||1,500 -2,500|
Ogden et al. (1996) report that, during the historical period between the 1930's and 1960's, over 90% of the wading birds of the five species listed in the table above nested in colonies that were located in the mainland, mangrove region, and along the mangrove/marsh ecotone downstream from the southern Everglades. The numbers which nested in this region during the base year, 1986-1995, ranged between 6% and 58% (mean 26%) of the total of these five species in the Everglades basin. Ogden et al. suggested that the former colony locations in the estuarine region, under pre- drainage conditions, provided higher densities of prey and a greater range of foraging site options for nesting wading birds than is possible in most years or under highly managed conditions, in other areas of the Everglades basin. A recommended restoration target for colony locations is for the percentage of birds nesting in the estuarine region to substantially increase above the percentages which occurred during the base years. The number of these five species which nesting in the estuarine colonies in 1996 was 9% of the total pairs which nested in the total Everglades basin.
The three year running averages (1994-1996) for number of nesting pairs of Great Egrets and Wood Storks in mainland, Everglades colonies were higher than any three year averages recorded for these two species during the base period (see table). The most recent three year average for the Great Egret essentially equaled the restoration target (4,000 pairs), while the average for the stork remains far below the target. The 1994-1996 running averages for the Snowy Egret/Tricolored Heron (combined) and White Ibis were within the ranges recorded during the base period; neither showed improvement in the direction of the targets. The relatively healthy nesting population of Great Egrets during the 1995- 1996 period seems to be a reflection of the strong nesting effort by this species during three high water years (P. Frederick. 1995. Wading bird nesting success studies in the Water Conservation Areas ofthe Everglades, 1992-1995. Annual Report, SFWMD; Gawlik & Ogden 1995, 1996), and the fact that Great Egrets have been less adversely impacted by water management practices during the past several decades than have he other common species in the basin (P. Frederick, pers. comm. ). The relatively high, 1994-1996 average for storks was largely due to the sharply increased nesting effort in 1996, which presumably was a response by storks to improved foraging conditions created by a rapid dry-down following three high water years. Why the smaller species of waders did not react similarly in 1996 is a question with no easy answer, although the differences among species in how they responded to water patterns during the 1996 dry season is another example of the fact that there are strong differences in feeding ecology among the common species of waders.
The percentage of pairs of birds which nesting in estuarine and ecotone colonies in 1996 was well below the mean for the base period of years, and thus showed no improvement in the direction of the restoration target for colony locations. The low number of birds nesting in the area of the traditional, estuarine colonies in 1996 was in contrast to the pattern in several high water years during the 1970's and 1980's, when higher percentages of the total nesting effort occurred in the estuarine colonies in the wetter years. Reasons for the low number of nesting birds in estuarine colonies in 1996 are not obvious, although the fact that relatively few of the smaller species of waders nested at any site in 1996 may provide one clue. For more discussion on this question, see J. Ogden 1991. Wading bird colony dynamics in the central and southern Everglades. Annual Report. SFRC.