Avian Response to Nutrient Enrichment in the Northern Everglades
Gaea E. Crozier, and Dale E. Gawlik
South Florida Water Management District, West Palm Beach, FL, USA
Abstract.- Years of nutrient enrichment from urban and agricultural runoff have
led to eutrophication of the northern Everglades. To examine the influence of
nutrient enrichment on the avian community, we surveyed birds along a nutrient
gradient in Water Conservation Area 2A (WCA 2A). Bird species were generally
more abundant in the nutrient enriched sites than unenriched sites. However,
differences in species composition were observed between enriched and
unenriched sites. In the naturally nutrient limited Everglades, nutrient enrichment
fundamentally changes the structure of the Everglades and is therefore
incompatible with ecosystem restoration.
Introduction.- The Everglades was originally an oligotrophic, phosphorus-limited
wetland with nutrient inputs restricted primarily to rainfall (Davis 1994).
Currently, phosphorus additions to the northern Everglades are about 10-20-fold
higher than historical levels (McCormick and O’Dell 1996, McCormick et al.
1999). Little is known about the effects of nutrient enrichment on birds in the
Everglades, and this is a particularly important issue in the face of the current
Everglades restoration efforts. We hypothesized that abundance of the nonwading birds would be higher in nutrient-enriched areas as the result of increased
productivity leading to increased food resources. We hypothesized that the
abundance of wading birds would be lower in nutrient-enriched areas as a result
of minimal slough habitat available for foraging due to cattail encroachment.
Methods.- The study was conducted in WCA 2A in the northern Everglades.
Nutrient enriched water flows from north to south in WCA 2A creating a nutrient
gradient extending approximately 8 km into the interior of the marsh (Payne et al.
2001). To gather information on the non-wading-bird community across this
nutrient gradient, we surveyed birds with point counts at 10 sampling sites in
1996 and 1997. A repeated measures ANOVA was used to examine differences
in bird abundance and species richness across the nutrient gradient. To gather
information on wading birds, aerial systematic reconnaissance flights (Bancroft
and Sawicki 1995) were flown in 1991 and 1995 to obtained bird abundance data
in 4-km2 cells across WCA 2A. An ANCOVA with water depth as a covariate
was used to compare wading-bird abundance in enriched and unenriched cells.
Results.- There were more individuals (all species combined) in enriched sites
compared to transitional and unenriched along the nutrient gradient (P = 0.05;
Fig. 1). There was not a significant effect of nutrient enrichment on species
richness. Of the four dominant species in the study, Boat-tailed Grackles and
Common Moorhens were more abundant in enriched sites (P  0.02) while
Common Yellowthroats were more abundant in unenriched sites (P < 0.01). Redwinged Blackbirds showed no effect. Examining species compositional
differences, the bird community in enriched areas tended to have more species
that prefer densely vegetated areas (i.e., rails, bitterns) whereas unenriched areas
had more species that utilize open areas (i.e., shorebirds). In the wading bird
community, Wood Storks and Great Egrets were more abundant (P < 0.01) in the
enriched compared to unenriched 4-km2 cells in a dry year (Fig. 2). Great Egrets
and all wading bird species combined had significantly more individuals (P <
0.01) in the enriched compared to the unenriched cells in a wet year. For White
Ibises and Wood Storks, the models did not converge in the wet year, however,
there tended to be more individuals in enriched compared to unenriched cells.
Discussion.- The results of this study suggest that some members of the bird
community tend to increase in abundance with nutrient enrichment. Changes in
habitat (i.e., increased amount of substrate for nesting or foraging) or changes in
the food web that increased food resources were presumably the cause. Overall,
water depth was the main factor affecting wading bird foraging. After accounting
for water depth, wading bird abundance generally increased in nutrient-enriched
areas even though cattail invasion left few open-water sloughs for foraging. We
suspect this was because the few open-water areas available to wading birds had a
higher availability of prey. However, the majority of open-water sites in enriched
areas in our study had been artificially created by airboat traffic. We hypothesize
that in dry years the abundance of wading birds is primarily driven by decreasing
water levels concentrating prey, but in wet years wading bird abundance is also
driven by habitat characteristics, such as nutrient enrichment, that may make prey
more available. Although bird abundance may generally increase in nutrientenriched areas of the Everglades, this is undesirable when indicative of an
unhealthy ecosystem. Nutrient enrichment fundamentally changes the Everglades
by changing the species composition and productivity of different components of
the system.
Fig. 1. The mean ± SE number of birds per site in enriched (n = 4; solid black),
transitional (n = 4; striped white), and unenriched (n = 2; solid white) areas.
Fig. 2. The mean ± SE number of four common wading bird species and all
wading species combined in enriched (n = 22 in 1991, n = 108 in 1995; solid
black) and unenriched (n = 128 in 1991, n = 342 in 1995; striped white) cells.
Gaea Crozier, South Florida Water Management District, Everglades Division
MS 4840, 3301 Gun Club Rd., West Palm Beach, FL 33406, Phone: 561-6868800 Ext. 4538, Fax: 561-682-6442, gcrozie@sfwmd.gov