Avian Seed Dispersers on Tree Islands in the Central Everglades

Dale E. Gawlik, Peg Gronemeyer, and Robert A. Powell

Everglades Systems Research Division

South Florida Water Management District

3301 Gun Club Road

West Palm Beach, FL 33406

INTRODUCTION

-   Importance of avian seed dispersal for tree islands in the
    Everglades?

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-   To predict where trees will develop (both potential colonization of
    new islands and maintenance of existing islands) requires knowledge
    of the quantity of seeds dispersed and their locations

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-   The probability of a seed dispersing to a particular site is
    dependent on characteristics of the dispersal agent (e.g., foraging
    behavior) and environmental factors (e.g., ground surface about
    water).

-   Many trees in the Everglades have fleshy fruits that are likely
    dispersed by birds.

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-   Model of seed dispersal system

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-   Figure 1 simplistic seed dispersal model and highlight of where
    birds fit in to the process. Alternately, use more complicated model
    presented by Wu and Sklar

-   Description of the model components relevant to avian seed
    dispersal.

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-   Seed characteristics: size, timing, quantity, nutritional quality,
    viability

-   Site characteristics: water levels, nutrients, elevation, distance
    to seed source, size (area)

-   Characteristics of dispersal mode

-   Animal: physiologic needs, diet selectivity, habitat selectivity,
    timing, movement patterns, foraging behavior

-   Wind: timing, speed, direction

-   Water: depth, timing, flow velocity, direction

-   Study objectives

-   1\. Identify trees/shrubs dispersed by birds

-   2\. Identify birds that disperse tree/shrubs

-   3\. Quantify seasonal patterns of avian dispersers

-   4\. Quantify habitat use patterns of avian dispersers

METHODS

-   Data from objectives 1 and 2 based on literature search

-   Data from objectives 3 and 4 from avian community study in central
    Everglades

-   Figure 2 -- map of Water Conservation Area 3 showing survey sites

-   Sites distributed between 2 habitats (description of bayheads and
    marsh)

-   Sampling periods: 4 each during summer 1996, summer 1997, winter
    1997, and spring 1998

-   Avian point-count methodology

-   Surveys starting within 30 minutes of dawn

-   Conducted from airboat

-   Two-minute settling period after arrival followed by a 6-minute
    survey period

-   Record all birds seen and heard

RESULTS

-   List of trees and shrubs occurring on tree islands and dispersed by
    birds

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-   Table 1 - Bird species names, percent of diet comprised of
    fruit/seeds of Everglades trees/shrubs, and guild based on percent
    fruit/seeds in diet (i.e., no fruit/seed guild, low fruit/seed
    guild, high fruit/seed guild)

-   Percent of avian community in each diet guild

-   Figure 3 -- Total bird abundance by guild, season, and habitat

-   Figure 4 -- Seasonal abundance of individual bird species in high
    fruit/seed guild

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-   ANOVA's on differences in abundance of birds among seasons and
    habitats for each guild

-   Range-wide population trends for individual species in high
    fruit/seed guild

DISCUSSION

-   Characterize Everglades avian dispersal system

-   Few highly specialized avian frugivores but large number of
    opportunistic frugivores

-   Low temporal variability of low fruit/seed guild and high temporal
    variability of high fruit/seed guild (high fruit/seed guild in
    Everglades is not as highly specialized as in tropical rainforest)

-   Potential importance of continental population declines of species
    in high fruit/seed guild

-   Strong habitat differences among avian dispersers are not evident in
    literature from tropics (importance and uniqueness of habitat
    differences in the Everglades)

-   Habitat heterogeneity among patches is high at meso scales (0.1-0.5
    km blocks) which can lead to habitat partitioning by birds

-   Colonization is one of island patches surrounded by marsh so seeds
    disperse among patches rather than incrementally from a seed source

-   Importance of species that are habitat generalists (low fruit/seed
    guild) or fly among islands in colonizing new tree island sites
    versus importance of species that stay on a single island (high
    fruit/seed guild) and disperse seeds within islands

-   In contrast to the Everglades, dispersal systems in tropical
    rainforest, where most avian dispersal theory was developed, ignore
    habitat differences and focus on diet and physiology

-   Habitat differences less important than diet and physiology in
    tropical rainforest because:

-   Tropics have large diversity of frugivores and large variety of
    fruits which lead to strong diet differences

-   In tropics, habitat heterogeneity among patches is high at small
    scales (1-10 m blocks) and provides little opportunity for habitat
    partitioning among bird species

-   In tropics, colonization is of gaps surrounded by forest where among
    patch movement of seeds is less important than incremental dispersal
    from edges

CONCLUSIONS

-   Describe "specialist versus generalist" paradigm

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-   Hypothesize where Everglades fits along that continuum

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-   Factors important for seed dispersal in Everglades should include
    patch dynamics and island biogeography processes