October 2003
Habitat loss has occurred at devastating rates throughout the world over the past two centuries. Yet these losses are not all equal, and some environments have been affected more than others. Wetland habitat in North America provides one such example. Throughout much of the United States, wetland habitat loss has been substantial:
Between 1780’s and 1980’s, the lower 48 states have lost 53% of the original wetland habitat, or about 104 million acres.1
Twenty-two states have lost 50% or more of their original wetlands, with California losing the largest percentage (91%) and Florida losing the most acreage (9.3 million acres).1
Most recent losses over the past two decades have been primarily due to agriculture and urban development.2
Most recent losses have been in freshwater wetlands (98%), and 95% of the remaining wetlands are freshwater (as opposed to coastal marshes).2
This severe wetland loss is probably not isolated to the United States. For example, in New Zealand only 8% of the original wetlands remain.3 However, for most areas of the world, little information exists on both wetland loss and the current status of wetlands.3
In some ways, this severe habitat loss is not entirely surprising. Public perception of wetlands was historically quite negative, with wetlands perceived as swampy wastelands hardly good for anything.4 This view shaped wetland policy until the early twentieth century when perceptions began to change slowly with the growing concern about declining migratory bird populations.4 Yet we now know that wetlands are key components in ecosystems, providing many services such as:
reducing erosion by potentially decreasing floodwater velocity and volume5
preventing downstream flood attenuation by spreading excess water flow in floodplains and shallow depressions5
improving water quality by removing nutrients and absorbing sediment loads6 (this potential benefit is highly dependent on the amount of time that water is retained within wetlands7)
acting as nutrient sources for fish and other aquatic organisms inhabiting downstream areas in riverine and coastal situations8
providing habitat for a variety of wildlife, such as waterfowl, amphibians, and insects (for example, the wetland landscapes in the Prairie Pothole Region of the Midwestern United States are thought to produce 50-75% of the total continental duck production9)
Fortunately, a variety of recent U.S. federal and state programs (e.g., the Wetland Reserve Program10) have increased wetland protection and provided incentives for habitat restoration. But did this protection come too late?
Patterns and potential causes of wetland bird population trends
Meanwhile, many migratory birds have experienced declines throughout the United States. Evidence for bird declines has been based primarily on the past 40 years; prior to the 1960’s little data exist for interpreting long-term trends. Data on wetland birds are particularly sparse, yet some evidence indicates that this group might indeed be experiencing population declines.11,12 What factors are contributing to these declines? And what are the ramifications?
Habitat loss and fragmentation: Both the loss and fragmentation (or “breaking apart”) of wetlands can have detrimental impacts on wetland bird populations by influencing habitat use, reproduction, and survival.13
Invasive species: Some non-native plant species have invaded wetlands, causing profound changes in wetland structure, which could make those wetlands unsuitable for some species of birds. For example, Purple Loosestrife (Lythrum salicaria) has invaded many wetlands in the midwestern United States, and avian diversity in Purple Loosestrife dominated wetlands can be lower than diversity in other wetland types.14
Global climate change: Climate can have particularly strong effects on wetland structure if precipitation declines or becomes more variable in the future. Although much disagreement remains in projecting future changes in precipitation, most models project global increases in precipitation along with some regional declines (including predicted declines for the Prairie Pothole Region of the United States) occurring due to poleward shifts in rain belts.15 Precipitation can affect wetland birds by altering water levels and habitat structure within wetlands.16 Also, in times of drought, climate can cause habitat loss because smaller, shallower wetlands will become dry, effectively changing into grassland or open habitat. This climate-caused habitat loss has been correlated with annual variation in some wetland birds, including many waterfowl.9,17
How can habitat loss influence wetland birds?
Habitat loss and fragmentation might be particularly important for bird populations. While evidence has mounted for habitat loss and fragmentation affecting bird populations using other habitats, evidence for effects on wetland birds is limited. Most research in wetlands has been inconclusive because of poor study design.13 However, a few patterns have emerged.
Bird abundance and habitat use: Some species, such as Seaside Sparrows, may not use small wetlands or may occur in lower densities than in larger wetlands.18,19 Other species, such as Black Terns, may be less likely to settle in landscapes with few wetlands.19 Wetlands that are relatively isolated from other wetlands might also contain fewer species.20 Another index of the relative amount of habitat fragmentation is the amount of perimeter (or edge) of wetland relative to its area. In Iowa, one third of the examined species occurred in lower densities in wetlands with more edge relative to its area.21 In addition, wetland loss can make the remaining wetlands in the landscape less suitable for some species. At a larger temporal scale, wetland loss from agricultural expansion was negatively correlated with annual abundances of some waterfowl, including Mallards and Northern Pintails.17
Reproduction: Breeding success for some species can potentially be lower near the perimeters (edges) of wetlands than toward the interior of wetlands because many nest predators cannot access deep water habitats within wetlands.22 These patterns in nest predation not only occur with habitat fragmentation but also with variation in precipitation. During dry periods, wetlands can be more easily accessed by nest predators, potentially resulting in lower breeding success. Although lower breeding success can occur as a function of water depth in wetlands, it remains unknown if and how anthropogenic habitat loss might further reduce breeding success in wetlands.
Survival: Fragmentation and habitat loss could also influence survival of some wetland birds because some predators, such as skunks, tend to forage near the edges of wetlands.16,23 Yet the influence of habitat loss on survival has not been well documented.
Patterns like these illuminate the importance of protecting complexes of wetlands in fragmented landscapes. Yet some of these influences are directly related to changes in habitat structure within wetlands. For example, as wetland size increases, habitat heterogeneity within wetlands tends to increase, leading to potential increases in bird diversity.16 Likewise, the structural changes of habitat near wetland edges probably produce the edge effects observed in some wetland systems. However, other factors influencing wetland birds, such as wetland isolation, are clearly operating at larger scales. Understanding local factors will be important for managing and conserving individual wetlands, but larger-scale perspectives will be critical for understanding and managing populations in fragmented landscapes.
Why worry about wetland birds?
If habitat loss is indeed having detrimental impact on wetland bird populations, what implications might these impacts have? Birds play critical roles in wetland systems by acting as herbivores, predators and prey, and being facilitators for plant dispersal — not to mention providing aesthetic values for bird watchers and other nature enthusiasts.
- Most wetland birds are insectivorous during the breeding season, foraging on aquatic invertebrates for themselves and their young. Birds (including adults as well as eggs, nestlings, and juveniles) act as prey for other vertebrates, such as snakes, skunks, and mink.
- Some wetland birds, such as Canada Geese, are also herbivores, feeding on aboveground plant parts. Herbivory by these species can have potentially strong impacts on habitat structure, plant species composition, and primary productivity in wetlands.7
- Wetland birds could be acting as dispersal agents for many wetland plants that do not use abiotic agents, such as wind, for dispersal into new areas. The importance of birds as dispersal agents will likely increase with loss and fragmentation of remaining wetlands because of the ability of birds to use landscapes at large scales.
- Finally, birds provide great intrinsic value to wetlands and other habitats. Birds play a highly visible role in wetland ecosystems. Observing vast numbers of ducks using wetlands during migration is a magnificent sight, as is hearing the raucous sounds of yellow-headed blackbirds singing to each other during the breeding season.
An answer: habitat restoration?
Habitat restoration is typically considered the alteration of an ecosystem back to its initial or pre-settlement state. Most wetlands are restored by breaking drainage tiles, thus returning hydrology to the system. However, areas are not typically replanted with wetland species.24 A key factor for restoring wetland areas will be determining which plant species are predominantly avian-dispersed and which are predominantly dispersed by other means. Those species that are not dispersed by birds might require more intensive restoration strategies than current practices typically entail. For example, recent comparisons of plant diversity and composition in restored and natural wetlands found that restored wetlands tend to have lower plant diversity than natural wetlands.24 Differences in restored wetlands were consistent with the hypothesis that differences were due to dispersal limitation. Regardless of the dispersal agent required, reducing isolation of wetlands by strategic planning or focusing on wetland complexes will likely improve restoration strategies focused on both plant and bird diversity.21
Restoration practices have undoubtedly produced habitat for some species of wetland-breeding birds. Recent work suggests that bird communities in restored wetlands can be similar to those found in natural wetlands,25 and that bird communities in restored wetlands have higher diversity and abundances than in habitats that were previously restored.12 For example, much of the restoration efforts in the midwestern United States has focused on restoring wetland habitat from agricultural areas. Agricultural areas tend to have lower diversity and abundances of breeding birds than restored wetlands.12 Yet it is still unclear whether populations in restored areas have high enough reproduction to make positive contributions to populations.
Conclusion
Public perceptions of wetlands have come a long way in the past few decades, yet wetlands are still being lost throughout the country. Habitat loss can directly impact birds using these areas by potentially influencing bird abundance and habitat use, reproduction, and survival. Habitat restoration is one of the only potential answers in counteracting wetland loss, but it remains unclear if and how restoration influences population dynamics of wetland birds. In addition, we need a better understanding of the ultimate roles birds play in these ecosystems to effectively address ongoing wetland loss and the strategies focused on mitigating this issue.
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