Douglas J. Levey

Habitat fragmentation and its lasting impact on Earth's ecosystems

Urgent need for conservation and restoration measures to improve landscape connectivity. We conducted an analysis of global forest cover to reveal that 70% of remaining forest is within 1 km of the forest’s edge, subject to the degrading effects of fragmentation. A synthesis of fragmentation experiments spanning multiple biomes and scales, five continents, and 35 years demonstrates that habitat fragmentation reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering nutrient cycles. Effects are greatest in the smallest and most isolated fragments, and they magnify with the passage of time. These findings indicate an urgent need for conservation and restoration measures to improve landscape connectivity, which will reduce extinction rates and help maintain ecosystem services.

Corridors affect plants, animals, and their interactions in fragmented landscapes

Among the most popular strategies for maintaining populations of both plants and animals in fragmented landscapes is to connect isolated patches with thin strips of habitat, called corridors. Corridors are thought to increase the exchange of individuals between habitat patches, promoting genetic exchange and reducing population fluctuations. Empirical studies addressing the effects of corridors have either been small in scale or have ignored confounding effects of increased habitat area created by the presence of a corridor. These methodological difficulties, coupled with a paucity of studies examining the effects of corridors on plants and plant–animal interactions, have sparked debate over the purported value of corridors in conservation planning. We report results of a large-scale experiment that directly address this debate. In eight large-scale experimental landscapes that control for patch area and test alternative mechanisms of corridor function, we demonstrate that corridors not only increase the exchange of animals between patches, but also facilitate two key plant–animal interactions: pollination and seed dispersal. Our results show that the beneficial effects of corridors extend beyond the area they add, and suggest that increased plant and animal movement through corridors will have positive impacts on plant populations and community interactions in fragmented landscapes.

Are Plant Populations Seed Limited? A Critique and Meta‐Analysis of Seed Addition Experiments

We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (establishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta‐analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degree are populations seed and establishment limited? (2) Under what conditions (e.g., habitats and life‐history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. However, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large‐seeded species, species in disturbed microsites, and species with relatively short‐lived seed banks. Most seed addition experiments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be overcome by increasing the number and range of seed addition treatments.

Spatial and temporal variation in Costa Rican fruit and fruit-eating bird abundance

Understory fruit and fruit-eating birds were censused monthly for a year in gaps, intact forest, and second-growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit-eating bird abundance. Fruits were most abundant in the mid-to-late rainy season (August-January). Crop sizes were larger on second-growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit-eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long-term maintenance of frugivore populations.

CORRIDOR USE BY DIVERSE TAXA

One of the most popular approaches for maintaining populations and con- serving biodiversity in fragmented landscapes is to retain or create corridors that connect otherwise isolated habitat patches. Working in large-scale, experimental landscapes in which open-habitat patches and corridors were created by harvesting pine forest, we showed that corridors direct movements of different types of species, including butterflies, small mam- mals, and bird-dispersed plants, causing higher movement between connected than between unconnected patches. Corridors directed the movement of all 10 species studied, with all corridor effect sizes .68%. However, this corridor effect was significant for five species, not significant for one species, and inconclusive for four species because of small sample sizes. Although we found no evidence that corridors increase emigration from a patch, our results show that movements of disparate taxa with broadly different life histories and functional roles are directed by corridors.

Arrival and Survival in Tropical Treefall Gaps

Most tropical tree species require light from a treefall gap at some time during their lives to reach maturity. Responses to light conditions have been dichotomized as shade-intolerant pioneers or shade-tolerant climax species (e.g., Whitmore 1975, 1982, 1989). The former typically have small, widely dispersed seeds from which juveniles establish only in gaps, while the latter typically have larger seeds that can germinate beneath the forest canopy and can persist as suppressed juveniles or grow slowly until a gap forms. According to this framework, a new gap promotes shade-intolerant regeneration through germination and shade-tolerant regeneration through release of suppressed juveniles. Truly shade-tolerant species can grow to maturity beneath the forest canopy, but even these are likely to benefit from any increases in light levels beneath the canopy (Uhl et al. 1988, Canham 1989, Lieberman et al. 1989b, Martinez-Ramos et al. 1989). Although useful, we believe this dichotomy limits views of gap dynamics by implying that each species is constrained to a specific pathway to the forest canopy. In reality, all species recruit to differing degrees from dispersal into new gaps and from release of dormant seed or juvenile banks beneath the canopy (see Martinez-Ramos et al. 1989). The probability that a tree of a given species will enter the forest canopy is a function of the joint probabilities of arriving and surviving in particular habitats. We emphasize three issues that, for any species, define probable regeneration at a given site: (1) pattern of seed arrival in gaps and beneath the canopy, (2) proportion of forest area in gap vs. closed canopy, and (3) survival to reproductive maturity of seeds landing in gaps and beneath the canopy. This view enables recruitment of tropical trees to be interpreted from the perspective of relative advantages of given characteristics within the context of those environments in which individuals with those characteristics are located.

SECONDARY METABOLITES OF FLESHY VERTEBRATE-DISPERSED FRUITS: ADAPTIVE HYPOTHESES AND IMPLICATIONS FOR SEED DISPERSAL

We discuss seven hypotheses to explain the adaptive significance of secondary metabolites in ripe fleshy fruits and their implications for seed dispersal. These hypotheses are the attraction/association, seed germination inhibition, attraction/repulsion, protein assimilation, gut retention time, directed toxicity, and defense trade‐off hypotheses. We examine evidence that supports or refutes these hypotheses and suggest further tests of each. In addition, we summarize recent work with Solanum fruit pulp glycoalkaloids that bears directly on three of these hypotheses (directed toxicity, gut retention time, and defense trade‐off). We conclude that evidence addressing many of these hypotheses is either observational or indirect, but most hypotheses find at least some level of support. Because many of the hypotheses are not mutually exclusive, we also conclude that synergistic interactions and multifunctionality in secondary metabolites may provide economical evolutionary solutions for plants facing disparate and temporally variable selective pressures that impinge on fruits and seeds.

Corridors Increase Plant Species Richness at Large Scales

Habitat fragmentation is one of the largest threats to biodiversity. Landscape corridors, which are hypothesized to reduce the negative consequences of fragmentation, have become common features of ecological management plans worldwide. Despite their popularity, there is little evidence documenting the effectiveness of corridors in preserving biodiversity at large scales. Using a large-scale replicated experiment, we showed that habitat patches connected by corridors retain more native plant species than do isolated patches, that this difference increases over time, and that corridors do not promote invasion by exotic species. Our results support the use of corridors in biodiversity conservation.

Tropical Wet Forest Treefall Gaps and Distributions of Understory Birds and Plants

I examined the patch dynamics of understory birds and fruiting plants in treefall gaps and intact forest sites to determine the importance of gaps in structuring a Costa Rican tropical wet forest community. Understory birds and fruiting plants were censused monthly for 12 mo in 13 gaps and 13 intact forest sites. Of species with adequate sample sizes, 40% of bird species (17 of 42) and 30% of fruiting plant species (10 of 33) were found significantly more often in gaps than in intact forest. In contrast, only 5% of the bird species (2 of 42) and no plant species were found significantly more often in intact forest sites. Frugivorous and nectarivorous bird species were especially common in gaps. Birds of second—growth and forest edge were proportionately more abundant in gaps than were birds of forest interior. Canopy birds were not prevalent in gaps. Plants in gaps tended to produce more fruit over a longer period of time than conspecifics fruiting under a closed canopy. Also, large gaps had higher densities of fruiting plants and fruit—eating birds than small gaps. The high diversity and long fruiting periods of plants in gaps suggest that gaps may be important sources of fruit during periods of fruit scarcity. Given the high density of fruit—eating birds in gaps, seed deposition patterns generated by these birds are probably nonrandom. In particular, seeds may be dispersed into or around the periphery of gaps more often than into closed understory sites. Thus, the distributional patterns of understory fruits and frugivores may reinforce each other; fruiting shrub density affects the distribution of fruit—eating birds, which then influences the distribution of fruiting plants and birds in the next generation.

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.