Effects Of Habitat Structure Research Articles

Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 442:59-70 (2011) - DOI: https://doi.org/10.3354/meps09393 Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions Rachel Lannin, Kevin Hovel* Department of Biology and Coastal & Marine Institute, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA *Email: hovel@sciences.sdsu.edu ABSTRACT: In seagrass habitats, high structural complexity often enhances prey survival by reducing predator–prey encounter rates and predator foraging success. However, faunal density and biomass typically increase with seagrass structural complexity, such that variation in prey or predator density may contribute to or alter relationships between structural complexity and prey survival. We determined whether variability in prey density alters relationships between seagrass structural complexity and (1) prey survival, (2) predator–prey encounter rates, and (3) behavioral components of predator–prey interactions in eelgrass Zostera marina habitat. Using transplanted eelgrass in laboratory mesocosms, we compared trends in mesopredator (juvenile giant kelpfish Heterostichus rostratus) feeding success and behavior, and predator avoidance behavior by prey (grass shrimp Hippolyte californiensis) between experiments in which we (1) held prey density constant while increasing eelgrass shoot density, or (2) increased prey density while increasing eelgrass shoot density. We found that increasing prey density (but not eelgrass density) enhanced proportional prey survival and decreased the probability that mesopredators would attack prey, whereas increasing eelgrass density (but not prey density) decreased the likelihood that prey would attempt to escape from mesopredator attack. Predator–prey encounter rates were influenced both by prey density and eelgrass density, whereas mesopredator attack success was not influenced by either eelgrass density or prey density. Our results suggest that variable prey density modifies relationships between habitat structure and prey survival through a variety of behavioral mechanisms, and that factors that covary with habitat structure, such as prey density, may alter the effects of habitat structure on predation. KEY WORDS: Epifauna · Habitat structure · Predation · Seagrass · Structural complexity · Zostera marina Full text in pdf format PreviousNextCite this article as: Lannin R, Hovel K (2011) Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions. Mar Ecol Prog Ser 442:59-70. https://doi.org/10.3354/meps09393 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 442. Online publication date: December 05, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Habitat structure influences below ground biocontrol services: A comparison between urban gardens and vacant lots

Urban agriculture offers a framework for local self-reliance by provisioning food security, employment opportunities, and other community benefits. However, urban agriculture must rely on the supporting and regulating services of the soil food web. Hence, we quantified belowground biocontrol activity in urban gardens and vacant lots in two post-industrial cities using an in situ insect baiting technique. Due to the differences in habitat structure, we hypothesized that belowground biocontrol services will differ between gardens and vacant lots and the influence of habitat structure would differ with the type of biocontrol organism. Results revealed that biocontrol activity, as assessed by % mortality of baited insects, varied between 51% and 98% with higher activity often recorded in vacant lots than gardens. Major contributions to bait insect mortality were by ants, followed by microbial pathogens and entomopathogenic nematodes, respectively. Ants showed higher ( p < 0.0001) % mortality in vacant lots (60% ± 33.4%) than in urban gardens (33.3% ± 22.2%) whereas microbial pathogens exhibited higher ( p < 0.0001) mortality in gardens (27.8% ± 15%) than vacant lots (8.3% ± 16.7%). Ants caused higher ( p < 0.0001) mortality when larger-mesh size cages were used compared with the smaller-mesh size cages, but mortality by microbial pathogens did not differ with cage type. The high biocontrol activity indicates the resilience of the soil food web in urban ecosystems and the differential effects of habitat structure on biocontrol activity can help guide landscape planning and vegetation management to enhance urban environments and boost local self-reliance.

Habitat type and schooling interactively determine refuge-seeking behavior in a coral reef fish throughout ontogeny

Habitat selection by coral reef fish is influenced by a variety of biological and abiotic factors, but the relative importance of these factors is expected to change throughout ontogeny, espe- cially for species that utilize different habitats during their life cycle. In this study, 2 in situ choice ex- periments were designed to test the individual as well as the interactive effects of habitat structure and schooling (with conspecifics or heterospecifics) on refuge-seeking behavior of a coral reef fish. The experiments were conducted for 3 size-classes of the common reef fish Haemulon flavolineatum, a species that shows multiple habitat shifts throughout its ontogeny. We tested the hypothesis that, due to the additive effects of schooling and structure in providing suitable refuge, fish would be more attracted to a microhabitat with conspecifics or heterospecifics than to a microhabitat or conspecifics alone. In the habitat-only experiment, early juveniles (<3 cm fork length) showed no preference for any microhabitat structures, whereas larger fish preferred complex mangrove structure to seagrass, rubble, or coral. In the second experiment, which offered various combinations of habitat, including habitat with both conspecifics and heterospecifics, the preference for mangrove structure was com- pletely replaced by attraction to conspecifics for all size-classes. Unexpectedly, the combination of conspecifics and habitat structure showed no additive effect. The results demonstrate that although H. flavolineatum makes multiple shifts in habitat throughout its life cycle, in the absence of other bio- logical or environmental drivers preference for shelter habitat past the early post-settlement stage remains the same. The study further demonstrates the critical role of the presence of conspecifics in microhabitat choice, and provides a better understanding of the relative importance of these factors, whether in isolation or additively, in selection of refuge habitat by a reef fish.

Testing the effect of habitat structure and complexity on nekton assemblages using experimental oyster reefs

Structurally complex habitats are often associated with more diverse and abundant species assemblages in both aquatic and terrestrial ecosystems. Biogenic reefs formed by the eastern oyster ( Crassostrea virginica) are complex in nature and are recognized for their potential habitat value in estuarine systems along the US Atlantic and Gulf of Mexico coasts. Few studies, however, have examined the response of nekton to structural complexity within oyster reefs. We used a quantitative sampling technique to examine how the presence and complexity of experimental oyster reefs influence the abundance, biomass, and distribution of nekton by sampling reefs 4 months and 16 months post-construction. Experimental oyster reefs were colonized immediately by resident fishes and decapod crustaceans, and reefs supported a distinct nekton assemblage compared to mud-bottom habitat. Neither increased reef complexity, nor age of the experimental reef resulted in further changes in nekton assemblages or increases in nekton abundance or diversity. The presence of oyster reefs per se was the most important factor determining nekton usage.

Regional vs local effects of habitat loss and fragmentation on two plant-animal interactions

The effects of habitat fragmentation on plant–animal interactions may emerge at different spatial scales, depending on the species-specific perception response of the interacting animals. Furthermore, changes in habitat cover and configuration commonly occur simultaneously, hampering efforts to understand and mitigate the impact of fragmentation on these biotic interactions. In order to account for the relative influence of habitat loss and fragmentation on plant–animal interactions, we quantified habitat structure in sixteen sectors (nested circular areas of 100 and 200 m radii) in four different localities (four sectors per locality) across the Cantabrian Range in NW Spain. In the center of each 100 m radius sector, we measured the magnitude of two ecologically opposite (mutualistic vs antagonistic) interactions in individual holly trees Ilex aquifolium which strongly determine the regeneration process in this plant species: frugivory by birds and seed predation by rodents. We found that habitat fragmentation, though not habitat loss, affected the magnitude of both plant–animal interactions. However, these effects were conditioned by the strong differences in spatial heterogeneity in habitat structure between localities. In fact, the effect of habitat fragmentation on both plant–animal interactions disappeared when the locality in which sectors were sited was taken into account. This study highlights that 1) habitat spatial configuration, far from being a negligible component of habitat structure, is in fact able to influence key ecological processes such as plant–animal interactions, and 2) the potential spatial and structural complexity of localities makes a regional approach (i.e. that involving many localities) indispensable in the quest for comprehensive understanding of the effects of habitat structure on biodiversity in real-world fragmented landscapes.

Rainfall and landscape features affect productivity in an alpine population of Eagle Owl Bubo bubo

We investigated the effects of habitat structure and composition, rainfall and nest spacing on the productivity of an Eagle Owl Bubo bubo population in Piedmont, northern Italy, at 10 sites from 1996 to 2007. We modeled the effects of the above factors on the productivity of 85 breeding attempts through a mixed model procedure. The number of fledged young per pair was affected positively by wetland interspersion index (a potential estimate of prey abundance) and negatively by rainfall during chick-rearing and by cover of urbanized land in the nest surroundings. Our results demonstrated that productivity can be affected by a variety of factors of different kinds, which should be considered together in studies on habitat quality.

Topographic complexity and landscape ­temperature patterns create a dynamic habitat structure on a rocky intertidal shore

Habitat complexity is a fundamental dri- ver of ecosystem structure and function. Rocky inter- tidal shores are the classic model system for investi - gating theoretical and mechanistic models of habitat complexity, because the structural topography is largely biologically independent and stable over time. In the present study, we investigate how static (topographic complexity and heterogeneity) and dynamic (tempera- ture landscape) components of habitat structure influ- ence the abundance and body-size distribution of in - vertebrates on a rocky shore. Using a novel approach that included high-resolution 3D fractal surface and temperature measurements, we show that topographic complexity, temperature landscape and habitat hetero- geneity had largely independent effects on species abundance and body size. Invertebrate abundance was associated with temperature landscape in 7 of 11 spe- cies, while variation in body-size was mostly driven by fractal topography in 7 of 10 species. Overall, the body sizeabundance relationship was not influenced by habitat structure, indicating that the effects of habitat structure on body size and abundance were largely independent. The results illustrate the value of combin- ing measures of static and dynamic habitat structure to explain and predict biological patterns on rocky shores.

Feeding response of a benthic copepod to ciliate prey type, prey concentration and habitat complexity

Summary 1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3–69 ciliates copepod−1 h−1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1/3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 μgC out of a maximum of 1.1 μgC copepod−1 h−1). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod–ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.

Habitat simplification affects nuclear-follower foraging association among stream fishes

Nuclear-follower interaction is a particular kind of interspecific foraging association that includes a nuclear species, which dig in or inspect the bottom, and follower species, which access the food items made available by the nuclear. In this study we examined the effect of habitat structure on nuclear-follower relationship in a stream of Bodoquena Plateau, Central-West Brazil. Foraging associations were registered while snorkeling in 24 observation sessions, totaling six hours in unaltered and altered sites. Overall, 272 nuclear-follower associations were registered, having four species acting as nuclear and seven as followers. The dominant nuclear species were different in each site. Prochilodus lineatus was the main nuclear species in the altered site and Leporinus macrocephalus in the unaltered site. The richness of follower species was similar between sites, however, follower species abundance per interaction were significantly higher in the unaltered site than in the altered site. These differences seem to be a consequence of the alterations in assemblage composition and feeding behavior of the nuclear species that presumably are affected by different substrate composition and food availability between the structurally distinct areas.

Linking amphibian call structure to the environment: the interplay between phenotypic flexibility and individual attributes

The structure of the environment surrounding signal emission produces different patterns of degradation and attenuation. The expected adjustment of calls to ensure signal transmission in an environment was formalized in the acoustic adaptation hypothesis. Within this framework, most studies considered anuran calls as fixed attributes determined by local adaptations. However, variability in vocalizations as a product of phenotypic expression has also been reported. Empirical evidence supporting the association between environment and call structure has been inconsistent, particularly in anurans. Here, we identify a plausible causal structure connecting environment, individual attributes, and temporal and spectral adjustments as direct or indirect determinants of the observed variation in call attributes of the frog Hypsiboas pulchellus. For that purpose, we recorded the calls of 40 males in the field, together with vegetation density and other environmental descriptors of the calling site. Path analysis revealed a strong effect of habitat structure on the temporal parameters of the call, and an effect of site temperature conditioning the size of organisms calling at each site and thus indirectly affecting the dominant frequency of the call. Experimental habitat modification with a styrofoam enclosure yielded results consistent with field observations, highlighting the potential role of call flexibility on detected call patterns. Both, experimental and correlative results indicate the need to incorporate the so far poorly considered role of phenotypic plasticity in the complex connection between environmental structure and individual call attributes.

Effects of Habitat Structure and Fragmentation on Diversity and Abundance of Primates in Tropical Deciduous Forests in Bolivia

Habitat structure and anthropogenic disturbance are known to affect primate diversity and abundance. However, researchers have focused on lowland rain forests, whereas endangered deciduous forests have been neglected. We aimed to investigate the relationships between primate diversity and abundance and habitat parameters in 10 deciduous forest fragments southeast of Santa Cruz, Bolivia. We obtained primate data via line-transect surveys and visual and acoustic observations. In addition, we assessed the vegetation structure (canopy height, understory density), size, isolation time, and surrounding forest area of the fragments. We interpreted our results in the context of the historical distribution data for primates in the area before fragmentation and interviews with local people. We detected 5 of the 8 historically observed primate species: Alouatta caraya, Aotus azarae boliviensis, Callithrix melanura, Callicebus donacophilus, and Cebus libidinosus juruanus. Total species number and detection rates decreased with understory density. Detection rates also negatively correlated with forest areas in the surroundings of a fragment, which may be due to variables not assessed, i.e., fragment shape, distance to nearest town. Observations for Alouatta and Aotus were too few to conduct further statistics. Cebus and Callicebus were present in 90% and 70% of the sites, respectively, and their density did not correlate with any of the habitat variables assessed, signaling high ecological plasticity and adaptability to anthropogenic impact in these species. Detections of Callithrix were higher in areas with low forest strata. Our study provides baseline data for future fragmentation studies in Neotropical dry deciduous forests and sets a base for specific conservation measures.

Effects of habitat structure and tidal height on epifaunal assemblages associated with macroalgae

Patterns of distribution and abundance of epifauna often differ markedly among macroalgal species. The hypotheses tested were that (1) assemblages of mobile epifauna associated with Laminaria ochroleuca and Sargassum muticum differed because they have different structure, and (2) assemblages of mobile epifauna associated with S. muticum differed between heights on the shore because tidal height affects physical and biological conditions. We also investigated the effect of epiphytic biomass on the composition of epifaunal assemblages. Hypotheses were tested with measuring and manipulative experiments using natural and artificial algae, and by measuring uni- and multivariate assemblage descriptors. The results indicated that epifaunal assemblages associated with natural L. ochroleuca and S. muticum differed, but only differences in epifaunal densities were likely to be related to the structure of algae since all other variables did not clearly differ between the two algae. Although structure might play an important role, other factors need to be taken into account and further experimental tests are necessary. Epifaunal assemblages associated with S. muticum did vary depending on the height on the shore, but inconsistently over time in the case of natural algae. In addition, epifaunal densities of natural algae were positively related to biomass of epiphytes in both species. Time of sampling, epiphytic load and height on the shore were the most important factors in structuring epifaunal assemblages rather than complexity of the host algae.

Effects of land abandonment and changing habitat structure on avian assemblages in upland pastures of Bulgaria

SummaryOver the past half century, grassland birds in Europe have declined dramatically and in order to maintain and restore their populations it is critical to understand how habitat structures and quality within pastures affect birds. This study investigated the effects of habitat structure and pasture abandonment on grassland birds in IBA Ponor, western Bulgaria. Birds were sampled using the point count method at 143 randomly located circular plots and a total of 1,401 observations of birds from 31 species were recorded. The results showed that habitat complexity, management and landscape position influenced bird community structure and species occurrence within the upland pastures. Extensively grazed pastures supported higher structural complexity of vegetation cover and higher bird-species richness and diversity compared with abandoned ones. Moreover, bird species with a preference for grazed rather than abandoned pastures had higher conservation status and most were associated with shrub cover. To maintain high levels of avian diversity, habitat complexity within pastures should be maintained through extensive grazing, to ensure availability of scrub vegetation wherever possible. Finally, this study provided evidence that agri-environmental schemes should not be directly extrapolated from one country or region to another without been tested first, because within the same management, differences in habitat structural characteristics may exist due to the landscape and socio-economic characteristics of the region.

Effects of habitat structure and shrub encroachment on bird species diversity in arid savanna in Northern Cape province, South Africa

Bird community diversity was assessed in semi-arid savanna in the Northern Cape province of South Africa and related to vegetation diversity and foliage height diversity. As this is an area in which shrub encroachment is common, the effects of shrub or bush encroachment on bird species diversity were specifically addressed. An experiment in which the dominant encroaching species, Acacia mellifera, was removed from ten 1 ha plots was used and compared with ten controls. A strong positive association with foliage height diversity was shown. No effect of experimental removal of the dominant encroacher, A. mellifera, on bird species diversity or species richness was found. However, four species increased in relative abundance in cut plots and two species decreased in relative abundance in these plots.

Preoutbreak Dynamics of a Recently Established Invasive Herbivore: Roles of Natural Enemies and Habitat Structure in Stage-Specific Performance of Gypsy Moth (Lepidoptera: Lymantriidae) Populations in Northeastern Wisconsin

A major challenge to addressing biological invasions is that the need for emergency responses often precludes opportunities to analyze the dynamics between initial establishment and population eruption. Thus, a broader understanding of underlying processes and management opportunities is often lacking. We examined the effects of habitat structure and natural enemies on recently established preeruptive gypsy moth, Lymantria dispar L., populations over 4 yr in northeastern Wisconsin. Forty-five sites were established across a range of habitat structures in oak-dominated northern hardwood forests. The number of egg masses was positively related to percent composition of oaks and other favored species. Other life stages were not related to habitat structure variables. Abundance of each life stage can predict the subsequent life stage with variable degrees of accuracy, but male moth densities were only a weak predictor of egg mass or larval densities the following year. The parasitic fly Compsilura concinnata (Meigen), an introduced generalist with deleterious nontarget effects, caused the highest mortality to larvae. The specialist pathogens Entomophaga maimaiga and nucleopolyhedrosis virus were widely distributed but caused less mortality than reported in the northeastern United States, where gypsy moth has been established much longer. Small mammals are the major predators of pupae as elsewhere, but invertebrates seem less important along the western than southern advancing front of gypsy moth. Overall habitat structure did not influence natural enemy populations. These results suggest that the pre-eruptive phase is distinct from the pre-establishment phase by high mating success and from the eruptive phase by the prominent role of generalist natural enemies. Improved understanding of these dynamics can help guide silvicultural and biocontrol strategies in newly invaded regions of the Midwest and provide general insight into invasive forest defoliators.

Territory characteristics and coexistence with heterospecifics in the Dartford warbler Sylvia undata across a habitat gradient

The study of successional gradients may help to understand the relative influence of habitat structure and competition on territory characteristics. Here, we evaluate the effects of vegetation cover, conspecific and heterospecific densities, and distance to the nearest neighbor on territory size, shape, and overlap in insectivorous birds. We studied these effects along a gradient of postfire habitat regeneration in which foliage cover and densities of focal species varied several-fold. We delineate 197 territories (minimum convex polygons) of the shrub-dwelling Dartford warbler (Sylvia undata) and 255 of the syntopic Sardinian (Sylvia melanocephala), subalpine (Sylvia cantillans) and melodious (Hippolais polyglotta) warblers at three plots in NE Catalonia (Spain and France) in 1987–2005. After accounting for the effect of the number of locations used to delineate polygons, generalized linear mixed models (GLMM) showed a reduction in territory area of the Dartford warbler as conspecific density increased and distance to nearest neighbor decreased, in accordance with the contender pressure hypothesis for territory size regulation. Heterospecific density was not included in the final model of territory size and the effect of habitat structure was marginal. Territory roundness was positively correlated with its size and with conspecific density, probably in relation to energetic constraints, and negatively with heterospecific density. Territorial exclusion was almost complete among Dartford warblers, whereas interspecific territory overlap was extensive and tended to increase with heterospecific density and with structural diversity along the gradient. Our results support the hypothesis that Mediterranean warbler coexistence derives from ecological segregation and not from interspecific territoriality.

Multitrophic effects of nutrient addition in upland grassland

Although the effects of nutrient enhancement on aquatic systems are well documented, the consequences of nutritional supplements on soil food webs are poorly understood, and results of past research examining bottom-up effects are often conflicting. In addition, many studies have failed to separate the effects of nutrient enrichment and the physical effects of adding organic matter. In this field study, we hypothesised that the addition of nitrogen to soil would result in a trophic cascade, through detritivores (Collembola) to predators (spiders), increasing invertebrate numbers and diversity. Nitrogen and lime were added to plots in an upland grassland in a randomised block design. Populations of Collembola and spiders were sampled by means of pitfall traps and identified to species. Seventeen species of Collembola were identified from the nitrogen plus lime (N+L) and control plots. Species assemblage, diversity, richness, evenness and total number were not affected by nutrient additions. However, there was an increase in the number of Isotomidae juveniles and Parisotoma anglicana trapped in the N+L plots. Of the 44 spider species identified, over 80% were Linyphiidae. An effect on species assemblage from the addition of N+L to the plots was observed on two of the four sampling dates (July 2002 and June 2003). The linyphiid, Oedothorax retusus, was the only species significantly affected by the treatments and was more likely to be trapped in the control plots.The increased number of juvenile Collembola, and change in community composition of spiders, were consequences of the bottom-up effect caused by nutrient inputs. However, despite efforts to eliminate the indirect effects of nutrient inputs, a reduction in soil moisture in the N+L plots cannot be eliminated as a cause of the invertebrate population changes observed. Even so, this experiment was not confounded by the physical effects of habitat structure reported in most previous studies. It provides evidence of moderate bottom-up influences of epigeic soil invertebrate food webs and distinguishes between nutrient addition and plant physical structure effects. It also emphasises the importance of understanding the effects of soil management practices on soil biodiversity, which is under increasing pressure from land development and food production.

Abundance, spatial distribution and habitat relationships of echinoderms in the Cabo Verde Archipelago (eastern Atlantic)

The analysis of spatial variability in distribution and abundance of echinoderms may help in identifying the range of processes that can explain the observed patterns of this important component of benthic communities. The distribution and abundance of the echinoderm assemblage inhabiting the shallow rocky reefs at the Cabo Verde archipelago (where few studies other than descriptive ones have been performed until now) was quantified at three spatial scales (among islands, between locations within islands, and among replicates), at two depth strata, and related to fine-scale variation of habitat structure. Total echinoderm abundance and the abundance of the sea urchins Diadema antillarum and Eucidaris tribuloides, and the holothurian Euapta lappa were heterogeneous at the largest considered scale. Most species and habitat descriptors exhibited spatial variability at finer scales. There were significant relationships between habitat architecture and depth and both assemblage parameters and species abundances. Although the effects of habitat structure were species-specific, the probability of occurrence of Asteroidea, Ophiuroidea and Holothuroidea species was higher in heterogeneous habitats. Meanwhile Echinoidea and Holothuroidea species showed higher correlations to complex habitats. The observed spatial patterns are inferred to reflect behavioural responses to fine-scale microhabitat complexity, as well as broad-scale oceanic variables and recruitment dynamics.

Avian diversity in a Kenyan agroecosystem: effects of habitat structure and proximity to forest

Tropical agroecosystems cover an increasingly large proportion of the Earth’s terrestrial surface. Yet, relatively little is known about the factors that influence their avifauna, especially in areas of high human population density. The potential of tropical farmland for sustaining bird biodiversity, including forest birds, can be influenced by habitat structure and the distance from the nearest forest. We investigated the effect of these two factors on the bird community in the farmland near Kakamega Forest, Kenya. Using point counts, we assessed the number of bird species and individuals on 56 study plots in distances up to about 2,100 m from the forest. We observed a total of 96 bird species in the farmland, 22 of which were forest, 58 shrub-land, and 16 open-country species. High vertical vegetation heterogeneity and a large number of woody plant individuals were related to high species richness of forest and shrub-land birds, whereas open-country birds avoided such areas. The species richness and total number of forest birds declined with increasing distance to the forest. A comparison with the bird community within Kakamega Forest indicated that only a fraction of the forest species could be sustained in the farmland. This suggests that agroecosystems with a diverse habitat structure can support a high diversity of birds, but have only a limited capacity to compensate for forest loss.

Deciduous habitat fragmentation caused by a larch plantation matrix matters to birds in deciduous habitats

Few studies have examined the relative importance of habitat loss and fragmentation caused by structurally complex matrices such as plantations. We examined the effects of the loss and fragmentation of the original deciduous habitat (secondary deciduous broadleaf forest) caused by a larch plantation matrix on bird occurrences in deciduous habitats in both the winter and breeding seasons in Chikuma Highland, Nagano prefecture, central Japan. Birds were counted using the plot-count method in 33 (winter) and 51 (breeding) deciduous habitats with a range of surrounding habitat loss and fragmentation at a 1600-m scale. Three species groups, for which larch plantations are likely to function as low-quality matrices, were analyzed. Effects of landscape structure were found only for flycatchers, which were negatively affected by habitat fragmentation. Flycatchers frequently occurred in habitats surrounded by elongated habitat patches in which between-patch distances were short. This effect was significant after the confounding effects of habitat structure were removed. Because habitat fragmentation was more important than habitat loss, the destruction of habitat connectivity, i.e., isolation and contraction of habitat patches, may not be compensated by the amount of habitat and may primarily be considered in low-contrast landscapes.