Herbivore Exclusion Experiment Research Articles

Limitations on reproductive success in endemic Aquilegia viscosa (Ranunculaceae) relative to its widespread congener Aquilegia vulgaris: the interplay of herbivory and pollination

Plant reproduction can be strongly affected by herbivory and different features of pollination ecology, such as pollinator visitation rates and capacity for self-pollination. The purpose of this study is to compare the relative impact of herbivory and pollination on maternal reproductive success in endemic Aquilegia viscosa and its widespread congener Aquilegia vulgaris. We conducted herbivore exclusion experiments in two populations of each species in 2 different years and showed that the maternal fertility of A. viscosa was significantly more limited by floral predation and pre-dispersal seed predation than its widespread congener. In the absence of herbivory, A. viscosa retained significantly lower maternal fertility than A. vulgaris. Experimental pollinations in an insect-free glasshouse showed that the two species have an equal seed/ovule ratio both in the absence of pollinators and in the presence of non-limiting outcross pollination. Pollinator visitation rates were significantly higher in populations of A. vulgaris than in populations of A. viscosa. In addition, path analyses showed that spur length, an important trait for pollinator attraction in Aquilegia, and, indirectly sepal and petal width, contribute positively to the seed/ovule ratio in A. vulgaris, but not in A. viscosa. These results indicate that maternal fertility of endemic A. viscosa is strongly reduced by flower and seed predation despite low rates of pollinator visitation, and that pollen or resource limitation in the wild may further reduce maternal fertility. Finally, floral trait variation appears to be decoupled from fertility variation in endemic A. viscosa, which possibly constrains the evolution of reproductive traits in this species.

Regulation of shrub dynamics by native browsing ungulates on East African rangeland

Abstract Herbivores, edaphic features and fire are primary factors regulating the balance between woody and herbaceous vegetation in savannas. Many observational studies have evaluated the potential effects of browsing herbivores on woody plant dynamics in African savannas, but few experimental studies have compared the dynamics of African savannas with and without browsers. A replicated herbivore exclusion experiment was used to assess the role that native ungulates play in regulating woody plant dynamics on commercial rangeland in central Kenya, where the indigenous fauna have been allowed to coexist with cattle. Exclusion of native browsing ungulates for just 3 years showed that they have dramatic effects at every scale from individual twig growth rates to overall rates of woody biomass accumulation in the ecosystem. At the scale of individual Acacia twigs, browsers significantly reduced leaf density, leaf biomass and growth rates of twigs < 50 cm above‐ground (within the foraging height of dik‐diks Madoqua kirkii), but browsers had no effects on twig leaf density or leaf biomass at a height of 1·0–1·25 m above‐ground. Reductions in the growth rate of twigs within the foraging height of dik‐diks was associated with a 6‐fold reduction in the rate at which shrubs < 0·5‐m tall were recruited into the 0·5–1·5 m height class. This reduced recruitment combined with measured rates of shrub mortality in larger height classes shows that browsers reduced the rate of increase in shrub density nearly to zero (7·1 ± 10·2 shrubs ha−1 year−1) compared to the rapid rate of increase in the absence of browsers (136·9 ± 13·6 shrubs ha−1 year−1). Damage to shrub canopies by elephants Loxodonta africana caused large, significant reductions in cover of A. mellifera and Grewia tenax, but lesser reductions in cover of A. etbaica. For Acacias, elephant damage was focused on shrubs > 2·5 m tall, such that Acacias in intermediate height classes (0·5–2·5 m) experienced minimal browser impacts. Elephants influenced shrubland dynamics by altering shrub height‐class distributions, shifting species composition from broad‐leaved Grewia tenax to fine‐leaved Acacia species, and suppressing woody biomass accumulation; but elephants had little influence on changes in shrub density. Synthesis and applications. Our results suggest that a community of native browsers that includes both small, selective species (e.g. dik‐diks) and large, bulk‐feeding species (elephants) can provide an important ecosystem service by suppressing shrub encroachment on commercial rangeland.

Convergent Demographic Effects of Insect Attack on Related Thistles in Coastal vs. Continental Dunes

Insect herbivory is common, but the conditions under which it reduces the fitness and population size of plants remain poorly understood. We quantified population-level impacts of floral herbivory by specialized insects on cobweb thistle (Cirsium occidentale var. occidentale) in a California coastal dune ecosystem, and then compared these demographic effects to those published for Platte thistle (Cirsium canescens) in similarly designed exclusion experiments in the continental sand dune ecosystem in the Great Plains. As a separate test of the strength of the seed-to-seedling linkage, we quantified seedling establishment rates in seed addition plots and compared these results to those obtained in herbivore exclusion experiments. This is the first test of direct vs. indirect methods of evaluating the potential impact of seed loss in plant dynamics. Floral herbivory on cobweb thistle in coastal dunes substantially reduced key components of plant fitness. Reduction of insect feeding within flower heads with insecticide increased mean per capita seed production by 144–316% and led to a 130–196% increase in cumulative seedling recruitment in the next generation, depending on dune habitat. Juvenile plant densities around insecticide-treated plants subsequently increased by >50%. Both seed and flower head addition experiments corroborated the seed limitation of recruitment and juvenile plant establishment for cobweb thistle; exclusion of postdispersal seed predators did not substantially alter this relationship. While seed addition results were qualitatively similar to herbivore exclusion results, seed addition overestimated the seed–seedling transition. These results are broadly parallel to those found previously for Platte thistle in continental dunes, where reduction of insect feeding in Platte thistle flower heads increased mean per capita seed production by 37–240%, depending on year and microhabitat. This reduction in seed translated into a 33–580% increase in cumulative seedling recruitment and juvenile plant density. Thus in both systems, insect exclusions demonstrated that floral herbivory limited numbers of seeds and seedlings and population density of juvenile plants. This unique comparison of effects of insect herbivory on plant dynamics suggests that for related plants, the character and outcome of such interactions are likely to be functionally similar in structurally convergent ecosystems.

CONVERGENT DEMOGRAPHIC EFFECTS OF INSECT ATTACK ON RELATED THISTLES IN COASTAL VS. CONTINENTAL DUNES

Insect herbivory is common, but the conditions under which it reduces the fitness and population size of plants remain poorly understood. We quantified population- level impacts of floral herbivory by specialized insects on cobweb thistle ( Cirsium occi- dentale var. occidentale) in a California coastal dune ecosystem, and then compared these demographic effects to those published for Platte thistle ( Cirsium canescens) in similarly designed exclusion experiments in the continental sand dune ecosystem in the Great Plains. As a separate test of the strength of the seed-to-seedling linkage, we quantified seedling establishment rates in seed addition plots and compared these results to those obtained in herbivore exclusion experiments. This is the first test of direct vs. indirect methods of evaluating the potential impact of seed loss in plant dynamics. Floral herbivory on cobweb thistle in coastal dunes substantially reduced key components of plant fitness. Reduction of insect feeding within flower heads with insecticide increased mean per capita seed production by 144-316% and led to a 130-196% increase in cumulative seedling recruit- ment in the next generation, depending on dune habitat. Juvenile plant densities around insecticide-treated plants subsequently increased by .50%. Both seed and flower head addition experiments corroborated the seed limitation of recruitment and juvenile plant establishment for cobweb thistle; exclusion of postdispersal seed predators did not sub- stantially alter this relationship. While seed addition results were qualitatively similar to herbivore exclusion results, seed addition overestimated the seed-seedling transition. These results are broadly parallel to those found previously for Platte thistle in continental dunes, where reduction of insect feeding in Platte thistle flower heads increased mean per capita seed production by 37-240%, depending on year and microhabitat. This reduction in seed translated into a 33-580% increase in cumulative seedling recruitment and juvenile plant density. Thus in both systems, insect exclusions demonstrated that floral herbivory limited numbers of seeds and seedlings and population density of juvenile plants. This unique comparison of effects of insect herbivory on plant dynamics suggests that for related plants, the character and outcome of such interactions are likely to be functionally similar in structurally convergent ecosystems.

Feeding selectivity of the herbivorous fish Scartichthys viridis:effects on macroalgal community structure in a temperate rocky intertidal coastal zone

The ecological importance of fish herbivory was examined in a temperate rocky intertidal zone along the central Chilean coast. In this system, the blenny Scartichthys viridis is the most abundant and only herbivorous fish. We describe its diet, determine its field food selectivity (gut contents vs macroalgal field availability comparison) and report on its laboratory dietary preferences in a multiple-choice feeding experiment. Finally, we evaluate the relative effects of fish herbivory and grazing invertebrates on macroalgal abundance, distribution and diversity patterns in the field, using replicated exclusion cages to manipulate grazing intensity. S. viridis was found to be a selective grazer; its diet was dominated by 2 macroalgae: the green Ulva and the red Gelidium, representing 32.6 and 41.4% of its total gut content biomass, respectively Ulva was consumed much above its field availability while Gelidium was selected only during fall-winter. In the laboratory, S. viridis selected the green macroalgae Ulva rigida and Codium dimorphum and avoided Gelidium chilense. We suggest that Gelidium may be included in its diet due to the limited availability of Ulva in the field. Experimental exclusion of this fish from rocky surfaces resulted in increased abundance of green foliose macroalgae (U. rigida and Enteromorpha linza), and, to a much lesser extent, of the red foliose macroalgae G. chilense, Porphyra columbina, and Mazzaella Iaminarioides, and of the brown macroalgae Colpomenia phaeodactyla and Petalonia fascia, as well as an extension of the distribution of P. columbina from the high intertidal to the mid intertidal zone, where it normally does not occur. Exclusion of grazing invertebrates resulted in a decrease in foliose macroalgal species richness, but had no effect on biomass. To our knowledge, this is the first experimental demonstration of the effect of an herbivorous fish on macroalgal community structure in a temperate rocky intertidal area. The selective feeding displayed by S. viridis, its high numerical abundance, and the results of the herbivore exclusion experiment suggest its importance in maintaining the low abundance of foliose macroalgae and the high relative cover of brown and red crustose macroalgae, characteristic of many low to mid rocky intertidal areas along the coast of central Chile.

Relaxation of an induced defense after exclusion of herbivores: spines on Acacia drepanolobium.

Descriptive and experimental evidence suggests that spine length is an inducible defense, with longer spines being produced by branches experiencing greater levels of herbivory. Here we present results from a replicated, controlled herbivore exclusion experiment in which cattle, wildlife (large mammalian herbivores), and megaherbivores (elephants and giraffes) were independently manipulated. Experimental wildlife barriers virtually eliminated herbivory on Acacia drepanolobium branches at all heights. Megaherbivore barriers reduced herbivory on branches more than 1.75 m from the ground by up to 80%, and reduced herbivory on lower branches by 40%. These patterns of herbivory were matched by patterns of relaxation of spine length that occurred in response to the treatments. After 22 months of herbivore exclusion, the lengths of newly produced spines were 19% shorter on branches protected from large mammal herbivory than on trees in control plots. On low branches, there was a steady increase in spine length from total exclusion plots (shortest spines) to plots with wildlife to plots with both megaherbivores and wildlife (longest spines). On higher branches, new spines were shorter in total exclusion plots and wildlife plots than in plots in which megaherbivores were allowed. This is the first replicated, controlled experimental demonstration that browsing by free-ranging herbivores is associated with greater spine lengths. Examination of trees incidentally protected from herbivory for several years suggests that reduction in spine length in the experimental plots will eventually exceed 70%. Initially slow relaxation of spine length may represent a cautious adaptive strategy in an environment where a given branch is likely to escape herbivory in a given growth season, even when herbivores are present.

KLEE: A long‐term multi‐species herbivore exclusion experiment in Laikipia, Kenya

Livestock and wildlife share much of their respective ranges throughout the semi‐arid ecosystems of the world. As the profitability of livestock production becomes more marginal and wildlife values increase, there is a need to understand the interactions between livestock and wild large mammalian herbivores (and other indigenous biodiversity). To address this, we have established a long‐term multi‐species herbivore exclusion experiment in the Laikipia ecosystem in Kenya. Using a series of semi‐permeable barriers, we are differentially excluding various combinations of cattle, large wild mammalian herbivores, and mega‐herbivores (giraffes and elephants) from a series of replicated four‐hectare plots. We are monitoring soil, plant, and animal responses to these treatment characteristics. This multi‐disciplinary project is one of the first to include controlled, replicated exclusion of combinations of multiple guilds of rangeland herbivores in the same place at the same time. We report here (1) the first quantitative vegetation analysis of this important grazing ecosystem; (2) details of the experimental design; (3) evidence of the effectiveness of the herbivore exclosures; and (4) a summary of some preliminary results.

Factors affecting regulation of maternal investment in an indeterminate flowering plant (Cercidium microphyllum; Fabaceae)

On palo verde trees, nearly 80% of potential offspring were lost during flower bud development. Flower buds at the base of racemes developed earliest and were more likely to survive depredations of gelechiid larvae or abortion from putative limited resources. Herbivory accounted for greater cumulative losses than putative resources; however, an herbivore exclusion experiment suggested that buds damaged by herbivores would have aborted anyway without damage. In a natural experiment, plants that received water runoff had significantly higher densities of racemes, with only slight increases in numbers of buds, flowers, or pods per raceme. Similarly, thinning racemes experimentally to assess limited resources among potentially competing racemes increased the number of viable flower buds per raceme only slightly, with negligible herbivore damage. The production of more racemes rather than more reproductive structures per raceme may be a mechanism to allocate limited resources more efficiently. Alternatively, in studies with animals, similar patterns in the production of broods are thought to be mechanisms to avoid nest predation. Thus, while putative limited resources and resource allocation patterns reduced the proximate effects of larvae, herbivory must be considered as a possible ultimate factor in the patterns observed here because limited resources may be allocated in ways to reduce herbivory.

Complexity of early and middle successional stages in a rocky intertidal surfgrass community.

A variety of simple models have been proposed to describe ecological succession (e.g., Connell and Slatyer 1977), but these models do not address some agents that may increase complexity. To determine the complexity of a natural sequence, four null hypotheses were tested: (1) seasonality of growth, recruitment, and mortality does not influence succession; (2) the interspecific interactions that produce successional change are the same throughout the sequence; (3) consumers have no influence on succession, and (4) small scale spatial variation in establishment and mortality does not occur. These hypotheses were tested in a low zone rocky intertidal community normally dominated by the surfgrass Phyllospadix scouleri on the Oregon coast.Succession experiments initiated in different seasons and an herbivore exclusion experiment falsified each of these hypotheses. At two sites (Boiler Bay and Squaw Island) experimental plots that were cleared in the spring were first colonized by Ulva sp., but those cleared in other seasons were first colonized by Phaeostrophion irregulare. Ulva appears adapted to colonize space made available by winter storms, whereas Phaeostrophion takes advantage of space made available by sand movement in the fall. Another seasonal pattern was the sharp decline in total algal cover in the fall, when wave action increased. At Squaw Island, the presence of Phaeostrophion established in fall and winter significantly inhibited the summer establishment of Ulva, though Ulva occupied some space epiphytically. In contrast, at Boiler Bay, a different, stronger type of interspecific interaction occurred: Phaeostrophion totally inhibited the establishment of filamentous diatoms. The cover of Ulva in the summer was also influenced by herbivores, but Ulva cover declined in the fall in both herbivore exclusions and controls, suggesting that herbivores were not solely responsible for its replacement. Local variation was demonstrated, because after three years of succession, replicate plots often differed. The early colonists, Phaeostrophion and Ulva, continued to dominate some plots; in other plots they had been replaced by middle successional species, including Cryptosiphonia woodii and Odonthalia floccosa. In still other plots Rhodomela larix had replaced other species. Similarly complex successional sequences occur in many natural communities. Thus, the features that simple models do not address may add complexity to succession, and for some communities different approaches must be developed.

Differential susceptibility to grazers in two phases of an intertidal alga: Advantages of heteromorphic generations

The intertidal red alga Gigartina papillata (C. Ag.) J. Ag. alternates between two major forms in its life cycle, an upright blade and a fleshy crust. The evolution and functional significance of such alternation of heteromorphic generations is little understood. This study, done on the outer coast of Washington state (U.S.A.), considers the rôle of physical factors, measured by intertidal position, and a biological factor, interactions with herbivores, as selective pressures potentially responsible for this pattern. Observations along transects and a herbivore exclusion experiment showed that the two phases of this alga simultaneously occupy similar intertidal positions but differ significantly and oppositely in their response to grazing. The number of crust plants declined in the absence of grazers, probably due to being overgrown and shaded by a thick algal film. It is, therefore, suggested that the crust form is grazer-dependent. In contrast, the number of blade phase plants increased in the absence of grazers. “Bet-hedging” is suggested in explanation of the maintenance of two phases in one cycle; the crust is more persistent but susceptible to overgrowth, while the more productive blade is more susceptible to grazing.