Ecological Effects of Interspecific Competition

Ecological Effects of Interspecific Competition: Definition - Individuals of one species suffer a reduction in fecundity, survivorship or growth as the result of either exploitation of resources or inference from individuals of another species These effects alter population dynamics and distribution of competing species, rendering their communities. Effects also affect a species evolution, influencing own distributions and dynamics. We focus on ecological and evolutionary effects on different ecological hierarchies. Adresses a difference between what it can do and what in practice it does. Competition among phytoplankton for phosphorous ● ● ● ● Plankton competed in pairs. Pop densities measured over time as resources were continually being added to medium but impact on limited resources (phosphorous) recorded. All were stable when grown in isolated conditions. The winner was a more efficient exploiter excluding less efficient ones for phosphorus. Coexistence and exclusion of competing for salmonid fishes: Dolly Valden Char & White Spotted Char - morphologically similar, closely related. White spotted futher upstream. Intermediate zone of overlap. Where one species is absent the fish expand their ranges. Aggressive alone when water temp brought to 12C (from 6C) but those temps were less aggressive in the presence of white spotted charr. No change in growth rate when alone but together, Dolly Varden would decrease with an increase in temp whereas whitespotted increased with temp. Supports at lower altitudinal Dolly Varden boundary the white spotted exclude Dolly’s due to more aggressive and foraging better and growing faster. Dolly varden did not outcompete white-spotted at lower boundaries (no evidence for upper boundary) - temperature mediated effects were minimal Key Points: Competing species coexist at one spatial scale but become more distinctive in their distribution at larger scales. Coexisted in the same stream but confined to different altitudinal zone. Species could survive perfectly well in locations where there was an absence of interspecific competition. Dolly Varden can live in white-spotted charr zone only when no white-spotted were there They both share parts of a similar fundamental niche - as the basic requirements of the Dolly Varden are also provided in white-spotted zone. But white-spotted does not provide a realized niche for Dolly Varden when present (Varden outcompeted). The fundamental niche is a list of conditions/resources allowing species to exist, grow and reproduce when considered in isolation. Realized niche is the same, but is able to grow in the presence of another species that is potentially harmful to its existence. Study shows experimental manipulations as a key to prodding nature's secrets. Coexistence of competing diatoms: Measured coexistence on two shared limiting resources. Bacterias limited by silicate and phosphate. One was a better competitor for phosphate and another microbe better for silicate. Creating realized niches when one of the factors was in short supply but when their was balanced supplies of both the diatoms coexisted. Coexistence of competing birds: Niche differentiation is not always conspicuous. 4 Warblers in meditaranian all feed on similar resources nad live near each other. All insectivores. However the bush height in this case creates a realized ecological niche. Different plant species and plant height creates preferences suited to some of the warblers and not others. Could conclude that they coexist and coptete via differential resource utilization (feed off slightly different resources) - difficult to conclude. You can measure competition if you remove one species and measure the effect. When this occurred, remaining species fledged 78%-129% more young due to having preferred nesting sites away from predators. Eliminate competition and you can measure the effect. Competition between unrelated species: Examples: Sea urchins removed off the virgin islands - increased parrotfish and surgeonfish abundance (formerly sharing seaweed). Interspecific competition across huge taxonomic divides. Competitive Exclusion Principle: If two competing species coexist in a stable environment, they do as a result of niche differentiation. If there is no such differentiation (precluded by habitat) - one will outcompete and exclude the other. Otherwise known mathematically as Lotka and Volterra model. Niche differentiation does not prove that a species is coexisting competitors, they may just not be competing at all and may have never done so in evolutionary history < competitive exclusion principle may not be that useful in practice as not many attempts to remove competitive species. Species that had once competed (evolutionary ancestors, left printed on evolution to give the impression. This principle also relies on the environment to be stable for long periods which is often not the case (constant supply of resources). Environmental heterogeneity: Patches are only available temporarily and appear at unpredictable times, at unpredictable places. Interspecific competition occurs under the influence of pathy, cyclic, impermanent and an unpredictable world. Some ants occur within the same 100m2 but are completely intolerable to each other and will not occupy the same tree. Trees become available after fire for recolonization. Exhibits competition-colonization trade off - when one improves, it's at the expense of another. The four different ants were optimized to different levels of tree damage, with the weakest ants being the quickest to colonize and larger ants being slowest. Presence of superior and inferior competitors suggests - intraspecific aggregated distributions whereby powers of the superior competitor were directed against members of its own species creating room for inferior to escape to. Natural world supports heterogeneity. Evolutionary Effects of Interspecific Competition: Invoking the ghost of competition past - coexisting species will exhibit differences in morphology/ physiology to ensure little competition between each other. Character displacement & ecological release of indian mongoose Where similar but larger mongoose species are present, the prey catching apparatus has been selected for reduced size (character displacement) - creating a realized niche because smaller predators take smaller prey. Ecological release - a response to the absence of ecological effects of other species. Canadian Sticklebacks: Evolution in action: selection on microorganisms > challenge to keep competition running for long enough > and for significant replication for changes to be detected. Often use competing strains of the same bacterial species rather than independent species for whole organism biologists. Interspecific competition and community structure: Inter.s provides potential to keep apart or drive apart the niches of coexisting competitors. Regulation of diversity of phytoplankton. Species diversity increased as potential limiting factors below survival threshold for growth. Species diversity increased as number of resources at physiologically limiting levels increased (more resources were more limiting) Niche complementarity: The tendency for coexisting species which occupy a similar position along one niche dimension to differ along with another. Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species cooccur, but are minimized or lost where the species’ distributions do not overlap. Species separated in space or time: Resources separated spatially - differential resource utilization expresses itself as microhabitat differentiation (fish feeding at different depths) or resources become available at different times of day / seasons. Additionally, species may share the same resources but can only compete at environmental conditions (high sunlight, low sunlight - each competitor arising to the superior competitor. Tends to have an effect within guilds - groups of species exploiting the same class of resource in a similar fashion. How significant is interspecific competition in practice? Need to distinguish between mere chance and interspecific competition driving species into niches. Connells review of prevalence of current competition - 215 species, 527 different experiments showed 40% influenced by interspecific competition. Though problems that species only studied if suspected of undergoing competition and if species don’t compete these results are not being reported. Interspecific competition - species differ more than chance alone To test this, null models are produced - if the actual data shows significant difference to null models, interspecific competition is strongly inferred.